News |  Sitemap |  Contact
PDF Export  | 

Holger Buhr: ImageJ based application for automatical analysis of a new X-ray phantom

Abstract

Background:

A program using ImageJ was developed for quality testing in radiography. In order to consolidate existing test procedures and to enable an automatically detection and evaluation a new test phantom including new features was designed. The image of this phantom is detected, pre-processed for some physical and technical influences of the x-ray device and analyzed finally. Results are saved in MS-Excel format for evaluation by medical physicists experts.

Material and Methods:

The phantom is a 300 x 300 x 60 mm PMMA block containing different elements. Elements inclide metal edges for the detection of the phantom itself, metal balls to measure the accuracy of the central projection of the x-ray device, aluminum rods and air pillars of different thickness for measurement of absorption characteristics. A 60 x 60 x 60 mm square cavity in the center of the phantom can hold four different test cartridges, containing a homogeneous PMMA block, a Siemens star, a test plate with metallic slanted edges and an aluminum step wedge. The software detects the phantom in the image by means of the Harris Corner detector. The positions of the metal edges denote the mechanical alignment of the phantom on the bucky table of the x-ray device. Eight metal balls, four on the top positioned perpendicular to four on the bottom side of the phantom are reproduced and differentiated in the image, due to the angular beam projection. When detected using the particle analyzer, the program can deduce the position of the radiation source in reference to the the position of the phantom based on the position of these balls in the image. In the regions of the aluminum rods and air pillars the contrast, the signal-to-noise (SNR) ratio is calculated. With the slanted edges the modulation transfer function (MTF) is measured. The MTF is a very sensitive and significant measurement for the sharpness in relation to spatial frequencies. In the homogeneous area of the PMMA block the noise-power spectrum (NPS) is calculated providing a measurement of image noise in relation to spatial frequencies. Using the mean pixel values of the aluminum steps the dynamic range of the x-ray device is measured. In addition to the above measurements the software also incorporates a pre-processing step aiming at equalizing inhomogeneities in the image caused by the so-called Heel effect. This effect can be described approximately by a 2-D polynomial of degree 2. This smoothing function is calculated by an optimization algorithm minimizing the entropy of the histogram in the homogeneous area of the test phantom.

Results:

This software allows for an automatic analysis of a new test phantom enabling a fast and accurate quality check of x-ray devices. Physical conditions of such devices having a negative impact on image analysis and subsequent measurements are eliminated by sophisticated pre-processing. This also establishes inter-system comparability.

Keywords

X-ray, radiology quality test, automatic detection, radiological phantom

Administrative data

Presenting author: Holger Buhr
Organisation: Cologne University of Applied Sciences

co-authors: Cenk Altinsoy, Christian Blendl, Marlen Badelt, Marta Gardel, Dietmar Kunz, Clemens Loos, Stefan Rudolf, Giannina Sauerwein, Sylvia Schmitt, Felix Spalthoff, Patrick Stein, Alexandra Tobisch, Velina Zhekova

Type: Poster (portrait)

© Luxembourg Institute of Science and Technology | Legal Notice