Gradient Distortion Correction Research Articles

Geometric Distortion Correction of Renal Diffusion Tensor Imaging Using the Reversed Gradient Method.

Renal echo planar diffusion tensor imaging (DTI) has clinical potential but suffers from geometric distortion. We evaluated feasibility of reversed gradient distortion correction in 10 diabetic patients and 6 volunteers. Renal area, apparent diffusion coefficient, fractional anisotropy, and tensor eigenvalues were measured on uncorrected and distortion-corrected DTI. Corrected DTI correlated better than uncorrected DTI (r = 0.904 vs 0.840, P = 0.002) with reference anatomic T2-weighted imaging, with no significant difference in DTI metrics.

Quantitative Evaluations of Geometrical Distortion Corrections in Cortical Surface-Based Analysis of High-Resolution Functional MRI Data at 7T.

BackgroundAlthough 7T functional MRI (fMRI) provides better signal‐to‐noise ratio and higher spatial resolution than 3T fMRI, geometric distortions become more challenging because fMRI is more susceptible to distortions than structural MRI. Accurate alignment of 7T fMRI to structural MRI data is critical for precise cortical surface‐based analysis.PurposeTo quantify the effectiveness of distortion corrections of 7T fMRI data.Study TypeProspective.SubjectsFifteen healthy individuals aged 19–26 years (mean: 21.9 years).Field Strength/SequenceMultiband gradient‐echo echo‐planar imaging sequence at 7T; 3D T1/T2‐weighted sequences (magnetization prepared rapid acquisition with gradient echo [MPRAGE] and sampling perfection with application optimized contrast using different flip angle evolution [SPACE]) at 3T.AssessmentfMRI data at 7T were registered to cortical surfaces reconstructed from 3T structural data acquired in the same subjects. Distortions induced by B0 inhomogeneity and gradient nonlinearity (B0 and gradient distortions) were evaluated as cortical fallout (misregistration of noncortical areas) and displacement (misregistration along gray matter).Statistical TestsRepeated measures analyses of variance with post‐hoc t‐tests with Bonferroni correction.ResultsThe accuracy of fully corrected fMRI images based on the intensity distribution was 89.2%. Without any corrections, 9.7% of vertices in the whole surfaces were fallout and the average displacement was 0.96 mm for the rest of the vertices. B0 and gradient distortion corrections significantly reduced the fallout (to 2.1% and 8.7%) and displacement (to 0.29 mm and 0.86 mm). These corrections were effective even around regions with moderate distortions (the somatosensory and visual cortices for B0 distortion, and the anterior frontal, inferior temporal, and posterior occipital cortices for gradient distortion).Data ConclusionB0 distortion correction is crucial for surface‐based analysis of fine‐resolution fMRI at 7T. Gradient distortion correction should be considered when regions of interest include regions distant from the isocenter of scanners.Evidence Level1Technical Efficacy Stage1

TU-F-CAMPUS-I-04: Head-Only Asymmetric Gradient System Evaluation: ACR Image Quality and Acoustic Noise

Purpose: A high-performance head-only magnetic resonance imaging gradient system with an acquisition volume of 26 cm employing an asymmetric design for the transverse coils has been developed. It is able to reach a magnitude of 85 mT/m at a slew rate of 700 T/m/s, but operated at 80 mT/m and 500 T/m/s for this test. A challenge resulting from this asymmetric design is that the gradient nonlinearly exhibits both odd- and even-ordered terms, and as the full imaging field of view is often used, the nonlinearity is pronounced. The purpose of this work is to show the system can produce clinically useful images after an on-site gradient nonlinearity calibration and correction, and show that acoustic noise levels fall within non-significant risk (NSR) limits for standard clinical pulse sequences. Methods: The head-only gradient system was inserted into a standard 3T wide-bore scanner without acoustic damping. The ACR phantom was scanned in an 8-channel receive-only head coil and the standard American College of Radiology (ACR) MRI quality control (QC) test was performed. Acoustic noise levels were measured for several standard pulse sequences. Results: Images acquired with the head-only gradient system passed all ACR MR image quality tests; Both even and odd-order gradient distortion correction terms were required for the asymmetric gradients to pass. Acoustic noise measurements were within FDA NSR guidelines of 99 dBA (with assumed 20 dBA hearing protection) A-weighted and 140 dB for peak for all but one sequence. Note the gradient system was installed without any shroud or acoustic batting. We expect final system integration to greatly reduce noise experienced by the patient. Conclusion: A high-performance head-only asymmetric gradient system operating at 80 mT/m and 500 T/m/s conforms to FDA acoustic noise limits in all but one case, and passes all the ACR MR image quality control tests. This work was supported in part by the NIH grant 5R01EB010065

