**Notes to the Slides**

***Development of Amendments to the U.S. Radiation-Safety Standard
for***

***Diagnostic X-Ray Computed Tomography (CT) Equipment***

S.H. Stern\*, R.M. Gagne, H.H. Knox, M.P. Divine, R.J. Doyle, C.A.
Finder,

R.G. Kaczmarek, R.V. Kaczmarek, H.L. Rourk, T.B. Shope, Jr., D.C.
Spelic,

O.H. Suleiman and S.A. Tucker

*U.S. Food and Drug Administration*

\*Presenting May 22, 2002, to the

*Technical Electronic Product Radiation Safety Standards Advisory
Committee*

**[Slide 1: Title]{.underline}**

This presentation grows out of the collaborative efforts of an FDA group
of science, regulation, and economics staff. We're working to facilitate
radiation dose reduction through consideration of amendments to the
existing CT performance standard. Our motivation is the proposition that
the current Federal regulations covering CT---in place since the
mid-1980s---have not kept pace with technological developments and with
the need to assure the lowest dose for the best image quality
practically achievable.

The work group's current thinking and my own personal ideas and analysis
presented here do not necessarily reflect any official position of the
FDA or its components. Many items in the slides are annotated with
superscripted numbers that cite references and notes listed at the end
of the presentation. Reference to any products, manufacturers, models of
CT systems, or external web sites does not imply FDA endorsement.

**[Slide 2: Advances and Concerns]{.underline}**

The theme of the introductory part of this presentation is the interplay
of technology and clinical practice in CT, how the rapid technological
and clinical advances of the past few years have increased CT use and
have led to public-health concerns. This theme is a basis for background
discussion and for updates on the activities CDRH has undertaken to
address these concerns since I spoke about them last year.

**[Slide 3: CT Applications]{.underline}**

Computed tomography is a vitally important, beneficial modality whose
radiation doses are relatively higher than those of other x-ray exams.
The scope of CT applications is broad, and CT is used in many different
ways---from diagnosis, to cancer staging, to treatment planning, and
more recently for real-time visualization during interventional
operations.

**[Slide 4: Predominant CT Technology]{.underline}**

This slide summarizes those physical, geometrical, and mechanical
aspects of currently predominant CT technology that bear on individual
radiation-dose delivery. Electron-beam CT is not covered here because
e-beam CT scanners make up perhaps only 1-2% of approximately 10,000 CT
units in the U.S.

The essential feature of x-ray CT irradiation is a thin, fan-shaped
x-ray beam that rotates around a patient. In most systems, x-ray
detectors are located beyond the patient diametrically opposite the
x-ray source, and the beam and detectors rotate together while the
detectors register x-rays transmitted through the patient. (In the
figure, the x-ray beam is indicated by the red shading, and the
detectors are indicated by green.) A single 360^o^ rotation typically
takes from one-half to one second, a relatively brief period compared to
rotation times of ten years ago. An important point is that while some
of the most recent models of scanners now offer different options that
enable a system to automatically adjust radiation output higher or lower
to account for a patient's circumference, in most systems, the
radiological techniques---such as the peak x-ray tube voltage (kVp), the
x-ray tube current (mA), the rotation time---need to be set manually by
the CT technologist. In an ideal workplace, these settings are based on
a technique chart which a facility would develop covering different
examination protocols and various sizes of patients.

What's referred to as a single "slice" corresponds to a thickness
usually between 1 and 10 mm along the length of a patient, and it yields
*one* cross-sectional image per single rotation. Single-slice scanners
are distinguished from CT systems that are capable of doing
"multi-slice" scanning. Spiral *multi-slice* scanners were introduced
only four years ago, and when they operate in multi-slice mode, they
produce 2 to 4 cross-sectional images *simultaneously* per rotation.
These images correspond to adjacent slices along the length of the
patient. Newer spiral scanner models can provide 8 and even 16 slices
simultaneously, and in the next few years they will probably replace
most of the axial-only models.

In axial CT, the table moves increment-by-increment following each
single rotation. Spiral scanning (also called "helical" scanning) refers
to table movement at a *constant rate* during *continuous* rotations.
(It's called "spiral" or "helical" because the combination of smooth
table movement and x-ray source rotation leads to the x-ray field
tracing out a helical path around the patient.) The direction along the
length of the patient is referred to as the "z-axis," the axis about
which the beam and detectors rotate. Typically in a single phase of a CT
examination the table movement spans a range covering on the order of 10
to 50 slices along the length of a patient.

