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K Hynynen

Publications and source records attributed to K Hynynen.

At least 73 records · Page 4Linked to original sources
PubMed1997

Cylindrical ultrasonic transducers for cardiac catheter ablation.

This study was designed to evaluate the feasibility of using cylindrical ultrasound transducers mounted on a catheter for the ablation of cardiac tissues. In addition, the effects of ultrasound frequency and power was evaluated both using computer simulations and in vitro experiments. Frequencies of 4.5, 6, and 10 MHz were selected based on the simulation studies and manufacturing feasibility. These transducers were mounted on the tip of 7-French catheters and applied in vitro to fresh ventricular canine endocardium, submerged in flowing degassed saline at 37 degree C. When the power was regulated to maintain transducer interface temperature at 90-100 degree C, the 10-, 6-, and 4.5-MHz transducers generated a lesion depth of 5.9 +/- 0.2 mm, 4.6 +/- 1.0 mm, and 5.3 +/- 0.9 mm, respectively. The 10-MHz transducer was chosen for the in vivo tests since the maximum lesion depth was achieved with the lowest power. Two dogs were anesthetized and sonications were performed in both the left and right ventricles. The 10-MHz cylindrical transducers caused an average lesion depth of 6.4 +/- 2.5 mm. In conclusion, the results show that cylindrical ultrasound transducers can be used for cardiac tissue ablation and that they may be able to produce deeper tissue necrosis than other methods currently in use.

Animals↗
PubMed1997

Temperature monitoring of ultrasonically heated muscle with RARE chemical shift imaging.

The ability to monitor tissue temperature in ultrasonically heated rabbit muscle is demonstrated using a chemical shift imaging approach based on the rapid acquisition with relaxation enhancement (RARE) fast imaging method [Hennig et al., Magn. Reson. Med. 3, 823-833 (1986)] applied in a line scan format. A three echo sequence with a 16 Hz spectral resolution with 64 ms echo readouts and 78 ms echo spacings is shown capable of measuring relevantly small water frequency shifts in phantoms. Applied to the in vivo model of ultrasonically heated rabbit muscle, water resonance frequencies at the ultrasonic focal point were found to be linearly related to temperature with a slope of -0.007 +/- 0.001 ppm/degree C (N = 6 studies). Measurements of the frequency shift in unheated tissue located away from the ultrasonically heated tissue varied by approximately 0.011 ppm over the course of the experiments, leading to an estimated temperature accuracy of +/- 1.6 degrees C in vivo.

Animals↗
PubMed1997

Thermal effects of focused ultrasound on the brain: determination with MR imaging.

PURPOSE: To determine the feasibility of the use of temperature-sensitive magnetic resonance (MR) imaging for the detection of local temperature elevations at the focus of a low-power ultrasound beam in the brain. MATERIALS AND METHODS: The brains in 28 rabbits were sonicated at acoustic power levels of 3.5-17.5 W. Four to five different locations were sonicated at different acoustic power levels in each rabbit. MR images were obtained 2 hours, 48 hours, 10 days, and 23 days after the sonications, depending on when the animals were sacrificed. Histologic evaluation of whole brain was performed. RESULTS: Forty of 43 (93%) of the lowest-power (3.5-W) sonications were visible on temperature-sensitive MR images and did not result in any short- or long-term histologic or MR imaging evidence of tissue damage. A contrast-to-noise ratio of approximately 6 and a temperature elevation of 7 degrees-8 degrees C were observed. CONCLUSION: Temperature elevations induced by means of focused ultrasound exposures that do not cause damage in the in vivo rabbit brain can be detected at temperature-sensitive MR imaging.

Animals↗
PubMed1996

Simultaneous magnetic resonance phase and magnitude temperature maps in muscle.

