Notes to "***NEXT* 2000: Survey of Computed Tomography (CT) Practice,
Workload, Dose**," by Stanley H. Stern, Ph. D., FDA/CDRH Office of
Health and Industry Programs, Division of Mammography Quality and
Radiation Programs, Radiation Programs Branch, to be presented to the
*Technical Electronic Product Radiation Safety Standards Committee
Rockville, Maryland, June 21, 2000*

## 

## Slide 1

This presentation is about how CDRH is leveraging the Nationwide
Evaluation of X-Ray Trends, whose acronym is "*NEXT*," to obtain
up-to-date information on clinical practice, patient workload, and
patient dose related to computed tomographic examinations and procedures
across the United States:

-   I will describe briefly what *NEXT* is, outline how it works, and
    cite some key findings.

-   I'll identify the most significant technological advances in CT
    since the last CT survey in 1990 and describe how innovations of
    this past decade and their promotion of related clinical
    applications have led us to revamp the CT survey.

-   The heart of this discussion is the 2000-CT survey; I'll highlight
    the parts of it intended to garner dose and dose-rate data
    associated with the most recently developed modes of operation for
    exams of the body as well as the head.

-   A primary motivation for characterizing x-ray trends is to
    understand how they affect individual and population radiation dose,
    and so I'll speak briefly about aspects of dosimetry that are
    peculiar to CT and how the survey is designed to facilitate
    inference of patient and population dose.

-   And finally, I will mention a complementary CDRH project underway to
    develop a compendium of patient tissue doses associated with CT
    exams.

### 

### **[Slide 2]{.underline}**

*NEXT* is a cooperative program encompassing national quality assurance
and radiological-health research. The program is administered through
the Conference of Radiation Control Program Directors, which is the
umbrella organization of State radiation-control agency directors.
CRCPD's *NEXT* Committee coordinates annual participation of over 40
States. Each year States provide personnel who recruit clinical
facilities, do on-site surveys, and perform x-ray equipment measurements
in approximately 350 locations across the country---private practices,
hospitals, clinics. Surveyors acquire x-ray system data on technique,
exposure, image-quality, and patient-workload associated with a
particular radiological examination whose selection for survey varies
from year to year.

CDRH underpins the program scientifically and technically:

-   We develop the survey instruments and protocols for measurement;

-   We identify, design, test, procure, and calibrate equipment and
    materials;

-   We select survey samples from State rosters of facilities;

-   We develop curricula for training surveyors;

-   We enter survey results into databases, analyze, and interpret the
    data;

-   And we publish *NEXT* findings as technical reports and as papers in
    peer-reviewed journals.

## Slide 3

The *NEXT* program is unique in the United States: It is the only
mechanism for obtaining medical radiology data that are *nationally
representative* of the amount x-ray exposure and numbers of people
exposed, image quality, and clinical practice related to patient dose.
Facilities that participate in the survey are solicited by random
sampling of State rosters. What each year's survey captures for a
particular type of examination are "snapshots" of the U.S. distributions
of the most important machine-generated radiological variables affecting
patient dose. Examples of such variables are radiation exposure at the
skin-entrance plane, x-ray tube peak voltage, tube current, exposure
time, x-ray spectrum half-value layer. Data have also been collected on
the quality of film processing. In acquiring these data, surveyors
employ patient-equivalent phantoms---some embedded with test-image
objects---to attenuate the radiation. Such phantoms standardize the
measurement of exposure and assessment of image quality across all
facilities surveyed.

CDRH composes brochures of these data that CRCPD provides to the States
for distribution to facilities when State personnel inspect them as part
of their routine radiation-control programs. These brochures serve as a
quality-assurance tool: they enable comparison of the facility's
radiological techniques and exposures to nationwide norms.

For a given kind of examination, national trends emerge from analysis of
data collected over time. Over of the course of a number of years,
information published from *NEXT* surveys has proved to be a seminal
resource, cited in scientific journals and used by researchers to
identify radiological health problems and to suggest solutions involving
changes in equipment technology, radiological techniques, and clinical
protocols. In recognition of the impact of the NEXT program in promoting
radiological health, the international journal *Applied Radiation and
Isotopes* invited CDRH to review *NEXT* findings in a special edition
published last year.^1^

## Slide 4

This slide indicates the scope and some principal findings of the *NEXT*
program since 1984, and I'd like to use it to highlight several points:

-   First, please look at the two columns on the left: In the past
    fifteen years *seven* different types of radiographic and
    fluoroscopic examination have been surveyed. Every few years an
    examination category is repeated, perhaps with some kind of variant
    introduced---hospitals vs. private practice, 4.7- vs. 4.2-cm
    compressed-breast phantom, adult vs. pediatric chest
    radiography---in order to keep pace with technological and clinical
    developments. In 1996, for example, along with the survey listed
    here for upper gastrointestinal fluoroscopy, there were pilot
    studies of fluoroscopy in cardiac catheterization labs and on mobile
    C-arm units. These kinds of technical pilots are useful as an
    "avant-garde" anticipating prospective widespread incorporation of
    such modalities into clinical practice.

