Ultrasound heating in a tissue-bone phantom

MRI-guided focused ultrasound surgery in musculoskeletal diseases: the hot topics

MRI-guided focused ultrasound surgery (MRgFUS) is a minimally invasive treatment guided by the most sophisticated imaging tool available in today's clinical practice. Both the imaging and therapeutic sides of the equipment are based on non-ionizing energy. This technique is a very promising option as potential treatment for several pathologies, including musculoskeletal (MSK) disorders. Apart from clinical applications, MRgFUS technology is the result of long, heavy and cumulative efforts exploring the effects of ultrasound on biological tissues and function, the generation of focused ultrasound and treatment monitoring by MRI. The aim of this article is to give an updated overview on a "new" interventional technique and on its applications for MSK and allied sciences.

Thermal effects generated by high-intensity focused ultrasound beams at normal incidence to a bone surface

Experiments and computations were performed to study factors affecting thermal safety when high-intensity focused ultrasound (HIFU) beams are normally incident (i.e., beam axis normal to the interface) upon a bone/soft-tissue interface. In particular, the temperature rise and thermal dose were determined as a function of separation between the beam focus and the interface. Under conditions representative of clinical HIFU procedures, it was found that the thermal dose at the bone surface can exceed the threshold for necrosis even when the beam focus is more than 4 cm from the bone. Experiments showed that reflection of the HIFU beam from the bone back into the transducer introduced temperature fluctuations of as much as +/-15% and may be an important consideration for safety analyses at sufficiently high acoustic power. The applicability of linear propagation models in predicting thermal dose near the interface was also addressed. Linear models, while underpredicting thermal dose at the focus, provided a conservative (slight overprediction) estimate of thermal dose at the bone surface. Finally, temperature rise due to absorption of shear waves generated by the HIFU beam in the bone was computed. Modeling shear-wave propagation in the thermal analysis showed that the predicted temperature rise off axis was as much as 30% higher when absorption of shear waves is included, indicating that enhanced heating due to shear-wave absorption is potentially important, even for normally incident HIFU beams.

MicroPET-compatible, small animal hyperthermia ultrasound system (SAHUS) for sustainable, collimated and controlled hyperthermia of subcutaneously implanted tumours

An external ultrasound system was developed for the heating of subcutaneously implanted tumours in small animals. This small animal hyperthermia ultrasound system (SAHUS) was designed to be compatible with a microPET (small animal positron emission tomography) scanner to facilitate studies of hyperthermia effects on tumour hypoxia. Collimation and localization of energy deposition, a specific goal for the new device to avoid regional and/or systemic heating of small animals, was demonstrated using thermoradiography following high-power short-time heating of a layered gel phantom. The in vivo heating capabilities of the SAHUS were tested using PC3 cell line tumours (2000-2700 mm(3)) grown in the lateral proximal thighs of Nu-/Nu- nuBR nude mice. Intratumour temperatures were recorded during heating trials with deep and superficial interstitial thermocouples. The experimental data showed that the SAHUS could produce hyperthermia in 8 +/- 2 mm diameter tumours in small animals to a target temperature of 41.5 degrees C and maintain it within a narrow temperature range (+/- 0.3 degrees C) for up to 4 h without raising the core temperature of the animals. PET imaging studies, data to be published separately, were conducted before and during SAHUS-induced hyperthermia. Both devices performed as expected and there was no significant decrease in image quality. In this paper, the new SAHUS is described and data from phantom and in vivo experiments presented.

Development and characterization of a blood mimicking fluid for high intensity focused ultrasound

A blood mimicking fluid (BMF) has been developed for the acoustic and thermal characterizations of high intensity focused ultrasound (HIFU) ablation devices. The BMF is based on a degassed and de-ionized water solution dispersed with low density polyethylene microspheres, nylon particles, gellan gum, and glycerol. A broad range of physical parameters, including attenuation coefficient, speed of sound, viscosity, thermal conductivity, and diffusivity, were characterized as a function of temperature (20-70 degrees C). The nonlinear parameter B/A and backscatter coefficient were also measured at room temperature. Importantly, the attenuation coefficient is linearly proportional to the frequency (2-8 MHz) with a slope of about 0.2 dB cm(-1) MHz(-1) in the 20-70 degrees C range as in the case of human blood. Furthermore, sound speed and bloodlike backscattering indicate the usefulness of the BMF for ultrasound flow imaging and ultrasound-guided HIFU applications. Most of the other temperature-dependent physical parameters are also close to the reported values in human blood. These properties make it a unique HIFU research tool for developing standardized exposimetry techniques, validating numerical models, and determining the safety and efficacy of HIFU ablation devices.

