Attenuation correction (AC) of whole-body PET data in combined PET/MRI tomographs is expected to be a technical challenge. In this study, a potential solution based on a segmented attenuation map is proposed and evaluated in clinical PET/CT cases. Methods: Segmentation of the attenuation map into 4 classes (background, lungs, fat, and soft tissue) was hypothesized to be sufficient for AC purposes. The segmentation was applied to CT-based attenuation maps from 18 F-FDG PET/CT oncologic examinations of 35 patients with 52 18 F-FDG-avid lesions in the lungs (n 5 15), bones (n 5 21), and neck (n 5 16). The standardized uptake values (SUVs) of the lesions were determined from PET images reconstructed with nonsegmented and segmented attenuation maps, and an experienced observer interpreted both PET images with no knowledge of the attenuation map status. The feasibility of the method was also evaluated with 2 patients who underwent both PET/CT and MRI. Results: The use of a segmented attenuation map resulted in average SUV changes of 8% 6 3% (mean 6 SD) for bone lesions, 4% 6 2% for neck lesions, and 2% 6 3% for lung lesions. The largest SUV change was 13.1%, for a lesion in the pelvic bone. There were no differences in the clinical interpretations made by the experienced observer with both types of attenuation maps. Conclusion: A segmented attenuation map with 4 classes derived from CT data had only a small effect on the SUVs of 18 F-FDGavid lesions and did not change the interpretation for any patient. This approach appears to be practical and valid for MRI-based AC.Key Words: instrumentation; PET/CT; PET/MRI; attenuation correction J Nucl Med 2009; 50:520-526 DOI: 10.2967/jnumed.108.054726 Int he same way in which PET/CT has been shown to be a powerful multimodality imaging tool, there are compelling reasons for combining PET and MRI. PET/MRI would have the following advantages: improved soft-tissue contrast; the possibility of performing truly simultaneous instead of sequential acquisitions; and the availability of sophisticated MRI sequences, such as diffusion and perfusion imaging, functional MRI, and MR spectroscopy, which can add important information. Moreover, the use of PET/MRI would result in a significant decrease in radiation exposure, which is of foremost importance for serial follow-up and pediatric imaging.Thus, a combined PET/MRI scanner would provide an alternative to a combined PET/CT scanner for whole-body oncologic imaging (1,2); improved accuracy could be achieved in the detection, staging, and characterization of several cancers (3-10). Moreover, the combination of PETand MRI is perfectly suited to neurologic imaging and offers new possibilities for cardiovascular imaging (11,12). Consequently, much research effort is being directed toward the development of combined imaging devices, and the initial results are promising (13-18).However, a still-unsolved technical challenge for combined whole-body PET/MRI is the correction of attenuation and scatter in the PET data (19). For this purpose, an...

These results compare favorably with other state-of-the-art PET/CT and PET/MR scanners, indicating that the integration of the PET detectors in the MR scanner and their operation within the magnetic field do not have a perceptible impact on the overall performance. The MR subsystem performs essentially like a standalone system. However, further work is necessary to evaluate the more advanced MR applications, such as functional imaging and spectroscopy.

