William R. Overall

Oscillating dual-equilibrium steady-state angiography.

A novel technique of generating noncontrast angiograms is presented. This method, called oscillating dual‐equilibrium steady‐state angiography (ODESSA), utilizes a modified steady‐state free precession (SSFP) pulse sequence. The SSFP sequence is modified such that flowing material reaches a steady state which oscillates between two equilibrium values, while stationary material attains a single, nonoscillatory steady state. Subtraction of adjacent echoes results in large, uniform signal from all flowing spins and zero signal from stationary spins. Venous signal can be suppressed based on its reduced T2. ODESSA arterial signal is more than three times larger than that of traditional phase‐contrast angiography (PCA) in the same scan time, and also compares favorably with other techniques of MR angiography (MRA). Pulse sequences are implemented in 2D, 3D, and volumetric‐projection modes. Angiograms of the lower leg, generated in as few as 5 s, show high arterial signal‐to‐noise ratio (SNR) and full suppression of other tissues. Magn Reson Med 47:513–522, 2002.

Steady-state sequence synthesis and its application to efficient fat-suppressed imaging.

A new synthesis algorithm, based on the Shinnar‐Le Roux (SLR) transform, can be used to generate fully refocused steady‐state pulse sequences with arbitrary magnetization profiles as a function of off‐resonant precession. This is accomplished by appropriate periodic oscillation of the RF excitation magnitude and phase from echo to echo. The technique is applied to the design of refocused steady‐state free precession (SSFP) sequences with flat profiles, providing the opportunity for banding‐artifact‐free imaging with steady‐state contrast. The algorithm is also used to generate refocused‐SSFP sequences with an arbitrarily broad region of attenuated signal. These sequences are implemented and applied to the problem of steady‐state fat suppression. Preliminary results show signal levels that agree well with theory, and a broad region of suppressed signal at each echo. Total imaging time is kept identical to that of a standard refocused‐SSFP experiment through echo equalization and interleaving. 3D images from the leg of a normal volunteer acquired in 44 s demonstrate the applicability of the technique to fat‐suppressed imaging. Magn Reson Med 50:550–559, 2003.

Positive Contrast with Alternating Repetition Time SSFP (PARTS): A Fast Imaging Technique for SPIO-Labeled Cells

There has been recent interest in positive‐contrast MRI methods for noninvasive tracking of cells labeled with superparamagnetic iron‐oxide nanoparticles. Low‐tip‐angle balanced steady‐state free precession sequences have been used for fast, high‐resolution, and flow‐insensitive positive‐contrast imaging; however, the contrast can be compromised by the limited suppression of the on‐resonant and fat signals. In this work, a new technique that produces positive contrast with alternating repetition time steady‐state free precession is proposed to achieve robust background suppression for a broad range of tissue parameters. In vitro and in vivo experiments demonstrate the reliability of the generated positive contrast. The results indicate that the proposed method can enhance the suppression level by up to 18 dB compared with conventional balanced steady‐state free precession. Magn Reson Med, 2010.

Fast phase‐contrast velocity measurement in the steady state

A new method of encoding flow velocity as image phase in a refocused steady‐state free precession (SSFP) sequence, called steady‐state phase contrast (SSPC), can be used to generate velocity images rapidly while retaining high signal. Magnitude images with refocused‐SSFP contrast are simultaneously acquired. This technique is compared with the standard method of RF‐spoiled phase contrast (PC), and is found to have more than double the phase‐signal to phase‐noise ratio (PNR) when compared with standard PC at reasonable repetition intervals (TRs). As TR decreases, this advantage increases exponentially, facilitating rapid scans with high PNR efficiency. Rapid switching between the two necessary steady states can be accomplished by the insertion of a single TR interval with no flow‐encoding gradient. The technique is implemented in a 2DFT sequence and validated in a phantom study. Preliminary results indicate that further TR reduction may be necessary for high‐quality cardiac images; however, images in more stationary structures, such as the descending aorta and carotid bifurcation, exhibit good signal‐to‐noise ratio (SNR) and PNR. Comparisons with standard‐PC images verify the PNR advantage predicted by theory. Magn Reson Med 48:890–898, 2002.

