Category: Liver

Kinetic-guided image reconstruction

Posted on by Michele ScipioniLeave a Comment on Kinetic-guided image reconstruction

In the brain, kinetic modeling is frequently employed to study neuroreceptor properties and behavior. The binding potential is one such property, reflecting the density of available receptors and their affinity for the PET ligand. It can be measured by fitting a kinetic model to the PET time-activity curve with the additional input of activity concentration in arterial blood. When arterial blood is not collected as part of the study design, the kinetic model may instead be fit using an image-derived reference region that is devoid of specific binding.

However, the kinetic model selected can introduce bias into the measurements of interest. In the context of a behavioral-task study of dopamine release in healthy volunteers, we have shown that the use of MRTM2 (one of the most common reference tissue models) introduces bias into estimates change in binding potential. (Levine 2019 – abstract) 

Relevant publications

Clinical performance of motion correction for upper abdominal PET/DCE-MRI

Posted on by Michele ScipioniLeave a Comment on Clinical performance of motion correction for upper abdominal PET/DCE-MRI

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.

After developing a novel self-navigated method that simultaneously corrects both PET data and DCE-MRI data for respiratory motion without increasing acquisition times, we focused on investigating its clinical performance and to compare the motion-corrected (MoCo) and uncorrected (non-MoCo) PET, MRI and fused PET/MRI data (Catalano 2018).

In this study, the quality of MoCo PET images was found to be higher than that of non-MoCo PET images. The improved quantitation allowed by MoCo PET might be clinically relevant. In fact, besides the contribution of PET quantitation to discriminating benign from malignant lesions, assessment of treatment response relies on measured differences in SUVmax and MTV along with possible complete visual disappearance of metabolically active lesions.

Furthermore, MoCo DCE-MRI might improve the overall quality of the data in patients unable to follow breathing instructions (deaf patients, patients with a different first language, or patients unable to hear the voice of the technologist while gradients are on), and in those with hemodynamic compromise in whom timing of arterial and portal venous phase imaging is challenging. The higher quality provided by fused PET/MRI data after MoCo has important clinical implications, as in the case of small but metabolically active lesions whose anatomic correlate might be difficult to identify. This is especially true when metabolic PET data are improperly fused over the MR anatomic overlay.

Relevant publications

Motion correction for abdominal imaging

Posted on by Michele ScipioniLeave a Comment on 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).

Siemens Biograph mMR

Posted on by Michele ScipioniLeave a Comment on 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.

https://www.siemens-healthineers.com/en-us/magnetic-resonance-imaging/mr-pet-scanner/biograph-mmr

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