2010-09-06

Receive Higher Accuracy when Measuring CTDIw

RTI strongly recommend the 1 point method when measuring CTDIw. Using the 1 point method only one exposure in the centre hole is performed for each CTDI_w estimation. This is possible by using know relations in the dose distribution between the centre hole and the 4 periphery holes. This relation is defined by the "k-factor".

How accurate is the k-factor?

The k-factors have been extracted from statistics during two or three years  from measurements of the CTDI_w and CTDI₁₀₀ measured with ion chambers on different scanners.

The variation between different scanners and bowtie filters are less than ±5%. This difference gives less distribution to the expanded uncertainty than influences by measuring the 5 periphery holes individually. Factors that add inaccuracy when measuring in the periphery holes are energy dependencies, low signal and high dynamic variation in signal during the tube rotation. When using the CT dose profiler in the periphery holes during a scan the pitch has to be less than 0,5 to avoid loss of information for the dose estimation.

Can I check the k-factor myself?

Yes; You can easily extract you k-factor for each of your CT scanners by using previous measurements with ion chambers.

RTI strongly recommends the 1 point method to be used for CTDI_w measurements with the CT Dose Profiler. The method saves time and gives a higher accuracy than the 5 point method, either using a pencil ion chamber or the CT Dose Profiler.