Medical and Statistical Review of N-13 Ammonia Positron Emission
Tomography---February 18, 1999

Florence Houn, MD MPH FACP, Medical Officer

Sonia Castillo, PhD, Statistician

I. Introduction

This is a primary medical and statistical review of N-13 ammonia
radiopharmaceutical used in positron emission tomography (PET) for the
indication of assessing myocardial blood perfusion in the evaluation of
coronary artery disease (CAD), following separate administration under
rest and stress conditions, for patients suspected of having the disease
or who have an existing diagnosis of CAD.

Positron emission tomography radiopharmaceuticals have been evaluated
and approved by the Food and Drug Administration (FDA) for use as
radiopharmaceutical diagnostic agents. Typically, PET
radiopharmaceuticals have a short physical half-life and they are
manufactured using local cyclotrons or generators. In addition, many of
the manufacturers of PET radiopharmaceuticals are located in hospitals
and clinics, thus differing from the traditional pharmaceutical and
device manufacturers that FDA regulates. In addition, many academic
institutions have developed several PET radiopharmaceuticals. FDA has
been working with the PET community to develop appropriate criteria and
procedures to evaluate PET products for safety and effectiveness. On
November 21, 1997, the President signed the Food and Drug Administration
Modernization Act (FDAMA) into law. Section 121 of FDAMA requires FDA to
develop procedures under subsections (b) or (j) of section 505 (21 USC
355) for the approval of PET radiopharmaceuticals. FDA believes at this
time that it could support the approval of some commonly used PET
radiopharmaceuticals under section 505(b)(2) of the act (21 CFR
355(b)(2)).

One of the more commonly used PET radiopharmaceuticals is N-13 ammonia.
There have been published reports of N-13 ammonia PET investigations
since the 1970s. While the clinical uses for N-13 ammonia PET are
diverse in the medical literature, one common theme in N-13 ammonia PET
studies is the use of this radiopharmaceutical in the evaluation of
myocardial blood flow. Over a flow range of 0 to 300 ml/min per 100
grams, N-13 ammonia tissue concentrations are almost linearly related to
flow. At higher flow rates, there is non-linear uptake of N-13 ammonia
in myocardium. In addition, hemodynamic and metabolic alterations have
little effect on the relationship between flow and net N-13 ammonia
extraction. Areas of impaired perfusion appear as defects or lower
counts of radioactivity on images. Coupling the scan to a rest and
stress component (i.e., exercise or drug-induced hyperemia), similar to
rest and stress thallium testing, further assists in assessment of
myocardial perfusion under these conditions to aid in the evaluation of
CAD.

N-13 ammonia PET has several important features: it allows for a less
invasive evaluation of myocardial blood flow; it has high temporal
resolution with sampling rates of several seconds, which allow in vivo
measurements of regional blood flow; and, using validated
radiopharmaceutical kinetic models, quantitative assessment of blood
flow is possible. Because of the ability to correct for photon
attenuation with PET, it is postulated that PET may be important for
patients with equivocal single-photon emission computed tomography
(SPECT), e.g., thallium-201 and technetium-99m-based agents, results
related to the possibility of photon attenuation, such as patients whose
body habitus may cause photon attenuation. All these characteristics of
N-13 ammonia PET provide clinicians with the potential of additional
diagnostic information to assist them in evaluating patients suspected
of having CAD.

The current Agency recommendation for evaluating the diagnostic
effectiveness of radiopharmaceuticals for imaging myocardial perfusion
in the evaluation of CAD is to conduct studies with an external
reference standard of truth, coronary angiography. Coronary angiography
involves intravenous iodinated contrast material traversing vessels
based in part on osmotic pressure differences and lumen diameter.
Vessels with lumen diameters of less than 100 micrometers, however, are
not defined by angiography. Coronary angiography reveals anatomy of
arteries and arterioles and degree of stenosis. It is used to identify
CAD and its distribution. Thus, N-13 ammonia PET published studies using
coronary angiography as a standard of truth, may be evaluated to help
determine diagnostic testing performance.

The Agency has approved radiopharmaceuticals to identify regional
differences in uptake that are associated with perfusion abnormalities
of angiographically-identified vessels. The location of these perfusion
defects is correlated with the location of CAD on angiography. Approved
radiopharmaceuticals also identify perfusion abnormalities from smaller
vessels (including microperfusion) not identified by angiography. For
example, the FDA has approved SPECT radiopharmaceuticals for indications
based on their ability to image microperfusion (also called functional
perfusion) that is not visible on coronary angiography. These
indications for SPECT radiopharmaceuticals include the assessment of
myocardial perfusion at rest, exercise, or pharmacologically-induced
hyperemia and the evaluation of myocardial function and wall motion.

In the literature, N-13 ammonia PET has been used for these same types
of functional myocardial perfusion evaluations. Because N-13 is a
radionuclide of a natural constituent of ammonia, as opposed to
technitium-labeled products, it is expected to participate in the normal
physiological uptake of ammonia. N-13 ammonia is, as mentioned above,
highly extractable from the circulation into myocardial cells where it
is rapidly metabolized. This property may provide information on
microperfusion, in addition to conveying anatomical information (patency
or occlusion) about larger vessels. Because myocardial perfusion in
coronary artery disease encompasses both anatomical and functional
properties, it is important to evaluate N-13 ammonia PET performance
with respect to both of these characteristics. For assessing functional
aspects, standards other than angiography may be useful, e.g. exercise
tolerance, ejection fraction, wall motion, or clinical outcomes, such as
cardiac-specific morbidity and mortality.

Of note, the patient population enrolled in N-13 ammonia PET clinical
trials, particularly the prospective trials, defined the population in
which the diagnostic test's safety and effectiveness were evaluated. No
population-based screening trials were published with N-13 ammonia PET
and none were reviewed for screening indications.

This review of the N-13 ammonia PET literature serves as the clinical
and statistical evaluation of the peer-reviewed scientific literature
regarding the effectiveness and safety of N-13 ammonia in PET imaging to
assess myocardial perfusion in the evaluation of coronary artery
disease, following separate administration under rest and stress
conditions, for patients suspected of having the disease or who have an
existing diagnosis of CAD. Because of the publicly available safety and
effectiveness data documenting the product's use cited in this review,
safety and effectiveness requirements of section 505(b)(2) of the act
(21 USC 355(b)(2)) and part 314 (21 CFR part 314) for this product and
this use may be met by citing the docket number (Docket No.\_\_\_\_\_\_)
of this review.

II. Evaluation of the Effectiveness Data for N-13 Ammonia PET Imaging to
    Assess Myocardial Perfusion in the Evaluation of Coronary Artery
    Disease

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A.  Data Sources

FDA's Center for Drug Evaluation and Research (CDER) conducted a
literature search of the recent peer-reviewed, medical journals to
evaluate N-13 ammonia PET effectiveness data. Search criteria included:
studies published from January 1990 to July 1, 1998 identified as human
clinical trials with N-13 ammonia in PET, written in English, found by
searching on-line databases of Medline, Cancerlit, Derwent Drug File,
Biosis Preview, International Pharmacology Abstracts and Embase. Review
articles on N-13 ammonia PET imaging were identified using the same
criteria in the Cochrane Database for Systemic Reviews and Cochrane
Controlled Trials Register. Finally, FDA solicited references from the
PET community on N-13 ammonia from any time period published in
peer-reviewed journals.

The search generated 76 articles involving clinical trials with N-13
ammonia. Of these, the medical reviewer selected articles which allowed
comparison of N-13 ammonia PET myocardial perfusion results to a
recognized standard of myocardial perfusion in the evaluation of CAD,
such as coronary angiography, or to appropriate comparators, such as
rubidium-82 PET, scans using technetium-99m-based agents, or other
tests. Other articles were obtained using the reference sections of the
primary articles identified in the above searches.

In all, 17 articles are included in this review of effectiveness. Two
articles were reviewed as being adequate and well-controlled published
studies, 3 were reviewed as controlled studies that were supportive, 9
were reviewed as other published studies that had supporting
information, and 3 served as references for a modeling standard that
quantified myocardial blood flow. The articles are listed in
chronological order under subheadings.

B.  Published Literature

1\. Well-controlled and adequate published clinical trials are described
below. These published trials allowed FDA to assess the effectiveness of
N-13 ammonia PET for myocardial perfusion in the evaluation of CAD by
meeting the following criteria: there was comparison between N-13
ammonia PET and an accepted "truth" standard of coronary perfusion or
stenosis imaging, coronary angiography; the study population was
prospectively enrolled; the entry criteria defined the target clinical
population in which N-13 ammonia PET was intended to be used; there were
clearly defined endpoints; detailed data on findings were presented; and
there were procedures to minimize interpretation bias, such as masking
(also referred to as blinding) and randomization, when flow measurement
was not performed quantitatively. The first article, Gould (1986), is
the initial report of findings and the second article, Demer (1989), is
the comprehensive report that includes data from the first study.

a\. *Gould LK, Goldstein RA, Mullani NA, Kirkeeide RL, Wong WH, Tewson
TJ, Berridge MS, Bolomey LA, Hartz RK, Smalling RW, Fuentes F, Nishikawa
A. Noninvasive assessment of coronary stenoses by myocardial perfusion
imaging during pharmacologic coronary vasodilation. Clinical feasibility
of positron cardiac imaging without a cyclotron using generator-produced
rubidium-82. J Am Coll Cardiol 1986;7:775-89.*

This study conducted at the University of Texas Medical School
determined the clinical feasibility of diagnosing significant coronary
artery disease by rubidium-82 PET; however, 23 of the 50 patients
studied were tested with N-13 ammonia. PET examinations were performed
at rest and after intravenous dipyridamole combined with handgrip
stress. Coronary angiography was performed.

[Inclusion Criteria]{.underline}: There were 50 patients admitted to the
protocol who were undergoing diagnostic coronary angiography for chest
pain syndromes, evaluation of myocardial infarction, before and after
percutaneous transfemoral coronary angioplasty, or as part of a protocol
of quantitative coronary angiography before and after plasmaphoresis for
cholesterol control.

[Dose]{.underline}: Two IV doses of 10-20 mCi N-13 ammonia or 30-50 mCi
rubidium-82.

[Schema of Trial]{.underline}: A comparison of patient coronary flow
reserve as measured by an automated, digitized coronary angiogram
program versus rubidium-82 or N-13 ammonia PET perfusion imaging.

[Image Protocol]{.underline}: PET images were either obtained before or
after cardiac angiography, but were not randomized. The selection of
rubidium-82 or N-13 ammonia was determined by availability of the
generator. When rubidium was unavailable, N-13 ammonia was used. All PET
images were interpreted visually by two observers masked as to the
clinical status and angiographic results of the patient. All images were
read masked three times in a procedure in which each reading session of
all studies utilized one of three different presentation formats: gray
scale, tricolor, or isocount color format. For the isocount color
format, which had the best correlation results, the composite
presentation of all nine heart slice images was interpreted in toto for
the entire heart on the rest and then stress studies. Slice-by-slice
comparisons were not relied on because of the problem of translation of
patient positions. The sessions were separated by 1 to 2 weeks. There
was no statement on how dispute resolution was handled. There was no
mention of inter-reader variability. Coronary angiograms were analyzed
using cine x-ray frames that were digitized using a computer program.
The program was automated to identify stenoses and measure stenosis
dimensions. From these dimensions, coronary flow reserve (CFR) was
calculated. There was no mention of the time interval between PET and
angiography examinations.

