Geometrical accuracy of magnetic resonance imaging for ocular proton therapy planning.

Background & purpose: Magnetic resonance imaging (MRI) is increasingly used in treatment preparation of ocular proton therapy, but its spatial accuracy might be limited by geometric distortions due to susceptibility artefacts. A correct geometry of the MR images is paramount since it defines where the dose will be delivered. In this study, we assessed the geometrical accuracy of ocular MRI. Materials & methods: A dedicated ocular 3 T MRI protocol, with localized shimming and increased gradients, was compared to computed tomography (CT) and X-ray images in a phantom and in 15 uveal melanoma patients. The MRI protocol contained three-dimensional T2-weighted and T1-weighted sequences with an isotropic reconstruction resolution of 0.3-0.4 mm. Tantalum clips were identified by three observers and clip-clip distances were compared between T2-weighted and T1-weighted MRI, CT and X-ray images for the phantom and between MRI and X-ray images for the patients. Results: Interobserver variability was below 0.35 mm for the phantom and 0.30(T1)/0.61(T2) mm in patients. Mean absolute differences between MRI and reference were below 0.27 ± 0.16 mm and 0.32 ± 0.23 mm for the phantom and in patients, respectively. In patients, clip-clip distances were slightly larger on MRI than on X-ray images (mean difference T1: 0.11 ± 0.38 mm, T2: 0.10 ± 0.44 mm). Differences did not increase at larger distances and did not correlate to interobserver variability. Conclusions: A dedicated ocular MRI protocol can produce images of the eye with a geometrical accuracy below half the MRI acquisition voxel (<0.4 mm). Therefore, these images can be used for ocular proton therapy planning, both in the current model-based workflow and in proposed three-dimensional MR-based workflows.

Preparation-based B 1 + mapping in the heart using Bloch-Siegert shifts.

PURPOSE: To develop and evaluate a robust cardiac B 1 + $$ {\mathrm{B}}_1^{+} $$ mapping sequence at 3 T, using Bloch-Siegert shift (BSS)-based preparations. METHODS: A longitudinal magnetization preparation module was designed to encode | B 1 + | $$ \mid {\mathrm{B}}_1^{+}\mid $$ . After magnetization tip-down, off-resonant Fermi pulses, placed symmetrically around two refocusing pulses, induced BSS, followed by tipping back of the magnetization. Bloch simulations were used to optimize refocusing pulse parameters and to assess the mapping sensitivity. Relaxation-induced B 1 + $$ {\mathrm{B}}_1^{+} $$ error was simulated for various T 1 $$ {\mathrm{T}}_1 $$ / T 2 $$ {\mathrm{T}}_2 $$ times. The effective mapping range was determined in phantom experiments, and | B 1 + | $$ \mid {\mathrm{B}}_1^{+}\mid $$ maps were compared to the conventional BSS method and subadiabatic hyperbolic-secant 8 (HS8) pulse-sensitized method. Cardiac B 1 + $$ {\mathrm{B}}_1^{+} $$ maps were acquired in healthy subjects, and evaluated for repeatability and imaging plane intersection consistency. The technique was modified for three-dimensional (3D) acquisition of the whole heart in a single breath-hold, and compared to two-dimensional (2D) acquisition. RESULTS: Simulations indicate that the proposed preparation can be tailored to achieve high mapping sensitivity across various B 1 + $$ {\mathrm{B}}_1^{+} $$ ranges, with maximum sensitivity at the upper B 1 + $$ {\mathrm{B}}_1^{+} $$ range. T 1 $$ {\mathrm{T}}_1 $$ / T 2 $$ {\mathrm{T}}_2 $$ -induced bias did not exceed 5.2 % $$ \% $$ . Experimentally reproduced B 1 + $$ {\mathrm{B}}_1^{+} $$ sensitization closely matched simulations for B 1 + ≥ 0 . 3 B 1 , max + $$ {\mathrm{B}}_1^{+}\ge 0.3{\mathrm{B}}_{1,\max}^{+} $$ (mean difference 0.031 ± $$ \pm $$ 0.022, compared to 0.018 ± $$ \pm $$ 0.025 in the HS8-sensitized method), and showed 20-fold reduction in the standard deviation of repeated scans, compared with conventional BSS B 1 + $$ {\mathrm{B}}_1^{+} $$ mapping, and an equivalent 2-fold reduction compared with HS8-sensitization. Robust cardiac B 1 + $$ {\mathrm{B}}_1^{+} $$ map quality was obtained, with an average test-retest variability of 0.027 ± $$ \pm $$ 0.043 relative to normalized B 1 + $$ {\mathrm{B}}_1^{+} $$ magnitude, and plane intersection bias of 0.052 ± $$ \pm $$ 0.031. 3D acquisitions showed good agreement with 2D scans (mean absolute deviation 0.055 ± $$ \pm $$ 0.061). CONCLUSION: BSS-based preparations enable robust and tailorable 2D/3D cardiac B 1 + $$ {\mathrm{B}}_1^{+} $$ mapping at 3 T in a single breath-hold.