-   The two columns on the right correspond to measures of radiation
    levels, either at skin-entrance^2^ or absorbed by the tissue of
    clinical interest.^3^ So for mammography surveys,^4,\ 6,\ 9^ the
    radiosensitive tissue is the glandular breast tissue, and the
    circled values show a clear trend of increasing mean
    glandular-tissue dose. Let me hasten to add that along with an
    increasing dose for film/screen mammography there are associated
    trends, which are not explicitly presented in this table, of
    progressively better scores for visualizing test-image objects, of
    the near disappearance of Xero-mammography as a relatively high-dose
    modality, and of the nearly universal adoption of the use of grids
    to suppress radiation scatter.^6^ For mammography, the bottom line
    is improved image quality over this period of time.

-   For this slide, the last item I want to cover is computed
    tomography, and the CT surveys are highlighted in yellow.^5,7^ There
    are *two* key ideas presented here: First, skin-entrance dose (41
    mGy) or internal-head dose (47 mGy) incurred on average by a patient
    in a routine CT head examination is the *largest dose* among those
    of all the radiographic exams listed, fluoroscopy^7,\ 8^ excluded.
    In fact CT may be the single modality which contributes most to the
    collective population x-ray dose arising from diagnostic
    radiological exams,^10^ and we are hoping that the year 2000 survey
    will enable estimation of just what the CT contribution is to
    population dose.

> For CT, the numbers in parentheses are the extrema of a range that
> includes 50% of all of the values of the sample distribution. In other
> words, 50% of the doses fall within this range, and 50% fall outside
> of the range. The second key idea then, is that the ranges are
> relatively *broad*. The implication is that through judicious
> selection of technique factors, it's possible to obtain satisfactory
> image quality and spare patient dose as well.^5^ So, these two
> points---large dose, broad range---represent our baseline for CT.

If this is our baseline, our starting point, what are the challenges we
face in mounting a survey in the year 2000? What's happened since 1990
that we need to capture in a new survey?

## Slide 5

The bulleted items correspond to the principal technological advances
that have already been incorporated or are being incorporated into most
CT systems in the field. They have led to profound changes in how CT is
applied in clinical practice:

-   Slip rings are housed in the gantries of CT systems, and they
    conduct electrical energy between the high-voltage power supply and
    the rotating x-ray tube or between the computer and the
    data-acquisition array (if the x-ray detectors are rotating with the
    tube). This technology is in common use in low-cost as well as
    top-of-the-line scanners.^11^

> Here's the important point: Slip rings eliminate the constraint of
> electrical cables, and so the x-ray tube or detectors can rotate
> continuously---over many 360^o^ revolutions---while the x-ray tube is
> energized. The technology has fostered a new mode of CT scanning
> referred to as "helical" or "spiral" scanning, where the patient table
> moves at a constant rate while the x-ray tube is rotating, and in
> effect the x-ray beam traces out a helical pattern around a patient as
> the patient table advances through the gantry opening. Helical
> scanning has spawned an explosion of new clinical applications of CT
> because of the advantages it holds over conventional, slice-by-slice,
> axial scanning, namely, speed, reduction of patient-motion artifacts,
> and facilitation volume rendering of images. Just a few examples of
> such applications are spiral CT angiography,^12,\ 13^ detection of
> spine fractures,^14^ evaluation of laryngeal disease,^15^
> cinematically displayed visualization of pancreatic vascular and
> ductal anatomy,^16^ detection and management of renal and ureteral
> calculi,^17^ and there are many other applications. One of the key
> objectives of this survey is to find out what percentage of CT systems
> in the field have helical-scanning capability.

