Biomedical subjects

K Hynynen

Publications and source records attributed to K Hynynen.

135 records · Page 8Linked to original sources

PubMedDate not supplied

Pre-focal plane high-temperature regions induced by scanning focused ultrasound beams.

A steady-state, three dimensional, bioheat transfer equation-based simulation program has been developed and used with single and multiple transducer arrangements to investigate the effects of spherical transducer characteristics (frequency and f-number) and scanning patterns on treatment temperature distributions. Results for uniformly perfused tissues show that: (1) a single circular scan induces radially non-uniform temperature distributions at the focal depth and undesirable pre-focal high temperature regions; and (2) multiple circular scans spaced by the focal diameter of the ultrasound beam and using feedback control can induce uniform temperature distributions in the focal plane, but still cause undesirable pre-focal plane high temperatures. These two results apply both to a single transducer scanned with its axis normal to the skin and to multiple tilted transducers with overlapping foci. The extent and magnitude of this pre-focal high-temperature region increases as frequency increases (from 0.5 to 2.0 MHz) and as the f-number increases (from 1.0 to 2.0). Finally, (3) when four tilted transducers with overlapping foci are used with multiple circular scans spaced by the focal diameter, if the transducer closest to the central axis of the scans is turned off (every transducer is turned off periodically) the pre-focal high-temperature region can be eliminated. From the results of this parametric study and practical considerations, a reasonable compromise of transducer choice for general use is one with a low f-number (about 1.0) and a frequency of about 1.0 MHz. For multiple tilted transducers with overlapping foci the choice of frequency is also 1.0 MHz and the f-number should be as low as possible.

Body Temperature↗

PubMedDate not supplied

The effects of tissue heterogeneities and large blood vessels on the thermal exposure induced by short high-power ultrasound pulses.

The lack of knowledge of blood perfusion distribution in a tumour and its surrounding tissues remains a major source of uncertainty of the induced temperature distribution during hyperthermia treatments. In addition, large blood vessels cool significant tissue volumes around them, making it very difficult to cover the whole tumour volume with therapeutic thermal exposure even with modern highly controllable hyperthermia systems. In this paper the earlier theoretical and experimental studies indicating the feasibility of inducing perfusion-independent thermal exposure using high-power, short ultrasound pulses have been continued. The purpose of this research was to investigate temperature elevation variations caused by tissue location, tissue interfaces, and large blood vessels through a series of in vivo experiments. Results show that high-temperature ultrasound hyperthermia does not suffer extensively from tissue heterogeneities, and even tissues within a few millimetres from an artery could be adequately exposed.

Animals↗

PubMedDate not supplied

Optimization of temperature distributions in scanned, focused ultrasound hyperthermia.

Scanned, focused ultrasound systems (SFUS) have considerable flexibility in shaping the power deposition field during hyperthermia treatments. When utilizing this adaptability many complicated, interacting decisions must be made to obtain an optimal steady-state temperature distribution. This optimization problem is studied using a 3-D, radially symmetric simulation program which searches for a set of optimal scan parameters. The conjugate-gradient optimization technique with a golden section search was used to obtain the optimal temperature distributions attainable with a single circular scan of a tumour. The variable scan parameters of the single transducer heating system optimized (and under the control of the therapist) are: transducer tilt and rotation angles, focal depth, output acoustical power, and scan radius. This single scan study includes the effects of tumour and normal tissue blood perfusions, tumour depth, skin temperature boundary condition, as well as tumour size and shape. A similar, but less comprehensive, study was done for larger tumours using two concentric circular scans. The results show that (1) the optimization process can produce a set of scan parameters that give a considerably better temperature distribution than could be obtained ad hoc, and (2) the optimal scan parameter configuration obtained produces a close-to-ideal tumour temperature distribution for a wide variety of clinically relevant conditions. Thus, when extended to include data from individual patients such optimization should be a very useful tool in patient treatment planning, and should enhance the present capabilities of clinical scanned, focused ultrasound systems.

Body Temperature↗

PubMedDate not supplied

Experimental evaluation of two simple thermal models using hyperthermia in muscle in vivo.

The predictions from two simple field equation models for calculating temperature distributions in tissue, namely, the Pennes' bioheat transfer equation (BHTE) and an effective thermal conductivity equation (ETCE), were compared to in vivo experimental temperature measurements made under hyperthermic conditions generated by scanned focused ultrasound. The models were kept simple (i.e. homogenous isotropic properties, no separate blood vessels included) in order to concentrate attention on the predictive abilities of these field equations using a minimum number of free parameters. Simulated results were fitted to the experimental data (multiple, linear temperature profiles in the thigh muscles of greyhound dogs) by minimizing a performance index using a golden section searth. This search determined a value for the single free parameter in each model (blood perfusion in the BHTE, and effective thermal conductivity in the ETCE) which minimized the square error difference between the experimental and simulated temperatures. The results showed that (a) the simple BHTE model could qualitatively reproduce the major features of the temperature patterns seen experimentally better than the ETCE model could, and (b) the simple BHTE model produced better quantitative fits to the experimental data than did the simple ETCE model. In addition, blood perfusion predictions from the BHTE model compared well to measurements done with coloured microspheres. Finally, the experimental results showed that individual, large blood vessels appeared to have a major influence in producing asymmetries in the experimental data in 21% of the measured temperature profiles.

Animals↗

PubMedDate not supplied

Small cylindrical ultrasound sources for induction of hyperthermia via body cavities or interstitial implants.

