Anthony Vu

Multiecho reconstruction for simultaneous water‐fat decomposition and T2* estimation

To describe and demonstrate the feasibility of a novel multiecho reconstruction technique that achieves simultaneous water‐fat decomposition and T2* estimation. The method removes interference of water‐fat separation with iron‐induced T2* effects and therefore has potential for the simultaneous characterization of hepatic steatosis (fatty infiltration) and iron overload.

Fast spin echo sequences with very long echo trains: design of variable refocusing flip angle schedules and generation of clinical T2 contrast.

Reducing and continuously varying the flip angle of the refocusing RF pulses in a rapid acquisition with relaxation enhancement (RARE; fast/turbo spin echo) sequence is a useful means of addressing high RF power deposition and modulation transfer function (MTF) distortion due to relaxation. This work presents a streamlined technique to generate a sequence of refocusing flip angles on a per‐prescription basis that produces relatively high SNR and limits blurring in a wide range of materials encountered in vivo. Since the “effective TE” (traditionally defined as the time at which the center of k‐space is sampled) no longer corresponds to the expected amount of spin‐echo T2 contrast due to the mixing of stimulated and spin echoes, a “contrast‐equivalent” TE is defined and experimentally demonstrated that allows annotation of a more accurate effective TE that matches the contrast produced by 180° refocusing. Furthermore, contrast is shown to be manipulable by the addition of magnetization preparation pulse sequence segments, such as T2‐prep, to produce clinically desirable contrast for routine head and body imaging. Magn Reson Med, 2006.

Effects of refocusing flip angle modulation and view ordering in 3D fast spin echo

Recent advances have reduced scan time in three‐dimensional fast spin echo (3D‐FSE) imaging, including very long echo trains through refocusing flip angle (FA) modulation and 2D‐accelerated parallel imaging. This work describes a method to modulate refocusing FAs that produces sharp point spread functions (PSFs) from very long echo trains while exercising direct control over minimum, center‐k‐space, and maximum FAs in order to accommodate the presence of flow and motion, SNR requirements, and RF power limits. Additionally, a new method for ordering views to map signal modulation from the echo train into ky‐kz space that enables nonrectangular k‐space grids and autocalibrating 2D‐accelerated parallel imaging is presented. With long echo trains and fewer echoes required to encode large matrices, large volumes with high in‐ and through‐plane resolution matrices may be acquired with scan times of 3–6 min, as demonstrated for volumetric brain, knee, and kidney imaging. Magn Reson Med 60:640–649, 2008.

Three-dimensional T1 mapping for dGEMRIC at 3.0 T using the Look Locker method.

Objective:The objective of this study was to implement a three-dimensional (3-D) T1 mapping sequence (3DLL) at 3.0 T for dGEMRIC based on the Look Locker scheme. Materials and Methods:Because all current reports on dGEMRIC are at 1.5 T and mostly using 2-D IR fast spin echo (FSE), data were acquired at 1.5 T and 3.0 T with both 3DLL and 2-D IR-FSE sequence. Phantoms with different concentrations of Gd(DTPA)2− were used and seven subjects (three asymptomatic, four symptomatic) were scanned using the dGEMRIC technique. Results:The T1 measurements obtained on the phantom with 3DLL show very good agreement with those acquired with 2-D IR-FSE. Using a two-tailed paired t test, the T1 (Gd) measurements in two sections obtained in all subjects with both sequences were found to be statistically indistinguishable at either field strength (P = 0.07 at 1.5 T and P = 0.07 at 3.0 T). Conclusions:The preliminary data presented here suggest that the 3DLL sequence provides accurate T1 values with sufficient in-plane resolution and allows full joint coverage in less than 10 minutes.

Fat-suppressed three-dimensional dual echo Dixon technique for contrast agent enhanced MRI.

To develop a fast T1‐weighted, fat‐suppressed three‐dimensional dual echo Dixon technique and to demonstrate its use in contrast agent enhanced MRI.

Magnetic resonance imaging of the pancreas at 3.0 tesla: qualitative and quantitative comparison with 1.5 tesla.

