Initial experience of MR/PET in a clinical cancer center.

Magentic Resonance/positron emission tomography (PET) has been introduced recently for imaging of clinical patients. This hybrid imaging technology combines the inherent strengths of MRI with its high soft-tissue contrast and biological sequences with the inherent strengths of PET, enabling imaging of metabolism with a high sensitivity. In this article, we describe the initial experience of MR/PET in a clinical cancer center along with a review of the literature. For establishing MR/PET in a clinical setting, technical challenges, such as attenuation correction and organizational challenges, such as workflow and reimbursement, have to be overcome. The most promising initial results of MR/PET have been achieved in anatomical areas where high soft-tissue and contrast resolution is of benefit. Head and neck cancer and pelvic imaging are potential applications of this hybrid imaging technology. In the pediatric population, MR/PET can decrease the lifetime radiation dose. MR/PET protocols tailored to different types of malignancies need to be developed. After the initial exploration phase, large multicenter trials are warranted to determine clinical indications for this exciting hybrid imaging technology and thereby opening new horizons in molecular imaging.

PET imaging of hepatocellular carcinoma with 18F-fluoroethylcholine and 11C-choline.

PURPOSE: Choline-based radiotracers have been studied for PET imaging of hepatocellular carcinoma (HCC). Using an (18)F-labeled choline analog, instead of the (11)C-labeled native choline, would facilitate its widespread use in the clinic. In this study, PET with (18)F-fluoroethylcholine (FEC) and (11)C-choline (CHOL) were compared using an animal model of HCC. The effects of fasting on the performance of choline-based tracers were also investigated. METHODS: A woodchuck model of HCC was used to compare the two tracers, which were administered and imaged in sequence during the same imaging session. Dynamic PET images were generated spanning 50 min starting from tracer injection. Time-activity curves and tracer contrast were calculated in liver regions with tracer accumulation, and the contrast at a late time-point with the two tracers, and between fasted and nonfasted states, were compared. RESULTS: Foci of HCC with increased uptake ranged in size from 1.0 to 1.6 cm, with mean tumor-to-background contrast of 1.3 with FEC and 1.5 with CHOL at 50 min after injection. The tracers show similar patterns of uptake immediately following administration, and both activities plateaued at 10 min after injection. No significant differences in uptake dynamics or final contrast were observed between the fasted and nonfasted states. CONCLUSION: PET imaging of HCC is possible with both CHOL and FEC. Fasting was not found to affect accumulation of either tracer. These results encourage further investigation into the clinical utility of FEC for HCC imaging.

The impact of orthopedic metal artifact reduction software on interreader variability when delineating areas of interest in the head and neck.

PURPOSE: Metal artifacts during computed tomography (CT) hinder the evaluation of diagnostic images and impair the delineation of tumor volume in treatment planning. Several solutions are available to minimize these artifacts. Our objective was to determine the impact of one of those tools on the interreader variability when measuring head and neck structures in the presence of metal artifacts. METHODS AND MATERIALS: Eleven patients were retrospectively selected from an institutional review board-approved study based on the presence of metallic artifacts in the head and neck region. CT raw data were postprocessed using a metal artifact reduction tool. A single matching CT slice from the filtered backprojection and postprocessed data sets was selected in the region of the metal artifact. Areas of selected anatomical structures were measured by independent readers, including an anatomical structure selected from a CT slice with no metal artifact in each patient as control. The intraclass correlation coefficient was calculated. RESULTS: Two extreme outliers were identified and the intraclass correlation coefficient was performed with and without them. The intraclass correlation on filtered backprojection, postprocessed, and control images was 0.903, 0.948, and 0.985 with outliers and 0.884, 0.971, and 0.989 without outliers, respectively, for all readers. On the other hand, the intraclass correlation on filtered backprojection, postprocessed, and control images for experienced readers was 0.904, 0.979, and 0.976 with outliers and 0.934, 0.975, and 0.990 without outliers, respectively. CONCLUSIONS: The interreader variability of areas measured in the presence of metal artifact was greatly decreased by the use of the metal artifact reduction tool and almost matched the variability observed in the absence of the metal artifact.

Performance evaluation of the Ingenuity TF PET/CT scanner with a focus on high count-rate conditions.

This study evaluated the positron emission tomography (PET) imaging performance of the Ingenuity TF 128 PET/computed tomography (CT) scanner which has a PET component that was designed to support a wider radioactivity range than is possible with those of Gemini TF PET/CT and Ingenuity TF PET/MR. Spatial resolution, sensitivity, count rate characteristics and image quality were evaluated according to the NEMA NU 2-2007 standard and ACR phantom accreditation procedures; these were supplemented by additional measurements intended to characterize the system under conditions that would be encountered during quantitative cardiac imaging with (82)Rb. Image quality was evaluated using a hot spheres phantom, and various contrast recovery and noise measurements were made from replicated images. Timing and energy resolution, dead time, and the linearity of the image activity concentration, were all measured over a wide range of count rates. Spatial resolution (4.8-5.1 mm FWHM), sensitivity (7.3 cps kBq(-1)), peak noise-equivalent count rate (124 kcps), and peak trues rate (365 kcps) were similar to those of the Gemini TF PET/CT. Contrast recovery was higher with a 2 mm, body-detail reconstruction than with a 4 mm, body reconstruction, although the precision was reduced. The noise equivalent count rate peak was broad (within 10% of peak from 241-609 MBq). The activity measured in phantom images was within 10% of the true activity for count rates up to those observed in (82)Rb cardiac PET studies.