-   Another significant advance in CT technology has been the
    development of high heat-capacity x-ray tubes. If we look for
    example at the x-ray tube specifications of one particular CT
    manufacturer, from 1986 to 1998 the heat capacity of their tubes
    increased by a factor of five, from 1.5 to 7.5 million heat
    units.^18^ This capacity in conjunction with the ability of scanners
    to rotate continuously has led to the growth of what's called "CT
    fluoroscopy."^19-21^ Manufacturers also refer to this mode as
    "dynamic scanning" or "continuous scanning." In this mode of
    operation, the x-ray source rotates continuously around the patient
    multiple times---in some systems up to 200 revolutions of 360^o^
    each. Contrary to the helical scanning movement, in CT fluoroscopic
    mode, table movement is *not* programmed to advance automatically,
    but it is under the control of the operator. The patient and table
    may stay fixed in place as the x-ray tube is activated and rotates
    continuously to enable low-resolution CT images to be acquired
    nearly in real time, at rates of 6 to 8 frames per second. The
    images are continuously reconstructed and displayed on a monitor
    typically located near the patient table. The mode is used to
    visualize interventional procedures involving biopsies or
    drainage.^27-29^

> Last year Captain Robert Gagne addressed this Committee about FDA
> concerns for potentially large skin-dose and injury from CT
> fluoroscopy,^21^ and this year the NEXT survey has begun to obtain an
> accurate picture of the nationwide prevalence of this mode and the
> dose rates and doses associated with its use.

-   A third major group of technological advances falls into a category
    that may be characterized as "ultra-fast" scanning. There are really
    two different technologies here: First "e-beam" CT refers to
    electron-beam computed tomography. Although it has really been
    around since the early 1980s,^22^ it wasn't followed at all in the
    1990 CT survey. In electron-beam CT, x-rays are produced by
    electromagnetically scanning an electron beam about a large,
    semicircular tungsten target underneath the patient; because there
    is no mechanical motion, scan times of 50 millisecond are
    possible.^22^ The extent to which electron-beam CT has caught on is
    not clear, and the NEXT survey is trying to answer this question.

The second technology---multi-slice helical CT---is relatively more
recent.^23^ It uses two to four parallel arcs of detectors to produce a
double or quadruple helix of volumetric data.^23,\ 24^ Some multi-slice
scanners have x-ray tubes that spin at two revolutions per second, so
that the 4-slice units can be eight times faster than most single-slice
scanners.^23^

**[Slide 6]{.underline}**

The single word that summarizes these changes is "complexity." Because
of these developments, there has been a shift in the types of exams
done: these days there are more clinical applications of CT to the body
than there are to the head. In 1983, 63% of the 5 to 5.5 million CT
procedures in the U.S. were head scans,^25^ whereas a study in 1997 of
the ten most frequently performed CT procedures at the Cleveland Clinic
Foundation indicates that head procedures account for only 41% of the
total.^26^ Because of the advent of CT fluoroscopy, computed tomography
is used to visualize interventional procedures as well as for
diagnosis.^27-29^ The proliferation of helical scanning, with many
different kinds of models and options such as CT fluoroscopy and
multi-slice scanning has resulted in a variety of different irradiation
and scanning conditions whose terminology itself is not completely
standardized.^23,\ 30^ This complexity has led us to try some major
changes^31^ in conducting the year-2000 CT survey compared to what was
done in 1990.

**[Slide 7]{.underline}**

The national sample size was initially set for 350 facilities randomly
distributed across the United States. This year 42 States are actually
participating in the survey, and they will cover 314 facilities.
Facilities are picked randomly from rosters submitted by States, and the
sample size within each State is proportional to its population. The
target in each of these facilities is the most frequently used CT
system.

Because of the advances in technology and clinical practice since 1990,
we've had to introduce several new aspects into the way the NEXT survey
is conducted. The major innovation is that the survey for each facility
is divided into *two* parts. One part is an on-site visit that focuses
on routine exams of the adult head. Even though the focus is the head
exam, several crucial features of the survey pertain to body exams also.
A second part of the survey is based on a questionnaire that the
facility fills out in advance of the surveyor visit. It is through this
questionnaire that we hope to obtain more complete information about
exams of the body, which, as I mentioned earlier, comprise the
preponderant^26^ part of the of CT universe.

This slide summarizes the important elements of the surveyor's visit on
site at a participating facility. The on-site visit represents the
traditional way a NEXT survey is done: a surveyor interviews a CT
technologist familiar with the system and makes measurements with a
standardized reference phantom. But there are several new twists as
well:

-   For example, we are going to determine what percentages of CT
    systems are capable of helical scanning and of CT fluoroscopy.
    Helical scanning wasn't even covered at all in the 1990-CT survey.

