Zixin Deng

Four-Dimensional Magnetic Resonance Imaging With 3-Dimensional Radial Sampling and Self-Gating-Based K-Space Sorting: Early Clinical Experience on Pancreatic Cancer Patients.

PURPOSE To apply a novel self-gating k-space sorted 4-dimensional MRI (SG-KS-4D-MRI) method to overcome limitations due to anisotropic resolution and rebinning artifacts and to monitor pancreatic tumor motion. METHODS AND MATERIALS Ten patients were imaged using 4D-CT, cine 2-dimensional MRI (2D-MRI), and the SG-KS-4D-MRI, which is a spoiled gradient recalled echo sequence with 3-dimensional radial-sampling k-space projections and 1-dimensional projection-based self-gating. Tumor volumes were defined on all phases in both 4D-MRI and 4D-CT and then compared. RESULTS An isotropic resolution of 1.56 mm was achieved in the SG-KS-4D-MRI images, which showed superior soft-tissue contrast to 4D-CT and appeared to be free of stitching artifacts. The tumor motion trajectory cross-correlations (mean ± SD) between SG-KS-4D-MRI and cine 2D-MRI in superior-inferior, anterior-posterior, and medial-lateral directions were 0.93 ± 0.03, 0.83 ± 0.10, and 0.74 ± 0.18, respectively. The tumor motion trajectories cross-correlations between SG-KS-4D-MRI and 4D-CT in superior-inferior, anterior-posterior, and medial-lateral directions were 0.91 ± 0.06, 0.72 ± 0.16, and 0.44 ± 0.24, respectively. The average standard deviation of gross tumor volume calculated from the 10 breathing phases was 0.81 cm(3) and 1.02 cm(3) for SG-KS-4D-MRI and 4D-CT, respectively (P=.012). CONCLUSIONS A novel SG-KS-4D-MRI acquisition method capable of reconstructing rebinning artifact-free, high-resolution 4D-MRI images was used to quantify pancreas tumor motion. The resultant pancreatic tumor motion trajectories agreed well with 2D-cine-MRI and 4D-CT. The pancreatic tumor volumes shown in the different phases for the SG-KS-4D-MRI were statistically significantly more consistent than those in the 4D-CT.

DANTE-prepared three-dimensional FLASH: A fast isotropic-resolution MR approach to morphological evaluation of the peripheral arterial wall at 3 Tesla.

To develop and assess a sequence using DANTE dark‐blood preparation combined with FLASH readout (DANTE‐FLASH) for rapid isotropic‐resolution three‐dimensional (3D) peripheral vessel wall imaging at 3 Tesla (T).

Changes in left ventricular function and coronary blood flow velocity during isocapnic hypoxia: A cardiac magnetic resonance imaging study

Background Cardiac stress testing is the standard of care for diagnosing ischemic heart disease. Traditional stress testing involves physical or pharmacological stress to induce hyperemia and/or increase myocardial oxygen demand. Physical stress is not possible in 100% of cases however, and pharmacological stress carries rare but serious risk. We asked whether acute isocapnic hypoxia could be utilized as an alternative cardiovascular stress test.

Geometric validation of self‐gating k‐space‐sorted 4D‐MRI vs 4D‐CT using a respiratory motion phantom

