Stephen D. Pye

The performance of ultrasound physiotherapy machines in Lothian Region, Scotland, 1992.

Eighty-five ultrasound therapy machines in use in Lothian Region, Scotland were tested for performance and their recalibration was expedited where necessary. The performance characteristics reported are the temporal average acoustic output power, the treatment frequency, pulse timing and treatment timer accuracy. Consideration is also given to the performance of the wide dynamic range radiation force balance that was used to make acoustic power measurements. Of the machines tested, 69% had power outputs that differed by more than 30% from the expected values. The results of power measurements are analysed according to continuous and pulsed output, single and dual frequency treatment heads and year of manufacture. Therapy machines more than 10-12 years old and modern dual frequency treatment heads performed particularly badly. This study suggests that the performance of ultrasound therapy machines has improved little in the last 20 years despite the introduction of IEC 601-2-5 in 1984.

Adaptive time gain compensation for ultrasonic imaging

The quality of ultrasonic images is often adversely affected by incorrect time gain compensation (TGC) settings. TGC set up by the operator is inadequate for two reasons: firstly, one gain function is unlikely to be appropriate for all the scan lines in an image and secondly, the operator may not have sufficient time or experience to optimise it. Adaptive processing offers a solution to this problem; it has the potential both to improve image quality and to let the operators make more effective use of their time. The literature concerned with adaptive TGC is briefly reviewed. A microcomputer-controlled system has been used to develop various algorithms for adaptive TGC. The algorithms operate in real-time and have been tested using a grey-scale test object, and clinically in routine abdominal and obstetric scanning. The imaging characteristics of each algorithm are determined largely by the value of a parameter beta, described in the text. It is concluded that adaptive TGC capable of applying a unique gain function to each part of the image can consistently produce better images than a single gain function set either automatically or manually.

Mechanisms responsible for diminished fragmentation of ureteral calculi : an experimental and clinical study

We molded 24 synthetic stones (mean weight 680 mg., range 641 to 715) from a commercial mixture of gypsum, silica, cellulose and polyvinyl acetate. Each stone was subjected to 400 shocks on a Wolf 2300 Piezolith and groups of 6 stones were treated in 4 different modes. Mean amounts fragmented were 243 +/- 18 mg. in a free environment, 62 +/- 18 mg. confined loosely in a latex tube, 22 +/- 8 mg. impacted in the tube and 30 +/- 8 mg. impacted alongside a 7F stent. During a 30-month period 118 patients received in situ extracorporeal shock wave lithotripsy for ureteral calculi using the same lithotriptor. The mean stone burden was 11.4 mm. (range 4 to 29). Success was greater for patients with calculi 10 mm. or less than for those with stones greater than 10 mm. (71% versus 51%, p less than 0.05), despite the former group receiving less shocks (5,404 versus 7,491). The influence of size was then excluded by studying the number of shocks delivered per mm. of calculus. Patients receiving 500 to 699 shocks per mm. showed a higher success rate than those receiving a smaller number of shocks per mm. Treatment with a greater number of shocks per mm. did not improve success rate. The experimental study demonstrated that confinement and impaction significantly diminish the rate of fragmentation of calculi. However, the clinical study suggested that there may be an optimum number of shocks per mm. that should be delivered. Treatment beyond this point fails to improve results. The 28% failure rate even in those receiving the highest number of shocks per mm. suggests that large, impacted calculi are unsuitable for treatment with in situ shock wave lithotripsy on this machine.

Ultrasound Therapy Equipment - Does it Perform?

Summary This article discusses the calibration and performance of ultrasound physiotherapy machines with reference to current national and international standards, current practices, and published surveys of machine performance. Recommendations are made to help ensure a rational basis for the use of machines.

Cerebral blood-flow velocity and intermittent intracranial pressure elevation during sleep in hydrocephalic children.

The clinical importance of intermittent intracranial pressure (ICP) elevations during sleep in hydrocephalic children is unclear. Eight studies of continuous ICP monitoring with simultaneous cerebral blood‐flow velocity (CBFV) measurements were recorded during sleep in seven hydrocephalic children aged between one and 10 years. ICP was measured directly through a frontal reservoir. There were two main patterns of CBFV change in response to raised ICP: a progressive decrease in mean flow velocity and increase in resistance index, suggesting impaired haemodynamic compensation to ICP elevation due to reduced circulatory reserve in patients with limited intracranial compliance; and an increase in mean flow velocity with raised ICP, suggesting that appropriate haemodynamic compensation with increased blood‐flow can occur to maintain adequate cerebral perfusion in those with sufficient circulatory reserve. Simultaneous CBFV and ICP measurements may help to identify those with reduced circulatory reserve who are at greater risk of ischaemic insult from episodic increases in ICP.

