Assessment of a novel commercial large field of view phantom for comprehensive MR imaging quality assurance of a 0.35T MRgRT system.

Consistent quality assurance (QA) programs are vital to MR-guided radiotherapy (MRgRT), for ensuring treatment is delivered accurately and the onboard MRI system is providing the expected image quality. However, daily imaging QA with a dedicated phantom is not common at many MRgRT centers, especially with large phantoms that cover a field of view (FOV), similar to the human torso. This work presents the first clinical experience with a purpose-built phantom for large FOV daily and periodic comprehensive quality assurance (QUASAR™ MRgRT Insight Phantom (beta)) from Modus Medical Devices Inc. (Modus QA) on an MRgRT system. A monthly American College of Radiology (ACR) QA phantom was also imaged for reference. Both phantoms were imaged on a 0.35T MR-Linac, a 1.5T Philips wide bore MRI, and a 3.0T Siemens MRI, with T1-weighted and T2-weighted acquisitions. The Insight phantom was imaged in axial and sagittal orientations. Image quality tests including geometric accuracy, spatial resolution accuracy, slice thickness accuracy, slice position accuracy, and image intensity uniformity were performed on each phantom, following their respective instruction manuals. The geometric distortion test showed similar distortions of -1.7 mm and -1.9 mm across a 190 mm and a 283 mm lengths for the ACR and MRgRT Insight phantoms, respectively. The MRgRT Insight phantom utilized a modulation transform function (MTF) for spatial resolution evaluation, which showed decreased performance on the lower B0 strength MRIs, as expected, and could provide a good daily indicator of machine performance. Both the Insight and ACR phantoms showed a match with scan parameters for slice thickness analysis. During the imaging and analysis of this novel MRgRT Insight phantom the authors found setup to be straightforward allowing for easy acquisition each day, and useful image analysis parameters for tracking MRI performance.

Clinical experience of MRI4D QUASAR motion phantom for latency measurements in 0.35T MR-LINAC.

PURPOSE: In MRgRT, accuracy of treatment depends on the gating latency, when real-time targeting and gating is enabled. Gating latency is dependent on image acquisition, processing time, accuracy, efficacy of target tracking algorithms, and radiation beam delivery latency. In this report, clinical experience of the MRI4D QUASAR motion phantom for latency measurements on a 0.35-T magnetic resonance-linear accelerator (MR-LINAC) with two imaging speeds and four tracking algorithms was studied. MATERIALS/METHODS: Beam-control latency was measured on a 0.35-T MR-LINAC system with four target tracking algorithms and two real-time cine imaging sequences [four and eight frames per second (FPS)]. Using an MR-compatible motion phantom, the delays between phantom beam triggering signal and linac radiation beam control signal were evaluated for three motion periods with a rigid target. The gating point was set to be 8 mm above the full exhalation position. The beam-off latency was measured for a total of 24 combinations of tracking algorithm, imaging FPS, and motion periods. The corresponding gating target margins were determined using the target motion speed multiplied by the beam-off latency. RESULTS: The largest measured beam-off latency was 302 ± 20 ms with the Large Deforming Targets (LDT) algorithm and 4 s motion period imaged with 8-FPS cine MRI. The corresponding gating uncertainty based on target motion speed was 3.0 mm. The range of the average beam-off latency was 128-243 ms in 4-FPS imaging and 47-302 ms in 8-FPS imaging. CONCLUSIONS: The gating latency was measured using an MRI4D QUASAR motion phantom in a 0.35-T MR-LINAC. The latency measurements include time delay related to MR imaging method, target tracking algorithm and system delay. The gating uncertainty was estimated based on the beam-off latency measurements and the target motion.

First clinical experience of correcting phantom-based image distortion related to gantry position on a 0.35T MR-Linac.

