AJR 2005; 184:S146-S151
© American Roentgen Ray Society
Establishing a PET/CT Practice
Paul Shreve1
1 Advanced Radiology Services, PC, 3264 North Evergreen Dr., Grand Rapids, MN
49525.
Received January 26, 2005;
accepted after revision March 14, 2005.
CONTINUING MEDICAL EDUCATION
This article is available for 1 hour of Category 1 CME credit. It is free
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Abstract
OBJECTIVE. This chapter discusses the differences in the operation
of dual-technique PET/CT compared with conventional PET tomography in clinical
practice.
CONCLUSION. Although PET/CT still is a relatively new medical
imaging technique, it is becoming the preferred method for body oncology
imaging and will play an increasingly important role in molecular imaging in
clinical practice. Therefore, it is imperative that facilities learn how to
establish a successful PET/CT practice.
Introduction
PET/CT combined scanners are rapidly replacing conventional
dedicated PET tomographs for body oncology imaging applications. Currently,
over 80% of new PET tomograph equipment delivered is in the PET/CT combined
technique configuration. Establishment of a successful PET/CT practice
requires integration of the two techniques on several levels, including
planning and design of the PET/CT facility, staffing, patient preparation and
management procedures, imaging protocols, image distribution and archiving,
reporting and billing of examinations, and patient and referring physician
education.
Design of a PET/CT Facility
A PET/CT facility differs from a conventional dedicated PET facility in
three ways: increased space requirements and scanner sitting considerations,
accommodations allowing for higher throughput of patients, and radiation
shielding requirements.
The increased space needs of the scanner room reflect both the greater
patient pallet length and linear travel distance and the deeper scanner gantry
of combined PET/CT units. Depending on the PET/CT scanner vendor, this can
require a room length 2 to 3 m deeper than stand-alone dedicated PET scanners
or CT scanners. In addition, the power supply and cooling requirements of the
CT unit are much higher (especially with the latest MDCT scanners) than are
those of a conventional dedicated PET scanner. PET/CT scanners also require a
particularly rigid, flat, and true floor to ensure proper and continued
alignment of the scanner gantries and extended travel patient pallet. Finally,
an IV contrast injector must be positioned to allow access to either end of
the longer scanner gantry. If radiation therapy planning applications are
anticipated, space at the ceiling and flanking walls must be reserved for
installation of laser positioning sources. With new construction, these
considerations can be incorporated into planning at the onset of facility
design. Mobile PET/CT providers already have adapted larger coaches to the
longer scanning room and additional ancillary equipment needed for mobile
PET/CT. Installing a new PET/CT in an existing medical imaging department,
such as replacing an existing stand-alone dedicated PET or CT scanner,
however, typically requires considerable renovation.
Further adding to the need for additional facility space is the potential
for higher patient throughput with the PET/CT scanner relative to conventional
dedicated PET scanner operation. Substituting a subminute CT acquisition for
the typical 15-20 min sealed-source transmission scan reduces whole torso scan
times to fewer than 30 min. Even with a relatively short patient FDG uptake
time of 1 hr (and the trend is to longer uptake times), these short scan times
result in a queue of injected (i.e., radioactive) patients who must be
properly isolated from the technologists and other patients and personnel.
Hence, while two or three uptake rooms may suffice for a stand-alone dedicated
PET scanner, a PET/CT facility will need four or five uptake rooms if the full
potential volume of patient throughput is anticipated (Figs.
1A,
1B, and
1C). Adding to space
requirements is the desirability of a dedicated bathroom(s) for the PET
patients, both for their convenience and to minimize technologists', other
workers', and other patients' exposure to radiation. These additional space
needs can be moderated to some extent in settings where high patient
throughput is not expected due to location, such as in a rural facility or a
highly competitive urban environment, where 4 to 6 rather than 12 or more
PET/CT scans are performed each day. In such instances, routine CT scans can
be interleaved between PET/CT scans, spacing out the FDG-injected patients,
such that only one or two of the uptake rooms are needed. This approach also
can be useful in the mobile environment, where the additional uptake rooms and
dedicated bathrooms would require ancillary site construction or
modification.
