Transcript RAD 354 Chapt. 23 Multi

RAD 354 Chapt. 23 Multi-slice CT
• In short – CT is a thin band (fan) of radiation
directed toward the pt. and the remnant
radiation emitted from the pt. is measured by
a receptor and the response transmitted to a
computer for reconstruction of the image(s)
“Slip rings”
Dates/Activities to Remember
• 1970 – Sir Godfrey Hounsfield,Ph.D..,
demonstrated the technique of CT scanning –
“EMI” head unit
Dates/Activities Con’t.
• 1982 Dr. Hounsfield and Dr. Alan Cormack
share the Nobel Peace Prize in physics
• CT Generations and important items:
– 1st – translate-rotate; “pencil beam” - one full
rotation for 1 image; head only; 5 min. per image!
– 2nd – translate-rotate, but with a “fan” beam;
produced INCREASED INTENSITY at the edge of
the beam – “bow-tie” filter used to remedy this;
30 sec. image time
Generations con’t.
– 3rd – used ARRAY of DETECTORS – produced “ring
artifacts”
– 4th rotate – stationary, fan beam, detector array;
short image time and NO re-con time!
– 5th is NOW here! “Spiral/slip ring” (It is slip ring
technology that has enabled this latest
generation!)
Major Components of CT
• Gantry – tube-receptor array, collimators (prept. and pre-detector), generator, pt. couch
• Computer – microprocessor – recon time was
a BIG issue until recently
• Operating console – usually two monitors –
one for RIS/HIS and the second for the
imaging portion (often times a “remote”
monitor is set up at the radiologist’s desk –
can really be a hassle)
Radiation Dose Comparisons
Current – 5th Generation Terms
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Spiral
Helical
16/32/64 slice
Slip Rings CT
Interpolation/Extrapolation
• Interpolation – information ALONG the axis
WITHIN established values
• Extrapolation – information OUTSIDE axis
values established, BUT an ESTIMATED value
based upon previously measured parameters
IF they continue
“Spiral” Terms
• Pitch – relationship between couch movement and xray beam collimation (pitch ratio)
– Increasing pitch ratio enables much MORE of the pt. to be
imaged in ONE BREATH HOLD! Breath holds =
misrepresentation of the images.
• Slip ring technology made “spiral CT” possible
• Three slip rings in the gantry (usually)
– One for hi voltage power to the tube and hi voltage
generator
– Low voltage power to the rotating gantry portion
– Transfer of digital data from gantry receptors to the
computer
Multi Slice CT
• Multi detector arrays
– Two at once with one beam = two slices per 360
degrees
– With “spiral” enables two detectors to produce 16
slices
• Envision a person on the table/couch in a
screw/nut fashion going through the gantry
Spiral/slip ring
Tube (fan beam)/receptors
Z-axis
Maximum Intensity Projection (MIP)
• Pre selecting pixel levels to be viewed AFTER
obtaining a “rubic’s cube of information”
– Then only exhibit those at the PRE-DETERMINED
value – like an MRA or CTA
Spiral Advantages of Conventional CT
• Motion blur is reduced – fewer artifacts
• Lowered imaging time (fewer breath holds
required = less motion and less misrepresentation)
• Larger volumes can be imaged with LESS part
overlap artifacts