Control of cavitation activity by different shockwave pulsing regimes.

The aim of the study was to control the number of inertial cavitation bubbles in the focal area of an electromagnetic lithotripter in water independently of peak intensity, averaged intensity or pressure waveform. To achieve this, the shockwave pulses were applied in double pulse sequences, which were administered at a fixed pulse repetition frequency (PRF) of 0.33 Hz. The two pulses of a double pulse were separated by a variable short pulse separation time (PST) ranging from 200 micros to 1500 ms. The number and size of the cavitation bubbles were monitored by scattered laser light and stroboscopic photographs. We found that the number of inertial cavitation bubbles as a measure of cavitation dose was substantially influenced by variation of the PST, while the pressure pulse waveform, averaged acoustic intensity and bubble size were kept constant. The second pulse of each double pulse generated more cavitation bubbles than the first. At 14 kV capacitor voltage, the total number of cavitation bubbles generated by the double pulses increased with shorter PST down to approximately 400 micros, the cavitation lifespan. The results can be explained by cavitation nuclei generated by the violently imploding inertial cavitation bubbles. This method of pulse administration and cavitation monitoring could be useful to establish a cavitation dose-effect relationship independently of other acoustic parameters.

Blood cell damage after in vitro irradiation of fresh whole blood with 630 nm laser light.

A study on blood cell damage after irradiation of fresh whole blood with 630 nm laser light was carried out in vitro. Various fluence rates of laser light were used with and without cooling of blood. Damage to the blood was assessed by blood cell counts, osmotic fragility measurements and examination of blood films. Exposure of a 1 mm blood layer to 630 nm laser light without cooling led to changes in blood counts first detected at fluence rates of 130 mW/cm2. Changes in osmotic fragility first became evident at 210 mW/cm2. Increasing cell damage with increasing fluence rates was evident in blood films. Using the cooling device changes in whole blood after irradiation first occurred at a fluence rate of 293 mW/cm2. Measurement of the fluence rates at which cell damage begins is important in laser induced fluorescence diagnostics and photodynamic therapy applications in blood or blood products using photosensitizers.

A chemical dosimeter for the determination of the photodynamic activity of photosensitizers.

Uric acid a known singlet oxygen scavenger, was investigated as a chemical dosimeter in physiological aqueous solution for use in photodynamic therapy. The uric acid test takes the decrease in uric acid (UA) absorbance at 293 nm after laser light irradiation of a solution containing UA and a photosensitizer as a rapid evaluation of relative photodynamic activities of the photosensitizer. A uric acid test standard procedure was defined. To compare photodynamic activity of different photosensitizers or irradiation conditions a proposal for a photodynamic activity scale based on the uric acid test is given. Examples of uric acid test operation are given by comparing the changes in UA absorbance decrease with respect to irradiation wavelength and to photosensitizer concentration of Photofrin II with that of two other photosensitizers (5,10,15,20-tetrakis-[4-hydroxyphenyl]-21H,23H-porphyrin (TOP) and 5,10,15,20-tetrakis-[4-carboxyphenyl)-21H, 23H-porphyrin (TCPP), both derivatized with methoxypolyethyleneglycol (TOP-MPV and TCPP-AMP) as a macromolecular carrier). The photodynamic activity of the three photosensitizers using the proposed photodynamic activity scale is given.

Photodynamic therapy as a tool for suppressing the haematogenous dissemination of tumour cells.

The chance of most cancer patients surviving their disease is to a high degree dependent on the status of the metastatic processes. One general route of cancer-cell dissemination is passive transport in the blood stream, i.e., haematogenous dissemination. In this study we try to find an answer to the following question: is it possible to use photodynamic therapy for suppressing the haematogenous dissemination of cancer cells? In first in vitro experiments we incubated CX1 cells (colon carcinoma cells) with two photosensitizers, Photofrin II and mesotetra(hydroxyphenyl)chlorin (mTHPC). We added the cells to fresh whole blood and irradiated the blood with suitable laser light in a flow-through irradiation system. The tumour-cell survival fraction (SF) was determined with plating efficiency. Using Photofrin II we observed a minimal tumour-cell survival in blood of SF = 3.5% and using mTHPC we measured SF = 0.02%. These results encourage further investigations concerning the use of photodynamic therapy for suppressing haematogenous dissemination.

