In this study, we demonstrate the proof of principle of the University Medical Center Utrecht (UMCU) robot dedicated to magnetic resonance imaging (MRI)-guided interventions in patients. The UMCU robot consists of polymers and non-ferromagnetic materials. For transperineal prostate interventions, it can be placed between the patient's legs inside a closed bore 1.5T MR scanner. The robot can manually be translated and rotated resulting in five degrees of freedom. It contains a pneumatically driven tapping device to automatically insert a needle stepwise into the prostate using a controller unit outside the scanning room. To define the target positions and to verify the needle insertion point and the needle trajectory, a high-resolution 3D balanced steady state free precession (bSSFP) scan that provides a T2/T1-weighted contrast is acquired. During the needle insertion fast 2D bSSFP images are generated to track the needle on-line. When the target position is reached, the radiation oncologist manually places a fiducial gold marker (small seed) at this location. In total two needle trajectories are used to place all markers. Afterwards, a high-resolution 3D bSSFP scan is acquired to visualize the fiducial gold markers. Four fiducial gold markers were placed transperineally into the prostate of a patient with a clinical stage T3 prostate cancer. In the generated scans, it was possible to discriminate the patient's anatomy, the needle and the markers. All markers were delivered inside the prostate. The procedure time was 1.5 h. This study proves that MRI-guided needle placement and seed delivery in the prostate with the UMCU robot are feasible.

ObjectivesTo demonstrate the feasibility of MRI-based assessment of the intrahepatic Ho-PLLA-MS biodistribution after radioembolisation in order to estimate the absorbed radiation dose.MethodsFifteen patients were treated with holmium-166 (166Ho) poly(L-lactic acid)-loaded microspheres (Ho-PLLA-MS, mean 484 mg; range 408–593 mg) in a phase I study. Multi-echo gradient-echo MR images were acquired from which R2* maps were constructed. The amount of Ho-PLLA-MS in the liver was determined by using the relaxivity r2* of the Ho-PLLA-MS and compared with the administered amount. Quantitative single photon emission computed tomography (SPECT) was used for comparison with MRI regarding the whole liver absorbed radiation dose.ResultsR2* maps visualised the deposition of Ho-PLLA-MS with great detail. The mean total amount of Ho-PLLA-MS detected in the liver based on MRI was 431 mg (range 236–666 mg) or 89 ± 19 % of the delivered amount (correlation coefficient r = 0.7; P < 0.01). A good correlation was found between the whole liver mean absorbed radiation dose as assessed by MRI and SPECT (correlation coefficient r = 0.927; P < 0.001).ConclusionMRI-based dosimetry for holmium-166 radioembolisation is feasible. Biodistribution is visualised with great detail and quantitative measurements are possible.Key Points• Radioembolisation is increasingly used for treating unresectable primary or metastatic liver tumours.• MRI-based intrahepatic microsphere biodistribution assessment is feasible after holmium-166 radioembolisation.• MRI enables quantification of holmium-166 microspheres in liver in a short imaging time.• MRI can estimate the whole liver absorbed radiation dose following holmium-166 radioembolisation.

In this article, we present a 3D imaging technique, applying center-out RAdial Sampling with Off-Resonance reception, to accurately depict and localize small paramagnetic objects with high positive contrast while suppressing long T 2 * components. The center-out RAdial Sampling with Off-Resonance reception imaging technique is a fully frequency-encoded 3D ultrashort echo time acquisition method, which uses a large excitation bandwidth and off-resonance reception. By manually introducing an offset, Df 0 , to the central reception frequency (f 0 ), the typical radial signal pileup observed in 3D center-out sampling caused by a dipolar magnetic field disturbance can be shifted toward the source of the field disturbance, resulting in a hyperintense signal at the magnetic center of the small paramagnetic object. This was demonstrated both theoretically and using 1D time domain simulations. Experimental verification was done in a gel phantom and in inhomogeneous porcine tissue containing various objects with very different geometry and susceptibility, namely, subvoxel stainless steel spheres, a puncture needle, and paramagnetic brachytherapy seeds. In all cases, centerout RAdial Sampling with Off-Resonance reception was shown to generate high positive contrast exactly at the location of the paramagnetic object, as was confirmed by X-ray computed tomography. Magn Reson Med 65:146-156,