Medical diagnostic applications and sources.

The ways in which ultrasound is used in medical diagnosis are reviewed, with particular emphasis on the ultrasound source (probe) and implications for acoustic exposure. A brief discussion of the choice of optimum frequency for various target depths is followed by a description of the general features of diagnostic ultrasound probes, including endo-probes. The different modes of diagnostic scanning are then discussed in turn: A-mode, M-mode, B-mode, three-dimensional (3D) and 4D scanning, continuous wave (CW) Doppler, pulse-wave spectral Doppler and Doppler imaging. Under the general heading of B-mode imaging, there are individual descriptions of the principles of chirps and binary codes, B-flow, tissue harmonic imaging and ultrasound contrast agent-specific techniques. Techniques for improving image quality within the constraints of real-time operation are discussed, including write zoom, parallel beam forming, spatial compounding and multiple zone transmission focusing, along with methods for reducing slice thickness. At the end of each section there is a summarising comment on the basic features of the acoustic output and its consequences for patient safety.

Estimated fetal cerebral ultrasound exposures from clinical examinations.

The results of a survey of worst-case in-water values for I(SPTA), acoustic power and peak negative pressure, from a wide range of machines in clinical use, have been used with simple "fixed-path" tissue models to estimate worst-case exposure values at the fetus (in situ). The distributions of the estimated in situ values are peaked more toward their low value ends than is the case for distributions of the corresponding in-water values, with the ratio of maximum to median values being higher. Although the upper tails of the derated I(SPTA) distributions are not heavily populated, they contain important instances of relatively high values, mainly for lower frequency probes. Spectral Doppler (SD) mode produces the highest in situ estimates for all three parameters, particularly for I(SPTA) values, which exceed 5000 mW cm(-2) in a third trimester minimum attenuation model, and 900 mW cm(-2) in a "typical" attenuation model. The maximum and median values of peak negative pressure do not show particularly large differences between modes for any of the attenuation models. Values up to -3 MPa are predicted for the third trimester minimum path model, and up to -1 MPa for the "typical" attenuation model. The maximum and median values of power are dependent on the mode, with the color flow imaging (CFI) and SD modes producing the largest values (approximately 200 mW in the third trimester minimum path model). In a previous publication, more accurate calculations of temperature elevations for a bone target third trimester model were calculated for some of the probes. Extrapolation of these results for different fixed attenuation models suggests that spectral Doppler exposure may produce temperature elevations of around 6 degrees C in minimum attenuation models, and over 1 degrees C in a more "typical" attenuation model. In CFI mode, a worst-case temperature elevation of approximately 1.8 degrees C is estimated for the third trimester, and 1.3 degrees C for the first and second trimesters. For B-mode, the corresponding figures are 1.4 and 1.1 degrees C, respectively. For the case of a "typical" attenuation model, worst-case temperature elevations of approximately 0.3 degrees C are predicted for both B-mode and color flow imaging modes.

Development of a system to record cardiac output continuously in the newborn.

Intermittent recordings of Doppler flow velocity and cardiac output are of value during intensive care of the sick newborn infant but result in repeated disturbance of the child. We describe a new device for making continuous precordial recordings of Doppler flow velocity from the pulmonary artery in healthy resting newborn infants. Optimal probe siting was evaluated in six babies, and signals were found to be best when the pulmonary artery was insonated from the mid left parasternum. Continuous recordings were made in 13 other babies. Pulmonary artery velocities and, by calculation, cardiac output were measured continuously over periods ranging from 24 to 60 min. Median right ventricular output ranged widely from 148 to 246 mL x kg(-1) x min(-1). In contrast, for individual babies, the values were remarkably stable: the interquartile ranges varied from 13.2 to 29.9 mL x kg(-1) x min(-1). The simultaneous display of signal power allowed independent assessment of artifactual changes in cardiac output. This technique is feasible in healthy term infants and now requires evaluation in the intensive care setting where it may provide useful information concerning trends and short-term variability in right ventricular output.

A comparison of AIUM/NEMA thermal indices with calculated temperature rises for a simple third-trimester pregnancy tissue model. American Institute of Ultrasound in Medicine/National Electrical Manufacturers Association.

Temperature rises due to diagnostic ultrasound exposures have been calculated for a simple third-trimester pregnancy tissue model. This consisted of a layer of soft tissue representing the abdominal/uterine wall, a layer of liquid and a layer of fetal bone. The ultrasound field parameter used in the calculations was the temporal average of the square of the acoustic pressure (p2TA), measured in water but corrected for attenuation in the tissue model. The three-dimensional (3-D) distribution of p2TA was measured for five probes operating in B-mode, and four probes operating in pulsed Doppler and color flow imaging modes. The calculated temperature rises were compared to the AIUM/NEMA-defined thermal indices appropriate to third-trimester scanning. In B-mode, the ratio of calculated temperature rise to thermal index varied between 0.62 and 1.25, with calculated temperature rises as high as 1.4 degrees C. In color-flow imaging mode, this ratio varied between 1.26 and 2.45 and, in pulsed Doppler mode, between 1.46 and 2.92, with calculated temperature rises as high as 1.8 degrees C and 5.8 degrees C, respectively. These results indicate that, for scanning situations where bone is insonated through an overlying low attenuation liquid layer, the thermal index may substantially underestimate the maximum temperature rise that could occur.