Noninvasive magnetic resonance temperature maps that are used to monitor thermal ablation of tissue are described. In magnetic resonance images, thermally induced proton nuclear magnetic resonance frequency shifts, and changes in the longitudinal relaxation time produce both phase and magnitude changes in the MR signal. Temperature maps with improved sensitivity are derived from the complex-difference nuclear magnetic resonance signal. Bovine muscle specimens were heated with focused ultrasound to model thermal surgery and create a known thermal distribution to test the method. Resulting MR images acquired in 2 s produce temperature maps with 1 min resolution and 2 degrees C temperature sensitivity. The temperature sensitivity was increased by extending the acquisition to 5 s, by decreasing the receiver bandwidth, and increasing the echo time.

Animals↗
PubMed1996

Optimization of spoiled gradient-echo phase imaging for in vivo localization of a focused ultrasound beam.

The parameters of a spoiled gradient-echo (SPGR) pulse sequence have been optimized for in vivo localization of a focused ultrasound beam. Temperature elevation was measured by using the proton resonance frequency shift technique, and the phase difference signal-to-noise ratio (SNR delta phi) was estimated in skeletal muscle and kidney cortex in 10 rabbits. Optimized parameters included the echo time equivalent to T2* of the tissue, the longest repetition time possible with a 20-s sonication, and the flip angle equivalent to the Ernst angle. Optimal SPGR phase imaging can detect a sonication beam with a peak phase difference of 0.55 radian, which corresponds to a temperature elevation of 7.3 degrees C. The sonication beam can be localized within one voxel (0.6 x 0.6 x 5 mm3) at power levels that are below the threshold for thermal damage of the tissue.

Animals↗
PubMed1996

Potential adverse effects of high-intensity focused ultrasound exposure on blood vessels in vivo.

The aim of the study was to evaluate the potential adverse effects of high intensity ultrasound exposure on blood vessels during noninvasive focused ultrasound surgery. A hydraulic MR-compatible positioning device was used to manipulate a focused ultrasound transducer (frequency 1.49 MHz, f-number = 0.8) in an MRI scanner. The system was used to sonicate a branch of the femoral artery and vein of 19 rabbits (26 thighs) in vivo at intensity levels above the threshold for transient cavitation; i.e., between 4400 and 8800 W cm-2 with multiple 1 s pulses stepped across the vessels (step size = 0.7 mm). The vessels were located and followed by MR angiography. In 13 rabbits, x-ray angiograms were also performed after the animals were euthanized. The results demonstrated that the 1 s high-intensity exposures caused the arteries to constrict at all exposure levels tested. At the intensity of 5800 W cm-2 and above, the MRI angiogram immediately after the sonications showed no flow. The x-ray angiograms (1-2 h later) showed that the blood vessels were open, but constricted to about 50% or less of their diameter. Both the MR and x-ray angiograms showed that the vessel diameters relaxed toward their initial diameter during the first week after sonication. In five cases, hemorrhage or vessel rupture was caused by the sonication. This study demonstrates that short, high-intensity focused ultrasound exposure can cause vessel spasm and hemorrhage when transient cavitation is present. This condition should be avoided during noninvasive focused ultrasound surgery.

Angiography↗
PubMed1996

Ultrasound surgery using multiple sonications--treatment time considerations.

The noninvasive surgery of a tumor by focused transducers or phased arrays with multiple sonications was simulated in this study. The effects of multiple sonications on near field heating were investigated by varying the delay time between consecutive pulses and the movement pattern of the transducer within the focal plane. To avoid normal tissue damage, sufficient delay time between consecutive pulses is necessary. It was found that the delay is dependent on acoustic power and pulse duration. The treatment time is often long when a strongly focused transducer is used to treat a large tumor, because the volume of necrosed tissue generated by the focused ultrasound beam is small requiring a large number of multiple sonications. To reduce treatment time, it is necessary to decrease the total number of pulses. This can be done by increasing the acoustic power and/or sonication time, Longer sonications induce a perfusion dependency on the temperature elevations. The acoustic power is limited by mammalian tissue temperatures which should not exceed 100 degrees C to avoid boiling and gas formation. An alternative way to increase the coagulated tissue volume is to modify the acoustic field such that a large tissue volume is exposed, while generating a more uniform temperature elevation to avoid high exposure limiting temperatures. This can be done effectively by using a phased array. This study demonstrated that phased arrays can offer significantly shorter treatment times than similar spherically curved transducers.