[Primary Endpoints]{.underline}: Coronary artery stenosis was based on
angiography and defined by using a scale of 0 to 5, with 0 being no flow
and 5 being normal. Significant CAD was determined to be a CFR of less
than 3.0 on angiography. The investigators discussed their rationale for
selection of 3.0 CFR. This endpoint was chosen prospectively. PET
detected perfusion defects were defined by the isocount color format and
the continuum of changing count densities as reflected in hue gradation
of each color corresponding to steps of 3% of maximal counts upon
evaluation of the rest and stress-hand grip images. All nine slices of
heart images had ranges of density counts scaled in 33 steps with black
for the lowest counts (\<20% of maximum counts), to blue (20-35% of
maximum counts), to green (35-50% of maximum counts), to yellow (50-68%
of maximum counts), to red (68-85% of maximum counts), to white (85-100%
of maximum counts). Reduction in coronary perfusion is manifested as
colors indicating the percentage reduction of maximum counts. Although
not specifically stated, it appears from the study's statement of, "...a
large anterior wall defect in which counts are 50% below the normal
areas, indicating at least a 50% reduction in coronary flow reserve,"
that percent count reduction is correlated to CFR.

[Results]{.underline}: Fifty patients were studied, but 6 patients were
excluded from analyses because of technical errors of positioning and
computer errors. These 6 patients were imaged as the first subjects and
the investigators learned from these initial experiences. There is no
mention of sex or age distribution. Data from rubidium-82 and N-13
ammonia studies were pooled. Twenty-two patients had CAD with an
angiographically determined CFR of less than 3.0 (significant CAD); 9
were without CAD on catheterization; and 13 had CAD with CFR of 3 or
greater (mild disease). PET sensitivity of identifying significant CAD
(less than 3.0 CFR) was 95% (21 out of 22), specificity (patients
without CAD) was 100% (9 out of 9), however, identifying insignificant
CAD (CFR of 3 or greater) was 31% (4 out of 13) for PET. This means only
31% of the patients with milder disease were identified by PET; 69% of
patients with milder disease were missed. Sensitivity for significant
left coronary artery system disease was 89% (16 of 18) and for right
coronary artery system disease it was 100% (9 of 9).

Rubidium and N-13 ammonia images were compared qualitatively. The study
states that the quality of images using the two radiopharmaceuticals
were comparable, although "the quality of images is a function of the
dose given to compensate for the greater decay of rubidium-82 up to the
time of data acquisition." In addition, N-13 ammonia images had more
activity in the liver than rubidium-82.

The study also presents results of angiography compared to all three
formats of image presentation (isocount, gray scale, and tricolor
format). The isocount color format had the most favorable agreement with
angiography results. The other formats had more problems with
interpreting intensity of gray or uni-color scale, image plane
displacement with slice by slice comparisons, and, thus, greater
variability of interpretation among readers.

[Safety Issues]{.underline}: Side effects of intravenous dipyridamole
were presented in a table: headache and flushing occurred in 46% and 52%
of patients respectively, followed by 24% with chest pain, 13% requiring
aminophylline, and 9% have ST segment changes. Nausea, dizziness and
PVCs/SVTs were reported less than 10% of the time. No safety issues
associated with administration of N-13 ammonia are mentioned.

[Commentary]{.underline}: The study has many strengths.

The study encompassed the proper study population for PET imaging: those
who had indications of possible CAD and were undergoing cardiac
catheterization. This appears to be a prospective collection of
patients. There were subjects with normal coronary anatomy included in
the selection. Although small in sample size, the study had the
appropriate enrollment criteria to encompass a board spectrum of the
disease, patients with possible CAD.

Another strength is that PET imaging is compared to an accepted
standard, cardiac angiography. Furthermore, the interpreting physicians
were masked and read images of the same patient three times. This
procedure permits determination of intra-observer variability. The
investigators also addressed the problems of comparing PET perfusion
images to single geometric measurements of stenosis severity obtained
during angiography by the calculation of CFR on angiograms. In 1986 when
the study was published, CFR could not be calculated from PET imaging.
The software to calculate CFR by quantifying and validating activity
within a three-dimensional reconstruction of cardiac radiopharmaceutical
distribution was under development.

Significant CAD was defined prospectively as a CFR from angiography of
less than 3.0 and then this was applied to sensitivity and specificity
of rest-stress PET imaging. This specific statement in the study
eliminates concerns about data manipulations to fit hypotheses. PET
sensitivity of identifying significant CAD (less than 3.0 CFR) was 95%
(21 out of 22), specificity (patients without CAD) was 100% (9 out of
9), however, identifying insignificant CAD (CFR of 3 or greater) was 31%
(4 out of 13) for PET.

Weakness in the study centered on the lack of information on
inter-reader variability, if images were read independently, frequency
of disagreement of PET interpretations, and dispute resolution. Results
were not stratified by radiopharmaceutical (rubidium versus N-13
ammonia), as well. However, the study did mention, "There was no
difference in sensitivity and specificity between the two tracers in
this small number of patients." This implies that data was analyzed to
support this statement, but, as is often the case with peer-reviewed
literature, was not published. The study is limited also by its small
number of subjects. It was reviewed in the context of being a
preliminary report to the more comprehensive study by Demer, which
follows. Finally, the study did not clearly state the relationship
between isocounts and CFR as measured by angiography. While rationale
for the selection of 3.0 CFR for significant CAD is discussed, there is
lack of discussion on what defined significant CAD for PET isocount
images. Inference is made from reading captions on figures that the
percent of maximum counts is directly correlated to the same percent
reduction in CFR. While these data and definitions often exist and are
documented in studies, they may be absent from the final published
study.

Overall, this study is well-designed and has results supporting N-13
ammonia PET is similar to the already FDA-approved rubidium-82
radiopharmaceutical for assessing coronary perfusion in the evaluation
of coronary artery disease, following separate administration under rest
and stress conditions, in patients suspected of having the disease.

b\. *Demer LL, Gould LK, Goldstein RA, Kirkeeide RL, Mullani NA,
Smalling RW, Nishikawa A, Merhige ME. Assessment of coronary artery
disease severity by PET: comparison with quantitative arteriography in
193 patients. Circulation 1989;79:825-35.*

This study from the University of Texas Medical School assessed the
accuracy of N-13 ammonia PET rest and stress (dipyridamole) tests in
evaluating CAD compared to coronary angiography in 193 patients (82
received rubidium-82 and 111 received N-13 ammonia). It is a follow up
to the preliminary Gould study and includes the patients in the Gould
article. It makes advances over other studies by assessing CAD with a
continuous scale to cover the range of disease severity rather than the
binary classification and using direct correlation rather than
sensitivity-specificity analysis. This study also enrolled larger
numbers of patients.

[Inclusion Criteria]{.underline}: All patients were undergoing cardiac
catheterization for: chest pain syndromes, myocardial infarction,
abnormal stress tests, coronary angioplasty, thrombolytic therapy or
acute infarction, evaluation before renal transplant, before or after
cholesterol lowering programs, or as part of screening feasibility
studies. Patients either received rubidium-82 or N-13 ammonia. There
were 143 men and 50 women studied. Sixty-six had a previous diagnosis of
myocardial infarction.

[Dose]{.underline}: Reference was made to previous articles, including
the published study in II(B)(1)(a) where the doses are stated as two IV
doses of 10-20 mCi N-13 ammonia or 30-50 mCi rubidium-82.

[Schema of Trial]{.underline}: A comparison of patient stenosis flow
reserve (SFR) as measured by an automated, digitized coronary angiogram
program versus rubidium-82 or N-13 ammonia PET perfusion imaging.
Scoring of both diagnostic tests was on a continuous scale.

[Image Protocol]{.underline}: PET images were either obtained before or
after cardiac angiography. There is no mention of randomization. The
selection of rubidium-82 or N-13 ammonia was determined by availability
of the generator. When rubidium was unavailable, N-13 ammonia was used.
All PET images were interpreted visually by two observers masked as to
the clinical status and angiographic results of the patient. All images
were read independently and they were displayed using the isocount color
format. Rest and dipyridamole stress images were displayed either
side-by-side or superimposed with adjustable color scales. Seven regions
of each cardiac image (anterior, apical, anteroseptal, posterseptal,
anterolateral, posterolateral, and inferior walls) were evaluated. PET
detected perfusion defects were defined by the color format and the
continuum of changing count densities as reflected in hue gradation of
each of five primary colors corresponding to steps of 3% of maximal
counts upon evaluation of the rest and stress-hand grip images.
Perfusion defects were defined as regions of subjectively-interpreted
lower counts in at least two contiguous slices compared to the remainder
of the heart. Perfusion defects were graded on a 0 to 5 scale: 0 is
normal, 1 is possible, 2 is probable, 3 is mild, 4 is moderate, and 5 is
severe. One score is assigned to each region and each step of the scale
is one primary color step. The average of the two readings was taken for
each region. Scores were assigned by each reader and then averaged to
define a PET defect severity score. Interobserver differences in PET
scan interpretation were defined, tracked, and analyzed. A dispute
resolution protocol was defined and images with qualitatively different
interpretations were reread independently (8 cases) and either had
agreement (2 cases) or were divergent and had scores averaged (6 cases).

Coronary angiograms were analyzed using cine x-ray frames that were
digitized using a computer program that automatically detects vessel
borders. Absolute and relative stenosis dimensions were measured. From
these dimensions, stenosis flow reserve was calculated. SFR is defined
as the intersection of flow at maximum coronary vasodilation relative to
rest flow, under standardized hemodynamic conditions. SFR scores ranged
from 0 (total occlusion) to 5 (normal). There was no mention of the time
interval between PET and angiography examinations.

[Primary Endpoints]{.underline}: Scores of PET defect severity on N-13
ammonia PET imaging (0 to 5; 0 is normal, 1 is possible, 2 is probable,
3 is mild, 4 is moderate, and 5 is severe) and scores of SFR (0 to 5; 0
being total occlusion; \<3 is significant CAD;
3[\<]{.underline}SFR[\<]{.underline}4 is mild CAD, and SFR
[\>]{.underline}4 is normal coronary artery) for patients and for
regions of the heart.

[Results]{.underline}: There were 115 patients with significant CAD (SFR
\<3), 37 patients with mild CAD (3[\<]{.underline}SFR\<4), and 41
patients with normal coronaries (SFR[\>]{.underline}4). There is no
mention of age or sex distribution of the subjects. Data for N-13
ammonia and rubidium-82 studies are pooled. With increasingly severe
impairment of stenosis flow reserve, subjective PET defect severity
scores increased. Despite wide scatter SFR scores per PET scores, a PET
score of 2 or more was highly predictive of significant CAD (SFR\<3).
The Spearman correlation coefficient was 0.77 [+]{.underline}0.06 for
patient scores of the most severe PET defect correlated with the SFR of
the patient's most severe stenosis (n=174 patients). Nineteen patients
were excluded from this analysis "because they had undergone
revascularization during acute infarction causing residual stenosis
severity that would not be comparable to the severity of the fixed
perfusion defect."

The study did supply the following information on rubidium-82 and N-13
ammonia performance: "Images obtained with rubidium-82 and N-13 ammonia
tracers were qualitatively similar. The two false positive cases
included one N-13 ammonia and one rubidium-82 image. Of the 7 false
negative scans, five were N-13 ammonia scans and 2 were rubidium-82.
Thus, 79 of 82 rubidium images and 105 of 111 ammonia images were
consistent with the arteriographic results. These ratios were not
significantly different (p=0.73)." Unfortunately, the total of 193 cases
includes cases that were excluded from the analysis. The study's
analysis only included 174 cases. The numbers provided above do not aid
in the calculation of a sensitivity or specificity for N-13 ammonia, but
do provide information that suggests the comparability of the N-13
ammonia and rubidium-82, an FDA-approved radiopharmaceutical.

The authors provided graphical representation that do permit an estimate
of N-13 ammonia PET sensitivity and specificity using data from the plot
of mean stenosis flow reserve score (5-0) versus subjective PET defect
severity score (0-5). See Appendix A: Figure 3. This data shows
considerable scatter SFR scores of each PET score. By visually
interpreting Figure 3's lower plot, there are a total of 96 patients
with significant CAD (SFR \<3) and 78 patients with SFR
[\>]{.underline}3. PET scores reveal 106 patients that are CAD-positive
(PET defect severity score [\>]{.underline}2) and 68 patients are PET
CAD-negative (PET defect severity score \<2). If one attributes 2 out of
the 4 patients with a PET score of 1 and a SFR of \<3 (significant CAD)
from the plot as being PET false positive results, we can calculate the
remaining cells of a 2 x 2 table. The 4 patients with PET score of 1 had
SFR values ranging from greater than 2.2 to about 3.8 and the exact
number of patients with a SFR \<3 (PET false negatives) is not stated.
Thus, as an arbitrary approach, the value of 2 out of 4 patients was
assigned as it represented about the same spatial percentage of error
bar below a SFR of 3.