MR-guided radiotherapy requires strong imaging spatial integrity to deliver high quality plans and provide accurate dose calculation. The MRI system, however, can be compromised by the integrated linear accelerator (Linac), resulting in inaccurate imaging isocenter position and geometric distortion. Dependence on gantry position further complicates the correction of distortions. This work presents a new clinical application of a commercial phantom and software system that quantifies isocenter alignment and geometric distortion, as well as providing a deformation vector field (DVF). A large distortion phantom and a smaller grid phantom were imaged at multiple gantry angles from 0 to 330° on a 0.35 T integrated MR-Linac. The software package was used to assess geometric distortion and generate DVFs to correct distortions within the phantom volume. The DVFs were applied to the grid phantom with resampling software then evaluated using structural similarity index measure (SSIM). Scans were also performed with a ferromagnetic clip near the phantom to investigate the correction of more severe artifacts. The mean magnitude isocenter shift was 0.67 mm, ranging from 0.25 to 1.04 mm across all angles. The DVF had a mean component value of 0.27 ± 0.02, 0.24 ± 0.01, and 0.19 ± 0.01 mm in the right-left (RL), anterior-posterior (AP), and superior-inferior (SI) directions. The ferromagnetic clip increased isocenter position error from 1.98 mm to 2.20 mm and increased mean DVF component values in the RL and AP directions. The resampled grid phantom had an increased SSIM for all gantry angles compared to original images, increasing from 0.26 ± 0.001 to 0.70 ± 0.004. Through this clinical assessment, we were able to correct geometric distortion and isocenter shift related to gantry position on a 0.35 T MR-Linac using the distortion phantom and software package. This provides encouragement that it could be used for quality assurance and clinically to correct systematic distortion caused by imaging at different gantry angles.

Development and evaluation of a novel MR-compatible pelvic end-to-end phantom.

MR-only treatment planning and MR-IGRT leverage MRI's powerful soft tissue contrast for high-precision radiation therapy. However, anthropomorphic MR-compatible phantoms are currently limited. This work describes the development and evaluation of a custom-designed, modular, pelvic end-to-end (PETE) MR-compatible phantom to benchmark MR-only and MR-IGRT workflows. For construction considerations, subject data were assessed for phantom/skeletal geometry and internal organ kinematics to simulate average male pelvis anatomy. Various materials for the bone, bladder, and rectum were evaluated for utility within the phantom. Once constructed, PETE underwent CT-SIM, MR-Linac, and MR-SIM imaging to qualitatively assess organ visibility. Scans were acquired with various bladder and rectal volumes to assess component interactions, filling capabilities, and filling reproducibility via volume and centroid differences. PETE simulates average male pelvis anatomy and comprises an acrylic body oval (height/width = 23.0/38.1 cm) and a cast-mold urethane skeleton, with silicone balloons simulating bladder and rectum, a silicone sponge prostate, and hydrophilic poly(vinyl alcohol) foam to simulate fat/tissue separation between organs. Access ports enable retrofitting the phantom with other inserts including point/film-based dosimetry options. Acceptable contrast was achievable in CT-SIM and MR-Linac images. However, the bladder was challenging to distinguish from background in CT-SIM. The desired contrast for T1-weighted and T2-weighted MR-SIM (dark and bright bladders, respectively) was achieved. Rectum and bone exhibited no MR signal. Inputted volumes differed by <5 and <10 mL from delineated rectum (CT-SIM) and bladder (MR-SIM) volumes. Increasing bladder and rectal volumes induced organ displacements and shape variations. Reproduced volumes differed by <4.5 mL, with centroid displacements <1.4 mm. A point dose measurement with an MR-compatible ion chamber in an MR-Linac was within 1.5% of expected. A novel, modular phantom was developed with suitable materials and properties that accurately and reproducibly simulate status changes with multiple dosimetry options. Future work includes integrating more realistic organ models to further expand phantom options.

Empirical validation of gradient field models for an accurate ADC measured on clinical 3T MR systems in body oncologic applications.

PURPOSE: To empirically corroborate vendor-provided gradient nonlinearity (GNL) characteristics and demonstrate efficient GNL bias correction for human brain apparent diffusion coefficient (ADC) across 3T MR systems and spatial locations. METHODS: Spatial distortion vector fields (DVF) were mapped in 3D using a surface fiducial array phantom for individual gradient channels on three 3T MR platforms from different vendors. Measured DVF were converted into empirical 3D GNL tensors and compared with their theoretical counterparts derived from vendor-provided spherical harmonic (SPH) coefficients. To illustrate spatial impact of GNL on ADC, diffusion weighted imaging using three orthogonal gradient directions was performed on a volunteer brain positioned at isocenter (as a reference) and offset superiorly by 10-17 cm (>10% predicted GNL bias). The SPH tensor-based GNL correction was applied to individual DWI gradient directions, and derived ADC was compared with low-bias reference for human brain white matter (WM) ROIs. RESULTS: Empiric and predicted GNL errors were comparable for all three studied 3T MR systems, with <1.0% differences in the median and width of spatial histograms for individual GNL tensor elements. Median (±width) of ADC (10-3mm2/s) histograms measured at isocenter in WM reference ROIs from three MR systems were: 0.73 ± 0.11, 0.71 ± 0.14, 0.74 ± 0.17, and at off-isocenters (before versus after GNL correction) were respectively 0.63 ± 0.14 versus 0.72 ± 0.11, 0.53 ± 0.16 versus 0.74 ± 0.18, and 0.65 ± 0.16 versus 0.76 ± 0.18. CONCLUSION: The phantom-based spatial distortion measurements validated vendor-provided gradient fields, and accurate WM ADC was recovered regardless of spatial locations and clinical MR platforms using system-specific tensor-based GNL correction for routine DWI.