Assessment of Fiducial-based 2D kV Orthogonal Imaging, Fiducial-based CBCT, and Soft-tissue-based CBCT for Prostate Cancer Patients With Implanted Fiducial Markers

Purpose/Objective(s): To assess the geometric accuracy and imaging performance of a newly developed phased array coil system optimized for MR imaging of patients immobilized with the type S frame for head and neck radiation therapy. Materials/Methods: Protocol optimization for the novel coil was undertaken using a GE Optima450-GEM 70cm wide bore unit adapted for radiation therapy MR-SIM localization. Pulse sequence parameters for T1FSE, T2 FRFSE and T1 and T2 CUBE (including matrix, slice thickness, bandwidth) were optimized to maximize SNR while minimizing the effect of chemical shift. Phantom measurements were then performed to quantify system related residual geometric distortions after application of a 3D commercial gradient distortion correction algorithm for the effective FOV and the pulse sequences utilized. The optimized protocols were used to image seven volunteers positioned using an MR compatible type S system with neutral head supports. MRI images (from the base of brain to below the clavicles) were acquired using the novel phased array coil combination placed over the head, neck and thoracic region. The volunteers were re-imaged without immobilization in a standard diagnostic head coil, which served as the benchmark for image quality. Signal to noise, image homogeneity and image artifacts were compared between the two image datasets in a blinded study. Visibility of ten anatomical structures was evaluated. For statistical analysis, Cohen’s kappa, Wilcoxon matched pairs and t-testing were utilized. Results: A geometrically accurate, high resolution MR-SIM imaging protocol (T1/T2CUBE. 2D T1/T2 FSE) was developed with 3mm slice thickness/no gap that provides excellent image quality from the base of brain to below the clavicles in the treatment position. From phantom studies, residual distortions were found to be 1.0mm within a 10 cm radius and < 1.5 mm within a 15 cm radius of the scan centre. Using a RBWof 50-62.5 minimized chemical shift (<1mm within a 10cm radius) at 1.5T while providing acceptable SNR. The coil provided higher SNR values in all anatomical structures, but less uniformity than the standard diagnostic head coil (full comparative SNR/CNR/geometric data will be presented). No significant difference for image quality and artifacts were demonstrated between the coils in the blinded study. Conclusions: The novel open architecture purpose built phased coil array provides unparalleled coverage and good image quality of the subject immobilized for head and neck radiation therapy and is a promising solution for MR-SIM in the head and neck region. Author Disclosure: G. Perkins: None. R. Hammoud: None. S. Pienaar: None. S. Paloor: None. N. Al Hammadi: None.

