ct seeram: chapter 1 computed tomography - an overview

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