Noise properties of a NMR transceiver coil array.

The use of multiple radiofrequency (RF) surface coil elements has applications in both fast parallel imaging and conventional imaging techniques. Through implementation of a simple magnetic decoupling network, 50 Omega matching can be achieved in both the transmitter and receiver chains, enabling the use of conventional RF power amplifiers and preamplifiers for transceive applications. Unlike phased array coil arrangements using low impedance preamplifiers for decoupling, the noise correlation between 50 Omega coils decoupled with discrete components has not been characterized. We have measured the dependence of coil quality factor (Q-factor) and noise correlation on coil separation and shown these quantities to be consistent with theoretical arguments, at least at 4 T (170 MHz). Our results suggest that a coil system for transmission and reception of NMR signals with 50 Omega coils can be built to take advantage of all the benefits of conventional array coils and with the added advantages of using conventional amplifiers.

Sodium T2*-weighted MR imaging of acute focal cerebral ischemia in rabbits.

Changes in T2*-weighted tissue sodium (23Na) signal following acute ischemia may help to identify necrotic tissue and estimate the duration of ischemia. Sodium signal was monitored in a rabbit model of acute (0-4 h) focal cerebral ischemia, using gradient echo 23Na MR images (echo time = 3.2 ms) acquired continuously in 20-min intervals on a 4-Tesla MRI. 2,3,5-Triphenyl-tetrazolium chloride staining was used to identify regions of necrosis. In necrotic tissue, average 23Na image signal intensity decreased by 11% +/- 8% during the first 40 min of ischemia followed by a linear increase (0.19%/min) to 25% +/- 14% greater than baseline after 4 h of ischemia. The time course of 23Na signal change observed in necrotic tissue following focal ischemia in this rabbit model is consistent with an initial decrease in 23Na T2* relaxation time followed by an increase in tissue sodium concentration and provides further evidence that tissue 23Na signal may offer unique information regarding tissue viability that is complementary to other MR imaging techniques.

Transceive surface coil array for MRI of the human prostate at 4T.

A novel torso transceive surface coil array for prostate magnetic resonance imaging (MRI) and spectroscopy (MRS) at 4T is presented. It is shown that with the use of a conformal transceive surface coil array with 50 Omega transmitter amplifiers and receiver preamplifiers, one can perform whole-volume torso imaging while maintaining the high signal-to-noise ratio (SNR) inherent to surface coil designs. Recent theoretical considerations have shown that by focusing the infringing radiofrequency (RF) electromagnetic field, one can achieve increased penetration and signal homogeneity compared to a conventional circularly polarized driving scheme. A variation of this driving scheme particular to the proposed coil design resulted in a twofold increase in SNR in the prostate compared to that achieved with a conventional circularly polarized driving scheme. The novel transceive surface coil array presented is capable of full-volume imaging of the human torso at 4T while maintaining signal penetration in the deep region of the prostate gland.

Transceive surface coil array for magnetic resonance imaging of the human brain at 4 T.

As the static magnetic field strength used in human magnetic resonance imaging increases, the wavelength of the corresponding radiofrequency field becomes comparable to the dimensions of the coil and volume of interest. The dielectric resonance effects that arise in this full wavelength regime may be partially compensated for through the use of surface coils. A novel high-field (4 T) transceive surface coil array is presented that allows arbitrary surface coil placement and size while maintaining the ability to independently transmit and/or receive through conventional 50-ohm power amplifiers and preamplifiers, respectively. A ninefold signal-to-noise ratio (SNR) increase is shown in close proximity to the transceive array and there is an overall 38% increase throughout the entire brain volume in comparison to the standard hybrid birdcage coil. Furthermore, the ability to independently transmit and receive through each surface coil within this array enables transmit and/or receive-only fast parallel imaging techniques to be employed while maintaining the increased SNR sensitivity inherent to surface coil designs.