Biomedical subjects

D R Wyman

Publications and source records attributed to D R Wyman.

9 recordsLinked to original sources

PubMed1992

Magnetic resonance imaging of interstitial laser photocoagulation in brain.

Magnetic resonance (MR) imaging can be used to monitor the development of thermal lesions induced in tissue using interstitial laser photocoagulation (ILP). A potential application for ILP is the treatment of surgically inaccessible brain tumors. For the successful clinical application of MR-monitored ILP, it is necessary to relate MR images of ILP lesions to the actual induced lesions. In this preliminary study we performed ILP in the normal brains of anesthetized cats by delivering interstitially 1.0, 1.5, and 2.0 W of continuous-wave Nd:YAG laser energy (1,064 nm) for 1,000 s via a plane-cut 400 microns core optical fiber. At 48 h post-irradiation the lesions consisted of four sharply demarcated concentric zones of thermal damage. Lesion diameter increased linearly with delivered power. T2-weighted proton spin-echo images acquired during ILP showed a region of complete or near signal loss that underestimated the actual lesion at 48 h. Gadolinium-enhanced T1-weighted spin-echo images acquired immediately post-irradiation showed the actual lesion precisely.

Animals↗

PubMed1992

Interstitial laser photocoagulation: Nd:YAG 1064 nm optical fiber source compared to point heat source.

Interstitial laser photocoagulation (ILP) was performed in vitro in lean bovine and chicken muscle by delivering 1.6 W of continuous-wave Nd:YAG laser energy (1064 nm) from a 400-microns core optical fiber for 300s. The resulting thermal coagulation lesion was consistently larger when the delivered energy was deposited into a small steel sphere than when it was delivered freely into the tissue. Mathematical modelling confirms this result. This preliminary study suggests that a point heat source produces a larger volume of thermal coagulation than a point optical source (1064 nm) delivering the same power.

Animals↗

PubMed1992

Sonographic changes during hepatic interstitial laser photocoagulation. An investigation of three optical fiber tips.

RATIONALE AND OBJECTIVES: Interstitial laser photocoagulation (ILP) destroys tumors thermally, using laser energy delivered from implanted optical fibers. The objectives of the study are to identify a fiber tip/delivered energy combination which produces lesions of useful size, visible on ultrasound (US) during ILP, and to compare ILP lesions and their US images. METHODS: Hepatic ILP was performed at laparotomy in six pigs, using three different fiber tips (cylindrical diffusing, spherical diffusing, plane-cut). US images were obtained during ILP, immediately after ("early" images), and before the animals were killed (2-2.5 hours, "late" images). Actual lesions were assessed histopathologically. RESULTS: Few US changes were seen around cylindrical diffusing and spherical diffusing tips until tip destruction. Plane-cut tips, at 1.5 to 2.0 W, produced prominent US images of the 1- to 2-cm thermal lesions. Early images tended to overestimate necrosis. Late images approximated necrosis. CONCLUSION: For US-controlled ILP, plane-cut tips are better than currently available cylindrical diffusing or spherical diffusing tips. Lesion image growth periods might enable control of lesion size. Further studies are needed to determine the consistency of the described relationship between lesion images and actual lesions.

Animals↗

PubMed1991

A control method for a nonlinear multivariable system: application to interstitial laser hyperthermia.

An original adaptive control method is presented for controlling a nonlinear multivariable system. The method, which could be described as a modified quasi-linear approach, involves dividing the source excitation into a series of pulsing rounds and is implemented as a control algorithm on a computer. The theory underlying the method is developed with reference to an application involving temperature control in interstitial laser hyperthermia. In this application, the method is both successful and necessary to achieve optimally uniform elevated temperatures in a ground beef phantom. Apart from variable and parameter definitions, the method is otherwise general and might be useful for controlling a nonlinear system in which no prior exact characterization of the system is possible. Simulations were conducted to assess the effectiveness of the method in systems for which the unit excitation response changes by factors ranging from zero to three over the total period of excitation. In each case the method has proven stable.

Algorithms↗

PubMed1990

Optical properties of normal and diseased human breast tissues in the visible and near infrared.

The optical absorption and scattering coefficients have been determined for specimens of normal and diseased human breast tissues over the range of wavelengths from 500 to 1100 nm. Total attenuation coefficients were measured for thin slices of tissue cut on a microtome. The diffuse reflectance and transmittance were measured for 1.0 mm thick samples of these tissues, using standard integrating sphere techniques. Monte Carlo simulations were performed to derive the scattering and absorption coefficients, as well as the mean cosine of the scattering angle. The results indicate that scatter exceeds absorption by at least two orders of magnitude. Absorption is most significant at wavelengths below 600 nm. The scattering coefficients lie in the range 30-90 mm-1 at 500 nm, and fall smoothly with increasing wavelength to between 10 and 50 mm-1 at 1100 nm. The scattering coefficient for adipose tissue differs, in that it is invariant with wavelength over this spectral range. For all tissues examined, the scattered light is highly forward peaked, with the mean cosine of the scattering angle in the range 0.945-0.985. Systematic differences between the optical properties of some tissue types are demonstrated.

Breast↗

PubMed1989

Monte Carlo modeling of light propagation in highly scattering tissue--I: Model predictions and comparison with diffusion theory.

Using optical interaction coefficients typical of mammalian soft tissues in the red and near infrared regions of the spectrum, calculations of fluence-depth distributions, effective penetration depths and diffuse reflectance from two models of radiative transfer, diffusion theory, and Monte Carlo simulation are compared for a semi-infinite medium. The predictions from diffusion theory are shown to be increasingly inaccurate as the albedo tends to zero and/or the average cosine of scatter tends to unity.

Light↗

PubMed1987

Assessment of a procedure for left ventricle volumetry.

A three-step radiographic procedure is described for the determination of left ventricle volumes, based on one lateral and two angled single plane images. The first angled image is taken to yield a minimally foreshortened LV image and the second image, at the same angle, is taken of a calibration sphere. Five sources of operator-related, or 'input', error are analysed, two of which are normally removable. The remaining three input errors are assessed using a commercial digital subtraction angiography system and comparisons with the analysis are given. A summary result is that relative errors in end-systolic and end-diastolic LV volumes, as well as cardiac output, should normally be less than 13% under reasonable operator care. The input errors should not contribute to errors in calculated ejection fractions.

Heart↗

PubMed1985

The reduction of renogram deconvolution to a direct method of transit time determination.

It is known that the intrarenal mean transit time (MTT) can be determined using renography by first deconvoluing the kidney retention function from the obtained time-activity curve and then integrating the retention function. A direct and approximate calculational method, based on an integral mathematical model, has also been employed to estimate the MTT. In this work it is shown that the direct approximate method is equivalent to the standard deconvolution method applied with the assumption of a time independent retention function. Potential errors incurred using the direct method are thus quantified and assessed over a range of representative decay parameters.

Humans↗

PubMedDate not supplied

Accuracy of blood transit time estimates using temporal filtering in digital radiology.

The application of recursive filtering in digital radiology can yield incorrect blood flow estimates if blood flow is calculated using a transit time, defined as the difference in times to peak contrast over the length of a vessel segment. The transit time error is calculated here, from computer simulations, as a function of filter design, image noise and contrast medium dispersion over the vessel segment. An error reduction procedure based on prefiltering is suggested and tested by simulation. Results indicate that the transit time error can be substantial and the reduction procedure effective, depending on the filter design and system noise level.

Angiography↗