CTCT

• Seeram: Chapter 1

Computed Tomography

- An Overview

GeorgeDavid

Early HistoryEarly History

• “tomostomos” Greek word meaning sectionsection

• Sectional imaging methods first Sectional imaging methods first developed in 1920’sdeveloped in 1920’s

Early History:Conventional Tomography

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 vs Axial Tomography

Conventional Cut

CT Axial Cut

Conventional Tomography BlurringConventional Tomography Blurring

• Image produced on film

• Objects above or below fulcrum plane change position on film & thus blur

CT ImageCT Image

• Not produced on film

• Mathematically reconstructed from many projection measurements of radiation intensity

• Digital Image calculated

AcmeMini-

Compu-ter

Digital Image

How Did We Go From…How Did We Go From…

The story concerns these men. What was their Link?The story concerns these

men. What was their Link?

Paul, Ringo, George, & John

Geoff

???

It Was the Late 1960’sIt Was the Late 1960’s

A lot of the money was going hereA lot of the money was going here

Follow the MoneyFollow the Money

Measure Intensity of a Pencil BeamMeasure Intensity of a Pencil Beam

X-Ray Source

Radiation Detector

CT ImageCT Image

• Measure a bunch of pencil beam intensities

CT ImageCT Image

• Now make measurements from every angle

CT ImageCT Image• When you get done, multiple pencil

beams have gone through every point in body

GeorgeDavid

Digital ImageDigital Image

• 2-dimensional array of individual image points calculated

• each point called a pixelpixel picture element

• each pixel has a value value represents x-ray transmission

(attenuation)

Digital Image MatrixDigital 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

GeorgeDavid

Numbers / Gray ShadesNumbers / Gray Shades

• Each number of a digital image corresponds to a gray shade for one pixelpixel

Image ReconstructionImage Reconstruction

AcmeMini-

Computer

Projection(raw)Data

Pixel(calculated)

Data

X-Ray Source

Radiation Detector

GeorgeDavid

Image ReconstructionImage Reconstruction

• CT math developed in 1910’s

• Other Applications astronomy (sun spot mapping) electron microscope imaging Nuclear medicine emission tomography MRI

CT HistoryCT History• First test images in 1967

• First clinical images ~ 1971

• First commercial scanner 1972

                                                                     

CT HistoryCT HistoryCT math developed in

1910’s

• First commercial scanner 1972

• What took so long?                                                                      

CT HistoryCT History

• CT made possible by high speed minicomputer

                                                                     

CT ComputersCT Computers• Old mainframe computers too expensive

& bulky to be dedicated to CT

GeorgeDavid

The 1st Computer BugThe 1st Computer Bug

CT history - Obsolete Terminology

CT history - Obsolete Terminology

• CTAT computerized transverse axial tomography

• CAT computerized axial tomography

• CTTRT computerized transaxial transmission

reconstructive tomography

• RT reconstructive tomography

GeorgeDavid

Data AcquisitionData Acquisition

• cross sectional image reconstructed from many straight line transmission measurements made in different directions

Tube

Detector

GeorgeDavid

Translate / RotateTranslate / Rotate

CT Early UnitsCT Early Units

• 4 minute scans• 5 minute reconstruction• 80 X 80 matrix• head only

water bag fit tightly around head Original computer software couldn’t

deal with transition from skull to air

Beam TranslationBeam Translation

• beam collimated to small round spot collimated at tube and collimator

X-ray Tube

Detector

Beam TranslationBeam Translation• Tube/detector translates left to right

• Entire assembly rotates 1o to right

• Tube/detector translates right to left

X-ray Tube

Detector

Translate - RotateTranslate - Rotate• 180 translations in alternate directions

• 1 degree rotational increments between translations

Projection MeasurementsProjection Measurements

• Radiation detector generates a voltage proportional to radiation intensity

Image ReconstructionImage Reconstruction• Minicomputer does its thing

Analog to Digital (A to D) conversion

Digital Image MatrixDigital 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

• Digital Matrix contains many numbers which may be Displayed on CRT Manipulated Stored

Digital Image ManipulationDigital Image Manipulation

• Window

• Level

• Smoothing

• Edge enhancement

• Slice reformatting

• 3D derived from multiple axial slices

Digital Image StorageDigital 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 - ImprovementsCT - 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 CT2nd Generation CT

• 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

2nd Generation CT2nd 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 CT3rd 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 CT3rd Generation CT

Patient

Z-axis orientation perpendicular to page

4th Generation CT4th 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) CT3rd & 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

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 CTSpiral 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 CTSpiral CT

Patient

Z-axis orientation perpendicular to page

Multi-slice CTMulti-slice CT

• Multiple rows of fan beam detectors

• Wider fan beam in axial direction

• Table moves much faster

• Substantially greater throughput

GeorgeDavid

Computer ImprovementsComputer 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)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)

GeorgeDavid

CT Patient DoseCT Patient Dose

• In theory only image plane exposed

• In reality adjacent slices get some exposure because x-ray beam diverges interslice scatter

GeorgeDavid

Dose ProtocolsDose Protocols

• Plain X-ray entrance skin exposure

• Mammography mean glandular dose

• CT Computer tomography dose index (CTDI) Multiple-scan average dose (MSAD)

GeorgeDavid

CT Dose depends onCT Dose depends on

• kVp

• mA

• time

• slice thickness

• filtration

• Noise

• detector efficiency

• collimation

• matrix resolution

• reconstruction algorithm

CT Patient DoseCT Patient Dose• Typically 2 - 4 rad

AAPM has a single slice protocol for measuring head & body doses

• More dose required to improve spatial resolution for same noise level smaller pixels need to increase does to get same dose /

pixel

• More dose required to improve noise for same spatial resolution

Resolution

Noise

Dose

GeorgeDavid

New StuffNew Stuff

• CT Angiography

• CT fluoroscopy

• CT virtual endoscopy / colonoscopy / ??scopy