Frank N Ranallo PhD, Presenter: Nothing to Disclose

To devise a test tool and method to simply and accurately measure off-focus radiation from clinical x-ray units. The measurement is presented as a percentage of the total radiation existing the tube within the collimated beam.

The fact that off-focus radiation degrades radiographic image quality has been long known. Various methods in equipment design have been used to reduce the amount of off-focus radiation contributing to medical images. The use of upper collimators placed as close as possible to the focal spot has been a prime method of reducing off-focus radiation. Improved tube design can also reduce off –focus radiation. However because of the lack of a generally available test method for the measurement of off-focus radiation, this important property of radiographic imaging systems has been generally ignored in equipment specifications, acceptance testing, and quality assurance testing. Many present day collimator designs provide less reduction of off-focus radiation in an effect to reduce costs, and this trend has not been noticed because of the lack of equipment testing for this property. The proposed method is able to accurately isolate the contribution of off-focus radiation using film or other image receptors and simply provide the fraction of the total radiation that is off-focal. The theory and use of this method will be described.

This test tool is used to determine the amount of off-focus radiation for various x-ray systems. This measurement is performed as a function of kV, mA, and collimated field size. The fraction of off-focus radiation is seen to vary by more than a factor of four from less than 5% to greater than 20 %.

Off-focus radiation is a significant component of image quality that can now be easily measured along with other common QA tests.. The amount of this degrading off-focus radiation is strongly affected by collimator design and also dependant of x-ray field size and exposure techniques. The specification and measurement of off-focus radiation should be a consideration in the evaluation of x-ray imaging equipment.

Ranallo, F, A Device and Method for the Measurement of Off-Focus Radiation in Clinical X-ray Units.  Radiological Society of North America 2004 Scientific Assembly and Annual Meeting, November 28 - December 3, 2004 ,Chicago IL. http://archive.rsna.org/2004/4417549.html