> Also, we will obtain information about CT fluoroscopy that will help
> us make informed regulatory decisions. The patient-workload data and
> technique factors sought for the CT fluoroscopy mode are not limited
> to head exams: they refer to the most typically-used settings for body
> or head scanning for all patients---pediatric as well as adult. We are
> explicitly asking for the average "beam-on" time for CT
> fluoroscopically-guided procedures, and by making measurements of the
> dose, duration of exposure, and slice width near the surface of a head
> phantom, we can estimate the skin-dose rate and typical values of skin
> dose in the CT fluoroscopic mode of operation. This information will
> give CDRH the most definitive insight to date on the pervasiveness and
> dosimetry for an increasingly popular mode of operation potentially
> associated with skin injury.

-   For the routine head exam in particular, surveyors are asking for
    the weekly patient workload and also for a breakdown of how a
    facility does those exams according to what clinical protocol. What
    percentage of exams involves axial CT exclusively? What percentage
    uses helical scanning? How many head exams consist of a scanning
    phase without contrast media followed by a scanning phase with
    contrast media? Of course, if there were two distinct phases to an
    examination---first without contrast then with contrast---the
    radiation dose would be double that for a single phase.

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-   We will find out the technique sets---tube voltage, current, scan
    time, number of slices, slice width, table increment, number of
    scanner rotations, and so on---applied for the most frequently used
    axial-scanning protocol and separately for the most frequently used
    helical-scanning protocol of the head.

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-   The survey entails three sets of exposure measurements, two of which
    are with the standard head phantom developed by CDRH. The head
    phantom is a 16-cm diameter cylinder approximately 16 cm long, made
    of polymethyl methacrylate, a material whose atomic composition and
    density make for reasonably good simulations of the
    radiation-scattering and attenuation properties of tissue.

> The first set of measurements will be made with an ionization chamber
> in the center hole of the phantom and will yield the multiple-scan
> average dose, abbreviated "MSAD," which is a descriptor of the central
> dose amongst a series of scans comprising a head exam. These values
> can be directly compared to those obtained from the 1990 survey, and
> they will offer the clearest indication of dose trend over this past
> decade.
>
> The second set of measurements will be made in a phantom hole located
> 1 cm from the phantom-entrance surface, and values from these
> measurements can be used to describe the average dose rate and dose at
> the skin surface when the CT unit operates in a CT fluoroscopic mode.
> These measurements can also be related to those obtained in the 1985
> survey whose values were also measured in a surface hole of the head
> phantom.
>
> The third set of measurements will have the ionization chamber aligned
> along the axis of rotation *free-in-air*. The great advantage of this
> last set of measurements is that because they are unencumbered by the
> attenuation and scattering introduced by the head phantom, the
> free-in-air values are not limited to descriptors of head dose.
> Free-in-air values can be applied to make estimates of internal-tissue
> doses to the *body* based on computer calculations by the National
> Radiological Protection Board of the United Kingdom.^32^ The NRPB
> calculations simulate radiation transport in an anthropomorphically
> modeled mathematical phantom.^32^ They offer a way for us to estimate
> what we expect will be the largest contribution to population dose
> from CT as it is practiced today, namely, doses to tissues from exams
> of the *body* not limited exclusively to exams of the head.

**[Slide 8]{.underline}**

Most of this slide summarizes features of the second principal aspect of
the year-2000 CT survey. The second part consists of a detailed
questionnaire addressed primarily to the CT technologist but also to the
medical physicist most familiar with the most frequently used CT unit.
These questions cover exams of the body as well as those of the head,
and although the focus is adult patients, there are some queries about
pediatric patient workload and techniques too.

-   First, we try to identify the types and numbers of CT units
    available at each facility surveyed---units with only axial-scanning
    capability, units that can do single-slice or multi-slice helical
    scanning, units capable of doing electron-beam CT.

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-   Second, we seek detailed enumeration of patient workload per week,
    of frequency of use of scanning protocols---axial *vs*. helical,
    contrast *vs*. no-contrast phases---and of x-ray system technique
    factors. Each data set is associated with an examination category
    for the most frequent types of examinations---abdomen and pelvis,
    head, simple sinus, chest-abdomen-pelvis, and so on. This
    information represents the core of CT practice and exposure as they
    relate to patient dose in the U.S., and its acquisition would enable
    a detailed estimation of population dose heretofore unavailable.

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-   Third, the questionnaire includes a group of queries about system
    maintenance and quality-assurance tests. Quality assurance is an
    important aspect of maintaining imaging integrity and radiological
    protection, and it simply was not covered in the previous CT survey.

