Francis A. Duck

Ultrasound in medicine

INTRODUCTION Ultrasound in medicine F.A. Duck THE PHYSICS OF MEDICAL ULTRASOUND Ultrasonic fields: structure and prediction V.F. Humphrey and F.A. Duck Nonlinear effects in ultrasound propagation A.C. Baker Radiation pressure and acoustic streaming F.A. Duck Ultrasound properties of tissues J.C. Bamber TECHNOLOGY AND MEASUREMENT IN DIAGNOSTIC IMAGING Transducer arrays for medical ultrasound imaging T.L. Szabo Doppler technology and techniques P.N.T. Wells The purpose and techniques of acoustic output measurement T.A. Whittingham ULTRASOUND HYPERTHERMIA AND SURGERY Ultrasound hyperthermia and the prediction of heating J.W. Hand Focused ultrasound surgery G.R. ter Haar Acoustic wave lithotripsy M. Halliwell ACOUSTIC BUBBLES An introduction to acoustic cavitation T.G. Leighton Echo-enhancing (ultrasound contrast) agents D.O. Cosgrove Sonochemistry and drug delivery G.J. Price SOME MODERN RESEARCH ISSUES IN MEDICAL ULTRASOUND Imaging elastic properties of tissue J.F. Greenleaf, R.L. Ehman, M. Fatema, and R. Muthupillai The signal-to-noise relationship for investigative ultrasound C.R. Hill Challenges in the ultrasonic measurement of bone J.G. Truscott and R. Strelitzki INDEX

NONLINEAR ACOUSTICS IN DIAGNOSTIC ULTRASOUND

The propagation of ultrasonic waves is nonlinear. Phenomena associated with the propagation of diagnostic ultrasound pulses cannot be predicted using linear assumptions alone. These include a progressive distortion in waveform, the generation of frequency harmonics and acoustic shocks, excess deposition of energy and acoustic saturation. These effects occur most strongly when ultrasound propagates within liquids with comparatively low acoustic attenuation, such as water, amniotic fluid or urine. Within soft tissues, similar effects occur, although they are limited by absorption and scattering. Nonlinear effects are of considerable importance during acoustic measurements, especially when these are used to predict in situ exposure. Harmonic generation may be used to create images. These offer improvements over conventional B-mode images in spatial resolution and, more significantly, in the suppression of acoustic clutter and side-lobe artifacts. B/A has promise as a parameter for tissue characterisation, but methods for imaging B/A have shown limited success.

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.

Electrical Properties of Tissue

This chapter discusses the electrical and dielectric properties of tissue, covering the frequency range from d.c. to over 10 GHz. The electrical character of tissues over a wide range of frequencies may be described by using the two properties relative permittivity, ∈′ (the charge) and conductivity, σ (current densities set up in response to an applied electric field of unit amplitude). From both of these, the complex relative permittivity, ∈*, can be defined by the equation ij , characterize the effect in terms of the charge generated for unit applied stress under short circuit conditions.

An experimental investigation of streaming in pulsed diagnostic ultrasound beams

Streaming is shown to occur in water in the focused beams produced by a number of medical pulse-echo devices. The use of hot film anemometry to measure the streaming velocity is described and velocities measured in water using commercial equipment are quoted. The highest velocities occur in pulsed Doppler mode with a maximum velocity of 14 cm s-1 being observed. An experimental set-up was used to investigate the parameters affecting streaming and it was found that the harmonic content of the pulse waveform had a major effect on the streaming velocity. The time taken for a stream to become established at the focus of the acoustic beams studied was typically approximately 0.5 s.

Evidence for ultrasonic finite‐amplitude distortion in muscle using medical equipment

Finite-amplitude distortion of ultrasonic waves from medical equipment has been observed to occur following transmission through calf muscle in human volunteers. Measurements were made using both dynamic pulse-echo imaging equipment and physiotherapy equipment. In both cases irradiation was carried out under operating conditions commonly used clinically. Pressure waveforms were measured at the skin surface using a broadband polyvinylidene difluoride membrane hydrophone. Using a pulsed, weakly focused 2.5-MHz beam with input peak pressure of 0.8 MPa and a pressure gain of 5.3 at the focus, the mean second harmonic peak magnitude (16 measurements) was 17 dB below the fundamental peak. A 1.1-MHz continuous wave therapy set with input peak pressure of 0.5 MPa showed mean second harmonic magnitude 23 dB below the fundamental.

