CT Users Group

M Dunn, E K Inness

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

Abstract

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.

Method
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.

Results
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.

References

1. Dixon R L. A new look at CT dose measurement: Beyond CTDI. Medical Physics, 2003: 30: 1272-1280.
2. Nakonechny K D, Fallone B G and Rathee S. Novel methods of measuring single scan dose profiles and cumulative dose in CT. Medical Physics, 2004: 32: 98-109.
3. Mori S, Endo M and Nishizawa K. Enlarged longitudinal dose profiles in cone-beam CT and the need for modified dosimetry. Medical Physics, 2005: 32: 1061-1069.
4. Dixon R L. Restructuring CT dosimetry- A realistic strategy for the future requiem for the pencil chamber. Medical Physics, 2006: 33: 3973-3976.
5. Boone J M. The trouble with CTDI₁₀₀. Medical Physics, 2007: 34: 1364-1371
6. Dixon R L and Ballard A C. Experimental validation of a versatile system of CT dosimetry using a conventional ion chamber: Beyond CTDI₁₀₀. Medical Physics, 2007: 34:3399-3413.

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