[photo-3d] Re: big mistake, 3D X-ray CT

[abram.klooswyk@xxxxxx]

Michael Galazin in his "big mistake" thread has made some other 
mistakes (15 Apr 2001).
>(...) was diagnosed with cancer almost two years ago 
>(the CT scan showed it quite clearly (...)).
(...)
>BTW, CT scans give a 3D view, sort of, in slices.

John A. Rupkalvis:
>Yes, and some very good stereoscopic images can be 
>generated on the computer using the z-axis information 
>that these scans give.

CT stands for (X-ray) Computed Tomography, and tomography is 
imaging of cuts. Standard CT images are flat, 2D, even more 
flat than you might think, for the 2D image is in fact a 
merging of data from a slice of the body in a picture without 
the thickness of that slice, squeezing it to zero thickness. 
The original body slices can be thinner than 1 mm, but many 
are more, up to 8 and 10 mm, but they are still shown with 
zero thickness in the pictures.

The computers have software to generate so-called 3D images 
from a consecutive series of such slices, but these 3D images 
are of the type you see everywhere as result of so-called 3D 
rendering CAD programs: perspective and shadowing give a depth 
effect, but the image _is_ 2D.

Only taking two of those images from slightly different angles 
(3 to 6 degrees) finally gives a true stereoscopic pair of 
images. But this last step is not routine in most hospitals, 
as stereo viewing means are often not available and free 
viewing is considered awkward.

The realism achieved by those images tend to convince some 
people (even doctors) that they are looking at reality.
However, despite computer manipulation, X-ray CT images are 
essentially only shadow images. When you make a rabbit on 
the wall from the shadows of your hands, some children might 
diagnose it as a real rabbit, but more sceptical children 
might ask for more proof.

The same holds for medical diagnoses "clearly" seen on CT 
scans. I have seen many so-called "false positive" diagnoses 
made on X-rays (CT and others). It is a big mistake to rely 
only on X-rays, 2D or 3D stereo, to make important decisions. 
"Treat the patient, not the X-ray" is the bottom line.

But stereo 3D CT images are sometimes helpful, and often
beautiful.

Abram Klooswyk 


 

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