Biomedical subjects

Gregory T Clement

Publications and source records attributed to Gregory T Clement.

4 recordsLinked to original sources

PubMed2006

Automated sonographic evaluation of testicular perfusion.

Contrast-enhanced ultrasound (US) imaging is potentially applicable to the investigation of vascular disorders of the testis. We investigated the ability of two automated computer algorithms to analyse contrast-enhanced pulse inversion US data in a rabbit model of unilateral testicular ischaemia and to correctly determine relative testicular perfusion: nonlinear curve fitting of the US backscatter intensity as a function of time; and spectral analysis of the intensity time trace. We compared (i) five metrics based on the algorithmic data to testicular perfusion ratios obtained with radiolabelled microspheres, a reference standard; (ii) qualitative assessment of the US images by two independent readers blinded to the side of the experimental and control testes to the radiolabelled microsphere perfusion ratios; and (iii) results of the algorithmically-derived metrics to the qualitative assessments of the two readers. For the curve fit method, the algorithmically-derived metrics agreed with the reference standard in 54% to 68% of all cases. For the spectral method, the results agreed in 70% of all cases. The two readers agreed with the reference standard in 40% and 35% of all cases, respectively. These results suggest that automated methods of analysis may provide useful information in the assessment of testicular perfusion.

Algorithms↗

PubMed2005

A magnetic resonance imaging-compatible, large-scale array for trans-skull ultrasound surgery and therapy.

OBJECTIVE: Advances in ultrasound transducer array and amplifier technologies have prompted many intriguing scientific proposals for ultrasound therapy. These include both mildly invasive and noninvasive techniques to be used in ultrasound brain surgery through the skull. In previous work, it was shown how a 500-element hemisphere-shaped transducer could correct the wave distortion caused by the skull with a transducer that operates at a frequency near 0.8 MHz. Because the objective for trans-skull focusing is its ultimate use in a clinical context, a new hemispheric phased-array system has now been developed with acoustic parameters that are optimized to match the values determined in preliminary studies. METHODS: The transducer was tested by focusing ultrasound through ex vivo human skulls and into a brain phantom by means of a phase-adaptive focusing technique. Simultaneously, the procedure was monitored by the use of magnetic resonance guidance and thermometry. RESULTS: The ultrasound focus of a 500-element 30-cm-diameter, 0.81-MHz array could be steered electronically through the skull over a volume of approximately 30 x 30 x 26 mm. Furthermore, temperature monitoring of the inner and outer surfaces of the skull showed that the array could coagulate targeted brain tissue without causing excessive skull heating. CONCLUSIONS: The successful outcome of these experiments indicates that intensities high enough to destroy brain tissue can be produced without excessive heating of the surrounding areas and without producing large magnetic resonance noise and artifacts.

Artifacts↗

PubMed2004

500-element ultrasound phased array system for noninvasive focal surgery of the brain: a preliminary rabbit study with ex vivo human skulls.

The aim of this study was to test a prototype MRI-compatible focused ultrasound phased array system for trans-skull brain tissue ablation. Rabbit thigh muscle and brain were sonicated with a prototype, hemispherical 500-element ultrasound phased array operating at frequencies of 700-800 kHz. An ex vivo human skull sample was placed between the array and the animal tissue. The temperature elevation during 20-30-sec sonications was monitored using MRI thermometry. The induced focal lesions were observed in T2 and contrast-enhanced T1-weighted fast spin echo images. Whole brain histology evaluation was performed after the sonications. The results showed that sharp temperature elevations can be produced both in the thigh muscle and in the brain. High-power sonications (600-1080 W) produced peak temperatures up to 55 degrees C and focal lesions that were consistent with thermal tissue damage. The lesion size was found to increase with increasing peak temperature. The device was then modified to operate in the orientation that will be used in the clinic and successfully tested in phantom experiments. As a conclusion, this study demonstrates that it is possible to create ultrasound-induced lesions in vivo through a human skull under MRI guidance with this large-scale phased array.

Animals↗

PubMed2003

Forward planar projection through layered media.

A planar forward projection algorithm is combined with ray theory to describe longitudinal propagation through an arbitrary number of randomly oriented isotropic layers. This method first measures the space-time pressure field in a plane, then uses wavevector frequency-domain methods to project the field through layered media and to an arbitrary new plane, not necessarily parallel to the initial plane. The approach is valid for longitudinal propagation through liquid layers and in solids, such as soft tissues, that can be approximated as viscous liquids. The algorithm is verified by propagating the field from a 0.5 MHz planar transducer through a combination of rubber, plastic, and water layers. Hydrophone measurements indicate correlation between measured and simulated fields for angles below the longitudinal critical angles of the layered materials.

Acoustics↗