The recently introduced first integrated whole-body PET/MR scanner allows simultaneous acquisition of PET and MRI data in humans and, thus, may offer new opportunities, particularly regarding diagnostics in oncology. This scanner features major technologic differences from conventional PET/CT devices, including the replacement of photomultipliers with avalanche photodiodes and the need for MRI-based attenuation correction. The aim of this study was to evaluate the comparability of clinical performance between conventional PET/CT and PET/MR in patients with oncologic diseases. Methods: Thirty-two patients with different oncologic diagnoses underwent a single-injection, dual-imaging protocol consisting of a PET/CT and subsequent PET/MR scan. PET/CT scans were performed according to standard clinical protocols (86 6 8 min after injection of 401 6 42 MBq of 18 F-FDG, 2 min/ bed position). Subsequently (140 6 24 min after injection), PET/ MR was performed (4 min/bed position). PET images of both modalities were reconstructed iteratively. Attenuation and scatter correction as well as regional allocation of PET findings were performed using low-dose CT data for PET/CT and Dixon MRI sequences for PET/MR. PET/MR and PET/CT were compared visually by 2 teams of observers by rating the number and location of lesions suspicious for malignancy, as well as image quality and alignment. For quantitative comparison, standardized uptake values (SUVs) of the detected lesions and of different tissue types were assessed. Results: Simultaneous PET/MR acquisition was feasible with high quality in short acquisition time (#20 min). No significant difference was found between the numbers of suspicious lesions (n 5 80) or lesion-positive patients (n 5 20) detected with PET/MR or PET/CT. Anatomic allocation of PET/ MR findings by means of the Dixon MRI sequence was comparable to allocation of PET/CT findings by means of low-dose CT. Quantitative evaluation revealed a high correlation between mean SUVs measured with PET/MR and PET/CT in lesions (r = 0.93) and background tissue (r = 0.92). Conclusion: This study demonstrates, for what is to our knowledge the first time, that integrated whole-body PET/MR is feasible in a clinical setting with high quality and in a short examination time. The reliability of PET/ MR was comparable to that of PET/CT in allowing the detection of hypermetabolic lesions suspicious for malignancy in patients with oncologic diagnoses. Despite different attenuation correction approaches, tracer uptake in lesions and background correlated well between PET/MR and PET/CT. The Dixon MRI sequences acquired for attenuation correction were found useful for anatomic allocation of PET findings obtained by PET/MR in the entire body. These encouraging results may form the foundation for future studies aiming to define the added value of PET/MR over PET/CT.

All of the proposed novel methods have an average global performance within likely acceptable limits (±5% of CT-based reference), and the main difference among the methods was found in the robustness, outlier analysis, and clinical feasibility. Overall, the best performing methods were MR-ACBOSTON, MR-ACMAXPROB, MR-ACRESOLUTE and MR-ACUCL, ordered alphabetically. These methods all minimized the number of outliers, standard deviation, and average global and local error. The methods MR-ACMUNICH and MR-ACCAR-RiDR were both within acceptable quantitative limits, so these methods should be considered if processing time is a factor. The method MR-ACSEGBONE also demonstrates promising results, and performs well within the likely acceptable quantitative limits. For clinical routine scans where processing time can be a key factor, this vendor-provided solution currently outperforms most methods. With the performance of the methods presented here, it may be concluded that the challenge of improving the accuracy of MR-AC in adult brains with normal anatomy has been solved to a quantitatively acceptable degree, which is smaller than the quantification reproducibility in PET imaging.

PET allows for quantitative, regional myocardial perfusion imaging. The short half-lives of the perfusion tracers currently in use limit their clinical applicability. Here, the biodistribution and imaging quality of a new 18 F-labeled myocardial perfusion agent ( 18 F-BMS-747158-02) in an animal model are described. Methods: The biodistribution of 18 F-BMS-747158-02 was determined at 10 and 60 min after injection. The first-pass extraction fraction of the tracer was measured in isolated rat hearts perfused with the Langendorff method. Small-animal PET imaging was used to study tracer retention. Results: The biodistribution at 10 min after injection demonstrated high myocardial uptake (3.1 percentage injected dose per gram [%ID/g]) accompanied by little activity in the lungs (0.3 %ID/g) and liver (1.0 %ID/g). The tracer showed a high and flow-independent myocardial first-pass extraction fraction, averaging 0.94 (SD 5 0.04). PET imaging provided excellent delineation of myocardial structures. The heartto-lung activity ratio increased from 4.7 to 10.2 between 1 and 15 min after tracer injection (at rest). Adenosine infusion (140 mg/ kg/min) led to a significant increase in myocardial tracer retention (from 1.68 [SD 5 0.23]) s 21 to 3.21 [SD 5 0.92] s 21 ; P 5 0.03). Conclusion: The observation of a high and flow-independent first-pass extraction fraction promises linearity between tracer uptake and myocardial blood flow. Sustained myocardial tracer uptake, combined with high image contrast, will allow for imaging protocols with tracer injection at peak exercise followed by delayed imaging. Thus, 18 F-BMS-747158-02 is a promising new tracer for the quantitative imaging of myocardial perfusion and can be distributed to imaging laboratories without a cyclotron.

