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Kullervo Hynynen

Publications and source records attributed to Kullervo Hynynen.

64 records · Page 4Linked to original sources
PubMed2002

Bio-acoustic thermal lensing and nonlinear propagation in focused ultrasound surgery using large focal spots: a parametric study.

It is well known that the acoustic properties of soft tissue have a dependence on tissue temperature. This is of particular interest in focused ultrasound surgery since the mechanism of action of focused ultrasound surgery is to kill targeted tissue by inducing localized heating by ultrasound absorption, and hence cautery of that tissue. However, the act of localized heating induces a change in the acoustic properties of the targeted tissue and tissue surrounding it. This phenomenon distorts the incoming acoustic wavefront, and has been termed the thermal lens effect for this reason. Furthermore, nonlinear effects in acoustic propagation become non-negligible at the ultrasound intensities required for therapeutic action. This paper examines the importance of the thermal lens effect and nonlinear tissue properties by simulating a variety of clinically applicable phased array transducer configurations that have not yet been appropriately analysed using a full three-dimensional nonlinear treatment of acoustic propagation. The significance of the thermal lens effect is characterized by comparing the simulation of coupled acoustic and thermal propagation with an uncoupled treatment; neglecting thermal lensing typically produces a movement of 1 to 2 mm in the predicted position of the focus towards the transducer. The results also show that the classical methods of acoustic propagation can produce grossly erroneous results under certain clinically relevant transducer configurations and that an acoustic field scan with a hydrophone may not accurately predict therapeutic effect.

Acoustics↗
PubMed2002

Effects of parameter errors in the simulation of transcranial focused ultrasound.

Previous numerical simulation work has supported experiments showing that a sharply focused transcranial ultrasound field can be generated for noninvasive therapy and surgery in the brain. The predicted pressure gain and optimal sonicating frequency could be affected by uncertainty in the simulation parameters. We estimate the effects of uncertainty in the speed of sound in the skull and brain, and in CT data that specifies the contour of the skull. The results of our simulations indicate that each of these errors may change the predicted pressure gain by up to a few percent, but the predicted optimal frequency is not significantly affected.

Brain↗
PubMed2002

The temperature dependence of ultrasound-stimulated acoustic emission.

Given the high variability of tissue properties during sonication, temperature monitoring is one of the most crucial components for accurate thermal treatment of tissues with focused ultrasound and other thermotherapy devices. Recently, the method of ultrasound-stimulated acoustic emission (USAE) has been introduced as a potential method for measurements of mechanical properties of tissues. In this paper, the dependence of USAE on tissue temperature is determined. Because USAE depends on the acoustic and mechanical properties, both of which vary with temperature, it is hypothesized that the USAE signal is also temperature-dependent and in such a way that it can be used to guide thermal therapy. In a series of experiments, ex vivo porcine muscle and fat samples were exposed to ultrasound at power levels that induce temperature elevation. In both tissue types, below the coagulation threshold, the USAE amplitude was found to vary linearly with temperature. However, at higher powers, the correlation with temperature was lost due mainly to the irreversible nature of the changes in the tissue properties. Theoretical simulations were used to interpret the USAE response change with temperature involving both reversible and irreversible changes and during both heating and cooling. These results indicate that USAE may have important promise as a potential method for localizing temperature elevation and, thus, thermal surgery monitoring, as well as detection of irreversible changes in tissues.

Adipose Tissue↗
PubMed2002

Correlation of ultrasound phase with physical skull properties.

Noninvasive treatment of brain disorders using focused ultrasound (US) requires a reliable model for predicting the distortion of the field due to the skull using physical parameters obtained in vivo. Previous studies indicate that control of US phase alone is sufficient for producing a focus through the skull using a phased US array. The present study concentrates on identifying methods to estimate phase distortion. This will be critical for the future clinical use of noninvasive brain therapy. Ten ex vivo human calvaria were examined. Each sample was imaged in water using computerized tomography (CT). The information was used to determine the inner and outer skull surfaces, thickness as a function of position, and internal structure. Phase measurement over a series of points was obtained by placing a skull fragment between a transducer and a receiver with the skull normal to the transducer. Correlation was found between the skull thickness and the US phase shift. A linear fit of the data follows that predicted by a homogeneous skull when average speed of sound 2650 m/s was used. Large variance (SD = 60 degrees, mean = 50 degrees ) indicates the additional role of internal bone speed and density fluctuations. In an attempt to reduce the variance, the skull was first studied as a three-layer structure. Next, density-dependent bone speed fluctuation was introduced to both the single-layer and three-layer models. It was determined that adjustment of the mean propagation speeds using density improves the overall phase prediction. Results demonstrate that it is possible to use thickness and density information from CT images to predict the US phase distortion induced by the skull accurately enough for therapeutic aberration correction. In addition, the measurements provide coefficients for phase dependence on skull thickness and density that can be used in clinical treatments.

Brain Diseases↗
PubMed2002

Intraoperative magnetic resonance imaging and magnetic resonance imaging-guided therapy for brain tumors.

Since their introduction into surgical practice in the mid 1990s, intraoperative MRI systems have evolved into essential, routinely used tools for the surgical treatment of brain tumors in many centers. Clear delineation of the lesion, "under-the-surface" vision, and the possibility of obtaining real-time feedback on the extent of resection and the position of residual tumor tissue (which may change during surgery due to "brain-shift") are the main strengths of this method. High-performance computing has further extended the capabilities of intraoperative MRI systems, opening the way for using multimodal information and 3D anatomical reconstructions, which can be updated in "near real time." MRI sensitivity to thermal changes has also opened the way for innovative, minimally invasive (LASER ablations) as well as noninvasive therapeutic approaches for brain tumors (focused ultrasound). Although we have not used intraoperative MRI in clinical applications sufficiently long to assess long-term outcomes, this method clearly enhances the ability of the neurosurgeon to navigate the surgical field with greater accuracy, to avoid critical anatomic structures with greater efficacy, and to reduce the overall invasiveness of the surgery itself.