Significant CAD per Angiography in Patients

(n=174 patients)

Diseased Non-diseased

(SFR \<3) (SFR [\>]{.underline} 3)

totals:

PET Score [\>]{.underline}2 94 12 106

CAD per PET

in patients PET Score \<2 2 66 68

totals: 96 78

> Sensitivity=98%
>
> (95% Confidence interval 92.1-99.7%)
>
> Specificity=85%
>
> (95% Confidence interval 74.7-91.7%)

These numbers, however, seem to differ from the data quoted for the
total 193 patients having a total of 2 false positive and 7 false
negative cases for N-13 ammonia and rubidium-82 combined.

For each of the 243 stenoses, PET defect severity score correlated with
the SFR of the corresponding artery with a Spearman coefficient of 0.63
[+]{.underline}0.08. There were variations in this relationship that
were attributed to subjective scoring of PET defects, anatomic
variations, and other reasons.

By visually interpreting the accompanying plot of stenosis flow reserve
score in reverse (5-0) versus the subjective PET defect severity score
(0-5), there are a total of 135 vessels with significant CAD (SFR \<3)
and 108 vessels with SFR [\>]{.underline}3. PET scores reveal 161
vessels that are positive (PET score [\>]{.underline}2) and 82 are PET
negative (PET score \<2). If one attributes 2 out of the 5 patients with
a PET score of 1 and a SFR of \<3 (significant CAD) from the plot as
being PET false positive results, we can calculate the remaining cells
of a 2 x 2 table. The 5 patients with PET score of 1 had SFR values
ranging from greater than 2.5 to about 4.8 and the exact number of
patients with a SFR \<3 is not stated. Thus, as an arbitrary approach,
the value of 2 out of 5 patients was assigned as it represented about
the same spatial percentage of the error bar below 3.0 SFR.

Significant CAD per Angiography in Vessels (n=243 vessels)

Diseased Non-diseased

(SFR \<3) (SFR [\>]{.underline} 3)

totals:

PET Score [\>]{.underline}2 133 28 161

CAD per PET

in vessels PET Score \<2 2 80 82

totals: 135 108

> Sensitivity=99%
>
> (95% Confidence interval 94.9-99.9%)

Specificity=74%

> (95% Confidence interval 64.5-81.7%)

Readers had agreement on whether the most severe PET defect was less
than or more than a score of 2 for 89% of patients. Overall agreement
data for all observations were presented for both rest and stress scans,
including the amount of disagreement. In 82% of rest scans and 83% of
stress scans the two numeric scores were in agreement, as defined by a
difference of no more than 2 in PET score.

[Safety Issues]{.underline}: No safety issues were reported.

[Commentary]{.underline}: This study expands on the earlier study by the
same authors with a larger number of patients. Many of the strengths of
the earlier study are also present here: use of coronary angiography as
the accepted standard, the entry criteria encompasses the appropriate
population for which PET may be indicated (those patients with suspected
CAD), and the use of a quantitative program to measure angiographic
results. Moreover, this study addresses weakness of the previous study
by reporting detailed information on readers' performances and
variability of interpretation, and by analyzing results in the presence
of variability. SFR is proposed as a better indicator of coronary
perfusion in angiographic images compared to percent stenosis. Dispute
resolution is described fully. The authors still acknowledge that
quantitative measurements of PET perfusion are preferable, but at the
time of the 1989 study, it was not validated. N-13 ammonia and
rubidium-82 data are pooled. The authors had stated that sensitivity and
specificity were comparable for both agents, although the analyses were
not published. While information is provided in the text about how N-13
ammonia PET performed, as opposed to rubidium-82, this information does
not allow estimates of N-13 ammonia PET performance and appears to
differ from information in Figure 3.

The study contained enough data to attempt to create 2 x 2 tables to
estimate sensitivity and specificity based on patients and vessels,
using the authors' cut off points for a positive and negative PET test
and the authors' definitions of significant CAD and non-significant CAD.
Because no data for the tables' cells were specified, an approach was
outlined above that allowed data to be generated based on a the
published graphical information.

This study further supports the ability of N-13 ammonia PET

to assess myocardial perfusion in the evaluation of CAD, following
separate administration under rest and stress conditions, for patients
suspected of having the disease.

2.  Other Controlled Published Studies

The following 3 studies of N-13 ammonia PET provided additional
information concerning the test's ability to assess myocardial perfusion
in the evaluation of CAD for patients suspected of having the disease.
The study hypotheses were, however, somewhat variable. The patient
populations were selected retrospectively based on previous angiographic
data. Normal volunteers were selected to represent CAD-free patients.
While there was direct comparison of PET results to other standards,
including angiography, or rubidium-82 PET, scans using
technetium-99m-based agents, or other tests, this comparison was at
times not the primary endpoint. Enough detailed data, however, were
presented to allow assessment of N-13 ammonia PET effectiveness in
myocardial perfusion in the evaluation of CAD, following separate
administration under rest and stress conditions.

a\. *Schelbert HR, Wisenberg G, Phelps ME, Gould KL, Henze E, Hoffman
EJ, Gomes A, Kuhl DE. Noninvasive assessment of coronary stenoses by
myocardial imaging during pharmacologic coronary vasodilation. Am J
Cardiol. 1982;49:1197-1207.*

This study from the University of California, Los Angeles, compared the
sensitivity of N-13 ammonia PET in detecting coronary artery disease in
normal volunteers and patients with angiographically documented coronary
artery disease (CAD) using rest and stress (dipyridamole) testing.

[Inclusion Criteria]{.underline}: Thirty-two patients with CAD, who had
all undergone routine coronary angiography prior to enrollment, were
chosen to participate. Twenty-two were men and 10 were women. The ages
ranged from 31 to 68 years with a mean of 53.1 years. In the 13 normal
volunteers (their ages ranged from 18 to 44 years with a mean of 25.2
years, all male), there was no clinical or EKG evidence of CAD. Eleven
patients with CAD underwent thallium-201 exercise stress testing within
an average of 15.8 days (range 1-42) of N-13 ammonia study.

[Dose]{.underline}: 0.22 ([+]{.underline} 0.09)mCi/kg N-13 ammonia
injected IV per dose given in two doses.

[Schema of Trial]{.underline}: Patients with CAD on angiography and
normal volunteer controls were randomized in sequence to undergo N-13
ammonia PET resting and stress testing using dipyridamole. Imaging
results were compared to angiographic results. Normal controls did not
undergo angiography. A subset of CAD patients, eleven, underwent
exercise-stress thallium-201 testing for comparison with angiography and
PET.

[Image Protocol]{.underline}: Patients and controls were randomly
sequenced to undergo PET. There were two independent observers for PET
readings who were masked to subject's clinical status or symptoms. A
study was considered abnormal if a regional defect in myocardial N-13
activity was observed in the hyperemic (post-dipyridamole) image that
either was not present or considerably larger than that in the control
image. Observer agreement was excellent. Four patient studies that had
disagreement over numbers of vessels involved were jointly reviewed and
agreement reached by consensus. Eleven patients also underwent
thallium-201 exercise stress testing within an average of 15.8 days.
Coronary angiograms were done within an average of 11 weeks in advance
of the PET studies.

[Primary Endpoints]{.underline}: The appearance of uniformity (normal)
or reduction of N-13 activity (abnormal) distributed in the myocardium
at various anatomically correlated regions of the heart's vasculature
system through angiographically defined stenoses. Hemodynamically
significant stenosis was defined as 50 percent or greater (diseased
vessel). Thallium-210 studies were defined as positive for
stress-induced ischemia if the immediate post-exercise images showed
regional reductions in myocardial thallium-201 activity that were either
completely or partially resolved on the delayed images.

[Results]{.underline}: In the 13 normal volunteers, N-13 activity was
uniform at rest and during hyperemia, whereas 31 out of 32 CAD patients
had regional defects on the hyperemic images not present during rest. No
false positive images were obtained on the normal volunteers. The data
can be represented in the following table:

Angiographically- Normal

> defined patients w/CAD Volunteers

(\>50% stenosis)

Patients with PET 31 0

perfusion defect

Patients without PET 1 13

perfusion defect

Sensitivity is about 97% and specificity is assumed to be 100%, if all
normals were, in fact, disease-free.

Of all 58 stenoses identified on coronary angiography, 52 (90 percent)
were correctly identified by N-13 ammonia PET. Of the 19
angiographically-identified, stenosed coronary arteries in the subgroup
who underwent exercise thallium-201 planar imaging, 11 (58 percent) were
correctly identified with thallium-201 and 17 (89 percent) with N-13
ammonia PET.

[Safety Issues]{.underline}: The article mentioned the purity of N-13
ammonia was greater than 99 percent. There were 12 patients with CAD
(37.5 percent) with chest discomfort or pain on infusion of dipyridamole
and 9 of these patients received aminophylline IV. No adverse events
were reported with N-13 ammonia infusion.

[Commentary]{.underline}: This study used both coronary angiograms and
exercise stress thallium testing to compare the diagnostic performance
of N-13 ammonia PET. The study provided detailed data on patient and
vessel comparative results. The study also included normal controls,
although these patients did not undergo coronary angiography. While the
study encompassed small numbers, the PET results were for the most part
consistent among the two observers and highly correlated with
angiographic data of patients with CAD (31 out of 32 (97%) CAD patients
were correctly identified by PET).

Although the mean time difference from angiography to PET was 11 weeks,
all CAD patients were in symptomatically stable condition with no
evidence of clinical progression of their disease.

The design was a comparative trial with angiography as the standard of
truth, and the study assumed normals did not have CAD. This means that
CAD patients were defined by their previous angiography results as
having stenoses and subsequent PET results were compared to angiography
results to assess presence of perfusion abnormalities correlating with
stenoses. As an initial study, investigators must first assess a new
technology on a pre-defined population where "truth" is known, and, for
specificity, where truth is assumed. The study does not allow us to
calculate the true specificity of PET, only the estimated specificity
based on an assumed truth. Only a subset of 11 patients with CAD
underwent stress-thallium testing. These thallium rests were compared to
both PET and angiographic data for patients and for individual vessels
to assess concordance of stenoses and perfusion defects. This
comparative study obtains important information on how the test performs
relative to thallium and then how both ultimately compared to the
accepted truth standard, angiography. The weakness of this approach is
it assumes angiography is, in fact, the ultimate truth. As stated
earlier, PET and SPECT radiopharmaceuticals and angiography may measure
different aspects of coronary anatomy. N-13 ammonia is a functional
examination of microperfusion. N-13 ammonia exchanges across the
capillary and cellular membranes via diffusion, while angiography is a
structural assessment of patency. Correlation of results between PET
perfusion defects and angiographic stenoses is acceptable, although it
may be intrinsically difficult.

A weakness of the study is retrospectively selected study populations do
not test how the test will be used in practice (along with or in lieu of
coronary angiography) nor do they provide information on how the test
performs in the target population. The sample size is also very limited.
The study, however, provides information as proof of concept.

Other concerns raised from the study center around entry criteria.
Selection criteria for angiograms of CAD were not stated. Selection
criteria for the 11 patients who underwent exercise thallium-201 were
also not provided. No information on patient refusal or dropouts is
provided.

Not having specific entry criteria for patients with CAD can have
advantages. There are a wide variety of patients with CAD, from diabetic
to hyperlipidemic to hypertensive, and vascular changes from these
different conditions may differ. Diabetic vessel disease is primarily a
small vessel disease while other processes affect larger vessels.
General entry criteria would serve to increase generalizability of the
results to a wider patient population. However, no criteria are
specifically stated in the study.

