Three-dimensional analytical magnetic resonance imaging phantom in the Fourier domain.

This work presents a basic framework for constructing a 3D analytical MRI phantom in the Fourier domain. In the image domain the phantom is modeled after the work of Kak and Roberts on a 3D version of the famous Shepp-Logan head phantom. This phantom consists of several ellipsoids of different sizes, orientations, locations, and signal intensities (or gray levels). It will be shown that the k-space signal derived from the phantom can be analytically expressed. As a consequence, it enables one to bypass the need for interpolation in the Fourier domain when testing image-reconstruction algorithms. More importantly, the proposed framework can serve as a benchmark for contrasting and comparing different image-reconstruction techniques in 3D MRI with a non-Cartesian k-space trajectory. The proposed framework can also be adapted for 3D MRI simulation studies in which the MRI parameters of interest may be introduced to the signal intensity from the ellipsoid.     

Three‐dimensional (3D) visualization of endolymphatic hydrops after intratympanic injection of Gd‐DTPA: Optimization of a 3D‐real inversion‐recovery turbo spin‐echo (TSE) sequence and application of a 32‐channel head coil at 3T

Purpose:

To enable volume visualization of endolymphatic hydrops of Ménière's disease via a volume rendering (VR) technique, a three‐dimensional (3D) inversion‐recovery (IR) sequence with real reconstruction (3D‐real IR) sequence after intratympanic injection of Gd‐DTPA was optimized for higher spatial resolution using a 32‐channel head coil at 3T.

Materials and Methods:

Pulse sequence parameters were optimized using a diluted Gd‐DTPA phantom. Then, 11 patients who had been clinically diagnosed with Ménière's disease and a patient with sudden hearing loss were scanned. Images were processed using commercially available 3D‐VR software. 3D‐real IR data was processed to produce endolymph and perilymph fluid volume images in different colors. 3D‐CISS data was processed to generate total fluid volume images.

Results:

While maintaining a comparable signal‐to‐noise ratio (SNR) and scan time, the voxel volume could be reduced from 0.4 × 0.4 × 2 mm³ with a 12‐channel coil to 0.4 × 0.4 × 0.8 mm³ with a 32‐channel coil. A newly‐optimized protocol allowed the smooth, three‐dimensional visualization of endolymphatic hydrops in all patients with Ménière's disease.

Conclusion:

Volumetrically separate visualization of endo‐/perilymphatic space is now feasible in patients with Ménière's disease using an optimized 3D‐real IR sequence, a 32‐channel head coil, at 3T, after intratympanic administration of Gd‐DTPA. This will aid the understanding of the pathophysiology of Ménière's disease. J. Magn. Reson. Imaging 2010;31:210–214. © 2009 Wiley‐Liss, Inc.     

Carotid arterial wall MRI at 3T using 3D variable‐flip‐angle turbo spin‐echo (TSE) with flow‐sensitive dephasing (FSD)

Purpose:

To evaluate the effectiveness of flow‐sensitive dephasing (FSD) magnetization preparation in improving blood signal suppression of three‐dimensional (3D) turbo spin‐echo (TSE) sequence (SPACE) for isotropic high‐spatial‐resolution carotid arterial wall imaging at 3T.

Materials and Methods:

The FSD‐prepared SPACE sequence (FSD‐SPACE) was implemented by adding two identical FSD gradient pulses right before and after the first refocusing 180°‐pulse of the SPACE sequence in all three orthogonal directions. Nine healthy volunteers were imaged at 3T with SPACE, FSD‐SPACE, and multislice T2‐weighted 2D TSE coupled with saturation band (SB‐TSE). Apparent carotid wall‐lumen contrast‐to‐noise ratio (aCNR_w‐l) and apparent lumen area (aLA) at the locations with residual‐blood (rb) signal shown on SPACE images were compared between SPACE and FSD‐SPACE. Carotid aCNR_w‐l and lumen (LA) and wall area (WA) measured from FSD‐SPACE were compared to those measured from SB‐TSE.

Results:

Plaque‐mimicking flow artifacts identified in seven carotids on SPACE images were eliminated on FSD‐SPACE images. The FSD preparation resulted in slightly reduced aCNR_w‐l (P = 0.025), but significantly improved aCNR between the wall and rb regions (P < 0.001) and larger aLA (P < 0.001). Compared to SB‐TSE, FSD‐SPACE offered comparable aCNR_w‐l with much higher spatial resolution, shorter imaging time, and larger artery coverage. The LA and WA measurements from the two techniques were in good agreement based on intraclasss correlation coefficient (0.988 and 0.949, respectively; P < 0.001) and Bland‐Altman analyses.

