Mm Of Isocenter Research Articles

Phantom-based field maps for gradient nonlinearity correction in diffusion imaging.

Gradient coils in magnetic resonance imaging do not produce perfectly linear gradient fields. For diffusion imaging, the field nonlinearities cause the amplitude and direction of the applied diffusion gradients to vary over the field of view. This leads to site- and scan-specific systematic errors in estimated diffusion parameters such as diffusivity and anisotropy, reducing reliability especially in studies that take place over multiple sites. These errors can be substantially reduced if the actual scanner-specific gradient coil magnetic fields are known. The nonlinearity of the coil fields is measured by scanner manufacturers and used internally for geometric corrections, but obtaining and using the information for a specific scanner may be impractical for many sites that operate without special-purpose local engineering and research support. We have implemented an empirical field-mapping procedure using a large phantom combined with a solid harmonic approximation to the coil fields that is simple to perform and apply. Here we describe the accuracy and precision of the approach in reproducing manufacturer gold standard field maps and in reducing spatially varying errors in quantitative diffusion imaging for a specific scanner. Before correction, median B value error ranged from 33 - 41 relative to manufacturer specification at 100 mm from isocenter; correction reduced this to 0 - 4. On-axis spatial variation in the estimated mean diffusivity of an isotropic phantom was 2.2% - 4.1% within 60 mm of isocenter before correction, 0.5% - 1.6% after. Expected fractional anisotropy in the phantom was 0; highest estimated fractional anisotropy within 60 mm of isocenter was reduced from 0.024 to 0.012 in the phase encoding direction (48% reduction) and from 0.020 to 0.006 in the frequency encoding direction (72% reduction).

Characterization of spatial distortion in a 0.35 T MRI-guided radiotherapy system

Spatial distortion results in image deformation that can degrade accurate targeting and dose calculations in MRI-guided adaptive radiotherapy. The authors present a comprehensive assessment of a 0.35 T MRI-guided radiotherapy system’s spatial distortion using two commercially-available phantoms with regularly spaced markers.Images of the spatial integrity phantoms were acquired using five clinical protocols on the MRI-guided radiotherapy machine with the radiotherapy gantry positioned at various angles. Software was developed to identify and localize all phantom markers using a template matching approach. Rotational and translational corrections were implemented to account for imperfect phantom alignment. Measurements were made to assess uncertainties arising from susceptibility artifacts, image noise, and phantom construction accuracy.For a clinical 3D imaging protocol with a 1.5 mm reconstructed slice thickness, 100% of spheres within a 50 mm radius of isocenter had a 3D deviation of 1 mm or less. Of the spheres within 100 mm of isocenter, 99.9% had a 3D deviation less than 1 mm. 94.8% and 100% of the spheres within 175 mm were found to be within 1 mm and 2 mm of the expected positions in 3D respectively. Maximum 3D distortions within 50 mm, 100 mm and 175 mm of isocenter were 0.76 mm, 1.15 mm and 1.88 mm respectively. Distortions present in images acquired using the real-time imaging sequence were less than 1 mm for 98.1% and 95.0% of the cylinders within 50 mm and 100 mm of isocenter. The corresponding maximum distortion in these regions was 1.10 mm and 1.67 mm.These results may be used to inform appropriate planning target volume (PTV) margins for 0.35 T MRI-guided radiotherapy. Observed levels of spatial distortion should be explicitly considered when using PTV margins of 3 mm or less or in the case of targets displaced from isocenter by more than 50 mm.

TH-AB-204-04: On the Feasibility of a Multi-3D Kinect V2 Camera System for Monitoring Radiation Therapy Treatment Setup and Improving Patient Safety