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Fig. 1C. —Examples of clinical PET/CT facility layouts. Layout for
ancillary facility to a mobile PET/CT site. Note dedicated bathrooms for
patients. IV catheters for infusion of FDG and IV contrast can be placed using
uptake rooms or common staging area elsewhere in the facility. Once patients
have been infused with FDG, they become a source of radiation exposure to
technologists and other ancillary personnel and need to be isolated from
others. (Courtesy of Michael L. Clark, H & H Systems and Design, New
Albany, Indiana)
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Perhaps the most complex aspect of PET/CT facility planning and design is
radiation shielding. Government oversight and regulation of medical facility
radiation shielding is typically regional (in the United States, by state
government) and therefore can vary considerably. Furthermore, oversight and
regulation of facility radiation shielding for PET and CT operation can even
be under different government bureaus. There are two completely different
radiation protection needs in a PET/CT facility. Radiation shielding
requirements for CT deal with shielding the intermittent high flux of
relatively low-energy scattered X-rays (typically under 60 keV) emanating from
the scanner gantry in a largely predictable pattern. The number of patients
scanned in a given time and the protocols used (total mAs) determine the
extent of shielding in the walls, windows, ceiling, and floor. In settings
where use of the CT scanner alone, and in high volumes such as with 20 or more
CT patients (not uncommon with contemporary MDCT scanners) is anticipated, the
shielding requirements will be substantially greater than in applications
where CT is only used in conjunction with PET.
In contrast to CT, the radiation shielding requirements for FDG PET deal
with shielding the high-energy (over 500 keV for unscattered photons) much
lower flux annihilation photon energy emanating continuously from the injected
patients. Unlike the CT-related radiation, which is confined to the scanner
room, the FDG PET-related radiation source moves with the injected patients.
This situation is no different from conventional nuclear medicine imaging
practice, except that the annihilation photon energy is so much more
penetrating than the typical single photon radionuclides such as
99mTc. Consequently, more lead or dense concrete is required for
direct shielding. Due to the 2-hr physical half-life of 18F and the
urinary excretory route of FDG, total patient activity at the time of scanning
is reduced at least 30% relative to immediately after the infusion of FDG.
This, along with the self-shielding effect of the scanner gantry and the
distances from the scanner table to walls and ceiling, results in relatively
moderate shielding requirements for the scanner room, approximately one-eighth
of an inch (3.75 mm) lead equivalent, similar to that required for a
high-volume CT scanner room. In general, distance is the best means of
attenuating the energetic radiation exiting the injected patients. Distance
unfortunately is not generously available in the small rooms occupied by the
patients after FDG infusion (uptake rooms), and indeed the uptake rooms
require the most extensive radiation shielding, often three-eighths of an inch
(11.25 mm) lead equivalent or more, depending on local regulations. Location
of the uptake rooms is an especially important consideration because they are
the locus of the greatest high-energy radiation source. Permitted exposure
rates for nonmonitored personnel, such as office workers, is very low under
federal regulations at less than 3 mrem/hr or 100 mrem accumulated dose/year
[1]. Hence, occupancy opposite
adjoining walls (and floors above and below) of a PET/CT facility,
particularly of the uptake rooms, needs to be carefully considered. In the
instance of an occupied office adjacent to an uptake room and occupied 8 hr
per day, for example, up to 5 cm (2 inches) of lead-equivalent shielding may
be required.
The increased throughput of PET patients possible with a PET/CT scanner
also increases the shielding needs of a facility over that of a conventional
clinical PET operation, and thus replacement of conventional PET tomographs
with a PET/CT may well require additional shielding, including in the floor or
ceiling in some instances. Shielding needs of the floor and ceiling can be
eliminated by locating a PET/CT facility with no occupied space above or
below.
Staffing a PET/CT Facility
As with facility radiation shielding regulations, licensure of
technologists performing CT scans or PET examinations varies depending on
local governing jurisdiction. In the United States, licensure of technologists
is a state function, but not all states even require licensure of
technologists. For example, 36 of 50 states fully or partially license
radiographers, while 23 of 50 states fully or partially license nuclear
medicine technologists. In states where no licensure is required, registry in
radiography with advanced certification in CT for CT and registry in nuclear
medicine for PET is at the discretion of the institution, facility, or
increasingly, third party payer. The training and certification requirements
and credentialing for the technologists at a PET/CT facility will thus depend
on these local government, institution, and payer regulations. In some states,
only technologists with registry in CT can operate a CT scanner and only
technologists with nuclear registry can administer radiotracers and perform
PET scans. Consequently, under such regulations, at least two technologists,
one from each background, would be needed to perform PET/CT scans. In states
with no licensure requirements, the individuals administering radiotracer and
performing the PET examination or the CT scans need only be under the
supervision of an appropriately trained and qualified physician, and
consequently technologists from either a radiography with advanced
certification in CT or from a nuclear medicine background can staff a PET/CT
facility.
It has been recognized that technologists performing PET/CT examinations
ideally should have competencies in both nuclear medicine and CT
[2]. A consensus regarding a
curriculum for PET/CT was established by a joint working group of the ARRT and
SNMTS [3], and now pathways
exist for dual registry. In addition, courses and preceptorships for
technologists in PET/CT now are offered by several professional and commercial
organizations [4].
