Biomedical subjects

Michael J Bronskill

Publications and source records attributed to Michael J Bronskill.

7 recordsLinked to original sources

PubMed2006

Analysis of factors important for transurethral ultrasound prostate heating using MR temperature feedback.

The feasibility of using MR thermometry for temperature feedback to control a transurethral ultrasound heating applicator with planar transducers was investigated. The sensitivity of a temperature-based feedback algorithm to spatial (control point area, slice thickness, angular alignment) and non-spatial (imaging time, temperature uncertainty) parameters was evaluated through numerical simulations. The angular alignment of the control point with the ultrasound beam was an important parameter affecting the average spatial error in heat delivery. The other spatial parameters were less influential, thus providing an opportunity to reduce spatial resolution for increased SNR in the MR imaging. The update time was the most important non-spatial parameter determining the performance of the control algorithm. Combined non-spatial and spatial parameters achieved acceptable performance with a voxel size of 3 mm x 3 mm, a 10 mm slice thickness and a 5 s update time. Temperature uncertainty of up to 2 degrees C had little effect on the performance of the control algorithm but did reduce the average error slightly due to a systematic, noise-induced overestimation of the boundary temperature. These simulations imply that MR thermometry performed on clinical 1.5 T imaging systems is of sufficient quality for use as thermal feedback for conformal prostate thermal therapy with transurethral ultrasound heating applicators incorporating planar transducers.

Body Temperature↗

PubMed2005

Method for MRI-guided conformal thermal therapy of prostate with planar transurethral ultrasound heating applicators.

A method for conformal prostate thermal therapy using transurethral ultrasound heating applicators incorporating planar transducers is described. The capability to shape heating patterns to the geometry of the prostate gland from a single element in a multi-element heating applicator was evaluated using Bioheat transfer modelling. Eleven prostate geometries were obtained from patients who underwent MR imaging of the prostate gland prior to radical prostatectomy. Results indicate that ultrasound heating applicators incorporating multi-frequency planar transducers (4 x 20 mm, f = 4.7 MHz, 9.7 MHz) are capable of shaping thermal damage patterns to the geometry of individual prostates. A temperature feedback control algorithm has been developed to control the frequency, rotation rate and applied power level from transurethral heating applicators based on measurements of the boundary temperature during heating. The discrepancy between the thermal damage boundary and the target boundary was less than 5 mm, and the transition distance between coagulation and normal tissue was less than 1 cm. Treatment times for large prostate volumes were less than 50 min, and perfusion did not have significant impact on the control algorithm. Rectal cooling will play an important role in reducing undesired heating near the rectal wall. Experimental validation of the simulations in a tissue-mimicking gel phantom demonstrated good agreement between the predicted and generated patterns of thermal damage.

Acoustics↗

PubMed2005

T1, T2 relaxation and magnetization transfer in tissue at 3T.

T1, T2, and magnetization transfer (MT) measurements were performed in vitro at 3 T and 37 degrees C on a variety of tissues: mouse liver, muscle, and heart; rat spinal cord and kidney; bovine optic nerve, cartilage, and white and gray matter; and human blood. The MR parameters were compared to those at 1.5 T. As expected, the T2 relaxation time constants and quantitative MT parameters (MT exchange rate, R, macromolecular pool fraction, M0B, and macromolecular T2 relaxation time, T2B) at 3 T were similar to those at 1.5 T. The T1 relaxation time values, however, for all measured tissues increased significantly with field strength. Consequently, the phenomenological MT parameter, magnetization transfer ratio, MTR, was lower by approximately 2 to 10%. Collectively, these results provide a useful reference for optimization of pulse sequence parameters for MRI at 3 T.

Animals↗

PubMed2004

Multi-modality tissue-mimicking phantom for thermal therapy.

A tissue-mimicking phantom material has been developed for use with thermal therapy devices and techniques. This material has magnetic resonance properties (primarily T2) which change drastically upon thermal coagulation, enabling its use for device characterization and treatment verification using simple T2-weighted imaging techniques. The coagulation temperature of the phantom can be changed from 50-60 degrees C by adjusting the pH from 4.3 to 4.7. The energy absorption properties can be adjusted to match the acoustical and optical properties of tissues. T2 relaxation measurements are provided as a function of temperature, along with T2-weighted MR images to illustrate the visualization of heating patterns. A complete recipe for fabricating phantoms is provided.

Acrylic Resins↗

PubMed2003

Multifrequency ultrasound transducers for conformal interstitial thermal therapy.

Control over the pattern of thermal damage generated by interstitial ultrasound heating applicators can be enhanced by changing the ultrasound frequency during heating. The ability to change transmission frequency from a single transducer through the use of high impedance front layers was investigated in this study. The transmission spectrum of multifrequency transducers was calculated using the KLM equivalent circuit model and verified with experimental measurements on prototype transducers. The addition of a quarter-wavelength thick PZT (unpoled) front layer enabled the transmission of ultrasound at two discrete frequencies, 4.7 and 9.7 MHz, from a transducer with an original resonant frequency of 8.4 MHz. Three frequency transmission at 3.3, 8.4, and 10.8 MHz was possible for a transducer with a half-wavelength thick front layer. Calculations of the predicted thermal lesion size at each transmission frequency indicated that the depth of thermal lesion could be varied by a factor of 1.6 for the quarter-wavelength front layer. Heating experiments performed in excised liver tissue with a dual-frequency applicator confirmed this ability to control the shape of thermal lesions during heating to generate a desired geometry. Practical interstitial designs that enable the generation of shaped thermal lesions are feasible.

Animals↗

PubMed2002

Utility of simultaneous brain, CSF and hyperintensity quantification in dementia.

Improved methods of quantifying MRI are needed to study brain-behavior relationships in dementia. Rating scales are variable; lesion-tracing approaches can be subjective and ignore atrophy; segmentation of MRI hyperintensities is complicated by partial volume effects; and hyperintense lesions in different anatomical areas may have different effects. The goal of this study was to extend existing segmentation approaches to include hyperintensities and to demonstrate the utility of simultaneously assessing atrophy and lesion compartments in dementia. A semi-automated method was applied to quantify brain and cerebrospinal fluid (CSF) compartments and to subclassify hyperintensities into periventricular, deep white matter, thalamic and basal ganglia compartments. Twenty MR scans from participants in an ongoing dementia study were used to generate intra- and inter-rater reliability estimates. High intra- and inter-class correlation coefficients (0.83-0.99) were obtained for all measures and the semi-automated measurements were highly correlated with traced volumes. Brain, CSF and specific lesion volumes were significantly correlated with neuropsychological functions. In models using only total hyperintensity volumes, the effects of lesion compartments (such as thalamic) were masked. Simultaneous quantification of atrophy and anatomically distinct hyperintensities is important for understanding cognitive impairments in dementia.

Aged↗

PubMed2002

Comparison of MR imaging breast coils.

In one volunteer, five breast coils were evaluated for signal-to-noise ratio (SNR), uniformity, comfort, subject orientation, access to the breast, and unilateral imaging options. The four-coil arrays provided superior SNR, imaging flexibility, and access. Uniformity and comfort were issues with all coils. Substantial design differences exist between coils; purchasers should ensure that their specific requirements are met.

Adult↗