Toward Implementing an MRI-Based PET Attenuation-Correction Method for Neurologic Studies on the MR-PET Brain Prototype

Abstract

A number of factors have to be considered for implementing an accurate attenuation correction (AC) in a combined MR-PET scanner. In this work, some of these challenges were investigated and an AC method based entirely on the MR data obtained with a single dedicated sequence was developed and used for neurological studies performed with the MR-PET human brain scanner prototype. Methods The focus was on the bone/air segmentation problem, the bone linear attenuation coefficient selection and the RF coil positioning. The impact of these factors on the PET data quantification was studied in simulations and experimental measurements performed on the combined MR-PET scanner. A novel dual-echo ultra-short echo time (DUTE) MR sequence was proposed for head imaging. Simultaneous MR-PET data were acquired and the PET images reconstructed using the proposed MR-DUTE-based AC method were compared with the PET images reconstructed using a CT-based AC. Results Our data suggest that incorrectly accounting for the bone tissue attenuation can lead to large underestimations (>20%) of the radiotracer concentration in the cortex. Assigning a linear attenuation coefficient of 0.143 or 0.151 cm−1 to bone tissue appears to give the best trade-off between bias and variability in the resulting images. Not identifying the internal air cavities introduces large overestimations (>20%) in adjacent structures. Based on these results, the segmented CT AC method was established as the “silver standard” for the segmented MR-based AC method. Particular to an integrated MR-PET scanner, ignoring the RF coil attenuation can cause large underestimations (i.e. up to 50%) in the reconstructed images. Furthermore, the coil location in the PET field of view has to be accurately known. Good quality bone/air segmentation can be performed using the DUTE data. The PET images obtained using the MR-DUTE- and CT-based AC methods compare favorably in most of the brain structures. Conclusion An MR-DUTE-based AC method was implemented considering all these factors and our preliminary results suggest that this method could potentially be as accurate as the segmented CT method and it could be used for quantitative neurological MR-PET studies.

MR image‐based synthetic CT for IMRT prostate treatment planning and CBCT image‐guided localization

“…Different methods for generating S‐CT have been proposed. For image intensity/segmentation‐based conversion methods, 7 , 8 , 9 , 10 , 11 specific MR sequences, such as the ultrashort echo time (UTE) 10 , 11 sequence, were used to achieve accurate separation of air and bony tissue for creating S‐CT. While providing promising results, they are not practical since these special sequences are not generally available in clinic.…”

The Use of Anatomical Information for Molecular Image Reconstruction Algorithms: Attenuation/Scatter Correction, Motion Compensation, and Noise Reduction

“…It was claimed that it is feasible to use discretized μ-maps clinically. Segmentation is performed on T1-weighted or T2-weighted MR images [29,85,86] or on ultrashort echo time (UTE) sequence MR images for better imaging bones with very short T 2 values [87,88]. The other is to use atlasbased methods with machine learning techniques [27,89].…”

Novel Quantitative Techniques in Hybrid (PET-MR) Imaging of Brain Tumors

“…29 A proof of principle of the use of dualecho ultra-short echo time MR imaging-based attenuation correction in brain imaging to discriminate air-filled cavities from bone on MR images was also reported. 30,31 An alternative to the image segmentation approach is the use of anatomic atlas registration for attenuation correction where the PET atlas is registered to the patient's PET and prior knowledge of the atlas' attenuation properties is used to build a patient-specific attenuation map. 32 Deformable image registration plays a key role in atlas-based attenuation correction, which may fail in situations with large deformations.…”