John Poletti MSc, Presenter: Nothing to Disclose

Donald McLean PhD, Abstract Co-Author: Nothing to Disclose

A number of recent studies have shown that an increase in the source to image distance (SID) used for plain x-ray projections will result in a reduction in effective dose to the patient. To take advantage of longer SIDs it is necessary to determine the optimum focal spot size for use with these SIDs, in order to optimise image quality. The factors to be considered include the effect of exposure time on movement blur, quantum noise, scatter and the degradation in spatial resolution due to geometric blur and the image receptor.

A Monte Carlo simulation was performed of the complete imaging chain for a digital radiography (DR) system with the inclusion of a contrast-detail test object. The EGSnrcMP code was used to model x-ray transport in a system including a test object, phantom, focussed grid, automatic exposure control and indirect DR image receptor. Light spread in the image receptor was incorporated using published data. Image generation required of the order of 1011 photon histories, taking eight days of CPU time per image using 2.4GHz AMD Athlon computers. Test images were generated for the standard SID of 100cm, with a range of focal spot sizes. Focal spot sizes at longer SIDs were determined geometrically. The images were scored by eight experienced observers and contrast-detail curves were plotted.

The contrast-detail results showed that focal spots smaller than those currently utilized produced the best contrast-detail performance, despite the presence of quantum noise and x-ray scatter in the images. Therefore the choice of focal spot size must be based on other criteria besides quantum limits. In particular the focal spot may need to act as an anti-aliasing filter due to the Nyquist limit of the DR image receptor.

To maximize image quality and dose advantages gained from the use of longer SIDs, x-ray tubes should have controllable focal spot sizes, linked automatically to the SID being selected.

Use of longer SIDs will enable reduction of doses to patients. The focal spot size must be tailored to the new SID.

Poletti, J, McLean, D, Optimization of Focal Spot Size to Enable Dose Reduction at Increased Source to Image Distances in Digital Radiography.  Radiological Society of North America 2007 Scientific Assembly and Annual Meeting, November 25 - November 30, 2007 ,Chicago IL. http://archive.rsna.org/2007/5007216.html