Changes in RBE of 14-MeV (d + T) neutrons for V79 cells irradiated in air and in a phantom: is RBE enhanced near the surface?

PURPOSE: The relative biological effectiveness (RBE) for inactivation of V79 cells was determined as function of dose at the Heidelberg 14-MeV (d + T) neutron therapy facility after irradiation with single doses in air and at different depths in a therapy phantom. Furthermore, to assess the reproducibility of RBE determinations in different experiments we examined the relationship between the interexperimental variation in radiosensitivity towards neutrons with that towards low LET 60Co photons. METHODS: Clonogenic survival of V79 cells was determined using the colony formation assay. The cells were irradiated in suspension in small volumes (1.2 ml) free in air or at defined positions in the perspex phantom. Neutron doses were in the range, Dt = 0.5-4 Gy. 60Co photons were used as reference radiation. RESULTS: The radiosensitivity towards neutrons varied considerably less between individual experiments than that towards photons and also less than RBE. However, the mean sensitivity of different series was relatively constant. RBE increased with decreasing dose per fraction from RBE = 2.3 at 4 Gy to RBE = 3.1 at 0.5 Gy. No significant difference in RBE could be detected between irradiation at 1.6 cm and 9.4 cm depth in the phantom. However, an approximately 20% higher RBE was found for irradiation free in air compared with inside the phantom. Combining the two effects, irradiation with 0.5 Gy free in air yielded an approximately 40% higher RBE than a dose of 2 Gy inside the phantom. CONCLUSION: The measured values of RBE as function of dose per fraction within the phantom is consistent with the energy of the neutron beam. The increased RBE free in air, however, is greater than expected from microdosimetric parameters of the beam and may be due to slow recoil protons produced by interaction of multiply scattered neutrons or to an increased contribution of alpha particles from C(n, alpha) reactions near the surface. An enhanced RBE in subcutaneous layers of skin combined with an increase in RBE at low doses per fraction outside the target volume could potentially have significant consequences for normal tissue reactions in radiotherapy patients treated with fast neutrons.

Dose-response relationship for late functional changes in the rat brain after radiosurgery evaluated by magnetic resonance imaging.

PURPOSE: Only few quantitative data are available on late effects in the healthy brain after radiosurgery. An animal model can contribute to systematically investigate such late effects. Therefore, a model applying radiosurgery at the rat brain was established. A long-term (19 months) follow up study with 66 animals after radiosurgery was carried out. METHODS AND MATERIALS: In 60 animals, an area in the frontal lobe of the brain was irradiated stereotactically with a 15 MV linac. Different doses of 20, 30, 40, 50, and 100 Gy with two field sizes (3.9 and 5.9 mm collimator) were selected, using the integrated logistic formula with input parameters from human brain. The induced alteration of the blood-brain barrier permeability was investigated by means of contrast enhanced magnetic resonance imaging. RESULTS: A first intracranial signal enhancement was observed in one animal 160 days after irradiation with 100 Gy. Beginning at 5 months all animals in the two 100 Gy groups homogeneously showed contrast enhancement, but none of the other groups. This remained until 13 months after irradiation. The volume of contrast enhancement as well as the increase of signal intensity were different between the two 100 Gy groups. After 19 months, the animals irradiated with lower doses also showed contrast enhancements, although not uniformly throughout one group. A maximum likelihood fit of the logistic formula P(D) = 1/[1 + (D50/D)k] to the incidence of late effects for the 5.9 mm collimator at 19 months after irradiation results in the parameters D50 = 37.4(-5.2,+6.1) Gy and k = 4.7 +/- 2.4. CONCLUSIONS: An animal model was established to study late normal brain tissue response. The observed late effects appeared very similar to the estimation of the integrated logistic formula for human brain. Based on these radiosurgery techniques, future experiments will focus on modifications in the irradiation modalities, i.e., irregular volumes, radiation quality or fractionation.