The study did not mention if the angiograms and thallium-201 exercise
tests were read by the same two observers as the PET studies or the
manner and sequence the three tests were read. The study did state that
readers were "unaware of the patient's clinical state or symptoms,"
implying that CAD status was not known. However, there was no mention if
images of rest and exercise PET, thallium-201, or coronary angiography
images were presented in an unpaired fashion to the readers. Because
none of the normal volunteers had angiograms or thallium tests, it is
preferable that all tests be read separately and unpaired. Otherwise, it
would be easy to determine case and control status, which may bias
readers.

This study appears to have a high degree of agreement between the two
observers for PET readings. In addition, the protocol specified the
exact types of disagreement encountered and how disagreements were
handled. The study used resolution by consensus reading, a common, but
potentially biased, means to address disagreement.

As with many studies relying on interpretation of visual images,
observer performance is important. Concerns for observer performance may
be reflected in the subjective and subtle nature of reading PET images
based on appearance of degrees of homogeneity. Performance may be highly
correlated to experience and skills of each interpreting physician,
which could be quite variable. In 1982, when this study was published,
PET was extremely limited in its use and experience was not widespread.
This published early experience may have raised concerns at that time
about reproducibility in the general imaging community, which lacked
exposure to the technology. However, it appears that over the next
fifteen years, other studies have been published supporting rather
consistent performance by interpreting physicians with N-13 ammonia PET.

In summary, this 1982 study was a study comparing PET, thallium, and
angiography readings in retrospectivly selected patients with known CAD
and normal volunteers. The strength of the study lies in its consistent
results across two independent readers, despite having a subjective,
dichotomous endpoint: normal or abnormal homogeneity of
radiopharmaceutical activity. The correlation between PET results and
angiography with respect to patients and vessels identified as diseased
is reproduced from Gould (1986) and Demer (1989). N-13 ammonia PET had
97% sensitivity (31 of 32 patients) for identifying patients with
angiographic CAD. The study supports the utility of N-13 ammonia PET for
assessing myocardial perfusion in the evaluation of CAD, following
separate administration under rest and stress conditions.

b\. *Di Carli M, Sherman T, Khanna S, Davidson M, Rokhsar S, Hawkins R,
Phelps M, Schelbert H, Maddahi J. Myocardial viability in asynergic
regions subtended by occluded coronary arteries: Relation to the status
of collateral flow in patients with chronic coronary artery disease. J
Am Coll Cardiol 1994; 23:860-68.*

This study was conducted at the University of California Los Angeles
School of Medicine. This study determined whether angiographically
visualized collateral vessels in patients with chronic CAD and left
ventricular dysfunction imply the presence of viable myocardium in
regions with wall motion defects subtended by completely occluded
coronary arteries as assessed with rest N-13 ammonia and fluorine-18
deoxyglucose (18-FDG) PET. Viability is defined in this study as the
characterization of myocardial metabolism with PET fluorine-18
deoxyglucose. The purpose was to provide data on the relationship
between angiographic collateral flow, myocardial wall motion
abnormalities, and myocardial viability as defined by preserved
metabolism of 18-FDG.

CAD patients considered for revascularization often have severe wall
motion abnormalities. This myocardial dysfunction may respond variably
to revascularization. The differentiation of reversible from
irreversible myocardial dysfunction is useful in decision-making about
revascularization for each patient candidate, and this is not
sufficiently achieved by use of angiographic data alone. Hypothetically,
this differentiation permits prediction of functional outcome after
revascularization. Areas of reversible dysfunction may be at high risk
for further ischemic events on medical therapy. Because angiography only
visualizes collateral flow in vessels greater than 100 micrometers in
diameter, N-13 ammonia perfusion was hypothesized as being able to
assess microperfusion in areas of wall motion dysfunction subtended by
completely occluded coronary arteries.

[Inclusion criteria]{.underline}: Forty-two consecutive patients with
confirmed CAD on catheterization and left ventricular dysfunction were
referred for assessment of myocardial viability defined by the pattern
of PET 18-FDG metabolism. The study group consisted of 37 men and 5
women, aged from 34 to 81 years. Thirty-five patients had a history of
previous myocardial infarction, 35 patients had symptoms of heart
failure, and 17 patients had angina.

[Dose]{.underline}: 20 mCi of N-13 ammonia and 10 mCi of 18-FDG injected
intravenously.

[Schema of trial]{.underline}: Comparisons of the extent of PET
viability patterns and the severity of perfusion defects between three
groups of angiographic collateral flow grades were performed.

[Image Protocol]{.underline}: All patients underwent coronary
angiography and contrast left ventriculography before PET imaging. Rest
regional myocardial perfusion and glucose metabolism were assessed with
N-13 ammonia and 18-FDG imaging, respectively. The N-13 ammonia PET
imaging was completed first followed by 18-FDG PET imaging. Two
experienced observers who were masked to the PET results assessed
independently segmental wall motion from the contrast left
ventriculography and collateral flow from the coronary angiograms.
Discrepancies between readers were resolved by consensus. PET image
myocardial perfusion and glucose uptake were scored separately for each
segment by consensus visual analysis of two observers masked to all
patient clinical data. The time interval between MI and angiography
ranged from 2 months to 14 years. The period between the PET study and
coronary angiography ranged from 4 to 70 days.

[Primary Endpoints]{.underline}: The PET measures of perfusion and
viability patterns were compared to the angiographic collateral flow
grade in the myocardial territories with totally occluded major coronary
arteries with corresponding myocardial wall motion defect.

The presence and severity of wall motion abnormalities was scored using
a 6-point scale from contrast ventriculography. From the angiograms, the
presence of collateral vessels to occluded arteries for each image was
classified into 3 collateral flow grades: 1 = absent, 2 = minimal, 3 =
well developed.

From the PET images, myocardial perfusion (N-13 ammonia) and glucose
uptake (18-FDG) were scored separately for each segment using the
following 5-point uptake scale: 0 = absence of background, 1 = severe
defect, 2 = moderate defect, 3 = mild defect, 4 = normal uptake.

PET viability patterns (match, mismatch, and normal) for each segment
were based on comparison of the N-13 ammonia and 18-FDG uptake scores.
Segments with perfusion defect (N-13 ammonia score [\<]{.underline}3)
were classified as [mismatch]{.underline} when the 18-FDG score was
higher than the N-13 ammonia score by at least 1 point. A
[match]{.underline} was defined as when both the N-13 ammonia and 18-FDG
uptake were both severely reduced or absent (score of 0 or 1) or, for a
[transmural match]{.underline}, both mild to moderately reduced (score 2
or 3). Normal was defined as both PET scores being 4.

The severity of perfusion deficit was assessed quantitatively by
calculating the average percent reduction of relative myocardial N-13
counts below the limits of a normal data base (percent perfusion below
normal value) in each region.

[Results]{.underline}: There was no significant correlation (p=0.14)
between the severity of perfusion deficit seen on N-13 ammonia PET and
the collateral grade assessed by angiography. The extent of mismatch was
unrelated to either the presence of the magnitude of collateral vessels.
Myocardial viability defined by 18-FDG uptake was present in severely
hypokinetic regions 82% of the time. Viability by 18-FDG uptake was
present in 49% of akinetic-dyskinetic regions. Of the 64 regions with
angiographic collateral vessels, 37 (58%) showed PET mismatch (18-FDG
score was higher than the N-13 ammonia perfusion score). In the 14
regions without collateral vessels on angiography, 7 (50%) exhibited PET
mismatch. The presence of angiographically visualised collateral vessels
was a sensitive (84%) but not specific (21%) marker for viability,
defined as 18-FDG metabolism (mismatch or matched presence of both
18-FDG and N-13 ammonia activity).

The study concluded that in patients with chronic coronary artery
disease, angiographically visualized collateral vessels to myocardial
regions with wall motion defect and subtended by occluded coronary
arteries do not always imply the presence of viable myocardium. The
study also concluded that PET studies may be a possible aid in assessing
patients' myocardial viability for decisions about revascularization.

[Safety Issues]{.underline}: No safety issues were reported.

[Commentary]{.underline}: By the time this article was written in 1993,
N-13 ammonia PET had been used frequently to measure myocardial blood
flow. In addition, N-13 ammonia had been coupled with 18-FDG to study
the potential viability of myocardial tissue. While this study went
beyond assessing myocardial perfusion in the evaluation of CAD with N-13
ammonia PET, this aspect can still be commented upon. In fact, this
study has many strengths with respect to myocardial blood flow
measurement by N-13 ammonia PET.

One strength was that the standards of coronary angiography and contrast
left ventriculography were used to grade collateral vessel supply to
those myocardial regions with completely occluded arteries and abnormal
wall motion. In this study, the set of readers for the angiogram and
ventriculogram were masked to the results of the PET study to decrease
bias. Another strength was that PET myocardium perfusion values were
quantitatively assessed by a machine-dependent program. These results
were then correlated to describe the relationship between collateral
grade and blood flow value. Finally, the study population, 42
consecutive patients with CAD, was appropriate for the demonstrating the
test's performance in the target population.

The study demonstrated that the presence of angiographic collateral
blood flow to akinetic myocardium may not predict living myocardial
tissue and the absence of collateral flow may not predict non-living
myocardial tissue. In 58% of cases where angiographic collateral vessels
were present, PET demonstrated reduced N-13 ammonia perfusion with
18-FDG metabolism (mismatch); in 50% of cases where angiographic
collateral blood flow was absent, there was still perfusion, although
reduced, with metabolism of 18-FDG. These findings appear to contradict
findings of Gould (1986) Demer (1989) and Schelbert (1982), which rely
on good correlation between angiographic stenoses and PET. However, this
study population was very different: patients had more severe CAD with
severe asynergy of the myocardium supplied by the occluded vessel.
Moreover, 83% had a history of previous myocardial infarction. It may be
that in this patient population, subendocardial and intramural
intercoronary vasculature are present, but, per the authors'
discussions, are not angiographically demonstrated because these vessels
are as small as 20 micrometers in diameter. In addition, the authors
state that there may be low pressure gradient between the supplying
occluded artery and the collateral vessels preventing angiographic
visualization. Thus, the study's results do not necessarily contradict
the previously mentioned ones.

A weakness of the study was absence of formal accounting for
inter-reader variability for the masked readers of the angiographic
images. Instead, the results of the two masked readers were condensed
into one response by consensus after the images were rated
independently. This method for handling inter-reader variability is not
completely informative; it would have been of interest to know how much
discrepancy was present and for what regions the discrepancies occurred.
Another weakness was the small numbers of subjects studied.

Overall, this study demonstrated that myocardial blood flow measured by
N-13 ammonia PET affords information different from angiography,
although complementary. By studying N-13 ammonia PET in this extreme end
of the spectrum of coronary artery disease, this study provides useful
information on PET performance in this population. There is detailed
information on PET image acquisition and analysis. Angiographically
visualized collateral vessels (grade 2, minimal, and grade 3,
well-developed collaterals) detected PET mismatch (reduced, but present
N-13 ammonia perfusion with 18-FDG metabolism) 84% of the time (37 of 44
territories). This data suggests that both angiography and N-13 ammonia
PET contribute information on myocardial perfusion in the evaluation of
CAD. The study contributed to the FDA assessment of N-13 ammonia PET.

c\. *Gewirtz H, Fischman AJ, Abraham S, Gilson M, Strauss HW, Alpert NM.
Positron emission tomographic measurements of absolute regional
myocardial blood flow permits identification of nonviable myocardium in
patients with chronic myocardial infarction. J Am Coll Cardiol 1994;
23:851-59.*

This single center study was conducted at Massachusetts General
Hospital, Boston, Massachusetts. The study hypothesized that a minimal
level of blood flow is required to sustain myocardial viability, below
which the myocardial region would consist mostly of scar. This study
assessed whether chronically infarcted, nonviable myocardium could be
identified by quantitative measurements of regional myocardial blood
flow obtained using N-13 ammonia and 18-FDG PET in conjunction with a
mathematical model of N-13 ammonia radiopharmaceutical kinetics.

