Attenuation correction for a combined 3D PET/CT scanner P. E. Kinahan, a) D. W. Townsend, T. Beyer, and D. Sashin Department of Radiology, University of Pittsburgh, 200 Lothrop Street, Pittsburgh, Pennsylvania 15213 Received 14 October 1997; accepted for publication 21 July 1998 In this work we demonstrate the proof of principle of CT-based attenuation correction of 3D positron emission tomography PET data by using scans of bone and soft tissue equivalent phan- toms and scans of humans. This method of attenuation correction is intended for use in a single scanner that combines volume-imaging 3D PET with x-ray computed tomography CT for the purpose of providing accurately registered anatomical localization of structures seen in the PET image. The goal of this work is to determine if we can perform attenuation correction of the PET emission data using accurately aligned CT attenuation information. We discuss possible methods of calculating the PET attenuation map at 511 keV based on CT transmission information acquired from 40 keV through 140 keV. Data were acquired on separate CT and PET scanners and were aligned using standard image registration procedures. Results are presented on three of the attenu- ation calculation methods: segmentation, scaling, and our proposed hybrid segmentation/scaling method. The results are compared with those using the standard 3D PET attenuation correction method as a gold standard. We demonstrate the efficacy of our proposed hybrid method for con- verting the CT attenuation map from an effective CT photon energy of 70 keV to the PET photon energy of 511 keV. We conclude that using CT information is a feasible way to obtain attenuation correction factors for 3D PET. © 1998 American Association of Physicists in Medicine. S0094-24059801710-6 Key words: dual modality, PET and CT, attenuation correction I. INTRODUCTION While still in an early phase, the role of PET imaging in oncology research and patient care is clearly growing. 1 The increased uptake of  18 Ffluorodeoxyglucose, or FDG, by neoplasms can significantly influence patient management and predict survival probability, particularly for breast, lung, and colorectal cancers. 2 In regions such as the thorax and abdomen, the demon- stration of increased FDG uptake is limited in value without an unambiguous localization of tracer uptake to a specific structure, for example, a tumor seen on a corresponding CT image. Post hoc PET-CT alignment, however, is difficult due to shifting of the position of organs, which can occur without patient motion and may even depend upon the relative cur- vature of the patient beds of the separate scanners. To address this problem, a single tomograph with the unique capability of acquiring both functional PET and anatomical CT images is being built as a collaboration be- tween the University of Pittsburgh and Siemens/CTI, and funded in part by the National Cancer Institute. The purpose of the project is to construct a scanner for oncology imaging that provides accurately aligned functional and anatomical images. The PET/CT scanner design is based on a third-generation rotating fan-beam CT scanner, the Siemens Somatom AR.SP. The AR.SP can operate in a rapid helical scanning mode and has an in-plane resolution of better than 1 mm. The PET component is based on the rotating ECAT ART CTI PET Systems Inc., 810 Innovation Drive, Knoxville, TN 37932-2571Advanced Rotating Tomograph scanner manufactured by Siemens/CTI. The ART scanner has a spa- tial resolution of 6 mm in all directions, and for the NEMA- PET phantom described below has a scatter-subtracted sen- sitivity of approximately 300 kcps/Ci/ml. 3 Unlike most commercial scanners the ART scanner operates without re- tractable shielding rings, or septa, and is thus a volume, or 3D, imaging system with a 16.2 cm axial field-of-view. The 3D imaging of the ART scanner also results in a scatter fraction scatter/total of 36% as compared to approximately 12% for PET scanners imaging in 2D mode with the septa extended into the field of view. The increased scatter frac- tion, however, is adequately removed from the image by a model-based scatter correction procedure. 4 For the combined PET/CT scanner, the two bismuth ger- manate BGO PET detector arrays from the ART scanner will be mounted on the rotating support of the AR.SP CT system, as is conceptually illustrated in Fig. 1. Data will be acquired during continuous rotation of the gantry and read out through an optical slip-ring system. The PET and CT data will not be acquired simultaneously, however, and the two imaging functions will operate independently. The PET detector array will collect emission data in 3D mode over a 16.2 cm axial field-of-view, whereas CT volumes corre- sponding to the PET field-of-view will be obtained by an appropriate patient bed motion during a separate spiral scan. The effect of photon attenuation in PET imaging is usu- ally directly measured in coincident transmission mode with 68 Ge/ 68 Ga sources of 511 keV annihilation photons. It is un- 2046 2046 Med. Phys. 25 „10…, October 1998 0094-2405/98/25„10…/2046/8/$10.00 © 1998 Am. Assoc. Phys. Med.