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Adding Structure to Function INVITED COMMENTARY Adding Structure to Function and staging of malignant neoplasm by rate results are obtained with attenua w.hen cross-sectional anatomic im FDG PET are technically demanding. tion-correction versus emission-only aging emerged over two decades ago, A large portion of the body must be images is lacking. Centers continue to Henry N. Wagner, Jr., recognized that imaged quickly, small deposits of neo use noncorrected images for whole- the nuclear medicine community should plasm detected accurately, and the PET body oncology FDG PET, and when focus on tissue and organ function findings interpreted in the context of attenuation correction is performed, rather than anatomy (7). The value of the patient's corresponding normal and noncorrected images are often con functional imaging has not only be abnormal anatomy. At present, 2 practi sulted, in addition to corrected images, come clear since that time but is now cal, related considerations drive the before the final interpretation is ren recognized as the future of diagnostic merging of PET and CT: the need for a dered (7). imaging. Structural and functional im rapid, noise-free transmission scan for In response to the above needs, rapid aging are also increasingly understood attenuation correction of the PET emis advances have been made in the past 5 as complementary rather than compet sion data and a need for an anatomic years in the attenuation-correction hard ing imaging modalities. Over the past framework for the physiologic informa ware and software. The capacity to decade, manufacturers have considered tion provided by PET. perform transmission scans after tracer developing combined CT and PET or Without attenuation correction, PET injection has made routine whole-body SPECT imaging devices. CT, PET, and images are spatially distorted, and there (cf. torso) attenuation-corrected FDG SPECT are, in fact, closely related is marked radial nonuniformity in the PET feasible in clinical practice. High- imaging devices and share a common depiction of tracer activity (6). The activity single sources have been devel origin. In some respects, x-ray CT is application of a 511-keV transmission oped to improve counting statistics of the stepchild of transmission computed map to the reconstruction of the PET the transmission scan (11), and segmen tomography (2), which was first used emission data yields a spatially and tation methods allow elimination of the by Kühland Edwards (2) with a colli- quantitatively accurate depiction of the noise propagation, although with risks mated radionuclide source, as a direct tracer activity. High-quality, attenua of segmentation error (72). However, extension of their work in emission tion-corrected FDG PET images con these developments have not fully computed tomography (3). The 2 ar tain recognizable anatomic structure; solved the limitations of attenuation ticles by Beyer et al. (4) and Patton et yet, debate continues over whether the correction. Ideally, attenuation correc al. (5) in this issue of The Journal of use of current technology for attenua tion would add very little to the scanner Nuclear Medicine mark the beginning tion correction is truly advantageous time imposed by the emission-image of a new era in the growing integration over noncorrected PET for clinical on acquisitions, and there would be essen of functional and structural imaging cology diagnosis (7). At a practical tially no noise and no possible segmen into clinical practice. level, the additional scanner time tation errors propagated into the final The work reported by Beyer et al. (4) needed for the transmission scan using attenuation-corrected emission images. and Patton et al. (5) is part of a broad current technology can nearly double Kinahan et al. (73) have shown that it is trend toward the fusion of complemen total PET study time, imposing in possible to use CT imaging data to tary diagnostic imaging modalities. The creased likelihood of patient-move construct an attenuation map scaled to merging of biologic imaging, embod ment artifacts and decreased patient 511 keV. This essentially noise-free ied in PET, with the rapid and detailed throughput, ultimately decreasing the attenuation map, in which there is no structural imaging provided by x-ray diagnostic accuracy and increasing the need for segmentation assumptions, can CT may well be among the most fruit cost of PET imaging, respectively. Be then be used for attenuation correction ful direction of this trend. The impetus cause the typical transmission scan is of the PET imaging data. The time to build combined scanners has emerged very count poor, substantial statistical imposition for this approach is limited with the rapid growth of whole-body noise is propagated into the recon only by the speed of contemporary oncology PET imaging. The diagnosis structed attenuation-corrected emission spiral CT technology, which is mea scan, resulting in apparent decreases in sured in seconds for each PET emis Received Dec. 16,1999; revision accepted