Humans↗
PubMed1996

Noninvasive arterial occlusion using MRI-guided focused ultrasound.

The purpose of this work was to test the hypothesis that reproducible and sustainable arterial occlusion can be induced by focused ultrasound energy deposition noninvasively within deep tissue. An MRI-compatible focused ultrasound transducer was used to sonicate a branch of the renal artery (diameter about 0.6 mm) in vivo (nine rabbits). An intravenous MRI contrast agent bolus was injected about 30 min and up to 7 days after the sonication. After follow-up, in vitro magnification x-ray angiograms were obtained and the kidneys were fixed in formaldehyde for histologic study. The ultrasound pulses resulted in complete cessation of blood flow, as shown by the gradient echo images. In seven of the nine rabbits, a wedge-shaped unenhanced area was seen at the part of the kidney that was perfused by the vessel after the contrast agent injection. This area extended laterally (outside of the sonicated volume) to the cortical surface of the kidney. The x-ray angiograms showed that the artery was completely occluded. Postmortem histologic evaluation showed an infarcted tissue volume corresponding to the wedge shape seen in the images. This study showed that appropriately focused ultrasound can be used to close arteries noninvasively. This finding has significant clinical potential.

Angiography↗
PubMed1996

A study of various parameters of spherically curved phased arrays for noninvasive ultrasound surgery.

The spherically curved square element phased array design for ultrasound surgery was studied in several of its important aspects. A method for determining the spatial limitations of the multiple-foci region was developed for spherically curved phased arrays. The effects on the ultrasound fields of varying the phases and the amplitudes at control points were investigated. It was found that the phases and the magnitudes of control points have an impact on the ultrasound field distributions. The effects of multiple-foci spacing, pulse duration, and maximum temperature on the size and shape of necrosed tissue were investigated. For a spherically curved phased array with an 8 cm radius of curvature, an 8.8 x 8.8 cm2 projected area, and a 1.5 MHz operating frequency, the minimum number of phased array elements that could produce the largest acceptable necrosed tissue volume was 256. The tissue volume necrosed during the sonication could be increased to approximately 1 x 1 x 3 cm3 with this array.

General Surgery↗
PubMed1996

Design and optimization of an aperiodic ultrasound phased array for intracavitary prostate thermal therapies.

A 57 element aperiodic linear phased array was designed and constructed to investigate the feasibility of using transrectal ultrasound for the thermal therapeutic treatment of prostate cancer and benign prostatic hyperplasia. A method of reducing grating lobe levels by using optimized random distributions of unequally sized elements is introduced. Using this technique, array periodicity is avoided, making it feasible to use larger elements and hence fewer elements and amplifier channels, while still achieving acceptable power field patterns. Acoustic power field simulations determined that the grating lobe levels associated with selected aperiodic element distributions were approximately 30%-45% less than those associated with periodic element spacing and the same average element width. Or by using aperiodic rather than periodic element distributions, the average element width could be increased by approximately 20%-35% (approximately lambda/4.4), while maintaining a constant grating lobe level. Prior to construction of the 57 element array, the power capabilities of this type of array were demonstrated with a 16 element aperiodic phased array, which delivered over 28 W of acoustical power per cm of array length while focused. The power field patterns produced by the 57 element array closely matched the field patterns predicted by the theoretical model used in the simulations. The array produced acceptable power field patterns for foci at depths up to 5 cm and up to 2 cm off the center axis, in addition to producing multiple foci simultaneously. Based on the power capabilities and field patterns, this aperiodic array design has the potential to be incorporated into a clinical heating device as a means of delivering thermal therapies to the prostate and other target volumes close to body cavities.

