Katherine L. Wright

Non-Cartesian Parallel Imaging Reconstruction

Non‐Cartesian parallel imaging has played an important role in reducing data acquisition time in MRI. The use of non‐Cartesian trajectories can enable more efficient coverage of k‐space, which can be leveraged to reduce scan times. These trajectories can be undersampled to achieve even faster scan times, but the resulting images may contain aliasing artifacts. Just as Cartesian parallel imaging can be used to reconstruct images from undersampled Cartesian data, non‐Cartesian parallel imaging methods can mitigate aliasing artifacts by using additional spatial encoding information in the form of the nonhomogeneous sensitivities of multi‐coil phased arrays. This review will begin with an overview of non‐Cartesian k‐space trajectories and their sampling properties, followed by an in‐depth discussion of several selected non‐Cartesian parallel imaging algorithms. Three representative non‐Cartesian parallel imaging methods will be described, including Conjugate Gradient SENSE (CG SENSE), non‐Cartesian generalized autocalibrating partially parallel acquisition (GRAPPA), and Iterative Self‐Consistent Parallel Imaging Reconstruction (SPIRiT). After a discussion of these three techniques, several potential promising clinical applications of non‐Cartesian parallel imaging will be covered. J. Magn. Reson. Imaging 2014;40:1022–1040.

Clinical evaluation of CAIPIRINHA: Comparison against a GRAPPA standard

To evaluate image quality when using a CAIPIRINHA sampling pattern in comparison to a standard GRAPPA sampling pattern in patients undergoing a routine three‐dimensional (3D) breathheld liver exam. CAIPIRINHA uses an optimized phase encoding sampling strategy to alter aliasing artifacts in 3D acquisitions to improve parallel imaging reconstruction.

Simultaneous magnetic resonance angiography and perfusion (MRAP) measurement: initial application in lower extremity skeletal muscle.

To obtain a simultaneous 3D magnetic resonance angiography and perfusion (MRAP) using a single acquisition and to demonstrate MRAP in the lower extremities. A time‐resolved contrast‐enhanced exam was used in MRAP to simultaneously acquire a contrast‐enhanced MR angiography (MRA) and dynamic contrast‐enhanced (DCE) perfusion, which currently requires separate acquisitions and thus two contrast doses. MRAP can be used to assess large and small vessels in vascular pathologies such as peripheral arterial disease.

Time-Resolved and Bolus-Chase MR Angiography of the Leg: Branching Pattern Analysis and Identification of Septocutaneous Perforators

OBJECTIVE The goal of this study was to compare time-resolved MR angiography (MRA) and bolus-chase MRA in the identification of peroneal artery septocutaneous perforators and for classification of the branching pattern of the arterial tree in the leg in a cohort of candidates for fibular free flap transfer operations. MATERIALS AND METHODS Retrospective analysis was performed on imaging data from 53 legs of 27 patients (age range, 27-88 years) who underwent time-resolved MRA (FLASH; TR/TE, 2.5/1.0; flip angle, 22°; voxel dimensions, 1.54 × 1.25 × 1.5 mm; acquisition time, 2.27 s/frame) and bolus-chase MRA (FLASH; 3.2/1.2; flip angle, 25°; voxel dimensions, 0.94 × 0.89 × 1 mm) at 3 T with gadobenate dimeglumine administered at 0.05 and 0.10 mmol/kg, respectively. The branching pattern was analyzed; the total number of septocutaneous perforators for each leg was calculated from the time-resolved and bolus-chase MRA data; and the results were combined. The total and average number of septocutaneous perforators per leg and the frequency of various branching patterns were calculated. The techniques were compared in terms of branching pattern and number of visible septocutaneous perforators. RESULTS A total of 84 septocutaneous perforators (1.58 ± 1.05 [SD] per leg) were identified. Pattern 1A was found in 42 legs; 1B, two legs; 2A, one leg; 2B, one; 3A, four; 3B, one; and 3D, two legs. Classification with time-resolved MRA was successful for 53 legs and with boluschase MRA for 51 legs (Z = 0.713, p = 0.24, one-tailed, not significant). Twenty-two septocutaneous perforators were identified with time-resolved MRA and 82 with bolus-chase MRA. CONCLUSION MRA of the leg can be used to investigate the branching pattern and identify septocutaneous perforators in a single step. With the imaging parameters and contrast dose used in this study, septocutaneous perforators can be better identified with boluschase MRA, although this result may be partially related to the higher gadolinium dose used in this technique.

