Ethan K. Brodsky

Multiecho water-fat separation and simultaneous R2* estimation with multifrequency fat spectrum modeling.

Multiecho chemical shift–based water‐fat separation methods are seeing increasing clinical use due to their ability to estimate and correct for field inhomogeneities. Previous chemical shift‐based water‐fat separation methods used a relatively simple signal model that assumes both water and fat have a single resonant frequency. However, it is well known that fat has several spectral peaks. This inaccuracy in the signal model results in two undesired effects. First, water and fat are incompletely separated. Second, methods designed to estimate T  2* in the presence of fat incorrectly estimate the T  2* decay in tissues containing fat. In this work, a more accurate multifrequency model of fat is included in the iterative decomposition of water and fat with echo asymmetry and least‐squares estimation (IDEAL) water‐fat separation and simultaneous T  2* estimation techniques. The fat spectrum can be assumed to be constant in all subjects and measured a priori using MR spectroscopy. Alternatively, the fat spectrum can be estimated directly from the data using novel spectrum self‐calibration algorithms. The improvement in water‐fat separation and T  2* estimation is demonstrated in a variety of in vivo applications, including knee, ankle, spine, breast, and abdominal scans. Magn Reson Med 60:1122–1134, 2008.

Generalized k-space decomposition with chemical shift correction for non-Cartesian water-fat imaging.

Chemical‐shift artifacts associated with non‐Cartesian imaging are more complex to model and less clinically acceptable than the bulk fat shift that occurs with conventional spin‐warp Cartesian imaging. A novel k‐space based iterative decomposition of water and fat with echo asymmetry and least‐squares estimation (IDEAL) approach is introduced that decomposes multiple species while simultaneously correcting distortion of off‐resonant species. The new signal model accounts for the additional phase accumulated by off‐resonant spins at each point in the k‐space acquisition trajectory. This phase can then be corrected by adjusting the decomposition matrix for each k‐space point during the final IDEAL processing step with little increase in reconstruction time. The technique is demonstrated with water‐fat decomposition using projection reconstruction (PR)/radial, spiral, and Cartesian spin‐warp imaging of phantoms and human subjects, in each case achieving substantial correction of chemical‐shift artifacts. Simulations of the point‐spread‐function (PSF) for off‐resonant spins are examined to show the nature of the chemical‐shift distortion for each acquisition. Also introduced is an approach to improve the signal model for species which have multiple resonant peaks. Many chemical species, including fat, have multiple resonant peaks, although such species are often approximated as a single peak. The improved multipeak decomposition is demonstrated with water‐fat imaging, showing a substantial improvement in water‐fat separation. Magn Reson Med 59:1151–1164, 2008.

Rapid fat-suppressed isotropic steady-state free precession imaging using true 3D multiple-half-echo projection reconstruction.

Three‐dimensional projection reconstruction (3D PR)‐based techniques are advantageous for steady‐state free precession (SSFP) imaging for several reasons, including the capability to achieve short repetition times (TRs). In this paper, a multi‐half‐echo technique is presented that dramatically improves the data‐sampling efficiency of 3D PR sequences while it retains this short‐TR capability. The k‐space trajectory deviations are measured quickly and corrected on a per‐sample point basis. A two‐pass RF cycling technique is then applied to the dual‐half‐echo implementation to generate fat/water‐separated images. The resultant improvement in the signal‐to‐noise ratio (SNR) and contrast‐to‐noise ratio (CNR) was demonstrated in volunteer studies. Volumetric images with excellent spatial resolution, coverage, and contrast were obtained with high speed. The non‐contrast‐enhanced SSFP studies show that this technique has promising potential for MR angiography (MRA). Magn Reson Med 53:692–699, 2005.

Contrast-enhanced peripheral magnetic resonance angiography using time-resolved vastly undersampled isotropic projection reconstruction

To investigate the application of time‐resolved vastly undersampled isotropic projection reconstruction (VIPR) in contrast‐enhanced magnetic resonance angiography of the distal extremity (single station), and peripheral run‐off vasculature in the abdomen, thigh, and calf (three stations).