The features of fast, multi-slice spiral CT have enabled scanning of
large volumes of patient anatomy, three-dimensional rendering of images,
angiography, single-breath-hold imaging and visualization of small lung
nodules. The bottom line is that these advances in CT technology have
been rapidly adopted into clinical practice and have led to an explosive
growth in the number of applications, to a capability of examining
patients quickly, and to a high rate of use.

**[Slide 5: Public Health Concerns Responses]{.underline}**

The items on the left-hand side of this slide underscore some
public-health concerns ensuing from the growth in use of CT. The
right-hand side lists the preliminary responses of CDRH in addressing
these concerns. First, we are faced with the problem of determining the
scope of radiological exposure from CT---how many CT examinations are
going on annually, and just how large are the doses from what particular
exams? CDRH provided the principal technical direction for a survey
conducted through the Nationwide Evaluation of X-Ray Trends program
administered by the Conference of Radiation Control Program Directors.
Between April 2000 and July 2001 State inspectors surveyed examination
doses and workloads in 263 CT facilities randomly selected in 39 States
to provide the first national understanding of the magnitude of
collective dose from CT since the first CT survey in 1990. A related
project is the ongoing development of a handbook of patient doses
associated with approximately 50 of the most common CT examinations.
Such a handbook would foster risk communication between medical staff
and patients, and it would enable medical physicists and radiologists to
evaluate patient tissue doses and effective dose for their facility's CT
systems and adjust their protocols as needed to reduce doses.

In February 2001 the *American Journal of Roentgenology* published a
series of papers describing the potential risk associated with
inappropriate equipment settings and scanning techniques in CT
examinations of children. A great deal of publicity resulted from these
studies, and our concerns were voiced at the last meeting of TEPRSSC.
Following the advice of TEPRSSC, last November CDRH issued a Public
Health Notification to radiologists, radiation health professionals,
risk managers, and hospital administrators alerting facilities to the
problem and providing practical advice on how to reduce risk associated
with CT dose in pediatric and small adult patients.

Since that time there has been burgeoning popularization of a group of
applications commonly referred to as CT "screening" of self-referred
individuals who are asymptomatic of any particular disease. Among these
applications are included "whole-body" examinations, examinations of the
lungs for cancer, and "calcium-scoring" of the heart as a purported
indicator of potential heart disease. Right now CT screening makes up
only a tiny fraction of the number of CT procedures performed annually
in the U.S. Our main concerns are the risks associated with false
positive results and with radiation dose. False positive results could
needlessly lead to follow-up tests or procedures that might be
invasive---associated with surgical risks of anesthesia, bleeding,
infection, scarring---or entail additional radiological exams. Radiation
doses in diagnostic CT are among the highest of those of all x-ray
modalities, and screening CT doses are significantly large even when
\"low-dose\" protocols might be applied.

There are no scientific studies demonstrating that whole-body CT
screening of asymptomatic people is efficacious. Were it a useful
screening test, it would be able to detect particular diseases early
enough to be managed, treated, or cured and advantageously spare a
person at least some of the detriment associated with serious illness or
premature death. At this time any such presumed benefit of whole-body CT
screening is in fact *uncertain*, and the benefit may not be great
enough to offset the potential harms such screening could cause.

FDA has recently posted a web page about CT screening. The page provides
information about our concerns, contains brief explanations of computed
tomography, radiation risks, radiation quantities and units, the
regulatory status of CT, and includes links to related resources. It is
hoped that an objective presentation from a government institution whose
fundamental mission is to protect public health will clarify the natures
of the risks and presumed benefits in a way that persuades people to
carefully consider these aspects of CT screening before deciding whether
or not to have such exams.

