Biomedical subjects

Steven M Conolly

Publications and source records attributed to Steven M Conolly.

11 recordsLinked to original sources

PubMed2006

Respiration-induced B0 field fluctuation compensation in balanced SSFP: real-time approach for transition-band SSFP fMRI.

In functional MRI (fMRI) the resonance frequency shift induced from respiration is a major source of physiological noise. In transition-band SSFP fMRI the respiration-induced resonance offset not only increases noise interference, it also shifts the activation band. This leads to a reduction in the contrast-to-noise ratio (CNR) and the potential for varying contrast levels during the experiment. A novel real-time method that compensates for the respiration-induced resonance offset frequency is presented. This method utilizes free induction decay (FID) phase information to measure the resonance offset. For compensation, one can update the resonant frequency in real time by changing the transmit RF pulse and receiver phases to track the measured offset. The results show decreased signal power in the respiration frequency band and increased numbers of activated voxels with higher Z-scores compared to uncompensated experiments.

Algorithms↗

PubMed2006

Magnetic resonance imaging with T1 dispersion contrast.

Prepolarized MRI uses pulsed magnetic fields to produce MR images by polarizing the sample at one field strength (approximately 0.5 T) before imaging at a much lower field (approximately 50 mT). Contrast reflecting the T(1) of the sample at an intermediate field strength is achieved by polarizing the sample and then allowing the magnetization to decay at a chosen "evolution" field before imaging. For tissues whose T(1) varies with field strength (T(1) dispersion), the difference between two images collected with different evolution fields yields an image with contrast reflecting the slope of the T(1) dispersion curve between those fields. Tissues with high protein content, such as muscle, exhibit rapid changes in their T(1) dispersion curves at 49 and 65 mT due to cross-relaxation with nitrogen nuclei in protein backbones. Tissues without protein, such as fat, have fairly constant T(1) over this range; subtracting images with two different evolution fields eliminates signal from flat T(1) dispersion species. T(1) dispersion protein-content images of the human wrist and foot are presented, showing clear differentiation between muscle and fat. This technique may prove useful for delineating regions of muscle tissue in the extremities of patients with diseases affecting muscle viability, such as diabetic neuropathy, and for visualizing the protein content of tissues in vivo.

Algorithms↗

PubMed2006

Prepolarized magnetic resonance imaging around metal orthopedic implants.

A prepolarized MRI (PMRI) scanner was used to image near metal implants in agar gel phantoms and in in vivo human wrists. Comparison images were made on 1.5- and 0.5-T conventional whole-body systems. The PMRI experiments were performed in a smaller bore system tailored to extremity imaging with a prepolarization magnetic field of 0.4 T and a readout magnetic field of 27-54 mT (1.1-2.2 MHz). Scan parameters were chosen with equal readout gradient strength over a given field of view and matrix size to allow unbiased evaluation of the benefits of lower readout frequency. Results exhibit substantial reduction in metal susceptibility artifacts under PMRI versus conventional scanners. A new artifact quantification technique is also presented, and phantom results confirm that susceptibility artifacts improve as expected with decreasing readout magnetic field using PMRI. This proof-of-concept study demonstrates that prepolarized techniques have the potential to provide diagnostic cross-sectional images for postoperative evaluation of patients with metal implants.

Adult↗

PubMed2006

Rapid polarizing field cycling in magnetic resonance imaging.

We describe the electronics for controlling the independently pulsed polarizing coil in a prepolarized magnetic resonance imaging (PMRI) system and demonstrate performance with free induction decay measurements and in vivo imaging experiments. A PMRI scanner retains all the benefits of acquiring MRI data at low field, but with the higher signal of the polarizing field. Rapidly and efficiently ramping the polarizing coil without disturbing the data acquisition is one of the major challenges of PMRI. With our modular hardware design, we successfully ramp the 0.4-T polarizing coil of a wrist-sized PMRI scanner at up to 100 T/s without causing image artifacts or otherwise degrading data acquisition.

Computer-Aided Design↗

PubMed2005

Echo time optimization for linear combination myelin imaging.

A 3-echo linear combination myelin imaging method is presented. The echo times and weights are chosen such that the signal-to-noise ratio (SNR) of myelin-water is maximized, and signals from other white matter components are sufficiently suppressed. Interfering tissue water and cerebrospinal fluid (CSF) signals are much stronger than myelin due to their longer T2 and abundance. By carefully optimizing the echo times a 50-fold tissue water suppression is achieved along with a 10-fold CSF suppression. For comparison 4, 5, and 32 echo filters are also designed using the same method. The SNR efficiency of these filters is very similar. The 3-echo filter design was validated by phantom scans. In addition, multislice in vivo myelin images were acquired from both a healthy volunteer and a multiple sclerosis patient. Total scan time was 5 min. A uniform T2 filter is also designed to pass all white matter species with uniform gain. The myelin-water fraction of the in vivo 3-echo data set is then measured by dividing the myelin image by the uniformly filtered image. Obtained myelin-water fractions compare well with previous work.