PURPOSE MRI is increasingly being used for radiotherapy planning, simulation, and in-treatment-room motion monitoring. To provide more detailed temporal and spatial MR data for these tasks, we have recently developed a novel self-gated (SG) MRI technique with advantage of k-space phase sorting, high isotropic spatial resolution, and high temporal resolution. The current work describes the validation of this 4D-MRI technique using a MRI- and CT-compatible respiratory motion phantom and comparison to 4D-CT. METHODS The 4D-MRI sequence is based on a spoiled gradient echo-based 3D projection reconstruction sequence with self-gating for 4D-MRI at 3 T. Respiratory phase is resolved by using SG k-space lines as the motion surrogate. 4D-MRI images are reconstructed into ten temporal bins with spatial resolution 1.56 × 1.56 × 1.56 mm(3). A MRI-CT compatible phantom was designed to validate the performance of the 4D-MRI sequence and 4D-CT imaging. A spherical target (diameter 23 mm, volume 6.37 ml) filled with high-concentration gadolinium (Gd) gel is embedded into a plastic box (35 × 40 × 63 mm(3)) and stabilized with low-concentration Gd gel. The phantom, driven by an air pump, is able to produce human-type breathing patterns between 4 and 30 respiratory cycles/min. 4D-CT of the phantom has been acquired in cine mode, and reconstructed into ten phases with slice thickness 1.25 mm. The 4D images sets were imported into a treatment planning software for target contouring. The geometrical accuracy of the 4D MRI and CT images has been quantified using target volume, flattening, and eccentricity. The target motion was measured by tracking the centroids of the spheres in each individual phase. Motion ground-truth was obtained from input signals and real-time video recordings. RESULTS The dynamic phantom has been operated in four respiratory rate (RR) settings, 6, 10, 15, and 20/min, and was scanned with 4D-MRI and 4D-CT. 4D-CT images have target-stretching, partial-missing, and other motion artifacts in various phases, whereas the 4D-MRI images are visually free of those artifacts. Volume percentage difference for the 6.37 ml target ranged from 5.3% ± 4.3% to 10.3% ± 5.9% for 4D-CT, and 1.47 ± 0.52 to 2.12 ± 1.60 for 4D-MRI. With an increase of respiratory rate, the target volumetric and geometric deviations increase for 4D-CT images while remaining stable for the 4D-MRI images. Target motion amplitude errors at different RRs were measured with a range of 0.66-1.25 mm for 4D-CT and 0.2-0.42 mm for 4D-MRI. The results of Mann-Whitney tests indicated that 4D-MRI significantly outperforms 4D-CT in phase-based target volumetric (p = 0.027) and geometric (p < 0.001) measures. Both modalities achieve equivalent accuracy in measuring motion amplitude (p = 0.828). CONCLUSIONS The k-space self-gated 4D-MRI technique provides a robust method for accurately imaging phase-based target motion and geometry. Compared to 4D-CT, the current 4D-MRI technique demonstrates superior spatiotemporal resolution, and robust resistance to motion artifacts caused by fast target motion and irregular breathing patterns. The technique can be used extensively in abdominal targeting, motion gating, and toward implementing MRI-based adaptive radiotherapy.

TU‐F‐17A‐04: Respiratory Phase‐Resolved 3D MRI with Isotropic High Spatial Resolution: Determination of the Average Breathing Motion Pattern for Abdominal Radiotherapy Planning

PURPOSE To develop a retrospective 4D-MRI technique (respiratory phase-resolved 3D-MRI) for providing an accurate assessment of tumor motion secondary to respiration. METHODS A 3D projection reconstruction (PR) sequence with self-gating (SG) was developed for 4D-MRI on a 3.0T MRI scanner. The respiration-induced shift of the imaging target was recorded by SG signals acquired in the superior-inferior direction every 15 radial projections (i.e. temporal resolution 98 ms). A total of 73000 radial projections obtained in 8-min were retrospectively sorted into 10 time-domain evenly distributed respiratory phases based on the SG information. Ten 3D image sets were then reconstructed offline. The technique was validated on a motion phantom (gadolinium-doped water-filled box, frequency of 10 and 18 cycles/min) and humans (4 healthy and 2 patients with liver tumors). Imaging protocol included 8-min 4D-MRI followed by 1-min 2D-realtime (498 ms/frame) MRI as a reference. RESULTS The multiphase 3D image sets with isotropic high spatial resolution (1.56 mm) permits flexible image reformatting and visualization. No intra-phase motion-induced blurring was observed. Comparing to 2D-realtime, 4D-MRI yielded similar motion range (phantom: 10.46 vs. 11.27 mm; healthy subject: 25.20 vs. 17.9 mm; patient: 11.38 vs. 9.30 mm), reasonable displacement difference averaged over the 10 phases (0.74mm; 3.63mm; 1.65mm), and excellent cross-correlation (0.98; 0.96; 0.94) between the two displacement series. CONCLUSION Our preliminary study has demonstrated that the 4D-MRI technique can provide high-quality respiratory phase-resolved 3D images that feature: a) isotropic high spatial resolution, b) a fixed scan time of 8 minutes, c) an accurate estimate of average motion pattern, and d) minimal intra-phase motion artifact. This approach has the potential to become a viable alternative solution to assess the impact of breathing on tumor motion and determine appropriate treatment margins. Comparison with 4D-CT in a clinical setting is warranted to assess the value of 4D-MRI in radiotherapy planning. This work supported in part by grant 1R03CA173273-01.