Finally, we are aware of the small but growing use of what's called "CT
fluoroscopy" or "dynamic CT" to visually guide interventional procedures
involving biopsy, drainage, and device placement. "CT fluoroscopy"
refers to the capability of a CT system to update images in nearly real
time as the x-ray field and detectors rotate multiple times around a
patient at a fixed z position, that is, without table movement. Recent
reports cite mean values of entrance skin dose of approximately 100 to
400 mGy, below the threshold for skin injury. Several years ago a small
CDRH group drafted guidance for reviewers and manufacturers of CT
systems capable of CT fluoroscopy, but the move to formal adoption of
final guidance has been on hold in view of the relatively small
probability for skin injury in the most common procedures and also since
preliminary findings of the 2000 CT survey indicated that only 5% of the
most frequently used CT units in facilities have the capability of doing
CT fluoroscopy.

**[Slide 6: Current Federal CT Equipment Standards]{.underline}**

The baseline of radiation protection with respect to CT *equipment* is
prescribed by the Federal government through performance standards
established under the Radiation Control for Health and Safety Act. The
regulations in place now date back approximately 20 years. These rules
apply to manufacturers of CT equipment, not to the facilities that *use*
the equipment. The basic mandate is *documentary*: Manufacturers must
provide users with specified *documentation* of dose values for CT
systems under typical operating conditions. Because this mandate
predates special or new modalities such as electron-beam, multi-slice,
spiral, fluoroscopic, or cone-beam CT, the doses manufacturers report
don't necessarily pertain to those modes of operation. There is no
regulatory ceiling on patient dose, and there are few major equipment
requirements particular to CT per se.

**[Slide 7: FDA CTDI]{.underline}**

The current FDA standard for CT dose documentation is represented by the
computed tomography dose index, abbreviated "CTDI." CTDI incorporates a
number of the physical aspects associated with the geometry and
irradiation conditions of computed tomography. These aspects include a
rotating fan-shaped beam, collimation of the primary radiation to a thin
slice along the z-axis (the axis of rotation), broad scattering of the
primary radiation by the material it passes through, and
scattered-radiation contributions to the dose that are *cumulative* with
multiple rotations.

CTDI is an *index* of dose, a *descriptor* or *indicator* of the
magnitude of dose associated with the radiation output of a specific CT
model. It is *not* a measure of patient dose on a *person-by-person*
basis. CTDI is a representation of dose which is *standardized* for
specific reference materials and reference-procedure conditions. It's
measured in a cylindrical phantom made of nearly solid acrylic, with
diameter either 16 cm to correspond to the adult head or 32 cm to the
adult body. The figure in the center of the slide depicts a cylindrical
phantom, and to the left is a face view of the phantom within the fan
beam indicated by the red shading. The x-ray source is at the apex on
the bottom, and the x-ray detectors are indicated by the green shading
at the top. In a single scan, the fan beam and detectors rotate as an
ensemble once around the central axis represented in the figure on the
left by the origin of the x-y coordinate system. This central axis of
rotation is the z axis.

Even though the CT radiation intended for image formation is collimated
within a relatively thin section along the z axis, much radiation
actually scatters throughout the phantom (or patient). In the center
figure, the red shading corresponds to the primary radiation passing
through the phantom to the detectors, and the dark blue-green shading
represents the scattered radiation. So the dose is actually
*distributed*, not localized exclusively to the narrow region
collimated. The figure on the right is called the dose "profile," and it
represents the distribution of dose along the z axis for a single slice.
The abscissa corresponds to position along the z-axis, where 0 mm is at
the center, and the ordinate is the dose in units of *rad*. For
single-slice scanners, the z-axis collimation of the system defines the
slice thickness, designated "T," and in this example T is 13 mm. One
sees that although most of the primary radiation is contained within the
13-mm-wide central zone of the phantom, the scattered radiation extends
*far beyond* the central zone, to more than 100 mm on either side.
Furthermore, when there are multiple scans extending over a range along
the patient length, as there are in most CT exams, at any one location
along the z axis, the scattered radiation from these other scans
cumulatively *adds* to the dose.

FDA therefore defined the dose index CTDI to be proportional to an
integral which includes the dose contributions from scattered as well as
primary radiation over a range of the dose profile extending from
negative seven to positive seven times the slice thickness T. In the
example depicted, for a slice thickness of 13 mm, the range of
integration is from -91 mm to +91 mm, covering practically all of the
dose contributions, and the CTDI here is 0.82 rad. An advantage of
defining a dose index this way is that mathematically CTDI is identical
to the *average dose* in the central plane of 14 contiguous axial scans.
In other words, the integral appropriately accounts for the dose
contributions of adjacent, nearby slices, each with its own single-slice
profile. So one can think of CTDI as the dose associated with a
reference procedure: It is the average central-plane dose for a 14-slice
exam, a reasonable representation of how exams were done 20 years ago.

