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The feasibility of small animal imaging using a clinical positron emission tomography/computed tomography (PET/CT) scanner with [F-18]-fluoro-2-deoxy-D-glucose (FDG) was evaluated. As tumor-bearing small animal models, rabbits with VX-2 liver tumors, rats with mammary tumors on the back, and mice with LS174T human colon tumor xenografts were prepared. Two-dimensional PET, CT, and fused PET/CT images were obtained and reconstructed with a combined PET/CT system using a conventional protocol for humans and dedicated high-resolution mode protocols specialized for each species. Estimated radioactivity concentrations in tumors and normal organs determined noninvasively on FDG-PET/CT were compared with the actual tissue radioactivity levels determined from gamma-counting after vivisection in rats. In addition, recovery-corrected radioactivity concentrations were calculated and evaluated using the tumor/normal organ sizes measured on CT. Tumors in rabbits and rats were clearly visualized by FDG-PET/CT in the dedicated protocols, and images were considered suitable for research purposes. With the aid of thin-slice CT-mapping images, FDG uptake was correctly localized in the viable tumor regions. In mice, increased FDG uptake in tumors with varying activity levels was observed, but detailed anatomical information was not optimally provided from the images, even using specialized protocols. The estimated radioactivity concentrations of tumors and normal organs were close to the actual radioactivity concentrations obtained by gamma-counting (r = 0.97, P < 0.001, the estimated/actual slope: 1) when recovery correction was applied using the sample sizes measured on CT. FDG-PET/CT imaging with a modern clinical scanner was demonstrated to be feasible, of excellent quality, and quite quantitatively accurate for research in rabbits or rats with tumors of appropriate size (>2 cm without recovery correction and >1 cm with recovery correction). Evaluation of FDG uptake within a tumor was possible with the aid of CT images. Dedicated small animal PET/CT scanner would be better suited for evaluating tumor-bearing mice and likely could enhance imaging smaller tumors in rabbits or rats. Although it has limitations, small animal imaging with a clinical PET/CT scanner may be quite adequate for sequential noninvasive imaging in oncology research because the CT is of high resolution, allowing for localization of PET findings and for more precise noninvasive estimation of radioactivity concentration through partial volume corrections.

Purpose: To investigate the effects of intravenous contrast agents on quantitative values obtained with a combined positron emission tomographic (PET) and computed tomographic (CT) scanner by using several phantoms and a dog., Materials and Methods: Fluorine 18 fluorodeoxyglucose (FDG) was mixed with different concentrations of contrast agent with the same syringe (phantom 1), and the phantom was scanned. After image reconstruction with various attenuation maps, radioactivity concentrations were compared. Then, FDG solutions with (phantom 2) or surrounded by (phantom 3) various concentrations of contrast agent were scanned repeatedly, and radioactivity concentration was compared. Finally, PET and CT with and without contrast agent were performed in a dog. PET images were reconstructed by using different attenuation maps, and radioactivity concentrations were compared. The radioactivity concentration on germanium 68 (68Ge)-based corrected images was regarded as standard, and percentage bias, defined as difference divided by measured activity of 68Ge-based corrected images, was assessed. The relationship between the concentration of contrast agent and the percentage bias was assessed with the Pearson coefficient r, and the significance of correlations was evaluated with the Fisher z test., Results: All phantom studies demonstrated that presence of a contrast agent resulted in overestimation of emission data. CT numbers showed a strong positive correlation with the percentage bias in phantoms 2 (r = 0.999) and 3 (r = 0.987); the maximum percentage bias at 1,360 HU reached approximately 45%. These effects were independent of FDG concentration. In a canine model, presence of a contrast agent also increased emission activity, but the percentage bias was less than 15% in the liver and smaller in all other organs except the kidney (26%)., Conclusion: High concentrations of a contrast agent caused considerable overestimation of apparent tracer activity in phantom studies; however, the emission bias was relatively modest in vivo, except in areas with very high contrast agent concentrations.