Generation and visualization of four-dimensional MR angiography data using an undersampled 3-D projection trajectory

Time-resolved contrast-enhanced magnetic resonance (MR) angiography (CE-MRA) has gained in popularity relative to X-ray Digital Subtraction Angiography because it provides three-dimensional (3-D) spatial resolution and it is less invasive. We have previously presented methods that improve temporal resolution in CE-MRA while providing high spatial resolution by employing an undersampled 3-D projection (3D PR) trajectory. The increased coverage and isotropic resolution of the 3D PR acquisition simplify visualization of the vasculature from any perspective. We present a new algorithm to develop a set of time-resolved 3-D image volumes by preferentially weighting the 3D PR data according to its acquisition time. An iterative algorithm computes a series of density compensation functions for a regridding reconstruction, one for each time frame, that exploit the variable sampling density in 3D PR. The iterative weighting procedure simplifies the calculation of appropriate density compensation for arbitrary sampling patterns, which improve sampling efficiency and, thus, signal-to-noise ratio and contrast-to-noise ratio, since it is does not require a closed-form calculation based on geometry. Current medical workstations can display these large four-dimensional studies, however, interactive cine animation of the data is only possible at significantly degraded resolution. Therefore, we also present a method for interactive visualization using powerful graphics cards and distributed processing. Results from volunteer and patient studies demonstrate the advantages of dynamic imaging with high spatial resolution.

3D hyperpolarized He-3 MRI of ventilation using a multi-echo projection acquisition

A method is presented for high‐resolution 3D imaging of the whole lung using inhaled hyperpolarized (HP) He‐3 MR with multiple half‐echo radial trajectories that can accelerate imaging through undersampling. A multiple half‐echo radial trajectory can be used to reduce the level of artifact for undersampled 3D projection reconstruction (PR) imaging by increasing the amount of data acquired per unit time for HP He‐3 lung imaging. The point spread functions (PSFs) for breath‐held He‐3 MRI using multiple half‐echo trajectories were evaluated using simulations to predict the effects of T2* and gas diffusion on image quality. Results from PSF simulations were consistent with imaging results in volunteer studies showing improved image quality with increasing number of echoes using up to 8 half‐echoes. The 8‐half‐echo acquisition is shown to accommodate lost breath‐holds as short as 6 sec using a retrospective reconstruction at reduced resolution and also to allow reduced breath‐hold time compared with an equivalent Cartesian trajectory. Furthermore, preliminary results from a 3D dynamic inhalation‐exhalation maneuver are demonstrated using the 8‐half‐echo trajectory. Results demonstrate the first high‐resolution 3D PR imaging of ventilation and respiratory dynamics in humans using HP He‐3 MR. Magn Reson Med 59:1062–1071, 2008.

Characterizing and correcting gradient errors in non‐cartesian imaging: Are gradient errors linear time‐invariant (LTI)?

Non‐Cartesian and rapid imaging sequences are more sensitive to scanner imperfections such as gradient delays and eddy currents. These imperfections vary between scanners and over time and can be a significant impediment to successful implementation and eventual adoption of non‐Cartesian techniques by scanner manufacturers. Differences between the k‐space trajectory desired and the trajectory actually acquired lead to misregistration and reduction in image quality. While early calibration methods required considerable scan time, more recent methods can work more quickly by making certain approximations. We examine a rapid gradient calibration procedure applied to multiecho three‐dimensional projection reconstruction (3DPR) acquisitions in which the calibration runs as part of every scan. After measuring the trajectories traversed for excitations on each of the orthogonal gradient axes, trajectories for the oblique projections actually acquired during the scan are synthesized as linear combinations of these measurements. The ability to do rapid calibration depends on the assumption that gradient errors are linear and time‐invariant (LTI). This work examines the validity of these assumptions and shows that the assumption of linearity is reasonable, but that gradient errors can vary over short time periods (due to changes in gradient coil temperature) and thus it is important to use calibration data matched to the scan data. Magn Reson Med, 2009.

