Biomedical subjects

D H Follett

Publications and source records attributed to D H Follett.

15 recordsLinked to original sources

PubMed1993

Blood flow measurement by Doppler ultrasound: a question of angles.

Ultrasound Doppler equipment is widely used to estimate blood velocity and volume flow. Recently there has been correspondence concerning the origin of the Doppler shift in blood vessels. The assumption that only the movement of the target need be considered is challenged by the observation that Doppler shifts occur where there is relative motion. In blood there is unlikely to be relative motion between a reflecting blood corpuscle and its supporting plasma. In the simple case of plug flow the relative movement takes place at the vessel wall. An investigation is described which uses a flow rig Doppler phantom to assess what effect, if any, this phenomenon would have on Doppler shift results obtained by insonating a vessel such as the aortic arch and range gating through a curved section where flow velocity gradients are encountered. It has been assumed that if the sample volume were to lie in that part of the vessel in which the blood velocity vector were directly towards the probe, then an angle of 0 degrees could be assumed for calculations using the Doppler equation. Our results indicate that the observed Doppler shifts lie midway between those expected if the shift were to occur solely at the first moving fluid boundary and those originating only from scatterers within the sample volume.

Blood Circulation↗

PubMed1993

The development of equipment for the technical assessment of respiratory motion induced artefacts in MRI.

A device and technique to study the effects of respiratory motion on the quality of magnetic resonance images is proposed. The construction of the device enables a variety of test objects to be mounted and used in the evaluation of imaging parameters that may be affected by motion. The equipment is constructed of cast acrylic and the movement is actuated and controlled pneumatically thus ensuring that there are no interactions with the magnetic field and radiofrequency detection system to cause further image artefacts. Separate studies have been performed, using ultrasound, to assess the degree and rate of movement of organs owing to respiration in order to derive the motion parameters for the apparatus. Preliminary results indicate that the technique produces motion induced artefacts simulating those which are the result of the effects of respiration.

Diaphragm↗

PubMed1991

Ultimate limits in ultrasonic imaging resolution.

According to elementary theory, the resolution of an ultrasonic imaging system increases with the ultrasonic frequency. However, frequency is limited by frequency-dependent attenuation. For imaging at any required depth, resolution improvement beyond the limit imposed by ultrasonic frequency can be obtained by increasing the ultrasonic intensity. This is itself, however, dependent on safety considerations and the effects of nonlinearity. In homogeneous media, image resolution increases with decreasing f-number. Particularly at low f-numbers, however, tissue inhomogeneity leads to a deterioration in image quality. Inhomogeneity may also be considered in terms of phase aberration. It has been found that for a given aperture, image degradation due to phase aberration is worse at higher frequencies. Schemes have been proposed for correction of this problem, but so far model systems do not lend themselves to clinical application. Deconvolution is unsatisfactory, speed correction is impracticable and synthetic aperture scanning and holography are virtually useless in biological tissues. Ultrasound-computed tomography has had only limited success. Speckle reduction can improve target detectability, but at the expense of resolution. Time-frequency control provides a useful partial solution to the problem of resolution reduction resulting from attenuation. It is clear that improved resolution would result in significant clinical benefits. An optimisation system for aperture size and ultrasonic frequency is proposed with signal averaging for resolution enhancement of a defined object area. This would have a compact ultrasonic beam and would allow frame rate to be traded for resolution, by means of signal averaging.

Humans↗

PubMed1991

Electromagnetic radiofrequency interference with Doppler equipment.

Doppler ultrasound equipments incorporate radiofrequency (RF) receivers operating at the microvolt level and are liable to interference from radiated fields and mains disturbances. The most significant interference routes are: radiation from radio broadcast, paging, communication and diathermy picked up on the transducer and patient acting as an aerial; and mains disturbances from diathermy, x-ray sets, motors and thyristor controls, etc, reaching the equipment interior. Direct mains interference can be reduced by proper design incorporating a mains filter, a low RF leakage enclosure, a ground plane, careful layout and further screening of the receiver circuits. Pick-up via the transducer occurs even if the signal leads are completely screened because an RF potential can exist between transducer and equipment enclosure due to the considerable impedance at RF of any wire or cable, typically 70 omega at 5 MHz for 2 m. This potential, reduced by about 50-60 dB, appears at the receiver input because of imperfect common mode rejection of the coupling cable. As a result, induced voltages above 100-300 microV may cause problems. It is shown that such levels can easily result from the interfering field strengths of 1 mV m-1 or more that may be experienced in a hospital environment. On the other hand, field strengths of 300 microV m-1, as generally allowed by regulatory standards at 3 m from interfering sources, should not cause much effect. Other interference can arise from modulation of a strong RF signal by mains frequency power components in the Doppler equipment and from associated computer circuits. These require careful layout to reduce electric and magnetic coupling, decoupling and filtering of power supplies and components, screening of RF circuits and particular attention to the reduction of power and computer signal currents flowing in RF signal earth paths. Finally, some initial simple acceptance tests for interference susceptibility are proposed, based on the application of 300 microV of RF signal between transducer body and equipment enclosure and 10 mV to the mains supply.

Equipment and Supplies↗

PubMed1988

Development of a penile rigidity indicator and new concepts in the quantification of rigidity.

A new non-invasive device has been developed for the continuous measurement of penile rigidity. We describe its design and performance in experimental and clinical studies, both of which have demonstrated its high degree of sensitivity, reliability and reproducibility. We also report two new concepts in quantifying penile rigidity.

Equipment Design↗

PubMed1987

Application of computerised penile arterial waveform analysis in the diagnosis of arteriogenic impotence. An initial study in potent and impotent men.

A new method is described for evaluating arteriogenic impotence by means of noninvasive quantification of penile Doppler arterial waveforms using computerised analysis based on the Laplace Transform model. The haemodynamic changes occurring during a papaverine-induced erection in healthy potent volunteers have been recorded by this technique, which has also been shown to be capable of discriminating between a normal and an abnormal penile arterial supply in an initial study of potent and impotent men.

Adult↗

PubMed1981

A frequency shift maximum frequency follower Doppler demodulator with amplitude correction.

A commonly used maximum-frequency follower Doppler demodulator uses a variable-frequency voltage controlled filter that requires two good quality multipliers and can be difficult to set up and stabilise. An alternative presented here has the advantage of cheapness, easy setting-up and amplitude independence over about 25 dB giving the potential for quantitative as well as qualitative results. A switching modulator is first used to convert the Doppler spectrum into a pair of sidebands disposed about the modulating frequency. The modulation frequency is continually adjusted so that the lower sideband edge, representing the highest Doppler frequency, just passes the skirt of a low pass filter. The filter output, after rectification and smoothing, provides drive to the voltage controlled oscillator (VCO) generating the modulation frequency and also forms the flow signal output. The fixed low pass filter results in a stable feed-back loop in which high loop gain can be used so that linearity depends mostly on the VCO. The characteristic slope is independent of signal amplitude above a threshold, but increasing signal causes an increasing offset voltage. A simple signal rectifier enables this offset to be largely cancelled, giving amplitude independence.

Arterial Occlusive Diseases↗

PubMed1978