Medical Imaging Systems 2
Computed
Tomography
Lecture No.3-4

 “tomos”
 Greek word meaning section
 Sectional imaging methods first developed in
1920’s

Early History:
Conventional Tomography
 first used in 1935
 image produced on film
 Image plane oriented parallel to film
 Anatomy in plane of fulcrum stays in focus
 anatomy outside of fulcrum plane mechanically blurred

Conventional vs Axial
Tomography
Conventional Cut
CT Axial Cut

Conventional Tomography Blurring
 Image produced on film
 Objects above or below fulcrum plane change position on film & thus blur

 Not produced on film
 Mathematically reconstructed from many projection measurements of radiation intensity
 Digital Image calculated
Acme
Mini-
Computer
Digital Image

The story concerns these men.
What was their Link?
???
Geoff
Paul, Ringo, George, & John

A lot of the money was going here

Measure Intensity of a Pencil Beam
X-Ray
Source
Radiation
Detector

CT Image
 Measure a bunch of pencil beam intensities

 Now make measurements from every angle

 When you get done, multiple pencil beams have gone through every point in body

X-Ray
Source
Projection
(raw)
Data
Radiation
Detector
Acme
Mini-
Computer
Pixel
(calculated)
Data

 2-dimensional array of individual image points calculated
 each point called a pixel
 picture element
 each pixel has a value
 value represents x-ray transmission
(attenuation)

Digital Image Matrix
125 25
199 192
311 111 182 222 176
85 69 133 149 112
77 103 118 139 154 125 120
145 301 256 223 287 256 225
178 322 325 299 353 333 300

 Each number of a digital image corresponds to a gray shade for one pixel

 CT math developed in 1910’s
 Other Applications
 astronomy (sun spot mapping)
 electron microscope imaging
 Nuclear medicine emission tomography
 MRI

 First test images in 1967
 First clinical images ~ 1971
 First commercial scanner 1972

 CT math developed in 1910’s
 First commercial scanner 1972
 What took so long?

CT History
 CT made possible by high speed minicomputer

 Old mainframe computers too expensive & bulky to be dedicated to CT

st

CT history - Obsolete
Terminology
 CTAT
 computerized transverse axial tomography
 CAT
 computerized axial tomography
 CTTRT
 computerized transaxial transmission reconstructive tomography
 RT
 reconstructive tomography

 cross sectional image reconstructed from many straight line transmission measurements made in different directions
Tube
Detector





4 minute scans
5 minute reconstruction
80 X 80 matrix head only
 water bag fit tightly around head

Beam Translation
 beam collimated to small round spot
 collimated at tube and collimator
X-ray
Tube
Detector

Beam Translation
 Tube/detector translates left to right
 Entire assembly rotates 1 o to right
 Tube/detector translates right to left
X-ray
Tube
Detector

 180 translations in alternate directions
 1 degree rotational increments between translations

Projection Measurements
 Radiation detector generates a voltage proportional to radiation intensity

 Minicomputer does its thing
Analog to Digital
(A to D) conversion

Digital Image Matrix
 Digital Matrix contains many numbers which may be
 Displayed on CRT


Manipulated
Stored
125 25 311 111 182 222 176
199 192
77 103
85
118
69
139
133
154
149
125
112
120
145 301 256 223 287 256 225
178 322 325 299 353 333 300

Digital Image Manipulation
 Window
 Level
 Smoothing
 Edge enhancement
 Slice reformatting
 3D
 derived from multiple axial slices

 Magnetic Disk
 CD
 Tape
 Optical Disk
 PACS archive
 picture archival and communications system


 not part of CT contains images from many modalities allows viewing on connected computers

CT - Improvements

 all CT generations measure same multi-line transmission intensities in many directions
Improvements




Protocol for obtaining many line transmissions
# of line transmissions obtained simultaneously detector location
Overall acquisition speed

 arc beam used instead of pencil beam
 several detectors instead of just one
 detectors intercepted arc
 radiation absorbent septa between detectors

 reduced scatter acted like grid
Tube
Detectors

 arc beam allowed 10 degree rotational increments
 scan times reduced
 20 sec - 2 min
 2 slices obtained simultaneously
 double row of detectors
10 o

3rd Generation CT
 Wide angle fan beam
 rotational motion only / no translation
 detectors rotate with tube
 30 o beam
 Many more detectors
 scan times < 10 seconds

3rd Generation CT
Z-axis orientation perpendicular to page
Patient

 Fixed annulus of detectors
 tube rotates (no translation) inside stationary detector ring
 only a fraction of detectors active at once

3 rd & 4 th Generation (Non-spiral) CT
 Tube rotates once around patient
 Table stationary
 data for one slice collected
 Table increments one slice thickness
 Repeat
 Tube rotates opposite direction

3 rd / 4 th Generation Image
Quality Improvements
 Faster scan times
 reduces motion artifacts
 Improved spatial resolution
 Improved contrast resolution
 Increased tube heat capacity
 less wait between scans / patients
 better throughput

Spiral CT
 Continuous rotation of gantry
 Patient moves slowly through gantry
 cables of old scanners allowed only
360 o rotation (or just a little more)
 tube had to stop and reverse direction
 no imaging done during this time
 no delay between slices
 dynamic studies now limited only by tube heating considerations

Spiral CT
Z-axis orientation perpendicular to page
Patient

Multi-slice CT
 Multiple rows of fan beam detectors
 Wider fan beam in axial direction
 Table moves much faster
 Substantially greater throughput

Computer Improvements
 Reconstruction time
 Auto-printing protocols
 Image manipulation
 Backup time
 Slice reformatting
 3D reconstruction
And the ability to do it all simultaneously



 four tungsten target rings surround patient
 replaces conventional x-ray tube

 no moving parts like 4 moving focal spots electron beam sweeps over each annular target ring
 can be done at electronic speeds
2 detector rings
 2 slices detected
 maximum scan rate
 24 frames per second

Imatron Cine CT
(scanned from Medical Imaging Physics, Hendee)

 In theory only image plane exposed
 In reality adjacent slices get some exposure because
 x-ray beam diverges
 interslice scatter

 Plain X-ray
 entrance skin exposure
 Mammography
 mean glandular dose
 CT
 Computer tomography dose index (CTDI)
 Multiple-scan average dose (MSAD)

 kVp
 mA
 time
 slice thickness
 filtration
• Noise
• detector efficiency
• collimation
• matrix resolution
• reconstruction algorithm

 Typically 2 - 4 rad
 AAPM has single slice protocol for measuring head & body doses
 More dose required at higher resolution for same noise level
 More dose required to improve noise at same spatial resolution
Resolution
Noise
Dose

To improve one requires compromise on another
Noise
Resolution
Dose

New Stuff
 CT Angiography
 CT fluoroscopy
 CT virtual endoscopy / colonoscopy / ??scopy
http://www.iacionline.net/ScannerCTDemo/CTScannerSimulator.html