Although PET/CT scanners have the potential for precise fused registration of structures visualized on both PET and CT, physiological motion during the acquisition of both studies may alter the appearance of organ shape, size or location. The degree of possible mismatch in abdominal organ size and position between PET and CT has not been evaluated. The aim of this study was to assess the consistency in location and measured size of upper abdominal organs with PET and CT using a combined PET/CT system. Forty-six consecutive inpatients who underwent clinical PET/CT scans for suspected cancer were evaluated. CT and PET images attenuation corrected by both CT and germanium-68 transmission scans were obtained, and we separately determined the location of the top and bottom (height), anterior and posterior margins (thickness), and right and left margins (width) for each organ, including liver, spleen, and bilateral kidneys, using CT and both sets of PET images. Differences between the two modalities in terms of location and measured organ size were investigated. In the upper margin of the liver and lower margin of the spleen, more than 10% of the cases showed a larger discrepancy (>20 mm) between CT-based and Ge-corrected PET-based measurements, although the differences in the positions of the edges were less than 10 mm in most cases. The center of the liver tended to be located cephalad and to the right of the body, and that of the spleen tended to be cephalad and posterior on PET, as compared with CT. Moreover, the center of both kidneys tended to be seen cephalad, posterior, and to the right on PET. The liver appeared slightly larger on PET than CT in thickness (CT vs CT-corrected PET vs Ge-corrected PET = 156 mm vs 162 mm vs 162 mm) and width (186 mm vs 189 mm vs 188 mm). By contrast, the spleen appeared slightly smaller on PET than CT in height (84 mm vs 77 mm vs 80 mm) and width (85 mm vs 81 mm vs 80 mm). A similar tendency was observed in the left kidney (105 mm vs 100 mm vs 99 mm in height, and 64 mm vs 59 mm vs 58 mm in width) and the right kidney (99 mm vs 93 mm vs 93 mm in height, and 64 mm vs 59 mm vs 60 mm in width). These differences between the two modalities were statistically significant ( P<0.05). In conclusion, minor mismatches in location and organ size were found to exist between CT and PET images, in part due to physiological motion. Although these differences could potentially affect the quality of the image registrations, they were generally of a modest nature.

Unlabelled: The aims of the study were to evaluate the effects of oral contrast on apparent tracer activity measured with PET/CT when using CT attenuation correction and to report our initial experience in the use of oral contrast with PET/CT., Methods: Phantom studies with (18)F activity and saline bags or syringes filled with barium or gastrografin of varying densities were performed using a PET/CT scanner (CT attenuation correction). In the study, 91 clinical patients received dilute oral contrast and were evaluated by whole-body (18)F-FDG PET., Results: A phantom experiment with CT contrast (1.3% weight/volume [w/v] barium) showed a "cold" area in the cold stomach whereas a phantom with high-density barium (98% w/v) showed an artifactual focus of intense "activity" in the cold stomach. In clinical studies, stomach and right colon were opacified by CT contrast. Maximal measured contrast density was 239 Hounsfield units., Conclusion: High-density barium causes overestimation of tissue (18)F-FDG concentration. Low-density barium does not cause significant artifacts and appears suitable for clinical use.

Unlabelled: This study evaluated lesion mislocalization between PET and CT on PET/CT studies when CT instead of germanium is used for attenuation correction (AC)., Methods: PET/CT scans were obtained for 300 clinical patients. Both CT and germanium scans were used to correct PET emission data. Cases were noted of suspected inaccurate localization of lesions on any of the 5 sets of images (PET using germanium AC [GeAC] fused and not fused with CT, PET using CT AC fused and not fused with CT, and PET with no AC [NAC]). Independent CT or MRI was used to determine true lesion locations., Results: Six of 300 patients (2%) had lesion mislocalization when CT was used for AC or fusion. True liver dome lesions were mislocalized to the right lung base on PET/CT, likely because of a respiratory motion difference between PET and CT. No mislocalization was present on NAC PET or non-CT-fused GeAC PET images., Conclusion: Serious lesion mislocalization on PET/CT studies may occur, albeit very infrequently, when CT is used for either AC or fusion.