From today's perspective, there are several problems with the regulatory
definition of CTDI. CTDI is simply not defined for spiral CT scanning,
which is how most body exams are done currently. (For spiral scanning
the irradiation geometry and dose profile are different than these
figures depict.) Also, spiral scanning or no, the regulatory definition
of CTDI does not account for CT procedures where the slices are *not
adjacent*, that is, where slices may be separated by gaps or where they
may overlap.

Over the years medical physicists have introduced a number of
non-regulatory variants of CTDI that have been adopted into practice and
to some extent by manufacturers. For example, it is much easier to
measure CTDI with a fixed-length, 100-mm long ionization chamber rather
than integrate a dose profile determined through thermoluminescent
dosimetry. "CTDI~100~" refers to the practice of using a 100-mm long
ionization chamber either in the center hole of a phantom or in any of
its peripheral holes to measure a value of CTDI integrated from -50 mm
to +50 mm irrespective of the slice thickness T. Although the ionization
chamber is contained entirely within the acrylic phantom, CTDI~100~
usually refers to dose to *air*, not dose to acrylic as in the FDA
definition. A variant of CTDI~100~ is what is called the "weighted"
CTDI, abbreviated "CTDI~w~," and it is based on a combination of values
of CTDI~100~ measured in the center hole and in the peripheral holes.
This combination approximates the CTDI~100~ average over the entire
central plane of the phantom. Another variant, the "volume" CTDI is
being introduced in an amendment to the current international
manufacturers' consensus standard covering the radiation safety of CT
equipment. The bottom line here can be broken into two parts: First,
variant quantities of CTDI that are either more easily determined, or of
broader generality, or of more utility, have by and large replaced the
FDA definition of CTDI for most practical purposes. Second, as a result
of this proliferation of non-standardized terms, there is confusion
amongst CT system users about precise definitions of CTDI values,
especially for values displayed by some CT systems.

**[Slide 8: Amendments Being Considered, Technical Features to Reduce
Dose]{.underline}**

Possible amendments to the current radiation-safety performance standard
would *require* particular technical features for CT equipment. Although
requiring such features through a mandatory standard applicable to all
new CT systems conceivably guarantees the largest and most systematic
dose reduction on a population-wide basis, there are a number of
associated issues that demand careful thought before we undertake such
change. We seek your comments, ideas, and questions on any aspect of
what is being suggested. The initial focus of the work group effort is
on three possible features--- display and recording of standardized dose
indices, automatic control of x-ray exposure according to individual
patient thickness, and x-ray field-size limitation for multi-slice
systems.

**[Slide 9: Dose-Index Standardization, Display,
Recording]{.underline}**

This amendment would require each new CT system to provide users with
options to display and record one or more dose indices for every
patient's examination. The dose indices and related terminology would be
standardized through formal definition in the regulations.

This amendment would enable an aspect of facility quality assurance that
today is feasible only with extra effort or through features available
on just some newer scanner models. The basis of this quality assurance
is the use of what are called "reference dose values" as norms to which
individual examination doses could be compared. If reference values are
exceeded, facilities could follow up anomalies by looking at possible
problems to see if exposures could be reduced without compromising image
quality. A reference dose value corresponds to the 75^th^ percentile of
the distribution of measured dose values for particular radiological
procedures. Reference values may be generated based on a facility's own
records of dose distributions for various CT exams or based on regional
or national dose distributions.

The concept of reference dose values, also called "reference levels,"
was introduced in the United Kingdom about 10 years ago and is being
adopted throughout Western Europe. It is being introduced into the U.S.
by the American College of Radiology with the aid of a task group of the
American Association of Physicists in Medicine. For example, the ACR
requires facility audits of dose values for comparison to reference
levels in its new CT accreditation program. There is no question about
the technical feasibility of simpler versions of such displays because
they already are available on some of the newer CT models, albeit with
ambiguous definitions. We assume that the systematic use of dose-index
display or recording in a facility audit program could reduce patient CT
dose on average on the order of 15%. This projection is based on the
range of dose reduction observed between 1985 and 1995 in the United
Kingdom for modalities *other* than CT, in a period before particular
indices of patient CT dose were introduced.

