Biomedical subjects

M T Alley

Publications and source records attributed to M T Alley.

8 recordsLinked to original sources

PubMed1999

Differentiation between the effects of T1 and T2* shortening in contrast-enhanced MRI of the breast.

The purpose of this study is to describe a technique for magnetic resonance imaging (MRI) that can potentially improve identification of malignant tissue in the human breast. The suggested MRI technique is based on the differentiation between two competing effects leading to opposite changes in image intensity, namely, T1 and T2* shortening caused by administration of gadolinium chelate. The proposed approach also allows calculation of changes in the R2* relaxation rate in breast tissue. The feasibility of the technique for in vivo MRI and increased lesion contrast is demonstrated. The results indicate that this technique may improve detection of malignant breast tissue.

Adult↗

PubMed1998

Gradient characterization using a Fourier-transform technique.

This paper describes a technique for characterizing the gradient subsystem of a magnetic resonance (MR) system. The technique uses a Fourier-transform analysis to directly measure the k-space trajectory produced by an arbitrary gradient waveform. In addition, the method can be easily extended to multiple dimensions and can be adapted to measuring residual gradient effects such as eddy currents. Several examples of gradient waveform and eddy-current measurements are presented. Also, it is demonstrated how the eddy-current measurements can be parameterized with an impulse-response formalism for later use in system tuning. When compared to a peak-fitting analysis, this technique provides a more direct extraction of the k-space measurements, which reduces the possibility of analysis error. This approach also has several advantages as compared to the conventional eddy-current measurement technique, including the ability to measure very short time constant effects.

Fourier Analysis↗

PubMed1997

Angiographic imaging with 2D RF pulses.

Magnetic resonance angiography (MRA) was performed by using RF pulses designed to excite a limited spatial extent in two orthogonal directions. The restriction in the second spatial dimension can be used to increase inflow enhancement and to improve small field-of-view imaging. A rectangular excitation was produced with an "echo-planar" k-space trajectory and a sinc-modulated RF waveform. In vivo images have demonstrated that vessels are more clearly delineated with the two-dimensional excitation. Aliasing artifacts in small field-of-view imaging are significantly reduced, although in some cases complete elimination is not possible due to the nature of the gradient trajectory.

Abdomen↗

PubMed1995

Coronary MRI with a respiratory feedback monitor: the 2D imaging case.

The inability to return the heart to the same position for all breath-holds during 2D coronary MR imaging can result in imaging different locations than desired. This can lead to problems such as (i) missing a whole vessel, or a part of it, (ii) misaligning segments of vessels imaged in different breath-holds, and (iii) degrading image quality when a single slice is acquired in multiple breath-holds. To reduce inconsistencies in the breath-hold level, we designed a respiratory feedback monitor (RFM) that uses a bellows to monitor the circumference of the subject's chest. When the circumference of the subject's chest is within preset limits, an audio signal alerts subjects to hold their breath at that position. Use of the RFM significantly reduces the problems caused by inconsistent breath-holds and the number of breath-holds for an examination in 2D coronary MR imaging.

Adult↗

PubMed1995

Respiratory blur in 3D coronary MR imaging.

3D MR imaging of coronary arteries has the potential to provide both high resolution and high signal-to-noise ratio, but it is very susceptible to respiratory artifacts, especially respiratory blurring. Resolution loss caused by respiratory blurring in 3D coronary imaging is analyzed theoretically and verified experimentally. Under normal respiration, the width for any Gaussian point spread function is increased to a new value that is at least several millimeters (about 3-4 mm). In vivo studies were performed to compare respiratory pseudo-gated 3D acquisition with breath-hold 2D acquisition. On average, the overall quality of a pseudo-gated 3D image is worse than that of the corresponding breath-hold 2D image (P = 0.005). In most cases, respiratory blur caused coronary arteries in pseudo-gated 3D data to have lower resolution than in breath-hold 2D data.

Adult↗

PubMed1995

Development of a phased-array coil for the lower extremities.

A phased-array coil was developed to facilitate imaging the vasculature of the lower extremities. The array consists of four surface coils placed in a Plexiglas "I-beam" frame that are configured to allow bilateral studies with up to a 40-cm field of view (FOV). Data from phantoms indicate an increase in signal-to-noise ratio (SNR) in the regions of interest by an average factor of 2.8 +/- 0.9 over that of the body coil. Preliminary in vivo data have also been obtained from n = 8 subjects and demonstrate significant improvements in image quality. The coil design described here should lead to reduced scan times through the ability to image both legs simultaneously with less need for patient repositioning.

Angiography, Digital Subtraction↗

PubMedDate not supplied

A complex-difference phase-contrast technique for measurement of volume flow rates.

Magnetic resonance (MR) phase-difference methods work well for measuring volumetric flow rates when the vessel diameter is large compared with the in-plane voxel dimensions. For small vessels (eg, coronary arteries), partial-volume effects introduce substantial errors in the measured volume flow rate. To correctly measure flow rates through a voxel, both the fraction of the voxel containing moving spins and the phase shift imparted to those spins must be known. The authors propose a flow measurement method that combines information obtained with both the complex-difference and phase-difference processing techniques and thereby provides the fractional volume occupied by the moving spins and the phase of those spins. The complex-difference flow map method proposed results in improved accuracy of MR phase-contrast flow measurements in the presence of partial-volume effects.

Blood Flow Velocity↗

PubMedDate not supplied

Ultrafast contrast-enhanced three-dimensional MR angiography: state of the art.

Ultrafast breath-hold contrast material-enhanced magnetic resonance (MR) angiography can be performed with a flexible imaging sequence. With the current generation of high-speed imaging gradients, it is possible to achieve sequence repetition times of 4 msec or less. These repetition times make it possible to obtain high-resolution (512 x 512 x 64) images in under 30 seconds. Applications of this versatile technique include imaging of aortic dissection, thoracic and abdominal aortic aneurysm, pulmonary embolus, carotid stenosis, and peripheral vascular disease. The administration of contrast material must be tailored to the vascular anatomy under examination to avoid venous enhancement. The rapid data acquisition times can be used to image multiple temporal phases or multiple locations. With this technique and administration of a T1-shortening contrast agent, high-quality MR angiography can be routinely performed in a variety of vascular regions (eg, thoracic and abdominal aorta, pulmonary arteries, carotid arteries, lower extremities).

Aortic Aneurysm↗