David L. Foxall

Inflow effect correction in fast gradient-echo perfusion imaging.

The purposes of this study were to assess the extent of the inflow effect on signal intensity (SI) for fast gradient‐recalled‐echo (GRE) sequences used to observe first‐pass perfusion, and to develop and validate a correction method for this effect. A phantom experiment with a flow apparatus was performed to determine SI as a function of Gd‐DTPA concentration for various velocities. Subsequently a flow‐sensitive calibration method was developed, and validated on bolus injections into an open‐circuit flow apparatus and in vivo. It is shown that calibration methods based on static phantoms are not appropriate for accurate signal‐to‐concentration conversion in images affected by high flow. The flow‐corrected calibration method presented here can be used to improve the accuracy and robustness of the arterial input function (AIF) determination for tissue perfusion quantification using MRI and contrast media. Magn Reson Med 50:885–891, 2003.

Frequency-modulated steady-state free precession imaging

Exploration of the possibilities of steady‐state free precession (SSFP) excitation has led to the discovery that it is tolerant of slow variations in spectral offset frequency. The effect has been used to eliminate banding artifacts from images obtained with the fully balanced SSFP imaging sequence. Magn Reson Med 48:502–508, 2002.

Dynamic B0 shimming at 7 T

Dynamic slice-wise shimming improves B0 field homogeneity by updating shim coil currents for every slice in a multislice acquisition, producing better field homogeneity over a volume than can be obtained by a single static global shim. The first aim of this work was to evaluate the performance of slice-wise field-map-based second-order dynamic shimming in a human high-field 7 T clinical scanner vis-à-vis image based second order static global shimming. Another goal was to characterize eddy currents induced by second and third order shim switching. A final aim was to compare global and dynamic shimming through shim orders to elucidate the relative benefits of going to higher orders and to dynamic shim updating from a static shimming regime. An external hardware module was used to store and dynamically update slice-optimized shim values during multislice data acquisition. High-bandwidth multislice gradient echo scans with B0 field mapping and low-bandwidth single-shot echo planar scans were performed on phantoms and humans using second-order dynamic and static global shims. For the measurement of second and third order shim induced eddy currents, step response temporal phase changes of individual shims were measured and fit to shim harmonics spatially and to multiexponential decay functions temporally. Finally, an order-wise field-map-based comparison was performed with first, second and third order global static shimming, first and second order dynamic shimming, as well as combined second or third order global and first order dynamic shim. Dynamic shimming considerably improved B0 homogeneity compared to static global shimming both in phantoms and in human subjects, reducing image distortion and signal dropout. The unshielded second and third order shims generated strong B0 and self and cross-term eddy fields, with multiple time constants ranging from milliseconds to seconds. Field homogeneity improved with increasing order of shim, with dynamic shimming performing better than global shimming. Hybrid global and dynamic shimming approach yielded field homogeneity better than global static shims but worse than dynamic shims.

Rapid iterative reconstruction for echo planar imaging

A rapid automated method for reconstructing echo planar imaging (EPI) data has been developed and is shown to improve image quality by suppressing the troublesome ghost artifact. The algorithm can be applied without prior knowledge obtained from either reference scans or operator intervention. It first estimates, then improves iteratively, the parameters for a linear phase correction applied directly to the complex image data derived from odd and even echoes. The theory used to derive the criteria employed in the iteration provides insight into mechanisms that allow the process to work. Magn Reson Med 42:541–547, 1999.

FAST sequences optimization for contrast media pharmacokinetic quantification in tissue

The purpose of this study was to investigate the influence of the fast gradient‐recalled echo (GRE) sequence parameters on the contrast dynamic range and signal sensitivity, to optimize the magnetic resonance (MR) sequence for contrast media pharmacokinetic assessment. Effects of the fast low‐angle shot (FLASH), Fast acquisition at steady rate (FAST), and radiofrequency‐spoiled (RF)‐FAST sequence parameters were studied in vitro. The FAST sequence had the highest sensitivity in low gadolinium (Gd) concentration. The FLASH and RF‐FAST sequences had a larger contrast dynamic range, but the FLASH images contained side band artifacts. Increasing the flip angle to 90° raised the sensitivity of the FAST sequence and the contrast dynamic range of the RF‐FAST sequence. The shortest possible TE was optimal for both contrast dynamics and imaging time. TI had an influence on the sensitivity of the FAST sequence only for small acquisition matrices. This study indicates the optimal parameters for contrast dynamics (RF‐FAST, 90° flip angle, shortest possible TE) and sensitivity (FAST, 90° flip angle, long TIeff). J. Magn. Reson. Imaging 2001;14:771–778.

Inflow effect in first‐pass cardiac and renal MRI

To estimate the effect of the inflow effect on the arterial input function in vivo in cardiac and renal MR perfusion imaging using fast gradient echo (GRE) sequences and contrast media.

Starter sequence for steady-state free precession imaging

The dynamic equilibrium exploited by balanced steady‐state free precession imaging develops slowly because its formation is dependent on both spin–spin and spin–lattice relaxation times. Attempting to image before steady state is established results in artifacts due to transient signal oscillations. Using a starter sequence to precondition the spin system can significantly reduce the delay before imaging. An improved design for a steady‐state starter sequence is presented. The new sequence has the advantage of uniformly exciting the steady‐state response for all resonance offsets and can be phase cycled to suppress banding artifacts. Magn Reson Med 53:919–929, 2005.

Sodium-23 and proton nuclear magnetic resonance imaging studies of carbon tetrachloride-induced liver damage in the rat

Magnetic resonance imaging techniques were used to investigate the response of the liver of the rat in situ to a toxicological challenge by carbon tetrachloride. Proton images were taken as transverse slices through the rat before and after intraperitoneal administration of the hepatotoxin. Two to three hours after carbon tetrachloride was given, a region of high proton signal intensity was observed where the portal vein enters the liver. Sodium-23 images were also taken, and a region of high sodium intensity was observed in the same location within the liver as the increased proton intensity. The results suggest that acute administration of carbon tetrachloride induces localized liver damage in the region where the hepatotoxin first enters the liver. This liver damage is manifest as edema with a buildup of sodium ion and water, which can be readily detected by sodium-23 and proton NMR imaging techniques, respectively.