Rapid time‐resolved magnetic resonance angiography via a multiecho radial trajectory and GraDeS reconstruction

Contrast‐enhanced magnetic resonance angiography is challenging due to the need for both high spatial and temporal resolution. A multishot trajectory composed of pseudo‐random rotations of a single multiecho radial readout was developed. The trajectory is designed to give incoherent aliasing artifacts and a relatively uniform distribution of projections over all time scales. A field map (computed from the same data set) is used to avoid signal dropout in regions of substantial field inhomogeneity. A compressed sensing reconstruction using the GraDeS algorithm was used. Whole brain angiograms were reconstructed at 1‐mm isotropic resolution and a 1.1‐s frame rate (corresponding to an acceleration factor > 100). The only parameter which must be chosen is the number of iterations of the GraDeS algorithm. A larger number of iterations improves the temporal behavior at cost of decreased image signal‐to‐noise ratio. The resulting images provide a good depiction of the cerebral vasculature and have excellent arterial/venous separation. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Contrast‐enhanced whole‐heart coronary magnetic resonance angiography at 3 T with radial EPI

Whole‐heart coronary magnetic resonance angiography is a promising method for detecting coronary artery disease. However, the imaging time is relatively long (typically 10–15 min). The goal of this study was to implement a radial echo planar imaging sequence for contrast‐enhanced whole‐heart coronary magnetic resonance angiography, with the aim of combining the scan efficiency of echo planar imaging with the motion insensitivity of radial k‐space sampling. A self‐calibrating phase correction technique was used to correct for off‐resonance effects, trajectory measurement was used to correct for k‐space trajectory errors, and variable density sampling was used in the partition direction to reduce streaking artifacts. Seven healthy volunteers and two patients were scanned with the proposed radial echo planar imaging sequence, and the images were compared with a traditional gradient echo and X‐ray angiography techniques, respectively. Whole‐heart images with the radial EPI technique were acquired with a resolution of 1.0 × 1.0 × 2.0 mm³ in a scan time of 5 min. In healthy volunteers, the average image quality scores and visualized vessel lengths of the RCA and LAD were similar for the radial EPI and gradient echo techniques (P value > 0.05 for all). Anecdotal patient studies showed excellent agreement of the radial EPI technique with X‐ray angiography. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Fast inversion recovery magnetic resonance angiography of the intracranial arteries

Inversion‐prepared pulse sequences can be used for noncontrast MR angiography (MRA) but suffer from long scan times when acquired using conventional nonaccelerated techniques. This work proposes a subtraction‐based spin‐labeling, three‐dimensional fast inversion recovery MRA (FIR‐MRA) method for imaging the intracranial arteries. FIR‐MRA uses alternating cycles of nonselective and slab‐selective inversions, leading to dark‐blood and bright‐blood images, respectively. The signal difference between these images eliminates static background tissue and generates the angiogram. To reduce scan time, segmented fast gradient recalled echo readout and parallel imaging are applied. The inversion recovery with embedded self‐calibration method used allows for parallel acceleration at factors of 2 and above. An off‐resonance selective inversion provides effective venous suppression, with no detriment to the depiction of arteries. FIR‐MRA was compared against conventional three‐dimensional time‐of‐flight angiography at 3 T in eight normal subjects. Results showed that FIR‐MRA had superior vessel conspicuity in the distal vessels (P < 0.05), and equal or better vessel continuity and venous suppression. However, FIR‐MRA had inferior vessel sharpness (P < 0.05) in four of nine vessel groups. The clinical utility of FIR‐MRA was demonstrated in three MRA patients. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Non‐contrast‐enhanced vascular magnetic resonance imaging using flow‐dependent preparation with subtraction

Recent concerns over contrast agent safety have encouraged new developments in non‐contrast‐enhanced vascular imaging techniques. This work investigates the potential for imaging both arteries and veins with vascular anatomy by nonenhanced static subtraction angiography (VANESSA), a method using controllable flow suppression together with subtraction of bright‐ and dark‐blood images. The lower legs of eight healthy volunteers and three patients were imaged using a modified motion‐sensitized driven equilibrium preparation, with three‐dimensional balanced steady‐state free precession readout. The vascular signal decreased with increasing motion‐suppression gradient amplitude, and was suppressed when the velocity‐encoding parameter was (approximately) less than the measured flow velocity. Selected pairs of images were subtracted to depict vessels with either fast flow (e.g. arteries), slow flow (e.g. veins), or both. Several methodological modifications improved image quality and reduced the background signal from static tissues. Subjectively assessed image quality in volunteers was rated as excellent for 56/64 arterial segments, and good or excellent for 35/64 veins. In conclusion, VANESSA enables rapid non‐contrast‐enhanced imaging of arteries and veins, combining information on both morphology and flow. This study demonstrates good technical performance in volunteers and evaluation in patients with vascular disease is warranted. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Non‐contrast‐enhanced flow‐independent peripheral MR angiography with balanced SSFP

