Henry Tsung Kak Liu MBChB, Presenter: Nothing to Disclose

Nigel Graeme Anderson MBChB, FRANZCR, Abstract Co-Author: Nothing to Disclose

To optimise radiation dose for adult CT head examination by determining the modulation characteristics of the CT scanner. To assess the dose and diagnostic acceptability of pre- and post-optimised CT adult head examinations.

In a dose survey of pediatric head scans in our tertiary hospital, we found the average dose (as measured by Dose-length-product or DLP) to be 83% higher than those reported in NRPB 2005.  Further investigations uncovered dose modulation did not occur when applying the scanning parameters of our GE Lightspeed VCT as supplied by the manufacturer. A phantom study was carried out to determine the modulation characteristics and the maximum noise index at which modulation occurs. Using the phantom study results, we selected NI = 11 for adult head examinations (previously NI = 6 as supplied by GE). Between 12/2007-2/2008, the dose (DLP) and 5-point diagnostic acceptability scale (including noise, sharpness, contrast) of a total of 100 randomly selected pre- and post-optimisation CT adult head examinations with comparable age (64.6 [18-100] vs 69.3 [26-91]) and gender ratio (40 male vs 44 male) were critically appraised by two independent neuroradiologists. DLP was analysed using Student T-test and the diagnostic acceptability using Chi-square test. 

2% of pre-and 1% of post-optimised examinations were deemed diagnostically unacceptable.  Amongst the diagnostic acceptable examinations, mean DLP dropped from 1259 mGy-cm (std dev 340) to 614 mGy-cm (std dev 207). The level of diagnostic acceptability was not statistically or clinically different when it dropped from 3.0 (std dev 0.6) to 2.8 (std dev 0.5). Figure 1 demonstrates that dose (DLP) reduction of over 50% was achieved with meaningful dose modulation without significant compromise to diagnostic acceptability.

Manufacturers’ preset CT scan parameters may not be dose-optimised. Marked dose reduction can be achieved by optimising scan parameters using a phantom.

Dose is the main source of harm of CT examinations. Phantom study should be part of QA of CT for dose optimisation for adult and pediatric head scans.  Dose reduction may apply to other body areas.

Liu, H, Anderson, N, Prospective Trial of CT Head Dose Optimization Technique Using Phantom Study.  Radiological Society of North America 2009 Scientific Assembly and Annual Meeting, November 29 - December 4, 2009 ,Chicago IL. http://archive.rsna.org/2009/8012772.html