[Inclusion criteria]{.underline}: The study group consisted of 26
patients (25 men, 1 woman; from 34 to 74 years of age) with documented
chronic myocardial infarction based on historical and ECG evidence of
previous MI. Patients were initially referred for a clinically indicated
thallium stress test. The referring physician also requested the PET
study. Patients who were diabetic, had myocardial infarction \<2 months
or had cardiac catheterization \>14 months before the PET study were
excluded from the study group.

All patients had cardiac catheterization and were clinically stable
without recent (2 weeks) unstable chest pain. The time between PET scan
and previous myocardial infarction ranged from 2 to 192 months. Cardiac
catheterization (angiogram) was done within a mean of 5 months for all
patients. Eleven patients had had revascularization of the infarct zone.
The mean time between revascularization and PET study was 11 months.
Cardiac and other medications were continued.

[Dose]{.underline}: Approximately 25 mCi of N-13 ammonia, 7.5 mCi of
18-FDG.

[Schema of trial]{.underline}: The study proposed to demonstrate that
chronic infarct zones with very low flow compared with normally perfused
regions of the heart under basal physiologic conditions should be
composed largely of scar (non-viable myocardium, defined by 18-FDG PET
metabolism).

[Image Protocol]{.underline}: PET myocardial perfusion study at rest
with N-13 ammonia and myocardial metabolism study with 18-FDG images
were acquired. Regional myocardial blood flow and regional 18-FDG uptake
were calculated using the same regions. All flow computations were made
without knowledge of regional wall motion data. Appropriate segments of
the PET images and radionuclide image were matched for comparisons.

For wall motion analysis, contrast biplane left ventriculography was
performed in 21 patients, multi-projection radionuclide ventriculogram
in 3 patients, and echocardiogram in 2 patients. All ventriculograms
were interpreted without knowledge of PET myocardial blood flow
measurements by the clinician who performed the cardiac catheterization.

[Primary Endpoints]{.underline}: Myocardial viability was evaluated both
by analysis of regional wall motion and by objective assessment of the
relation between myocardial glucose metabolism and myocardial blood
flow. Specific results for regional myocardial blood flow were
correlated with descriptions of various states of coronary artery
stenosis, and there was myocardial blood flow correlation with the
results of metabolic (18-FDG) imaging and with regional wall motion.

There was no mention of how angiograms were evaluated or by whom.

Myocardial regions were designated as normal, border, or infarct zones
on the basis of percent of maximum regional myocardial flow via N-13
ammonia PET. All infarct zones corresponded to regions of previous MI as
determined by historical and ECG criteria.

Regional myocardial blood flow and 18-FDG uptake were compared to
determine if a region had a mismatch outcome, a presumed surrogate of
myocardial viability. A mismatch was defined objectively as a difference
between relative 18-FDG uptake and relative flow of \>0.34.

Regional wall motion was another indicator of myocardial viability.
Segments were classified as: viable if they had normal wall motion or
only mild hypokinesia; scar if they had dyskinesia; or either viable,
scar, or both if they had severe hypokinesia or akinesia.

[Results]{.underline}: In patients with chronic myocardial infarction,
normal blood flow (0.81 [+]{.underline}0.32 ml/min per g) was greater (p
\<0.02) than that of border zones (0.59 [+]{.underline}0.29 ml/min per
g), which also exceed (p\<0.001) that of infarct zone flow (0.27
[+]{.underline}0.17 ml/min per g). Mismatch between blood flow and
18-FDG uptake, with a single exception, was not observed in any segment
with blood flow \<0.25 ml/min per g. All dyskinetic segments (n=5) also
had blood flow \<0.25 ml/min per g. In 23 patients with 45 myocardial
segments with normal contraction or only mild hypokinesia, 43 of the 45
segments had flow [\>]{.underline}0.39 ml/min per g (average flow 0.78
[+]{.underline}0.35 ml/min per g).

In patients with chronic myocardial infarction, myocardial viability is
unlikely when basal regional myocardial blood flow was \<0.25 ml/min/g.
Average basal flow in segments with normal or nearly normal wall motion
was 0.78 [+]{.underline}0.35 ml/min/g. PET measurement of regional
myocardial blood flow was helpful in identifying nonviable myocardium in
these patients.

[Safety issues]{.underline}: The radiation dose associated with the N-13
ammonia study was 5 mR/mCi for whole body and 51 mR/mCi to the bladder
(target organ). The radiation dose associated with the F-18 deoxyglucose
study was 39 mR/mCi for whole body and 440 mR/mCi to the bladder (target
organ).

[Commentary]{.underline}: Although the primary hypothesis of the study
does not relate to a direct assessment of myocardial perfusion in the
evaluation of CAD using N-13 ammonia PET as compared to another
perfusion standard, myocardial perfusion assessment can still be
commented upon. Indirect measurements such as regional wall motion on
ventriculograms (21 of 26 patients), radionuclide ventriculograms (3 of
26 patients), and echocardiograms (2 of 26 patients) were obtained to
permit comparison of PET perfusion results.

Because myocardial blood flow is needed to maintain aerobic metabolism
in normal contractile myocardial cells, lack of flow leads to oxygen
deprivation in myocardial cells and impaired contractility. Abnormal
wall motion is one of many consequences of ischemia. This study uses
wall motion as a marker for ischemic conditions and consequently
compares PET perfusion results to severity of wall motion abnormality.
Study results support the correlation that the more severe the wall
motion abnormality, the lower the perfusion rate. All dyskinetic
segments (n=5) had perfusion \<0.25 ml/min per g; 43 out of the 45
myocardial segments with normal contraction or only mild hypokinesia had
flow [\>]{.underline}0.39 ml/min per g.

Several strengths of the study are evident. The PET blood flow results
were objectively computed using a mathematical model and were made
without knowledge of regional wall motion results. The ventriculograms,
for wall motion, were assessed without knowledge of the PET results.
Thus, the description of PET blood flow values by wall motion
abnormalities provided objective information that could be used in the
final assessment of myocardial viability. The patient population of
those with documented myocardial infarction is an appropriate one to
identify nonviable myocardium quantitatively by PET measurements.
Lastly, coronary angiography was used to assess stenosis severity and
the PET blood flow data were correlated to these results. These results
described the blood flow for non-stenosed, [\>]{.underline}50%, and
\<50% stenosed vessels.

One concern the study raises is that PET scans and angiograms were done
at differing points in the patients' clinical course. The time from
myocardial infarction to PET scan (2-192 months, mean 44 months) and the
time from PET scan to cardiac catheterization (mean of 5 [+]{.underline}
4 months (SD), with 72% of patients having catheterization performed
within 3 months of PET study) is quite variable. Study endpoints such as
wall motion, perfusion, and myocardial metabolism may be time-sensitive
from infarct and results may need to be stratified by time rather than
analyzed as a group. It may also be important to minimize time between
each test to facilitate comparisons.

The study did not mention how stenoses were determined or how these were
calculated from the coronary angiograms. Another concern is
ventriculograms were interpreted by only one reader. This reader was,
however, masked to the PET results. While this eliminates inter-reader
variability, there is subjectivity in reading ventriculograms. However,
as ventriculography is a common and familiar procedure, the expectation
is that the reader would be accurate. Of note, while most
ventriculograms were obtained during catheterization, 5 patients had
either radionuclide ventriculograms or echocardiogram wall motion
assessment.

This study concluded that the rate of myocardial blood flow measured by
N-13 ammonia PET was correlated with degree of wall motion abnormality.
The results of PET in this study demonstrated biological consistency
with current understanding of consequences of low perfusion states on
myocardial wall motion. It supports the functional uses of N-13 ammonia
PET perfusion results.

3\. Other published studies

Although their study hypotheses differed widely, the studies below
provided supporting information of N-13 ammonia efficacy for assessing
myocardial perfusion in the evaluation of CAD.

*a. Laubenbacher C, Rothley J, Sitomer J, Beanlands R, Sawada S, Sutor
R, Muller D, Schwaiger M. An automated analysis program for the
evaluation of cardiac PET studies: Initial results in the detection and
localization of coronary artery disease using nitrogen-13-ammonia. J
Nucl Med 1993; 34:968-978.*

This study was conducted at the University of Michigan Medical Center.
An automated quantitative analysis method applied to myocardial N-13
ammonia PET rest/pharmacological stress data was evaluated to determine
N-13 ammonia PET's diagnostic performance. The study also developed
diagnostic criteria for stress-induced perfusion abnormalities for each
vascular territory. A control group of 23 subjects without evidence

of coronary artery disease (and who did not undergo angiography) and 29
patients with angiographically confirmed coronary artery disease were
studied.

The study concluded that high diagnostic accuracy for detection and
localization of coronary artery stenosis in predefined vascular
territories were possible using this new methodology of volumetric data
sampling and mathematical constraints of activity sampling to the
expected shape of the left ventricle. Detailed sensitivity and
specificity data stratified by each of the major coronary arteries under
stress and rest conditions for detecting CAD using a range of
angiographic stenosis percentages are presented. Using a threshold of
[\>]{.underline}75% stenosis diameter within a given vascular territory,
the authors report sensitivity was 93% and the specificity was 80%.
Receiver operator curves for overall detection and localization of CAD
in individual vascular territories are given. Interobserver and
intraobserver variability for each vessel territory is plotted as a
linear regression. Detailed charts of inter and intraobserver agreement
on detection and localization of CAD shows good agreement (between
80-97%).

Strong points of the study include use of angiography as the standard of
truth, use of computerized PET perfusion quantification, and the use of
an agreement study between two masked readers versus the computerized
PET quantification program. Readers re-read images with a high degree of
reproducibility. Reader agreement was calculated for different
anatomical regions. The high level of details in the published report
about methodology, data (including vessel territory data), and inter and
intraobserver performance support the high quality of this study. The
correlation of PET results and angiographic stenosis of
[\>]{.underline}90% in rest to stress evaluations range from 77% to 100%
depending upon vessel.

*b. Di Carli MF, Davidson M, Little R, Khanna S, Mody FV, Brunken RC,
Czernin J, Rokhsar S, Stevenson LW, Laks H, Hawkins R, Schelbert HR,
Phelps ME, Maddahi J. Value of metabolic imaging with positron emission
tomography for evaluating prognosis in patients with coronary artery
disease and left ventricular dysfunction. Am J Cardiol 1994; 73:527-33.*

This study was conducted at the University of California Los Angeles
School of Medicine. This study was done with 93 consecutive patients
with angiography-defined coronary artery disease and severe left
ventricular dysfunction (mean left ventricular function of 0.25). It
assessed the prognostic value of PET mismatch of the ammonia and glucose
uptake scores and the interrelation between PET mismatch pattern and
choice of medical therapy or revascularization for predicting survival
and improvement in symptoms of heart failure. Follow up was for an
average of 13.6 months (range 2 to 31 months).

Patients with severe left ventricular dysfunction and CAD may have less
mortality with revascularization. However, surgical mortality is high
for these patients. The ability to predict beneficial effects of
revascularization would be to improve blood flow to hypoperfused regions
that have living myocardium (but may be contracting less optimally due
to ischemia) to improve left ventricular function. N-13 ammonia
perfusion assessment coupled with 18-FDG metabolism was hypothesized to
have a role in this prediction.

All patients were potential candidates for myocardial revascularization
and had a resting, myocardial perfusion (N-13 ammonia) and glucose
uptake (18-FDG) PET study. Diabetics, hypertensives, patients with
pulmonary disease, heart failure, or angina, were included. Patients
were followed up for death and its cause. Fifty patients eventually
underwent medical therapy and the rest (43) revascularization.

The results suggest that the presence of mismatch (segmental glucose
uptake by 18-FDG was increased relative to perfusion) in patients with
CAD and severe LV dysfunction is associated with poor annual survival
with medical therapy \[i.e., are at higher risk of cardiac death during
follow-up\] compared to those who had revascularization.
Revascularization in patients with PET mismatch appeared to be
associated with improved survival and decreased symptoms of heart
failure.

