Evaluation of experimental methods for assessing safety for ultrasound radiation force elastography.

Standard test tools have been evaluated for the assessment of safety associated with a prototype transducer intended for a novel radiation force elastographic imaging system. In particular, safety has been evaluated by direct measurement of temperature rise, using a standard thermal test object, and detection of inertial cavitation from acoustic emission. These direct measurements have been compared with values of the thermal index and mechanical index, calculated from acoustic measurements in water using standard formulae. It is concluded that measurements using a thermal test object can be an effective alternative to the calculation of thermal index for evaluating thermal hazard. Measurement of the threshold for cavitation was subject to considerable variability, and it is concluded that the mechanical index still remains the preferred standard means for assessing cavitation hazard.

Bone vibration measurement using ultrasound: application to detection of hip prosthesis loosening.

Hip prosthesis loosening can be determined in vivo using a vibration-based technique called vibrometry. In this technique, a low frequency (<1000Hz) sinusoidal vibration is applied to the femoral condyles and the resulting vibration is measured at the greater trochanter. If the prosthesis is securely fixed, the output vibration signal matches that of the input vibration, whereas if the prosthesis is loose, the output vibration signal is distorted and shows the marked presence of harmonics of the input frequency. One of the main problems with this application of this technique is in measuring the output vibration where significant amounts of soft tissue cover the measurement site. In order to circumvent this problem, an ultrasound probe, normally used for the measurement of blood flow, has been used to measure the output vibration. This has been evaluated by comparing the results obtained from the ultrasound probe with those from a conventional accelerometer in models representing a tight and loose hip prosthesis under simulated clinical conditions. The ultrasound probe was able to consistently detect the output vibration, for both the loose and secure prostheses. Under the test conditions used (which attempted to simulate a large thickness of soft tissue), the ultrasound probe was able to produce a greatly enhanced output vibration signal compared to the accelerometer. This suggests that the use of an ultrasound probe to detect mechanically induced vibration through significant amounts of soft tissue appears to be viable and could lead to enhanced detection of prosthesis loosening using this technique.

Comparison of the acoustic streaming in amniotic fluid and water in medical ultrasonic beams.

AIM: Acoustic streaming in amniotic fluid has been investigated under a variety of conditions relevant to the diagnostic use of ultrasound. METHOD: An ultrasonic Doppler method has been used for measurement. Streaming velocities have been compared with those generated in water for the same exposure conditions. Beams were generated by laboratory equipment simulating beams from clinical systems. The fluids were insonated IN VITRO using 3.5 MHz, 5 MHz and 7.5 MHz transducers in continuous wave (CW) and pulsed mode. RESULTS: Acoustic streaming was measured in both amniotic fluid and water at the power levels 50 mW and 140 mW. Enhancement of velocities due to non-linear effects in high amplitude pulses was demonstrated for amniotic fluid as well as for water. The potential and limitations of present numeric methods for the prediction of acoustic streaming were explored. CONCLUSION: Pulsed ultrasound caused similar streaming velocities in amniotic fluid and water while continuous wave beams induced significantly faster streaming in amniotic fluid than in water.

Automated quantitative volumetric breast ultrasound data-acquisition system.

This paper describes the development and initial testing of an automated ultrasound imaging technique to acquire quantitative volumetric breast data; the clinical application being breast cancer diagnosis and management. A novel mechanical scanner has been designed and constructed to constrain the breast tissue without compromising the image, to acquire images of the majority of the breast using a conventional B-mode scanner and to maintain patient comfort. An algorithm to improve upon simple depth-dependent amplification by compensating for tissue-dependent attenuation is applied to the images, making the grey-scale values represent local scattering properties more closely. Registration techniques have been developed to correct for geometric errors arising in the data set because of tissue movement and variations in speed of sound in the tissues. The data sets are reconstructed into volumes and viewed interactively. A pilot study of seven patients was performed and selected results are presented to illustrate lesion features. The automated scan reduces operator-dependence, provides clear information on the 3-D tissue boundaries and provides a full record for monitoring or surgical planning.

Heating caused by selected pulsed Doppler and physiotherapy ultrasound beams measured using thermal test objects.

OBJECTIVE: To assess the heating caused by selected pulsed Doppler and physiotherapy ultrasound beams by measurements made using thermal test objects. METHOD: Thermal test objects were used to measure temperature rises in selected ultrasound fields. These were compared with theoretical predictions based on standard exposure measurements. A separate thermocouple was used to measure heating at the transducer surface. RESULTS: Temperature rises of up to 6 degrees C were measured for Doppler fields using a thermal test object. The attenuation-corrected temperature rises that were measured generally agreed with calculated Thermal Indices. Temperature rises of up to 2 degrees C were observed for physiotherapy ultrasound fields in pulsed operation. CONCLUSION: Significant overlap between the measured temperature rises of selected pulsed Doppler and physiotherapy ultrasound fields was observed.

Ultrasonic power balances--effect of a coupling window on the power measured from physiotherapy ultrasound units.