Performance evaluation of a whole-body PET scanner using the NEMA protocol. National Electrical Manufacturers Association.

UNLABELLED: This study evaluates the performance of the newly developed high-resolution whole-body PET scanner ECAT EXACT HR+. METHODS: The scanner consists of four rings of 72 bismuth germanate block detectors each, covering an axial field of view of 15.5 cm with a patient port of 56.2 cm. A single block detector is divided into an 8 x 8 matrix, giving a total of 32 rings with 576 detectors each. The dimensions of a single detector element are 4.39 x 4.05 x 30 mm3. The scanner is equipped with extendable tungsten septa for two-dimensional two-dimensional measurements, as well as with three 68Ge line sources for transmission scans and daily quality control. The spatial resolution, scatter fraction, count rate, sensitivity, uniformity and accuracy of the implemented correction algorithms were evaluated after the National Electrical Manufacturers Association protocol using the standard acquisition parameters. RESULTS: The transaxial resolution in the two-dimensional mode is 4.3 mm (4.4 mm) in the center and increases to 4.7 mm (4.8 mm) tangential and to 8.3 mm (8.0 mm) radial at a distance of r = 20 cm from the center. The axial slice width measured in the two-dimensional mode varies between 4.2 and 6.6 mm FWHM over the transaxial field of view. In the three-dimensional mode the average axial resolution varies between 4.1 mm FWHM in the center and 7.8 mm at r = 20 cm. The scatter fraction is 17.1% (32.5%) for a lower energy discriminator level of 350 keV. The maximum true event count rate of 263 (345) kcps was measured at an activity concentration of 142 (26.9) kBq/ml. The total system sensitivity for true events is 5.7 (27.7) cps/Bq/ml. From the uniformity measurements, we obtained a volume variance of 3.9% (5.0%) and a system variance of 1.6% (1.7%). The implemented three-dimensional scatter correction algorithm reveals very favorable properties, whereas the three-dimensional attenuation correction yields slightly inaccurate results in low- and high-density regions. CONCLUSION: The ECAT EXACT HR+ has an excellent, nearly isotropic spatial resolution, which is advantageous for brain and small animal studies. While the relatively low slice sensitivity may hamper the capability for performing fast dynamic two-dimensional studies, the scanner offers a sufficient sensitivity and count rate capacity for fully three-dimensional whole-body imaging.

Pharmacokinetic analysis of 5-[18F]fluorouracil tissue concentrations measured with positron emission tomography in patients with liver metastases from colorectal adenocarcinoma.