This study supports the use of N-13 ammonia in assessment of myocardial
perfusion in the evaluation of CAD by contributing clinical endpoints of
survival and mortality to the interpretation of PET. The results
obtained are consistent with the biologic plausibility that PET
metabolism and perfusion mismatch assesses physiologic aspects of
microperfusion not appreciated by angiography alone. This study is
consistent with the PET studies of Gewirtz on wall motion. It goes
further by having outcomes data that support N-13 ammonia PET's
assessment of perfusion. The results support the clinical significance
of PET myocardial perfusion measurements.

*c. Gould LK, Martucci JP, Goldberg DI, Hess MJ, Edens RP, Latifi R,
Dudrick SJ. Short-term cholesterol lowering decreases size and severity
of perfusion abnormalities by positron emission tomography after
dipyridamole in patients with coronary artery disease; a potential
noninvasive marker for healing coronary endothelium. Circulation
1994;89:1530-38.*

This study from the University of Texas Medical School is a randomized
study. Fifteen patients with CAD documented by a 50% or more diameter
stenosis in one or more major epicardial coronary arteries on coronary
angiography were assessed for short-term (90 day) effects of cholesterol
lowering programs on perfusion as measured by rest-dipyridamole N-13
ammonia PET. Patients served as their own control in pre- and post-
intervention studies. Completely automated, objective measures of size
and severity of perfusion abnormalities on rest-dipyridamole PET images
were made by computer algorithm. Results of PET image evaluations, serum
lipid profiles, and clinical evaluation using the Bruce protocol showed
improvement during treatment and reversal of improvement on treatment
withdrawal. The authors suggest that myocardial perfusion and CAD can be
followed non-invasively by dipyridamole PET.

This study's strength lies in the use of multiple complementary
endpoints, including clinical performance on the Bruce protocol, to
support PET findings of altered perfusion. Use of an automated system to
measure PET perfusion addresses concerns of observer bias and
variability. While the study is small, the consistent results across
treatment groups and across endpoints supports the usefulness of PET in
myocardial perfusion imaging to assess CAD. This study also supports the
clinical significance of PET myocardial perfusion data.

*d. Beanlands RSB, Muzik O, Melon P, Sutor R, Sawada S, Muller D, Bondie
D, Hutchins GD, Schwaiger M. Noninvasive quantification of regional
myocardial flow reserve in patients with coronary atherosclerosis using
nitrogen-13 ammonia positron emission tomography: Determination of
extent of altered vascular reactivity. J Am Coll Cardio 1995;
26:1465-75.*

This study was conducted at the University of Michigan Medical Center.
The study evaluated patients with coronary artery disease to determine
the relation between regional myocardial blood flow reserve measured by
N-13 ammonia kinetic modeling and stenosis severity assessed by
quantitative angiography, and examine whether flow reserve is impaired
in regions supplied by vessels without significant angiographic disease.

Twenty-seven subjects were enrolled and classified into three groups: 1)
5 young healthy volunteers, 2) 7 middle aged volunteers with low
likelihood for CAD, and 3) 15 patients with angiographically confirmed
coronary artery disease ([\>]{.underline}50% stenosis by quantitative
coronary angiography with 9 of the 15 having at least one vessel disease
[\>]{.underline}90% stenotic). All subjects underwent N-13 ammonia PET
rest/pharmacological stress studies and group 3 patients underwent
coronary angiography (PET and coronary angiography were performed within
a mean of 3.5 [+]{.underline}7 days of each other). All of group 3
patients' data on vessel anatomy, percent area stenosis, minimal lumen
diameter, flow rates at rest and with stress, myocardial flow reserve,
collateral flow and stenosis severity category were presented in detail.

Results indicated flow reserve as measured by PET had a correlation
coefficient of r=-0.56 with percent stenosis on angiogram (fair inverse
correlation-the higher the flow reserve the smaller the percent
stenosis) and a correlation coefficient of r=0.75 with minimal lumen
diameter on angiogram (good correlation).

The study concluded that there was a progressive reduction in myocardial
flow reserve, as measured by N-13 ammonia PET, with increasing
angiographic disease severity. Among patients with coronary disease,
myocardial regions without significant angiographic stenoses displayed
reduced flow reserve in regions as compared to control subjects.

The study used quantitative angiography data compared to quantitative
PET results. A variety of subjects with different clinical conditions
were tested, however, only 9 of 27 subjects had severe CAD of
[\>]{.underline}90% stenosis. The study also used a variety of
angiographic stenosis parameters, studied multiple vessels per patient
using angiography and PET, determined PET flow rates and myocardial flow
reserve for each vessel, and compared PET to angiography using these
diverse factors. These parameters supported correlation between
angiographic results and PET results. This study's consistent finding of
good correlation, r=0.75, between angiography stenosis and PET flow
rates, its detailed presentation of vessel and patient data, and the use
of objective measurements provide support for the use of N-13 ammonia
PET in assessing myocardial perfusion in the evaluation of CAD,
following separate administration under rest and stress conditions.

*e. Czernin J, Barnard RJ, Sun KT, Krivokapich J, Nitzsche E, Dorsey D,
Phelps ME, Schlebert HR. Effects of short-term cardiovascular
conditioning and low-fat diet on myocardial blood flow and flow reserve.
Circulation 1995;92:197-204.*

This study from the University of California School of Medicine at Los
Angeles measured response of myocardial blood flow and flow reserve to
cardiovascular conditioning together with a low-fat diet. The study
hypothesis was that cardiovascular conditioning with a low-fat diet
would lead to changes in hemodynamics, serum lipids, and exercise
performance, along with increased blood flow as measured by N-13 ammonia
PET. The study group consisted of 13 volunteers, of whom 4 had CAD, none
of whom were on any lipid lowering, cardiac, or antihypertensive
medications. The nonrandomized and non-concurrent control group
consisted of 8 normal volunteers. Quantification of myocardial blood
flow at rest and during dipyridamole-induced stress was done using a
semi-quantitative image analysis program. With intervention, resting
rate-pressure product, serum cholesterol, and resting flow rate
decreased while hyperemic blood flow increased resulting in improved
myocardial flow reserve. No changes were observed with controls.
Correlation coefficients were calculated between PET measurements and
clinical parameters listed above and results were moderate to good.

The results of this study provide support for clinical endpoints (such
as rate-pressure product and improved exercise capacity) correlating
with PET myocardial flow reserve measurements. While it may seem
intuitive that increased myocardial perfusion should lead to improved
rate-pressure product and improved exercise capacity, these results
confirm the biologic plausibility of PET results. While small in size,
the endpoints are clinically relevant to coronary perfusion, they are
assessed objectively, and there is a high level of detailed data on
patients. This study supports N-13 ammonia PET in evaluating myocardial
perfusion in the evaluation of CAD, following separate administration
under rest and stress conditions.

*f. Gould LK, Ornish D, Scherwitz L, Brown S, Edens RP, Hess MJ, Mullani
N, Bolomey L, Dobbs F, Armstrong WT, Merritt T, Ports T, Sparler S,
Billings J. Changes in myocardial perfusion abnormalities by PET after
long, intense risk factor modification. JAMA 1995;274:894-901.*

This randomized, controlled trial quantified changes in size and
severity of myocardial perfusion abnormalities by rest-dipyridamole PET
in patients with documented CAD on initial coronary angiography who
either underwent risk factor modification (n=20) or usual care (n=15).
Both quantitative coronary angiography and quantitative PET imaging were
done at baseline and 5 years after randomization. Results showed the
risk factor modification group had improved size and severity of
perfusion abnormalities on dipyridamole PET compared to controls. The
relative magnitude of changes in size and severity of PET perfusion
abnormalities was comparable to or greater than the magnitude of changes
in percent diameter stenosis, absolute lumen area, or stenosis flow
reserve documented by quantitative coronary angiography.

The study's design is well-conceived. The report contained agreement
data on visual interpretation versus the automated PET program results.
It also contained the drop out rates for each group (5 of 20 controls
dropped out and 8 of 28 patients in the intervention group dropped out).
Because the trial is itself very small, there is concern about bias
introduced with these drop outs. However, there are high quality,
quantitative comparison data involving PET imaging compared to coronary
angiography data in pre- and post-intervention which support use of N-13
ammonia myocardial perfusion in the evaluation of CAD, following
separate administration under rest and stress conditions.

*g. Sambuceti G, Parodi O, Giorgetti A, Salvadori P, Marzilli M, Dabizzi
P, Marzullo P, Neglia D, L'Abbate A. Microvascular dysfunction in
collateral-dependent myocardium. J Am Coll Cardiol 1995; 26:615-23.*

This study was conducted at a clinical site in Pisa, Italy. The study
evaluated myocardial blood flow regulation in collateral-dependent
myocardium of patients with angina and coronary artery disease. Nineteen
patients with angina and angiographically confirmed coronary artery
disease as well as a comparison group of 13 normal subjects were
enrolled. Rest/pharmacologic and/or external pacemaker (pacing-induced
tachycardia) stress (dipyridamole) N-13 ammonia PET imaging studies were
performed on all subjects.

Patients with stable angina and single-vessel disease may have a
dysfunction of the resistive vessels that decrease the vasodilatory
capacity in areas supplied by angiographically normal coronary arteries.
This microvascular dysfunction might affect blood flow regulation. The
authors hypothesized that use of N-13 ammonia to assess perfusion in
collateral-dependent areas at rest and stress may provide more
functional information of the microvasculature in these regions.

The angiographic score of collateral circulation was not associated with
differences in baseline myocardial perfusion or function. During pacing,
myocardial blood flow was similar in the six patients with well
visualized and in the five patients with poorly visualized collateral
circulation. However, the angiographic score of collateral circulation
was associated with differences in the degree of flow inhomogeneity. The
ratio between flow in collateral-dependent regions and in remote regions
used as "control" areas was lower in patients with poorly developed
collateral vessels during pacing and after dipyridamole compared to
those with good collaterals.

The study concluded that despite a reduced blood flow at rest,
collateral-dependent myocardium maintains a residual blood flow reserve
that is almost fully used during increased oxygen consumption.

This study allowed comparison of angiographic data and quantitative PET
imaging data in a variety of subjects, including those with severe CAD.
(Di Carli (1994) and Haas (1997) identified lack of correlation between
angiography and PET in patients with severe CAD.) Normal subjects who
had atypical chest pain who were referred to coronary angiography were
also included. All angiograms were done at or less than two weeks before
the PET study. This makes it unlikely that the patient's CAD may have
worsened or improved, impacting on the comparability of the patient's
PET and angiogram. The study is consistent with Di Carli and Haas.
Angiography and PET contribute different information about perfusion
making complete correlation between these tests throughout the vast
clinical spectrum of the disease unlikely.

*h. Soufer R, Dey HM, Lawson AJ, Wackers FJT, Zaret BL. Relationship
between reverse redistribution on planar thallium scintigraphy and
regional myocardial viability: A correlative PET study. J Nucl Med 1995;
36:180-187.*

This study was conducted at the Department of Veterans Affairs Medical
Center, West Haven, Connecticut. A study group of 32 patients with
documented chronic coronary artery disease and demonstrated reverse
redistribution, defined as a new or increased defect on planar
thallium-201 scintigraphy after stress, were enrolled. The mean age was
66 years. All patients were maintained on existing medication. All
patients underwent both planar thallium-201 rest/stress scintigraphy and
N-13 ammonia and F-18 deoxyglucose (18-FDG) PET viability imaging.
Radionuclide angiography to obtain wall motion was performed on 23 of 32
patients. After study, all patients were followed up for 9 to 19 months.

