GantryMate: A Modular MR-Compatible Assistance System for MR-Guided Needle Interventions.

Percutaneous minimally invasive interventions are difficult to perform in closed-bore high-field magnetic resonance systems owing to the limited space between magnet and patient. To enable magnetic resonance-guided needle interventions, we combine a small, patient-mounted assistance system with a real-time instrument tracking sequence based on a phase-only cross-correlation algorithm for marker detection. The assistance system uses 2 movable plates to align an external passive marker with the anatomical target structure. The targeting accuracy is measured in phantom experiments, yielding a precision of 1.7 ± 1.0 mm for target depths up to 38 ± 13 mm. In in vivo experiments, the possibility to track and target static and moving structures is demonstrated.

3D CMRO2 mapping in human brain with direct 17O MRI: Comparison of conventional and proton-constrained reconstructions.

Oxygen metabolism is altered in brain tumor regions and is quantified by the cerebral metabolic rate of oxygen consumption (CMRO2). Direct dynamic 17O MRI with inhalation of isotopically enriched 17O2 gas can be used to quantify CMRO2; however, pixel-wise CMRO2 quantification in human brain is challenging due to low natural abundance of 17O isotope and, thus, the low signal-to-noise ratio (SNR) of 17O MR images. To test the feasibility CMRO2 mapping at a clinical 3 T MRI system, a new iterative reconstruction was proposed, which uses the edge information contained in a co-registered 1H gradient image to construct a non-homogeneous anisotropic diffusion (AD) filter. AD-constrained reconstruction of 17O MR images was compared to conventional Kaiser-Bessel gridding without and with Hanning filtering, and to iterative reconstruction with a total variation (TV) constraint. For numerical brain phantom and in two in vivo data sets of one healthy volunteer, AD-constrained reconstruction provided 17O images with improved resolution of fine brain structures and resulted in higher SNR. CMRO2 values of 0.78 - 1.55µmol/gtissue/min (white brain matter) and 1.03 - 2.01µmol/gtissue/min (gray brain matter) as well as the CMRO2 maps are in a good agreement with the results of 15O-PET and 17O MRI at 7 T and at 9.4 T. In conclusion, the proposed AD-constrained reconstruction enabled calculation of 3D CMRO2 maps at 3 T MRI system, which is an essential step towards clinical translation of 17O MRI for non-invasive CMRO2 quantification in tumor patients.