The purpose of our study was to develop a pharmacokinetic model to quantify the intracellular 5-fluorouracil (5-FU) concentration in liver metastases, which is expected to be closely correlated to therapy response. In addition, the influence of the biomodulator folinic acid on the action of 5-FU in the metastases was investigated. After i.v. application of 5-FU labeled with the positron emitter fluorine-18 (5-[18F]FU), the kinetics of the regional 5-[18F]FU/uptake was measured dynamically with positron emission tomography over 120 min in 14 patients with a total of 27 liver metastases from colorectal adenocarcinoma. Activity-time curves were evaluated in the metastases, the normal liver tissue, as well as in the aorta and analyzed by a six-compartment model. The catabolic breakdown of 5-FU to alpha-fluoro-beta-alanine (FBAL) in the normal liver tissue was modeled to separate the catabolites from the cytostatic agent 5-[18F]FU and the active 5-[18F]fluorodeoxyuridine nucleotides. With our model, all measured activity-time courses could be described adequately with only small interindividual variations in parameters connected with liver and blood. Extrahepatic clearance of 5-FU was estimated as 0.66 +/- 0.33 liters/min, whereas the hepatic clearance was 0.52 +/- 0.25 liters/min. The Michaelis-Menten parameters describing the nonlinear conversion of 5-FU to FBAL were Km = 11.3 +/- 6.4 micromol and Vmax = 147.1 +/- 130.7 micromol/min. The maximum FBAL concentration in the liver was reached between 35 and 65 min after i.v. 5-FU infusion. The most sensitive parameters for therapy monitoring were k(in) and k(out), which characterize the transport in and out of the intracellular volume of the metastases, respectively. Tumor response can only be expected if k(in) is high and k(out) is low ("trapping"). These criteria were met by 6 of the 27 metastases, which were identical to those with high values for the area under the intracellular 5-FU concentration curve (AUC[meta,IC]5-FU). The parameters k(in) and k(out) were also used to investigate the influence of the biomodulating agent folinic acid on drug effect. Five of the six metastases that showed trapping belonged to patients who received folinic acid. With the exception of one patient, however, all patients who received folinic acid had multiple metastases, of which only one was able to trap 5-FU. Because patient response can only be expected when all metastases trap 5-FU, folinic acid showed no effect on the overall clinical response. With the quantitative modeling approach used, trapping of 5-FU can be assessed noninvasively and on an individual basis. This makes it possible to adjust the dose for each individual patient to optimize the treatment schedule.

Monte Carlo-based analysis of PET scatter components.

UNLABELLED: This study quantifies the different scatter components in PET and examines how the different components degrade reconstructed PET images. METHODS: We simulated the measurement of various phantoms using Monte Carlo (MC) calculations and compared the MC-generated projections and images with the corresponding experimental data. The coincidences were subdivided in four classes: primaries, object scatter (scattered in the object only), gantry scatter (scattered in the scanner only) and mixed scatter (scattered both in the object and the scanner). RESULTS: In the projections of the line sources, the gantry scatter was closely located around the source position, whereas the object scatter was smeared over the whole field of view and could be parameterized well by a monoexponential function. The mixed scatter had nearly the same distribution as the object scatter, but with a smaller amplitude. The calculations and experimental data were in excellent agreement; i.e., led to the same parameterization of the scatter distribution functions and to a similar localization of the scatter components in the reconstructed images. CONCLUSION: The spatial distribution of the scatter components justifies the widely-used assumption that it is sufficient to restrict experimental scatter correction techniques to the object scatter. Furthermore, it is possible to derive the parameters for the scatter kernels, which are needed for the convolution-subtraction algorithm, by MC simulations.

Texture analysis of differently reconstructed PET images.

The insufficient number of projections and the statistics of radioactive decay are supposed to lead to different textures in the images reconstructed by the diverse algorithms used in positron emission tomography (PET). In our study, four reconstruction methods were examined, including the filtered backprojection and three algebraic approaches. Grey-level morphological operators were used to extract the texture. To quantify the observations, we used the texture parameters directionality, fractal dimension, and lacunarity. By increasing the number of iteration steps, each of the algebraic reconstruction methods evolve the texture from coarse to fine granularities, but surprisingly, during the iteration process all methods follow a quite similar 'trail' of possible textures within the active zone. The texture of the termination points of the movement have been compared with the result that the 'hils' and 'valleys' of the activity distribution are located in the same positions. The texture derived from the filtered backprojection is included in the set of textures produced by the algebraic methods. Artefacts are restricted to zones without activity. We found that the texture is independent of the number of counts when more than ten counts are registered in each pixel within the examined region of interest. A high reliability in the reconstructed images is given. All methods allow texture based tissue classification.

Assessment of the biodistribution and metabolism of 5-fluorouracil as monitored by 18F PET and 19F MRI: a comparative animal study.