New and future developments in ultrasonic imaging.

In the first part of the review, recent developments in medical imaging technology are described. Developments in transducer materials and matching, leading to improvements in band-width and sensitivity are discussed. Improvements in dynamic range due to increased transducer sensitivity, lower electronic noise levels and more efficient filtering are then considered. The benefits of the application of digital signal processing (DSP) techniques to radiofrequency (RF) echo signals are described, including more precise filtering and beam forming, synthetic aperture and parallel receive beam forming. Finally, the current situation in regard to 1.5 D arrays, 3 D scanning, ultrasound computed tomography (UCT), harmonic imaging with contrast agents and elastography are discussed. In the second part, some predictions for future developments are made. These will be possible largely due to the power of DSP. Parallel transmissions will make more efficient use of time, allowing greater spatial and temporal resolution, and greater accuracy in Doppler imaging. Adaptive transmission tailoring will be used, where the pulse characteristics to each part of the image field are independently optimized, as will adaptive receive processing in which echo sequences from each part of the image are independently and optimally processed. An important potential development will be automatic feature recognition, making possible accurate compound scanning with high spatial resolution, and quantitative information about the spatial distribution of acoustic speed. Compound scanning will provide more complete visualization of all structures and, particularly when incorporated into intravascular probes, should greatly aid the investigation of arterial plaque morphology. Feature recognition will also make it possible to have UCT systems (array based in future) which require less than 360 degrees access. Harmonic imaging without contrast agents, based simply on the inherent non-linearity of sound propagation in tissue, will become common. 2 D phased array transducer will permit symmetric beam focusing and scanning throughout a solid cone, greatly facilitating the development of 3 D scanning applications. Large 2 D arrays would have the potential to produce a five-fold increase in spatial resolution of a limited volume of tissue, or to measure the variation of backscatter with angle, as an aid to tissue characterization. Finally, ultrasound will be increasingly used to measure the elastic and dynamic properties of local regions of tissue.

Comparison of lower limb arterial assessments using color-duplex ultrasound and ankle/brachial pressure index measurements.

The strength of agreement between two noninvasive methods of assessing lower limb arterial disease and their relationship to patient symptoms following exercise have been investigated. Color-duplex ultrasound (CDU) and ankle/brachial pressure index (ABPI) (before and afer exercise) measurements were obtained from 200 consecutive patients referred to a vascular investigations laboratory. From these patients, 290 limbs were available for study, comprising limbs without previous vascular surgery, from patients without diabetes and who could attempt a walking exercise test. The overall level of agreement between CDU and resting ABPI measurements was 83% (Kappa 0.66). The ABPI technique identified the more serious disease; a resting ABPI of less than 0.6 gave 100% agreement with CDU. With higher resting ABPIs the level of agreement became poorer: 83% (0.6 < or = ABPI <0.9) and 76% (normal ABPI > or = 0.9). The addition of postexercise ABPI measurements in determining significant arterial disease increased the strength of relationship between the two techniques by only 2% (85%, Kappa 0.69). The exercise test was generally limited by the most symptomatic limb in each patient, and the agreement between CDU and postexercise ABPI measurements in these limbs was higher at 93% (Kappa 0.81). In comparison, agreement for the least symptomatic group of limbs was found to be poor (69%, Kappa 0.37). Compared with symptoms after exercise, overall agreements with CDU and ABPI were both 67% (Kappa 0.27). The agreement was better (91%) when the resting ABPI was less than 0.6. The ABPI is biased toward the detection of more severe disease and is more consistent with CDU when the most symptomatic limbs are compared. The relationship between either test and symptoms after exercise is strong only for limbs with major disease.

The use of duplex ultrasound in the assessment of arterial supply to the penis in vasculogenic impotence.

Impotence may be caused by arterial disease affecting the vessels supplying the corpora cavernosa. Color duplex ultrasound was used to measure the peak systolic velocity and systolic rise time in the deep penile arteries in 22 impotent men following papaverine stimulation. The results were compared with the findings of selective internal pudendal pharmaco-arteriography. A further comparison was made using color duplex ultrasound with 37 impotent men who all responded well to papaverine. A systolic rise time of 110 msec. or more was found to be the best discriminant of disease in the arteries supplying the corpora giving a positive predictive value of 0.92. A long systolic rise time in a papaverine responder may indicate that the arterial supply is borderline or that the arterial flow is maximal and that the problem lies on the sinusoidal-venous side. It appears that in the absence of a pathological condition there is a large surplus arterial supply.