The output from physiotherapy ultrasound (US) probes is usually measured using a radiation force balance. Some balances are sealed and incorporate a membrane coupling window. The probes are highly resonant, and the output may be affected by changes in acoustic loading caused by the membrane. A model has been developed to examine this effect, and predicts that a polythene membrane 100-microm thick can change the output power by up to 17%.

A review of therapeutic ultrasound: biophysical effects.

Almost 2 decades ago, it was pointed out that physical therapists tended to overlook the tenuous nature of the scientific basis for the use of therapeutic ultrasound. The purpose of this review is to examine the literature regarding the biophysical effects of therapeutic ultrasound to determine whether these effects may be considered sufficient to provide a reason (biological rationale) for the use of insonation for the treatment of people with pain and soft tissue injury. This review does not discuss articles that examined the clinical usefulness of ultrasound (see article by Robertson and Baker titled "A Review of Therapeutic Ultrasound: Effectiveness Studies" in this issue). The frequently described biophysical effects of ultrasound either do not occur in vivo under therapeutic conditions or have not been proven to have a clinical effect under these conditions. This review reveals that there is currently insufficient biophysical evidence to provide a scientific foundation for the clinical use of therapeutic ultrasound for the treatment of people with pain and soft tissue injury.

Comparison of finite element and heated disc models of tissue heating by ultrasound.

This paper compares different techniques used to model the heating caused by ultrasound (US) in a phantom containing a layer of bone mimic covered by agar gel. Results from finite element (FE) models are compared with those from two techniques based on the point-source solution to the bioheat transfer equation (BHTE): one in which the bone mimic is considered to be an absorbing disc of infinitesimal thickness and the other in which the region through which the US travels is considered to be a volume heat source. The FE results are also compared with experimental measurements. The results from the models differed by up to 40% compared with those from the FE model. Furthermore, for the intensity distribution considered, which corresponds to that in the focal zone of a single-element transducer, the top hat distribution predicts a temperature rise 1.8 times greater than that for a more realistic one based on measured values.

International recommendations and guidelines for the safe use of diagnostic ultrasound in medicine.

Modern sophisticated ultrasonographic equipment is capable of delivering substantial levels of acoustic energy into the body when used at maximum outputs. The risk of producing bioeffects has been studied by international expert groups during symposia supported by the World Federation for Ultrasound in Medicine and Biology (WFUMB). These have resulted in the publication of internationally accepted conclusions and recommendations. National ultrasound safety committees have published guidelines as well. These recommendations and safety guidelines offer valuable information to help users apply diagnostic ultrasound in a safe and effective manner. Acoustic output from ultrasound medical devices is directly regulated only in the USA and this is done by the Food and Drug Administration (FDA). However, there is also a modern trend towards self-regulation which has implications for the worldwide use of diagnostic ultrasound. It has resulted in a move away from the relatively simple scheme of FDA-enforced, application-specific limits on acoustic output to a scheme whereby risk of adverse effects of ultrasound exposure is assessed from information provided by the equipment in the form of a real-time display of safety indices. Under this option, the FDA allows a relaxation of some intensity limits, specifically approving the use of medical ultrasound devices that can expose the fetus or embryo to nearly eight times the intensity that was previously allowed. The shift of responsibility for risk assessment from a regulatory authority to the user creates an urgent need for awareness of risk and the development of knowledgeable and responsible attitudes to safety issues. To encourage this approach, it is incumbent on authorities, ultrasound societies and expert groups to provide relevant information on biological effects that might result from ultrasonographic procedures. It is obvious from the continued stream of enquiries received by ultrasound societies that effective dissemination of such knowledge requires sustained strenuous effort on the part of ultrasound safety committees. There is a strong need for continuing education to ensure that appropriate risk/benefit assessments are made by users based on an appropriate knowledge of the probability of biological effects occurring with each type of ultrasound procedure. The primary purpose of this paper is to draw attention to current safety guidelines and show the similarities and areas of general agreement with those issued by the parent ultrasound organisation, the WFUMB. It is equally important to identify gaps in our knowledge, where applicable.

Measurement of acoustic streaming using magnetic resonance.

Magnetic resonance imaging (MRI) has been used to explore acoustic streaming caused in water under ultrasonic exposure conditions similar to those used for diagnostic applications. Streaming was established in an enclosed tube with acoustically transparent end windows, using a pulsed, weakly-focused transducer of acoustic frequency 3.5 MHz. Phase-detection MRI was used to image and quantify streaming profiles in the region of the acoustic focus. Acoustic powers in the range 0.4 mW to 100 mW were used. The sensitivity of the technique enabled streaming velocities down to 0. 1 mm s(-1) to be measured, generated by acoustic power less than 1 mW. In addition, acoustic streaming generated within open meshes with minimum pore dimensions of 3.0 mm and 2.0 mm was measured. The flow velocity in the coarser mesh reached 0.9 mm s(-1) at 95 mW total acoustic power. These observations demonstrate that acoustic streaming is probably a much more general phenomenon in diagnostic ultrasound (ultrasound) than previously recognised. The combination of magnetic resonance and ultrasound shows promise as a diagnostic method for the differentiation of cystic lesions in vivo, and for their characterisation, with sensitivity significantly greater than using ultrasound alone.

