[photo-3d] Re: X-ray stereos

[abram.klooswyk@xxxxxx]

Peter Abrahams:
>If I remember correctly, pseudo mounting causes the spiral to
> reverse -- but sometimes even that was hard to tell...

John Rupkalvis:
>I doubt that the direction of the spiral would be much of an 
>identifier, since your visual impression could be that of 
>looking at the spiral from either point of view. Sort of like 
>looking at an X-ray of a human head, where you cannot tell if 
>you are looking at it from the front or the back without an 
>identifier.

When Peter first posted his stereo X-rays of shells, one of 
them was at first "pseudo", as I wrote him at the time, which 
was clear because almost all shells have a right handed spiral 
direction.
For shells are _asymmetrical_ objects, mirroring makes them 
pseudo. John's example of a human head does not work, because 
heads are symmetric (most of them!). A better example would be 
a chest stereo X-ray, because totally mirrored anatomy in man 
(situs inversus) is probably as rare as in shells. Such a pair 
(in frontal projection) would be easily identified as ortho or 
pseudo.
In the same way a stereo x-ray of a left hand would appear as 
a right hand when the images are interchanged or reversed. 
It is in fact the same as holding a hand before a mirror.
(A funny book on this topic is Martin Gardner's "The 
Ambidextrous Universe", it touches upon stereochemistry, 
but not on stereoscopy, if memory serves.) 

Now I don't know how far we get off topic, 
>The further from the source, the less penetration.  Now 
>things get interesting.  The far side of an X-rayed object of 
>equal density will appear very slightly more dense than the 
>side closest to the source.

This not often of much help, but sometimes definition is.
Because details closer to the tube are slightly more magnified,
they also become slightly less sharp. But this only works when
object size is relatively large compared to tube-film distance.
This is sometimes the case in lateral skull x-rays.

Being off topic, a comment on  Ralph Johnston's friend who 
said on the flower stereo x-rays:
>"It requires very low energy (kilovoltage) X rays and high 
>resolution, fine grain industrial X ray film, and manual 
>processing.  It is just the opposite of what's done these days 
>in medical X ray where you want high energy, very rapid 
>exposures and fast film to reduce radiation exposure to the 
>patient. (...)"

This is not always true, and especially not when you compare 
it with flower X-rays, which is quite comparable to a special 
medical x-ray technique. 
Radiologist have a simple idea of the body (:-)) with three 
densities: there is bone, there is air (gas) and there are 
"soft tissues", which is everything else. For imaging air, 
that is mostly: lungs, the highest voltages are used, 140 
kiloVolt. For bone, say ribs, the voltage is lower, with 
the same chest projection say 80 kV is used.

But for the soft tissues it should be considerable less, the 
example is of course mammography, x-ray imaging of the breast, 
mostly of females (indeed, not always!), about 30 kV is used. 
Moreover, breast imaging requires the highest possible 
resolution, so fine grain films and a very small focus of 0.1 mm.
[>X-rays cannot be focused by a conventional lens.
Indeed, but the word "focus" is used here for the spot from where 
the x-rays come. Funny terminology, isn't it.]  

In fact dedicated mammography x-ray machines can be used 
for objects like flowers, onions, grapes...
I have also seen stereo x-rays of female breasts. This are 
cases of accidental stereo, for taking two x-rays of the same 
body part on the same day is not justified, but sometimes two 
x-rays one year apart form a reasonable stereopair.  

Abram Klooswyk 


 

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