Zero TE‐based pseudo‐CT image conversion in the head and its application in PET/MR attenuation correction and MR‐guided radiation therapy planning

• Published in: Magnetic resonance in medicine Vol. 80; no. 4; pp. 1440 - 1451
• Main Authors: Wiesinger, Florian, Bylund, Mikael, Yang, Jaewon, Kaushik, Sandeep, Shanbhag, Dattesh, Ahn, Sangtae, Jonsson, Joakim H., Lundman, Josef A., Hope, Thomas, Nyholm, Tufve, Larson, Peder, Cozzini, Cristina
• Format: Journal Article
• Language: English
• Published: United States Wiley Subscription Services, Inc 01.10.2018
• Subjects: Aged; Attenuation; attenuation correction; Bone density; Bone imaging; Brain Neoplasms - diagnostic imaging; Brain Neoplasms - radiotherapy; Computed tomography; Conversion; Female; Head; Humans; Image acquisition; Image Processing, Computer-Assisted - methods; Magnetic resonance imaging; Magnetic Resonance Imaging - methods; Male; Mapping; Medical imaging; Middle Aged; Multimodal Imaging; Neck; Patients; PET/MR; Phantoms, Imaging; Positron emission; Positron-Emission Tomography - methods; Proton density (concentration); pseudo-CT; Radiation; Radiation therapy; radiation therapy planning; Radiotherapy Planning, Computer-Assisted - methods; RTP; RUFIS; synthetic CT; Tomography; ZTE; radiation therapy planning; synthetic CT; PET/MR; attenuation correction; RUFIS; pseudo-CT; ZTE; RTP
• ISSN: 0740-3194; 1522-2594; 1522-2594
• DOI: 10.1002/mrm.27134

Purpose To describe a method for converting Zero TE (ZTE) MR images into X‐ray attenuation information in the form of pseudo‐CT images and demonstrate its performance for (1) attenuation correction (AC) in PET/MR and (2) dose planning in MR‐guided radiation therapy planning (RTP). Methods Proton density‐weighted ZTE images were acquired as input for MR‐based pseudo‐CT conversion, providing (1) efficient capture of short‐lived bone signals, (2) flat soft‐tissue contrast, and (3) fast and robust 3D MR imaging. After bias correction and normalization, the images were segmented into bone, soft‐tissue, and air by means of thresholding and morphological refinements. Fixed Hounsfield replacement values were assigned for air (‐1000 HU) and soft‐tissue (+42 HU), whereas continuous linear mapping was used for bone. Results The obtained ZTE‐derived pseudo‐CT images accurately resembled the true CT images (i.e., Dice coefficient for bone overlap of 0.73 ± 0.08 and mean absolute error of 123 ± 25 HU evaluated over the whole head, including errors from residual registration mismatches in the neck and mouth regions). The linear bone mapping accounted for bone density variations. Averaged across five patients, ZTE‐based AC demonstrated a PET error of ‐0.04 ± 1.68% relative to CT‐based AC. Similarly, for RTP assessed in eight patients, the absolute dose difference over the target volume was found to be 0.23 ± 0.42%. Conclusion The described method enables MR to pseudo‐CT image conversion for the head in an accurate, robust, and fast manner without relying on anatomical prior knowledge. Potential applications include PET/MR‐AC, and MR‐guided RTP.