Flow‐independent angiography is a non‐contrast‐enhanced technique that can generate vessel contrast even with reduced blood flow in the lower extremities. A method is presented for producing these angiograms with magnetization‐prepared balanced steady‐state free precession (bSSFP). Because bSSFP yields bright fat signal, robust fat suppression is essential for detailed depiction of the vasculature. Therefore, several strategies have been investigated to improve the reliability of fat suppression within short scan times. Phase‐sensitive SSFP can efficiently suppress fat; however, partial volume effects due to fat and water occupying the same voxel can lead to the loss of blood signal. In contrast, alternating repetition time (ATR) SSFP minimizes this loss; however, the level of suppression is compromised by field inhomogeneity. Finally, a new double‐acquisition ATR‐SSFP technique reduces this sensitivity to off‐resonance. In vivo results indicate that the two ATR‐based techniques provide more reliable contrast when partial volume effects are significant. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Highly accelerated contrast‐enhanced MR angiography: Improved reconstruction accuracy and reduced noise amplification with complex subtraction

Contrast‐enhanced magnetic resonance angiography is routinely performed using parallel imaging to best capture the first pass of contrast material through the target vasculature, followed by digital subtraction to suppress the appearance of unwanted signal from background tissue. Both processes, however, amplify noise and can produce uninterpretable images when large acceleration factors are used. Using a phantom study of contrast‐enhanced magnetic resonance angiography, we show that complex subtraction processing prior to partially parallel reconstruction improves reconstruction accuracy relative to magnitude subtraction processing for reduction factors as large as 12. Time‐resolved contrast‐enhanced magnetic resonance angiographic data obtainedwith complex subtraction in volunteers supported the results of the phantom study and when compared with magnitude subtraction processing demonstrated reduced geometry factors as well as improved image quality at large reduction factors. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Non‐contrast‐enhanced four‐dimensional (4D) intracranial MR angiography: A feasibility study

T₁‐shortening contrast agents have been widely used in time‐resolved magnetic resonance angiography. To match imaging data acquisition with the short time period of the first pass of contrast agent, temporal resolution and/or spatial resolution have to be compromised in many cases. In this study, a novel non‐contrast‐enhanced technique was developed for time‐resolved magnetic resonance angiography. Alternating magnetization preparation was applied in two consecutive acquisitions of each measurement to eliminate the need for contrast media. Without the constraint of contrast media kinetics, temporal resolution is drastically improved from the order of a second as in conventional contrast‐enhanced approach to tens of milliseconds (50.9 msec) in this study, without compromising spatial resolution. Initial results from volunteer studies demonstrate the feasibility of this method to depict anatomic structure and dynamic filling of main vessels in the head. Magn Reson Med 63:835–841, 2010. © 2010 Wiley‐Liss, Inc.     

Interleaved variable density sampling with a constrained parallel imaging reconstruction for dynamic contrast‐enhanced MR angiography

For MR applications such as contrast‐enhanced MR angiography, it is desirable to achieve simultaneously high spatial and temporal resolution. The current clinical standard uses view‐sharing methods combined with parallel imaging; however, this approach still provides limited spatial and temporal resolution. To improve on the clinical standard, we present an interleaved variable density (IVD) sampling method that pseudorandomly undersamples each individual frame of a 3D Cartesian k_y–k_z plane combined with parallel imaging acceleration. From this dataset, time‐resolved images are reconstructed with a method that combines parallel imaging with a multiplicative constraint. Total acceleration factors on the order of 20 are achieved for contrast‐enhanced MR angiography of the lower extremities, and improvements in temporal fidelity of the depiction of the contrast bolus passage are demonstrated relative to the clinical standard. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Targeted ROTational magnetic resonance angiography (TROTA).

An MR angiographic method is presented in which a rotating 2D slice is centered on and targets a region or vessel of interest. Collecting a series of slices rotating about the center of the targeted region yields projection data sufficient for the calculation of 3D volumetric data of the region using conventional backprojection reconstruction techniques. These volumetric data depict the internal structure of the vessel and can be processed and displayed with multiplanar reformation, maximum intensity projections, and 3D rendering algorithms. The rotational angiographic acquisition preserves the high temporal resolution of 2D-MR digital subtraction angiography with the added benefit of 3D reformatting and display. The method is explained in detail and results from phantom and human experiments are presented.