Computer Simulation↗
PubMed1996

A parametric study of the concentric-ring transducer design for MRI guided ultrasound surgery.

Noninvasive surgery using high-powered, focused ultrasound transducers in conjunction with magnetic resonance imaging has been shown to be feasible in previous studies. For clinical treatments, the geometry of standard MRI equipment limits the space available for ultrasound surgical equipment. This space requirement can be reduced in one dimension by using phased arrays to control the focal depth, thus eliminating the space required for the motion of a fixed focus transducer. Because of its symmetry, an annular array is ideal for changing the focal depth. Previous works have simulated, built, and characterized various concentric-ring transducers; however, no study has thoroughly examined the potential and limitations of the concentric-ring design for MRI guided ultrasound surgery. The present work is a systematic examination of the capabilities of the concentric-ring array, using numerical simulations to predict the power field, temperature distribution, and accumulated thermal dose. The results presented here illustrate the effects of ring size, center-to-center spacing configurations, number of rings, and radius of curvature on transducer performance. A 10-cm radius of curvature transducer with 14 evenly spaced rings has been built and characterized in order to verify the accuracy of the numerical simulations. The pressure-squared fields produced by this transducer are in excellent agreement with the simulated fields.

Equipment Design↗
PubMed1996

A clinical, noninvasive, MR imaging-monitored ultrasound surgery method.

A noninvasive method of tissue ablation that is guided and monitored with magnetic resonance (MR) imaging has been developed. The method uses sharply focused ultrasound transducers of different focal lengths to induce a localized temperature elevation during a short exposure (1-20 seconds). A hydraulic, computer-controlled positioning device moves the transducer in an MR imager. The positioner is built into a standard cradle in the imager. The system includes cavitation detection and power monitoring circuitry for patient safety. The target volume is outlined with cross-sectional MR images obtained immediately before sonication. By means of the software, the focus is moved to ablate the volume defined with the images. The temperature elevation during the exposure is monitored by means of the proton resonance frequency shift with fast gradient-echo sequences, and the necrosed volume is demonstrated with T2-weighted fast spin-echo images. This method has been extensively tested in in vivo animal experiments and is now undergoing clinical trial.

Animals↗
PubMed1995

Histologic effects of high intensity pulsed ultrasound exposure with subharmonic emission in rabbit brain in vivo.

In this study, the threshold for subharmonic emission during in vivo sonication of rabbit brain was investigated. In addition, the histologic effects of pulsed sonication above this threshold were studied. Two spherically curved focused ultrasound transducers with a diameter of 80 mm and a radius of curvature of 70 mm were used in the sonications. The operating frequencies of the transducers were 0.936 and 1.72 MHz. The sonication duration was varied between 0.001 and 1 s and the repetition frequency between 0.1 and 5 Hz. The threshold for subharmonic emission at the frequency of 0.936 MHz was found to be approximately 2000 W cm-2 and 3600 W cm-2 for pulse durations of 1 s and 0.001 s, respectively. The threshold was approximately 1.5-fold as high at a frequency of 1.72 MHz. However, there was considerable variation from experiment to experiment. The multiple pulse experiments at a frequency of 1.72 MHz and an intensity of 7000 W cm-2 showed that the histologic effects ranged from no observable damage of the tissue, to blood-brain barrier breakage, to local haemorrhagia, to local destruction of the tissue, to gross hemorrhage resulting in the death of the animal. The severity of the tissue damage increased as the pulse duration, number of pulses and their repetition frequency increased. The results indicate that the end point of the tissue damage may be controlled by selecting the sonication parameters. Such control over tissue effects can have several different applications when brain disorders are treated.

Animals↗
PubMed1995

Application of ultrasound energy for intracardiac ablation of arrhythmias.

