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Biomedical subjects

K Hynynen

Publications and source records attributed to K Hynynen.

At least 91 records · Page 5Linked to original sources
PubMed1994

MR temperature mapping of focused ultrasound surgery.

Deep lying soft tissue tumors may be treated by a nonincisional surgical procedure executed inside an MR imaging system using a thermal effect delivered by a focused ultrasound transducer. A prototype system is constructed to assess MRI thermal monitoring and the localization of the heat zone in muscle. The temperature distribution of the focal spot is imaged with MRI while mechanically moving the transducer with an hydraulic 3-axis positioner. Acoustic power is applied with a spherical shell transducer using 1- to 10-s duration pulses at frequencies of 1.5 MHz to selectively coagulate tissue at 60-70 degrees C. The procedure is monitored with a series of fast second gradient echo, T1-weighted, temperature sensitive MR sequences. Acquisitions are optimized for high temperature sensitive images that yield the thermal diffusivity, heat flow time constant and the focal spot size in muscle. MR temperature maps of muscle provide localization and dosimetry both in the focal region and near field.

Animals↗
PubMed1994

Pulse duration and peak intensity during focused ultrasound surgery: theoretical and experimental effects in rabbit brain in vivo.

The goal of this study was to establish the exposure parameters that will generate predictable thermally induced lesions in brain. In addition, the accuracy of a theoretical model for prediction of the lesion size was tested. To do this, 160 adult rabbits were sonicated (frequency 0.936 and 1.72 MHz) and then sacrificed at various intervals after the sonications. The results showed that predictable thermal lesions could be induced if the exposure durations were between 0.5 and 2 s. Dimensions of the necrosed tissue volume were roughly predictable by the theoretical calculations based on purely thermal effects. Shorter sonications required higher intensities (above 3700 W cm-2 at 1.72 MHz) resulting in mechanical effects with extensive vascular damage. Lesion size varied more at longer exposures (5 and 10 s), perhaps due to the increased effect of tissue perfusion. As a conclusion, focused ultrasound can be used for destruction of tissues deep in brain without causing undesirable mechanical effects, if the exposure parameters are selected properly.

Animals↗
PubMed1994

The usefulness of a contrast agent and gradient-recalled acquisition in a steady-state imaging sequence for magnetic resonance imaging-guided noninvasive ultrasound surgery.

RATIONALE AND OBJECTIVES: The ability of magnetic resonance imaging to detect small temperature elevations from focused ultrasound surgery beams was studied. In addition, the value of a contrast agent in delineating the necrosed tissue volume was investigated. MATERIALS AND METHODS: Gradient-recalled acquisition in a steady state (GRASS) T1-weighted images were used to follow the temperature elevation and tissue changes during 2-minute sonications in the thigh muscles of 10 rabbits. The effects of the treatment on the vascular network was investigated by injecting a contrast agent bolus before or after the sonication. RESULTS: The signal intensity decreased during the sonication, and the reduction was directly proportional to the applied power and increase in temperature. The signal intensity returned gradually back to baseline after the ultrasound was turned off. Injection of the contrast agent increased the signal intensity in muscle, but not in the necrosed tissue. The dimensions of the delineated tissue volume were the same as measured from the T2-weighted fast-spin-echo images and postmortem tissue examination. CONCLUSIONS: These results indicate that magnetic resonance imaging can be used to detect temperature elevations that do not cause tissue damage and that contrast agent can be used to delineate the necrosed tissue volume.

Animals↗
PubMed1994

Design and experimental evaluation of an intracavitary ultrasound phased array system for hyperthermia.

For evaluating the feasibility of treating prostate cancer, a 64-element linear ultrasound phased array applicator for intracavitary hyperthermia was designed and constructed. A 64-channel ultrasound driving system including amplifiers, phase shifters, and RF power meters was also developed to drive the array. The design of the array and driving equipment are presented, as are the results of acoustical field measurements and in vitro perfused phantom studies performed with the array. Several techniques for heating realistically sized tumor volumes were also investigated, including single focus scanning and two techniques for producing multiple stationary foci. The results show that the operation of the array correlated closely with the theoretical model. When producing a single stationary focus, the array was able to increase tissue temperature by 12 degrees C in vitro in perfused phantom. With some minor improvements in array design, intracavitary phased arrays could be evaluated in a clinical environment.

Acoustics↗
PubMed1994

The effects of curved tissue layers on the power deposition patterns of therapeutic ultrasound beams.

