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First published online September 17, 2007
British Journal of Radiology (2007) 80, 835-840
© 2007 British Institute of Radiology
doi: 10.1259/bjr/41664978
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Accurate geometric calibration in stepping-table digital subtraction angiography

M A Schmidt, PhD 1 S L Nayak, DCR(R) 2 A-M Belli, FRCR 2 and A J Britten, PhD 1

Departments of 1 Medical Physics and 2 Radiology, St George's Hospital, London SW17 0QT, UK


Figure 1
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  		Figure 1. Diagram of the triangulation process leading to the definition of the calibration factor C as the ratio between the object displacement d(in millimeters) and the projected object displacement in the images D (in pixels). The object size (s) and the projected object size (S) are shown.

 

Figure 2
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  		Figure 2. Images of the grid of ball bearings(top) and test object SFS TO.M1 (bottom) placed on the patient couch. Correction of the pincushion image distortion associated with image intensifiers restores the known geometry of both test objects. Residual distortion is seen only at the outer edges.

 

Figure 3
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  		Figure 3. Central part of three images acquired within the same breath hold, prior to contrast agent injection, with a 2 cm table step. The black arrow points to the tip of the pigtail catheter and the white line indicates the position of the image profile used to estimate the projected catheter diameter by calculating the FWHM. Parallax changes surrounding the catheter tip position are noticeable in the background.

 

Figure 4
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  		Figure 4. Two examples of the registration process by cross-correlation in different subjects. The leftmost columns of both (a) and (b) show the image chosen as a reference (bottom) and the selected region of interest used for calculation (top), excluding most of the background area. The other two columns show the initial position that the user provided by clicking on the subsequent images (bottom row) and the result of the registration process (top row).

 




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