Positron Emission Tomography/Computed Tomography David W. Townsend, PhD Accurate anatomical localization of functional abnormalities obtained with the use of positron emission tomography (PET) is known to be problematic. Although tracers such as 18 F-fluorodeoxyglucose ( 18 F-FDG) visualize certain normal anatomical structures, the spa- tial resolution is generally inadequate for accurate anatomic localization of pathology. Combining PET with a high-resolution anatomical imaging modality such as computed tomography (CT) can resolve the localization issue as long as the images from the two modalities are accurately coregistered. However, software-based registration techniques have difficulty accounting for differences in patient positioning and involuntary movement of internal organs, often necessitating labor-intensive nonlinear mapping that may not converge to a satisfactory result. Acquiring both CT and PET images in the same scanner obviates the need for software registration and routinely provides accurately aligned images of anatomy and function in a single scan. A CT scanner positioned in line with a PET scanner and with a common patient couch and operating console has provided a practical solution to anatomical and functional image registration. Axial translation of the couch between the 2 modalities enables both CT and PET data to be acquired during a single imaging session. In addition, the CT images can be used to generate essentially noiseless attenuation correction factors for the PET emission data. By minimizing patient movement between the CT and PET scans and accounting for the axial separation of the two modal- ities, accurately registered anatomical and functional images can be obtained. Since the introduction of the first PET/CT prototype more than 6 years ago, numerous patients with cancer have been scanned on commercial PET/CT devices worldwide. The commercial designs feature multidetector spiral CT and high-performance PET components. Experience has demonstrated an increased level of accuracy and confidence in the interpretation of the combined study as compared with studies acquired separately, particularly in distinguish- ing pathology from normal, physiologic tracer uptake and precisely localizing abnormal foci. Combined PET/CT scanners represent an important evolution in technology that has helped to bring molecular imaging to the forefront in cancer diagnosis, staging and therapy monitoring. Semin Nucl Med 38:152-166 © 2008 Elsevier Inc. All rights reserved. H istorically, instrumentation for tomographic imaging of function (single-photon emission computed tomogra- phy [SPECT], positron emission tomography [PET]) evolved along a path somewhat different from that of anatomical im- aging devices (computed tomography [CT] and magnetic res- onance imaging [MRI]) and the corresponding clinical stud- ies were performed and interpreted separately in different clinical services, ie, nuclear medicine and radiology, respec- tively. Despite this segregation, the usefulness of combining anatomical and functional planar images was evident to phy- sicians even in the 1960s, preceding the invention of CT. The alignment of tomographic images is a complex procedure owing to the large number of degrees of freedom and, with- out some common features, such coregistration, may be problematic. In addition to simple visual alignment or the use of stereotactic frames that are undesirable or inconvenient in a diagnostic setting, sophisticated image fusion software was developed from the late 1980s onwards. 1 For (relatively) rigid objects, such as the brain, software can successfully align images from MR, CT, and PET, whereas in more flexible Departments of Medicine and Radiology, University of Tennessee Medical Center, Knoxville, TN. Financial support for the original PET/CT development was provided by NCI Grant CA 65856. Address reprint requests to David W. Townsend, PhD, Departments of Med- icine and Radiology, University of Tennessee Medical Center, 1924 Alcoa Highway, Knoxville, TN 37920-6999. E-mail: dtownsend@mc. utmck.edu 152 0001-2998/08/$-see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1053/j.semnuclmed.2008.01.003