Free-breathing liver perfusion imaging using 3-dimensional through-time spiral generalized autocalibrating partially parallel acquisition acceleration.

Objectives The goal of this study was to develop free-breathing high–spatiotemporal resolution dynamic contrast-enhanced liver magnetic resonance imaging using non-Cartesian parallel imaging acceleration, and quantitative liver perfusion mapping. Materials and Methods This study was approved by the local institutional review board and written informed consent was obtained from all participants. Ten healthy subjects and 5 patients were scanned on a Siemens 3-T Skyra scanner. A stack-of-spirals trajectory was undersampled in-plane with a reduction factor of 6 and reconstructed using 3-dimensional (3D) through-time non-Cartesian generalized autocalibrating partially parallel acquisition. High-resolution 3D images were acquired with a true temporal resolution of 1.6 to 1.9 seconds while the subjects were breathing freely. A dual-input single-compartment model was used to retrieve liver perfusion parameters from dynamic contrast-enhanced magnetic resonance imaging data, which were coregistered using an algorithm designed to reduce the effects of dynamic contrast changes on registration. Image quality evaluation was performed on spiral images and conventional images from 5 healthy subjects. Results Images with a spatial resolution of 1.9 × 1.9 × 3 mm3 were obtained with whole-liver coverage. With an imaging speed of better than 2 s/vol, free-breathing scans were achieved and dynamic changes in enhancement were captured. The overall image quality of free-breathing spiral images was slightly lower than that of conventional long breath-hold Cartesian images, but it provided clinically acceptable or better image quality. The free-breathing 3D images were registered with almost no residual motion in liver tissue. After the registration, quantitative whole-liver 3D perfusion maps were obtained and the perfusion parameters are all in good agreement with the literature. Conclusions This high–spatiotemporal resolution free-breathing 3D liver imaging technique allows voxelwise quantification of liver perfusion.

Quantitative high-resolution renal perfusion imaging using 3-dimensional through-time radial generalized autocalibrating partially parallel acquisition

ObjectivesDynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) examinations of the kidneys provide quantitative information on renal perfusion and filtration. However, these examinations are often difficult to implement because of respiratory motion and their need for a high spatiotemporal resolution and 3-dimensional coverage. Here, we present a free-breathing quantitative renal DCE-MRI examination acquired with a highly accelerated stack-of-stars trajectory and reconstructed with 3-dimensional (3D) through-time radial generalized autocalibrating partially parallel acquisition (GRAPPA), using half and quarter doses of gadolinium contrast. Materials and MethodsData were acquired in 10 asymptomatic volunteers using a stack-of-stars trajectory that was undersampled in-plane by a factor of 12.6 with respect to Nyquist sampling criterion and using partial Fourier of 6/8 in the partition direction. Data had a high temporal (2.1–2.9 seconds per frame) and spatial (approximately 2.2 mm3) resolution with full 3D coverage of both kidneys (350–370 mm2 × 79–92 mm). Images were successfully reconstructed with 3D through-time radial GRAPPA, and interframe respiratory motion was compensated by using an algorithm developed to automatically use images from multiple points of enhancement as references for registration. Quantitative pharmacokinetic analysis was performed using a separable dual-compartment model. ResultsRegion-of-interest (ROI) pharmacokinetic analysis provided estimates (mean (SD)) of quantitative renal parameters after a half dose: 218.1 (57.1) mL/min per 100 mL; plasma mean transit time, 4.8 (2.2) seconds; renal filtration, 28.7 (10.0) mL/min per 100 mL; and tubular mean transit time, 131.1 (60.2) seconds in 10 kidneys. The ROI pharmacokinetic analysis provided estimates (mean (SD)) of quantitative renal parameters after a quarter dose: 218.1 (57.1) mL/min per 100 mL; plasma mean transit time, 4.8 (2.2) seconds; renal filtration, 28.7 (10.0) mL/min per 100 mL; and tubular mean transit time, 131.1 (60.2) seconds in the 10 kidneys. Three-dimensional pixelwise parameter maps were also evaluated. ConclusionsHighly undersampled data were successfully reconstructed with 3D through-time radial GRAPPA to achieve a high-resolution 3-dimensional renal DCE-MRI examination. The acquisition was completely free breathing, and the images were registered to compensate for respiratory motion. This allowed for an accurate high-resolution 3D quantitative renal functional mapping of perfusion and filtration parameters.