A Novel MRI-SIM Technique for Localization of Head and Neck Cancer Patients Using Immobilization and Surface Coils: Initial Clinical Experience

Purpose/Objective(s): To investigate the feasibility of developing a geometrically accurate MR imaging protocol at 1.5T for patients immobilized with the type S frame for head and neck radiotherapy. Materials/Methods: Protocol optimization was undertaken using a GE Optima 450 70cm wide bore MR-SIM unit commissioned for radiotherapy planning. MRI images (from the base of brain to below the clavicles) were acquired in five volunteers using an 8 channel phased array coil placed over the head and neck region while each subject was immobilized in the treatment position with a five point thermoplastic device. Pulse sequence parameters for 3DT1, T2 FRFSE and T2 CUBE (including matrix, slice thickness, bandwidth) were optimized to maximize SNR while minimizing the effect of chemical shift. MR image quality was rated by two independent Radiation Oncologists benchmarked against “gold standard” reference diagnostic images. Phantom measurements were then performed to quantify system related residual geometric distortions after application of a commercial gradient distortion correction algorithm for the effective FOV and the pulse sequences utilized. The optimized protocols were used to image five patients undergoing head and neck radiotherapy. MR and planning CT images were fused in the ECLIPSE treatment planning system using mutual information algorithm. Image quality, distortion and accuracy of image registration using internal anatomical reference landmarks (multiple regions of interest in the head, neck and thorax) were evaluated. Results: A high resolution MR-SIM imaging protocol (3DT1, T2 FRFSE and T2 CUBE) was developed with 3mm slice thickness/no gap that covers the base of brain to below the clavicles. From phantom studies, residual distortions were found to be < 1.0 mm within a 8 cm radius and < 1.5mm within a 12 cm radius of the scan centre. Image deformation was minimized in the head and neck region due to the reproducibility of image localization between CT and MRI. In terms of registration accuracy, anatomic landmark displacements in the x, y and z direction relative to CT were <2mm for all landmarks in the head and neck region but up to 6 mm in the thorax. Conclusions: We have developed a novel MRI-SIM technique that provides good image quality for patients immobilized for head and neck radiotherapy and is now in clinical use.

Gradient distortions in MRI: Characterizing and correcting for their effects on SIENA-generated measures of brain volume change

Precise and accurate quantification of whole-brain atrophy based on magnetic resonance imaging (MRI) data is an important goal in understanding the natural progression of neurodegenerative disorders such as Alzheimer's disease and multiple sclerosis. We found that inconsistent MRI positioning of subjects is common in typically acquired clinical trial data – particularly along the magnet's long (i.e., Z) axis. We also found that, if not corrected for, the gradient distortion effects associated with such Z-shifts can significantly decrease the accuracy and precision of MRI-derived measures of whole-brain atrophy – negative effects that increase in magnitude with (i) increases in the Z-distance between the brains to be compared and (ii) increases in the Z-distance from magnet isocenter of the center of the pair of brains to be compared. These gradient distortion effects can be reduced by accurate subject positioning, and they can also be corrected post hoc with the use of appropriately-generated gradient-distortion correction fields. We used a novel DUPLO-based phantom to develop a spherical-harmonics-based gradient distortion field that was used to (i) correct for observed Z-shift-associated gradient distortion effects on SIENA-generated measures of brain atrophy and (ii) simulate the gradient distortion effects that might be expected with a greater range of Z-shifts than those that we were able to acquire. Our results suggest that consistent alignment to magnet isocenter and/or correcting for the observed effects of gradient distortion should lead to more accurate and precise estimates of brain-related changes and, as a result, to increased statistical power in studies aimed at understanding the natural progression and the effective treatment of neurodegenerative disorders.

Investigation of MR image distortion for radiotherapy treatment planning of prostate cancer

