ct seeram: chapter 1 computed tomography - an overview
TRANSCRIPT
CT
Seeram: Chapter 1
Computed Tomography
- An Overview
Early History“tomostomos”
Greek word meaning sectionsectionSectional imaging methods first Sectional imaging methods first
developed in 1920’sdeveloped in 1920’s
Early History:Conventional Tomographyfirst used in 1935image produced on
filmImage plane oriented
parallel to filmAnatomy in plane of
fulcrum stays in focusanatomy outside of
fulcrum plane mechanically blurred
Conventional Cut
CT Axial Cut
Conventional Tomography Blurring
Image produced on filmObjects above or below
fulcrum plane change position on film & thus blur
CT ImageNot produced on filmMathematically reconstructed from many
projection measurements of radiation intensity
Digital Image calculated
AcmeMini-
Compu-ter
Digital Image
How Did We Go From…
The story concerns these men. What was their Link?
Paul, Ringo, George, & John
Geoff
???
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 ImageNow make measurements from every
angle
CT ImageWhen you get done, multiple
pencil beams have gone through every point in body
Image Reconstruction
AcmeMini-
Computer
Projection(raw)Data
Pixel(calculated)
Data
X-Ray Source
Radiation Detector
Digital Image2-dimensional array of individual
image points calculatedeach point called a pixelpixel
picture elementeach 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 ShadesEach number of a digital image
corresponds to a gray shade for one pixelpixel
Image ReconstructionCT math developed in 1910’sOther Applications
astronomy (sun spot mapping)electron microscope imagingNuclear medicine emission tomographyMRI
CT History
First test images in 1967First clinical images ~ 1971First commercial scanner 1972
CT History
CT math developed in 1910’s
First commercial scanner 1972
What took so long?
CT HistoryCT made possible by high speed minicomputer
CT ComputersOld mainframe computers too expensive & bulky
to be dedicated to CT
The 1st Computer Bug
CT history - Obsolete Terminology
CTATcomputerized transverse axial
tomographyCAT
computerized axial tomographyCTTRT
computerized transaxial transmission reconstructive tomography
RTreconstructive tomography
Data Acquisitioncross sectional image
reconstructed from many straight line transmission measurements made in different directions
Tube
Detector
CT Early Units
4 minute scans5 minute reconstruction80 X 80 matrixhead only
water bag fit tightly around head
Beam Translation
beam collimated to small round spotcollimated at tube and collimator
X-ray Tube
Detector
Beam TranslationTube/detector translates left to rightEntire assembly rotates 1o to rightTube/detector translates right to left
X-ray Tube
Detector
Translate - Rotate180 translations in alternate directions1 degree rotational increments between
translations
Projection Measurements
Radiation detector generates a voltage proportional to radiation intensity
Image ReconstructionMinicomputer does its thing
Analog to Digital (A to D) conversion
Digital Image MatrixDigital Matrix contains many
numbers which may beDisplayed on CRTManipulatedStored
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 ManipulationWindowLevelSmoothingEdge enhancementSlice reformatting3D
derived from multiple axial slices
Digital Image Storage
Magnetic DiskCDTapeOptical DiskPACS archive
picture archival and communications system not part of CT contains images from many modalities allows viewing on connected computers
CT - Improvementsall CT generations measure same
multi-line transmission intensities in many directions
ImprovementsProtocol for obtaining many line
transmissions# of line transmissions obtained
simultaneouslydetector locationOverall acquisition speed
2nd Generation CTarc beam used instead
of pencil beamseveral detectors
instead of just onedetectors intercepted
arcradiation absorbent
septa between detectors reduced scatter acted like grid
Tube
Detectors
2nd Generation CTarc beam allowed 10
degree rotational increments
scan times reduced20 sec - 2 min
2 slices obtained simultaneouslydouble row of
detectors
10o
3rd Generation CT
Wide angle fan beamrotational motion
only / no translationdetectors rotate with tube
30o beamMany more detectorsscan times < 10
seconds
3rd Generation CT
Patient
Z-axis orientation perpendicular to page
4th Generation CTFixed 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 thicknessRepeat
Tube rotates opposite direction
3rd / 4th Generation Image Quality Improvements
Faster scan timesreduces motion artifacts
Improved spatial resolutionImproved contrast resolutionIncreased tube heat capacity
less wait between scans / patientsbetter throughput
Spiral CTContinuous rotation of gantryPatient moves slowly through
gantrycables of old scanners allowed
only 360o rotation (or just a little more) tube had to stop and reverse directionno imaging done during this time
no delay between slicesdynamic studies now limited only by tube
heating considerations
Spiral CT
Patient
Z-axis orientation perpendicular to page
Multi-slice CTMultiple rows of fan beam detectorsWider fan beam in axial directionTable moves much fasterSubstantially greater throughput
Computer Improvements
Reconstruction timeAuto-printing protocolsImage manipulationBackup timeSlice reformatting3D reconstruction
And the ability to do it all
simultaneously
Cine CT (Imatron)four tungsten target
rings surround patientreplaces conventional x-ray tubeno moving partslike 4 moving focal spots
electron beam sweeps over each annular target ringcan 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 DoseIn theory only image plane exposedIn reality adjacent slices get some
exposure becausex-ray beam divergesinterslice scatter
Dose ProtocolsPlain X-ray
entrance skin exposureMammography
mean glandular doseCT
Computer tomography dose index (CTDI)Multiple-scan average dose (MSAD)
CT Dose depends onkVpmAtimeslice thicknessfiltration
• Noise
• detector efficiency
• collimation
• matrix resolution
• reconstruction algorithm
CT Patient Dose
Typically 2 - 4 radAAPM 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
Fundamental CT TradeoffFundamental CT Tradeoff
Resolution
Noise
Dose
To improve one requires compromise on another
New Stuff
CT AngiographyCT fluoroscopyCT virtual endoscopy / colonoscopy / ??scopy