Quantitative 3D dynamic contrast-enhanced (DCE) MR imaging of carotid vessel wall by fast T1 mapping using Multitasking

To develop a dynamic contrast‐enhanced (DCE) MRI method capable of high spatiotemporal resolution, 3D carotid coverage, and T1‐based quantification of contrast agent concentration for the assessment of carotid atherosclerosis using a newly developed Multitasking technique.

MO-FG-CAMPUS-JeP2-01: 4D-MRI with 3D Radial Sampling and Self-Gating-Based K-Space Sorting: Image Quality Improvement by Slab-Selective Excitation

PURPOSE A recent 4D MRI technique based on 3D radial sampling and self-gating-based K-space sorting has shown promising results in characterizing respiratory motion. However due to continuous acquisition and potentially drastic k-space undersampling resultant images could suffer from low blood-to-tissue contrast and streaking artifacts. In this study 3D radial sampling with slab-selective excitation (SS) was proposed in attempt to enhance blood-to-tissue contrast by exploiting the in-flow effect and to suppress the excess signal from the peripheral structures particularly in the superior-inferior direction. The feasibility of improving image quality by using this approach was investigated through a comparison with the previously developed non-selective excitation (NS) approach. METHODS Two excitation approaches SS and NS were compared in 5 cancer patients (1 lung 1 liver 2 pancreas and 1 esophagus) at 3Tesla. Image artifact was assessed in all patients on a 4-point scale (0: poor; 3: excellent). Signal-tonoise ratio (SNR) of the blood vessel (aorta) at the center of field-of-view and its nearby tissue were measured in 3 of the 5 patients (1 liver 2 pancreas) and blood-to-tissue contrast-to-noise ratio (CNR) were then determined. RESULTS Compared with NS the image quality of SS was visually improved with overall higher signal in all patients (2.6±0.55 vs. 3.4±0.55). SS showed an approximately 2-fold increase of SNR in the blood (aorta: 16.39±1.95 vs. 32.19±7.93) and slight increase in the surrounding tissue (liver/pancreas: 16.91±1.82 vs. 22.31±3.03). As a result the blood-totissue CNR was dramatically higher in the SS method (1.20±1.20 vs. 9.87±6.67). CONCLUSION The proposed 3D radial sampling with slabselective excitation allows for reduced image artifact and improved blood SNR and blood-to-tissue CNR. The success of this technique could potentially benefit patients with cancerous tumors that have invaded the surrounding blood vessels where radiation therapy is needed to remove tumor from those regions prior to surgical resection. This work is partially supported by NIH R03CA173273; and CTSI core voucher award.

TH-EF-BRA-07: Evaluation of Internal Target Volume Derived From a Prototype 4D-MRI Sequence with 3D Radial Stack-Of-Stars Trajectory and K-Space Self-Gating

