Whole-Body PET/MR Imaging: Quantitative Evaluation of a Novel Model-Based MR Attenuation Correction Method Including Bone

Abstract

In routine whole-body PET/MR hybrid imaging, attenuation correction (AC) is usually performed by segmentation methods based on a Dixon MR sequence providing up to 4 different tissue classes. Because of the lack of bone information with the Dixon-based MR sequence, bone is currently considered as soft tissue. Thus, the aim of this study was to evaluate a novel model-based AC method that considers bone in whole-body PET/MR imaging. Methods The new method (“Model”) is based on a regular 4-compartment segmentation from a Dixon sequence (“Dixon”). Bone information is added using a model-based bone segmentation algorithm, which includes a set of prealigned MR image and bone mask pairs for each major body bone individually. Model was quantitatively evaluated on 20 patients who underwent whole-body PET/MR imaging. As a standard of reference, CT-based μ-maps were generated for each patient individually by nonrigid registration to the MR images based on PET/CT data. This step allowed for a quantitative comparison of all μ-maps based on a single PET emission raw dataset of the PET/MR system. Volumes of interest were drawn on normal tissue, soft-tissue lesions, and bone lesions; standardized uptake values were quantitatively compared. Results In soft-tissue regions with background uptake, the average bias of SUVs in background volumes of interest was 2.4% ± 2.5% and 2.7% ± 2.7% for Dixon and Model, respectively, compared with CT-based AC. For bony tissue, the −25.5% ± 7.9% underestimation observed with Dixon was reduced to −4.9% ± 6.7% with Model. In bone lesions, the average underestimation was −7.4% ± 5.3% and −2.9% ± 5.8% for Dixon and Model, respectively. For soft-tissue lesions, the biases were 5.1% ± 5.1% for Dixon and 5.2% ± 5.2% for Model. Conclusion The novel MR-based AC method for whole-body PET/MR imaging, combining Dixon-based soft-tissue segmentation and model-based bone estimation, improves PET quantification in whole-body hybrid PET/MR imaging, especially in bony tissue and nearby soft tissue.

Hybrid Imaging: Instrumentation and Data Processing

“…At present, standard AC approaches for whole-body imaging in clinical systems rely on segmentation-based methods [218,219], recently also in combination with atlas-based approaches incorporating the major bone structures into the AC [220,221]. In general, these approaches perform reasonably well, although for specific application their accuracy may not be sufficient [222].…”

Comparison of atlas-based techniques for whole-body bone segmentation

“…Various strategies were proposed to incorporate bone tissue in PET/MRI attenuation maps in whole-body imaging ( Hofmann et al, 2011 ;Bezrukov et al, 2015 ;Ay et al, 2014 ;Arabi and Zaidi, 2016a ;Bezrukov et al, 2013 ;Marshall et al, 2013 ;Paulus et al, 2015 ). In whole-body imaging, almost all proposed methods, except joint attenuation-activity reconstruction techniques, rely on prior knowledge present in atlas images to predict bone from MRI.…”

“…It should be noted that this trend holds for nonselective atlas fusion schemes since an increased number of atlases would increase the likelihood of finding more similar cases to the target image, for instance in local weighting atlas fusion schemes which leads to a asymptotically rising curve. In some studies, only one atlas image is used to carry out the segmentation procedure, which involves a single online registration procedure ( Greer et al, 2011 ;Paulus et al, 2015 ). Marshall et al (2013 ) proposed to select the most similar subject for PET/MRI attenuation correction on the basis of available metadata, such as sex and age and some imagederived features, such as body volume, lung volume, etc.…”

The Effect of Susceptibility Artifacts Related to Metallic Implants on Adjacent-Lesion Assessment in Simultaneous TOF PET/MR

“…Moreover, external positron sources for transmission scanning can be used (28,29), as can the background radiation from the 176 Lu present in Lu 2 Sio 5 :Ce scintillators (15,30). Other solutions might include, for example, whole-body AC models with bone tissue and possibly implants (31). MR sequences such as zero echo time allow the imaging of bone tissue and improve the AC maps (32)(33)(34).…”