**[Slide 10: Promising Indices of Patient Dose]{.underline}**

There are several prospective indices of patient dose that could be
displayed and recorded for the purpose of dose audits. For the two
indices described in this slide, equivalent quantities are recommended
in quality criteria guidelines published by the European Commission,
although not quite with the same nomenclature as used here. In the first
amendment to the second edition of the International Electrotechnical
Commission safety standard for CT equipment, the "volume" computed
tomography dose index is introduced. It is based essentially on the
weighted CTDI, which is a weighted sum of CTDI~100~ measured in the
central and peripheral holes of an acrylic phantom. For axial scanning
the denominator in the expression for volume CTDI is Δz/nT, the ratio of
the table increment per rotation to the total thickness of tomographic
sections imaged. In axial scanning the volume CTDI is essentially what's
known as the "multiple scan average dose," abbreviated "MSAD." "Pitch"
is the analogous denominator for spiral scanning. The important point
here is that these denominators account for modifications to the
weighted dose index arising from possible gaps between multiple scans or
their possible overlap for examination protocols that may differ
according to the particular exam being performed. This accounting makes
the volume CTDI more sensitive to differing examination protocols than
either CTDI~w~ alone, or CTDI~100~ alone, or the FDA regulatory CTDI.

Another possible index for dose-display and recording is called the
"dose-length product," and it may hold more promise than the volume
CTDI. Dose-length product is simply the product of the volume CTDI and
the length of the irradiated volume. Here is its chief advantage:
Because the length of the irradiated volume depends on the region of the
body being studied, different examinations will be associated more
uniquely with characteristic values of dose-length product than with
values of volume CTDI. This result is evident from the table on the
left, which compares values of volume CTDI to those of dose-length
product. The dose-length product values are relatively sensitive to
differences in exams, whereas for the kinds of exams listed, volume CTDI
is practically constant between 30 and 35 mGy. The implication is that
facility audits of dose-length product could be exquisitely sensitive to
anomalously large doses for each different kind of examination; each
kind of examination could be associated with its own unique distribution
of dose-length product values. Another point in favor of the use of
dose-length product is that it is approximately proportional to the
total energy imparted and is therefore a better indicator of radiation
risk than is the volume CTDI. Using anatomy-specific coefficients
derived from computer simulations, one can estimate effective dose from
the dose-length product, and effective dose is the closest indicator we
have for overall radiation detriment. It is my understanding that one
manufacturer already displays values for effective dose on newer CT
models in Europe.

**[Slide 11: Automatic Exposure Control]{.underline}**

Of the three technical areas that we are considering, probably the
largest dose reduction---at least for thinner patients---would be
brought about by requiring every newly manufactured CT system to provide
the capability of automatically adjusting the amounts of x-ray emissions
to those needed to image particular patient anatomy. In other words, as
the x-ray beam probes a thinner portion of the anatomy, which would not
require as much radiation as a thicker portion would in order to reach
the detectors, the CT system would *automatically reduce* the average
tube current, or voltage, or some combination of radiological variables
to *spare* that thinner part unnecessary dose. And, conversely, when the
beam encounters thicker anatomy, the CT system would automatically
*increase* the tube output to levels needed for adequate visualization.
An automatic exposure control system offers a technical answer to
facilities where for practical or clinical reasons it is not the
practice to change manual techniques on a patient-by-patient basis let
alone readjust techniques within a single patient exam. With an AEC
system in place, the presumption is that pediatric and thinner adult
patients would receive lower doses than thicker patients.

A number of different approaches for modulating x-ray tube output are
available on newer scanner models, and these approaches span a range of
technical complexity. For example, at one end of the range are systems
that offer *recommendations* of specified technique settings for tube
current-time product and tube potential that the user may choose to
apply. Such recommendations are not automatic adjustments per se, but
they are based on anterior-posterior and lateral scan projection
radiograph data. Scan projection radiographs are the scout views
obtained prior to regular CT scanning. At the other end of the range of
approaches to AEC is truly automated, continuously updated tube-current
modulation in three dimensions based on measurements of x-ray
attenuation at the corresponding angles of the previous rotation. In
between these two extremes are several other algorithms offering, for
example, automated tube-current modulation axially for various image
qualities that may be selected by a user.

