Computed TomographyHistorical Perspective
MeaningTomography from the Greek word tomos meaning sectionEarly images lacked enough detail and clarity to be useful CT had to wait until the discovery of higher order mathematics and computersCT now overcomes limitations by using image computer reconstruction from projections to produce sharp, clear cross-sectional images
Conventional TomographyRadiograph obtained with a moving x-ray tubeResults in an image with superimposed tissues (CT does not)Tissues are blurred above and below the focal plane no improvement in spatial resolution
Image Reconstruction from Projections1917 Radon proved that a 2d or 3d model could be produced by collecting a large number of projections from different projectionsThis method is used in a variety of applications including astronomy and electron microscopyCormack developed reconstruction by back projection in the 1950s and 1960s
ProjectionsIn CT radiation passed through each cross-section in a specific way and is projected onto a detector that sends signals to a computer for processing. After processing a clear, sharp, digital image is produced.
Technical Definition Herman (1980) Image reconstruction from projections is the process of producing an image of a two dimensional distribution from estimates of its line integrals long a finite number of lines of known locations.
Image Reconstruction in Medicine Hounsfield (1967) applied reconstruction techniques to produce the worlds first useful CT scanner for imaging the brain.Hounsfields studies resulted in emission CT (nuclear medicine) and transmission CT (computed tomography).Hounsfields first machine used a gamma source Output too lowSource too largeImage reconstruction is also used in Ultrasound and MRI
Evolution of TermsHounsfield first coined the term computerized transverse axial scanning.Other terms include: Computerized transverse axial tomography, computerized transverse axial tomography, computer-assisted tomography, computerized axial tomography, computerized transaxial transmission reconstructive tomography.
TermsThe term Computed Tomography was established by the Radiological Society of North America in their major journal RadiologyAdditionally the American Journal of Roentgenology accepted this term.The term Computed Tomography is considered the correct term
ProcessData acquisitionImage reconstructionImage display, manipulation, storage, recording, and communication
Data AcquisitionData acquisition refers to the collection of x-ray transmission measurements from the patientPatient -> detector = transmission values/attenuation valuesInitial scanners took an inordinate amount of time to complete one slice
Image ReconstructionTransmission measurements are sent to a computerThe computer uses mathematical techniques to reconstruct the CT image in a finite number of steps called reconstruction algorithmsHounsfield developed an algorithm called the algebraic reconstruction technique
Image ReconstructionA variety of computers are integral to the reconstruction processComputer equipment includes array processor, minicomputer, and microprocessors
Image Display, Manipulation, Storage, Recording, and CommunicationsAfter reconstruction the images can be displayed, recorded, and analyzedTypically images are displayed on a cathode ray tube.Monitors allow a variety of individuals to view and manipulate the images
ManipulationMany computer packages allow images to be manipulated after the scan is complete (post-processing)Images can be reconstructed in a variety of planes, can be colored, and 3d models may be created.
StorageImages can be recorded and stored on a variety of archive mediaArchive media include radiographic film, mag tapes, optical disks, and cd-rom
CommunicationsCT scanners can be connected to a wide array of devices:Laser printers, diagnostic workstations, display monitors, and computers outside the hospital. Many different types of CT systems and equipment can communicate through a standard protocol called Digital Imaging and Communication in Medicine(dicom)
Brief History of CT1895 Roentgen discovers x-rays1917 Radon develops reconstruction mathematics1963 Cormack formulates x-ray absorption in tissue1972 Hounsfield demonstrates CT1974 Convolution and back projection
Brief History of CT1975 First whole body CT1976 Somatom scanner operating on fan beam principle, 5s scan with instant image reconstruction1978 Topogram1979 Hounsfield and Cormack Nobel Prize1981 512 squared matrix1983 High frequency generators & EBCT
Brief history of CT1984 Opti 155 CT tube with 1.75 MHU, 70cm gantry opening, +/- 25 degree tilt1986 Osteo CT (BMD), xenon CT (measures cerebral blood flow)1987 Continuous rotation flying focal spot CT tube1989 Spiral CT1991 Multislice CT introduced
Brief history of CT1991 intuitive mouse-driven Windows interface1992 Integrated CT angiography1994 Routine sub second spiral CT1996 Spiral 4 everything; neuro and high-resolution spiral1997 multi detector arrays; volume scans
Nobel Laureate Godfrey HounsfieldAssociated Press Monday, August 23, 2004; Page B05 Godfrey Hounsfield, 84, who developed the first practical CAT scan machine and shared a Nobel Prize in 1979 for inventing CAT scan technology, died Aug. 12 at a hospital in Kingston upon Thames, England. The cause of death was not reported.
Nobel Laureate Godfrey HounsfieldThe Nobel committee described Mr. Hounsfield, who worked at EMI laboratories' medical research division, as "the central figure in computer-assisted tomography." The device uses X-rays to scan from different angles and a computer to assemble the images into a cross section.
EMIElectric and Musical IndustriesNot only did EMI employ Hounsfield and market the first UK CAT Scanner, they also signed contracts with
Emission vs. TransmissionEmission CT involves nuclear medicine and Gamma-ray emission from the patientComputed Tomography utilizes x-ray transmission through a patient
Limitations of CTSpatial resolutionRelatively high patient doseZ-axis reformationDistinct artifacts
Advantages of CTBetter contrast resolutionNo superimposition of tissuesLess scatter radiation3D imagingBone mineral assay