Ensuring safety of implanted devices under MRI using reversed RF polarization.

Patients with long‐wire medical implants are currently prevented from undergoing magnetic resonance imaging (MRI) scans due to the risk of radio frequency (RF) heating. We have developed a simple technique for determining the heating potential for these implants using reversed radio frequency (RF) polarization. This technique could be used on a patient‐to‐patient basis as a part of the standard prescan procedure to ensure that the subjects device does not pose a heating risk. By using reversed quadrature polarization, the MR scan can be sensitized exclusively to the potentially dangerous currents in the device. Here, we derive the physical principles governing the technique and explore the primary sources of inaccuracy. These principles are verified through finite‐difference simulations and through phantom scans of implant leads. These studies demonstrate the potential of the technique for sensitively detecting potentially dangerous coupling conditions before they can do any harm. Magn Reson Med, 2010.

Self-gated fat-suppressed cardiac cine MRI.

To develop a self‐gated alternating repetition time balanced steady‐state free precession (ATR‐SSFP) pulse sequence for fat‐suppressed cardiac cine imaging.

High spatiotemporal resolution hyperpolarized 13C angiography

Background Sub-millimeter resolution, background-free magnetic resonance angiography (MRA) has been performed previously using dynamic nuclear polarization (DNP)enhanced C labeled small molecules [1-4]. This approach of contrast-enhanced MRA is appealing since many of the commonly used DNP substrates are endogenous and could be potentially used in large doses in patients with renal insufficiency. The aim of this study was to combine high spatial resolution MRA with a high frame rate spiral readout [5], and to test the feasibility of C magnetic resonance fluoroscopy in rats on a clinical imaging system.

Multi-echo, multi-slice, cardiovascular T2* spiral imaging in a single breath-hold

Background Elevated levels of iron in the body can be detected in various organs including the heart. If not treated, high iron levels can lead to serious health conditions including heart failure and cirrhosis. Cardiovascular MR provides a non-invasive and repeatable alternative method to endomyocardial biopsy for monitoring iron levels within the myocardium. Multi-echo, single slice, Cartesian acquisitions have previously been developed for cardiovascular iron assessment with MR, but the presence of iron within the myocardium can be heterogeneous [1]. In this work, we present a single breath-hold, multi-echo, multi-slice, spiral acquisition providing large coverage of the heart for the assessment of cardiovascular iron deposition.

Rapid left ventricular function MRI with an accelerated real-time-based spiral acquisition

Background Breath-held (BH) cardiac cine MRI is a widely used technique for the assessment of cardiac left ventricular (LV) function. Image quality can be severely compromised in subjects who cannot perform the required breath holds or in subjects with arrhythmia. Additionally, the series of breath holds required for full ventricular coverage leads to prolonged exam times. These drawbacks can be mitigated with real-time imaging approaches, but the limited spatial resolution of many real-time techniques can limit their utility for functional assessment. In this work, we develop a real-time-based multi-slice steady-state free precession (SSFP) sequence that utilizes an accelerated spiral acquisition and non-Cartesian SPIRiT reconstruction to increase spatial resolution.

Initial in vivo validation of real-time phase-contrast sequence

Background Phase-Contrast MRI (PC-MRI) is a standard tool used for the quantitation of flow in everyday cardiac MR studies. However current methods require multiple breathholds and post-acquisition analysis. A PC-MRI sequence that provides real-time acquisition and display of quantitative flow rates has the potential to substantially reduce scan times while providing the clinician with the immediate ability to query flow trends and disturbances. The feasibility of using real-time MRI for the quantitation of flow has been previously reported (Joseph, et al. JMRI 2014, 40:206-213). We have implemented a real-time sequence using the Heartvista cardiac package (HeartVista, Inc., Menlo Park, CA) for this initial, in vivo validation study.