Abstract
Medical linac based imaging modalities such as portal imaging can be utilized for highly accurate measurements. An intensity-weighted centroid method for determining object center is proposed that can detect the position of small object at subpixel accuracy. The principles and algorithms of the intensity-weighted centroid method are presented. Analytical results are derived for positional accuracy of a rod and a sphere in digital images, and the theoretical accuracy limits are calculated. The method was experimentally examined using phantoms with embedded ball bearings (BBs). Images of the phantoms were taken by the MV portal imager of a medical linac. The image pixel size was 0.26 mm when projected at the linac isocenter plane. The BB coordinates were calculated by applying the intensity-weighted centroid method after removing the background. The reproducibility of BB position detection was measured with 3 monitor unit (MU) exposures at various dose rates. A stationary BB, of 0.25 image contrast, showed position reproducibility in the range of 0.004 - 0.013 mm. When the method was used to measure the displacement of a moving BB, the difference between the measured and expected BB position had a standard deviation of 0.006 mm. The effect of image noise on the BB detection accuracy was measured using a phantom with multiple BBs. The overall detection accuracy, represented by standard deviation, steadily improved from 0.13 mm at 0.03 MU to 0.008 mm at 5.0 MU, and showed an inverse correlation with contrast-to-noise ratio. We demonstrated that intensity-weighted centroid method can achieve subpixel accuracy in position detection. With a linac based imaging system, precise mechanical measurement with accuracy of microns could be achieved.
Highlights
New radiotherapy delivery modalities, especially stereotactic radiosurgery, increasingly demand better mechanical accuracy of medical linear accelerators and imaging devices [1]
The ultimate results of the quality assurance and calibration done in this way depend upon the accuracy with which the ball bearings (BBs) position is measured on the images
We propose using an intensity-weighted centroid of a BB to represent its center on the image
Summary
Especially stereotactic radiosurgery, increasingly demand better mechanical accuracy of medical linear accelerators (linacs) and imaging devices [1]. Phantoms with built-in ball bearings (BBs) are commonly used in radiology and radiation oncology for quality assurance and mechanical calibration [3]-[9]. During these procedures, the phantoms are imaged using the portal imager (MV therapy beam) or kV imager attached to the linac gantry (e.g., On-Board Imager). The ultimate results of the quality assurance and calibration done in this way depend upon the accuracy with which the BB position is measured on the images. The purpose of this study was to determine both the theoretical and practical accuracy with which a given linac imaging system can detect the position of BBs in a phantom
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