MR-Based Attenuation Correction Methods for Improved PET Quantification in Lesions Within Bone and Susceptibility Artifact Regions

Abstract

Hybrid PET/MR systems have recently entered clinical practice. Thus, the accuracy of MR-based attenuation correction in simultaneously acquired data can now be investigated. We assessed the accuracy of 4 methods of MR-based attenuation correction in lesions within soft tissue, bone, and MR susceptibility artifacts: 2 segmentation-based methods (SEG1, provided by the manufacturer, and SEG2, a method with atlas-based susceptibility artifact correction); an atlas-and pattern recognition-based method (AT&PR), which also used artifact correction; and a new method combining AT&PR and SEG2 (SEG2wBONE). Methods: Attenuation maps were calculated for the PET/MR datasets of 10 patients acquired on a whole-body PET/MR system, allowing for simultaneous acquisition of PET and MR data. Eighty percent iso-contour volumes of interest were placed on lesions in soft tissue (n 5 21), in bone (n 5 20), near bone (n 5 19), and within or near MR susceptibility artifacts (n 5 9). Relative mean volume-of-interest differences were calculated with CT-based attenuation correction as a reference. Results: For soft-tissue lesions, none of the methods revealed a significant difference in PET standardized uptake value relative to CT-based attenuation correction (SEG1, 22.6% 6 5.8%; SEG2, 21.6% 6 4.9%; AT&PR, 24.7% 6 6.5%; SEG2wBONE, 0.2% 6 5.3%). For bone lesions, underestimation of PET standardized uptake values was found for all methods, with minimized error for the atlas-based approaches (SEG1, 216.1% 6 9.7%; SEG2, 211.0% 6 6.7%; AT&PR, 26.6% 6 5.0%; SEG2wBONE, 24.7% 6 4.4%). For lesions near bone, underestimations of lower magnitude were observed (SEG1, 212.0% 6 7.4%; SEG2, 29.2% 6 6.5%; AT&PR, 24.6% 6 7.8%; SEG2wBONE, 24.2% 6 6.2%). For lesions affected by MR susceptibility artifacts, quantification errors could be reduced using the atlas-based artifact correction (SEG1, 254.0% 6 38.4%; SEG2, 215.0% 6 12.2%; AT&PR, 24.1% 6 11.2%; SEG2w-BONE, 0.6% 6 11.1%). Conclusion: For soft-tissue lesions, none of the evaluated methods showed statistically significant errors. For bone lesions, significant underestimations of 216% and 211% occurred for methods in which bone tissue was ignored (SEG1 and SEG2). In the present attenuation correction schemes, uncorrected MR susceptibility artifacts typically result in reduced attenuation values, potentially leading to highly reduced PET standardized uptake values, rendering lesions indistinguishable from background. While AT&PR and SEG2wBONE show accurate results in both soft tissue and bone, SEG2wBONE uses a two-step approach for tissue classification, which increases the robustness of prediction and can be applied retrospectively if more precision in bone areas is needed. Hybr id PET/MR systems have recently been introduced into clinical practice and are attracting increasing interest in the research and clinical communities (1-3). While PET/CT is an established hybrid modality with a wide range of applications, specific application trends for PET/MR are currently under investigation (4-7). MR-based atten...

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

“…Since bone tissue generates a void signal when using common MR sequences, it is indistinguishable from air. As such, bony structures are commonly replaced by soft-tissue in current methods, thus leading to significant underestimation of tracer uptake in the vicinity of bony structures ( Bezrukov et al, 2013 ;Hofmann et al, 2011 ).…”

“…Basically two categories have emerged: atlas-guided attenuation map generation approaches ( Hofmann et al, 2011 ;Bezrukov et al, 2013 ;Arabi and Zaidi, 2016a ;Marshall et al, 2013 ;Arabi and Zaidi, 2016b ) and emission-based approaches ( Rezaei et al, 2012 ;Mehranian and Zaidi, 2015 ). Atlas-guided methods primarily rely on prior information provided by registration of an atlas into target image coordinates to allow classification of bone tissues.…”

“…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.…”

Feasibility of using respiration‐averaged MR images for attenuation correction of cardiac PET/MR imaging

“…In brain PET, ignoring bone has been suggested to cause quantification bias (29) . In whole‐body PET/MR imaging, however, neglecting bone in segmented attenuation images has been suggested to cause large errors only in regions that are inside or near bones 30 , 31 , 32 . In one example demonstrated by Samarin et al, (32) classifying bone as soft tissue resulted in less than 6% difference for PET voxels in the heart region.…”

PET/MR attenuation correction: where have we come from and where are we going?

“…In an attempt to improve overall robustness, a modified version of this same approach including atlasbased artefact detection has recently been applied to wholebody imaging and has led to mean activity underestimations of <6 % [12]. Variants of this approach consider the use of multiple MR sequences to improve the identification of different tissues classes and hence to improve the overall atlas registration process and the subsequent pseudo-CT prediction model [13,14].…”