In vitro heating of human fetal vertebra by pulsed diagnostic ultrasound.

The temperature rise generated at the surface of unperfused human fetal vertebrae in vitro by an ultrasound beam with characteristics typical of those used in pulsed Doppler examinations has been measured. The bone samples were from fetuses that ranged in age from 14 to 39 weeks, dating from the last menstrual period. The samples were embedded in agar gel and the temperature rise at their surface was measured using a 50-microm diameter K-type thermocouple. The power in the ultrasound beam was 50 +/- 2 mW and the -6 dB diameter was 2.9 mm. The temperature rise at 295 s ranged from 0.6 degrees C in the youngest sample to 1.8 degrees C in the oldest. Approximately 70% of the temperature rise occurred in the first min.

Acoustic saturation and output regulation.

Acoustic saturation pressures are predicted for ultrasonic beams of a range of frequencies and focal depths. Using reasonable approximations, saturation values for mechanical index (MI) and derated spatial-peak, time-average and pulse-average intensities are calculated. These are compared with thresholds set for regulatory purposes by the U. S. Food and Drug Administration (FDA), and by the International Electrotechnical Commission (IEC). It is concluded that there are many conditions for which acoustic saturation in water prevents the values of MI and regulated intensities from exceeding thresholds set by the FDA. These conditions are particularly associated with higher frequencies and deeper focal lengths. The thresholds for action set by IEC 61157 are sufficiently low that similar problems do not arise. It is concluded that present regulations are not fully effective in limiting the output from diagnostic ultrasound equipment, and that some conditions exist that are not subject to output control.

Estimating in situ exposure in the presence of acoustic nonlinearity.

Evidence is presented for excess attenuation of pulsed ultrasound due to finite amplitude effects in water. Measurements on a modern scanner are used to demonstrate that linear derating can underestimate many exposure quantities, including all safety indices apart from the cranial thermal index. More appropriate methods for estimating in situ exposure are reviewed. A preferred procedure that requires exposure measurements to be made in water under "small signal" conditions is selected. A spectral index is defined that is proposed as an indicator of finite amplitude effects, where spectral index = 0.1 defines the threshold between nonlinear and quasi-linear conditions.

A possible approach to the treatment of polycystic ovarian syndrome using focused ultrasound.

A new method of treatment for infertility caused by polycystic ovarian syndrome (PCOS) using thermal lesion formation by high intensity ultrasound has been suggested. Current options for PCOS therapy and the achievements of focused ultrasound surgery technique are reviewed and discussed. A prototype of a low-cost commercial device for the treatment of PCOS has been designed and tested to prove the feasibility of the method. A transducer with curvature radius of 36 mm and aperture diameter of 36 mm, operating at 0.97 MHz, was designed and tested. It provided a maximum acoustic power output of 180 W. Well-defined tissue damage was obtained within 10 s in a pig's liver in vitro at 3 cm depth within an area 5 mm in diameter and 12 mm in length without damaging the surrounding tissue. Evaluation of the size of the lesions produced at different frequencies, sonication times and output power has been carried out by visual inspection of the colour changes in cut tissue sections. Results demonstrate that a surgical tool based on the method suggested should be feasible and warrants further investigation.

Studies of acoustic streaming in biological fluids with an ultrasound Doppler technique.

Acoustic streaming generated by diagnostic ultrasound fields is an important area for study both for safety reasons and because of its potential application as a diagnostic tool. A method of investigating streaming in biological fluids is reported. A number of fluids were insonated using a 3.5 MHz weakly focused single element transducer which was driven in pulsed mode. Streaming was detected in each fluid using an 8 MHz continuous wave Doppler system. The maximum streaming velocity was obtained by spectral analysis of the Doppler signal. Using this system longitudinal streaming profiles were measured. At an acoustic power of 150 mW the maximum streaming velocities detected were: 9.3 cm s-1 in water, 6.8 cm s-1 in 4.5% human serum albumin (HSA) solution and 4.9 cm s-1 in blood, when transmission was through a water path of approximately 10 cm into a 3 cm sample of fluid. When measurements were made in the biological fluids alone, without a water path, the maximum streaming velocities were reduced.

Determining the conditions for measurement of spatial-peak temporal-averaged intensity in scanned ultrasound beams.

This paper describes a new three stage approach which simplifies the process of measurement of spatial-peak temporal-averaged intensity for scanned ultrasound beams. Firstly, the conditions delivering the maximum total acoustic power are determined. Secondly, out-of-plane beam-widths are measured which, together with knowledge of the in-plane scan widths, are used to locate the depth at which the scanned area is at a minimum. Finally, the spatial-peak temporal-average intensity is measured using the conditions and depth identified in the first two stages. Experimental results justify the use of this new procedure.

Measurement of radical production as a result of cavitation in medical ultrasound fields.

This paper describes the use of the terephthalate dosimeter to measure the threshold and extent of cavitation arising from medical ultrasound sources at high frequencies. Significant hydroxyl radical production was noted from a physiotherapy source and low level activity was also detected when using a pulsed, diagnostic type transducer system. The effect of sound intensity and the type of field is described. The possibility of using polymer degradation to monitor the cavitation is also discussed.