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.

Effects of oxygen deprivation on parapyramidal neurons of the ventrolateral medulla in the rat.

We characterized the electrophysiological properties and responses of neurons located in the parapyramidal region of the ventral aspect of the rat medulla oblongata (parapyramidal neurons, PP neurons) to oxygen deprivation, in order to understand the mechanisms involved in hypoxia induced respiratory depression. The responses of PP neurons to oxygen deprivation were compared to those of the functionally dissimilar neurons of the dentate gyrus (DG). Neurons from the PP region were found to fire spontaneously with a frequency of 3-3.5 spikes/sec in both adults and neonates and responded to an anoxic insult with a complete loss of spontaneous firing. Discrete metabolite analysis showed a small (about 17%) decrease in tissue adenosine triphosphate (ATP) levels of the PP neurons during an anoxic insult and the decrease was significantly smaller than in the DG cell region (28%). In contrast to the DG neurons, the PP neurons recovered from an anoxic insult lasting more than 30 min, indicating a greater survival capacity of the PP neurons during oxygen deprivation. The PP neurons were also capable of withstanding successive anoxic insults better than the DG cells as demonstrated by their complete recovery following reoxygenation. It is suggested that the PP neurons may depress their electrical activity as an energy conservation mechanism, and thereby survive anoxic insults longer than the dentate neurons, whereas the loss of cellular activity in the DG neurons may be a result of energy depletion.

Substrates of energy metabolism attenuate methamphetamine-induced neurotoxicity in striatum.

High doses of methamphetamine (METH) produce a long-term depletion in striatal tissue dopamine content. The mechanism mediating this toxicity has been associated with increased concentrations of dopamine and glutamate and altered energy metabolism. In vivo microdialysis was used to assess and alter the metabolic environment of the brain during high doses of METH. METH significantly increased extracellular concentrations of lactate in striatum and prefrontal cortex. This increase was significantly greater in striatum and coincided with the greater vulnerability of this brain region to the toxic effects of METH. To examine the effect of supplementing energy metabolism on METH-induced dopamine content depletions, the striatum was perfused directly with decylubiquinone or nicotinamide to enhance the energetic capacity of the tissue during or after a neurotoxic dosing regimen of METH. When decylubiquinone or nicotinamide was perfused into striatum during the administration of METH, there was no significant effect on METH-induced striatal dopamine efflux, glutamate efflux, or the long-term dopamine depletions measured 7 days later. However, a delayed perfusion with decylubiquinone or nicotinamide for 6 h beginning immediately after the last METH injection attenuated the METH-induced striatal dopamine depletions measured 1 week later. These results support the hypothesis that the compromised metabolic state produced by METH administration predisposes dopamine terminals to the neurotoxic effects of glutamate, dopamine, and/or free radicals.

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.

ATP catabolism and adenosine generation during ischemia in the aging heart.

Myocardial injury following ischemia and reperfusion is increased in the aging heart. The mechanisms underlying the increased susceptibility of the aging heart to ischemic injury remain unknown. We investigated whether decreased glycogen utilization with a more rapid depletion of ATP occurred during ischemia in the aging heart. Isolated buffer-perfused hearts from adult (6 months old) and aging (24 months old) Fischer 344 rats were subjected to 0, 2, 5, 10, 15 or 25 min of global stop-flow ischemia following a 15 min equilibration period (n = 5-6 for each ischemic time at each age). ATP level were decreased at preischemic baseline in aging hearts. ATP levels remained lower in the aging heart throughout ischemia (P < 0.001) with a similar pattern of decrease in both age groups. The decrease in tissue glycogen and increase in lactate contents was similar during ischemia in both age groups, suggesting that comparable glycogen utilization occurred during ischemia in adult and aging hearts. ATP catabolism leads to ADP, AMP and then adenosine. Tissue levels of adenosine, an important cardioprotective metabolite, were measured during ischemia. Tissue adenosine levels were decreased by 50% in the aging heart at 5 and 10 min, and remained depressed at 15 min and 25 min of ischemia compared to adult controls. Thus, increased ischemic injury in the aging heart is not related to differences in glycogen consumption. Lower tissue ATP levels and decreased adenosine levels were observed during ischemia. The differences in ATP content between adult and aging hearts occurred only during early ischemia and are unlikely to provide a mechanism for the increased damage observed following more prolonged periods of ischemia in the aging heart. The potential contribution of these decreases in tissue adenosine content to the increased injury observed in the aging heart will require further study.

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.

Creatine and nucleoside triphosphates in rat cerebral gray and white matter.

In vivo localized nuclear magnetic resonance spectroscopy has shown three-fold higher phosphocreatine/nucleoside triphosphate (PCr/NTP) ratios in cerebral white compared to gray matter. To interpret these results, total creatine (Cr) and ATP concentrations were measured enzymatically in samples taken from rapidly frozen rat cerebral cortex and corpus callosum. Total Cr (PCr plus Cr) and ATP concentrations were the same in the two regions. High performance liquid chromatography showed similar concentrations of total NTP in the two regions. These results suggest that higher in vivo PCr/NTP ratios in white compared to gray matter may be due to a higher PCr/Cr ratio in white matter and/or higher percentages of non-adenine mono- or diphosphate nucleotides in gray matter.

The influence of age, liver size and enantiomer concentrations on warfarin requirements.

1. We have tested the hypothesis that the fall in hepatic mass with age influences the age related increase in sensitivity to warfarin. In 39 otherwise healthy outpatients, aged 50-87 years, stabilised on warfarin for prophylaxis of thromboembolism, age, mean International Normalised Ratio (INR), and mean warfarin dosage were recorded. Liver volume was measured by ultrasound, and plasma was assayed for trough concentrations of (R)- and (S)-warfarin. 2. There was a negative correlation between age and liver volume (r = -0.41; P = 0.01) and age and dose (r = -0.53; P = < 0.001) and a positive correlation between liver volume and dose (r = 0.49; P = 0.002). There was no significant correlation between dosage and (R)- and (S)-warfarin concentrations, nor between dosage and INR. 3. The regression model including both age and liver volume data showed a better fit for estimation of warfarin dosage requirement than regression models based on age and liver volume data alone. Ninety-five per cent prediction intervals for warfarin dose requirements were wide, whether age alone, or age and liver volume were used in calculations. 4. Due to inter-individual variation in warfarin dosage requirements related to other influences, both explained and unexplained, routine measurement of age and liver volume would not contribute further clinically useful information to that obtained by the INR test currently used for predicting warfarin dosage requirements.

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.