Three-dimensional imaging of ventilation dynamics in asthmatics using multiecho projection acquisition with constrained reconstruction.

The purpose of this work is to detect dynamic gas trapping in three dimensions during forced exhalation at isotropic high spatial resolution and high temporal resolution using hyperpolarized helium‐3 MRI. Ten subjects underwent hyperpolarized helium‐3 MRI and multidetector CT. MRI was performed throughout inspiration, breath‐hold, and forced expiration. A multiecho three‐dimensional projection acquisition was used to improve data collection efficiency and an iterative constrained reconstruction was implemented to improve signal to noise ratio (SNR) and increase robustness to motion. Two radiologists evaluated the dynamic MRI and breath‐held multidetector CT data for gas and air trapping, respectively. Phantom studies showed the proposed technique significantly improved depiction of moving objects compared to view‐sharing methods. Gas trapping was detected using MRI in five of the six asthmatic subjects who displayed air trapping with multidetector CT. Locations in disagreement were found to represent small to moderate regions of air trapping. The proposed technique provides whole‐lung three‐dimensional imaging of respiration dynamics at high spatial and temporal resolution and compares well to the current standard, multidetector CT. While multidetector CT can provide information about static regional air trapping, it is unable to depict dynamics in a setting more comparable to a spirometry maneuver and explore the longitudinal time evolution of the trapped regions. Magn Reson Med, 2009.

Overexpressing Corticotropin-Releasing Factor in the Primate Amygdala Increases Anxious Temperament and Alters Its Neural Circuit

BACKGROUND Nonhuman primate models are critical for understanding mechanisms underlying human psychopathology. We established a nonhuman primate model of anxious temperament (AT) for studying the early-life risk to develop anxiety and depression. Studies have identified the central nucleus of the amygdala (Ce) as an essential component of ATs neural substrates. Corticotropin-releasing factor (CRF) is expressed in the Ce, has a role in stress, and is linked to psychopathology. Here, in young rhesus monkeys, we combined viral vector technology with assessments of anxiety and multimodal neuroimaging to understand the consequences of chronically increased CRF in the Ce region. METHODS Using real-time intraoperative magnetic resonance imaging-guided convection-enhanced delivery, five monkeys received bilateral dorsal amygdala Ce-region infusions of adeno-associated virus serotype 2 containing the CRF construct. Their cagemates served as unoperated control subjects. AT, regional brain metabolism, resting functional magnetic resonance imaging, and diffusion tensor imaging were assessed before and 2 months after viral infusions. RESULTS Dorsal amygdala CRF overexpression significantly increased AT and metabolism within the dorsal amygdala. Additionally, we observed changes in metabolism in other AT-related regions, as well as in measures of functional and structural connectivity. CONCLUSIONS This study provides a translational roadmap that is important for understanding human psychopathology by combining molecular manipulations used in rodents with behavioral phenotyping and multimodal neuroimaging measures used in humans. The results indicate that chronic CRF overexpression in primates not only increases AT but also affects metabolism and connectivity within components of ATs neural circuitry.

High‐spatial and high‐temporal resolution dynamic contrast‐enhanced perfusion imaging of the liver with time‐resolved three‐dimensional radial MRI

Detection, characterization, and monitoring the treatment of hepatocellular carcinomas (HCC) in patients with cirrhosis is challenging because of their variable and rapid arterial enhancement. Multiphase dynamic contrast‐enhanced MRI is used clinically for HCC assessment; however, the method suffers from limited temporal resolution and difficulty in coordinating imaging and breath‐hold timing within a narrow temporal window of interest. In this article, a volumetric, high‐spatial resolution, and high‐temporal resolution dynamic contrast‐enhanced liver imaging method for improved detection and characterization of HCC is demonstrated.