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Chapter 11

MEDRADSC 2Z03 Chapter Notes - Chapter 11: Godfrey Hounsfield, Cerebrospinal Fluid, Johann Radon

Medical Radiation Sciences
Course Code
Dawn Danko

of 4
CT History Summary
Limitation of standard radiographic technique
2 dimensional - Superposition of shadow
Film - poor medium to record different x-ray attenuation
Contrast resolution - reduction in strength of densities
a. Air - less dense
b. Fat - dark grey
c. Water/ soft tissue - only easy to tell on CT
d. Bone
e. Metal
Cerebral spinal fluid - provides nutrients as a cushion
Water in brain appear similar on a plane film image
Homogeneous shade of grey
Godfrey Hounsfield
based on Johann Radon's technique
Multiple projections:
o1d projection to 2d object
o2d projections to a 3d object
Reconstruct the object in space
The more projections taken, the more information
Advantages of x-rays --> can pass through the body, get content info
1. Lead box with a pinhole
oPinhole poked into the front
oRadioactive material Americium inside, provided a low-level constant x-
ray source
oRadiation came out in a pencil beam configuration
oAttenuation information would be recorded by the single detector on the
other side
oOnce they passed all the way through, rotate one degree
oScan all the way around 180˚
oOperated 9 days continuously, took a supercomputer 2.5 hours to
oSolving 28,000 simultaneous equations
2. Rotate and translate prototype
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oX-ray generator instead of radioactive material
oIncrease number of radiation beams available
oPlaced whole bed on lathe bed scanner
oRotate and translate
2. Filtered back projection
oImages created after each patch
oGet a series of grayscale
oReconstruct by passing the grayscale images back to the imaging plane
oAll the way around 180˚
First clinical scanner
oVery slow, couldn’t image chest, abdomen, develop the brain
oBite block for holding still
oSkull is dense, radio-opaque
Standard angiography
oPut a catheter into groin, up into vessels of the brain
oInject contrast and look at the vessels of the brain
oLook for shift, hyper-vascular tumour with a lot of blood supply
oIndirectly make assumption of the brain
oNeedle was placed into the back
oAir was introduced into needle
oPatient placed upright and air bubble up through the spine into the
ventricles (central portion)
oFilled with air
oExtrinsic contrast to see the inside (air) against the background
oPatient were strapped into chair and flipped in all different directions to
move air throughout the ventricles
o--> painful and nauseating
o--> throw up
First generation scanner
Just for the brain
Too slow, for things without motion
20 mins for ONE image
Rotate and translate mechanism
Pencil beam configuration moves across the brain, rotate one degree and across
Images could be sent to an off-site computer, information sent over a phone
line, image is reconstructed
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Sufficient for accurate clinical diagnosis
Second generation scanners
Rotate and translate mechanism
Fan-beam instead of pencil-beam
Multiple detectors instead of just one
Reduce the scanner time to a couple of minutes
Third generation scanners
Arc array of detectors instead of just multiple detector
Start in one position, rotate and scan 180˚, stop and move in, rotate in other
All equipment was tethered by a power supply and wires, and would be ripped
Helical Scanner
Exact same configuration as previous
Power supply is connected to a track, could continuously wind in one direction
CT gantry can wind in one direction non-stop
More and more detectors: 4 > 16 > 64> 128 >>512
Less than one minute
Gives stop motion of a moving structure (e.g. heart)
Maximize medical care
Eliminate unnecessary invasive procedures
oE.g. exploratory surgery
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