CT Users Group

N Weir

Department of Medical Physics,
NHS Lothian,
Royal Infirmary of Edinburgh

Abstract

The limitations of the computed tomography dose index (CTDI) in representing patient dose from multislice and cone beam CT acquisitions have been well documented in the recent literature. The standard 100 mm integration length of pencil ionisation chambers is insufficient to measure the scatter tails of beams which may exceed several centimetres in length along the axis of rotation. CTDI_w is expressed as the average dose to air in standard Perspex phantoms and therefore may not be representative of dose in the typical patient's body. Use of CTDI is also based on the assumption that the scanner table moves during the examination, while many modern cone-beam imaging systems operate using a fixed table. A number of alternative methods have been proposed, including using longer pencil chambers and phantoms, or point dose measurements. Despite their advantages, these methods have the respective drawbacks of requiring expensive and cumbersome equipment or failing to give information along the entire scanner beam. This presentation investigates Dose Area Product (DAP) as a potential dose metric in CT. By utilising a large area ionisation chamber a DAP meter can provide coverage across the full extent of the CT beam and may therefore provide a more sensitive quality assurance measurement than CTDI₁₀₀ or point dose in cone beam systems. Preliminary comparative measurements of DAP versus CTDI₁₀₀ are presented for one wide beam CT scanner. DAP also provides a means of estimating the total energy absorbed by the patient and therefore directly relates to the radiation risk from the exposure. Preliminary results of using CT DAP measurements in patient dosimetry will also be discussed.

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