78 Australasian Physical & Engineering Sciences in Medicine Volume 25 Number 2, 2002 TECHNICAL NOTE Verification of CT number to density conversion for a simulator-CT attachment A. Chapman 1,2 , M. Butson 2 , K. Quach 1,2 , A. Rozenfeld 1 , P. Metcalfe 2 1 Physics Department, University of Wollongong, Wollongong, NSW 2 Department of Medical Physics, Illawarra Cancer Care Centre, Wollongong, NSW Abstract The calculation and verification of a CT number to density conversion table for a simulator-CT attachment known as Scanvision, which provides CT images for radiotherapy treatment planning, is presented. While the linear fit for CT-number-to-density is similar to most conventional doughnut gantry CT scanners, an offset of approximately 178 Hounsfield units was found for air using a polyethylene normalisation, and approximately 262 for air using air normalisation. The offset continues for other low-density samples. Results show that the simulator-CT reproducibly measures CT numbers. However a separate calibration line needs to be entered into the radiotherapy planning computer to ensure accurate CT-number-to-density conversion. Key words simulator-CT, radiotherapy Introduction Computed tomography (CT) is a technique used to create images of an object by rotating a narrow x-ray beam around its circumference, and measuring the subsequent attenuation. Computer software is used to reconstruct a cross-sectional slice image from the resulting data. Multiple slices can be used to form a three-dimensional image based on electron densities. The images obtained of a patient are used in radiotherapy to plan the treatment of the patient, and to check the set-up and target coverage of the plan before treatment on a linear accelerator. Hounsfield units (HU) (or CT numbers) can be theoretically related to attenuation coefficients 1 , which in turn can be related to cross sections and electron densities 2 . Routinely, however, CT numbers are related to electron densities via a linear function, obtained experimentally, which places the relative electron density of water at the zero point. It is often more convenient in practice to add 1000 HU to this number to create usable CT numbers in order to bring the relative electron density of air to the zero point, thus avoiding negative values for computer calculations. The CT numbers can be compared with the CT numbers (and therefore electron densities) of known Corresponding author: Alison Chapman, Department of Medical Physics, PO Box 1798, Wollongong NSW 2500 Australia, Fax: 61242 265397, Received: 3 October, 2000; Accepted: 8 May, 2002 materials, so that the composition of the body can be approximated. The relationship between CT number and relative electron density is shown below in equations (1) and (2). Although diagnostic CT scanners have the advantage of fast (and therefore the ability for multiple) and accurate scans, they do not account for beam divergence, so simulator beam’s eye view (BEV) software must be included to allow checks of the planned beam coverage, in which case the CT scanner is called a CT-simulator, and the software is called “virtual simulation” software. Because of the speed of the scans performed by the CT-simulators, the gantry is enclosed in a ‘doughnut’ shape, which limits table movement, and can limit the positioning of the patient; for example, during breast treatment the way the arm must be held away from the body, but this is not always possible on the CT-simulator. The clearance diameter on the Marconi ACQSIM CT large-bore scanner is 85 cm. The alternative to a CT-simulator is a device that attaches to a conventional x-ray simulator (which provides an x-ray BEV of the treatment area), and collimates the x- ray simulator’s beam and image intensifier. This is called a simulator-CT. The gantry, being unenclosed, has much more freedom of movement, and can be rotated around the patient to obtain a CT image. The clearance diameter is 100 cm from the end of the collimator to the image intensifier in extended mode. The simulator-CT does not produce scans with the same quality as the CT-simulator, but the images are sufficient for radiotherapy treatment planning. The purpose of this investigation was to use a Varian simulator-CT to measure the CT numbers of various body analogues in different scanner set-ups. This information was used to create a CT-to-density conversion table for use