A survey of the acoustic outputs of diagnostic ultrasound equipment in current clinical use.

Surveys published up to 1991 have highlighted a steady increase in the acoustic outputs from diagnostic ultrasound equipment. Since 1991 we have made measurements of the maximum peak negative pressure (p-) and spatial peak temporal average intensity (ISPTA) produced by 223 probes from 82 scanning systems in current clinical use in the Northern Region in the UK. Measurements have also been made of the maximum total acoustic power generated by 45 probes from 17 scanners. The results from these measurements are presented in this article and compared to the results of a similar survey of equipment from both the Northern and Wessex Regions in the UK and published in 1991. The comparison shows that measured ISPTA values have increased approximately sixfold in B mode and approximately threefold in colour Doppler mode. Also, measured total acoustic power values have doubled in pulsed Doppler mode. The present survey also draws attention to some particularly high ISPTA values obtained from a number of probes and scanning systems. This survey has shown that measurements of acoustic outputs from diagnostic ultrasound scanners in current clinical use are substantially higher than reported in earlier surveys and, for certain scanners, the acoustic outputs from scanned beam modes of operation can reach levels hitherto only found in pulsed Doppler mode.

Serial investigation of balloon angioplasty induced changes in the superficial femoral artery using colour duplex ultrasonography.

Percutaneous transluminal balloon angioplasty (PTA) of superficial femoral artery lesions is associated with similar initial success rates in coronary and iliac artery angioplasty but its application is limited by a much higher incidence of restenosis. To improve understanding of the trauma caused to the vessel by balloon angioplasty and the mechanisms contributing to the subsequent processes of healing and restenosis requires serial investigations of the treated arteries in vivo. This paper describes a prospective study using colour duplex ultrasonic imaging to assess arterial changes in 51 patients with atherosclerotic disease undergoing PTA of superficial femoral artery stenoses and occlusions. Each patient was scanned prior to angioplasty and at intervals up to 6 months post-angioplasty. On each scan, measurements were made of the overall vessel and lumen diameters at each site of angioplasty. These measurements indicate that angioplasty improves vessel patency mainly by stretching of the vessel wall, with compression and/or redistribution of the atherosclerotic plaque contributing less than 25% to the improvement of lumen diameter. Serial measurements after angioplasty show complex patterns of change at the angioplasty sites indicating that several mechanisms may be contributing to the processes of vessel healing and subsequent restenosis. Possible mechanisms which could explain the measured changes in overall vessel and lumen diameters are discussed.

The influence of ultrasound scanner beam width on femur length measurements.

Systematic differences have been found in measurements of femur lengths made with (1) a Hitachi EUB 25, representative of older machines in use when most of the femur length-gestation curves were produced; and (2) a modern high resolution machine (Acuson 128). It is proposed that these differences may be accounted for by the different beam widths of the two systems. Six operators measured the femur lengths of several normal fetuses using both machines. Analysis of the results revealed a highly significant (p < 0.005) difference between machines, that did not correlate with femur length (r = -0.022). The mean difference was 1.02 +/- 0.33 mm (Hitachi larger). The mean femur length was 46.44 mm (n = 57). Pulse-echo beam plots showed that, for the probes used, the Hitachi 6 dB beam width is greater than the Acuson beam width by typically 2 mm, depending on the range and Acuson focus setting. This beam width difference can account for the difference in femur length measurements, if allowance is made for the variable angle between femurs and ultrasound beams. The results suggest that femur length-gestation curves for use with modern machines should be based on measurements made with modern machines.

Towards a protocol for measurement of maximum spatial peak temporal average acoustic intensity from diagnostic B mode ultrasound scanners in the field.

The Northern Regional Medical Physics Department has been involved in measurement of the acoustic output of diagnostic ultrasound equipment for several years. As the complexity of diagnostic ultrasound equipment has increased, so have the problems of measuring the acoustic outputs of this equipment in the field. Measurements made in the field are often made on unfamiliar pieces of equipment and under tight constraints of time. In these circumstances the magnitude and the conditions under which the true maximum Ispta value occurs may not always be found. The aim of a measurement protocol is therefore to facilitate the measurement of Ispta in the field, so that the measured maximum Ispta value is as close as possible to the 'true maximum' Ispta value. To be of practical benefit the protocol must be succinct and easy to use, as well as applicable to most if not all types of scanner. Our experience has led us to believe that this is possible and that the benefits of a well designed measurement protocol will far outweigh any disadvantages. The development of two measurement protocols is discussed in this paper. The time required to carry out each measurement depends on the number of assumptions made about the operation of the scanner in the protocol used. The first protocol makes very few assumptions about the operation of a scanner; the results from measurements made using this protocol can be used to assess the validity of the much larger number of assumptions made in the second protocol. The results from measurements on three types of scanner using the two protocols are presented. The results demonstrate the validity of most of the assumptions made by the protocols and the potential benefits of using a protocol for measurement of maximum Ispta in the field in terms of reduced measurement time and greater consistency.