At the bottom of the slide I have underlined a major initiative intended
to help the *NEXT* program transition to an era of electronic file
transmission. For the first time, surveyors are being provided with
diskettes containing pre-formatted spreadsheets for their data entry. We
are encouraging electronic transmission of these files as a time-saving
efficiency, and in the future we are planning to work with the
Conference of Radiation Control Program Directors to establish web-based
data entry of *NEXT* survey results.

**[Slide 9]{.underline}**

Previous CT surveys relied on what are called "dose descriptors" to
characterize an amount of radiation energy representative of what's
absorbed per mass of generic tissue during a typical exam of the adult
head. The best known dose descriptors for CT are the *multiple-scan
average dose*, abbreviated "MSAD," which I mentioned earlier, and a
related quantity called the *computed tomography dose index*,
abbreviated "CTDI."^2,\ 3,\ 5,\ 7^ These descriptors are derived from
measurements of dose within a physical phantom intended to approximate
the radiation scattering and attenuation qualities of the head.
(Incidentally, there is also a 32-cm diameter CT phantom that can be
used for the indication of body dose.) The year-2000 CT survey will
obtain measures of MSAD using a head phantom.

But what we are *really* interested in is estimating doses to the
radiosensitive tissues of the body. Evaluation of tissue dose is the
foundation of ionizing radiation risk assessment, and the principal
risks associated with absorbed radiation are morbidity and mortality of
induced cancer, transmission of genetic defects, fetal mental
retardation, and acute skin injury.^33,^ ^34^ This slide^34^ is meant to
illustrate several considerations involved in CT tissue dosimetry.
What's plotted is a single set of calculated dose values corresponding
to one particular grouping of scanners---the GE series 8800 and
9000---modeled by the National Radiological Protection Board in computer
simulations of radiation transport through an anthropomorphic
phantom.^32^ The ordinate corresponds to the doses to the tissues
indicated, that is, any particular color indicates the dose to the
*whole* tissue, the dose averaged over the *entire mass* of the tissue
wherever it's distributed. Tissue dose is plotted as a function of the
*location* of a 5-mm wide scan-slice along the length of a person. "Zero
centimeters" here corresponds to the base of the trunk, and "94
centimeters" corresponds to the top of the head. In other words, if
there were a single axial scan 50 cm above the base of the trunk, then
the average dose to the breasts would be 0.015 relative units, to the
lungs 0.007 units, and to the active bone marrow 0.002 units. There are
two points I want to make:

-   First, the unit of measure indicated by the ordinate is not tissue
    dose *per se*; it is actually a *ratio* of tissue dose to dose free
    in air. In other words, internal tissue doses are represented by
    ratios *normalized* to the radiation output of the CT unit so that
    the doses themselves can be evaluated only if one knows how much
    radiation the CT unit emits in the first place. This situation
    reflects several important facts that many people are not aware of:
    Internal-tissue doses *cannot be practicably measured;* they are
    generally *not known* during the actual radiological exam; but they
    can be estimated from *computer simulations*. The radiation output
    of a CT unit---evaluated in terms of dose free-in-air---is really
    just the *starting point* in the estimation of internal-tissue
    doses.

-   Second, the radiosensitive tissues are distributed throughout body,
    and in order to estimate tissue doses we would need to know the
    anatomical ranges covered by various scanning protocols. The
    year-2000 survey is obtaining this information, as well as x-ray
    technique factors, dose free in air, exam frequency, frequency of
    use of contrast phases, and patient workload---hopefully all of the
    ingredients that we need to infer population dose from CT.

**[Slide 10]{.underline}**

Finally, I would like briefly to mention a CDRH project that is
complementary to the *NEXT* program CT survey, and that is the
development of a handbook of tissue-dose values from CT
examinations.^33,\ 34^ The handbook will be targeted to medical
physicists and radiologists in a format entailing look-up of dose values
according to the type of examination. We will try to have a generally
applicable set of tables---one table for each kind of exam, and that
table should be valid for any CT model. We would like to include options
for estimating dose for all the current and upcoming CT technologies,
including multi-slice helical scanning and fluoroscopic CT as well as
axial scanning. NEXT survey results will give us insight into how to
accurately parameterize dose values in terms of these new scanning
modalities. We also want to include ways to estimate pediatric and fetal
doses.

Our initial approach^35^ in handbook development is to characterize and
generalize the existing data set of normalized CT doses computed by the
National Radiological Protection Board of the United Kingdom.^32^ We
have already mapped anatomical scanning regions to corresponding
mathematical coordinates of the NRPB anthropomorphic phantom for
approximately 50 distinct CT exams. In addition to dose free-in-air, we
would like to normalize tissue doses to a reference parameter commonly
measured in the U.S., namely, the computed tomography dose index. Our
goal is to have this handbook available in approximately one year.

