Biomedical subjects

P B Greer

Publications and source records attributed to P B Greer.

9 recordsLinked to original sources

PubMed2003

The effect of leaf width and sampling distance on the "stair-stepping" of field borders defined by multileaf collimators.

Standard multileaf collimators (MLCs) are now available with 1.0 and 0.5 cm leaf widths. The aim of this work is to compare the dose-undulation and effective penumbra of field edges formed by these MLC leaf widths and to determine how reducing the sampling distance (centre-to-centre distance between adjacent MLC leaves) for the 1.0 cm leaf width compares to the smaller leaf width. The undulation of the 50% isodose line and the 80-20% and 80-30% effective penumbra were compared for a field edge angled at 45 degrees to the MLC leaf motion direction at 8 cm depth. The larger leaf width field was also segmented to form field edges with 0.5, 0.33 and 0.2 cm sampling distance. Random setup variation of 2 mm standard deviation was also incorporated. Dose undulation was 1.5 mm for the 0.5 cm MLC leaf width compared to 4.5 mm for the 1.0 cm width. The 80-20% effective penumbra was 2 mm less for the 0.5 cm leaf width and the 80-30% effective penumbra was approximately 3 mm less. When random setup variation was incorporated the 0.5 cm leaf width isodoses were straight compared with approximately 3 mm undulation for the larger MLC. Reducing the sampling distance for the 1.0 cm MLC leaf width to 0.33 cm resulted in penumbra only slightly greater than the 0.5 cm leaf width and removed the undulation. Effective penumbra and dose undulation are reduced for the 0.5 cm leaf width compared to a 1.0 cm leaf width. Reducing the sampling distance for the 1.0 cm MLC leaf can approximate the 0.5 cm leaf width, at the expense of longer treatment times, and increased quality assurance investment.

Computer Simulation↗

PubMed2000

Evaluation of an algorithm for the assessment of the MTF using an edge method.

An algorithm to calculate the presampling modulation transfer function (MTF) of an imaging system from an angled edge image has its own inherent transfer function. Factors such as the angle of the sampling aperture to the edge, registration of edge function profiles using the determined edge angle, differentiation, smoothing, and folding all combine to produce the frequency response of the algorithm. In this work, the profile registration transfer function accounting for an error in the determined edge angle has been derived. This has been incorporated with other, previously reported, algorithm component transfer functions to fully characterize the MTF calculation algorithm. When registering profiles, small errors in the edge angle determination were found to result in large errors in the MTF, as the misalignment errors increase with the number of profiles. For example, registering 50 profiles a 0.07 degree error in a 7 degree edge angle (1% error) produces a 36% error in the MTF at the system cutoff frequency f=f(c) when profiles are oversampled at a frequency f(s)=8f(c)(f(c) is defined as the maximum frequency reproducible without aliasing when sampling at the limiting system Nyquist frequency f(s) = 2f(c)). These results highlight the importance of quantifying the transfer function of the algorithm used to determine an imaging system modulation transfer function. The MTF calculation algorithm and the transfer function analysis have been incorporated into a Windows-based software program to be made available for general use.

Algorithms↗

PubMed2000

A design for a dual assembly multileaf collimator.

A multileaf collimator for radiation therapy has been designed that splits each leaf bank into two vertically displaced levels with each level consisting of alternate leaves and leaf spaces. The leaves in the upper level shield the spaces in the lower level. Each level can move laterally, in the direction perpendicular to leaf motion by one leaf width. Following lateral movement of one level, the leaves align with the other level and radiation is transmitted through the collimator as multiple slit fields in a grid pattern. This transmission can be used to form an image of the external anatomy and would enable double-exposure portal images to be acquired much more rapidly than at present. These could potentially be acquired during the treatment delivery. The radiation profiles transmitted for image formation through the collimator design were investigated. Individual and grid pattern slit field profiles formed by tungsten and lead alloy collimators were measured with varying slit width, source-collimator distance, collimator-detector distance, and collimation thickness. The slit width was found to have the major influence on the transmitted profiles. As the slit width decreases the profiles become broader than the geometric slit projection resulting in increasing overlap of adjacent profiles. The overlap results in a modulated image of the external anatomy for small slit widths, rather than a sampled or "grid" image for larger widths. The shielding of this design was found to be adequate provided the leaf faces of the adjacent vertically displaced leaves are at least aligned, therefore an overlap or tongue and groove is not required.

Equipment Design↗

PubMed1998

Comparison of two methods for anterior-posterior isocenter localization in pelvic radiotherapy using electronic portal imaging.