The effective clinical use of the anticancer drug 5-fluorouracil (5-FU) requires the non-invasive assessment of its transport and metabolism, particularly in the tumor and the liver, where the drug is catabolized to alpha-fluoro-beta-alanine (FBAL). In this study, the potentials and limitations of dynamic 18F PET and metabolic 19F MRI examinations for noninvasive 5-FU monitoring were investigated in ACI and Buffalo rats with transplanted MH3924A and TC5123 Morris hepatomas, respectively. Selective 5-[19F]FU and [19F]FBAL MR images were acquired 5 and 70 min after 5-FU injection using a CHESS MRI sequence. After administration of 5-[18F]FU, the kinetics of the regional 5-[18F]FU uptake were measured by dynamic PET scanning over 120 min. To allow a comparison between PET and MRI data, standardized uptake values (SUV) were computed at the same points in time. The TC5123 hepatoma showed a significantly (p < 0.002) higher mean SUV at 5 and 70 min post-5-FU injection than the MH3924A cell lines, whereas there were no significant differences between the mean SUV measured in the liver of both animal populations. In contrast to the PET data, no significant differences in the mean 5-[19F]FU and [19F]FBAL MR signal values in the tumor of both models were observed. The MR images, however, yielded the additional information that 5-FU is converted to FBAL only in the liver and not in the hepatomas.

Improved visualization of breast lesions with gadolinium-enhanced magnetization transfer MR imaging.

A pulse sequence with magnetization transfer as the main contrast mechanism (MT-FLASH) was developed for improved imaging of breast lesions that requires neither fat suppression nor postprocessing. After optimization of the sequence in phantom and volunteer studies, a clinical pilot study with 14 patients was performed. In carcinomas the relative signal increase after Gd-DTPA administration was on average 34% in MT-FLASH images compared with 169% in conventional T1-weighted (T1W) three-dimensional FLASH images. In MT-FLASH images, all lesions demonstrated a signal intensity higher than that of fat; in T1W images, all lesions have a lower signal intensity. The average postcontrast carcinoma-to-fat contrast-to-noise ratios were +11.6 and -14.2, respectively. The conspicuity of 12 of 13 carcinomas was improved in postcontrast MT-FLASH imaging enables excellent visualization of Gd-DTPA-enhancing breast lesions.

A variety of fast neutron beams for radiobiological research.

The design and construction of a new fast neutron facility and first dosimetric results obtained from seven neutron beams are presented. The neutrons are produced by bombarding beryllium targets with protons and deuterons from our K = 32 negative ion cyclotron. The dose rate in air 1 m distance from the thick target within a 13 x 13 cm2 field amounts to about 50 cGy/min at 30 microA of 32 MeV protons.

Functional magnetic resonance imaging in a stereotactic setup.

The localization of critical structures within the brain is important for the planning of therapeutic strategies. Functional MRI is capable to assess functional response of cortical structures to certain stimuli. The authors present two techniques for functional MRI (fMRI) in a stereotactic set-up. The skull of the patients has been immobilized for stereotactic treatment planning either with a self developed stereotactic ceramic frame and bony fixation or with an individual precision mask system made of light cast. It has been shown that this frame does not produce any image distortion. fMRI was performed using a modified FLASH sequence on a conventional 1.5 T MRI scanner with a specially developed linear polarized head coil. The imaging technique used was an optimized conventional 2D and 3D, first order flow rephased, gradient echo sequence (FLASH) with fat-suppression and reduce bandwidth (16-28 Hz/pixel) and TR = 80-120 ms, TE = 60 ms, flip angle = 40 degrees, matrix = 128 x 128, FOV = 150-250 mm, slice-thickness = 2-5 mm, NEX = 1, and a total single scan time for one image of about 7 sec. The motor cortex stimulation was achieved by touching each finger to thumb in a sequential, self-paced, and repetitive manner. Statistical parametric maps based on student's test were calculated. Pixels with a highly significant signal increase (p < 0.001) are overlaid on T1w SE images. The primary motor and sensory cortex could be visualized with this method in all 10 patients that were imaged in this study. Due to tight fixation of the patient's skull there have been no motion artifacts. These results show that functional MRI is feasible in an stereotactic set-up with an standard 1.5 T scanner. This is a prerequisite for the exact pre therapeutic assessment of the function of cortical centers.