Purpose: Iterative reconstruction (IR) is a statistical reconstruction method that may be influenced by high background activity such as in the liver. Recently developed combined positron emission tomography (PET) and computerized tomography (CT) instrumentation utilizes CT attenuation correction that may also differ compared to 68Germanium (68Ge) segmented attenuation correction (IR SAC). Quantitative differences could affect the clinical interpretation of metastatic foci and subsequent response to therapy. The purpose of this study is to characterize potential quantitative differences specifically in hepatic metastases., Procedures: Thirty-three metastatic liver lesions in 23 patients were evaluated. Whole-body 2-deoxy-2[18F]fluoro-D-glucose(FDG) PET images were obtained on a dedicated PET-CT device and reconstructed with IR and CT measured attenuation correction (IR CT MAC), IR and 68Ge segmented attenuation correction, and also with filtered back-projection and segmented attenuation correction (FBP SAC). Regions of interest over the liver lesions (L) and liver background (B) were drawn on FBP SAC images and superimposed on the co-registered IR CT MAC and IR SAC images for comparison. To identify the individual effects of IR and CT attenuation correction, IR SAC was first compared to FBP SAC, followed by a comparison of IR CT MAC and IR SAC. Differences were expressed as bias (%) and compared to FBP SAC., Results: Compared to FBP SAC, IR SAC showed significantly lower liver lesion (mean bias -7.1%; P < 0.00001), significantly higher liver background (mean bias -3.5%; P = 0.005), and significantly lower L/B ratio (mean bias -10.1%; P < 0.00001). Compared to IR SAC, the IR CT MAC showed small but not statistically significant increases in liver lesion (mean bias 3.0%; P = 0.1) and liver background (mean bias 4.3%; P = 0.09), and no significant difference in L/B ratio (mean bias -1.3%; P = 0.5). The overall effect of IR CT MAC compared to FBP SAC was a significant decrease in liver lesion (mean bias -4.2%; P = 0.002), significant increase in liver background (mean bias 8.7%; P = 0.002), and significantly lower L/B (mean bias -11.1%; P < 0.00001) compared to FBP SAC., Conclusion: Compared to FBP, IR resulted in significantly lower mean hepatic tumor activity, higher mean liver background activity, and lower L/B activity, but these differences were modest. These differences were similarly seen in the CT corrected IR CT MAC when compared to FBP SAC. CT attenuation correction did not result in significant differences when compared to 68Ge correction. Although the differences in IR CT MAC were modest, consistent reconstruction methods are important to decrease the measurement variability and improve reproducibility.

Unlabelled: In PET, transmission scanning for attenuation correction has most commonly been performed with an external positron-emitting radionuclide source, such as (68)Ge. More recently, combined PET/CT scanners have been developed in which the CT data can be used for both anatometabolic image formation and attenuation correction of the PET data. The purpose of this study was to assess the quantitative differences between CT-based and germanium-based attenuation-corrected PET images., Methods: Twenty-eight patients with known or suspected cancer underwent whole-body (18)F-FDG PET/CT scanning for clinical diagnostic purposes. For each patient, attenuation maps were obtained from both the CT scan and the (68)Ge transmission data, and 2 different attenuation-corrected emission datasets were produced. Measured activity concentrations (both mean and maximum) from identical regions of interest in representative normal organs and in 36 pathologic foci of uptake were compared., Results: CT-corrected emission images generally showed slightly higher radioactive concentration values than did germanium-corrected images (P < 0.01) for all lesions and all normal organs except the lung. Mean and maximum radioactivity concentrations were 4.3%-15.2% higher for CT-corrected images than for germanium-corrected images. Calculated radioactivity concentrations were significantly greater in osseous lesions than in nonosseous lesions (11.0% vs. 2.3%, P < 0.05, for mean value; 11.1% vs. 2.1%, P < 0.01, for maximum value). A weak positive correlation was observed between the CT Hounsfield units within the regions of interest and the percentage difference in apparent tracer activity in the CT-corrected images., Conclusion: Although quantitative radioactivity values are generally comparable between CT- and germanium-corrected emission PET images, CT-based attenuation correction produced radioactivity concentration values significantly higher than the germanium-based corrected values. These effects, especially in radiodense tissues, should be noted when using and comparing quantitative PET analyses from PET and PET/CT systems.