The figures in the slide depict how emissions would vary according to
patient sizes in three dimensions. On the left is a cross section of the
torso in the x-y plane, and the thickness or thinness of each red arrow
corresponds to the relatively greater or lesser amount of radiation
needed for reconstructing an image as the x-ray tube rotates around the
z axis. Not only is there tube-current modulation for the x and y
dimensions, there is also modulation corresponding to changes in average
anatomical thickness along the z axis---as the table moves. The graph on
the right shows how the tube current is reduced or increased by this
*additional* current-normalization factor that accounts for the average
anatomical thickness which the fan-beam slice encounters along the
length of the patient. For example, the x-ray output would be relatively
small when the patient's neck is passing through the fan beam, but
increases rapidly when the shoulders are in the beam and decreases as
the beam probes the lungs. Calculations and measurements suggest that
use of a sophisticated automatic exposure control system could reduce
patient dose by approximately 30% compared to systems where the
techniques are set manually.

**[Slide 12: Concern---"Over-beaming" in Multi-slice CT]{.underline}**

We are concerned that a number of different multi-slice CT models
produce images with a technologically inefficient application of
radiation. This inefficient technology has been dubbed "over-beaming."
The two figures represent a comparison of the spatial distributions of
radiation incident along the length of a patient. The figure on the left
depicts the distribution for a single-slice CT scanner, whereas the one
on the right corresponds to that of a multi-slice scanner. The CT system
represented on the left produces one image associated with a single
slice, while the model on the right can produce four images
simultaneously, each associated with a thinner slice. In each figure the
gradient in area and intensity of shading from dark red to light pink is
a representation of the falloff in radiation exposure from the central
umbra of the collimated x-ray field to the peripheral penumbra. On the
left, a single detector (indicated by the green rectangle) captures
essentially the entire radiation distribution. On the right, however,
the system of four detectors captures only the radiation of the umbra
region.

The *total* width of the tomographic section imaged---5 mm in this
example---for the slice associated with the one image produced on the
left is *equal* to the sum of the widths of the *four* 1.25-mm wide
slices respectively associated with the four images produced on the
right. In other words, in either figure the amount of visual information
that can be used for image reconstruction is approximately the same,
and, in fact, in the case of the multi-slice CT system, a user could
*elect* to trade off the resolution offered by four adjacent 1.25-mm
wide slices for a single 5-mm wide slice with relatively less image
noise than in each of the thinner-slice images.

Here's the important point in this comparison: Although the amount of
radiation applied to *construct one image* with the single-slice scanner
or to construct a *set of images* with the multi-slice system is the
*same* for each configuration, for the multi-slice CT system the
radiation distribution is much wider than that of the single-slice
system. Why? Multi-slice CT imaging requires that radiation incident on
the patient be consistently distributed across *each* of the separate
areas subtended by the detectors. Such consistency can be achieved by
opening up the z-collimation of the source radiation so that only the
most spatially uniform region of the x-ray field---the umbra---is
subtended by the detectors, and the spatially varying penumbral regions
are excluded. Furthermore, since the x-ray focal spot tends to wander
around spatially, multi-slice models *broaden* the umbra by opening the
collimation *even more* to compensate for x-ray source excursions. In
the example depicted by these figures, the width of the z-collimation
for the multi-slice system is 15 mm versus 5 mm for the single-slice
system. The problem of consistent spatial irradiation is not encountered
in single-slice systems because the single detector is longer than the
extent of the incident radiation, and it simply integrates the whole
distribution incident. However, multi-slice systems are not efficient
users of radiation in this sense: All of the radiation that falls beyond
the spatial extent of the detectors is *not* used by the detectors for
image construction, but it is nevertheless incident on the patient, and
it contributes to the dose.

**[Slide 13: X-Ray-Field Size Limitation]{.underline}**

To mitigate the inefficient use of radiation in multi-slice computed
tomography, we suggest consideration of an x-ray-field-size limitation.
Such an amendment would require that all new CT systems be capable of
automatically limiting field sizes to those no larger than needed to
construct multi-slice images.

