Computed TomographyComputed Tomography

Historical PerspectiveHistorical Perspective

MeaningMeaning Tomography – from the Greek word Tomography – from the Greek word tomos tomos

meaning sectionmeaning section Early images lacked enough detail and clarity to Early images lacked enough detail and clarity to

be useful be useful CT had to wait until the discovery of higher order CT had to wait until the discovery of higher order

mathematics and computersmathematics and computers CT now overcomes limitations by using image CT now overcomes limitations by using image

computer reconstruction from projections to computer reconstruction from projections to produce sharp, clear cross-sectional imagesproduce sharp, clear cross-sectional images

Conventional TomographyConventional TomographyRadiograph obtained with a moving x-ray Radiograph obtained with a moving x-ray

tubetubeResults in an image with superimposed Results in an image with superimposed

tissues (CT does not)tissues (CT does not)Tissues are blurred above and below the Tissues are blurred above and below the

focal plane – no improvement in spatial focal plane – no improvement in spatial resolutionresolution

Image Reconstruction from Image Reconstruction from ProjectionsProjections

1917 – Radon proved that a 2d or 3d 1917 – Radon proved that a 2d or 3d model could be produced by collecting a model could be produced by collecting a large number of projections from different large number of projections from different projectionsprojections

This method is used in a variety of This method is used in a variety of applications including astronomy and applications including astronomy and electron microscopyelectron microscopy

Cormack developed reconstruction by Cormack developed reconstruction by back projection in the 1950’s and 1960’sback projection in the 1950’s and 1960’s

ProjectionsProjections In CT radiation passed through each In CT radiation passed through each

cross-section in a specific way and is cross-section in a specific way and is projected onto a detector that sends projected onto a detector that sends signals to a computer for processing. signals to a computer for processing. After processing a clear, sharp, digital After processing a clear, sharp, digital image is produced.image is produced.

Technical Definition Technical Definition Herman (1980) – Image reconstruction Herman (1980) – Image reconstruction

from projections is the process of from projections is the process of producing an image of a two dimensional producing an image of a two dimensional distribution from estimates of its line distribution from estimates of its line integrals long a finite number of lines of integrals long a finite number of lines of known locations.known locations.

Image Reconstruction in Image Reconstruction in Medicine Medicine

Hounsfield (1967) – applied reconstruction Hounsfield (1967) – applied reconstruction techniques to produce the worlds first useful CT techniques to produce the worlds first useful CT scanner for imaging the brain.scanner for imaging the brain.

Hounsfield’s studies resulted in emission CT Hounsfield’s studies resulted in emission CT (nuclear medicine) and transmission CT (nuclear medicine) and transmission CT (computed tomography).(computed tomography).

Hounsfields first machine used a gamma source Hounsfields first machine used a gamma source Output too lowOutput too low Source too largeSource too large

Image reconstruction is also used in Ultrasound Image reconstruction is also used in Ultrasound and MRIand MRI

Evolution of TermsEvolution of Terms Hounsfield first coined the term Hounsfield first coined the term

computerized transverse axial scanning.computerized transverse axial scanning. Other terms include: Computerized Other terms include: Computerized

transverse axial tomography, computerized transverse axial tomography, computerized transverse axial tomography, computer-transverse axial tomography, computer-assisted tomography, computerized axial assisted tomography, computerized axial tomography, computerized transaxial tomography, computerized transaxial transmission reconstructive tomography.transmission reconstructive tomography.

TermsTermsThe term Computed Tomography was The term Computed Tomography was

established by the Radiological Society of established by the Radiological Society of North America in their major journal North America in their major journal RadiologyRadiology

Additionally the American Journal of Additionally the American Journal of Roentgenology accepted this term.Roentgenology accepted this term.

The term Computed Tomography is The term Computed Tomography is considered the correct termconsidered the correct term

ProcessProcessData acquisitionData acquisition Image reconstructionImage reconstruction Image display, manipulation, storage, Image display, manipulation, storage,

recording, and communicationrecording, and communication

Data AcquisitionData AcquisitionData acquisition – refers to the collection Data acquisition – refers to the collection

of x-ray transmission measurements from of x-ray transmission measurements from the patientthe patient

Patient -> detector = transmission Patient -> detector = transmission values/attenuation valuesvalues/attenuation values

Initial scanners took an inordinate amount Initial scanners took an inordinate amount of time to complete one sliceof time to complete one slice

Image ReconstructionImage ReconstructionTransmission measurements are sent to a Transmission measurements are sent to a

computercomputerThe computer uses mathematical The computer uses mathematical

techniques to reconstruct the CT image in techniques to reconstruct the CT image in a finite number of steps called a finite number of steps called reconstruction algorithmsreconstruction algorithms

Hounsfield developed an algorithm called Hounsfield developed an algorithm called the algebraic reconstruction techniquethe algebraic reconstruction technique

Image ReconstructionImage ReconstructionA variety of computers are integral to the A variety of computers are integral to the

reconstruction processreconstruction processComputer equipment includes array Computer equipment includes array

processor, minicomputer, and processor, minicomputer, and microprocessorsmicroprocessors

Image Display, Manipulation, Image Display, Manipulation, Storage, Recording, and Storage, Recording, and

CommunicationsCommunicationsAfter reconstruction the images can be After reconstruction the images can be

displayed, recorded, and analyzeddisplayed, recorded, and analyzedTypically images are displayed on a Typically images are displayed on a

cathode ray tube.cathode ray tube.Monitors allow a variety of individuals to Monitors allow a variety of individuals to

view and manipulate the imagesview and manipulate the images

ManipulationManipulationMany computer packages allow images to Many computer packages allow images to

be manipulated after the scan is complete be manipulated after the scan is complete (post-processing)(post-processing)

Images can be reconstructed in a variety Images can be reconstructed in a variety of planes, can be colored, and 3d models of planes, can be colored, and 3d models may be created.may be created.

