Evaluation of the attenuation properties of MR equipment for its use in a whole-body PET/MR scanner

Abstract

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.

PET/MR Imaging in the Detection and Characterization of Pulmonary Lesions: Technical and Diagnostic Evaluation in Comparison to PET/CT

“…The lungs were identified by connected-component analysis of the air in the inner part of the body. By application of a morphologic closing filter, virtual air artifacts induced by the absence of an MR imaging signal in cortical bone, heart, and aorta (because of blood flow) were corrected (12).…”

“…Moreover, the techniques for attenuation correction of PET data in PET/MR imaging are different from those in PET/CT. However, quantitative analysis of tracer uptake using the standardized uptake value (SUV) is essential for correct analysis of PET tracer distribution (9)(10)(11)(12)(13). Currently, in the commercially available whole-body fully integrated PET/MR imaging system (Biograph mMR; Siemens Medical Solutions) only a 2-point Dixon-based MR sequence is approved by the Food and Drug Administration for attenuation correction.…”

MRI-Based Attenuation Correction for Whole-Body PET/MRI: Quantitative Evaluation of Segmentation- and Atlas-Based Methods

“…Therefore, in whole-body applications, AC methods that do not account for bone may be sufficient for a wide variety of clinical applications (10). In addition to generating a suitable map from the MR images, the attenuation of MRI radiofrequency coils and ancillary devices must also be accounted for (11,12).…”

PET/MR — a rapidly growing technique of imaging in oncology and neurology

“…An alternative approach proposed for PET data attenuation correction in PET/MR is the one based on an atlas (map) [21]. It should be noted that the proposed correction techniques apply only to attenuation generated by the patient, and not to that caused by scanner equipment components, such as the bed on which the patient lies and the massive MR coils [22,23]. For that reason equipment attenuation maps are generated, which are stored in the system and included with human attenuation maps.…”