Purpose: To employ a multi-3D camera system for patient and treatment equipment tracking using the Kinect v2 camera system. Methods: The system has two cameras, a color camera (RGB) and a time-of-flight infrared camera (IR). The IR is used to measure the distances between the camera and the subject for each IR pixel producing a point cloud of fraxels (fragment pixel, our terminology for a measured location + color generated by a 3D camera). We intend to use multiple Kinect cameras to generate a real-time digital model of the treatment room. To enable this, each camera’s fraxels need to be quantitative and registered to the linac coordinate system. We developed a calibration system consisting of a large flat board with IR and visible markers at measured locations. Images of the board were acquired with both cameras and the relative geometry of the board to the camera physically measured. The board images were used to characterize the geometric lens distortion and the marker locations used to calibrate the fraxel depth scales. The cameras were placed in the same room and pointed to a localization jig consisting of a checkerboard aligned to the room coordinate system. The camera rotations and translations to the board were determined for each camera and were applied to the camera fraxels to generate a single point cloud of the room. Results: The transformations between the cameras yielded a 3D treatment space accuracy of < 2 mm error in a radiotherapy setup within 500mm of isocenter. A novel-computing framework using multiple intel-NUC systems and a core dual-E6 processors allowed multiple cameras to be used simultaneously, greatly reducing occlusions. Conclusions: The proposed 3D camera system has the potential for providing real-time access to the treatment room from outside and remote locations, and automate processes such as collision detection.

Initial clinical experience with a radiation oncology dedicated open 1.0T MR-simulation.

The purpose of this study was to describe our experience with 1.0T MR‐SIM including characterization, quality assurance (QA) program, and features necessary for treatment planning. Staffing, safety, and patient screening procedures were developed. Utilization of an external laser positioning system (ELPS) and MR‐compatible couchtop were illustrated. Spatial and volumetric analyses were conducted between CT‐SIM and MR‐SIM using a stereotactic QA phantom with known landmarks and volumes. Magnetic field inhomogeneity was determined using phase difference analysis. System‐related, in‐plane distortion was evaluated and temporal changes were assessed. 3D distortion was characterized for regions of interest (ROIs) 5–20 cm away from isocenter. American College of Radiology (ACR) recommended tests and impact of ELPS on image quality were analyzed. Combined ultrashort echotime Dixon (UTE/Dixon) sequence was evaluated. Amplitude‐triggered 4D MRI was implemented using a motion phantom (2–10 phases, ~2 cm excursion, 3–5 s periods) and a liver cancer patient. Duty cycle, acquisition time, and excursion were evaluated between maximum intensity projection (MIP) datasets. Less than 2% difference from expected was obtained between CT‐SIM and MR‐SIM volumes, with a mean distance of <0.2 mm between landmarks. Magnetic field inhomogeneity was <2 ppm. 2D distortion was <2 mm over 28.6–33.6 mm of isocenter. Within 5 cm radius of isocenter, mean 3D geometric distortion was 0.59±0.32 mm (maximum=1.65 mm) and increased 10–15 cm from isocenter (mean=1.57±1.06 mm, maximum=6.26 mm). ELPS interference was within the operating frequency of the scanner and was characterized by line patterns and a reduction in signal‐to‐noise ratio (4.6–12.6% for TE=50−150 ms). Image quality checks were within ACR recommendations. UTE/Dixon sequences yielded detectability between bone and air. For 4D MRI, faster breathing periods had higher duty cycles than slow (50.4% (3 s) and 39.4% (5 s), p<0.001) and ~ fourfold acquisition time increase was measured for ten‐phase versus two‐phase. Superior–inferior object extent was underestimated 8% (6 mm) for two‐phase as compared to ten‐phase MIPs, although <2% difference was obtained for ≥4 phases. 4D MRI for a patient demonstrated acceptable image quality in ~7 min. MR‐SIM was integrated into our workflow and QA procedures were developed. Clinical applicability was demonstrated for 4D MRI and UTE imaging to support MR‐SIM for single modality treatment planning.PACS numbers: 87.56.Fc, 87.61.‐c, 87.57.cp

SU‐E‐J‐171: Surface Imaging Based Intrafraction Motion Assessments for Whole Brain Radiotherapy