Algorithms↗

PubMed2005

Positive contrast magnetic resonance imaging of cells labeled with magnetic nanoparticles.

Contrast agents incorporating superparamagnetic iron-oxide nanoparticles have shown promise as a means to visualize labeled cells using MRI. Labeled cells cause significant signal dephasing due to the magnetic field inhomogeneity induced in water molecules near the cell. With the resulting signal void as the means for detection, the particles behave as a negative contrast agent, which can suffer from partial-volume effects. In this paper, a new method is described for imaging labeled cells with positive contrast. Spectrally selective RF pulses are used to excite and refocus the off-resonance water surrounding the labeled cells so that only the fluid and tissue immediately adjacent to the labeled cells are visible in the image. Phantom, in vitro, and in vivo experiments show the feasibility of the new method. A significant linear correlation (r = 0.87, P < 0.005) between the estimated number of cells and the signal was observed.

Animals↗

PubMed2005

Automatic tuning of flexible interventional RF receiver coils.

Microcontroller-based circuitry was built and tested for automatically tuning flexible RF receiver coils at the touch of a button. This circuitry is robust to 10% changes in probe center frequency, is in line with the scanner, and requires less than 1 s to tune a simple probe. Images were acquired using this circuitry with a varactor-tunable 1-inch flexible probe in a phantom and in an in vitro porcine knee model. The phantom experiments support the use of automatic tuning by demonstrating 30% signal-to-noise ratio (SNR) losses for 5% changes in coil center frequency, in agreement with theoretical calculations. Comparisons between patellofemoral cartilage images obtained using a 3-inch surface coil and the surgically-implanted 1-inch flexible coil reveal a worst-case local SNR advantage of a factor of 4 for the smaller coil. This work confirms that surgically implanted coils can greatly improve resolution in small-field-of-view (FOV) applications, and demonstrates the importance and feasibility of automatically tuning such probes.

Animals↗

PubMed2004

Time-optimal multidimensional gradient waveform design for rapid imaging.

Magnetic resonance imaging (MRI) is limited in many cases by long scan times and low spatial resolution. Recent advances in gradient systems hardware allow very rapid imaging sequences, such as steady-state free precession (SSFP), which has repetition times (TRs) of 2-5 ms. The design of these rapid sequences demands time-optimal preparatory gradient waveforms to provide maximum readout duty-cycle, and preserve spatial resolution and SNR while keeping TRs low. Time-optimal gradient waveforms can be synthesized analytically for certain simple cases. However, certain problems, such as time-optimal 2D and 3D gradient design with moment constraints, either may not have a solution or must be solved numerically. We show that time-optimal gradient design is a convex-optimization problem, for which very efficient solution methods exist. These methods can be applied to solve gradient design problems for oblique gradient design, spiral imaging, and flow-encoding using either a constant slew rate or the more exact voltage-limited gradient models. Ultimately, these methods provide a time-optimal solution to many 2D and 3D gradient design problems in a sufficiently short time for interactive imaging.

Magnetic Resonance Imaging↗

PubMed2004

Variable-rate selective excitation for rapid MRI sequences.

Balanced steady-state free precession (SSFP) imaging sequences require short repetition times (TRs) to avoid off-resonance artifacts. The use of slab-selective excitations is common, as this can improve imaging speed by limiting the field of view (FOV). However, the necessarily short-duration excitations have poor slab profiles. This results in unusable slices at the slab edge due to significant flip-angle variations or aliasing in the slab direction. Variable-rate selective excitation (VERSE) is a technique by which a time-varying gradient waveform is combined with a modified RF waveform to provide the same excitation profile with different RF power and duration characteristics. With the use of VERSE, it is possible to design short-duration pulses with dramatically improved slab profiles. These pulses achieve high flip angles with only minor off-resonance sensitivity, while meeting SAR limits at 1.5 T. The improved slab profiles will enable more rapid 3D imaging of limited volumes, with more consistent image contrast across the excited slab.

Computer Simulation↗

PubMed2004

Shim design using a linear programming algorithm.

The advent of open magnetic resonance imaging (MRI) scanners and dedicated MRI scanners tailored to specific body parts has led to an increasing number of noncylindrical MRI scanner geometries, for which noncylindrical gradients and shims are needed. These new scanner geometries are driving the need for fast, flexible shim design methods that can design shim coils for any geometry. A linear programming (LP) algorithm was developed to design minimum-power resistive shim coils on an arbitrary surface. These coils can be designed to produce any order shim field over an arbitrarily shaped target region, which can be placed anywhere within the coil. The resulting designs are relatively sparse and can be readily constructed. This algorithm was used to design and construct a seven-coil cylindrical shim set for a knee imaging magnet with a cylindrical homogeneous region. The algorithm was then used to design shim coils for a biradial head imager with an asymmetrically located spherical target region for brain imaging.

Algorithms↗

PubMed2002

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.

Artifacts↗