That completes this talk. It was presented for your information, and I
would be pleased to address any questions that you may have.

**Cited references and notes**

1\. The table slide 3 has been adapted from Orhan H. Suleiman, Stanley
H. Stern, and David C. Spelic, "Patient dosimetry activities in the
United States: the Nationwide Evaluation of X-Ray Trends (NEXT) and
tissue dose handbooks," *Applied Radiation and Isotopes*, Vol. 50, No.
1, pp. 247-259 (January 1999).

2\. For all examinations except Head CT, the air kerma is determined
from measurements made free in air at the skin-entrance plane. For the
1985 Head CT survey, values cited in this column correspond to the mean
multiple-scan average dose (MSAD), which is the average dose at the
middle of a series of scans comprising (in this case) a head exam,
inferred from measurements made with a pencil ionization chamber in the
***top-surface hole*** of CDRH head phantoms (*cf*. ref. 5). Values in
parentheses correspond to the range of the distribution including 50% of
sampled values. The quantity MSAD is discussed by Thomas B. Shope,
Robert M. Gagne, and Gordon C. Johnson, "A method for describing the
doses delivered by transmission x-ray computed tomography," *Medical
Physics* Vol. 8, No. 4, pp. 488-495 (July/August 1981). For the
fluoroscopy surveys, the values cited in this column correspond to the
air kerma *rate* (free in air).

3\. For mammography surveys, the tissue dose is the mean glandular dose
inferred from measured values of free-in-air skin-entrance air kerma
applied to simulations of radiation-transport through a mathematical
phantom modeled with a uniform distribution of 50% glandular tissue (by
weight) and 50% adipose tissue (by weight). For the 1990 Head CT survey,
values cited in this column correspond to the mean MSAD (and range of
the distribution including 50% of sampled values) inferred from
measurements made with a pencil ionization chamber in the ***central,
interior hole*** of CDRH head phantoms (*cf*. ref. 7).

4\. The 1985 mammography surveys used a physical phantom to represent
the x-ray attenuation of a breast compressed to a uniform thickness of
***4.7 cm***. See Burton J. Conway, *Nationwide Evaluation of X-Ray
Trends (NEXT) Tabulation and Graphical Summary of Surveys 1984 through
1987,* CRCPD Publication 89-3, Conference of Radiation Control Program
Directors, Inc., Frankfort, Kentucky, August 1989.

5\. The 1985 survey was an independent project organized by CDRH and the
National Cancer Institute, and it was conducted by 26 States, not part
of the NEXT program. That survey did not use a randomly selected sample,
but the sample size of 250 facilities was approximately the same as that
used in the 1990 NEXT CT survey (*cf.* ref. 7). The 1985 values of "air
kerma" are actually values of the MSAD, but they are inferred from
measurements made with a pencil ionization chamber in the ***top-surface
hole**,* that is, near the skin surface, of a CDRH head phantom. The
MSAD value cited has been adapted from John L. McCrohan, Jack F.
Patterson, Robert M. Gagne, and Howard A. Goldstein, "Average Radiation
Doses in a Standard Head Examination for 250 CT Systems," *Radiology*
Vol. 163, No. 1, pp. 263-268 (April 1987).

6\. The 1988, 1992, and 1997 mammography surveys used a physical phantom
to represent the x-ray attenuation of a breast compressed to a uniform
thickness of ***4.2 cm***. See Burton J. Conway, *Nationwide Evaluation
of X-Ray Trends (NEXT) Tabulation and Graphical Summary of 1988
Mammography Survey,* CRCPD Publication 90-7, Conference of Radiation
Control Program Directors, Inc., Frankfort, Kentucky, October 1990;
Burton J. Conway, *Nationwide Evaluation of X-Ray Trends (NEXT)
Tabulation and Graphical Summary of 1992 Mammography Survey,* CRCPD
Publication 95-2, Conference of Radiation Control Program Directors,
Inc., Frankfort, Kentucky, March 1995; Orhan H. Suleiman, David C.
Spelic, John L. McCrohan, Gordon R. Symonds, and Florence Houn,
"Mammography in the 1990s: The United States and Canada," *Radiology*
Vol. 210, No. 2, pp. 345-351 (February 1999).