Fetal Thermal Effects of Diagnostic Ultrasound

Processes that can produce a biological effect with some degree of heating (ie, about 1°C above the physiologic temperature) act via a thermal mechanism. Investigations with laboratory animals have documented that pulsed ultrasound can produce elevations of temperature and damage in biological tissues in vivo, particularly in the presence of bone (intracranial temperature elevation). Acoustic outputs used to induce these adverse bioeffects are within the diagnostic range, although exposure times are usually considerably longer than in clinical practice. Conditions present in early pregnancy, such as lack of perfusion, may favor bioeffects. Thermally induced teratogenesis has been shown in many animal studies, as well as several controlled human studies; however, human studies have not shown a causal relationship between diagnostic ultrasound exposure during pregnancy and adverse biological effects to the fetus. All human epidemiologic studies, however, were conducted with commercially available devices predating 1992, that is, with acoustic outputs not exceeding a spatial‐peak temporal‐average intensity of 94 mW/cm2. Current limits in the United States allow a spatial‐peak temporal‐average intensity of 720 mW/cm2 for fetal applications. The synergistic effect of a raised body temperature (febrile status) and ultrasound insonation has not been examined in depth. Available evidence, experimental or epidemiologic, is insufficient to conclude that there is a causal relationship between obstetric diagnostic ultrasound exposure and obvious adverse thermal effects to the fetus. However, very subtle effects cannot be ruled out and indicate a need for further research, although research in humans may be extremely difficult to realize.

American Institute of Ultrasound in Medicine consensus report on potential bioeffects of diagnostic ultrasound: Executive summary

The continued examination of potential biological effects of ultrasound and their relationship to clinical practice is a key element in evaluating the safety of diagnostic ultrasound. Periodically, the American Institute of Ultrasound in Medicine (AIUM) sponsors conferences bringing experts together to examine the literature on ultrasound bioeffects and to develop conclusions and recommendations related to diagnostic ultrasound. The most recent effort included the examination of effects whose origins were thermal or nonthermal, with separate evaluations for potential effects related to fetal ultrasound. In addition, potential effects due to the introduction of ultrasound contrast agents were summarized. This information can be used to assess risks in comparison to the benefits of diagnostic ultrasound. The conclusions and recommendations are organized into 5 broad categories, with a comprehensive background and evaluation of each topic provided in the corresponding articles in this issue. The following summary is not meant as a substitute for the detailed examination of issues presented in each of the articles but rather as a means to facilitate further study of this consensus report and implementation of its recommendations. The conclusions and recommendations are the result of several rounds of deliberations at the consensus conference, subsequent review by the Bioeffects Committee of the AIUM, and approval by the AIUM Board of Governors.

The output of pulse-echo ultrasound equipment: a survey of powers, pressures and intensities

A survey of the powers, pressures and intensities generated by ultrasonic pulse-echo equipment in clinical use has been carried out. Three conventional B-scanners, four linear-array scanners and four mechanically sectored scanners were included in the study. Measurements were made on a total of 22 transducers covering the nominal frequency range 2.25-7.5 MHz. On those instruments where an output power control was provided, two measurements were made: one at the maximum available power and a second at a lower power. On arrays with a variable transmit focus control, measurements were made at all available focus settings. In all, measurements were made on 38 separate focused pulsed ultrasonic fields. The measurements were carried out using a calibrated ultrasonic force balance, and a calibrated polyvinylidene difluoride (PVdF) membrane hydrophone. A very wide range of maximum powers, pressures and intensities were found. Powers from 0.5-80 mW were measured; spatial-average temporal-peak positive pressures at the transducer varied between 30 kPa and 1.15 MPa, and spatial-peak pulse-average intensities were in the range 3.6 X 10(3)-1.1 X 10(7) Wm-2.

Optical Properties of Tissue including Ultraviolet and Infrared Radiation

This chapter examines the optical properties of tissue in the spectrum from ultraviolet to infrared, approximately 100 nm to 1 mm. The measurement of the effective attenuation coefficient and penetration depth can be performed on thick samples of tissue by standard narrow-beam experiments. Heterogeneities in tissues cause considerable spread in the measured values. Two optical windows are seen to exist in tissue: the main one lies between 600 and 1300 nm and a second bounded by two water absorption bands lies between 1600 to 1850 nm. At infrared wavelengths, the optical absorption coefficient becomes increasingly strongly dependent on the tissue water content. This can be used to predict the optical attenuation of tissue, αt, at 10.6 μm, using the simple expression αt= α w W. W is the percentage water content in the tissue and α w is the absorption coefficient of water at the wavelength of interest. Penetration depth δ in mm may be calculated from the values of α using the expression δ = 1/α. Also, the optical attenuation of a tissue or fluid sample is sometimes given in terms of its optical density, OD, which is log 10 (1/attenuation).