PETCT Imaging Systems

The first combined PET/CT system was developed by Townsend and co-workers at the University of Pittsburgh in 1998.28,33,34 The system was configured by combining a Somatom AR.SP spiral CT scanner (Siemens Medical Systems) in tandem with the PET detectors from an ECAT ART PET system (CTI/Siemens). The PET subsystem consisted of two arrays of bismuth germanate (BGO) block detectors covering 16.2 cm in the axial direction with 24 partial detector rings operated without septa, allowing the PET data to be acquired in a fully 3-dimensional mode. The CT scanner was a third generation helical CT scanner that has an x-ray tube operated at 110-130 kVp with a 6.5 mm Al-equivalent filtration and having a xenon x-ray detector with 512 elements. Both the CT components and the PET detectors are mounted on opposite surfaces of the rotating stage of the CT system, and during imaging are rotated continuously at a rate of 30 rpm. The system has a common patient table, with the patient translated between the centers of the CT and PET imaging planes which were offset axially by 60 cm. On the prototype system, an axial extent of 100 cm in the patient could be covered by simple table translation33,34 with the PET and CT images acquired sequentially rather than simultaneously. The PET/CT prototype was operational at the University of Pittsburgh from May 1998 to August 2001, during which over 300 cancer patients were scanned. These pioneering studies by Townsend and his colleagues demonstrated both the feasibility and the clinical benefit of combined PET/CT scanning, and prompted significant interest from the major medical imaging equipment manufacturers who now all have introduced commercial PET/CT scanners for clinical use.

PET/CT scanners now are available from all of the major medical imaging equipment manufacturers (GE Medical Systems, CTI/Siemens Medical Systems, and Philips Medical Systems).35,107 Current systems have up to 16 slice CT capability and have radionuclide detectors with either 2D or 3D PET imaging capability. The PET scanner can have either bismuth germinate (BGO), lutetium oxyorthosilicate (LSO), or gadolinium oxyorthosilicate (GSO) scintillators. The CT study typically is used for both localization of the FDG uptake38 4161 as well as for attenuation correction of the PET image. In addition, the use of CT in comparison to external transmission rod sources for producing the attenuation data increases patient throughput by approximately 30%.107 As noted above, the PET/CT system also has a specially designed patient table that is designed to minimize deflection when it is extended into the patient port.

Figure 8 shows the 7 steps identified by Beyer et al.108 that comprise a typical PET/CT scan, demonstrating the degree of integration available in a modern dual-modality imaging system. (1) The patient is prepared for imaging which commonly includes administration both with contrast media86 and with the radiopharmaceutical, typically 370 to 555 MBq (10 to 15 mCi) of 18F-fluro-deoxyglucose (FDG) in adults. (2) The patient then is asked to remove all metal objects that could introduce artefacts in the CT scan and then is positioned on the patient table of the dual/modality imaging system. (3) The patient then undergoes an "overview" or "scout" scan during which x-ray projection data are obtained from the patient to identify the axial extent of the CT and radionuclide study. (4) The patient undergoes a CT acquisition. (5) The patient then undergoes the nuclear medicine study approximately 1 hour after FDG administration. (6) The CT and PET data then are reconstructed and registered, with the CT data used for attenuation correction of the reconstructed radionuclide tomograms. (7) The images are reviewed by a physician who can view the CT scan, the radionuclide images, and the fused x-ray/radionuclide data, followed by preparation of the associated clinical report.

Figure 8. Illustration of standard PET/CT scanning protocol. The patient is positioned on a common specially designed patient table in front of the combined mechanical gantry. First, a topogram is used to define the co-axial imaging range (orange). The spiral CT scan (grey box) preceded the PET scan (green box). The CT data are reconstructed on-line and used for the purpose of attenuation correction of the corresponding emission data (blue box). Black and blue arrows indicate acquisition and data processing streams, respectively. Reprinted with permission from ref.108

Figure 8. Illustration of standard PET/CT scanning protocol. The patient is positioned on a common specially designed patient table in front of the combined mechanical gantry. First, a topogram is used to define the co-axial imaging range (orange). The spiral CT scan (grey box) preceded the PET scan (green box). The CT data are reconstructed on-line and used for the purpose of attenuation correction of the corresponding emission data (blue box). Black and blue arrows indicate acquisition and data processing streams, respectively. Reprinted with permission from ref.108

There have been multiple studies which have demonstrated the role of PET/CT especially for oncologic applications.110 For example, in a clinical study of PET/CT for the evaluation of cancer in 204 patients with 586 suspicious lesions, PET/CT provided additional information over separate interpretation of PET and CT in 99 patients (49%) with 178 sites (30%).109 Furthermore, PET/CT improved characterization of equivocal lesions as definitely benign in 10% of sites and as definitely malignant in 5% of sites. It precisely defined the anatomical location of malignant FDG uptake in 6%, and led to the retrospective lesion detection on PET or CT in 8%. As a result, PET/CT had an impact on the management of 28 patients (14%), obviated the need for further evaluation in 5 patients, guided further diagnostic procedures in 7 patients, and assisted in planning therapy in 16 patients. Figure 9 shows an example of a patient study where the combined PET/CT images provided additional information, thus impacting the characterization of abnormal FDG uptake.

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