7\. For the 1990-CT head exam, the "tissue dose" is actually the MSAD
inferred from values measured with a pencil ionization chamber in the
***central, interior hole*** of the phantom. The MSAD value cited has
been adapted from the CT results of Burton J. Conway, *Nationwide
Evaluation of X-Ray Trends (NEXT) Summary of 1990 Computerized
Tomography Survey and 1991 Fluoroscopy Survey*, Conference of Radiation
Control Program Directors, CRCPD Publication 94-2, Frankfort, Kentucky,
January 1994. Also see Burton J. Conway *et al*., "Average Radiation
Dose in Standard CT Examinations of the Head: Results of the 1990 NEXT
Survey," *Radiology* Vol. 184, No. 1, pp. 135-140 (July 1992).

8\. The 1996 fluoroscopy air-kerma rate cited comes from a preliminary
data analysis presented by David Spelic and Jack Ferruolo, "1996 NEXT
Fluoroscopy Survey," *Annual Meeting of the Conference of Radiation
Control Program Directors*, Tacoma, Washington, May 1997.

9\. The 1997 mammography data were obtained from inspections conducted
under the *Mammography Quality Standards Act*.

10\. Data from the United Kingdom have indicated that computed
tomography is "the most significant source of exposure to diagnostic
X-rays for the UK population...." See P.C. Shrimpton, D. Hart, B.F.
Wall, and K. Faulkner, *Survey of CT Practice in the UK Part 1: Aspects
of Examination Frequency and Quality Assurance*, NRPB-R248, National
Radiological Protection Board, Chilton, Didcot, Oxon, p. 1, December
1991; Paul C. Shrimpton, "Computed Tomography---A Spiralling Challenge
in Radiological Protection," in *Current Topics in Radiography Number
2*, edited by Audrey Paterson and Richard Price, (W.B. Saunders Company
Ltd., London, 1996), pp. 26-34; P.C. Shrimpton and S. Edyvean, "CT
scanner dosimetry. Report on a round table meeting establishing a survey
of standard dosimetric measurements on CT scanners, held at St George's
Hospital, London, 18 April 1997," *The British Journal of Radiology*,
Vol. 171, pp. 1-3, January 1998.

11\. Willi A. Kalender, "Principles and Performance of Spiral CT," in
*Medical CT and Ultrasound: Current Technology and Applications,
Proceedings of the 1995 Summer School on CT and US Technology and
Applications of the American Association of Physicists in Medicine,
Connecticut College, New London, June 25-29, 1995,* edited by Lee W.
Goldman and J. Brian Fowlkes, (published for the American Association of
Physicists in Medicine by Advanced Medical Publishing, Madison,
Wisconsin, 1995), pp. 379-410.

12\. Christoph G. Diederichs *et al*., "Blurring of Vessels in Spiral CT
Angiography: Effects of Collimation Width, Pitch, Viewing Plane, and
Windowing in Maximum Intensity Projection," *Journal of Computer
Assisted Tomography* Vol. 20, No. 6, pp. 965-974 (1996).

13\. Steven L. Novick and Elliot K. Fishman, "Three-Dimensional CT
Angiography of Pancreatic Carcinoma: Role in Staging Extent of Disease,"
*American Journal of Roentgenology* Vol. 170, No. 1, pp. 139-143
(January 1998).

14\. Thomas M. Link *et al*., "Artificial Spine Fractures: Detection
with Helical and Conventional CT," *Radiology* Vol. 198, No. 2, pp.
515-519 (February 1996).

15\. Y. Robert *et al*., "Helical CT of the Larynx: A Comparative Study
With Conventional CT Scan," *Clinical Radiology* Vol. 51, pp. 882-885
(1996).

16\. Vincent M. Bonaldi *et al.*, "Helical CT of the Pancreas: A
Comparison of Cine Display and Film-Based Viewing," *American Journal of
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17\. Glenn M. Preminger *et al.*, "Urolithiasis: Detection and
Management with Unenhanced Spiral CT---A Urologic Perspective,"
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18\. Compare *Whole Body CT Scanner Model TCT-400S*, Toshiba Product
Data No. 2N20108, 86-8-AC/A, Toshiba Corporation, Tokyo, Japan, 1986 to
*Half-Second Helical CT Scanner Aquilion*, Toshiba Product Data No.
MPDCT0122EAB, 99-01 TME/SB, Toshiba Corporation, Tokyo, Japan, 1998. For
general information on CT x-ray tube technology, see Stanley H. Fox, "CT
Tube Technology," in *Medical CT and Ultrasound: Current Technology and
Applications, op. cit.,* pp. 349-357.

19\. Ryoichi Kato *et al*., "Radiation Dosimetry at CT Fluoroscopy:
Physician's Hand Dose and Development of Needle Holders," *Radiology*
Vol. 201, No. 2, pp. 576-578 (November 1996).

