Download Computed Tomography

Computed Tomography
Historical Perspective
Meaning
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Tomography – from the Greek word tomos
meaning section
Early images lacked enough detail and clarity to
be useful
CT had to wait until the discovery of higher order
mathematics and computers
CT now overcomes limitations by using image
computer reconstruction from projections to
produce sharp, clear cross-sectional images
Conventional Tomography
Radiograph obtained with a moving x-ray
tube
 Results in an image with superimposed
tissues (CT does not)
 Tissues are blurred above and below the
focal plane – no improvement in spatial
resolution
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Image Reconstruction from
Projections
1917 – Radon proved that a 2d or 3d
model could be produced by collecting a
large number of projections from different
projections
 This method is used in a variety of
applications including astronomy and
electron microscopy
 Cormack developed reconstruction by
back projection in the 1950’s and 1960’s
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Projections
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In 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
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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
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Hounsfield (1967) – applied reconstruction
techniques to produce the worlds first useful CT
scanner for imaging the brain.
Hounsfield’s studies resulted in emission CT
(nuclear medicine) and transmission CT
(computed tomography).
Hounsfields first machine used a gamma source
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Output too low
Source too large
Image reconstruction is also used in Ultrasound
and MRI
Evolution of Terms
Hounsfield 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.
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Terms
The term Computed Tomography was
established by the Radiological Society of
North America in their major journal
Radiology
 Additionally the American Journal of
Roentgenology accepted this term.
 The term Computed Tomography is
considered the correct term
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Process
Data acquisition
 Image reconstruction
 Image display, manipulation, storage,
recording, and communication
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Data Acquisition
Data acquisition – refers to the collection
of x-ray transmission measurements from
the patient
 Patient -> detector = transmission
values/attenuation values
 Initial scanners took an inordinate amount
of time to complete one slice
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Image Reconstruction
Transmission measurements are sent to a
computer
 The computer uses mathematical
techniques to reconstruct the CT image in
a finite number of steps called
reconstruction algorithms
 Hounsfield developed an algorithm called
the algebraic reconstruction technique
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Image Reconstruction
A variety of computers are integral to the
reconstruction process
 Computer equipment includes array
processor, minicomputer, and
microprocessors
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Image Display, Manipulation,
Storage, Recording, and
Communications
After reconstruction the images can be
displayed, recorded, and analyzed
 Typically images are displayed on a
cathode ray tube.
 Monitors allow a variety of individuals to
view and manipulate the images
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Manipulation
Many 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.
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Storage
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Images can be recorded and stored on a variety
of archive media
Archive media include radiographic film, mag
tapes, optical disks, and cd-rom
Communications
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CT scanners can be connected to a wide
array of devices:
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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 CT
1895 – Roentgen discovers x-rays
 1917 – Radon develops reconstruction
mathematics
 1963 – Cormack formulates x-ray
absorption in tissue
 1972 – Hounsfield demonstrates CT
 1974 – Convolution and back projection
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Brief History of CT
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1975 – First whole body CT
1976 – Somatom scanner operating on fan
beam principle, 5s scan with instant image
reconstruction
1978 – Topogram
1979 – Hounsfield and Cormack Nobel Prize
1981 – 512 squared matrix
1983 – High frequency generators & EBCT
Brief history of CT
1984 – Opti 155 CT tube with 1.75 MHU,
70cm gantry opening, +/- 25 degree tilt
 1986 – Osteo CT (BMD), xenon CT
(measures cerebral blood flow)
 1987 – Continuous rotation flying focal
spot CT tube
 1989 – Spiral CT
 1991 – Multislice CT introduced
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Brief history of CT
1991 – intuitive mouse-driven Windows
interface
 1992 – Integrated CT angiography
 1994 – Routine sub second spiral CT
 1996 – Spiral “4” everything; neuro and
high-resolution spiral
 1997 – multi detector arrays; volume
scans
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Nobel Laureate
Godfrey Hounsfield
Associated 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 Hounsfield
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The 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.
EMI
Electric and Musical Industries
 Not only did EMI employ Hounsfield and
market the first UK “CAT” Scanner, they
also signed contracts with
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The Beatles
Pink Floyd
Queen
Emission vs. Transmission
Emission CT involves nuclear medicine
and Gamma-ray emission from the patient
 Computed Tomography utilizes x-ray
transmission through a patient
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Limitations of CT
Spatial resolution
 Relatively high patient dose
 Z-axis reformation
 Distinct artifacts
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Advantages of CT
Better contrast resolution
 No superimposition of tissues
 Less scatter radiation
 3D imaging
 Bone mineral assay
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