C - T  Scout  Images

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Test exposures were made on 5 and 11 January 1994 to explore the use of C-T scout images for input of brachytherapy source locations to the treatment planning computer system. We used a 15 cm polystyrene cube calibration check phantom as a test object, with 10 cm lengths of teflon tube taped to the surface to determine magnifications.

It was apparent that the scout images were anamorphic projections since they were produced by an axial scan of the patient with a stationary source and detector array. In the axial direction the magnification is independent of depth in the object, while in the transverse direction the diverging rays from target to detector array produce a magnification that varies with distance from the source.

A source - axis distance of 61.3 cm was calculated from measurements of the image, with an equivalent source - film distance of 42 cm for the recording camera geometry used. The scales shown are identical and accurate for axial magnification. At a source to object distance of 65.3 cm the transverse scale is accurate; at other distances a different magnification would be needed.

For this Picker PQ2000 C-T scanner, with adjustments as found, we could position the patient with the region of interest 4 cm from the gantry axis away from the xray target, laterally and vertically, to produce orthogonal scout images with identical magnifications in axial and transverse directions. This would be appropriate for input to a computer program that assumes a single magnification throughout each film image; a program that models radiographic projection from the image points would be confused.

Posted 6 August 1998 by  [Glen Sandberg]