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...

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.

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.

The combination of magnetic resonance imaging (MR) and positron emission tomography (PET) scanners can provide a powerful tool for clinical diagnosis and investigation. Among the challenges of developing a combined scanner, obtaining attenuation maps for PET reconstruction is of critical importance. This requires accounting for the presence of MR hardware in the field of view. The attenuation introduced by this hardware cannot be obtained from MR data. We propose the creation of attenuation models of MR hardware, to be registered into the MR-based attenuation map prior to PET reconstruction. Two steps were followed to assess the viability of this method. First, transmission and emission measurements were performed on MR components (RF coils and medical probes). The severity of the artifacts in the reconstructed PET images was evaluated. Secondly, a high-exposure computed tomography (CT) scan was used to obtain a model of a head coil. This model was registered into the attenuation map of PET/CT scans of a uniform phantom fitted with the coil. The resulting PET images were compared to the PET/CT reconstruction in the absence of coils. The artifacts introduced by misregistration of the model were studied. The transmission scans revealed 17% count loss due to the presence of head and neck coils in the field of view. Important sources of attenuation were found in the lock, signal cables and connectors. However, the worst source of attenuation was the casing between both coils. None of the measured medical probes introduced a significant amount of attenuation. Concerning the attenuation model of the head coil, reconstructed PET images with model-based correction were comparable to the reference PET/CT reconstruction. However, inaccuracies greater than 1-2 mm in the axial positioning of the model led to important artifacts. In conclusion, the results show that model-based attenuation correction is possible. Using a high-exposure scan to create an attenuation model of the coils has been proved feasible. However, adequate registration of the model is mandatory.

Abstract-We present a novel method for joint reconstruction of both image and motion in positron-emission-tomography (PET). Most other methods separate image from motion estimation: They use deformable image registration/optical flow techniques in order to estimate the motion from individually reconstructed gates. Then, the image is estimated based on this motion information. With these methods, a main problem lies in the motion estimation step, which is based on the noisy gated frames. The more noise is present, the more inaccurate the image registration becomes.As we show in a simulation study, our joint reconstruction approach overcomes these drawbacks and results in both visually and quantitatively better image quality. We attribute these results to the fact that for motion estimation always the currently best available image estimate is used and vice versa. Additionally, results for real dual respiratory and cardiac gated patient data are presented.

Dual gated cardiac PET studies were performed successfully and showed better resolved myocardial walls as compared with ungated acquisitions. The respiratory motion of the heart presented a significant elastic component and was of the same magnitude as the spatial resolution of current PET cameras.

Learning Objectives: On successful completion of this activity, participants should be able to describe (1) special requirements for planning whole-body PET/MRI examinations; (2) general aspects of whole-body PET/MRI protocols in oncology; and (3) possible artifacts in PET/MRI.Financial Disclosure: An author of this article is a meeting participant or lecturer for Siemens AG. No other relevant relationships that could be perceived as a real or apparent conflict of interest were reported. CME Credit: SNMMI is accredited by the Accreditation Council for Continuing Medical Education (ACCME) to sponsor continuing education for physicians. SNMMI designates each JNM continuing education article for a maximum of 2.0 AMA PRA Category 1 Credit. Physicians should claim only credit commensurate with the extent of their participation in the activity. For CE credit, participants can access this activity through the SNMMI Web site (http:// www.snmmi.org/ce_online) through September 2013.Integrated PET/MRI systems open exciting possibilities for clinical and research applications. However, compared with PET/CT, PET/MRI is a complex technique resulting in new problems and challenges, especially regarding workflow, scan protocols, and data analysis. This complexity applies in particular to examinations in oncology with partial-or whole-body coverage extending over several bed positions. Unlike diagnostic PET/CT, for which the clinical CT protocols can largely be copied from stand-alone CT, the design of a diagnostic MRI protocol for partial-or whole-body coverage is more complex and has to be adapted to the special requirements of PET/MRI to be both time-efficient and comprehensive. Here, we describe basic considerations concerning workflow, imaging protocols, and image analysis for whole-body PET/MRI in oncology, based on our experience with the first integrated PET/MRI scanner. The aim is to fully and optimally make use of the combined PET/MRI measurements in oncology, including identifying and reducing image artifacts as well as optimizing workflow beyond the mere fusion of 2 image datasets. Thesuccessofcombi ned PET and CT has demonstrated the clinical value of multimodality imaging technology, providing both anatomic and molecular information within a single imaging session. However, whereas CT and especially modern multislice CT are a powerful imaging tool, they still have shortcomings, such as low soft-tissue contrast when compared with MRI. Thus, the combination of PET and MRI might be clinically advantageous over PET/ CT for some indications. Moreover, because MRI has more potential for functional and molecular imaging than does CT, the combination of PET with MRI offers completely new opportunities for research applications and for molecular imaging in general.As a consequence, a lot of effort has been put into the development of combined PET and MRI. The first preclinical combined PET/MRI systems were introduced in the 1990s (1-3). Clinically, a PET insert for a standard 3.0-T MRI scanner for simultaneous PET/MRI of t...

MR FOV restrictions can indeed make the reconstructed PET data unacceptable for diagnostic purposes. Biases can be globally compensated by automatic preprocessing of the attenuation map. However, inaccuracies in the correction will result in small artifacts near the periphery of the image that could lead to false-positive findings.