1. How to interpret x,y,z of localizer images
I had someone in our group ask me this question, but I couldn't give him
a good answer. Maybe someone who works with CT or other slice data can
answer it for me.
Part of the question is explaining to me what a cylindrical projection
is:
Imagine a volume positioned above a flat target. A point source
emitting a flat fan-shaped beam moves above the volume in a line
perpendicular to the plane of the fan. The resulting projection through
the volume onto the flat target is a cylindrical projection. In fact
the target is curved (I think) and it is the volume which moves, not the
source. But you get the idea.
Pixels in the localizer are the result of summing along rays in the
fan. The result is that only z locations in the localizer correspond to
z locations in the volume. x or y locations in the localizer (depending
on whether it is an AP or lateral localizer) cannot be attributed in a
simple fashion to x or y locations in the volume (that is, I assume, in
the CT slices).
So, assuming that the localizer and slices are defined using the same
DICOM frame of reference, how do we relate the x,y,z coordinates of the
localizer to those of the slices? Are there any corrections that need
to be made to the pixel data if the pixels are not quite rectangular (or
uniform in size)? Do all the CT vendors do this in a standard way, or
is this an exercise left to the reader?
Thanks in advance for your help.
Steve Moore (the Mallinckrodt one, not the other one)
Mallinckrodt Institute of Radiology
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