StorageStorage

Images can be recorded and stored on a variety Images can be recorded and stored on a variety of archive mediaof archive media

Archive media include radiographic film, mag Archive media include radiographic film, mag tapes, optical disks, and cd-romtapes, optical disks, and cd-rom

CommunicationsCommunicationsCT scanners can be connected to a wide CT scanners can be connected to a wide

array of devices:array of devices:Laser printers, diagnostic workstations, Laser printers, diagnostic workstations,

display monitors, and computers outside the display monitors, and computers outside the hospital. hospital.

Many different types of CT systems and Many different types of CT systems and equipment can communicate through a equipment can communicate through a standard protocol called Digital Imaging standard protocol called Digital Imaging and Communication in Medicine(dicom)and Communication in Medicine(dicom)

Brief History of CTBrief History of CT1895 – Roentgen discovers x-rays1895 – Roentgen discovers x-rays1917 – Radon develops reconstruction 1917 – Radon develops reconstruction

mathematicsmathematics1963 – Cormack formulates x-ray 1963 – Cormack formulates x-ray

absorption in tissueabsorption in tissue1972 – Hounsfield demonstrates CT1972 – Hounsfield demonstrates CT1974 – Convolution and back projection1974 – Convolution and back projection

Brief History of CTBrief History of CT 1975 – First whole body CT1975 – First whole body CT 1976 – Somatom scanner operating on fan 1976 – Somatom scanner operating on fan

beam principle, 5s scan with instant image beam principle, 5s scan with instant image reconstructionreconstruction

1978 – Topogram1978 – Topogram 1979 – Hounsfield and Cormack Nobel Prize1979 – Hounsfield and Cormack Nobel Prize 1981 – 512 squared matrix1981 – 512 squared matrix 1983 – High frequency generators & EBCT1983 – High frequency generators & EBCT

Brief history of CTBrief history of CT1984 – Opti 155 CT tube with 1.75 MHU, 1984 – Opti 155 CT tube with 1.75 MHU,

70cm gantry opening, +/- 25 degree tilt70cm gantry opening, +/- 25 degree tilt1986 – Osteo CT (BMD), xenon CT 1986 – Osteo CT (BMD), xenon CT

(measures cerebral blood flow)(measures cerebral blood flow)1987 – Continuous rotation flying focal 1987 – Continuous rotation flying focal

spot CT tubespot CT tube1989 – Spiral CT1989 – Spiral CT1991 – Multislice CT introduced 1991 – Multislice CT introduced

Brief history of CTBrief history of CT1991 – intuitive mouse-driven Windows 1991 – intuitive mouse-driven Windows

interfaceinterface1992 – Integrated CT angiography1992 – Integrated CT angiography1994 – Routine sub second spiral CT1994 – Routine sub second spiral CT1996 – Spiral “4” everything; neuro and 1996 – Spiral “4” everything; neuro and

high-resolution spiralhigh-resolution spiral1997 – multi detector arrays; volume 1997 – multi detector arrays; volume

scansscans

Nobel Laureate Nobel Laureate Godfrey HounsfieldGodfrey Hounsfield

Associated PressAssociated PressMonday, August 23, 2004; Page B05 Monday, August 23, 2004; Page B05

Godfrey Hounsfield, 84, who developed the Godfrey Hounsfield, 84, who developed the first practical CAT scan machine and first practical CAT scan machine and shared a Nobel Prize in 1979 for inventing shared a Nobel Prize in 1979 for inventing CAT scan technology, died Aug. 12 at a CAT scan technology, died Aug. 12 at a hospital in Kingston upon Thames, hospital in Kingston upon Thames, England. The cause of death was not England. The cause of death was not reported. reported.

Nobel Laureate Nobel Laureate Godfrey HounsfieldGodfrey Hounsfield

The Nobel committee described Mr. The Nobel committee described Mr. Hounsfield, who worked at EMI Hounsfield, who worked at EMI laboratories' medical research division, as laboratories' medical research division, as "the central figure in computer-assisted "the central figure in computer-assisted tomography." The device uses X-rays to tomography." The device uses X-rays to scan from different angles and a computer scan from different angles and a computer to assemble the images into a cross to assemble the images into a cross section. section.

EMIEMIElectric and Musical IndustriesElectric and Musical IndustriesNot only did EMI employ Hounsfield and Not only did EMI employ Hounsfield and

market the first UK “CAT” Scanner, they market the first UK “CAT” Scanner, they also signed contracts withalso signed contracts with

The BeatlesThe Beatles

Pink FloydPink Floyd

Queen Queen

Emission vs. TransmissionEmission vs. TransmissionEmission CT involves nuclear medicine Emission CT involves nuclear medicine

and Gamma-ray emission from the patientand Gamma-ray emission from the patientComputed Tomography utilizes x-ray Computed Tomography utilizes x-ray

transmission through a patienttransmission through a patient

Limitations of CTLimitations of CTSpatial resolutionSpatial resolutionRelatively high patient doseRelatively high patient doseZ-axis reformationZ-axis reformationDistinct artifactsDistinct artifacts

Advantages of CTAdvantages of CTBetter contrast resolutionBetter contrast resolutionNo superimposition of tissuesNo superimposition of tissuesLess scatter radiationLess scatter radiation3D imaging3D imagingBone mineral assayBone mineral assay