Motion correction for abdominal imaging

Respiratory motion correction for abdominal PET-MRI studies

PET and MRI are two powerful imaging technologies that are characterized by high sensitivity and the ability to provide superior anatomic detail, respectively, which might make them ideal for evaluating the upper abdomen. However, PET requires long acquisition times, including the acquisition of data from moving organs, which may result in image blurring. On the other hand, MRI, especially standard DCE-MRI, can scan the chosen field of view in a shorter time, but requires the patient’s cooperation with the respiratory instructions and the ability to suspend respiration for the acquisition time of breath-hold sequences, usually in the range 14–20 s. Moreover, even in patients with an adequate respiratory breath-hold ability, the quality and the diagnostic information of DCE-MRI are also dependent on the hemodynamics of the patient and the timing of contrast agent injection and data acquisition. These variables explain the occurrence of respiratory artifacts and erroneous phases of contrast enhancement imaging in DCE-MRI.

We presented and evaluated in vivo a comprehensive approach for self-gated MR motion modeling applied to concurrent respiratory motion compensation of PET and DCE-MRI data acquired simultaneously in an integrated PET/MR system.

Fully registered, motion-corrected PET images and diagnostic DCE-MR images were obtained with negligible acquisition time prolongation compared with standard breath-hold techniques. Both the MR and the PET image quality and tracer uptake quantification were improved when compared with conventional methods (Fuin 2018).

Comparison of PET images reconstructed before and after motion correction using motion vector fields obtained from 1- or 6-minutes of MR data

This approach was subsequently evaluated clinically in collaboration with Dr. Onofrio Catalano to demonstrate that motion-corrected PET/MRI produced better PET images and reduced the spatial mismatch between the two modalities (Catalano 2018).

PET-based motion correction

When MR-derived motion estimates are unavailable, either in the gaps between sequences or when a given MR sequence is not amenable to motion estimation, motion can also be derived directly from the PET images themselves. Motion estimates from both the MR and the PET can be unified to provide continuous estimates of head motion throughout the duration of the scan, leveraging the higher resolution and more accurate MR-based estimates when they are available. (Levine 2017 Abstract)

Relevant publications

Transmission imaging for attenuation correction

Transmission imaging for validation of MR-based attenuation correction methods​

While transmission-based techniques are still considered the true gold standard for PET attenuation correction, traditional rotating transmission sources have not been integrated into the PET/MRI scanners due to obvious engineering challenges and the desire to reduce the radiation exposure. As such techniques would be valuable for improving and validating MR-based approaches, several stationary transmission sources have been suggested as alternatives.

In the case of scanners without time-of-flight capabilities (e.g. Biograph mMR), we showed that a single torus source filled with [18F]FDG allows the acquisition of highly accurate transmission images before radiotracer administration (Bowen et al 2016)

Attenuation correction in presence of metal implants

Attenuation correction in the presence of metallic implants

The numerous foreign objects (e.g. dental implants, surgical clips and wires, orthopedic screws and plates, prosthetic devices, etc.) that can be present in the subject lead to susceptibility artifacts in the MR images that propagate as signal voids in the corresponding attenuation maps. 

We developed a method to estimate the location, shape, and linear attenuation coefficient of the implant using a joint reconstruction of the activity and attenuation algorithm. The implant PET-based attenuation map completion (IPAC) method performs a join reconstruction of radioactivity and attenuation from the emission data (Fuin et al 2017). 

MR-, CT- and IPAC-based attenuation map estimation

Siemens Biograph mMR

The Biograph mMR scanner (Siemens Healthineers, Erlangen, Germany) consists of a 3T whole-body superconductive magnet with active shielding and external interference shielding and a whole-body PET scanner. It is equipped with a gradient system with a maximum gradient amplitude of 45 mT/m and a maximal slew rate of 200 T/m/s.  Separate cooling channels that simultaneously cool primary and secondary coils allow the application of extremely gradient intensive techniques. 

This scanner is equipped with the “TIM” RF coils that were custom designed to minimize the 511 keV photons attenuation. The fully-integrated PET detectors use avalanche photodiode (APD) technology and LSO scintillator crystals (eight rings with 56 detectors blocks per ring, each consisting of 8×8 arrays of 4×4×20 mm3crystals read out by a 3×3 array of APDs).  The PET scanner’s transaxial and axial fields of view are 594 mm and 25.8 cm, respectively.

The Biograph mMR was installed at the Martinos Center in June 2011.