Rapid dual‐echo ramped hybrid encoding MR‐based attenuation correction (dRHE‐MRAC) for PET/MR

• Published in: Magnetic resonance in medicine Vol. 79; no. 6; pp. 2912 - 2922
• Main Authors: Jang, Hyungseok, Liu, Fang, Bradshaw, Tyler, McMillan, Alan B.
• Format: Journal Article
• Language: English
• Published: United States Wiley Subscription Services, Inc 01.06.2018
• Subjects: Air; Attenuation; attenuation correction; Biomarkers - metabolism; Bone and Bones - diagnostic imaging; Brain; Brain - diagnostic imaging; Coding; Computed tomography; Fluorodeoxyglucose F18 - chemistry; Glucose - metabolism; Humans; hybrid encoding; Image classification; Image processing; Image Processing, Computer-Assisted; Image quality; Image reconstruction; Image segmentation; Magnetic Resonance Imaging; Medical imaging; MRAC; Multimodal Imaging; Neuroimaging; PET/MR; Phantoms, Imaging; Photons; Positron emission; Positron emission tomography; Protons; Spatial discrimination; Spatial resolution; Tissue Distribution; Tomography; Tomography, X-Ray Computed; UTE; hybrid encoding; MRAC; PET/MR; attenuation correction; UTE
• ISSN: 0740-3194; 1522-2594; 1522-2594
• DOI: 10.1002/mrm.26953

Purpose In this study, we propose a rapid acquisition for MR‐based attenuation correction (MRAC) in positron emission tomography (PET)/MR imaging, in which an ultrashort echo time (UTE) image and an out‐of‐phase echo image are obtained within a single rapid scan (35 s) at high spatial resolution (1 mm3), which allows accurate estimation of a pseudo CT image using 4‐class tissue classification (discrete bone, discrete air, continuous fat, and continuous water). Methods In dual‐echo ramped hybrid encoding (dRHE), a UTE echo is directly followed by a second out‐of‐phase echo, in which hybrid spatial encoding combining single‐point imaging and 3‐dimensional radial frequency encoding is used to improve the quality of both images. Two‐point Dixon reconstruction is used to estimate fat‐ and water‐separated images, and UTE images are used to estimate bone. Air and bone segmentation is improved by using multiple UTE images with an advanced hybrid‐encoding scheme that allows reconstruction of multiple UTE images. To evaluate the proposed method, dRHE‐MRAC PET/MR brain imaging was performed in 10 subjects. Dice coefficients and PET reconstruction errors relative to CT‐based attenuation correction were compared with existing system MRAC approaches. Results In dRHE‐MRAC, the Dice coefficients for soft tissue, air, and bone were respectively 0.95 ± 0.01, 0.62 ± 0.06, and 0.78 ± 0.05, which was a significantly improved result compared with existing approaches. In most brain regions, dRHE‐MRAC showed significantly reduced PET error (less than 1%) with P values less than 0.05. Conclusions Dual‐echo ramped hybrid encoding enables rapid and robust imaging for MRAC with a very rapid acquisition. Magn Reson Med 79:2912–2922, 2018. © 2017 International Society for Magnetic Resonance in Medicine.