Brain↗
PubMed2002

Micro-receiver guided transcranial beam steering.

A new method for focusing ultrasound energy in brain tissue through the skull is investigated. The procedure is designed for use with a therapeutic transducer array and a small catheter-inserted hydrophone receiver placed in the brain to guide the array's focus. When performed at high-intensity, a focal intensity on the order of several hundred watts per centimeter-squared is achieved, and cells within a target volume are destroyed. The present study tests the feasibility and range of the method using an ex vivo human skull. Acoustic phase information is obtained from the stationary receiver and used to electrically shift the beam to new locations as well as correct for aberrations due to the skull. The method is applied to a 104-element 1.1 MHz array and a 120-element 0.81 MHz array. Using these array configurations, it is determined that the method can reconstruct and steer a focus over a distance of 50 mm. Application of this minimally invasive technique for ultrasound brain therapy and surgery also is investigated in vitro with a 64-element 0.664 MHz hemisphere array designed for transskull surgery. Tissue is placed inside of a skull and a catheter-inserted receiver is inserted into the tissue. A focus intense enough to coagulate the tissue is achieved at a predetermined location 10 mm from the receiver, the maximum distance that this large element array can electronically steer the focus.

Animals↗
PubMed2002

The use of quantitative temperature images to predict the optimal power for focused ultrasound surgery: in vivo verification in rabbit muscle and brain.

In this study, we investigated the use of MRI-derived thermal imaging for determining the exposure parameters for focused ultrasound (FUS) surgery. Since the temperature rise induced by a FUS beam scales linearly with power, the temperature maps acquired during subthreshold sonications can be used to determine the power necessary to produce thermal tissue damage with a desired size. Thermal images acquired during multiple sonications delivered at different locations in rabbit thigh muscle and brain tissue in vivo were analyzed to test this hypothesis. First, the linearity of the induced temperature rise with the acoustic power was tested. Next, the temperature maps acquired during preliminary low power sonications were scaled up until the estimated size of the tissue damage was equal to the tissue damage size of subsequent high power sonications. A threshold thermal dose was used to estimate the onset of thermal damage. The predicted power (based on amount of scaling required to reach the target size) was then compared to the true high power value. Overall, the temperature rise varied linearly with power (slope of deltaThigh/deltaTlow vs Power(high)/Power(low) = 0.97, 0.93 for pairs of sonications at each location in brain, muscle). The predicted power matched the true high power in the brain sonications (slope = 1.04). The predicted power underestimated the true high power in the muscle sonications (slope = 0.87). This under-prediction was due to a deviation from linearity in those cases where tissue damage was detected in subsequent MR images (slope of deltaThigh/deltaTlow vs Power(high)/Power(low) = 1.02, 0.84 for no tissue damage, tissue damage). The source of this deviation was not clear from these experiments. Even with this underestimation of the power, this method will be useful because it will allow an estimate of the proper power to use during FUS surgery without exact knowledge of the tissue parameters.

Animals↗
PubMedDate not supplied

Magnetic resonance image-guided focused ultrasound surgery.

The powerful union of focused ultrasound and magnetic resonance imaging (MRI) has created a new approach to noninvasive surgery. By using this integrated therapy delivery system, the physician can correctly localize tumors, optimally target acoustic energy, monitor energy deposition in real time, and accurately control the deposited thermal dose within the entire tumor volume. This satisfies the requirements for "ideal surgery." In a real sense, MRI provides the "road map" by which focused ultrasound surgery (FUS) is followed. The advantages of MRI over ultrasound guidance in controlling FUS lie in the more sensitive detection of tumor target, the real-time detection of tissue temperature, and the confirmation of thermally induced tissue changes-powerful features that eventually can replace the traditional surgical approach. Applying software that connects the therapy and imaging system (the "Dosimetry Workstation"), the physician can generate an entire treatment plan from quantifying temperature changes to positioning the therapy transducer. The noninvasive debulking of tumors without disturbing adjacent, functionally intact structures is thereby accomplished.

Body Temperature↗
PubMedDate not supplied

Ultrasound stimulates proteoglycan synthesis in bovine primary chondrocytes.

Mechanical forces can stimulate the production of extracellular matrix molecules. We tested the efficacy of ultrasound to increase proteoglycan synthesis in bovine primary chondrocytes. The ultrasound-induced temperature rise was measured and its contribution to the synthesis was investigated using bare heat stimulus. Chondrocytes from five cellular isolations were exposed in triplicate to ultrasound (1 MHz, duty cycle 20%, pulse repetition frequency 1 kHz) at average intensity of 580 mW/cm2 for 10 minutes daily for 1-5 days. Temperature evolution was recorded during the sonication and corresponding temperature history was created using a controllable water bath. This exposure profile was used in 10-minute-long heat treatments of chondrocytes. Heat shock protein 70 (Hsp70) levels after one-time treatment to ultrasound and heat was analyzed by Western blotting, and proteoglycan synthesis was evaluated by 35S-sulfate incorporation. Ultrasound treatment did not induce Hsp70, while heat treatment caused a slight heat stress response. Proteoglycan synthesis was increased approximately 2-fold after 3-4 daily ultrasound stimulations, and remained at that level until day 5 in responsive cell isolates. However, chondrocytes from one donor cell isolation out of five remained non-responsive. Heat treatment alone did not increase proteoglycan synthesis. In conclusion, our study confirms that pulsed ultrasound stimulation can induce proteoglycan synthesis in chondrocytes.

Animals↗