CT Users Group
CT Users Group meeting information
Some thoughts on CT Dosimetry Methods
C J Martin, D Gentle
Gartnavel Royal Hospital,
A key component of dosimetry and quality assurance for CT scanners is the CT dose index (CTDI). Conventionally the CTDI utilises absorbed dose in air measured across CT slices with a 100 mm long pencil ionisation chamber. The suitability of the CTDI for measurements in phantoms with multi-slice CT and helical scanning is questionable. A practical investigation into CT dosimetry has been carried out using Gafchromic film and a 20 mm rod CT chamber. Scatter dose distributions within standard head and body phantoms have been measured to determine levels of the absorbed dose to air beyond the end of standard 100 mm CT chambers. The influence of the limited length of the phantoms has also been studied. Results show that the CTDI at the centre of a phantom from a longer 300 mm chamber would be 25% and 59% higher for head and body phantoms respectively, while the weighted CTDI used in patient dosimetry would be 20% and 25% higher, respectively.
An alternative approach to dosimetry would be the use of shorter rod chambers in the middle of phantoms to measure equilibrium doses for helical scans. However, there are periodic variations in dose along the phantoms due to areas either lying outside the primary beam or being exposed to overlapping beams. The beam widths and pitches used must be considered carefully for such measurements in order to minimise errors resulting from these variations. Gafchromic film could be a valuable aid in assessing the uniformity of dose from helical scans.
Is it appropriate to consider new methods of dosimetry at this time? If so, then perhaps it is also time to consider using a different design of phantom and depths of measurement. The large discrepancy between the CTDI measured with chambers 100 mm and 300 mm long results partly because of the large dimension of the body phantom. An elliptical phantom of slightly smaller dimension would be a better approximation to the human torso and would have a smaller level of scatter beyond the 100 mm length. Is it time to start the collection of dosimetry data for elliptical CT phantoms of agreed dimensions?