A numerical model for calculating the ultrasonic power deposition in layered media was developed and experimentally tested. The new model takes into account the ultrasound wave reflection and refraction occurring at tissue interfaces. In order to study the effects of soft tissue interfaces on the resulting ultrasonic power deposition patterns, the tested model was applied to simulate four different treatments situations in ultrasound therapy (such as hyperthermia). It was found that in most cases the effect of soft tissue interfaces could be ignored during ultrasound treatments. However, strongly curved tissue interfaces such as those encountered during brain treatments could have significant effects on the location of the focus and the magnitude of local power deposition if sharply focused ultrasound beams are used. The maximum lateral displacement of the focus in a brain treatment was 3 mm. This is too large to be ignored during therapy and thus the effect of tissue interfaces should be taken into account when certain ultrasound treatments are planned and executed.

Acoustics↗
PubMed1994

The effect of various physical parameters on the size and shape of necrosed tissue volume during ultrasound surgery.

The purpose of this study was to test the concept of using calculated thermal dose as a predictor for the necrosed tissue volume. A parametric study was conducted where the sonication parameters (pulse duration, power), transducer parameters (frequency, F number) and tissue properties (perfusion rate, attenuation) were varied and their effect on the lesion size was investigated. In vivo experiments where a focused ultrasound beam was used to induce tissue necrosis in thigh muscle of dog and rabbit were also conducted to obtain the reliability of the predictions. The experimental and simulated lesion sizes compared well. From the parametric study the threshold intensity for 1- and 5-s sonications were found to be about 1000 and 400 W/cm2, respectively. It was found that the lesion size was practically perfusion independent for pulses 5 s or shorter. The lesion size increases with increased pulse duration, acoustical power, and F number, but decreases with increased frequency provided that the focal intensity is kept constant. It was found also that the deeper the focus is in the tissue, the smaller the frequency range that causes selective tissue necrosis in the focal zone.

Animals↗
PubMed1993

Feasibility and toxicity of transrectal ultrasound hyperthermia in the treatment of locally advanced adenocarcinoma of the prostate.

PURPOSE: This Phase I trial tests the ability of a new hyperthermia device, the transrectal ultrasound probe, to heat the prostate gland, and evaluates the toxicity of transrectal ultrasound hyperthermia (TRUSH) given with concurrent standard external beam irradiation in the treatment of locally-advanced adenocarcinoma of the prostate. METHODS AND MATERIALS: Between June, 1990 and August, 1991, 14 patients with American Urological Society Stage C2 or D1 adenocarcinoma of the prostate were treated with TRUSH concurrently with standard external beam radiotherapy to the prostate. Twenty-two heat treatments were delivered in 14 patients; 8 patients received two TRUSH procedures, each separated by 1 week. Patient age ranged between 53-86 (mean: 72) years. Three patients had well-, 6 patients had moderately-, and 5 patients had poorly-differentiated adenocarcinoma of the prostate. Karnofsky status ranged from 70-90. Standard radiotherapy to the prostate and periprostatic tissues was delivered using a four-field approach with 1.8-2 Gy daily fractions delivered 5 x/week to a total dose of 67-70 Gy calculated to the minimum tumor volume. TRUSH was delivered after transperineal placement of multipoint thermometry probes by a urologist, under transrectal ultrasound guidance. Two to three thermocouple probes containing seven sensors each were placed in the prostate in an attempt to sample temperatures throughout the gland. The sensor depth from the rectal wall ranged from 5-25 mm. RESULTS: Thirty-six percent of all sensors were heated above 42.5 degrees C averaged over 30 min; and all patients had at least some sensors within the prostate heated to temperatures > or = 42.5 degrees C. The average temperature of all sensors of all sensors (T(ave) +/- s.d.) over all treatments, however, was only 41.9 degrees C +/- 0.9 degrees C over 30 min. The maximum temperature for normal tissues outside the gland was 41.1 degrees C +/- 1.3 degrees C. Treatments have been well-tolerated with few complications. Tolerance has been "good" in 17/22, "fair" in 3/22, and "treatment limiting" in 2/22 treatments secondary to position intolerance and/or pain. There has been one episode of hypotension related to narcotic administration and three episodes of rapidly resolving pain during hyperthermia treatment. Mild hematuria has occurred in 5/22, and moderate hematuria has occurred in 2/22 transperineal thermometer catheter placements. CONCLUSION: In conclusion, TRUSH is well-tolerated and has great potential for consistently heating the prostate gland. We anticipate that further equipment modifications will improve our ability to heat the entire prostate to temperatures > 42.5 degrees C.