Conclusion:

FSD‐SPACE is a time‐efficient 3D imaging technique for carotid arterial wall with superior spatial resolution and blood signal suppression. J. Magn. Reson. Imaging 2010;31:645–654. © 2010 Wiley‐Liss, Inc.     

Sodium MRI using a density‐adapted 3D radial acquisition technique

A density‐adapted three‐dimensional radial projection reconstruction pulse sequence is presented which provides a more efficient k‐space sampling than conventional three‐dimensional projection reconstruction sequences. The gradients of the density‐adapted three‐dimensional radial projection reconstruction pulse sequence are designed such that the averaged sampling density in each spherical shell of k‐space is constant. Due to hardware restrictions, an inner sphere of k‐space is sampled without density adaption. This approach benefits from both the straightforward handling of conventional three‐dimensional projection reconstruction sequence trajectories and an enhanced signal‐to‐noise ratio (SNR) efficiency akin to the commonly used three‐dimensional twisted projection imaging trajectories. Benefits for low SNR applications, when compared to conventional three‐dimensional projection reconstruction sequences, are demonstrated with the example of sodium imaging. In simulations of the point‐spread function, the SNR of small objects is increased by a factor 1.66 for the density‐adapted three‐dimensional radial projection reconstruction pulse sequence sequence. Using analytical and experimental phantoms, it is shown that the density‐adapted three‐dimensional radial projection reconstruction pulse sequence allows higher resolutions and is more robust in the presence of field inhomogeneities. High‐quality in vivo images of the healthy human leg muscle and the healthy human brain are acquired. For equivalent scan times, the SNR is up to a factor of 1.8 higher and anatomic details are better resolved using density‐adapted three‐dimensional radial projection reconstruction pulse sequence. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

A 20‐channel receive‐only mouse array coil for a 3 T clinical MRI system

A 20‐channel phased‐array coil for MRI of mice has been designed, constructed, and validated with bench measurements and high‐resolution accelerated imaging. The technical challenges of designing a small, high density array have been overcome using individual small‐diameter coil elements arranged on a cylinder in a hexagonal overlapping design with adjacent low impedance preamplifiers to further decouple the array elements. Signal‐to‐noise ratio (SNR) and noise amplification in accelerated imaging were simulated and quantitatively evaluated in phantoms and in vivo mouse images. Comparison between the 20‐channel mouse array and a length‐matched quadrature driven small animal birdcage coil showed an SNR increase at the periphery and in the center of the phantom of 3‐ and 1.3‐fold, respectively. Comparison with a shorter but SNR‐optimized birdcage coil (aspect ratio 1:1 and only half mouse coverage) showed an SNR gain of twofold at the edge of the phantom and similar SNR in the center. G‐factor measurements indicate that the coil is well suited to acquire highly accelerated images. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Spatially encoded phase‐contrast MRI—3D MRI movies of 1D and 2D structures at millisecond resolution

This work demonstrates that the principles underlying phase‐contrast MRI may be used to encode spatial rather than flow information along a perpendicular dimension, if this dimension contains an MRI‐visible object at only one spatial location. In particular, the situation applies to 3D mapping of curved 2D structures which requires only two projection images with different spatial phase‐encoding gradients. These phase‐contrast gradients define the field of view and mean spin‐density positions of the object in the perpendicular dimension by respective phase differences. When combined with highly undersampled radial fast low angle shot (FLASH) and image reconstruction by regularized nonlinear inversion, spatial phase‐contrast MRI allows for dynamic 3D mapping of 2D structures in real time. First examples include 3D MRI movies of the acting human hand at a temporal resolution of 50 ms. With an even simpler technique, 3D maps of curved 1D structures may be obtained from only three acquisitions of a frequency‐encoded MRI signal with two perpendicular phase encodings. Here, 3D MRI movies of a rapidly rotating banana were obtained at 5 ms resolution or 200 frames per second. In conclusion, spatial phase‐contrast 3D MRI of 2D or 1D structures is respective two or four orders of magnitude faster than conventional 3D MRI. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.