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2. THE CT SCAN SYSTEM 
The CT scan produces two types of images: 
(i) a series of CT scan slices at known intervals through the body. The system allows for, discrete and selected slices, 
so that it is possible to obtain a slice on which a single ball bearing is imaged. The slice is a true map of the body 
segment being scanned. X and Y coordinates are determined from the slice, with the table position giving the Z 
coordinate. The accuracy of the CT scan slice coordinate system is dependent on two factors - the slice thickness, with 
a minimum thickness of 2mm not being uncommon, and a measuring resolution of 1mm on the actual slices. This 
therefore restricts the accuracy of the CT slice coordinate system to approximately 1.5mm. 
  
Figure 1: An AP surview (left) showing four ball bearings marking the three leg points and the entry point 
and a CT scan slice (right) through a single ball bearing 
(ii) two "upright", usually perpendicular, AP and LAT surviews - somewhat analogous to X-rays produced in ordinary 
biplanar radiography - are normally used for planning the slice positions. The geometric property of X-rays - the central 
projection in which all rays pass through a perspective centre (Hallert 1970) - only partly applies to the surview as the 
central projection only applies in the horizontal axis, whereas the vertical axis is linearly mapped. This occurs as the 
scanner only operates in a single plane orthogonal to the CT scan table and a surview is "compiled" by moving the 
table through this plane. All the horizontal sections, conforming to the central projection, are "stacked" together to form 
the surview. 
LAT Surview 
      
  
L P = Point of Intersection 
f = Principal Distance 
pp = Principal Point 
  
AP Surview L = Perspective Centre 
of LAT Surview 
i / A = Perspective Centre 
of AP Surview 
Figure 2 : Scanner operating in single orthogonal plane to the CT scan table, the Z axis is orthogonal to the page. 
IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop “From Pixels to Sequences”, Zurich, March 22-24 1995