The number of technologists needed in a clinical PET/CT facility will
depend on the anticipated throughput and the complexity of scans performed.
Generally, two technologists will be required, although one technologist with
skills in both techniques can handle a modest throughput of uncomplicated
patients. When high throughput (more than 12 patients a day) is anticipated,
and especially when more complex patients such as inpatients, pediatric
patients, or radiation therapy planning patients are included in the patient
mix, an additional technologist or patient assistant may be needed. Since
technologist radiation exposure at a PET/CT facility occurs during close
proximity to the patients after FDG infusion, in facilities with high patient
throughput duties, such as radiotracer infusion and escorting patients to and
from the scanner, these duties may need to be rotated among technologists,
nurses, or patient assistants to minimize individual radiation exposure. While
it is desirable for the interpreting physician to be onsite and closely
involved with the technologists in the day-to-day operation of PET/CT, this is
not mandatory depending on government or institutional polices and payer
requirements. PET/CT facilities designated as independent testing facilities
(IDTF) require direct (onsite) physician supervision of CT while PET does not
[5].
As with conventional dedicated PET practices, it is best to have personnel
exclusively assigned to scheduling, reception, and especially billing, if
possible. Since the patient preparation and precautions for the CT aspect of
PET/CT are essentially the same as stand-alone CT, it can be advantageous to
have CT and PET/CT scheduling, reception, and billing commonly shared;
however, an incremental full-time equivalent (FTE) is almost mandatory due to
the inherent complexity of the combined procedure and current complexities in
insurance coverage and billing for PET and contemporaneous CT. Such personnel
will need special training regardless of their previous experience. Special
forms for ordering the PET/CT scan must be developed borrowing from PET and CT
materials (Fig. 2). Similarly,
patient instructions should include a description of the entire procedure in
terms of both the PET and CT scans, including the expected time on the
scanner, total time one would expect to be at the facility, use of an IV
catheter for the radiotracer and IV contrast material administration, and any
anticipated use of anxiolytics. Instructions also should cover preparations
for the PET scan (see below), and specific instructions for diabetics and
patients with a history of contrast allergy.
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Fig. 2. —Example of examination order form developed for PET/CT. Order
form has explicit entries for type of PET and CT scans requested, and
check-off for patient conditions relevant to both FDG PET and contrast CT.
(Courtesy of PET Medical Imaging Center, Grand Rapids, MI)
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Patient Preparation and Management at a PET/CT Facility
Preparation for a PET/CT scan follows much the same scheme as stand-alone
dedicated FDG PET, with the added precautions regarding renal function and
contrast reaction history when IV contrast CT will be used. Diabetic patients
require special attention both to ensure acceptable serum glucose levels at
the time of FDG infusion and management after IV contrast administration for
patients on oral diabetes therapy. Restraint from intense physical activity
and an all-protein diet commonly are advocated the day before the procedure to
minimize skeletal muscle and myocardial muscle FDG uptake, respectively.
Commonly, patients are instructed to be NPO (nothing by mouth) for a specified
time (typically 6 hr or more) before PET or CT, but this is to ensure basal
endogenous serum insulin levels (for PET) and a stomach empty of solids (for
CT) at time of scanning. Hydration, however, is desirable for both the PET and
CT scans to facilitate clearing of radiotracer and IV contrast material,
respectively, and thus tap or bottled water can be taken orally even up to the
time of the examination.
Limited physical activity, at least during the initial 20-30 min after FDG
infusion, is advocated at most centers to minimize spurious muscle FDG tracer
accumulation. For dedicated head and neck examinations, further care in
minimizing speech and swallowing activities before and after FDG infusion also
is advocated at many centers. The role of anxiolytics to reduce spurious
muscle and brown fat uptake of FDG tracer will depend on local experience, as
with conventional PET. Management of patients during a PET/CT examination
again follows much the same scheme as with a conventional FDG PET examination,
except the greater throughput potential of PET/CT places greater emphasis on
radiation control procedures for the technologist and other personnel in close
proximity to patients after FDG infusion. It is highly advantageous to have
properly designed and shielded patient rooms for FDG injection and subsequent
uptake phase before the scan procedure. Since the uptake time can range from
1-2 hr or more, such rooms ideally provide a comfortable setting for the
patient and have audio- and videotape links to the scanning control room or
technologist work area to help minimize near-proximity patient encounters once
the patient has been injected with the radiotracer
(Fig. 3).
