Christopher T. Sica

Measurement of SAR-induced temperature increase in a phantom and in vivo with comparison to numerical simulation.

To compare numerically simulated and experimentally measured temperature increase due to specific energy absorption rate from radiofrequency fields.

Fast conjugate phase image reconstruction based on a Chebyshev approximation to correct for B0 field inhomogeneity and concomitant gradients.

Off‐resonance effects can cause image blurring in spiral scanning and various forms of image degradation in other MRI methods. Off‐resonance effects can be caused by both B0 inhomogeneity and concomitant gradient fields. Previously developed off‐resonance correction methods focus on the correction of a single source of off‐resonance. This work introduces a computationally efficient method of correcting for B0 inhomogeneity and concomitant gradients simultaneously. The method is a fast alternative to conjugate phase reconstruction, with the off‐resonance phase term approximated by Chebyshev polynomials. The proposed algorithm is well suited for semiautomatic off‐resonance correction, which works well even with an inaccurate or low‐resolution field map. The proposed algorithm is demonstrated using phantom and in vivo data sets acquired by spiral scanning. Semiautomatic off‐resonance correction alone is shown to provide a moderate amount of correction for concomitant gradient field effects, in addition to B0 imhomogeneity effects. However, better correction is provided by the proposed combined method. The best results were produced using the semiautomatic version of the proposed combined method. Magn Reson Med 60:1104–1111, 2008.

Optimization of spiral‐based pulse sequences for first‐pass myocardial perfusion imaging

Although spiral trajectories have multiple attractive features such as their isotropic resolution, acquisition efficiency, and robustness to motion, there has been limited application of these techniques to first‐pass perfusion imaging because of potential off‐resonance and inconsistent data artifacts. Spiral trajectories may also be less sensitive to dark‐rim artifacts that are caused, at least in part, by cardiac motion. By careful consideration of the spiral trajectory readout duration, flip angle strategy, and image reconstruction strategy, spiral artifacts can be abated to create high‐quality first‐pass myocardial perfusion images with high signal‐to‐noise ratio. The goal of this article was to design interleaved spiral pulse sequences for first‐pass myocardial perfusion imaging and to evaluate them clinically for image quality and the presence of dark‐rim, blurring, and dropout artifacts. Magn Reson Med, 2011.

Radiofrequency field enhancement with high dielectric constant (HDC) pads in a receive array coil at 3.0T

To investigate the use of a new high‐dielectric constant (HDC) material for improving SNR and transmission efficiency for clinical MRI applications at 3 Tesla (T) with cervical spine imaging.

Improved first‐pass spiral myocardial perfusion imaging with variable density trajectories

To develop and evaluate variable‐density spiral first‐pass perfusion pulse sequences for improved efficiency and off‐resonance performance and to demonstrate the utility of an apodizing density compensation function (DCF) to improve signal‐to‐noise ratio (SNR) and reduce dark‐rim artifact caused by cardiac motion and Gibbs Ringing.

Permittivity and performance of dielectric pads with sintered ceramic beads in MRI: early experiments and simulations at 3 T.

Passive dielectric materials have been used to improve aspects of MRI by affecting the distribution of radiofrequency electromagnetic fields. Recently, interest in such materials has increased with the number of high‐field MRI sites. Here, we introduce a new material composed of sintered high‐permittivity ceramic beads in deuterated water. This arrangement maintains the ability to create flexible pads for conforming to individual subjects. The properties of the material are measured and the performance of the material is compared to previously used materials in both simulation and experiment at 3 T. Results show that both permittivity of the beads and effect on signal‐to‐noise ratio and required transmit power in MRI are greater than those of materials consisting of ceramic powder in water. Importantly, use of beads results in both higher permittivity and lower conductivity than use of powder. Magn Reson Med, 2013.

Complex-Difference Constrained Compressed Sensing Reconstruction for Accelerated PRF Thermometry with Application to MRI Induced RF Heating

Introduce a novel compressed sensing reconstruction method to accelerate proton resonance frequency shift temperature imaging for MRI‐induced radiofrequency heating evaluation.

Bloch‐based MRI system simulator considering realistic electromagnetic fields for calculation of signal, noise, and specific absorption rate

To describe and introduce new software capable of accurately simulating MR signal, noise, and specific absorption rate (SAR) given arbitrary sample, sequence, static magnetic field distribution, and radiofrequency magnetic and electric field distributions for each transmit and receive coil.

Concomitant gradient field effects in balanced steady‐state free precession

Linear magnetic field gradients spatially encode the image information in MRI. Concomitant gradients are undesired magnetic fields that accompany the desired gradients and occur as an unavoidable consequence of Maxwells equations. These concomitant gradients result in undesired phase accumulation during MRI scans. Balanced steady‐state free precession (bSSFP) is a rapid imaging method that is known to suffer from signal dropout from off‐resonance phase accrual. In this work it is shown that concomitant gradient phase accrual can induce signal dropout in bSSFP. The spatial variation of the concomitant phase is explored and shown to be a function of gradient strength, slice orientation, phase‐encoding (PE) direction, distance from isocenter, and main field strength. The effect on the imaging signal level was simulated and then verified in phantom and in vivo experiments. The nearest signal‐loss artifacts occurred in scans that were offset from isocenter along the z direction with a transverse readout. Methods for eliminating these artifacts, such as applying compensatory frequency or shim offsets, are demonstrated. Concomitant gradient artifacts can occur at 1.5T, particularly in high‐resolution scans or with additional main field inhomogeneity. These artifacts will occur closer to isocenter at field strengths below 1.5T because concomitant gradients are inversely proportional to the main field strength. Magn Reson Med 57:721–730, 2007.

Muscle oxygenation during dynamic plantar flexion exercise: combining BOLD MRI with traditional physiological measurements

Blood‐oxygen‐level‐dependent magnetic resonance imaging (BOLD MRI) has the potential to quantify skeletal muscle oxygenation with high temporal and high spatial resolution. The purpose of this study was to characterize skeletal muscle BOLD responses during steady‐state plantar flexion exercise (i.e., during the brief rest periods between muscle contraction). We used three different imaging modalities (ultrasound of the popliteal artery, BOLD MRI, and near‐infrared spectroscopy [NIRS]) and two different exercise intensities (2 and 6 kg). Six healthy men underwent three separate protocols of dynamic plantar flexion exercise on separate days and acute physiological responses were measured. Ultrasound studies showed the percent change in popliteal velocity from baseline to the end of exercise was 151 ± 24% during 2 kg and 589 ± 145% during 6 kg. MRI studies showed an abrupt decrease in BOLD signal intensity at the onset of 2 kg exercise, indicating deoxygenation. The BOLD signal was further reduced during 6 kg exercise (compared to 2 kg) at 1 min (−4.3 ± 0.7 vs. −1.2 ± 0.4%, P < 0.001). Similarly, the change in the NIRS muscle oxygen saturation in the medial gastrocnemius was −11 ± 4% at 2 kg and −38 ± 11% with 6 kg (P = 0.041). In conclusion, we demonstrate that BOLD signal intensity decreases during plantar flexion and this effect is augmented at higher exercise workloads.