Objectives:We sought to perform a preliminary comparison of signal-to-noise ratio (SNR) and image quality for magnetic resonance imaging (MRI) of the pancreas at 1.5 and 3 T. Materials and Methods:Two imaging cohorts were studied using a T2-weighted, single-shot fast spin-echo pulse sequence and a T1-weighted, fat-suppressed 3D gradient-echo pulse sequence. In the first cohort, 4 subjects were imaged using identical imaging parameters before and after contrast administration at 1.5 and 3.0 T. The SNR was quantified for the pancreas as well as for the liver, spleen, and muscle. In a second cohort of 12 subjects in whom the receiver bandwidth was adjusted for field strength, SNR measurements and qualitative rankings of image quality were performed. Results:In the study cohort using identical imaging parameters at both magnetic field strengths, the mean (SD) ratios of SNR at 3.0 to 1.5 T of the single-shot fast spin-echo images for the pancreas, liver, spleen, and muscle were 1.63 (0.39), 1.82 (0.39), 1.45 (0.18), 2.01 (0.16), respectively. For the precontrast fat-suppressed 3D gradient-echo sequence, the corresponding ratios were 1.28 (0.29), 1.26 (0.30), 1.16 (0.27), and 1.76 (0.45), respectively; for the arterial phase, the corresponding ratios were 2.02 (0.28), 1.60 (0.42), 1.47 (0.26), and 1.94 (0.32), respectively; and for the delayed postcontrast phase, the corresponding ratios were 1.63 (0.51), 2.01 (0.25), 1.66 (0.06), and 2.31 (0.47), respectively. The SNR benefit of 3.0 T was significantly greater on contrast-enhanced as compared with noncontrast T1-weighted 3D gradient-echo images. In the second study cohort, SNR was superior at 3.0 T, although the use of a reduced readout bandwidth at 1.5 T substantially diminished the advantage of the higher field system. With qualitative comparison of images obtained at the 2 magnetic field strengths, the fat-suppressed 3D gradient-echo images obtained at 3.0 T were preferred, whereas the single shot fast spin-echo images obtained at 1.5 T were preferred because of better signal homogeneity. Conclusions:Our results in a small cohort of volunteers and patients demonstrate a marked improvement in SNR at 3.0 T compared with 1.5 T (by a factor of 2 in some cases) when identical imaging parameters were used. The SNR advantage at 3.0 T is diminished but persists when the receiver bandwidth is adjusted for magnetic field strength. The results suggest that 3.0 T may offer promise for improved body MRI, although further technical development to optimize SNR and improve signal homogeneity will be needed before its full potential can be achieved.

Accuracy of T1 measurement with 3‐D Look‐Locker technique for dGEMRIC

To validate the accuracy of T1 measurement by three‐dimensional Look‐Locker method (3D LL) for delayed gadolinium‐enhanced MRI of cartilage (dGEMRIC) of human subjects with and without osteoarthritis (OA), as compared with two‐dimensional inversion recovery fast spin‐echo (2D IR‐FSE) technique.

Gadolinium-enhanced off-resonance contrast angiography.

We describe a novel physical basis and methodology for gadolinium (Gd)‐enhanced MRA, which we call “off‐resonance contrast angiography” (ORCA). Unlike standard contrast‐enhanced (CE) MR angiography (MRA), ORCA contrast depends not on T1 but on Gd‐induced shifts in intravascular resonance frequency due to the bulk magnetic susceptibility (BMS) effects of Gd. The method was tested at 3 Tesla in phantoms with a range of dilutions of Gd‐DTPA and ultrasmall iron oxide contrast agent (CA). With the use of ORCA, complete background suppression was obtained without image subtraction. As a result, catheters filled with various Gd dilutions proved to be highly conspicuous in ORCA projection images. This feature may make ORCA particularly attractive for passive catheter tracking during MR‐guided endovascular procedures. Gd‐induced intravascular frequency shifts were measured in human subjects and found to be in the expected range. ORCA was used to create angiograms of forearm veins that were comparable in quality to standard CE‐MRA. In addition, ORCA images of the extracranial carotid bifurcation were successfully acquired during intravenous contrast administration. However, significant technical restrictions also exist, including a dependence on vessel orientation with respect to B0, and sensitivity to static field inhomogeneities. Further study is needed to determine the practicality and potential clinical utility of this method. Magn Reson Med 57:475–484, 2007.

Evaluation of Intrarenal Oxygenation at 3.0 T Using 3-Dimensional Multiple Gradient-Recalled Echo Sequence

Objective:The objective of this study was to validate quantitation of R2* and &Dgr;R2* measurements obtained with a 3-dimensional (3-D) multiple gradient-recalled echo (mGRE) sequence for evaluating intrarenal oxygenation in humans. Materials and Methods:Validation was accomplished (1) by comparing R2* values with previously established 2-D techniques (n = 5, mean age = 33.6 years) and (2) by measuring change in &Dgr;R2* after furosemide (20 mg intravenously) administration (n = 5, mean age = 22 years). Additional pre- and postfurosemide scans were done at 1.5 T for comparison purposes. Results:R2* measurements with the 3-D technique showed good agreement with the 2-D techniques. The baseline medullary R2* at 3.0 T was about twice the value found at 1.5 T. Furosemide-induced change in R2* was observed within 5 minutes after administration. Conclusions:R2* measurements with 3-D mGRE were comparable with those reported using 2-D techniques. The 3-D implementation facilitates observation of temporal changes in the medullary oxygenation without compromising spatial coverage.

Three-dimensional fast spoiled gradient-echo dual echo (3D-FSPGR-DE) with water reconstruction: Preliminary experience with a novel pulse sequence for gadolinium-enhanced abdominal MR imaging

To compare three‐dimensional fast spoiled gradient‐echo dual‐echo (3D‐FSPGR‐DE) with water reconstruction to conventional 3D‐FSPGR for gadolinium‐enhanced abdominal imaging.