A model for estimating ultrasound attenuation along the propagation path to the fetus from backscattered waveforms

Accurate estimates of the ultrasound pressure and/or intensity incident on the developing fetus on a patient-specific basis could improve the diagnostic potential of medical ultrasound by allowing the clinician to increase the transmit power while still avoiding the potential for harmful bioeffects. Neglecting nonlinear effects, the pressure/intensity can be estimated if an accurate estimate of the attenuation along the propagation path (i.e., total attenuation) can be obtained. Herein, a method for determining the total attenuation from the backscattered power spectrum from the developing fetus is proposed. The boundaries between amnion and either the fetus' skull or soft tissue are each modeled as planar impedance boundaries at an unknown orientation with respect to the sound beam. A mathematical analysis demonstrates that the normalized returned voltage spectrum from this model is independent of the planes orientation. Hence, the total attenuation can be estimated by comparing the location of the spectral peak in the reflection from the fetus to the location of the spectral peak in a reflection obtained from a rigid plane in a water bath. The independence of the attenuation estimate and plane orientation is then demonstrated experimentally using a Plexiglas plate, a rat's skull, and a tissue-mimicking phantom.

Thermal assessment of 40-MHz ultrasound at soft tissue-bone interfaces

Tissue exposure to diagnostic ultrasound (US) can cause significant temperature rises. However, little has been reported on thermal effects of high-frequency US, and guidelines for the use of US do not necessarily apply to higher frequencies. Temperature rise induced by US biomicroscopy (UBM) was measured in phantoms containing mouse skulls and in anesthetized mice and mice post mortem, with a 50-microm K-type thermocouple. The operating frequency was 40 MHz with a free field I(SPTA) of 2.6 mW/cm(2) (B-mode) and 11.9 W/cm(2) (Doppler). Peak negative pressures were 5.22 MPa (B mode) and 7.32 MPa (Doppler), resulting in a mechanical index (MI) of 0.83 (B-mode) and 1.05 (Doppler mode). In Doppler mode, mean temperature rises of 1.80 degrees C and 1.73 degrees C were measured for proximal and distal skull phantom surfaces after a 3-min insonation. In vivo, the proximal mouse skull surface showed a mean temperature rise of 2.1 degrees C, with no statistically significant differences post mortem. Our results indicate temperature rise from insonation of bone interfaces using similar exposure parameters should not cause adverse bioeffects.

Thermal contribution of compact bone to intervening tissue-like media exposed to planar ultrasound

The presence of bone in the ultrasound beam path raises concerns, both in diagnostic and therapeutic applications, because significant temperature elevations may be induced at nearby soft tissue-bone interfaces due the facts that ultrasound is (i) highly absorbed in bone and (ii) reflected at soft tissue-bone interfaces in various degrees depending on angle of incidence. Consequently, in ultrasonic thermal therapy, the presence of bone in the ultrasound beam path is considered a major disadvantage and it is usually avoided. However, based on clinical experience and previous theoretical studies, we hypothesized that the presence of bone in superficial unfocused ultrasound hyperthermia can actually be exploited to induce more uniform and enhanced (with respect to the no-bone situation) temperature distributions in superficial target volumes. In particular, we hypothesize that the presence of underlying bone in superficial target volume enhances temperature elevation not only by additional direct power deposition from acoustic reflection, but also from thermal diffusion from the underlying bone. Here we report laboratory results that corroborate previous computational studies and strengthen the above-stated hypothesis. Three different temperature measurement techniques, namely, thermometric (using fibre-optic temperature probes), thermographic (using an infrared camera) and magnetic resonance imaging (using proton resonance frequency shifts), were used in high-power short-exposure, and in low-power extended-exposure, experiments using a 19 mm diameter planar transducer operating at 1.0 and 3.3 MHz (frequencies of clinical relevance). The measurements were performed on three technique-specific phantoms (with and without bone inclusions) and experimental set-ups that resembled possible superficial ultrasound hyperthermia clinical situations. Results from all three techniques were in general agreement and clearly showed that significantly higher heating rates (greater than fourfold) were induced in soft tissue-like phantom materials adjacent (within approximately 5 mm) to a bovine bone as compared to similar experiments without bone inclusions. For low-power long-exposure experiments, where thermal conduction effects are significant, the thermal impact of bone reached at distances > 10 mm from the bone surface (upstream of the bone). Therefore, we hypothesize that underlying bone exposed to planar ultrasound hyperthermia creates a high-temperature thermal boundary at depth that compensates for beam attenuation, thus producing more uniform temperature distribution in the intervening tissue layers. With appropriate technology, this finding may lead to improved thermal doses in superficial treatment sites such as the chest wall and the head/neck.