A study of the directional response of ultraviolet radiometers: II. Implications for ultraviolet phototherapy derived from computer simulations

A theoretical model has been used to simulate irradiances for ultraviolet (UV) phototherapy cabinets and other sources. The accuracy of the simulation results has been checked by comparison with experimental measurements. The simulations have been used to study the influence of different factors on UV phototherapy exposure and to develop recommendations for the operation and calibration of phototherapy cabinets. Many radiometers used in the evaluation of skin doses have input optics with directional responses that are not proportional to the cosine of the angle of incidence for the UV radiation. Data on radiometer directional responses have been incorporated into the simulations, which show that the poor directional responses for some radiometers currently in use will give errors of 20-50% in the assessment of irradiance. The influence of lamp source geometries employed for radiometer calibration has been investigated. UV phototherapy dosimetry commonly uses a spectroradiometer and a radiometer in the transfer of irradiance calibrations from a small standard UV lamp to a large-area source with a different UV spectrum. Recommendations are given on the range of acceptability for radiometer directional responses and a method is described for determining whether these are fulfilled. Recommendations are made on the techniques that should be used for calibration.

Robust electromagnetic probe for the monitoring of lithotriptor output

The ability to measure the powers and pressures generated by extracorporeal lithotriptors is important for both patient safety and treatment planning. Regular measurements of output have been impossible in the past because there have been no instruments available that could withstand prolonged or repeated exposure to lithotriptor fields. The literature contains reports of various devices that have been used to make measurements of lithotriptor output, and these are briefly described here. The authors report the use of a new electromagnetic probe as a robust monitoring device. It has been tested by comparison with a PVdF coplanar membrane hydrophone, by comparison with the results of disintegrating a series of synthetic stones and by impact measurements. The electromagnetic probe has also been used to monitor the output of a piezoelectric extracorporeal lithotriptor over a 12-month period. Fluctuations in pulse energy of up to 350% were observed.

A Simple Calorimeter for Monitoring the Output Power of Ultrasound Therapy Machines

Summary Many ultrasound therapy machines have poor performance. A very simple and inexpensive calorimeter is described which can be used by physiotherapists to monitor the output powers of therapy machines as an aid to the safe and effective use of ultrasound. The design of the calorimeter and a protocol for its use are outlined, and results from therapy machines obtained over a six month period are presented.

An ultrasound mini-balance for measurement of therapy level ultrasound

This paper describes a cost-effective method for measuring acoustic power using a radiation force balance. The device is based around a long established balance design with a gantry arrangement fitted with an absorbing target. The notion of this balance design is that it can easily be constructed from materials that would be readily available within a clinical or industrial environment. The mini-balance was calibrated using a transfer standard against an NPL Reference balance, so a comparison of the performance between the two systems could be assessed. The measurements were completed at 1 MHz and 3 MHz and over the acoustic power range of 1 W to 15 W. The results show the acoustic power measured on the mini-balance to be within 5% of the reference measurements made on the NPL Balance. A separate systematic uncertainty budget is also presented based on studies made on the balance and on similar systems. The overall expanded uncertainty was calculated to be within 14% at 1 W level, decreasing with increasing power level to 7.4% above 5 W.

Characterization of cavitational activity in lithotripsy fields using a robust electromagnetic probe

A robust electromagnetic probe has been used to investigate cavitational activity in vitro in the fields of two extracorporeal lithotripters and one intracorporeal lithotripter. Some aspects of the electromagnetic probe design and characteristics are described. A series of experiments have been carried out with results indicating that the probe head moves in response to the pressure gradient generated by radial motion of cavitation bubbles. Empirical expressions have been derived for the cavitational force acting on the probe head, and for the low-frequency sawtooth pressure wave generated by inertial cavitation. This is the first time that the existence of a low-frequency sawtooth wave produced by inertial cavitation has been described. A linear relationship exists between the negative pressure amplitude of the sawtooth wave and the lifetime of the bubbles. Close to the cavitation site, substantial negative pressure is maintained throughout bubble expansion. This can easily exceed the tensile strength of urinary calculi, and may be considered to be an important mechanism for disintegrating these relatively weak structures. A pilot study has also been carried out involving three patients treated by extracorporeal lithotripsy. Signals similar to those recorded during the in vitro cavitation experiments were detected. We conclude that the electromagnetic probe is capable of detecting and quantifying aspects of cavitational activity in vitro, and potentially also in vivo. The observation that the probe responds directly to the motion of cavitation bubbles, coupled with its ability to detect cavitation at a distance, give it the potential for use in a range of medical and industrial applications.