Feb. lesion contrast and small-lesion detect- sion bed position. Thus, PET/CT 16,2000. ability (8-10). Anecdotal evidence of presents a plausible solution to the For correspondence or reprints contact: Paul D. Shrove, Division of Nuclear Medicine, University the value of attenuation correction in need for a very rapid, low-noise, and Hospital, University of Michigan, B1G 505-UH-Box FDG PET imaging is widespread, but quantitatively correct method of PET 2008, 1500 E. Medical Center Dr., Ann Arbor, Ml 48109. generalized evidence that more accu attenuation correction. Furthermore, 1380 THEJOURNALOFNUCLEARMEDICINE•Vol. 41 •No. 8 •August 2000 anatomic information in the CT images performed in response to a finding, or dural planning, and communication of can be incorporated into the image lack of findings, on CT. For larger findings to practitioners. reconstruction process, potentially en neoplasms (i.e., > 1.5-2 cm), anatomic There are arguments against the hancing the speed and accuracy of correlation often can be performed by merger of PET and CT in the form of a numerical reconstruction algorithms visual reference within the attenuation- single scanner. If PET is expensive, and scatter and partial volume correc corrected PET images and to the CT would not the addition of CT make it tions (14,15). images. For example, fused PET and more so? First, PET is not expensive. Many technical hurdles still remain. CT images added little to diagnostic Even high-end PET scanners have been, The rapid acquisition capabilities of accuracy in an early study of mediasti- and remain, priced in the mid to upper contemporary spiral CT can effectively nal lymph node staging in lung cancer range of MRI scanners. Second, PET freeze respiratory motion in the chest (17). As the size of detected neoplasm scanners already have transmission CT and upper abdomen, resulting in misreg gets smaller, however, "eyeball" corre capability built in them in the form of istration of the transmission scan with lation becomes increasingly inadequate, the transmission scan sources. Substi the PET emission data, for which respi particularly in the neck, abdomen, and tuting a widely used CT tube and ratory motion is averaged. Intravenous pelvis. Data showing that fused PET detector, for which development and contrast when given as a bolus can and CT images improve accuracy com manufacturing costs are distributed over result in intravascular attenuation dur pared with separate interpretation of many units, for the transmission scan ing the CT acquisition, which inadver the FDG PET and CT images in these sources and related mechanicals results tently scales to bone rather than soft more challenging situations have yet to in a modest increase in cost for a new tissue, creating attenuation-correction be published, because the hardware generation PET scanner. If the incorpo errors. These are solvable problems, and software for true image fusion is ration of CT reduces overall scan time however. CT for radiation therapy plan only now becoming available. to roughly 30 min for imaging the ning is conducted without the person Clinical PET imaging in oncology is torso, patient throughput becomes com holding their breath and with slowed already moving beyond cases requiring parable with MRI and effectively re table motion to average respiratory straightforward interpretive skills, such duces the cost of PET imaging. Incorpo movement. Currently, it is not uncom as pulmonary nodules, enlarged medi- ration of CT into the next generation of mon to perform spiral CT sequentially astinal lymph nodes, and well-circum PET should be seen, then, as an up during 2 or 3 phases of intravenous scribed masses. Increasingly, practitio grade in the transmission scan capabil contrast enhancement in some body ners are asked to identify and locate ity over that of the current generation applications (e.g., precontrast, arterial deposits of malignant neoplasm that PET scanners. phase, venous phase). Thus, a CT of are not detected on CT scans, evaluate The same concept applies to SPECT the whole torso for PET attenuation the precise location of recurrent neo and coincidence-SPECT devices. The correction (with lowered beam current plasm for biopsy, and provide accurate work by Patton et al. (5) demonstrates to reduce dosimetry) could be followed delineation of true neoplasm extent for how a low-cost CT tube and detector by a rapid intravenous contrast-en radiation therapy planning. Experience system incorporated into a dual-head hanced CT as needed. In any case, the at the University of Michigan is that SPECT gamma camera can provide need for the aggressive use of intrave true image fusion would be highly relatively rapid low-noise transmission nous contrast in CT for oncology diag valuable and possibly indispensable for scans and provide useful anatomic in nosis may need reconsideration in light interpretation or subsequent procedural formation for interpretation of the func of the superior ability of FDG PET to applications in up to 20% of our cur tional images.
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