Adenocarcinoma↗
PubMed1993

Magnetic resonance-guided thermal surgery.

A demonstration of MR guided thermal surgery involved experiments with imaging of focused ultrasound in an MRI system, measurements of the thermal transients and a thermal analysis of the resulting images. Both the heat distribution and the creation of focused ultrasound lesions in gel phantoms, in vitro bovine muscle and in vivo rabbit muscle were monitored with magnetic resonance imaging. Thermal surgical procedures were modeled by an elongated gaussian heat source where heat flow is controlled by tissue thermal properties and tissue perfusion. Temperature profiles were measured with thermocouples or calculated from magnetic resonance imaging in agreement with the model. A 2-s T1-weighted gradient-refocused acquisition provided thermal profiles needed to localize the heat distribution produced by a 4-s focused ultrasound pulse. Thermal analysis of the images give an effective thermal diffusion coefficient of 0.0015 cm2/s in gel and 0.0033 cm2/s in muscle. The lesions were detected using a T2-weighted spin-echo or fast spin-echo pulse sequence in agreement with muscle tissue sections. Potential thermal surgery applications are in the prostate, liver, kidney, bladder, breast, eye and brain.

Acoustics↗
PubMed1993

Focal spacing and near-field heating during pulsed high temperature ultrasound therapy.

It has been proposed that high temperature short duration hyperthermia treatment would be perfusion insensitive and thus, significantly improved thermal exposure uniformity could be achieved. This study investigates the execution of such a treatment, which utilizes single spherically curved transducer and multiple sonications to cover the complete target volume. The spacing of neighboring pulses as a function of the transducer characteristics was studied utilizing computer simulations. In addition, the temperature elevation in front of the focal zone during multiple sonications was evaluated. It was found that significant delays (20 s or longer) between the sonications must be introduced in order to avoid unwanted tissue damage in front of the focal zone. In addition, decreasing the pulse duration and F-number reduced the temperature build-up in front of the focus. The results were verified in vivo in dog's thigh muscle. This study is important not only for hyperthermia but also for ultrasound surgery, and indicates that each sonication system must be carefully evaluated for potential thermal damage outside of the target volume prior to implementation in therapy.

Animals↗
PubMed1993

Tissue thermometry during ultrasound exposure.

In order to quantify ultrasound therapy it is important to measure the tissue temperature during the treatment. Invasive probes induce several artifacts in ultrasound fields. The magnitude of these artifacts is probe dependent. Several different probes were evaluated for hyperthermia purposes in this study. An alternative noninvasive method to evaluate the temperature elevations and tissue damage is to use magnetic resonance imaging. The fast imaging sequences used in this study are marginally useful for monitoring hyperthermia. However, these imaging sequences can be utilized to guide and monitor ultrasound surgery.

Animals↗
PubMed1992

A comparison of theoretical and experimental ultrasound field distributions in canine muscle tissue in vivo.

Relative ultrasound field distributions were measured using thermal techniques in canine thighs in vivo and in water. The experimental results were compared with distributions obtained from a numerical model based on the one-dimensional integration of the Rayleigh-Sommerfeld diffraction integral. The comparisons showed that the theoretical model is a good approximation to the distributions measured in water, with the agreement decreasing for regions in front of the acoustic focus. The main lobe profiles obtained in the muscle tissue also agreed well with both theoretical results and results measured in water (focussing was not lost). However, these in vivo distributions showed enlargement of the side lobes indicating scattering of the waves. It was also found that the interfaces between muscle groups produced considerable beam distortions as well as increased side lobe levels. Scattering of energy from the main lobe to the side lobes was verified by measurements of the peak intensity and the total acoustic power attenuation coefficients for passage of the beams through the thighs which showed that the former was about 40% higher than the latter. Also, absolute intensity values at the acoustic focus were measured in water using a hydrophone (0.5 mm active diameter) for 11 transducers ranging in frequency from 0.246 to 3.54 MHz. When these absolute values were compared with the model predictions, it was found that the model consistently overestimated the experimental data by a factor of less than 2. That is, the model can also be used to obtain upper bounds for absolute intensity values. Consequences of these results on ultrasound hyperthermia treatments are discussed.

Animals↗
PubMed1992

The effect of wave reflection and refraction at soft tissue interfaces during ultrasound hyperthermia treatments.

