C.R. Hill

Ultrasonic attenuation and propagation speed in mammalian tissues as a function of temperature

Abstract Ultrasonic attenuation in the frequency range 1–7 MHz, and the speed of sound, were determined experimentally in both fresh and fixed bovine and human soft tissues for various temperatures in the range 5–65°C. At temperatures below 40°C the attenuation coefficient behaves similarly for fixed and fresh tissues where, at high frequencies, it has a negative dependence on temperature, the value at 20°C being about 21% higher than that at 37°C. As the frequency is reduced, the temperature coefficient of attenuation progressively decreases until, after passing a transition frequency (this varies with the tissue specimens but is around 1–2 MHz), a positive dependence on temperature may be observed. At temperatures above about 40°C, the attenuation coefficient of freshly excised tissues increases with temperature, whereas for fixed tissues the attenuation coefficient continues to decrease. These observations help to resolve a possible discrepancy evident in previous reports of the temperature dependence of attenuation. The speed of sound in non-fatty tissues increases with temperature and exhibits a maximum at around 50°C, while for fatty tissues a negative dependence is observed. The implications of this result for improved diagnostic procedures is discussed.

Effect of blood perfusion on the ablation of liver parenchyma with high-intensity focused ultrasound

This paper discusses the effect of blood perfusion on the ablation of rat liver tissue with high-intensity focused ultrasound (HIFU). For this study a practical method has been developed, in which the liver blood flow can be reduced by ligation of the hepatic artery and portal vein. During the treatment the rat liver was mobilized out of the abdomen and the blood flow was measured using both the radioactive microsphere method and a laser Doppler blood-flow monitor. The results show that the hepatic blood flow was about 23 ml/100 g min-1 via the hepatic artery and about 227 ml/100 g min-1 via the portal vein. The total liver blood flow was reduced by 98% when both the hepatic artery and portal vein were ligated. Comparative lesions were made on the same liver lobes of rats with both normal and reduced blood flow using a focused ultrasound beam of 1.7 MHz, 67-425 W cm-2 spatially averaged focal intensity ISAL and 2-20 s exposure duration. A marked difference has been found between the lesion dimensions obtained with normal blood flow and that with reduced blood flow. For exposures at 169 W cm-2 the lesion diameter with normal blood flow was reduced by 14% for 3 s exposure duration compared to that obtained with both hepatic artery and portal vein ligated, while the reduction was more than 20% for longer durations.

LESION DEVELOPMENT IN FOCUSED ULTRASOUND SURGERY: A GENERAL MODEL

An analytical model has been constructed for the process of formation of thermal lesions in tissue, resulting from exposure to intense, highly focused ultrasound beams such as may be used in minimally invasive surgery. The model assumes a Gaussian approximation to beam shape in the focal region and predicts, for any such focal beam, the time delay to initiation of a lesion and the subsequent time course of growth of that lesion in lateral and axial dimensions, taking into account the effects of thermal diffusion and blood perfusion. The necessary approximations and assumptions of the model are considered. Comparison of predictions with experimentally measured data on excised pig liver indicate generally good agreement. Comparisons are also made of this theory with previously published data on exposure-time dependence of lesioning threshold intensity. Deficiencies are identified in existing practice for measuring and reporting acoustic exposures for focused ultrasound surgery, and the proposal is therefore made that a quantity that would be more satisfactory, from the viewpoints both of metrology and biophysical relevance, is the intensity spatially averaged over the area enclosed by the half-pressure-maximum contour in the focal plane, as determined under linear conditions, provisionally denoted as ISAL.

Measurement of soft tissue motion using correlation between A-scans.

The advent of real-time B-scanning has led to interest in the diagnostic value of the dynamic properties of soft tissue. Present ultrasonic techniques for investigating motion cannot measure the motion of homogeneous tissues. A technique has been developed which uses the correlation coefficient between A-scans to measure the amplitude and frequency of their motion, both in water tank experiments and in vivo. The success of the technique, which is digitally implemented, supports the validity of stochastic models for the acoustic structure of soft tissues. The motion pattern observed in vivo can be correlated with the arterial pressure pulse.