MR imaging based treatment planning for radiotherapy of prostate cancer is limited due to MR imaging system related geometrical distortions, especially for patients with large body sizes. On our 0.23 T open scanner equipped with the gradient distortion correction (GDC) software, the residual image distortions after the GDC were <5 mm within the central 36 cm × 36 cm area for a standard 48 cm field of view (FOV). In order to use MR imaging alone for treatment planning the effect of residual MR distortions on external patient contour determination, especially for the peripheral regions outside the 36 cm × 36 cm area, must be investigated and corrected. In this work, we performed phantom measurements to quantify MR system related residual geometric distortions after the GDC and the effective FOV. Our results show that for patients with larger lateral dimensions (>36 cm), the differences in patient external contours between distortion-free CT images and GDC-corrected MR images were 1–2 cm because of the combination of greater gradient distortion and loss of field homogeneity away from the isocentre and the uncertainties in patient setup during CT and MRI scans. The measured distortion maps were used to perform point-by-point corrections for patients with large dimensions inside the effective FOV. Using the point-by-point method, the geometrical distortion after the GDC were reduced to <3 mm for external contour determination and the effective FOV was expanded from 36 cm to 42 cm.

SU‐EE‐A1‐06: Investigation of MR Image Distortion for Radiotherapy Treatment Planning of Prostate Cancer

Purpose: To implement MRI based treatment planning clinically for prostate cancer, the machine generated geometrical distortions on MR images must be investigated to ensure the accuracy of dose calculation and treatment delivery. Method and Materials: The MR system used for this study was a 0.23 Tesla open scanner (Philips Medical System, Cleveland, OH) operating clinically as an MR simulator. Based on the fact that there was no observable patient induced MR image artifact with 0.2 T low magnetic fields, the patient induced geometrical distortions were negligible for our MR unit and the machine induced geometrical distortions could be detected, studied and corrected accurately. A distortion evaluation phantom (F18) was employed to measure the distortions after a gradient distortion correction (GDC) software was installed to post‐process the images and a point‐by‐point correction method was then developed to remove the residual distortion based on precise measurements. Results: With our routine clinical 3‐dimensinal fast spin echo sequences, our work shows that there are no patient induced distortions detected and the machine specific geometrical distortions can be quantified and corrected first by the GDC post‐processing software and then by the point by point correction technique. The residual distortions after the corrections are reduced from the original &gt; 2 cm to &lt; 3 mm in the external contour determination for patients with large lateral dimensions. An effective FOV has been established, which can be used clinically to set up patient selection criteria for MR based treatment planning. Conclusion: MR geometrical distortions on MR scanners can be corrected and, with the GDC post‐processing and our point by point correction technique, 2% to 5% improvement in dose accuracy can be expected in the treatment planning for prostate cancer.

Dosimetric evaluation of MRI-based treatment planning for prostate cancer**The materials in this paper have been presented in part at the San Diego 2003 AAPM.

The purpose of this study is to evaluate the dosimetric accuracy of MRI-based treatment planning for prostate cancer using a commercial radiotherapy treatment planning system. Three-dimensional conformal plans for 15 prostate patients were generated using the AcQPlan system. For each patient, dose distributions were calculated using patient CT data with and without heterogeneity correction, and using patient MRI data without heterogeneity correction. MR images were post-processed using the gradient distortion correction (GDC) software. The distortion corrected MR images were fused to the corresponding CT for each patient for target and structure delineation. The femoral heads were delineated based on CT. Other anatomic structures relevant to the treatment (i.e., prostate, seminal vesicles, lymph notes, rectum and bladder) were delineated based on MRI. The external contours were drawn separately on CT and MRI. The same internal contours were used in the dose calculation using CT- and MRI-based geometries by directly transferring them between MRI and CT as needed. Treatment plans were evaluated based on maximum dose, isodose distributions and dose–volume histograms. The results confirm previous investigations that there is no clinically significant dose difference between CT-based prostate plans with and without heterogeneity correction. The difference in the target dose between CT- and MRI-based plans using homogeneous geometry was within 2.5%. Our results suggest that MRI-based treatment planning is suitable for radiotherapy of prostate cancer.