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.

Anthropomorphic carotid bifurcation phantom for MRI applications

Anthropomorphic carotid bifurcation flow phantoms that incorporate different stenotic geometries within the internal carotid artery have been developed. This technique produces high-fidelity, life-size vascular flow models that are compatible with magnetic resonance techniques. The models, in conjunction with a computer-controlled flow pump, address the need for a complex vascular geometry that can be used to verify magnetic resonance angiography (MRA) techniques that quantify stenosis severity and blood flow. Stenotic geometries, with up to 80% diameter reduction, have been fabricated in two different phantom materials. Plastic phantoms provide a durable, rigid geometry where the absolute dimensions of the model are well known. Agar gel phantoms provide tissue-like signal (T1, T2) up to the lumen boundary and are also compatible with ultrasound techniques. In this paper the technique to produce vascular flow phantoms is outlined and the compatibility of these phantoms with MRA techniques is demonstrated. J. Magn. Reson. Imaging 1999;10:533-544.

Proposed standard thermal test object for medical ultrasound

A general design for a thermal test object (TTO) is proposed. A number of novel features make the design particularly suitable for use as a standardised device for assessing the heating capability of diagnostic ultrasound beams. To assess performance, soft-tissue TTOs have been made containing thin-film thermocouples sandwiched between discs of tissue-mimicking gel. Installed in an appropriate measurement system, these TTOs exhibit excellent thermal and spatial resolution, allowing the ultrasound beam to be located rapidly and reproducibly. The measured temperature rise after 3 minutes of heating has been compared with theoretical predictions based on measured pressure distributions, and agreement is within 10%. Other studies have shown that soft-tissue- and bone-mimicking TTOs can be used to evaluate a wide range of ultrasound fields and that different physical tissue models can be simulated. It is concluded that this design would be suitable for providing reference assessments of the thermal hazard posed by diagnostic ultrasound under standardised conditions.

Application of low-intensity ultrasound to growing bone in rats

Low-intensity pulsed ultrasound recently has been shown to accelerate long bone fracture healing, but its effect on bone growth and development is unknown. The longitudinal growth and bone density of the femur and tibia in young rats was measured after application of an ultrasound transducer emitting 1.5-MHz pulsed ultrasound (30 mW/cm2, SATA) for 20 min/day. After 28 days, no length difference was detected (< or = 2%) compared to the sham-treated leg or to unexposed controls. Also, no significant difference in bone mineral density (BMD) of the femur or tibia was found (< or = 6%). In a repeated experiment in which a periosteal trauma stimulus was created in the femoral diaphysis, the ultrasound also had no effect on growth or BMD. This results suggests that physeal bone growth is far less sensitive to this level of ultrasound application than is fracture repair. This may be related to the cascade of cellular events and regulatory factors that are present after a fracture.

Perfusion corrections for ultrasonic heating in nonperfused media

The linear bio-heat transfer equation proposed by Pennes is widely used to predict temperature rise in perfused media. In this article, a mathematical relationship between the predicted temperature increase under perfused and nonperfused conditions is derived based on an analysis of the Pennes equation. The perfused temperature at time t and position r, Tperf(r,t) can be calculated from the unperfused temperature history Tunperf(r,t) and the time constant for perfusion, tau: [formula: see text] When the full nonperfused temperature history is not available, a simpler approximate method of estimating the perfused temperature is also suggested, requiring only a knowledge of the time constant for perfusion. Results are given showing the effects of perfusion after different exposure times for a range of beamwidths and perfusion time constants.

Ultrasound heating in a tissue-bone phantom

Temperature rise generated by focused ultrasound beams was tested on semipermanent tissue-bone phantoms. The phantoms were capped (sealed) plastic hollow cylindrical containers filled with tissue-mimicking material (TMM), in which were imbedded 25 microns diameter copper-constantan thermocouples (TC) and a piece of compact human or cow bone. The acoustic frequency specific attenuation coefficient of TMM was adjusted to be 0.3 dB cm-1 MHz-1 as specified by the FDA for a frequency range of 1-5 MHz. A high density 0.318 cm thick polyethylene sheet was chosen as the material to make caps of the phantoms. A formula developed to estimate the upper limit of temperature rises at tissue-bone interfaces generated by focused ultrasound has been proved to be appropriate experimentally using the semipermanent phantoms.