Optimized dynamic framing for PET-based myocardial blood flow estimation.

An optimal experiment design methodology was developed to select the framing schedule to be used in dynamic positron emission tomography (PET) for estimation of myocardial blood flow using (82)Rb. A compartment model and an arterial input function based on measured data were used to calculate a D-optimality criterion for a wide range of candidate framing schedules. To validate the optimality calculation, noisy time-activity curves were simulated, from which parameter values were estimated using an efficient and robust decomposition of the estimation problem. D-optimized schedules improved estimate precision compared to non-optimized schedules, including previously published schedules. To assess robustness, a range of physiologic conditions were simulated. Schedules that were optimal for one condition were nearly-optimal for others. The effect of infusion duration was investigated. Optimality was better for shorter than for longer tracer infusion durations, with the optimal schedule for the shortest infusion duration being nearly optimal for other durations. Together this suggests that a framing schedule optimized for one set of conditions will also work well for others and it is not necessary to use different schedules for different infusion durations or for rest and stress studies. The method for optimizing schedules is general and could be applied in other dynamic PET imaging studies.

Imaging lipid synthesis in hepatocellular carcinoma with [methyl-11c]choline: correlation with in vivo metabolic studies.

UNLABELLED: PET with [methyl-(11)C]-choline (11C-choline) can be useful for detecting well-differentiated hepatocellular carcinoma (HCC) that is not 18F-FDG-avid. This study was designed to examine the relationship between choline metabolism and choline tracer uptake in HCC for PET with 11C-choline. METHODS: Dynamic PET scans of 11C-choline were acquired using the woodchuck models of HCC. After imaging, [methyl-(14)C]-choline was injected, and metabolites from both HCC tissue samples and the surrounding hepatic tissues were extracted and analyzed by radio-high-performance liquid chromatography. The enzymatic activities of choline kinase and choline-phosphate cytidylyltransferase were assayed for correlation with the imaging and metabolism data. RESULTS: PET with 11C-choline showed an HCC detection rate of 9 of 10. The tumor-to-liver ratio for the 9 detected HCCs was 1.89±0.55. Hematoxylin-eosin staining confirmed that all spots with high tracer uptake were well-differentiated HCCs. Variation of radioactivity distribution within HCCs indicated a heterogeneous uptake of choline. The activities of both choline kinase and choline-phosphate cytidylyltransferase were found to be significantly higher in HCC than in the surrounding hepatic tissues. The major metabolites of 11C-choline were phosphocholine in HCC and betaine and choline in the surrounding hepatic tissues at 12 min after injection; in HCC, phosphocholine rapidly converted to phosphatidylcholine at 30 min after injection. CONCLUSION: HCCs display enhanced uptake of radiolabeled choline despite a moderate degree of physiologic uptake in the surrounding hepatic tissues. Initially, increased radiolabeled choline uptake in HCCs is associated with the transport and phosphorylation of choline; as time passes, the increased uptake of radiolabeled choline reflects increased phosphatidylcholine synthesis derived from radiolabeled cytidine 5'-diphosphocholine (CDP-choline) in HCCs. In contrast, the surrounding hepatic tissues exhibit extensive oxidation of radiolabeled choline via the phosphatidylethanolamine methylation pathway, a major contributor to the observed physiologic uptake.

[(Methyl)1-(11)c]-acetate metabolism in hepatocellular carcinoma.

PURPOSE: Studies have established the value of [(methyl)1-(11)C]-acetate ([(11)C]Act) combined with 2-deoxy-2[(18)F]fluoro-D-glucose (FDG) for detecting hepatocellular carcinoma (HCC) using positron emission tomography (PET). In this study, the metabolic fate of [(11)C]Act in HCC was characterized. METHODS: Experiments with acetic acid [1-(14)C] sodium salt ([(14)C]Act) were carried out on WCH-17 cells and freshly derived rat hepatocytes. PET scans with [(11)C]Act were also carried out on woodchucks with HCC before injection of [(14)C]Act. The radioactivity levels in different metabolites were quantified with thin-layer chromatography. RESULTS: In WCH-17 cells, the predominant metabolite was phosphatidylcholine (PC). Regions of HCCs with the highest [(11)C]Act uptake had higher radioactivity accumulation in lipid-soluble compounds than surrounding hepatic tissues. In those regions, PC and triacylglycerol (TG) accumulated more radioactivity than in surrounding hepatic tissues. CONCLUSIONS: High [(11)C]Act uptake in HCC is associated with increased de novo lipogenesis. PC and TG are the main metabolites into which the radioactive label from [(11)C]Act is incorporated in HCC.