Biomedical subjects

Michael B Smith

Publications and source records attributed to Michael B Smith.

40 records · Page 3Linked to original sources

PubMed2002

Polarization of the RF field in a human head at high field: a study with a quadrature surface coil at 7.0 T.

The RF field intensity distribution in the human brain becomes inhomogeneous due to wave behavior at high field. This is further complicated by the spatial distribution of RF field polarization that must be considered to predict image intensity distribution. An additional layer of complexity is involved when a quadrature coil is used for transmission and reception. To study such complicated RF field behavior, a computer modeling method was employed to investigate the RF field of a quadrature surface coil at 300 MHz. Theoretical and experimental results for a phantom and the human head at 7.0 T are presented. The results are theoretically important and practically useful for high-field quadrature coil design and application.

Brain↗

PubMed2002

Numerical calculations of the static magnetic field in three-dimensional multi-tissue models of the human head.

Susceptibility-induced perturbation of the static magnetic field by the human body during magnetic resonance imaging (MRI) leads to undesirable artifacts as well as valuable physiological information, as in functional MRI. The ability to calculate these perturbations for a multi-tissue human body model provides a powerful tool in designing hardware and acquisition methods for reducing the artifacts, and in relating observed image contrast to physiological origins. We have developed a method for calculating the static field in arbitrary 3D magnetic susceptibility distributions and performed calculations in a complete model of the human head and shoulders. The accuracy of our method was validated in regular geometries with known analytical solutions and in comparison with experimental results acquired from the head of the same human subject used for computer modeling. Results are presented in parts per million (ppm) deviation from the applied field strength and are valid for any imaging or spectroscopy system.

Computer Simulation↗

PubMed2002

Intrinsic muscle atrophy and toe deformity in the diabetic neuropathic foot: a magnetic resonance imaging study.

OBJECTIVE: The objectives of this study were to compare intrinsic foot muscle cross-sectional area (CSA) in patients with diabetic polyneuropathy and nondiabetic control subjects and to examine the association between intrinsic muscle CSA and clawing/hammering of the toes in neuropathic feet. RESEARCH DESIGN AND METHODS: High-resolution T2-weighted fast spin-echo images and parametric T2 multiple spin-echo images were acquired using multiple spin-echo magnetic resonance imaging (MRI) of frontal plane sections of the metatarsal region of the foot in a sample of eight individuals with diabetic polyneuropathy and eight age- and sex-matched nonneuropathic nondiabetic control subjects. The configuration of joints of the second toe was obtained using a three-dimensional contact digitizer. RESULTS: Remarkable atrophy was found in all the intrinsic muscles of neuropathic subjects as compared with nondiabetic control subjects. Quantitative T2 analysis showed a 73% decrease in muscle tissue CSA distally in the neuropathic subjects. Muscle comprised only 8.3 +/- 2.9% (means +/- SD) of total foot CSA compared with 30.8 +/- 3.9% in control subjects. No significant differences were found between the groups in the metatarso-phalangeal and proximal and distal interphalangeal joint angles of the second ray. Moreover, clawing/hammering of the toes was found in only two of eight neuropathic subjects. CONCLUSIONS: Although sensory neuropathy is often emphasized in considerations of diabetic foot pathology, our results show that the consequences of motor neuropathy in the feet are profound in people with diabetes. This has implications for foot function and may play a significant role in postural instability. However, intrinsic muscle atrophy does not necessarily appear to imply toe deformity.

Adult↗

PubMedDate not supplied

Two dimensional prolate spheroidal wave functions for MRI.

The tradeoff between spatial and temporal resolution is often used to increase data acquisition speed for dynamic MR imaging. Reduction of the k-space sampling area, however, leads to stronger partial volume and truncation effects. A two dimensional prolate spheroidal wave function (2D-PSWF) method is developed to address these problems. Utilizing prior knowledge of a given region of interest (ROI) and the spatial resolution requirement as constraints, this method tailors the k-space sampling area with a matching 2D-PSWF filter so that optimal signal concentration and minimal truncation artifacts are achieved. The k-space sampling area is reduced because the shape and size of the sampling area match the resolution posed by the non-rectangular shape of a convex ROI. The 2D-PSWF method offers an efficient way for spatial and temporal tradeoff with minimal penalty due to truncation, and thus, it promises a wide range of applications in MRI research.

Adult↗