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Fig. 3. —FDG PET uptake room. This example measures 2.4 x 3.5 m
(8 x 11.5 ft), and in addition to comfortable reclining patient lounge
chair, has entertainment system, call intercom, and videotape camera monitor.
Room is extensively shielded as is door, to minimize radiation exposure to
technologists, ancillary personnel, and nonmonitored workers in adjoining
space.
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Imaging Protocols in PET/CT
Overall scan acquisition time for an attenuation-corrected PET scan
performed on a PET/CT scanner is reduced by more than one-third compared with
a conventional attenuation-corrected PET scan performed using a sealed
radiosource-acquired transmission scan. Improvements in PET tomography
sensitivity have reduced emission acquisition times to 2-5 min per bed
position. Hence contemporary PET/CT scanners can perform whole-torso
examinations (axial coverage from neck to below hips) in typically 15-25 min.
Such shorter image acquisition times allow most body examinations to be
performed with the arms raised, improving both PET and CT image quality.
Although the gantry tunnel in a PET/CT scanner is nearly twice the length of a
conventional PET scanner, the shorter scanning time generally results in
greater overall patient comfort and a reduced sense of claustrophobia.
Dedicated head and neck PET/CT scans require special attention to stable
positioning of the head and neck during the sequence of CT and PET image
acquisitions to ensure accurate image registration and alignment. In addition,
both CT and PET acquisitions for dedicated head and neck examinations are
modified in terms of matrix, zoom, and acquisition time for PET, and
collimation, zoom and IV contrast protocol for CT.
Since the introduction of combined PET/CT scanners, the understood role,
and consequently, the protocol used for the CT portion of the examination, has
evolved from a role of rapid transmission scan with some anatomic reference
information to a fully merged anatomic diagnosis. Hence, the CT protocols for
PET/CT increasingly are identical to routine body or dedicated neck
stand-alone CT protocols in use of contrast material, detector collimation,
and beam current. Concerns regarding the effect of hyper-concentrated positive
oral contrast or undiluted IV contrast on the CT-derived
attenuation-correction map causing "hot-spot" artifacts on the
attenuation-corrected emission PET images prompted many centers to perform
separate CT acquisitions, one low-beam current without contrast for the
attenuation-correction map, and after the PET emission acquisition, a
contrast-enhanced full diagnostic CT scan acquisition. The actual spurious
effect of oral and IV contrast on the PET attenuation-corrected images is
clinically negligible [6,
7], and when the contrast is
used according to contemporary protocols (e.g., saline chase technique), no
perceptible artifacts are encountered, allowing for a single full-diagnostic
CT acquisition with the PET emission acquisition. The need for high rates
(>3.0 mL/sec) of IV contrast material injection, multiphase
contrast-enhanced CT images of visceral organs, and the role of positive oral
contrast versus negative oral contrast have yet to be resolved in the setting
of PET/CT applications in oncology. Contrast material likely will be used
somewhat less aggressively in PET/CT oncology examinations than in other
stand-alone CT examinations
[8]. In any case, the CT
portion of the PET/CT examination increasingly is understood as not only as a
source of anatomic localization for PET interpretation but also as a source of
anatomic diagnosis for a fully merged metabolic-anatomic examination.
Image Distribution and Archiving in PET/CT
As discussed above, the operation of a PET/CT facility follows the same
general procedures as a conventional, dedicated PET facility. Image
distribution and archiving, however, parallels CT due to the much higher data
storage needs of a CT image data set relative to PET image data sets. A 128
x 128 matrix PET image occupies approximately 50 kB of memory space,
while a 512 x 512 matrix CT image requires roughly 10-fold more memory
(500 kB). A typical whole-torso scan acquisition then generates a set of PET
images requiring 13 MB of memory, while a CT image set of 5 mm contiguous
reconstructed slice thickness requires 90 MB.
Since PET/CT studies typically are interpreted using coronal and sagittal
image reconstructions and transaxial images, archived CT image reformats
should have slice overlap, increasing the data volume to over 100 MB.