Ultrasound is a potential energy source for cardiac ablation. Small ultrasound applicators were tested for their ability to create lesions in cardiac tissue. Ultrasound applicators were designed, constructed and tested in canine cardiac tissue in degassed normal saline, and both in vitro and in vivo, lesions were produced by using transducers with frequencies of about 10 MHz. Lesion depth increased with longer duration of energy delivery from 15-60 s, and there was a linear relationship between increasing power and depth of lesions. Seven in vivo experiments in open-chest dogs were performed, and the ultrasound transducers were mounted on the tip of 7-French angiographic catheters. On the epicardium the maximum lesion depth was 9 mm. When the transducer was inserted into the left ventricle, lesions of 8.7 +/- 2.9 mm (n = 4) were produced. It is concluded that an ultrasound transducer mounted on a cardiac catheter can produce lesions that may be useful for ablation of cardiac arrhythmias.

Adult↗
PubMed1995

The feasibility of using ultrasound for cardiac ablation.

The feasibility of using ultrasound to induce cardiac tissue necrosis for the treatment of arrhythmias was investigated. A theoretical model was used to optimize the operating frequency for necrosis of highly perfused muscle tissue. From these simulations it appeared that frequencies from 10-15 MHz produce the deepest lesions at ultrasound intensities between 15 and 30 W/cm2. Test catheters with a planar ultrasound transducer (diameter 2.3 mm = 7 F) were also constructed and in vitro and in vivo tests with canine heart muscle were performed. Both of these tests showed that the ultrasound catheters could deliver adequate energy to necrose cardiac tissue. The in vivo lesion depths of 5-9 mm indicated that ultrasound has significant potential for cardiac ablation for the treatment of arrhythmias.

Animals↗
PubMed1995

Control of the necrosed tissue volume during noninvasive ultrasound surgery using a 16-element phased array.

Focused high-power ultrasound beams are well suited for noninvasive local destruction of deep target volumes. In order to avoid cavitation and to utilize only thermal tissue damage, high frequencies (1-5 MHz) are used in ultrasonic surgery. However, the focal spots generated by sharply focused transducers become so small that only small tumors can be treated in a reasonable time. Phased array ultrasound transducers can be employed to electronically scan a focal spot or to produce multiple foci in the desired region to increase the treated volume. In this article, theoretical and experimental studies of spherically curved square-element phased arrays for use in ultrasonic surgery were performed. The simulation results were compared with experimental results from a 16-element array. It was shown that the phased array could control the necrosed tissue volume by using closely spaced multiple foci. The phased array can also be used to enlarge a necrosed tissue volume in only one direction at a time, i.e., lateral or longitudinal. The spherically curved 16 square-element phased array can produce useful results by varying the phase and amplitude setting. Four focal points can be easily generated with a distance of two or four wavelengths between the two closest peaks. The maximum necrosed tissue volume generated by the array can be up to sixteen times the volume induced by a similar spherical transducer. Therefore the treatment time could be reduced compared with single transducer treatment.

Biophysical Phenomena↗
PubMed1995

Focused US system for MR imaging-guided tumor ablation.

PURPOSE: To measure the performance characteristics of a focused ultrasound (US) system for magnetic resonance (MR) imaging-guided tumor ablation. MATERIALS AND METHODS: The authors constructed a focused US system for MR imaging-guided tumor ablation. The location of the heated region and thermal dose were monitored with temperature-sensitive MR images obtained in phantoms and rabbit skeletal muscle after application of each sonic pulse. RESULTS: The region heated by the focused ultrasound beam was within 1 mm of that observed on temperature-sensitive fast gradient-echo MR images of in vivo rabbit skeletal muscle. Analysis of heat flow and the rate of coagulation necrosis provided an estimate of the size of the ablated region that was in agreement with experimental findings. CONCLUSION: MR imaging provides target definition and control for thermal therapy in regions of variable perfusion or in tissues that are not well characterized.

Animals↗