Several technical approaches to enable such limitation have been
patented, and one in fact has been implemented. The approach implemented
uses some of the x-ray detectors lying beyond those capturing the
clinically useful signal to track the wandering of the penumbral regions
of the x-ray field and feed back instructions to motor-driven collimator
cams to readjust their positions. Tracking and updated instructions are
done in real time to maintain the narrowest needed umbra incident on the
detectors. This system is represented by the figure on the left. The
x-ray field borders demarcated by dashed lines are set by the collimator
cams---also indicated with dashes---for an initial position of the x-ray
source so that the umbra is subtended by the clinical-signal detectors.
As the x-ray source wanders to the right, other detectors (not depicted
here) pick-up the movement of the penumbra and instruct the collimator
cams to re-adjust their positions to those indicated by the solid lines.
The result is that the umbra remains subtended by the clinical-signal
detectors. Had the collimation position remained unchanged, there would
have been an inconsistent spatial distribution of the x-ray radiation
across the clinical-signal detectors.

The chart on the right represents two multi-slice dose profiles measured
in a head phantom on the same CT system. For the same 5-mm wide
imaging-sensitivity profile, the dose profile in black is obtained when
there is no tracking and collimation-update system, whereas the dose
profile in fuchsia is obtained when the tracking-update system is
activated. It is evident that the non-tracking dose profile is
approximately 50% wider than the tracking profile. All of the radiation
represented by the difference between the two profiles would correspond
to radiation which is absorbed by a patient but *not* used to construct
images. Data suggest that the kind of x-ray-field size limitation
enabled by tracking and collimation adjustment could reduce dose in
multi-slice CT systems on the order of 30%.

**[Slide 14: Projected Benefits Introduction]{.underline}**

I will present quantitative projections of benefits that could result
from the relative amounts of dose reduction associated with the possible
implementation of amendments to the Federal radiation-safety standard in
each of the technical areas just described. The principal benefit would
be a population-wide *reduction* in morbidity and mortality associated
with avoidance of cancers produced by CT radiation.

**[Slide 15: Annual CT Dose, U.S.]{.underline}**

Projections are based on preliminary estimates of the current annual CT
dose in the United States derived from the 2000-2001 *NEXT* survey. The
survey results indicate that the total number of CT exams annually is
approximately 58 million, where 79% of all exams are comprised of
scanning in 6 anatomical regions or combinations of regions---brain,
abdomen-pelvis, chest, abdomen, chest-abdomen-pelvis, and pelvis alone.
Approximately 29% of all CT units in the U.S. can do multi-slice spiral
scanning, a remarkably large percentage since this technology was
introduced to the market in 1998. The effective dose average for the 6
exam regions is approximately 6.2 millisievert, and the product of this
average and the number of exams corresponds to a collective annual dose
of approximately 360,000 person-sievert per year.

**[Slide 16: Projected Benefits---collective dose, cancer mortality,
pecuniary savings]{.underline}**

If *all* CT equipment were to include the technical features just
proposed for consideration as mandatory standards, then, based on the
relative dose reductions and the collective dose attributable to CT, one
can estimate an annual collective dose savings of 193,000
person-sieverts per year---54,000 for dose-index display and recording
in a quality-assurance program, 108,000 for automatic exposure control,
and 31,000 for x-ray-field size limitation. All of these values are
uncertain, and they're based on a number of assumptions detailed in the
slides, references, and notes.

For an annual collective dose savings of 193,000 person-sieverts, on the
order of 8,700 radiation-induced cancer mortalities are projected to be
avoided per year beginning 20 years after each annual collective
exposure. The yellow shading is intended to highlight the uncertainty in
this projection which is based on an extrapolation to the CT-dose region
of a mortality risk estimate derived from larger-dose epidemiological
data. Other methods of extrapolation could yield higher or lower
estimates of the number of radiation-induced cancer deaths, and it is
even possible that the estimated dose savings would not result in any
avoidance of cancer death at all. In the United States in the year 2000,
the annual number of deaths linked to cancer from all causes not
specifically associated with radiation is approximately 550,000 \[Minino
and Smith 2001\].

There would also be a significant benefit in the pecuniary savings
associated with societal willingness to pay to avoid mortality risk.
Economists have estimated that society is willing to pay on the order of
\$5 million per premature mortality that it perceives might be avoided.

