MRI-Based Attenuation Correction for PET/MRI Using Ultrashort Echo Time Sequences

Abstract

One of the challenges in PET/MRI is the derivation of an attenuation map to correct the PET image for attenuation. Different methods have been suggested for deriving the attenuation map from an MR image. Because the low signal intensity of cortical bone on images acquired with conventional MRI sequences makes it difficult to detect this tissue type, these methods rely on some sort of anatomic precondition to predict the attenuation map, raising the question of whether these methods will be usable in the clinic when patients may exhibit anatomic abnormalities. Methods: We propose the use of the transverse relaxation rate, derived from images acquired with an ultrashort echo time sequence to classify the voxels into 1 of 3 tissue classes (bone, soft tissue, or air), without making any assumptions on patient anatomy. Each voxel is assigned a linear attenuation coefficient corresponding to its tissue class. A reference CT scan is used to determine the voxel-by-voxel accuracy of the proposed method. The overall accuracy of the MRI-based attenuation correction is evaluated using a method that takes into account the nonlocal effects of attenuation correction. Results: As a proof of concept, the head of a pig was used as a phantom for imaging. The new method yielded a correct tissue classification in 90% of the voxels. Five human brain PET/CT and MRI datasets were also processed, yielding slightly worse voxel-by-voxel performance, compared to a CT-derived attenuation map. The PET datasets were reconstructed using the segmented MRI attenuation map derived with the new method, and the resulting images were compared with segmented CT-based attenuation correction. An average error of around 5% was found in the brain. Conclusion: The feasibility of using the transverse relaxation rate map derived from ultrashort echo time MR images for the estimation of the attenuation map was shown on phantom and clinical brain data. The results indicate that the new method, compared with CTbased attenuation correction, yields clinically acceptable errors. The proposed method does not make any assumptions about patient anatomy and could therefore also be used in cases in which anatomic abnormalities are present. A new generation of medical imaging scanners, which will combine the high sensitivity of functional imaging by PET with the wide range of applications of MRI scanners, is currently being developed (1-4). Combined PET/MRI offers several advantages over PET/CT, of which the following relate to the particular strength of the MRI method. First, it is possible to acquire anatomic images with better soft-tissue contrast than CT. Second, the radiation dose to the patient is significantly smaller. Third, other applications such as spectroscopy or molecular imaging are also possible with MRI. A PET/MRI scanner will also be a useful research tool, for example, enabling researchers to validate molecular MRI protocols against PET, which can be considered the current gold standard in molecular imaging.One of the difficulties in the design of a m...

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