Biomedical subjects

B Haywood

Publications and source records attributed to B Haywood.

4 recordsLinked to original sources

PubMed2003

Controlled E-field gradient coils.

Peripheral neural stimulation is a major problem in current gradient coil designs. Induced current problems in patients relate directly to gradient strength and modulation frequency. Current designs of gradient coil tend to limit ultra-high-speed imaging methods such as echo-planar imaging through the effect of induced currents which produce tingling sensations and involuntary muscle twitch. Neural stimulation could also trigger epileptic fits and/or cardiac fibrillation. For reduction of induced currents, an important aspect is the coil geometry. It is desirable to design the gradient coil in such a way as to prevent closed loop circulating currents within the body. Preliminary results using a four-sector gradient coil with rectangular geometry, operating in a low mutual coupling mode, indicate significant reduction in the E-field within the subject volume of the coil. Reduction in induced currents in the patient allows safer operation at higher magnetic field strengths together with faster scans currently prohibited through neural stimulation effects in standard coil geometries.

Computer Simulation↗

PubMed2003

Optimized gradient pulse for use with EPI employing active acoustic control.

The concept of active acoustic control was recently introduced by Mansfield and Haywood (MAGMA 2000:10:147-151) to ameliorate the problem of acoustic noise from MRI, particularly that from high-speed EPI. A 30 dB reduction in noise was previously achieved with the use of acoustic control operating at spot frequencies within a narrow band. In this work, a new acoustic gradient pulse is introduced that comprises an oscillating gradient of finite duration, incorporating a combination of frequencies within this band designed for use as the switched read gradient in echo-planar imaging (EPI). Employing this pulse with active acoustic control results in a reduction of acoustic noise by 50 dB.

Acoustics↗

PubMed2001

Active acoustic control in gradient coils for MRI.

The new principles of active acoustic control in gradient coil design recently introduced by Mansfield and Haywood (MAGMA 1999;8(Suppl 1):55) are further developed theoretically for the far-field acoustic output for a single sector of a coil system comprising four or more flat rectangular coil sectors. Each sector consists of a split plate arrangement in which are embedded two windings, an outer primary winding and a narrow inner re-entrant loop control winding immediately adjacent to and surrounding the split or air gap. The wire spacing of the control winding is made small so as not to affect substantially the magnetic field created by the primary winding. Experimental results are presented for two sectors each made of a different readily available plastic material and tested over a range of frequencies. They both show substantial average reductions in acoustic output over the full output when the control winding is appropriately driven. New theoretical expressions are derived for particular frequencies based on normal mode expansions for the plate. This new approach is better able to explain the acoustic output difference between the full and reduced output modes. Empirical expressions are also developed which include longitudinal as well as transverse plate characteristics and used to fit the experimental acoustic output data as a function of frequency and indicate good agreement with regard to both the form and amplitude of the acoustic output response.

Acoustics↗

PubMed2000

Principles of active acoustic control in gradient coil design.

The new principles of active acoustic control in gradient coil design are introduced and theoretical expressions are developed for the far field acoustic output for a coil system comprising four or more flat rectangular coil sectors. Each sector consists of a split plate arrangement in which are embedded two windings, an outer primary winding and a narrow inner re-entrant loop control winding immediately adjacent to and surrounding the split or air gap. The wire spacing of the control winding is made small so as not to affect substantially the magnetic field created by the primary winding. Experimental results are produced for one sector which show an average difference in acoustic output of 34.9 dB when the control winding is appropriately driven. The theoretical expressions developed are used to fit the experimental data and indicate good agreement with regard to the form of the output response.

Acoustics↗