PURPOSE 4D-MRI based on resorting of 2D-cine-MRI images shows great potential to assess tumor motion more accurately compared to 4D-CT, however, it suffers from low through-plane resolution and stitching artifacts. 4D-MRI based on 3D acquisition results in stitching-artifact-free images with high in-plane and through-plane resolutions. In this study, we report our early clinical experience of a prototype 4D-MRI sequence with 3D stack-of-stars (SOS) trajectory for internal target volume (ITV) definition. METHODS Ten patients with 13 total lesions (7 pancreatic, 1 lung with 2 lesions, 1 liver with 3 lesions and 1 esophagus) were recruited. 4D-MRI used in-plane radial and through-plane Cartesian sampling. Two imaging orientations, i.e. axial slab (A) and coronal slab (B), were compared. ITVs were derived from 3-bin (ITV3), 5-bin (ITV5) and 10-bin (ITV10) reconstruction protocols. ITV5 was set as standard and minimum expansion of ITV3 needed to encompass ITV5 was derived. Similarity index was calculated from ITV3 and ITV5 (SI3/5), and ITV10 and ITV5 (SI10/5). Imaging noise was calculated for both method A and B. Wilcoxon-rank-sum test was performed with a p value <0.05 deemed as significant. RESULTS No significant difference (p=0.34) was observed from method A and B, indicating a uniform imaging noise distribution from 3D acquisition. Imaging noise and artifacts were visually different from different binning protocols, with more bins resulting in more noise, more artifacts and larger ITV. On average, ITV differs by 7% (1%-19%) comparing ITV3 with ITV5 for the patient cohort. For the pancreas sub-group, ITV differs by 4% (1%-6%). An average of 2.3mm (2mm-3mm) expansion was needed for ITV3 to encompass ITV5. SI10/5 was 0.93±0.03 (mean±σ) and SI3/5 was 0.95±0.03. CONCLUSION 4D-MRI with 3D SOS trajectory was evaluated on 10 patients. Significant difference in ITV was observed with different binning protocols. Imaging noise was similar irrespective of the imaging orientations. This work is partially supported by NIH R03CA173273 and CTSI core voucher award.

Development of a clinically practical whole-brain intracranial vessel wall MRI technique at 3 Tesla

Background T1-weighted variable-flip-angle 3D TSE has emerged as a promising intracranial vessel wall imaging technique. To increase spatial coverage and cerebrospinal fluid (CSF) attenuation, a whole-brain 0.5-mm-reoslution protocol based on an inversion-prepared 3D TSE sequence has recently been proposed at 3T. However, its 11-12-min scan time renders it clinically impractical. This work aimed to develop an expedited protocol and validate it on patients.

SU-F-303-09: Identifying Abdominal Inter-Organ Motion Correlations Using 4D-MRI and 4D-Image Registration

Purpose: This study aims to identify motion correlations between multiple abdominal organs using high spatial resolution 4D-MRI images and 4D-image registration, and to model internal organ motion using known surface motions for individual patients. Methods: 4D-MRI images were acquired at 3T using a spoiled gradient echo sequence with self-gated 3D projection reconstruction. Six healthy human subjects and a MRI motion phantom were scanned with the 4D-MRI imaging sequence. The images were reconstructed into 10 phases with isotropic spatial resolution [1.56 mm]^3. The 4D images were registered with an in-house 4D registration algorithm which uses the self-gating respiratory signals as the temporal regularization energy. A 4D deformation field was generated from the registration. In the human study, we examined motion of three regions of interest (ROIs), including abdominal skin, upper liver, and pancreas head. Each ROI contains 8,000–20,000 voxels. The primary motion trajectory was extracted by principle components analysis. The correlation of inter-organ motion was modeled by 3D orientation matching between motion trajectories. Liver and pancreas motions were projected onto the motion plane of abdominal surface resulting in rotation and scaling parameters. The correlation of inter-organ motions was evaluated using RMS errors of trajectory matching. Results: The accuracy of 4D-registration for the phantom data had RMS error 0.5±0.3mm. The human data registration accuracy, using manual tracking of landmarks, had RMS error 0.9±0.7mm. The motion ranges were 23±16mm (upper liver), 9.5±5mm (pancreas), and 10.2±6mm (surface). Compared to the surface motion, trajectories of liver and pancreas had 85±34 and 63±41 degree angle-of-motion and scale factors 2.4±1.3 and 1.5±1.1, respectively. The trajectory matching errors were 1.0±0.6mm (liver) and 0.7±0.5mm (pancreas). Conclusion: Abdominal inter-organ motion can be accurately characterized using 4D-MRI images and 4D-image registration techniques. The proposed method provides a feasible approach to more accurately use patient surface surrogates to estimate internal organ motions.