Three-dimensional through-time radial GRAPPA for renal MR angiography

Purpose To achieve high temporal and spatial resolution for contrast-enhanced time-resolved MR angiography exams (trMRAs), fast imaging techniques such as non-Cartesian parallel imaging must be employed. In this study, the 3D through-time radial GRAPPA method is used to reconstruct highly accelerated stack-of-stars data for time-resolved renal MRAs.To achieve high temporal and spatial resolution for contrast‐enhanced time‐resolved MR angiography exams (trMRAs), fast imaging techniques such as non‐Cartesian parallel imaging must be used. In this study, the three‐dimensional (3D) through‐time radial generalized autocalibrating partially parallel acquisition (GRAPPA) method is used to reconstruct highly accelerated stack‐of‐stars data for time‐resolved renal MRAs.

Rapid volumetric t1 mapping of the abdomen using three-dimensional through-time spiral GRAPPA

To develop an ultrafast T1 mapping method for high‐resolution, volumetric T1 measurements in the abdomen.

Quantification of left ventricular functional parameter values using 3D spiral bSSFP and through-time Non-Cartesian GRAPPA

BackgroundThe standard clinical acquisition for left ventricular functional parameter analysis with cardiovascular magnetic resonance (CMR) uses a multi-breathhold multi-slice segmented balanced SSFP sequence. Performing multiple long breathholds in quick succession for ventricular coverage in the short-axis orientation can lead to fatigue and is challenging in patients with severe cardiac or respiratory disorders. This study combines the encoding efficiency of a six-fold undersampled 3D stack of spirals balanced SSFP sequence with 3D through-time spiral GRAPPA parallel imaging reconstruction. This 3D spiral method requires only one breathhold to collect the dynamic data.MethodsTen healthy volunteers were recruited for imaging at 3 T. The 3D spiral technique was compared against 2D imaging in terms of systolic left ventricular functional parameter values (Bland-Altman plots), total scan time (Welch’s t-test) and qualitative image rating scores (Wilcoxon signed-rank test).ResultsSystolic left ventricular functional values were not significantly different (i.e. 3D-2D) between the methods. The 95% confidence interval for ejection fraction was −0.1 ± 1.6% (mean ± 1.96*SD). The total scan time for the 3D spiral technique was 48 s, which included one breathhold with an average duration of 14’s for the dynamic scan, plus 34’s to collect the calibration data under free-breathing conditions. The 2D method required an average of 5min40s for the same coverage of the left ventricle. The difference between 3D and 2D image rating scores was significantly different from zero (Wilcoxon signed-rank test, p < 0.05); however, the scores were at least 3 (i.e. average) or higher for 3D spiral imaging.ConclusionThe 3D through-time spiral GRAPPA method demonstrated equivalent systolic left ventricular functional parameter values, required significantly less total scan time and yielded acceptable image quality with respect to the 2D segmented multi-breathhold standard in this study. Moreover, the 3D spiral technique used just one breathhold for dynamic imaging, which is anticipated to reduce patient fatigue as part of the complete cardiac examination in future studies that include patients.

Time-resolved MR angiography of the legs at 3 T using a low dose of gadolinium: Initial experience and contrast dynamics

OBJECTIVE This article describes our initial clinical experience with time-resolved MR angiography (MRA) of the legs using the time-resolved imaging with stochastic trajectories (TWIST) technique with a half dose of gadolinium. MATERIALS AND METHODS Thirty-four patients underwent a TWIST examination of the legs at 3 T. Thirty-three patients also underwent a bolus-chase MRA examination in the same setting. Times elapsed between the start of contrast injection and the appearance of contrast material (t(A)) and peak enhancement of the arteries in the legs (t(B)) were analyzed. The number of patients with examinations affected by venous contamination was determined. The differences in t(A) and t(B) between cases in which venous contamination was present or absent were evaluated using a two-tailed Student t test. RESULTS The TWIST technique using a half dose of gadolinium provided diagnostic-quality images of all patients. The mean t(A) was 35.5 ± 8.8 (SD) seconds (range, 17.8-60.4 seconds), and the mean t(B) was 59.1 ± 15.1 seconds (range, 31-98.8 seconds). Venous contamination was observed in bolus-chase MRA images of 52.9% of patients. The relationship between venous contamination and t(A) was not statistically significant (p = 0.13). The incidence of venous contamination was higher in patients with lower values of t(B) (p = 0.01). CONCLUSION The described low-dose clinical experience with TWIST and the contrast dynamics information gained from this study could aid radiologists in planning protocols for leg MRA examinations.