Combined PET/CT was the first integrated dual-modality solution to reach clinical practice. Clinical PET/CT scanners have now been operating for almost a decade. Their quasisimultaneous acquisition of complementary anatomic and functional information has revolutionized oncology diagnosis and therapy assessment.
Combined PET/CT was the first integrated dual-modality solution to reach clinical practice.1 Clinical PET/CT scanners have now been operating for almost a decade. Their quasisimultaneous acquisition of complementary anatomic and functional information has revolutionized oncology diagnosis and therapy assessment.2
PET/CT also makes use of available CT transmission data to correct absorption of emitted annihilation photons during the PET emission scan.3 Such CT-based attenuation correction obviates the need for lengthy transmission scans that previously accounted for 40% of the total PET examination time.
Whole-body oncology examinations with PET/CT, covering a coaxial imaging range of about 100 cm from midear to upper thigh, now take less than 30 minutes. These short scan times are possible on any state-of-the-art PET/CT equipment. So it seems logical to increase patient throughput, assuming that the examination is reimbursable. But as with high-powered cars, driving "fast and furious" is not always-even ever-the best way to arrive at your destination.
THEORY AND PRACTICE
Incorporation of two complementary examinations-PET and CT-into one helps overcome the methodological shortcomings of both tomographic imaging modalities. PET/CT scanners can thus be regarded as a technical evolution. PET/CT acquisition protocols are not that different from those for stand-alone PET, which should make the adoption of PET/CT imaging straightforward, at least in nuclear medicine.
In practice, however, operating a PET/CT scanner to the highest diagnostic standards is not that easy (Figure 1), unless you have friends in radiology or make them quickly. Such friendship can help nuclear medicine physicians overcome the methodological hardship of routine PET/CT imaging. Lessons that must be learned about respiratory motion artifacts, for example, were held up as a major reason not to pursue combined imaging in the early days of PET/CT. Practitioners now have the know-how to minimize problems with these artifacts (Figure 2).
Patient positioning also plays a pivotal role in reducing beam-hardening and truncation artifacts from CT. These are particularly apparent when imaging large patients whose arms are inside the field-of-view. PET/CT users have learned that CT-based attenuation correction is also prone to amplification of high-density artifacts from focal accumulation of IV contrast,4 positive oral contrast,5 or metal implants.6
All of these pitfalls can diminish image quality, possibly leading to false diagnostic interpretation and making it hard to sell the power of combined imaging to referring physicians. Several groups that have investigated the challenges of CT-based attenuation correction have proposed modified acquisition schemes. These include breath-hold protocols and the use of extended FOV reconstructions to reduce CT truncation. Their use should improve the accuracy of the attenuation-corrected PET data.7
Nevertheless, a judicious choice of PET/CT referrals and imaging protocols is still required.8 This includes mandatory contrast-enhanced CT in previously undiagnosed patients and in selected patients during follow-up. New protocols for administration of CT contrast have been proposed to optimize enhancement and provide sufficient bowel distention, without excessively increasing photon attenuation.9-10 Metal implants, unfortunately, still give rise to significant artifacts that can be corrected only retrospectively.11
These pitfalls necessitate careful selection, and possible modification, of a standard whole-body PET/CT acquisition protocol. Modifying scan parameters12 and considering external advice related to patient preparation and positioning, as for PET(/CT)-guided radiation therapy planning, may be required. The following are some approaches to disease-specific imaging on PET/CT:
Head and neck imaging. A typical whole-body PET/CT examination involves craniocaudal CT followed by a multibed PET scan in the same direction. Total scan time will be around 30 minutes. It is highly likely that the patient's neck muscles will relax during this period, leading to spatial misalignment of the PET and CT images. Inadequate patient restraint may lead to particularly large misalignment effects that can invalidate the results of image fusion.
Rigid positioning aids, such as vacuum bean bags, can reduce the average head/neck motion in whole-body PET/CT examinations.13 A split protocol should also be adopted for patients in whom a head/neck malignancy is indicated. This comprises a thorax/abdomen scan (arms raised above the head) followed by head/neck scanning of the contiguous imaging range with a slight axial overlap and modified scan parameters such as narrower CT collimation, increased emission scan times, and finer image reconstruction into larger matrices.12 CT contrast volume can be split between the torso and neck scans.
Thoracic imaging. Imaging the thorax of cancer patients is a crucial step in whole-body oncology staging and follow-up. The diagnostic accuracy of PET is known to be inferior to dedicated CT for the detection of small lung lesions. Standard whole-body PET/CT examinations consist of a scout scan followed by a single spiral CT scan, then a multibed emission scan covering the same coaxial range. Reasonable coregistration accuracy, in the absence of respiratory gating options, typically involves acquiring the spiral CT scan during normal expiration or shallow breathing.
Respiration-induced artifacts from earlier PET/CT systems incorporating a single- or dual-row spiral CT scanner, for example, were reduced by following a limited breath-hold protocol.14 This required patients to hold their breath in normal expiration while CT covered the lower thorax and upper abdomen. The use of faster multislice CT systems means that the CT and PET portions can be acquired during shallow breathing. This has been shown to yield satisfactory coregistration accuracy in the region of maximum respiratory mobility-the diaphragm.15
Acquisition of CT data with or without breath-hold instructions during a standard whole-body PET/CT scan still generates blurred CT images that are of inferior radiological quality when compared with dedicated thoracic CT images.16 This problem prompted radiologists at the University of California, Los Angeles to try a new approach, in which they acquired a second ultralow-dose thoracic CT scan in full inspiration.17 The method detected previously unknown micrometastatic lung disease in 13% of patients who had already undergone standard PET/CT. The effective dose from the additional thoracic CT can be limited to 1 mSv if novel dose reduction schemes and efficient CT detector technology are used. This amount adds relatively little to the effective dose of 20 to 24 mSv for a standard diagnostic whole-body PET/CT examination.18
PET/CT colonography. Colorectal cancer constitutes a significant portion of cancer-induced deaths in the industrialized world. Abdominal CT and MRI, and FDG-PET in selected cases, are used together with optical colonoscopy for colorectal cancer staging.19 Clinical experience, however, has shown that nonspecific PET/CT can produce inconclusive findings. Side-by-side viewing of independent CT and PET images may deliver the same diagnostic accuracy but prolong time to diagnosis and treatment.
