MRI-Assisted PET Motion Correction for Neurologic Studies in an Integrated MR-PET Scanner

Abstract

Head motion is difficult to avoid in long PET studies, degrading the image quality and offsetting the benefit of using a high-resolution scanner. As a potential solution in an integrated MR-PET scanner, the simultaneously acquired MR data can be used for motion tracking. In this work, a novel data processing and rigid-body motion correction (MC) algorithm for the MR-compatible BrainPET prototype scanner is described and proof-of-principle phantom and human studies are presented. Methods To account for motion, the PET prompts and randoms coincidences as well as the sensitivity data are processed in the line or response (LOR) space according to the MR-derived motion estimates. After sinogram space rebinning, the corrected data are summed and the motion corrected PET volume is reconstructed from these sinograms and the attenuation and scatter sinograms in the reference position. The accuracy of the MC algorithm was first tested using a Hoffman phantom. Next, human volunteer studies were performed and motion estimates were obtained using two high temporal resolution MR-based motion tracking techniques. Results After accounting for the physical mismatch between the two scanners, perfectly co-registered MR and PET volumes are reproducibly obtained. The MR output gates inserted in to the PET list-mode allow the temporal correlation of the two data sets within 0.2 s. The Hoffman phantom volume reconstructed processing the PET data in the LOR space was similar to the one obtained processing the data using the standard methods and applying the MC in the image space, demonstrating the quantitative accuracy of the novel MC algorithm. In human volunteer studies, motion estimates were obtained from echo planar imaging and cloverleaf navigator sequences every 3 seconds and 20 ms, respectively. Substantially improved PET images with excellent delineation of specific brain structures were obtained after applying the MC using these MR-based estimates. Conclusion A novel MR-based MC algorithm was developed for the integrated MR-PET scanner. High temporal resolution MR-derived motion estimates (obtained while simultaneously acquiring anatomical or functional MR data) can be used for PET MC. An MR-based MC has the potential to improve PET as a quantitative method, increasing its reliability and reproducibility which could benefit a large number of neurological applications.

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

“…Motion estimation from gated 4D CT was considered in [35,36], but radiation dose and motion mismatch due to sequential acquisition of CT and PET (or SPECT) were major concerns. Recent advances in simultaneous PET-MR showed potentials to estimate accurate motion from highresolution MR images without additional radiation dose and achieved substantial quantitative improvements in PET imaging [37][38][39][40]. Figure 2 shows a motion-corrected image reconstruction example using simultaneous PET-MR. A balloon phantom with attached radioactive spheres was in the gel with background activity and the motion of the balloon mimics a cardiac motion with a 1-s period.…”

“…More recently, using the fact that molecular and anatomical images are affected by the same motion, motion correction for emission tomography has been investigated. Motion information from high-resolution anatomical information such as CT [35,36] or MR [37][38][39][40] was incorporated into reconstruction for motion-corrected PET or SPECT. Ideas of using structural couplings between molecular and anatomical images for reconstruction have been studied a couple of decades ago [41][42][43].…”

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

“…Tsoumpas and colleagues 48 studied the potential of using MR-derived motion fields to correct nonrigid motion in PET and showed that combined PET-MR acquisitions could potentially allow motion compensation in whole-body PET acquisitions without prolonging acquisition time or increasing radiation dose. In neurologic simultaneous PET-MR studies, Catana and colleagues 49 showed, using 3-dimensional Hoffman brain phantom and human volunteer studies, that high temporal-resolution MR imaging-derived motion estimates acquired simultaneously on the hybrid brain PET-MR scanner can be used to improve PET image quality, therefore increasing its reliability, reproducibility, and quantitative accuracy. Imaging in vivo primates, Chun and colleagues 47 have recently shown that tagged MR imagingbased motion correction in simultaneous PET-MR significantly improves lesion detection compared with respiratory gating and no motion correction while reducing radiation dose.…”

Kinetic Modelling in Human Brain Imaging

“…Some groups are currently investigating how one can use tracking devices (e.g. Montgomery et al (2006)) or, in case of the new MRI-PET scanners, simultaneous MRI scanning (Catana et al (2011)) to continuously monitor any movement and to correct the list mode using these measurements. This is still work in progress but the initial results look promising.…”