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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
Abstract
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.
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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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.
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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Patient Preparation and Management at a PET/CT Facility
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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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
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
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
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Abstract
Introduction