ACOUSTIC PROPERTIES OF NORMAL AND CANCEROUS HUMAN LIVER---I. DEPENDENCE ON PATHOLOGICAL CONDITION

Abstract Ultrasonic speed, attenuation and backscattering were measured as functions of frequency and orientation in specimens of exercised human liver, with a view to establishing the usefulness of such measurements to characterize the pathology and structure of the tissue. It is observed that acoustic speed is superior to any attenuation or backscattering characteristics for distinguishing in vitro between specimens of tumours and normal human liver selected at random. However, when the data are corrected for variations between one subject and another, sound speed, attenuation and the mean back-scattering coefficient at a given frequency show a comparable degree of usefulness in this respect. Analysis of the periodicities present in the backscattering diffraction patterns did not contribute any improvement in the ability to distinguish between tissue states. On average, by comparison with normal liver, ultrasound travels about 1.5% (± 1%) slower, is attenuated by about 20% (± 30%) less at 3 MHz and is backscattered by about 80% (±115%) less at 3 MHz in the tumor specimens that were measured. Livers infiltrated by diffuse malignant disease appear to possess quite different ultrasonic propagation properties to normal liver although insufficient data are yet available for firm conclusions to be drawn.

Acoustic properties of normal and cancerous human liver—II Dependence on tissue structure

Abstract Ultrasonic speed, attenuation and backscattering were measured as a function of frequency and compared with measurements of water content, fat content and collagen content in specimens of excised human liver. It is observed that the ultrasonic velocity decreases with both increasing water and fat contents, although the water content appears to have the overriding influence. An increase in the water content correlates well with decreasing attenuation and backscattering coefficients (and the slope of their frequency dependence), but positive dependences are found between these acoustic characteristics and the fat content. It is believed that the dependences on fat content are of secondary importance to those on water content and possibly arise as a result of an inverse relationship between the fat content and the water content. For the collagen content of liver specimens, positive correlations, barely significant, were found for attenuation and backscattering only after the data had first been corrected for variations in water content, while no significant velocity dependence was seen.

Ultrasonic propagation through fixed and unfixed tissues

Abstract Ultrasonic attenuation and backscattering coefficients and speed of sound were determined experimentally, as functions of frequency, in samples of fresh mammalian brain, liver and spleen, and in the same specimens fixed histochemically. It is observed that 4% formalin and 5% potassium dichromate are greatly superior to ethyl alcohol for consistently preserving the ultrasonic propagation properties to within only a few percent of those of the original fresh, unfixed material.

Ultrasonic study of in vivo kinetic characteristics of human tissues

A method is described for quantifying tissue movement in vivo from the computation of correlation coefficient between pairs of A-scans with appropriate time separation. The method yields quantifiable and repeatable secondary patterns of soft tissue movement in response to primary cardiac movement in a given subject, shows consistently different results as between normal livers and a variety of abdominal tumours, and is sensitive to either progress or therapeutically-induced regression of malignant disease. While the results reported here have been obtained using somewhat simple and crude equipment, the method is well suited to implementation on a commercial real-time scanner.

Ultrasonic attenuation and backscattering by mammalian organs as a function of time after excision.

Abstract Ultrasonic attenuation and backscattering were measured for bovine brain, spleen and liver and for porcine iiver as a function of time after excision for times up to 120 hr. The attenuation coefficient exhibits insignificant changes while the mean backscattering amplitude decreases substantially during the period such specimens are likely to be used. The changes in the two parameters are believed to reflect in large measure, their origins, viz., the molecular level for attenuation and macrostructural level for backscattering.

Application of fourier analysis to clinical study of patterns of tissue movement

An analysis is made of the kinetics of human liver parenchyma in response to mechanical impulses arising in the heart and aorta, and the results are applied to predicting the time course of the correlation between two time-separated A-scans derived from various regions of the liver. Such predictions are found to correspond well with data derived clinically, both from volunteers and from patients with liver metastases, using a commercial, real-time sector scanner. On the basis of Fourier spectral features of the clinically derived correlation patterns, a clear quantitative separation was demonstrated between the kinetic response of three classes of tissue: normal liver in volunteers, metastatic deposits in liver of cancer patients, and histologically normal liver regions in the same patients.