**[Slide 17: Amendments? Initial Steps]{.underline}**

Will there be amendments to the CT radiation-safety standard? Here are
the initial steps in this process: We've come up with a framework for
analysis that will lead to what is called a "concept paper" for
amendments, which will be the basis for CDRH decisions on how to
proceed.

**[Slide 18: Framework of Analysis]{.underline}**

This slide represents a framework for analyzing prospective technical
areas with respect to issues that need to be addressed in decisions on
how to proceed. In the block on the right, the region shaded in green
lists the technical areas summarized in this presentation, and the
region shaded in pink lists areas where we have an interest that is
deferred for the time being. The yellow-shaded block on the left lists
some general categories of issues---technical feasibility, impact on
clinical aspects such as efficacy and frequency of utilization,
harmonization with international consensus standards, CDRH resources
required to develop test methods and to incorporate the administration
of new rules in a compliance program. The arrows indicate that in
principle each of these issues can be applied as a basis of assessment
to each technical area under consideration.

We would like to hear your thoughts about *any* of these issues.
Although the equipment features that I've discussed today may all be
technically feasible, there remain a number of particular questions
outstanding. Here are a few examples: First, for the purpose of display
or recording in a quality-assurance program, not only would we have to
select a representative index of patient dose, we would need to specify
whether the dose index could be based on average values determined by
manufacturers for all models of scanners or whether it must be specific
to the particular unit be used in a facility. Should the dose index
displayed or recorded be based on real-time measurements made during
actual patient examinations? How would the index represent values in an
automatic exposure control mode? Parameters based on CTDI may not be
good candidates to represent skin dose, particularly for CT fluoroscopy.
What is a good index for skin dose? What impact might a dose-index
recording capability have on practice and use? Would there be any
inhibitions fostered by the possibility of associating recorded values
with patient medical records?

Second, with respect to automatic exposure control, in addition to
specifying what kind of technological approach is best, perhaps the key
question is how to define the optimal amounts of radiation needed by the
detectors for particular imaging tasks. These amounts would effectively
set the points of detection equilibrium driving the modulation of
emissions from the x-ray source according to patient anatomy thickness.
Should standards be set to optimize detection? Who should set the
equilibrium points and how would that be done? By manufacturers? By
radiologists? By FDA? Philip Judy, a prominent medical physicist, has
posed a related question \[Judy 2001\]: If automatic exposure control
reduces dose to thinner patients, would it on average increase dose to
thicker patients? The answer is not obvious.

Third, a primary challenge in developing an amendment for
x-ray-field-size limitation or for automatic exposure control and most
likely other areas as well would be how to prescribe performance
standards---not a design standards---forward-looking enough to transcend
limitations that might be present in current technological approaches.

**[Slide 19: Conclusion]{.underline}**

In conclusion, an FDA work group has identified several areas for
possible development of mandatory CT-equipment radiation-safety
performance standards. The initial focus is on technically feasible
features that would reduce patient dose---dose-index standardization,
display, and recording, automatic exposure control, and x-ray-field size
limitation. Were these features implemented on all CT systems, the
projected collective dose savings in the United States would be
approximately 193,000 person-sievert yearly. The work group has
established a framework of issues for analysis that would be detailed in
a regulatory concept paper for decisions on how to proceed. In the
development process we need input from industry, professional and other
stakeholder groups, the Conference of Radiation Control Program
Directors and States, as well as TEPRSSC. Our timeline for the initial
stage of this process is the completion of a concept paper by the end of
this year for CDRH review and decision making and a follow-up briefing
for TEPRSSC next year.

**[References cited in these notes]{.underline}**

Judy, Philip F., "Letter to the Editor. Comment on 'In X-Ray Computed
Tomography, Technique Factors Should Be Selected Appropriate to Patient
Size' \[*Medical Physics* Vol. 28, pp. 1543-1545, August 2001\],"
*Medical Physics* Vol. 28, No. 11, p. 2389, November 2001.

Minino, Arialdi M. and Smith, Betty L., "Deaths: Preliminary Data for
2000," *National Vital Statistics Reports* Vol. 49, No. 12, (National
Center for Health Statistics, Hyattsville, Maryland, October 9, 2001).