In this study, small (outside diameter 1 mm) cylindrical ultrasound sources were investigated for induction of hyperthermia in tumours. These ultrasound transducers could be placed in small-diameter body cavities, or they could be used interstitially in brachytherapy catheters. The ultrasound field measurements showed that the field is fairly uniform as a function of the length of the applicator except at the ends where sharp peaks were located. However, there were significant field variations as a function of rotation angle around the transducers. The degree of these non-uniformities varied from transducer to transducer, and also as a function of frequency. The temperature measurements in vitro perfused kidneys showed that therapeutic temperature elevations could be induced in perfused tissues. The radial extent of the therapeutic zone could be increased by circulating water around the applicators, thus avoiding high temperatures on the applicator surface. It was also shown that some control over the temperature distribution along the length of the applicator could be achieved by using a two-element applicator. An array of four applicators implanted in a square pattern with the spacing of 25 mm between the catheters, was able to heat the tissue volume inside of the implant. The results showed that these small ultrasound applicators may offer significant improvement over existing techniques by increasing the penetration depth and the control over the power deposition pattern.

Animals↗

PubMedDate not supplied

Simulation of bidirectional ultrasound hyperthermia treatments of neck tumours.

The temperature distributions produced in neck tumours by using either a single, scanned transducer (a unidirectional scan) or two separate transducers whose axis are perpendicular (a bidirectional scan) were simulated. The three-dimensional neck model included separate anatomical regions for the normal neck muscle tissue, the tumour, the spinal column and the trachea (no large blood vessels). The effects of variations in the transducer frequency and f number, the tumour size and location, and the normal and tumour blood perfusion rates were studies. The best simulated temperature distributions were produced by bidirectionally scanned, 2 MHz, f number 2.0 ultrasound transducers whose powers were modulated as a function of position. The simulated temperature distributions from such modulated bidirectional scans were significantly better than those of both unidirectional and unmodulated bidirectional scans. The 1-MHz transducers generally produced hot spots at the tissue-spine and/or tissue-trachea interface. The 3-MHz transducers eliminated those deep hot spots but created other hot spots close to the skin surface, and did not adequately heat the deeper regions of the tumour. These results from the simplified computer simulations may be used to guide the construction of improved ultrasound hyperthermia systems for the treatment of neck tumours.

Biophysical Phenomena↗

PubMedDate not supplied

A system for the simultaneous delivery of intraoperative radiation and ultrasound hyperthermia.

A multi-element ultrasonic applicator, utilizing an ultrasound reflector, is presented which enables the controlled delivery of ultrasonic energy for the induction of hyperthermia while allowing the simultaneous application of orthovoltage ionizing radiation. A temperature-controlled water circulating system allows for acoustic coupling, additional control over the tumour surface temperature and depth of maximum temperature into the tissue. In vitro and in vivo testing supported the applicators objective of effectively controlling temperature elevation. Therapeutic target temperatures of 42 degrees C can be achieved within a 5-min period and maintained for a 60-min treatment time. The depth of heat penetration could be varied as a function of surface temperature and ultrasound frequency. Heating was achieved to at least a depth of 3 cm. Radiation measurement methods verified the expected radiation dose uniformity and distribution.

Humans↗

PubMedDate not supplied

Patterns of changes of tumour temperatures during clinical hyperthermia: implications for treatment planning, evaluation and control.

The patterns of changes in tumour temperatures were studied at selected times throughout 104 hyperthermia sessions. Temperature change patterns were analysed in the context of the known patterns of change of the applied power. First, of 69 extracranial treatments analysed, 74% indicated relatively flat temperatures at constant applied power during a major portion of the treatment, thereby indicating that during that time there were no major changes in any of the physical or physiological tissue parameters which contribute to the ability of the tumour tissue to remove energy (Pattern 1). Second, after reaching an initial steady state, approximately 14% of these extracranial treatments showed either steadily decreasing temperatures at constant power, or constant temperatures at steadily increasing applied power, thereby indicating that the tumour's ability to remove energy was steadily increasing in time following the initial steady state (Pattern 2). Finally, after reaching an initial steady state, the remaining 12% of these treatments showed a pronounced decrease in temperature occurring about 10-20 min into the treatment followed by increasing temperatures or levelling off of temperatures at a higher value than the temperature minimum that had occurred, all at constant applied power (Pattern 3). Of 35 brain treatments analysed, 80% followed Pattern 1, 14% followed Pattern 2, and 6% followed Pattern 3. Intratumoral heterogeneity was evident in some cases with approximately 44% of all treatments having at least one individual temperature sensor change in a manner that did not follow the average direction of change when all sensors were combined. For seven patients with permanent probes, the patterns of change presented in the first treatments were also observed during six out of seven of the second treatments. In addition, three out of the five patients who had an evaluable third treatment showed a pattern of change during that third treatment that was similar to the pattern observed in both treatment one and treatment two.

Body Temperature↗

PubMedDate not supplied

The feasibility of MRI feedback control for intracavitary phased array hyperthermia treatments.

Temperature feedback control has the potential to enhance hyperthermia treatments by providing more uniform heating of the target volume and improving the transient temperature response. A multivariable least squares batch algorithm was used to estimate system parameters for simulated prostate hyperthermia treatments. A multi-input, multi-output (MIMO) linear quadratic regulator (LQR) controller was designed for prostate hyperthermia treatments with an intracavitary phased array. A parametric study was performed for the one-dimensional control case, investigating factors relevant to magnetic resonance imaging (MRI) feedback control such as spatial resolution of temperature measurements (size of the averaging volume), sampling rate (image acquisition time), thermometry noise, control width, control depth, physiological parameter changes and reference input structure. Simulations utilizing the two dimensional (2-D) thermometry of MRI and the 2-D focusing capabilities of phased arrays demonstrated that near field heating can be controlled such that the size and shape of the heated volume can be tailored in 2-D. The control algorithms developed in this study show promising potential for incorporation into a non-invasive prostate hyperthermia system utilizing MRI feedback.

Acoustics↗