The purpose of this study was to assess myocardial viability via PET in
the area of reverse distribution on planar thallium-201 scintigraphic
images and determine the prognostic value of reverse distribution and
PET imaging after a mean follow-up period of 14 months. The mechanism
that explains reverse redistribtution is uptake of thallium-201 by the
interstitial compartment of a necrotic area and faster washout rate of
thallium-201 from this compartment than adjacent normal myocardium. The
defect on imaging is only seen on redistribution imaging. This
phenomenon has been associated with higher mortality for patients with
previous myocardial infarction. Additional information on
pathophysiology was obtained through application of PET to the
phenomenon of reverse redistribution in this study.

Fifty segments showed reverse redistribution on thallium-201 images.
Nineteen segments were normal on PET 18-FDG and N-13 ammonia scan.
Seventeen had mismatch defined as a regions with severely reduced blood
flow with a relative increase of FDG that was greater than 50% of
activity in the reference region and within two standard deviations of
reference mean values for that segment as defined by a normal database.
A total of 36 of 50 segments (72%) of reverse redistribution were PET
defined as viable. Sixty-one percent (11 of 18) of segments with
abnormal regional wall motion and reverse distribution were PET scar.
After mean follow up of 14 months, 5 of 10 patients that had cardiac
events had severe reverse redistribution and PET mismatch (viability)
compared to 2 of 22 patients without cardiac events.

Using measurements of PET regional blood flow, the study concluded that
the majority of thallium reverse redistribution segments was PET viable
as judged by

18-FDG uptake. PET viability in areas of reversed distribution is not
inferred by regional wall motion analysis. Regional PET mismatch,
indicating viable myocardium with perfusion abnormality, and severe
redistribution were both associated with an increased frequency of
cardiac events.

This study used careful anatomical correlation of planar thallium
imaging and N-13 ammonia/FDG data. It demonstrates different
pathophysiologic information is obtained from these two modalities
concerning reverse redistribution found in CAD patients undergoing
thallium-201. Clinical significance of this information is obtained
through patient follow up to track cardiac events. The study supports
the physiologic assessment of perfusion by N-13 ammonia PET.

*i. Haas F, Haehnel CJ, Picker W, Nekolla S, Martinoff S, Meisner H,
Schwaiger M. Preoperative positron emission tomographic viability
assessment and perioperative and postoperative risk in patients with
advanced ischemic heart disease. J Am Coll Cardiol 1997; 30:1693-700.*

This retrospective study was conducted at a clinical site in Munich,
Germany. The study was masked and investigated whether determination of
tissue viability by means of PET imaging before coronary artery bypass
graft surgery affects clinical outcome with respect to both in-hospital
mortality and 1-year survival rate.

The basis for this hypothesis is stated above in the review of Di Carli,
Davidson, Little, et al., 1994.

A group of 76 patients with advanced three vessel and angiographically
confirmed CAD and severe LV dysfunction who were considered candidates
for coronary artery bypass graft (CABG) surgery were retrospectively
selected and had N-13 ammonia and 18-FDG PET imaging studies performed.

The study concluded selection of patients with

impaired left ventricular function based on extent of PET- defined
viability (perfusion/metabolism mismatch) supplementary to clinical and
angiographic data may lead to postoperative recovery with a low early
mortality and promising short-term survival.

This study is consistent with that of Di Carli, which showed collateral
circulation is not fully detected by angiography. PET results provide
somewhat different profusion information in patients with severe CAD
from those obtained by angiography. This consistency of results in
patients with severe CAD across institutions and across continents
supports the use of N-13 ammonia for perfusion assessment. It is also
consistent in demonstrating that myocardial perfusion in the evaluation
of CAD can not be completely understood through angiography results
alone. In this study, PET perfusion data was associated with a clinical
outcome, mortality. This study supports the clinical significance of PET
perfusion data.

4\. The following articles were reviewed for support of N-13 ammonia PET
myocardial perfusion quantification by computer programming.

*Krivokapich J, Smith GT, Huang SC, Hoffman EJ, Ratib O, Phelps ME,
Schelbert HR. N-13 ammonia myocardial imaging at rest and with exercise
in normal volunteers: Quantification of absolute myocardial perfusion
with dynamic positron emission tomography. Circulation 1989;
80:1328-37.*

*Hutchins GD, Schwaiger M, Rosenspire KC, Krivokapich J, Schelbert H,
Kuhl DE. Noninvasive quantification of regional blood flow in the human
heart using N-13 ammonia and dynamic positron emission tomographic
imaging. J Am Coll Cardiol 1990; 15:1032-42.*

*Gerwirtz H, Skopicki HA, Abraham SA, Castano H, Dinsmore RE, Alpert NM,
Fischman AJ. Quantitative PET measurement of regional myocardial blood
flow: Observations in humans with ischemic heart disease. Cardiology
1997: 88:62-70.*

These are three of several papers that have described and validated
radiopharmaceutical kinetic compartmental models to determine regional
myocardial blood flow with rest/stress PET imaging using N-13 ammonia in
normal volunteers. In addition to studying normal volunteers, the third
paper studies patients with angiographically confirmed ischemic heart
disease and proposes a new methodology that uses a single measurement of
adenosine stimulated myocardial blood flow to assess coronary artery
stenosis severity.

These papers form the basis and extension of the methodology for
quantitatively determining regional myocardial blood flow with PET N-13
ammonia as used in subsequent studies of cardiovascular disease.

C.  Discussion of Efficacy Data for Coronary Perfusion

The effectiveness data supporting the indication for N-13 ammonia PET
imaging for myocardial perfusion in the evaluation of CAD, following
separate administration under rest and stress conditions, is derived
from the published medical literature. As outlined in FDA's guidance for
industry, *Providing Clinical Evidence of Effectiveness for Human Drug
and Biological Products* (May 1998), FDA can, in certain circumstances,
rely on published reports alone to support approval of a new product.
FDA has outlined factors that increase the possibility of reliance on
published reports alone to support approval of a new product. These
include: 1) multiple studies conducted by different investigators where
each of the studies clearly has an adequate design and where findings
across studies are consistent; 2) a high level of detail in the
published reports, including clear and adequate descriptions of
statistical plans, analytic methods (prospectively determined), study
endpoints, and a full accounting of all enrolled patients; 3) clearly
appropriate endpoints that can be objectively assessed and are not
dependent on investigator judgment; 4) robust results that yield a
consistent conclusion of efficacy and do not require selected post hoc
analyses; 5) conduct of studies by groups with properly documented
operating procedures. As a body of literature taken together, the
reviewed studies provide the cumulative evidence to support
effectiveness of N-13 ammonia PET to assess myocardial perfusion in the
evaluation of CAD, following separate administration under rest and
stress conditions, for patients suspected of having the disease or who
have an existing diagnosis of CAD.

Five masked, controlled, comparison studies of N-13 ammonia PET imaging
for myocardial perfusion in the evaluation of CAD and recognized
standard for perfusion, angiography, or with appropriate clinical
functional standards, such as wall motion, are discussed above in
II(B)(1) and II(B)(2). Nine supporting studies in II(B)(3) are also
cited where myocardial perfusion in the evaluation of CAD is compared to
a variety of standards including exercise tolerance as well as coronary
angiography, however, the main hypotheses of these studies varied
considerably. Nevertheless, these studies provide direct or indirect
information on the functional assessment abilities of N-13 ammonia PET
and have some clinical outcomes data. All the studies, conducted by
different investigators in different populations with different disease
spectrum, performed over a number of years, allowed evaluation of N-13
ammonia myocardial blood flow through comparison with an identified
perfusion standard or clinical outcome.

New diagnostic tests are ideally evaluated in clinical trials with the
target population, meaning people with the suspicious condition,
including those who are suspicious for having the disease but don't have
it. Initial phase 1 and phase 2 studies on a new technology typically
are carried out with pre-defined populations of diseased and
non-diseased patients. Confirmatory clinical assessment of the test in
the larger target population provides more meaningful information on the
test's performance ability. This is because sensitivity and specificity
will vary depending on the characteristics of the population in which
the test is used. In the literature search and review of articles, no
studies were found performed with a healthy, asymptomatic population to
detect CAD as part of a screening setting. Therefore, use of N-13
ammonia PET to assess myocardial perfusion in the evaluation of CAD,
following separate administration under rest and stress conditions, is
only supported in the diagnostic setting, where patients are suspected
of having the disease or have an existing diagnosis of CAD.

The majority of studies did not include a prospective population for
which the test is intended. However, the two studies that did enroll
patients prospectively and utilized a direct comparison of anatomy as
measured by angiography to perfusion measured by PET were in II(B)(1):
Gould (1986) and Demer (1989) (the latter being the continuation of the
initial report of the former). Both Gould and Demer found consistent and
strong correlation with PET perfusion measurements and angiographically
defined CAD. Because perfusion and CAD were continuous in nature, the
authors did not calculate sensitivity and specificity by forcing
findings into dichotomous outcome variables. However, 2 x 2 tables were
constructed by FDA using the detailed data presented in Demer's study,
along with an assumption. The sensitivity and specificity of N-13
ammonia using the stated approach was acceptable.

Schelbert (1982) retrospectively selected 32 patients with CAD who had
angiography results, along with 13 normal volunteers. Di Carli (1994)
enrolled 42 consecutive patients with CAD proven by a previous
catheterization study to assess myocardial viability correlation with
wall motion, 18-FDG metabolism, and stenosis. Gewirtz (1994) assembled a
prospective cohort of 26 patients to study correlation of flow rates and
myocardial viability, but enrollment was limited to patients who had a
previous catheterization study and documented CAD on catheterization.
These latter three studies either presented data (Schelbert (1982)), or
may contain raw unpublished data (Di Carli (1994) and Gewirtz (1994)) on
sensitivity of PET with respect to angiography in detecting CAD.
Schelbert (1982) also included a subgroup of 11 patients who underwent
thallium-201 exercise testing and presented detailed study results:
numbers and locations of diseased vessels were identified and
comparisons of PET results (for the entire study and specific vessels)
to angiography, "truth."

The Gould (1986) and Demer (1989) studies can be viewed as comparative
studies between N-13 ammonia and angiography allowing for calculation of
sensitivity and specificity. In other studies, angiography defined
"truth" with respect to CAD for sensitivity, but specificity was
calculated based on an "assumed truth" that normal volunteers did not
have CAD. The reviewed studies of Schelbert (1982), Laubenbacher (1993),
Beansland (1995) and Gould (1995), supported reproducible and robust
correlation between angiography and N-13 ammonia PET, particularly in
patients with mild and moderate CAD. These results suggest that N-13
ammonia PET perfusion results are comparable to angiography for
anatomical assessment in particular patients.

Other comparative trials assessed the correlation of N-13 ammonia PET to
a variety of standards to investigate functional microperfusion. The
latter studies did use angiography to identify collaterals, but as
mentioned in the beginning of this review, angiography may not be the
appropriate standard for functional perfusion.

Studies that focused on functional hypotheses, such as Gewirtz (1994),
Haas (1997), Czernin (1995), Di Carli (both 1994 studies), Sambucetti
(1995), Gould (1994), and Soufer (1995), demonstrated that correlation
with angiography results was not good in patients with severe CAD and
left ventricular dysfunction. However, these authors and others used
standards to assess cardiac function, such as exercise tests, and
clinical outcomes to provide additional information with which N-13
ammonia results were consistent. These other standards of assessment
provided clinical correlation regarding the N-13 ammonia PET perfusion
results. These multiple studies conducted by different investigators in
different institutions, in different time periods contained detailed
information on study plans and results that support PET's use in
functional perfusion assessment.

The reviewed studies did not state a priori criteria for the amount of
correlation between N-13 ammonia PET and the comparative standard
considered satisfactory, sample size calculation based on an a priori
power, nor type I and type II error limits. However, enough raw data was
presented to allow adequate determination of N-13 ammonia testing
performance.

