Download CT Overview

CT
 Seeram: Chapter 1
Computed
Tomography
- An
Overview
Early History
 “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
CT Image
 Not produced on film
 Mathematically reconstructed from many
projection measurements of radiation intensity
 Digital Image calculated
Acme
MiniComputer
Digital Image
How Did We Go From…
The story concerns these men.
What was their Link?
???
Geoff
Paul, Ringo, George, & John
It Was the Late 1960’s
A lot of the money was going here
Follow the Money
Measure Intensity of a Pencil Beam
X-Ray
Source
Radiation
Detector
CT Image
 Measure a bunch of pencil beam intensities
CT Image
 Now make measurements from every angle
CT Image
 When you get done, multiple pencil
beams have gone through every point
in body
Image Reconstruction
X-Ray
Source
Acme
MiniComputer
Radiation
Detector
Projection
(raw)
Data
Pixel
(calculated)
Data
Digital Image
 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
311
111
182
222
176
199
192
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
Numbers / Gray Shades
 Each number of a digital image corresponds to
a gray shade for one pixel
Image Reconstruction
 CT math developed in 1910’s
 Other Applications
 astronomy (sun spot mapping)
 electron microscope imaging
 Nuclear medicine emission tomography
 MRI
CT History
 First test images in 1967
 First clinical images ~ 1971
 First commercial scanner 1972
CT History
 CT math developed in 1910’s
 First commercial scanner 1972
 What took so long?
CT History
 CT made possible by high speed minicomputer
CT Computers
 Old mainframe computers too expensive & bulky to be
dedicated to CT
The
st
1
Computer Bug
CT history - Obsolete
Terminology
 CTAT
 computerized transverse axial tomography
 CAT
 computerized axial tomography
 CTTRT
 computerized transaxial transmission
reconstructive tomography
 RT
 reconstructive tomography
Data Acquisition
 cross sectional image reconstructed from
many straight line transmission
measurements made in different
directions
Tube
Detector
Translate / Rotate
CT Early Units




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 1o to right
 Tube/detector translates right to left
X-ray
Tube
Detector
Translate - Rotate
 180 translations in alternate directions
 1 degree rotational increments between translations
Projection Measurements
 Radiation detector generates a voltage
proportional to radiation intensity
Image Reconstruction
 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
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
Digital Image Manipulation
 Window
 Level
 Smoothing
 Edge enhancement
 Slice reformatting
 3D
 derived from multiple axial slices
Digital Image Storage
 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
2nd Generation CT
 arc beam used instead of
Tube
pencil beam
 several detectors instead of
just one
 detectors intercepted arc
 radiation absorbent septa
between detectors


reduced scatter
acted like grid
Detectors
2nd Generation CT
 arc beam allowed 10 degree
rotational increments
 scan times reduced
 20 sec - 2 min
 2 slices obtained
simultaneously
 double row of detectors
10o
3rd Generation CT
 Wide angle fan beam
 rotational motion only /
no translation
 detectors rotate with tube
 30o beam
 Many more detectors
 scan times < 10 seconds
3rd Generation CT
Z-axis orientation
perpendicular to page
Patient
4th Generation CT
 Fixed annulus of detectors
 tube rotates (no translation) inside
stationary detector ring
 only a fraction of detectors active
at once
3rd & 4th 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
3rd / 4th 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
360o 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
Cine CT (Imatron)
 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)
CT Patient Dose
 In theory only image plane exposed
 In reality adjacent slices get some exposure
because
 x-ray beam diverges
 interslice scatter
Dose Protocols
 Plain X-ray
 entrance skin exposure
 Mammography
 mean glandular dose
 CT
 Computer tomography dose index (CTDI)
 Multiple-scan average dose (MSAD)
CT Dose depends on
 kVp
 mA
 time
 slice thickness
 filtration
• Noise
• detector
efficiency
• collimation
• matrix resolution
• reconstruction
algorithm
CT Patient Dose
 Typically 2 - 4 rad
 AAPM has single slice protocol for
measuring head & body doses
 More dose required at higher
Noise
resolution for same noise level
 More dose required to improve noise
at same spatial resolution
Resolution
Dose
Fundamental CT Tradeoff
To improve one requires
compromise on another
Noise
Resolution
Dose
New Stuff
 CT Angiography
 CT fluoroscopy
 CT virtual endoscopy / colonoscopy / ??scopy