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Practical investigations into using a small ion chamber and realistic phantom length for CT dosimetry

M Dunn, E K Inness

Medical Physics and Clinical Engineering,
Nottingham University Hospitals NHS Trust,
Nottingham, NG5 1PB


Background and purpose
The most commonly used dose descriptor in CT is CTDI100, which is the integration of the dose profile over the length of a 100 mm pencil chamber [1]. Recently the value of CTDI100 for specifying CT dose has been questioned by several investigators [3,4,5,6,]. A new methodology which uses a small ion chamber has been proposed [2]. This allows the integration to be over any arbitrary length (-L/2,L/2). The other concern regarding the current CT dosimetry method is the length of the standard CTDI phantoms (typically 14 or 15 cm). This sized phantom does not realistically represent the scatter in a patient [3, 4].

The aim of this study was to experimentally investigate the use of a small ion chamber and realistic phantom length for CT dosimetry. In particular, we wanted to investigate the CTDI100 shortfall in estimating the dose for large scan lengths.

All measurements were performed at Nottingham University Hospitals on a GE Lightspeed-16 slice scanner. Measurements were performed in either a 140 mm or 600 mm long, 32 cm diameter, PMMA body phantom. A pencil chamber with an active length of 100 mm and 3 cm³ volume was used, as well as a small ion chamber with an active length of 23 mm and 0.6cm³ volume.

For a scan length equal to 100 mm (made in a 600 mm body phantom), the small ion chamber and pencil chamber methodologies were found to agree to within 1.2%. The measured equilibrium doses obtained were CTDI = 1.84×CTDI100 on the central axis and CTDI = 1.2×CTDI100 on the peripheral axis.


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