An improved numerical model for calculating the ultrasonic power deposition in layered medium was developed and experimentally tested. The new model takes into account the ultrasound wave reflection and refraction at the tissue interfaces thereby providing improved accuracy in ultrasound hyperthermia treatment planning. The model was compared with a simplified model to evaluate when the tissue interfaces could be ignored in the hyperthermia treatment planning and evaluation. The effect of variations in water and tissue temperatures, the fat layer thicknesses, and the beam entrance angle were also investigated to establish guidelines for treatment execution. It was found that in most cases the effects of the soft tissue interfaces can be ignored. However, in some instances the acoustic focus may be shifted several millimeters off axis in layered medium. This is important when sharply focused transducers are used for ultrasound surgery or under the condition of pulsed, high-temperature hyperthermia treatments.

Humans↗
PubMed1991

The threshold for thermally significant cavitation in dog's thigh muscle in vivo.

In this study the threshold of thermally significant transient cavitation in vivo in dog's thigh muscle was investigated as a function of frequency from 0.246 MHz to 1.68 MHz. Cavitation, evidenced by strong emission of wide band noise monitored by a hydrophone, appeared to increase the energy absorption in tissue at the focal zone of a focused ultrasound beam as measured with an embedded thermocouple. This was indicated by a significant increase in the temperature, a loss of smooth temperature rise during the 1 s sound pulse and a significant reduction in the acoustic power transmitted through the thigh. This thermal phenomenon was associated with a strong emission of wide band noise which was monitored by a hydrophone. In addition, strong echoes appeared in ultrasound images during the pulses that caused the noise emission and the thermal effect. These echoes appeared preferentially at locations where there was acoustic heterogeneity. The measured cavitation pressure amplitude threshold was found to depend almost linearly on frequency with a slope of about 5.3 MPa MHz-1. (The extrapolated static pressure threshold was 0.6 MPa). When these measured levels are compared to those typical of clinical application, it appears that the transient cavitation can be avoided when perfusion independent high temperature hyperthermia is induced with focused and pulsed ultrasound fields. However, intensities required during scanned focused ultrasound hyperthermia, where sharply focused transducers are used to heat large tumors at low frequencies (1 MHz or below), could rise above the threshold. Thus, care should be taken when focused ultrasound systems are designed so that the maximum peak pressure is below the threshold in order to avoid unpredictable biological effects induced by transient cavitation. Finally it is unlikely that the present diagnostic ultrasound units which operate at higher frequencies and in pulsed mode could cause transient cavitation in vivo.

Acoustics↗
PubMed1990

Effects of physical parameters on high temperature ultrasound hyperthermia.

The purpose of this research was to investigate the feasibility of inducing perfusion independent, predictable therapeutic thermal dose using high power ultrasonic pulses. Computer simulations were used to study the effects of blood perfusion, tissue properties, transducer characteristics, and treatment geometry on the temperature elevation and thermal dose delivered by short ultrasonic pulses. Experiments were conducted in vitro and in vivo to investigate the effects of blood perfusion changes. Results show that short pulse lengths (less than or equal to 2 s) and small focal diameters (approximately 3 mm) give temperature elevations and thermal doses which are nearly perfusion independent. Normal fluctuations in tissue properties should not have a significant effect on the treatment provided that proper choice of transducer is made for each individual application.

Animals↗
PubMed1990

Hyperthermia in cancer treatment.

Hyperthermia, used as an adjunct to radiation therapy, can increase tumor regression significantly. When used as a local (as opposed to regional or systemic) modality, proper treatment requires detailed knowledge of tumor and normal tissue geometry, and physiologic parameters such as perfusion and thermal conductivity. A brief review of local heating techniques and details of two techniques used to treat brain tumors are provided: Scan Focused Ultrasound and Interstitial Ferromagnetic Seed Implants. These techniques require the most sophisticated use of diagnostic radiology methods. Data from several modalities such as CT, MRI, angiography, and xenon CT perfusion studies must be merged into a consistent data set. This data set must be indexed precisely relative to the treatment apparatus. Real-time noninvasive temperature monitoring of the treatment field has not been achieved at this time, but is of interest to researchers in hyperthermia.

Humans↗
PubMed1990

Computer-controlled dynamic phantom for ultrasound hyperthermia studies.

A dynamic phantom system for use in evaluating hyperthermia heating equipment has been designed, constructed, and tested. It consists of four in vitro preserved canine kidneys perfused with an 80% ethanol preparation by a single pump, plus four computer-controlled valves and four flow meters. The flow rate to each kidney is computer-controlled, giving flexibility in the types of spatial and temporal flow patterns possible. Examples are given for both step and ramp changes in flow, and for a temperature dependent flow which has been used to simulate physiological responses to elevated temperatures.

Animals↗