20\. Richard D. Nawfel *et al*., "Patient and Personnel Exposure During
Fluoroscopic CT-Guided Procedures," presentation at *the 1998 Annual
Meeting of the American Association of Physicists in Medicin*e.

21\. Robert M. Gagne, "An Overview of Computed Tomography Fluoroscopy
(CTF)," presentation to the *Technical Electronic Product Radiation
Safety Standards Committee*, Silver Spring, Maryland, September 16,
1999.

22\. Cynthia H. McCollough, "Principles and Performance of Electron Beam
CT," in *Medical CT and Ultrasound: Current Technologies and
Applications*, *op. cit.,* pp. 411-436.

23\. Kenyon K. Kopecky, Kenneth A. Buckwalter, and Roman Sokiranski,
"Multi-slice CT spirals past single-slice CT in diagnostic efficacy,"
*Diagnostic Imaging* Vol. 21, No. 4, pp. 36-42 (April 1999).

24\. Cynthia H. McCollough and Frank E. Zink, "Performance evaluation of
a multi-slice CT system," *Medical Physics* Vol. 26, No. 1, pp.
2223-2230 (November 1999).

25\. Ronald G. Evens and Fred A. Mettler, "National CT Use and Radiation
Exposure: United States 1983," presented at the *Conference on CT
Scanning Radiation Dose, Washington, DC, June 1984*, sponsored by the
American College of Radiology and the National Center for Devices and
Radiological Health, published in the *American Journal of
Roentgenology* Vol. 144, pp. 1077-1081 (May 1985).

26\. Brian R. Herts *et al*., "Comparison of Examination Times between
CT Scanners: Are the Newer Scanners Faster?" *American Journal of
Roentgenology* Vol. 170, No. 1, pp. 13-18 (January 1998).

27\. Barry Daly and Phillip A. Templeton, "Real-time CT Fluoroscopy:
Evolution of an Interventional Tool," *Radiology* Vol. 211, No. 2, pp.
309-315 (May 1999).

28\. Stuart G. Silverman *et al*., "CT Fluoroscopy-guided Abdominal
Interventions: Techniques, Results, and Radiation Exposure," *Radiology*
Vol. 212, No. 3, pp. 673-681 (September 1999).

29\. Barry Daly and Thorsten L. Krebs, guest editors, *Seminars in
Interventional Radiology* Vol. 16, No. 3: *Interventional/CT
Fluoroscopy,* 1999.

30\. Willi A. Kalender, *op. cit*., p. 382.

31\. Stanley H. Stern, David C. Spelic, Richard V. Kaczmarek, *NEXT 2000
Protocol for Computed Tomography (CT)*, Food and Drug Administration,
Center for Devices and Radiological Health, Rockville, Maryland, March
31, 2000 (unpublished).

32\. D.G. Jones and P.C. Shrimpton, *Survey of CT Practice in the UK.
Part 3: Normalised Organ Doses Calculated Using Monte Carlo Techniques*,
NRPB-R250, National Radiological Protection Board, Chilton, UK, 1991;
D.G. Jones and P.C. Shrimpton, *Normalised Organ Doses for X-ray
Computed Tomography Calculated Using Monte Carlo Techniques*,
NRPB-SR250, National Radiological Protection Board, Chilton, UK, 1993;
John C. Le Heron, *CTDOSE*, computer program, National Radiation
Laboratory, Christchurch, New Zealand, 1993.

33\. Stanley H. Stern and Orhan Suleiman, "Ionizing Radiation Risk
Assessment in Diagnostic and Interventional Radiology: Tissue Dose
Handbooks," poster paper presented at the *2000 FDA Science Forum, FDA
and the Science of Safety: New Perspectives, Washington, DC, February
14-15, 2000* (unpublished) and references cited therein.

34\. S.H. Stern and M. Rosenstein, "Reduction of Monte Carlo Data for
Convenient Evaluation of Tissue Doses to Reference Patients in Computed
Tomography (CT)," poster paper TAM-E5, *Thirty-Eighth Annual Meeting of
the Health Physics Society, Atlanta, Georgia, July 11-15, 1993*,
abstracted in *Health Physics* Vol. 64, No. 6 supplement, p. S35, June
1993.

35\. S. Stern and J. Yoon, "Development of a Handbook of Patient Tissue
Doses for X-Ray Computed Tomographic Examinations," poster paper to be
presented at the *World Congress on Medical Physics and Biomedical
Engineering, Chicago, July 2000* (unpublished).