In view of the commercial success of integrated PET/CT scanners, there is an increasing interest in comparable SPECT/CT systems. SPECT in combination with CT enables a direct correlation of anatomic information and functional information, resulting in better localization and definition of scintigraphic findings. Besides anatomic referencing, the added value of CT coregistration is based on the attenuation correction capabilities of CT. The number of clinical studies is limited, but pilot studies have indicated a higher specificity and a significant reduction in indeterminate findings. The superiority of SPECT/CT over planar imaging or SPECT has been demonstrated in bone scintigraphy, somatostatin receptor scintigraphy, parathyroid scintigraphy, and adrenal gland scintigraphy. Also, rates of detection of sentinel nodes by biopsy can be increased with SPECT/CT. This review highlights recent technical developments in integrated SPECT/CT systems and summarizes the current literature on potential clinical uses and future directions for SPECT/CT in cardiac, neurologic, and oncologic applications.

Prostate-specific membrane antigen (PSMA)-targeted radioligand therapy is increasingly used in metastatic castration-resistant prostate cancer. We aimed to estimate the absorbed doses for normal organs and tumor lesions using 177 Lu-PSMA I&T (I&T is imaging and therapy) in patients undergoing up to 4 cycles of radioligand therapy. Results were compared with pretherapeutic Glu-NH-CO-NH-Lys-(Ahx)-[ 68 Ga(HBEDCC)] ( 68 Ga-PSMA-HBED-CC) PET. Methods: A total of 34 cycles in 18 patients were analyzed retrospectively. In 15 patients the first, in 9 the second, in 5 the third, and in 5 the fourth cycle was analyzed, respectively. Whole-body scintigraphy was performed at least between 30-120 min, 24 h, and 6-8 d after administration. Regions of interest covering the whole body, organs, and up to 4 tumor lesions were drawn. Organ and tumor masses were derived from pretherapeutic 68 Ga-PSMA-HBED-CC PET/CT. Absorbed doses for individual cycles were calculated using OLINDA/EXM. SUVs from pretherapeutic PET were compared with absorbed doses and with change of SUV. Results: The mean whole-body effective dose for all cycles was 0.06 6 0.03 Sv/GBq. The mean absorbed organ doses were 0.72 6 0.21 Gy/GBq for the kidneys; 0.12 6 0.06 Gy/GBq for the liver; and 0.55 6 0.14 Gy/GBq for the parotid, 0.64 6 0.40 Gy/ GBq for the submandibular, and 3.8 6 1.4 Gy/GBq for the lacrimal glands. Absorbed organ doses were relatively constant among the 4 different cycles. Tumor lesions received a mean absorbed dose per cycle of 3.2 6 2.6 Gy/GBq (range, 0.22-12 Gy/GBq). Doses to tumor lesions gradually decreased, with 3.5 6 2.9 Gy/GBq for the first, 3.3 6 2.5 Gy/GBq for the second, 2.7 6 2.3 Gy/GBq for the third, and 2.4 6 2.2 Gy/GBq for the fourth cycle. SUVs of pretherapeutic PET moderately correlated with absorbed dose (r 5 0.44, P , 0.001 for SUV max ; r 5 0.43, P , 0.001 for SUV mean ) and moderately correlated with the change of SUV (r 5 0.478, P , 0.001 for SUV max , and r 5 0.50, P , 0.001 for SUV mean ). Conclusion: Organ-and tumorabsorbed doses for 177 Lu-PSMA I&T are comparable to recent reports and complement these with information on an excellent correlation between the 4 therapy cycles. With the kidneys representing the critical organ, a cumulative activity of 40 GBq of 177 Lu-PSMA I&T appears to be safe and justifiable. The correlation between pretherapeutic SUV and absorbed tumor dose emphasizes the need for PSMA-ligand PET imaging for patient selection.

Dixon-based MR imaging for MR AC allows for anatomical allocation of PET-positive lesions similar to low-dose CT in conventional PET/CT. Thus, this approach appears to be useful for future MR/PET for body regions not fully covered by diagnostic MRI due to potential time constraints.