[Therapeutic ultrasound in tumor therapy. Principles, applications and new developments]. / Therapeutischer ultraschall in der tumortherapie.

We are interested in the interaction of high-energy ultrasound with biological tissue with regard to cancer therapy. The physical and biological aspects of this are reviewed and summarized in this paper. The preliminary results give reason to hope, as ultrasound is easy to focus [correction of focuse] and might be applicable as a supplementary therapeutic technique in ultrasound-accessible tumors. The changes observed in biological studies are clearly a function of the physical parameters. Therefore, it is possible to calculate and predict the biological effects. Recent results have shown the potential of magnetic resonance imaging in monitoring ultrasound therapy online. The capability of MRI to measure temperature will be an important tool for the safe and reliable application of this new therapeutic approach. The role of ultrasound therapy in the management of human tumors must be clarified in carefully conducted clinical trials.

Functional MR imaging of visual and motor cortex stimulation at high temporal resolution using a FLASH technique on a standard 1.5 Tesla scanner.

Functional magnetic resonance imaging (fMRI) was performed on a conventional 1.5 T scanner by means of a modified FLASH-technique at temporal resolutions of 80 and 320 ms. The method's stability was assessed by phantom measurements and by investigation of three volunteers resulting in a low amplitude (3%) periodic (4 s) signal modulation for the in vivo measurements, which was not observable in the phantom experiments. fMRI activation studies of motor and visual cortices of four adjacent slices were carried out on 12 healthy right-handed volunteers. Stimulation was performed by a triggered single white light flash or single finger-to-thumb opposition movement, respectively. Event-related response of visual and motor activation was traced over 10.24 s with a temporal resolution of 320 ms for the four slice measurements. Brain activation maps were calculated by correlation of measured signal time course with a time-shifted boxcar function. Activation was quantified by calculation of percentual signal change in relation to the baseline. Observed signal magnitudes were about 5-7% in visual and about 8-12% in primary motor cortex. While photic response was delayed by about 2 s, motor stimulation showed an instantaneous increase of the MR signal. MR signal responses for both stimuli had decayed completely after about 5 s. Our results show that event-related fMRI enables mapping of brain function at sufficient spatial resolution with a temporal resolution of up to 80 ms on a conventional scanner.

Monitoring of task performance during functional magnetic resonance imaging of sensorimotor cortex at 1.5 T.

Functional magnetic resonance imaging (fMRI) has found widespread clinical interest. Difficulties in clinical use of the fMRI technique arise, considering the lack of knowledge about activation task performance. This accounts especially for sensorimotor activation studies, in which performance of the sensorimotor activation task is-if at all-usually rated visually by subjective or semiquantitative methods (i.e., defining categories of performance such as neurological soft signs scales). Recently, instrumental methods for the measurement and analysis of motor performance have been developed. Pronation/supination (hand rotation) movement was shown to be an easily measurable and promising motor task. We have adapted a mechanic device (pronation/supination device, PSD) to monitor motor performance during the fMRI experiment. In a feasibility study, an investigation of fMRI activation strength dependence of sensorimotor cortices and supplementary motor area upon task frequency (25, 50, and 75 cycles/min) was carried out on 10 right-handed healthy volunteers. Furthermore, the authors report the observation of stimulus-induced activation changes in the cerebellum during pronation/supination movement.

A comparison of magnetization prepared 3D gradient-echo (MP-RAGE) sequences for imaging of intracranial lesions.