Generalized inhomogeneity-resilient relaxation along a fictitious field (girRAFF) for improved robustness in rotating frame relaxometry at 3T.

PURPOSE: To optimize Relaxation along a Fictitious Field (RAFF) pulses for rotating frame relaxometry with improved robustness in the presence of B 0 $$ {\mathrm{B}}_0 $$ and B 1 + $$ {\mathrm{B}}_1^{+} $$ field inhomogeneities. METHODS: The resilience of RAFF pulses against B 0 $$ {\mathrm{B}}_0 $$ and B 1 + $$ {\mathrm{B}}_1^{+} $$ inhomogeneities was studied using Bloch simulations. A parameterized extension of the RAFF formulation was introduced and used to derive a generalized inhomogeneity-resilient RAFF (girRAFF) pulse. RAFF and girRAFF preparation efficiency, defined as the ratio of the longitudinal magnetization before and after the preparation ( M z ( T p ) / M 0 $$ {M}_z\left({T}_p\right)/{M}_0 $$ ), were simulated and validated in phantom experiments. T RAFF $$ {T}_{\mathrm{RAFF}} $$ and T girRAFF $$ {T}_{\mathrm{girRAFF}} $$ parametric maps were acquired at 3T in phantom, the calf muscle, and the knee cartilage of healthy subjects. The relaxation time maps were analyzed for resilience against artificially induced field inhomogeneities and assessed in terms of in vivo reproducibility. RESULTS: Optimized girRAFF preparations yielded improved preparation efficiency (0.95/0.91 simulations/phantom) with respect to RAFF (0.36/0.67 simulations/phantom). T girRAFF $$ {T}_{\mathrm{girRAFF}} $$ preparations showed in phantom/calf 6.0/4.8 times higher resilience to B 0 $$ {\mathrm{B}}_0 $$ inhomogeneities than RAFF, and a 4.7/5.3 improved resilience to B 1 + $$ {\mathrm{B}}_1^{+} $$ inhomogeneities. In the knee cartilage, T girRAFF $$ {T}_{\mathrm{girRAFF}} $$ (53 ± $$ \pm $$ 14 ms) was higher than T RAFF $$ {T}_{\mathrm{RAFF}} $$ (42 ± $$ \pm $$ 11 ms). Moreover, girRAFF preparations yielded 7.6/4.9 times improved reproducibility across B 0 $$ {\mathrm{B}}_0 $$ / B 1 + $$ {\mathrm{B}}_1^{+} $$ inhomogeneity conditions, 1.9 times better reproducibility across subjects and 1.2 times across slices compared with RAFF. Dixon-based fat suppression led to a further 15-fold improvement in the robustness of girRAFF to inhomogeneities. CONCLUSIONS: RAFF pulses display residual sensitivity to off-resonance and pronounced sensitivity to B 1 + $$ {\mathrm{B}}_1^{+} $$ inhomogeneities. Optimized girRAFF pulses provide increased robustness and may be an appealing alternative for applications where resilience against field inhomogeneities is required.