Biomedical subjects

C B Saw

Publications and source records attributed to C B Saw.

42 records · Page 3Linked to original sources

PubMed1989

Dosimetric considerations of stereotactic brain implants.

Dose distributions of stereotactic brain implants performed by four institutions were analyzed. In these implants 192Ir or 125I sources were used. The analyses involved an evaluation of the isodose distributions in two orthogonal planes, the dose gradient outside, and the dose homogeneity within the target volume. Quantitative evaluation of the dose homogeneity was performed using three volumetric irradiation indices. The dose homogeneity was observed to improve as the number of catheters increased. However, the number of catheters used is influenced by neurosurgical considerations. Thus, it is necessary to make a compromise between dose homogeneity and the maximum number of catheters to be used. The dose gradient, a centimeter outside the target volume, was found to depend on the geometry of the implant and at distances beyond, it was found to depend on the type of radioisotopes used.

Brachytherapy↗

PubMed1987

Influence of zoom factor on centre-of-rotation of the SPECT system and on the resolution of tomographic images.

The influence of zooming on centre-of-rotation (COR) of the rotating camera-based SPECT system and on the resolution of tomographic images was investigated. COR values were measured as a function of zoom factor varying from x1.0 to x3.0. Measurements were performed using two SPECT systems collimated with three collimators. In this study, the COR was observed to shift linearly with zoom factor. The influence of zooming on COR depends on the location and the deviation of the COR from the axis-of-rotation (AOR). We recommend that the deviation of the COR from the AOR be kept to within 0.4 pixels corresponding to 2.4 mm. Unlike zooming, the magnitude of COR shift is uncorrelated to the type of collimator used. Contrast and spatial resolution variations of the tomographic images were quantitated as a function of COR shift for clinical data acquired using no zoom factor and a zoom factor of 2.50. The resolution indices were observed to be more sensitive to COR shift for clinical data acquired using no zoom factor. The sensitivity appeared to be inversely dependent on the zoom factor utilized. However, zooming was found not to preserve the same resolution degradation. Optimum resolution images can be reconstructed by requiring the COR be determined and clinical study be acquired using the same zoom factor and collimator. For qualitative evaluation of tomographic images, the zoom factor requirement can be removed provided that the deviation of the COR from the AOR is within the recommended limit. This limit allows for a maximum spatial resolution degradation of 5%.(ABSTRACT TRUNCATED AT 250 WORDS)

Image Enhancement↗

PubMed1986

Effects of centre-of-rotation shift on contrast and spatial resolution of the SPECT system.

The effects of centre-of-rotation (COR) shift of the rotating camera-based single photon emission commuted tomography (SPECT) system on tomographic images were investigated quantitatively. Contrast and spatial resolution variations as a function of COR shift were measured for all shifts less than 1 pixel, in steps of 0.1 pixels. This region encompasses the typical COR variations. The behaviour of these indices was analysed. In addition, the validity of the limit imposed on the allowable COR shift, as utilized in the current SPECT quality control protocols, was examined. The value of either +/- 0.5 or +/- 0.25 pixels is presently in use as the limit. From this study, both limits result in the loss of resolution. The resolution loss will increase the uncertainties of volume and concentration measurements. For a COR shift of 0.25 pixels, the spatial resolution loss is approximately 5%. This small spatial resolution loss may not affect the qualitative evaluation of tomographic images. However, any loss of resolution should be avoided. For a COR shift of 0.1 pixels, there is no observed loss of resolution. This value should be a desirable choice as the maximum allowable COR shift since it ensures the optimum performance of the SPECT system.

Rotation↗

PubMed1985

SPECT imaging of 131I (364 keV): importance of collimation.

A low sensitivity medium energy collimator (LSMEC) designed with thick septa and long bore (theoretical leakage less than 3% 364 keV), was evaluated using 131I for a SPECT system operated in both planar and tomographic imaging modes. The collimator was designed to minimize the influence of photon penetration on spatial resolution, in particular the resolution index FWTM. Overall spatial resolution for the planar imaging mode at 10 cm from the collimator face was found to be 11.6 mm FWHM and 21.6 mm FWHM. The corresponding transverse plane and slice thickness resolution was of the order of 17 mm FWHM and 31 mm FWTM, for a radius of rotation of 16 cm. A SPECT resolution phantom was imaged. Two quadrants of cold rods were well resolved, with rod dimensions of 16 and 12.7 mm respectively, the resolution being comparable to that obtained using 99Tcm (140 keV) and a low-energy high-resolution collimator. NEMA sensitivity obtained was 75 cpm/microCi 131I. The resolution measurements obtained suggest that this collimator should be useful for SPECT imaging with 131I in either radioimmunoimaging or radioimmunotherapy.

Equipment Design↗

PubMedDate not supplied

Coordinate transformations and calculation of the angular and depth parameters for a stereotactic system.

Stereotactic systems have been used to assist in the precise implantation of radioactive sources in selected brain tumors. Use of such systems requires an algorithm that transforms spatial points in computed tomography coordinates into stereotactic frame coordinates. A simple algorithm performing the coordinate transformations, intended for inclusion in treatment-planning software packages for interstitial brain implants, has been developed. This algorithm was formulated using the geometrical configurations of the Brown-Roberts-Wells (BRW) stereotactic system. After the transformations, the BRW angular coordinates and depth specifying the probe direction, defined from the entry point to the target point, are determined from their respective cartesian coordinates. These angular coordinates and depth on the BRW stereotactic system allow accurate neurosurgical implantations of catheters into the brain, and thereafter the insertion of radioactive sources.

Brachytherapy↗

PubMedDate not supplied

Reference dose rates for single- and double-plane 192Ir implants.

The proper selection of the reference dose rate is critical in the irradiation of a tumor or tumor bed using interstitial implant. The selected reference dose rate should result in the delivery of highly homogeneous dose throughout the target volume, adequately cover the target volume, and minimize irradiation of the surrounding normal tissues. In this paper, the influence of the reference dose rate on the adequacy of the irradiation of idealized target volumes using single- and double-plane 192Ir implants was studied in terms of three volumetric irradiation indices. The results show that maximum relative dose homogeneity can be attained if the proper reference dose rate is chosen. This isodose rate contour exhibits a width larger than the thickness of the target volume in the central plane. The lowest dose rate within the idealized target volume is not recommended as the reference dose rate to avoid a large volume of the surrounding tissues from receiving dose rates equal to or greater than this dose rate, and also the inhomogeneous irradiation of the target volume.

Brachytherapy↗