In a pilot study including 64 patients with different types of brain tumors we investigated four types of MP-RAGE sequences. The sequences differ in the length of the recovery period and the data acquisition mode (sequential vs. centric phase-encoding). The sequence with sequential encoding and a short recovery period provided images that reached the quality and reliability of spin-echo images. The other MP-RAGE sequences failed in providing equivalent information. In particular, a considerable number of small lesions identified in spin-echo images were not detected in MP-RAGE images. The impact of the evolving magnetization on the point spread function was analyzed by performing simulation calculations. It was found that lesions with short T1 times are rendered with low spatial resolution when sequence parameters are not set appropriately. The low overall quality of images obtained by sequences applying centric encoding may be explained by eddy current effects as reported in other recently published studies.

Improved target volume definition in radiosurgery of arteriovenous malformations by stereotactic correlation of MRA, MRI, blood bolus tagging, and functional MRI.

In this methodological paper the authors report the stereotactic correlation of different magnetic resonance imaging (MRI) techniques [MR angiography (MRA), MRI, blood bolus tagging (STAR), and functional MRI] in 10 patients with cerebral arteriovenous malformations (AVM) and its application in precision radiotherapy planning. The patient's head was fixed in a stereotactic localization system that is usable at the MR and the linear accelerator installations. By phantom measurements different materials (steel, aluminium, titanium, plastic, wood, ceramics) used for the stereotactic system were tested for mechanical stability and geometrical MR image distortion. All metallic stereotactic rings (closed rings made of massive metal) led to a more or less dramatic geometrical distortion and signal cancellation in the MR images. The best properties-nearly no distortion and high mechanical stability-are provided by a ceramic ring. If necessary, the remaining geometrical MR image distortion can be "corrected" (reducing displacements to the size of a pixel) by calculations based on modeling the distortion as a fourth-order two-dimensional polynomial. Using this method multimodality matching can be performed automatically as long as all images are acquired in the same examination and the patient is sufficiently immobilized. Precise definition of the target volume could be performed by the radiotherapist either directly in MR images or in calculated projection MR angiograms obtained by a maximum-intensity projection algorithm. As a result, information about the hemodynamics of the AVM was provided by a three-dimensional (3D) phase-contrast flow measurement and a dynamic MRA with the STAR technique leading to an improved definition of the size of the nidus, the origin of the feeding arteries, and the pattern of the venous drainage. In addition, functional MRI was performed in patients with lesions close to the primary motor cortex area leading to an improved definition of structures at risk for high-dose application in radiosurgery. The different imaging techniques of MR provide a sensitive, noninvasive, 3D method for defining target volume, critical structures, and for calculating dose distributions for radiosurgery of cerebral arteriovenous malformations, because dose calculation of radiosurgery at sufficient accuracy can be based on 3D MR data of the geometrical conformation of the patient's head.

A new method of quantitative cavitation assessment in the field of a lithotripter.

Transient cavitation seems to be a very important effect regarding the interaction of pulsed high-energy ultrasound with biologic tissues. Using a newly developed laser optical system we are able to determine the life-span of transient cavities (relative error less than +/- 5%) in the focal region of a lithotripter (Lithostar, Siemens). The laser scattering method is based on the detection of scattered laser light reflected during a bubble's life. This method requires no sort of sensor material in the pathway of the sound field. Thus, the method avoids any interference with bubble dynamics during the measurement. The knowledge of the time of bubble decay allows conclusions to be reached on the destructive power of the cavities. By combining the results of life-span measurements with the maximum bubble radius using stroboscopic photographs we found that the measured time of bubble decay and the predicted time using Rayleigh's law only differs by about 13% even in the case of complex bubble fields. It can be shown that the laser scattering method is feasible to assess cavitation events quantitatively. Moreover, it will enable us to compare different medical ultrasound sources that have the capability to generate cavitation.