MRI-based treatment planning for radiotherapy: Dosimetric verification for prostate IMRT

Magnetic resonance (MR) and computed tomography (CT) image fusion with CT-based dose calculation is the gold standard for prostate treatment planning. MR and CT fusion with CT-based dose calculation has become a routine procedure for intensity-modulated radiation therapy (IMRT) treatment planning at Fox Chase Cancer Center. The use of MRI alone for treatment planning (or MRI simulation) will remove any errors associated with image fusion. Furthermore, it will reduce treatment cost by avoiding redundant CT scans and save patient, staff, and machine time. The purpose of this study is to investigate the dosimetric accuracy of MRI-based treatment planning for prostate IMRT. A total of 30 IMRT plans for 15 patients were generated using both MRI and CT data. The MRI distortion was corrected using gradient distortion correction (GDC) software provided by the vendor (Philips Medical System, Cleveland, OH). The same internal contours were used for the paired plans. The external contours were drawn separately between CT-based and MR imaging-based plans to evaluate the effect of any residual distortions on dosimetric accuracy. The same energy, beam angles, dose constrains, and optimization parameters were used for dose calculations for each paired plans using a treatment optimization system. The resulting plans were compared in terms of isodose distributions and dose-volume histograms (DVHs). Hybrid phantom plans were generated for both the CT-based plans and the MR-based plans using the same leaf sequences and associated monitor units (MU). The physical phantom was then irradiated using the same leaf sequences to verify the dosimetry accuracy of the treatment plans. Our results show that dose distributions between CT-based and MRI-based plans were equally acceptable based on our clinical criteria. The absolute dose agreement for the planning target volume was within 2% between CT-based and MR-based plans and 3% between measured dose and dose predicted by the planning system in the physical phantom. Magnetic resonance imaging is a useful tool for radiotherapy simulation. Compared with CT-based treatment planning, MR imaging-based treatment planning meets the accuracy for dose calculation and provides consistent treatment plans for prostate IMRT. Because MR imaging-based digitally reconstructed radiographs do not provide adequate bony structure information, a technique is suggested for producing a wire-frame image that is intended to replace the traditional digitally reconstructed radiographs that are made from CT information.

MRI simulation: effect of gradient distortions on three-dimensional prostate cancer plans

Purpose : To quantify the dosimetric consequences of external patient contour distortions produced on low-field and high-field MRIs for external beam radiation of prostate cancer. Methods and Materials : A linearity phantom consisting of a grid filled with contrast material was scanned on a spiral CT, a 0.23 T open MRI, and a 1.5 T closed bore system. Subsequently, 12 patients with prostate cancer were scanned on CT and the open MRI. A gradient distortion correction (GDC) program was used to postprocess the MRI images. Eight of the patients were also scanned on the 1.5 T MRI with integrated GDC correction. All data sets were fused according to their bony landmarks using a chamfer-matching algorithm. The prostate volume was contoured on an MRI image, irrespective of the apparent prostate location in those sets. Thus, the same target volume was planned and used for calculating the anterior-posterior (AP) and lateral separations. The number of monitor units required for treatment using a four-field conformal technique was compared. Because there are also setup variations in patient outer contours, two different CT scans from 20 different patients were fused, and the differences in AP and lateral separations were measured to obtain an estimate of the mean interfractional separation variation. Results : All AP separations measured on MRI were statistically indistinguishable from those on CT within the interfractional separation variations. The mean differences between CT and low-field MRI and CT and high-field MRI lateral separations were 1.6 cm and 0.7 cm, respectively, and were statistically significantly different from zero. However, after the GDC was applied to the low-field images, the difference became 0.4 ± 0.4 mm (mean ± standard deviation), which was statistically insignificant from the CT-to-CT variations. The mean variations in the lateral separations from the low-field images with GDC would result in a dosimetric difference of <1%, assuming an equally weighted four-field 18-MV technique for patient separations up to ∼40 cm. Conclusions : For patients with lateral separations <40 cm, a homogeneous calculation simulated using a 1.5 T MRI or a 0.23 T MRI with a gradient distortion correction will yield a monitor unit calculation indistinguishable from that generated using CT simulation.