A PET/CT protocol specific to colorectal cancer has now been suggested.20 Patient preparation includes bowel cleansing and pharmacological relaxation. The acquisition protocol is labeled whole body but actually split into a torso and an abdomen scan. Both are slightly overlapping, and the patient is scanned in supine and prone position for the torso and abdomen scan, respectively. The colon is distended with lukewarm water prior to the abdominal scan. Contrast-enhanced CT data are acquired for both scans.
We have found that using this protocol can limit overall patient turnaround time to 37 minutes, only slightly longer than for a conventional whole-body PET/CT scan. Initial staging with PET/CT colonography has indicated a high diagnostic accuracy for the combined imaging protocol with respect to TNM staging and its effect on therapy management (Figure 3).
This optimized PET/CT protocol can be recommended in patients with incomplete colonoscopy or those who refuse colonoscopy altogether. PET/CT colonography could become the method of choice for restaging patients with a suspected recurrence at the anastomotic site, given that optical colonoscopy is impaired by its strict endoluminal view. PET/CT colonography will provide referring physicians with an integrated report specific to the local disease that also includes an assessment of metastatic disease. Whole-body PET/CT colonography consequently offers a complete diagnostic workup for colorectal cancer patients prior to decisions on therapy management.
Special imaging protocols. Supine patient positioning is adequate for whole-body imaging in general, but the breast is often compressed when patients are imaged supine, making it difficult to differentiate lesions in the thoracic wall and breast (Figure 4A). Several groups have suggested acquiring an independent PET scan in prone position, with the breast hanging and the arms at the side or raised above the head. Coregistration of functional PET information with breast MRI is complicated, however, by differences in breast shape, based on different patient positioning on the resulting images.
Dedicated PET/CT examinations benefit from the use of additional positioning devices to hold the patient in a particular position and restrict the patient or a specific body region from moving during acquisition. One solution would be to acquire an independent PET scan with patients lying prone, arms at their sides or raised above their head. The coaxial scan could be split into separate scan ranges, as with the head/neck protocol. Alternatively, a dedicated scan range would allow for disease-specific patient positioning (Figure 4B) and scan parameters, though patient turnaround would then be limited by the time required to set up and initiate the independent scan. Further innovation by hardware providers is clearly required to enable timely and efficient modification of PET/CT scan protocols.
Special patient positioning is often necessary when incorporating PET and PET/CT into radiotherapy planning and follow-up. A flat tabletop is mounted on the PET/CT patient pallet, and additional positioning aids such as breast boards or an abdominal cast can then be fixed to this tabletop. PET/CT acquisitions for radiation therapy planning do not usually exceed three or four bed positions. Gating mechanisms are now being adopted in lung cancer patients to freeze the tumor and surrounding lung.21 The resulting volumetric anatomic-metabolic information should correspond to the morphology of the patient during gated treatment delivery.
Therapy assessment. PET and CT are both valuable tools for assessing the response of various cancers to therapy. In CT-based evaluations, changes to tumor size or to enhancement patterns are used to indicate any change to underlying morphology.22 PET, on the other hand, uses a radioisotope tracer, usually FDG, to depict metabolic changes.23 Functional information may include a standardized uptake value or total lesion glycolysis.24
The use of PET/CT to monitor therapy response is still not widespread, eight years after the first combined unit was installed in a radiology department. This is due mainly to a lack of finalized consensus guidelines on combined imaging,25 fear of overexposing patients to ionizing radiation from CT, and dearth of prospective studies using patient survival data as the benchmark for therapy response.
More and more PET/CT systems are being installed worldwide, and clinical experience in their use is growing exponentially. We may expect future studies to examine the use of PET/CT in extended follow-up as well as relatively short-term diagnosis and staging of cancer. These studies may mandate novel approaches to quantifying therapy response that go beyond criteria set out by the World Health Organization and the European Organization for Research and Treatment of Cancer.26 Visualization tools that can handle simultaneous display and analysis of baseline and follow-up studies may be performed.27
Clinical adoption and integration of a new imaging technology such as PET/CT is clearly more efficient if imaging experts-nuclear medicine physicians, radiologists, technologists, and physicists-cooperate during the preparation, installation, and operation of the combined system. The ultimate aim is to provide patients with the best diagnosis possible. This process should begin with the adaptation of imaging protocols to suit patients' needs. It should extend to providing referring physicians with a timely single integrated report, detailing anatomic and functional findings together. PET/CT will become a modality of choice only if it is used knowledgeably and reliably by healthcare professionals.
The authors would like to thank Sonja Kinner for preparing parts of the images.
DR. BEYER is an affiliated research associate in nuclear medicine at the University Hospital Essen in Germany and head of medical operations at Timaq Medical Imaging in Zurich, Switzerland. DR. VEIT-HAIBACH is a radiologist in the department of diagnostic and interventional radiology and neuroradiology at the University Hospital Essen. Since July 2006, he has worked in the nuclear medicine department at the University Hospital Zurich.
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