An experimental single-sideband acoustical re-injection test method for Doppler systems.

An experimental acoustical re-injection system based on a wideband single-sideband generator has been developed. This has been used to present a clinical Doppler system with a simulated Doppler shift, as would be obtained from a unidirectional moving reflector. Development of this approach should permit independent electronic testing of forward and reverse channels, including mean and peak frequency calculations and measurement of channel separation. Sideband suppression of 28 dB minimum has been obtained for a range of ultrasound frequencies from 2.5 MHz to 10 MHz and audio frequency shifts up to 10 kHz.

The assessment of carotid and vertebral arteries: a comparison of CFM duplex ultrasound with intravenous digital subtraction angiography.

The use of duplex ultrasound with colour flow mapping is compared with that of intravenous digital subtraction angiography (IVDSA) in the investigation of carotid and vertebral artery disease. Ninety-nine carotid and vertebral arteries were examined independently by IVDSA and ultrasound for location and degree of disease in 30 men (mean age 58) and 20 women (mean age 53). For purposes of comparison the carotid tree was divided into six segments. In 46% of cases no disease was detected by either modality. In 36% of cases where disease was found, ultrasound found mild disease in arteries reported as normal by IVDSA. Both modalities detected the six cases of total occlusion found in the study. IVDSA underestimated five cases of major disease found by ultrasound at the bifurcation or in the bulb. Overall agreement in grading vessel segments was good, with 74.5% in complete agreement and 90.3% grading stenoses to within +/- 25% of the other modality. There was good qualitative agreement in the findings for vertebral arteries. In four patients conventional arteriography was also available for comparison.

Resolution and information limitations from transducer arrays.

The factors limiting the resolution achievable with array-based imaging methods are considered. The limitations on beamwidth and field of view for beam scanning systems are first examined using plane wave diffraction theory, treating focusing and any array curvature as separable to the plane wave analysis. It is found that in all cases the number of half-beamwidths (maximum to null) across the field of view cannot exceed the number of electrically addressable elements in the array. The minimum beamwidth achievable by strong focusing is shown to be limited to twice the inter-element pitch. Digital reconstruction is then considered as an alternative to beam scanning. Here the number of image points recoverable across the field of view is equal to the number of elements in the array. The mathematical equivalence of the two methods is demonstrated, and the inference is made that the half-beamwidth represents a fundamental measure of lateral resolution. Alternative array configurations, suitable for CT methods, are then compared to probe arrays and it is found that the total number of resolution cells within a two-dimensional image is dependent on the pulse length and number of elements, rather than on the array configuration and imaging method. This again reveals the fundamental similarity of the various methods, essentially in combining position line information from the separate elements, albeit in different orders. Finally an expression for the information capacity of a scanning system is presented which relates the limitations on spatial and contrast resolution to the bandwidth and dynamic range of the system and the number of elements in the array.

Experimental studies in transmission ultrasound computed tomography.

The reconstruction of the speed-of-sound distribution within a target can be achieved by CT techniques from measurements on transmitted ultrasonic pulses. The mathematical relationship between speed-of-sound imaging and the conventional CT situation is explained. An experimental system, which has been developed to investigate speed-of-imaging and other forms of in-vivo ultrasound CT, is described, along with the techniques used for data acquisition and image reconstruction. These include measurement of pulse time-of-flight by the threshold or cross-correlation methods. Techniques for reducing artifacts in speed-of-sound images are also described, such as median filtering and modified Shepp-Logan filtering. These techniques have been used to obtain high quality speed-of-sound images of various phantoms. Images of tissue in-vitro have been less satisfactory, because of refraction and attenuation effects. Ways of overcoming these difficulties in an improved system are proposed.

Mixing Apple microcomputer graphics for ultrasound scan measurement.

A modern microcomputer with high-resolution graphics can provide an inexpensive method for measurement on video images from a real-time ultrasound scanner. The problem which has to be overcome to allow the computer graphics to be superimposed on the ultrasound video image and permit subsequent analysis is that of synchronization. The video signals must be synchronized before they can be mixed, but neither microcomputers nor ultrasound scanners provide facilities for external synchronization of their video output. A mixer has been designed which uses a buffer memory and allows the graphics of an Apple II microcomputer to be synchronized and mixed with an external video image; we used a Hitachi EUB22 real-time ultrasound scanner. The resulting combination is a versatile instrument which permits a wide range of measurements on ultrasonic images.