PURPOSE: The two setup methods commonly used to determine the anterior-posterior isocenter location in pelvic radiotherapy are to align lateral localization lasers with lateral skin tattoos on the patient, or to set the couch height so that the isocenter is at a fixed height (determined during simulation or treatment planning) above the couch top. This study was implemented to determine which technique gives more accurate patient treatment by comparison of the anterior-posterior setup variation measured with electronic portal imaging. METHODS AND MATERIALS: Eleven supine prostate patients were treated with tattoo localization and 159 left-lateral portal images were taken during the treatments. The field displacements were then determined by template matching. These patients were compared to nine patients (205 images) set up to a fixed isocenter height. Similarly, eight prone rectal patients (136 right-lateral images) set up to tattoos were compared to six patients (108 images) set up to a fixed height. The patients were not immobilized and were all treated with three field techniques on a hard couch top. The overall mean treatment position deviation and the standard deviation of the displacements (total setup variation) were calculated for each patient group along with the systematic (simulator-to-treatment) and the random (treatment-to-treatment) setup variation. RESULTS: The mean treatment position deviations were 3.3 mm anterior and 5.2 mm posterior with the tattoo method for the prostate and rectal patients, respectively. These mean position deviations were 0.4/0.1 mm anterior with the fixed height technique. The total setup variations were 4.6/5.2 mm (1 SD) with tattoo localization and 1.7/1.5 mm (1 SD) with the fixed height method. Similarly, random variation was 2.3/3.3 mm (1 SD) with the tattoo method compared to 1.3/1.2 mm (1 SD) with the fixed height method. Systematic variation was 3.7/4.5 mm (1 SD) compared to 1.2/1.1 mm (1 SD). CONCLUSION: The fixed height technique gives much more accurate localization of the anterior-posterior isocenter in pelvic radiotherapy than lateral skin tattoos.

Humans↗

PubMed1998

Set-up variation of patients treated with radiotherapy to the prostate measured with an electronic portal imaging device.

The set-up variation of 11 patients treated supine with radical radiotherapy for carcinoma of the prostate was measured with an electronic portal imaging device to determine the adequacy of set-up techniques and current margins, as well as the need for immobilization. During the treatments 172 images of the anterior fields and 159 images of the left-lateral fields were taken and the errors in treatment placement were measured by template matching. The variation in the superior-inferior direction was small, 1.4-1.6 mm (1 SD), while the medio-lateral variation was 2.8 mm (1 SD). The anterior-posterior variation was largest, 4.6 mm (1 SD) with an offset of 3.3 mm anterior. This anterior offset and large anterior-posterior variation suggests that set-up techniques were not optimal for this direction. The 1 cm margin used was adequate for set-up variation except in a small number of cases, which was mainly due to the anterior trend. Random (treatment-to-treatment) variations were small (1.1-2.3 mm; 1 SD), indicating that immobilization would result in only modest improvement in reproducibility for these supine patients.

Humans↗

PubMed1997

Anterior-posterior treatment localization in pelvic radiotherapy: tattoos or fixed couch-to-isocentre distance.

The methods of determining the anterior-posterior isocentre location in pelvic radiotherapy are either by aligning lateral localization lasers to tattoo marks on skin, or by setting a constant daily couch-to-isocentre distance. While using the former method the day-to-day vertical couch movement was recorded and combined with measurements of day-to-day anterior-posterior patient movement made with an electronic portal imaging device to determine whether couch vertical movement contributes to anterior-posterior setup variation. Seven unimmobilized patients were studied, four supine prostate and three prone rectum patients. The two motions were found to be highly correlated (correlation coefficient = 0.82) which supports the constant couch-to-isocentre distance approach. When the day-to-day couch vertical movement was subtracted from the anterior-posterior movement results the setup variation was reduced in six of the seven patients.

Humans↗

PubMed1996

Image timing and detector performance of a matrix ion-chamber electronic portal imaging device.

Image timing and input/output characteristics of a Varian PortalVision matrix liquid ion-chamber electronic portal imaging device (EPID) used with a Varian CL2100C/D linear accelerator were investigated to examine the variation in image timing and detector performance with acquisition mode. Contrast-detail curves showed that the input/output results were indicative of the image quality variation with acquisition mode. Overall for this accelerator/EPID arrangement the 500 MU/min repetition rate gives the best image quality for the 6 MV and 18 MV acquisition modes.

Equipment Design↗

PubMed1994

Point-spread function for light scattered in the human ocular fundus.

A model based on the framework of fundal reflectance model III is developed for the lateral spreading of light that emerges from the human ocular fundus after scattering in the choriocapillaris and the choroidal stroma. Blood in these layers is modeled as a forward scatterer, and the Kubelka-Munk diffuse radiation environment used for the choroid in model III is replaced by an arrangement of embedded reflectors that gives the same remittance. An equation is derived for the point-spread function, and sample calculations illustrate the dependence of the spreading function on retinal site and wavelength and identify the characteristic influences of the long-wavelength edge of the 575-nm oxyhemoglobin absorption band and the choroidal melanin.

Choroid↗

PubMed1993

Reflected line spread function of the human ocular fundus.

Characterisation of the reflection from the fundus of the eye is important for the development of an improved theoretical description of photoscreening. Measurement has been made of the reflected line spread function of the human fundus in vivo using a slit-lamp microscope with video camera and frame-grabber. Line spread functions representative of red, green, and blue light were obtained from fundus sites just temporal to the macula, 10 degrees temporal to the macula, away from the macula temporal, and 30 degrees temporal to the macula. The results show that the lateral spreading of light is greatest for red light, and support the view that red light penetrates further into the fundus before being reflected than green or blue.

Adult↗