The types of enrolled subjects varied widely in the studies. Normal
volunteers were studied with N-13 ammonia PET along with retrospectively
selected, angiographically-defined patients, and consecutively enrolled
CAD patients. Definitions of significant stenosis varied as did
definitions of CAD. CAD patient selection varied from study to study on
percent stenosis on angiogram, presence of left ventricular dysfunction
of a certain amount, and exercise tolerance results. For example,
prospective subjects from the Gould (1986) and Demer (1989) papers were
enrolled with indications for angiography, such as chest pain or
abnormal ECG findings, as part of the study entry criteria. These
included subjects who ultimately had normal angiographic results as well
as subjects who had coronary artery stenoses. Taken together, the
studies did contain patients that appeared to reflect the diverse
spectrum of disease and disease-free states to support the broad
application of N-13 ammonia in the population in which further
diagnostic testing concerning myocardial perfusion in the evaluation of
CAD might be used. These studies also demonstrated that in certain
patient disease circumstances, N-13 ammonia functional perfusion testing
provides additional information from angiography.

While many of the initial studies originated from a few institutions,
the recent N-13 ammonia PET studies are from many different institutions
and countries with independently supportive results. Although the total
numbers of patients in most studies were small, taken as a whole the
numbers are meaningful. There are additional studies published in the
medical literature, not included in this review because they did not
allow a comparative evaluation. Many of these studies support the use of
N-13 ammonia in myocardial perfusion in the evaluation of CAD by
providing consistent information on functional assessment. For example,
nitroglycerin use and N-13 ammonia PET showed topical nitroglycerin
alters myocardial perfusion by preferentially increasing flow to areas
of reduced perfusion with little or no change in global perfusion in
patients with angina responsive to nitroglycerin (Fallen, E. et al.
Redistribution of myocardial blood flow with topical nitroglycerin in
patients with coronary artery disease. Circulation. 1995;91:1381-1388.)
This study had no imaging or functional standard comparator but the
results support the biologic plausibility of N-13 ammonia PET to assess
functional perfusion.

The selection of measurement outcomes with N-13 ammonia has varied from
study to study. Early studies had to handle problems of ascertainment
bias because of subjective scoring or evaluation of images both from
N-13 ammonia PET and angiography or other imaging. There were problems
with translating the "gold standard's" two-dimensional picture to
accurately correlate with flow and there were usually issues of patient
positioning. N-13 ammonia also has known issues: potential dependence of
extraction and retention of N-13 ammonia in myocardium on the metabolic
status of the heart and a reduced retention on N-13 ammonia on the
posterolateral wall of the normal heart. Evolution in the imaging and
medical physics field has allowed for digitized images with computer
modeling to address many concerns and to generate calculations of flow
reserve for both angiographic data and PET scans. This allows for
objective quantification of MBF, which enhances reproducibility.

Reader performance is also important in studies. The amount of
experience readers had with N-13 ammonia imaging in the cited studies is
not mentioned. It is an assumption that performance as reported in the
studies can be replicated in the practicing community. Oftentimes,
however, studies report the better spectrum of reader performance
reflecting publication bias. Nevertheless, the results are consistent
across the different studies, different institutions, and different time
periods supporting independence of the findings.

Inter-reader variability is also important to assess if a study utilizes
more than one reader. Studies using one reader do not have inter-reader
variation; however, these studies do not capture the scope of variation
possible in reader assessment of scans when N-13 ammonia PET is used
more widely. The reviewed studies cited often made attempts to quantify
and address inter-reader variability in N-13 ammonia PET readings. Many
utilized more than one reader. Multiple readers decrease variability,
because as the number of readers increases, variability of the median
rating or score usually decreases. Many studies, however, did not
provide information on how angiograms were read. These might also be
subject to inter-reader variability, but readers would most likely have
more familiarity and experience with angiographic images. Criteria for
dispute resolution, which is needed whenever a study uses more than one
reader, were also only occasionally mentioned. As technology improved to
allow for computerized measurement and reproducibility, issues of
inter-reader variability and dispute resolution became less important.
These latter studies, utilizing objective quantification of myocardial
perfusion, demonstrated the results found in earlier studies were
robust. These latter studies suggest that inter-reader variability
issues in the earlier studies, which used subjective interpretation by
readers, are not a major influence.

Some studies cited also made attempts to minimize bias by using
randomization and masking. Randomization in these studies was done in
the time period when computerized readings were not available. N-13
ammonia PET images were presented to interpreting physicians in a random
manner. The readers were also masked to the clinical data of patients,
or their angiography (or other comparative technology) results. In more
recent times, computerized quantification of PET and angiographic
results has made randomization and masking less important. Nevertheless,
the study protocol should ensure that biases from technicians, as well
as investigators, who acquire, process, and record computerized results
are controlled. These studies are from the literature and it is not
possible to conduct scientific audits at this time. Again, because the
latter studies using computerized measurements obtained consistent
results as earlier studies using subjective criteria, bias appears to be
a minimal concern in the studies.

Given the consistency of findings for anatomical and function perfusion
assessment throughout the almost twenty years of published studies
reviewed, the reproducibility of results in many different institutions
with different investigators, the reproducible correlation with
angiography (an accepted external truth standard) in patients with mild
to moderate CAD, the correlation with current functional standards of
perfusion in patients with severe CAD and left ventricular dysfunction,
this review supports the effectiveness of N-13 ammonia for assessing of
myocardial perfusion in the evaluation of CAD, following separate
administration under rest and stress conditions, for patients suspected
of having the disease.

III. Safety Evaluation

N-13 ammonia PET radiopharmaceutical is radioactively labeled ammonia.
It is administered intravenously for PET imaging. The overall adverse
reaction profile of N-13 ammonia is well understood after roughly a
quarter century of clinical use. N-13 ammonia PET is performed in North
America, Japan, Europe, Australia, and other countries.

Because ammonia is a ubiquitous substance in the body, its metabolism
and excretion is well understood. N-13 ammonia is rapidly metabolized to
N-13 urea, N-13 neutral amino acids, and small amounts of N-13 acidic
amino acids. N-13 urea, the main circulating metabolite of N-13 ammonia,
is eliminated through the urine.

The FDA biopharmaceutical reviewer of N-13 ammonia published studies
reports that the dose range was from 8 to 25 mCi. The specific activity
range was reported as 200-400 mCi per micromole from Rosenspire (1990).
The biopharmaceutical reviewer estimates that the amount of N-13 ammonia
administered for the 8 mCi dose is 0.02-0.040 micromoles and
0.0625-0.125 micromoles of N-13 ammonia for the 25 mCi dose. Thus, the
amount of ammonia administered is very small compared to the amount of
ammonia already produced in the body. The blood ammonia concentration in
a normal person is less than 35 micromoles per liter. Thus the amount
introduced by N-13 ammonia PET imaging is not significant compared to
the circulating level of ammonia in the body.

Radiation exposure from N-13 ammonia is a known risk. The amount of N-13
ammonia injected in the above reviewed studies was usually one
intravenous dose of 10-20 mCi N-13 ammonia for the rest portion and
another similar dose for the stress portion of the PET scan. For doses
up to 25 mCi, data provided by the International Commission on Radiation
Protection allows estimation that the urinary bladder absorbs 0.75 rem,
an acceptable level. Higher radiation values are stated in two studies
found in this literature review (Gewirtz (1994) and Meyer (1995))
suggesting there are differences in dosimetry data determined by
different investigators in different laboratories.

The physical half-life of N-13 is about 10 minutes for decay from N-13
ammmonia to carbon-13. The effective blood elimination half-life based
on data from Rosenspire (1990) is 2.2 minutes. The short physical
half-life of N-13 permits sequential evaluations and avoids high
radiation exposure for patients. However, the short half-life requires
an on-site cyclotron for manufacture and sophisticated radiochemical
synthesis and purification procedures.

The N-13 ammonia PET studies which had a rest and a stress component,
involved monitoring of cardiovascular parameters, such as heart rate,
blood pressure, and the rate pressure product. These measurements are
reported in many papers for normal and CAD patients. These reports show
an increase in the blood pressure and heart rate associated with the
hyperemic/stress component of the test, as is expected. There were no
comments on any increase in blood pressure or heart rate with injection
of N-13 ammonia. There is no specific reference to any patient having an
acute effect with N-13 injections.

In the published literature concerning safety of PET drugs, in 1996,
Edward Silberstein, Janet Ryan and the Pharmacopeia Committee of the
Society of Nuclear Medicine published in the Journal of Nuclear Medicine
in a five year prospective study of 18 collaborating institutions. The
study used a questionnaire that enumerated monthly the number of
procedures used and the adverse reactions noted for radiopharmaceuticals
and non-radioactive drugs used in nuclear medicine. The study utilized
operational definitions for adverse reactions and significant adverse
reactions and devised an algorithm to categorize probability of
causation. The published study included a copy of the actual
questionnaire, which required itemization of any and all
radiopharmaceuticals administered, adverse reactions to
radiopharmaceuticals, dose, route, reaction, etc., as well as total
non-radiopharmaceuticals (such as adenosine or dipyridamole)
administered and adverse reactions to these agents. For
radiopharmaceuticals, there were a reported 18 total adverse reactions
for 773,525 injections. No reactions are reported for N-13 ammonia. The
study also performed a reference check of listed adverse reactions by
references and no adverse reactions were listed by the U.S.
Pharmacopeial Convention's Drug Information for the Health Care
Professional, 1995. Dr. Silberstein and the Pharmacopeia Committee of
the Society of Nuclear Medicine also conducted a retrospective and
prospective study of the prevalence of adverse reactions to PET
radiopharmaceuticals published in 1998 in the Journal of Nuclear
Medicine. In this updated study, Dr. Silberstein reported 22 PET centers
provided monthly adverse reaction data from 1994 to 1997 related to PET
drug administration in 47,876 dosages. In addition, retrospective data
was collected from the opening of these centers on 33,925
radiopharmaceutical dosages. In no case were there any adverse
reactions.

The published clinical trials literature is not an appropriate data base
to evaluate chemistry and manufacturing safety issues related to drug
purity and identity. Different production pathways may lead to different
concentrations of N-13 ammonia and to different by-products and
impurities. These specific topics will be addressed by the FDA PET
Chemistry and PET Good Manufacturing Practice working groups.

Because radiation is a known carcinogenic and mutagenic agent, standard
radiation precautions for using PET with respect to patients, pregnant
women, and occupational exposure are needed.

IV. Conclusions

The well-controlled studies of Gould (1986) and Demer (1989) permit an
estimate of N-13 ammonia's performance for assessing myocardial
perfusion in the evaluation of CAD, following separate administration
under rest and stress conditions, for patients undergoing cardiac
catheterization for the diagnosis of CAD. These two studies together
provide correlation with angiography data. This is independently
corroborated in studies by Schelbert (1982), Laubenbacher (1993) and
Beansland (1995). In other comparative studies of N-13 ammonia PET with
angiography, thallium-201, exercise stress testing, and clinical
outcomes of survival and cardiac-specific mortality, N-13 ammonia PET's
ability to detect myocardial perfusion in microvasculature is supported
by myocardial regions with scarring, wall motion abnormality,
non-angiographically defined collaterals or angiographically present
collaterals, thallium-201 reverse redistribution, and effects of
lifestyle interventions such as low fat diet with exercise on coronary
circulation. N-13 ammonia is a minimally invasive test, requiring
intravenous injections, and may be used to complement or provide an
alternative to existing coronary perfusion tests for certain patients.

The above referenced published medical literature and the cumulative
experience support the claim for the use of N-13 ammonia used in
positron emission tomography to assess myocardial perfusion in the
evaluation of CAD, following separate administration under rest and
stress conditions, for patients suspected of having the disease or who
have an existing diagnosis of CAD. Safety of N-13 ammonia in the medical
literature is supported by the absence of documented adverse events, the
knowledge of ammonia metabolism and its safety profile, and the known
radiation risk associated with its production, handling, and use.
Specific chemistry and manufacturing issues are deferred to the FDA
working groups on these issues.

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

Florence Houn, MD MPH date

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

Sonia Castillo, PhD date

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Appendix A: Figure 3 Plots of SFR and PET Defect Severity Scores from
Demer LL, Gould LK, Goldstein RA, Kirkeeide RL, Mullani NA, Smalling RW,
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