Abstract
Positron emission tomography/magnetic resonance imaging (PET/MRI) potentially offers several advantages over positron emission tomography/computed tomography (PET/CT), for example, no CT radiation dose and soft tissue images from MR acquired at the same time as the PET. However, obtaining accurate linear attenuation correction (LAC) factors for the lung remains difficult in PET/MRI. LACs depend on electron density and in the lung, these vary significantly both within an individual and from person to person. Current commercial practice is to use a single‐valued population‐based lung LAC, and better estimation is needed to improve quantification. Given the under‐appreciation of lung attenuation estimation as an issue, the inaccuracy of PET quantification due to the use of single‐valued lung LACs, the unique challenges of lung estimation, and the emerging status of PET/MRI scanners in lung disease, a review is timely. This paper highlights past and present methods, categorizing them into segmentation, atlas/mapping, and emission‐based schemes. Potential strategies for future developments are also presented.
Highlights
Sequential acquisitions.[1,2] Using MRI can circumvent artifacts from the iodinated contrast agents potentially used in
A toroidal transmission source has been demonstrated for evaluation of attenuation coefficient measures,[12] the limited space within the scanner bore and the need to measure over the whole-lung volume prevent use of transmission sources during clinical imaging
This review has identified the key challenges associated with determining lung attenuation maps
Summary
Sequential acquisitions.[1,2] Using MRI can circumvent artifacts from the iodinated contrast agents potentially used in. Alternatives include trilinear scaling,[8] the quadratic polynomial calibration curve,[9] and (virtual-) dual energy CT methods.[10,11] a toroidal transmission source has been demonstrated for evaluation of attenuation coefficient measures,[12] the limited space within the scanner bore and the need to measure over the whole-lung volume prevent use of transmission sources during clinical imaging This view is supported by the work of Bowen et al.[13] who stated that their developed transmission scan method is not for routine subject imaging, and by the study of Mollet et al.[14] who noted that the reduced diameter of the PET/MR system would affect the PET count rate performance of the simultaneous emission/ transmission scan. Given the under-appreciation of lung attenuation estimation as an issue, the inaccuracy of PET quantification due to the use of single-valued lung LACs, the unique challenges of lung estimation (later stated in Section 3), and the emerging status of PET/MRI scanners in both nodular and diffuse lung disease, this review seeks to outline technologies for improved lung LAC determination. The use of non-FDG radionuclides is reported where information is available
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