Purpose:To quantify and characterize intrafraction motion for whole brain radiotherapy treatments in open face masks using 3D surface imaging.Methods:Fifteen whole brain patients were monitored with 3D surface imaging over a total of 202 monitoring sessions. Mean translations and rotations were calculated over each minute, each session, and over all sessions combined. The percentage of each session that the root mean square (RMS) of the linear translations were outside of 2 mm, 3 mm, 4 mm, and 5 mm were determined for each patient. Correlations between mean translations per minute and time and between standard deviation per minute and time were evaluated using Pearson's r value.Results:The mean RMS translation averaged over all patients was 1.45 mm +/− 1.52 mm. The patients spent an average of 18%, 10%, 6%, and 3% of the monitoring time outside of 2 mm, 3 mm, 4 mm, and 5 mm RMS tolerances, respectively. The RMS values averaged over all patients were 1.31 mm +/− 0.98 mm, 1.52 +/‐ 1.04, and 1.30 mm +/− 0.71 mm over the 1th, 5th, and 10th minutes of monitoring, respectively. Neither, the RMS values (p = 0.15) or the standard deviations of the RMS values (p = 0.16) showed significant correlations with time.Conclusion:The patients were positioned within 2 mm of isocenter, which was the initial set‐up tolerance, for the majority of their treatments. The average position changed by &lt; 0.3 mm over 10 minutes of monitoring. Short term movements, reflected by the standard deviations, where on the order of 1 mm. This immobilization system provides adequate immobilization over a course of treatment for whole brain radiotherapy. This system may also be suitable for head and neck or stereotactic radiosurgery treatments as well.

A multi-center inter-manufacturer study of the temporal stability of phase-contrast velocity mapping background offset errors.

BackgroundPhase-contrast velocity images often contain a background or baseline offset error, which adds an unknown offset to the measured velocities. For accurate flow measurements, this offset must be shown negligible or corrected. Some correction techniques depend on replicating the clinical flow acquisition using a uniform stationary phantom, in order to measure the baseline offset at the region of interest and subtract it from the clinical study. Such techniques assume that the background offset is stable over the time of a patient scan, or even longer if the phantom scans are acquired later, or derived from pre-stored background correction images. There is no published evidence regarding temporal stability of the background offset.MethodsThis study assessed the temporal stability of the background offset on 3 different manufacturers’ scanners over 8 weeks, using a retrospectively-gated phase-contrast cine acquisition with fixed parameters and at a fixed location, repeated 5 times in rapid succession each week. A significant offset was defined as 0.6 cm/s within 50 mm of isocenter, based upon an accuracy of 10% in a typical cardiac shunt measurement.ResultsOver the 5 repeated cine acquisitions, temporal drift in the baseline offset was insignificant on two machines (0.3 cm/s, 0.2 cm/s), and marginally insignificant on the third machine (0.5 cm/s) due to an apparent heating effect. Over a longer timescale of 8 weeks, insignificant drift (0.4 cm/s) occurred on one, with larger drifts (0.9 cm/s, 0.6 cm/s) on the other machines.ConclusionsDuring a typical patient study, background drift was insignificant. Extended high gradient power scanning with work requires care to avoid drift on some machines. Over the longer term of 8 weeks, significant drift is likely, preventing accurate correction by delayed phantom corrections or derivation from pre-stored background offset data.

SU‐FF‐T‐276: Improving the Accuracy of Linac‐Based Stereotaxy

Purpose: Stereotactic alignment accuracy of couch mount linac systems can be improved by analyzing the film test with a computer program and repositioning the setup appropriately. Method and Materials: The Winston‐Lutz style film test is scanned and analyzed with a computer program that can estimate the 3D alignment error of the stereotactic quality assurance (QA) setup to within 0.1 mm. The program gives repositioning advice, and digital cameras are used by another program to provide laser guided repositioning. A final film test is taken whenever the couch mount needs to be repositioned. The programs remember laser, cone, and gantry offsets, so if the system is used frequently enough the laser guided repositioning often makes it possible to get within 0.2 mm of isocenter on the first couch mount film test. This system can make the couch mount more accurate than the mechanical isocenter standard (MIS). When the MIS has alignment problems it typically takes weeks or months to resolve, and often a site visit from a field service engineer is required. Conversely, our new approach adapts every treatment day to slight gantry, couch, and collimator movements, and always adapts to minimize the overall alignment error. Results: Clinical tests with a Varian 600c at Mercy Hospital in Scranton, Pennsylvania, and a Siemens Mevatron MXE 2 at Christiana Care in Newark, Delaware have shown that 0.2 mm isocenter alignment of the couch mount QA setup can be achieved using this system. This is 7.5 times more accurate than the current 1.5 mm radial alignment specification of a commercial linear accelerator based stereotactic system. Conclusion: This research shows the potential of linac based stereotaxy to approach the accuracy of the dedicated stereotactic systems.