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Biotech / Medical : IMAT - ultrafast tomography for coronary artery disease
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To: john couch who wrote (2641)12/18/1998 4:50:00 PM
From: john couch  Read Replies (2) of 3725
 
Imatron X-ray detector arrangement.
Interest has been expressed in how the detectors are arranged in the
Imatron scanner. Here is how the detectors are deployed for coronary
calcium screening in the latest scanners. There is also a brief
discussion of the X-ray source and the creation of the image.

There are 1728 detectors arranged 0.125 degrees apart along a ring
which is 675 mm in radius around the patient. The detectors cover an
arc of 216 degrees, which is the minimum for reconstruction of a CT
picture. The minimum is more than 180 degrees because of a requirement
of reconstruction mathematics to cover 180 degrees plus the width of
the X-ray fan. The detector arc is a circle with a 144 degree gap below
the patient. Each channel of the detector is read 864 times during the
scan. There are about 700,000 X-ray measurements made per picture. Not
all detectors are active all the time.

The X-ray source moves 210 degrees around the patient on a radius of
900 mm. This is also the minimum arc so that X-rays pass at all angles
through the patient and hit all the detectors. The source starts above
the patient to one side, then it moves under and around the patient
until it is above the patient on the other side. The path is more than
half of a circle around the patient with a 150 degree gap at the top.

The X-ray source is created by a 650 ma beam of electrons with an
energy of 130 KeV. When the electrons strike a tungsten target X-rays
are emitted. The electron beam is moved along the target by the
changing magnetic field from deflection coils. In a mechanical scanner
the target and the electron beam, inside an X-ray tube, are mechanically
rotated around the patient.

In any one CT picture made for coronary calcium diagnosis all the
X-rays lie in a plane through the patient. The acquisition of the data
for this picture takes 100 milliseconds. After the data for one picture
is made, the patient is moved and another picture is made in a new
parallel plane. After the patient is moved, the scanner waits until it
receives a trigger from an ECG connected to the patient. This is so that
each slice is made at about the same point in the cardiac cycle.

In a CT scanner the image is not a form of shadow cast by the patient
onto a flat surface as in a film X-ray. The image is conceptually a
slice of the patient in which all the X-rays went edge-wise through the
plane of the imaged slice. The image exists only after considerable
computation on the original data. If you scan the trunk of a tree with
a scanner you see a picture of the circular growth rings of the wood,
which looks very much like a photograph of the top of a stump, although
the wood was not cut. This is the reason for the term "slice". You can
imagine that the X-rays passed edge-wise through only the wood that
would be turned to sawdust by the saw in an imaginary cross-cut of the
trunk.
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