Furthermore, the increased use of MDCT where body CT images are commonly
reconstructed to 3-mm slice thickness with overlap, can yield CT data volume
over 200 MB. Allowing for multiple image reconstructions commonly used in PET
and CT image interpretation (attenuation and non-attenuation PET images and
soft-tissue and lung-reconstruction algorithm CT images) a total PET/CT image
file can occupy over 300 MB uncompressed. Hence, a PET/CT facility cannot be
readily integrated into a nuclear medicine department image display and
archiving infrastructure without substantially increasing archival storage
capacity. Ideally, PET/CT, like CT, MRI, sonography, digital radiography, and
so forth, should be integrated into a PACS system equipped routinely to handle
such large image files. Similarly, data and image workflow in PET/CT parallels
CT rather than conventional dedicated PET or general nuclear medicine
workflows. For example, it is common in nuclear medicine to store image raw
data, while in routine CT practice, the raw data usually are discarded
immediately or after a specified time once the usual set of images has been
reconstructed. Raw data occupies several times more storage space than the
reconstructed images (300 MB for six bed positions of PET-emission sinograms
and 800 MB for CT for a whole-torso examination CT sinogram), and hence cannot
be stored on the PET/CT scanner memory and must be transferred to fixed media
or discarded after a specified time. Images files archived on a local PACS or
institutional PACS can be retrieved for viewing on dedicated PET/CT
workstations, or viewed on PACS viewing stations within the network, although
currently the full functionality of a PET/CT workstation is not available as a
fully disseminated functionality on PACS system workstations.
Reporting On and Billing for PET/CT
Because PET/CT involves merging two complementary but separate imaging
techniques, there are added challenges associated with reporting on and
billing for the PET/CT examinations. For the CT portion of the examination to
have reliable value as a source of anatomic localization for the FDG PET
interpretation, the image quality will render the CT images diagnostic. Even
without contrast at one-third the beam current of optimized CT scan protocols,
there is ample anatomic diagnostic information both complementary to the
metabolic images of the PET scan and independent of the PET-derived diagnostic
information. Hence, interpretation of a PET/CT examination always involves not
just anatomic localization, but anatomic diagnosis as well. A PET/CT report,
therefore, always must combine both PET and CT findings and render an
integrated interpretation merging the anatomic and metabolic findings.
Optimal CT diagnostic technique, including the use of contrast material,
would be expected except when a properly performed diagnostic CT was performed
recently and is readily available for comparison. It should be noted that the
recently released PET/CT CPT code descriptions only specify the CT being used
for anatomic localization and attenuation correction and hence do not properly
account for the interpretation workload of the CT part of the examination.
When a diagnostic CT medically is necessary and explicitly is ordered by the
referring physician, appropriate CT scan CPT codes are used for each CT
portion of the examination. The physician order explicitly should state the
individual CT scans ordered, such as CT thorax with contrast, CT abdomen with
contrast, and so forth (Fig.
2), and the PET/CT report should involve either specific report
heading for PET and CT procedure and findings or explicitly should include
separate reports. The required use of G codes by the Centers for Medicare and
Medicaid Services (CMS) for Medicare patients adds to the complexity of
ordering and billing for PET/CT. This requirement, however, will be rescinded
as of April 2005, and now CPT codes can be used for CMS-approved indications
when billing for patients covered under the Medicare program
[9].
Referring Physician Education with PET/CT
PET is still a new medical imaging technique. Even though the clinical
value of PET has been documented for well over a decade, both the lay public
and even many physicians still do not understand the appropriate applications
of PET medical imaging (Fig.
4). Education of physicians and the general public regarding
PET/CT consequently remains largely focused on what PET is and how it is used
for patient diagnosis and management. Grand rounds, tumor boards, and special
continuing medical education events are the usual formats for educating
referring physicians. In many ways, PET/CT makes the PET metabolic images more
accessible to physicians who have familiarity with the anatomic depiction of
CT images, and hence PET/CT will likely play an important role in the
emergence of molecular imaging in clinical practice
[8].
References
- Federal Register 40CFR93
- PET-CT Consensus Conference: SNMTS; American Society of Radiologic
Technologists (ASRT). Fusion Imaging: a new type of technologist for a new
type of imaging. J Nucl Med Technol2002; 30:201
-204[Free Full Text]
- www.crcpd.org/PET-CT
Fusion Imaging/PET-CT%20Consensus%20Paper.doc
- See education and meetings under:
www.snm.org;
www.ami-imaging.org;
www.arrt.org
- Medicare Part B Reference Manual, appendix F
- Antoch G, Freudenberg L, Beyer T, Bockisch A, Debatin J. To enhance
or not enhance? F-18-FDG and CT contrast agents in dual-modality F-18-FDG
PET/CT. J Nucl Med2004; 45:25S
-35S.
- Yau YY, Chan W-S, Tam Y-M, et al. Applications of intravenous
contrast in PET/CT: does it really introduce significant attenuation
correction error? J Nucl Med2005; 46:283
-291[Abstract/Free Full Text]
- Shreve PD. Adding structure to function. J Nucl
Med 2000;41:1380
-1383[Free Full Text]
- www.cms.hhs.gov/Manuals/pmtrans/R475CP.pdf
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Abstract
Introduction
