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MED 107- MEDICAL PHYSICS II > 25. CT: IMAGE FORMATION > Flashcards

25. CT: IMAGE FORMATION Flashcards

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1
Q
  1. What is CT (Computed Tomography)?
A
  • it is a volumetric imaging modality
  • it is based on X-Ray Absorption
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2
Q
  1. When was CT introduced into medicine?
A
  • in the early 1970s
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3
Q
  1. What does CT allow for?
A
  • it allows for the reconstruction of a 2D or 3D absorber
    map
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4
Q
  1. In which sense it CT superior to X-Ray imaging?
A
  • the contrast in the soft tissue of the images is superior
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5
Q
  1. In which sense is X-Ray imaging superior to CT?
A
  • the spatial resolution of the images is superior
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6
Q
  1. Who invented the CT?
A
  • Sir Godfrey Hounsfield
  • he received the Nobel Prize in Medicine in 1979
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7
Q
  1. Where did the first clinical CT scan take place?
A
  • it took place in Atkinson Morley’s Hospital
  • in London, England
  • it took place on the 1st October 1971
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8
Q
  1. What essential role does the computer play in CT scans?
A
  • the computer is essential for Image Reconstruction
  • this is the first ever imaging modality to be this way
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9
Q
  1. What adjective can describe the rate at which Modern Clinical CT scanners work?
A
  • these scanners are very fast
  • they can produce a 2D cross-sectional image in less
    than one second
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10
Q
  1. How costly are CT scanners?
A
  • they are very expensive
  • this ranges into the millions
  • there is a relatively high cost per CT scan
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11
Q
  1. Why do Radiologists often take two perpendicular projections?
A

IN STANDARD X-RAY PROJECTION IMAGES:
- the exact location of an area of interest cannot be
determined

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12
Q
  1. Which are the names of the 2 perpendicular projections?
A
  1. Lateral
  2. Anterior-Posterior (AP)
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13
Q
  1. What does Data Acquisition refer to?
A
  • it refers to the collection of X-Ray transmission
    measurements
  • this is done through the patient
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14
Q
  1. What kind of source does Data Acquisition require?
A
  • it requires an X-Ray source
  • this X-Rays move in a parallel manner
  • they combine into the shape of a fan or a cone
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15
Q
  1. What does this image show?
A
  • a possible geometry for the CT scanner source and
    detectors

BOTH THE SOURCE AND THE ARC-SHAPED DETECTOR:
- rotate in a tandem
- they record the projections through a single plane
within the body
- this brings about many different angles

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16
Q
  1. What happens when an X-Ray beam passes through an object?
A
  • some of the photons are absorbed or scattered
17
Q
  1. What does the reduction of X-ray transmission (attenuation) depend on?

ATTENUATION: reduction of the effect or force

A
  1. THE ATOMIC COMPOSITION OF THE CROSSED
    TISSUES
  2. THE DENSITY OF THE CROSSED TISSUES
  3. THE ENERGY OF THE PHOTONS
18
Q
  1. What happens after an X-Ray beam passes through an object?
A
  • the partially attenuated X-Rays are collected
  • they are collected by the X-Ray detectors on the
    opposite side
19
Q
  1. What happens when these X-Rays are collected by the X-Ray detectors?
A
  • they are converted from X-Ray photons
    TO electrical signals
20
Q
  1. What happens to these electrical signals?
A
  • they are converted into digital data
  • the attenuation value is then calculated
  • these attenuations values are converted into different
    shades of grey
  • this results in contrast in the final image
21
Q
  1. What happens while the X-Ray tube and detectors are rotating around the patient?
A
  • many projections are being collected
  • they are being collected from consecutive angular
    orientations
22
Q
  1. What do these images show?
A
  • they show the Filtered Back
    Projection Method
  • this is the most common method to construct the
    image from the Data Acquisition
  • the forward projection
  • the detectors record these projections during the scan
  • there are 5 absorbing regions in the horizontal plane
  • grey colour indicated absorbtion
  • white indicated no absorption
23
Q
  1. What happens at the Forward Projection?
A
  • the detectors record these projections during the scan
  • there are 5 absorbing regions in the horizontal plane
  • the grey colour indicates that there is absorption
  • the white colour indicates that there is no absorption
24
Q
  1. What happens at the Back Projection?
A
  • we will measure the absorption projections
  • we measure each region that has overlaps
  • these numbers are translated into shades of grey
25
Q
  1. Explain the meaning of the different colour shades and number in this image.
A

HIGH NUMBERS OF ATTENUATION:
- translate as white
- they mean that the object is very dense
- it absorbs many of the photons
(high absorption)
- very few photons reach the detector

LOW NUMBERS OF ATTENUATION:
- translate as greys
- they mean that the object is not as dense
- it does not absorb many of the photons
(low absorption)
- many more photons reach the detector

26
Q
  1. What is the aim of a CT?
A
  • the aim is to obtain a spatially resolved map of
    absorption coefficients in one slice
  • this slice comes is of the patient’s body
27
Q
  1. What happens when a resolved map of absorption coefficients in samples at a finite resolution?
A
  • it provides an image
28
Q
  1. What is happening in this image?
A
  • we have obtained four different projections
  • we then determine the overall attenuation
29
Q
  1. What is happening in this image?
A
  1. WE HAVE 4 ATTENUATED BEAMS
    • I₁, I₂, I₃, I₄
  2. EACH PROJECTION FOLLOWS LAMBER BEER’S LAW
    - this allows us to obtain a linear equation system
    - this helps us to solve for µ₁, µ₂, µ₃, µ₄
  3. THE SIMPLE EQUATION SYSTEM CAN ONLY BE
    SOLVED:
    - when one of the Absorption Coefficient is known
30
Q
  1. What does this image show?
A

AN ARBITRARY OBJECT IS FORMED:
- it is composed of n-by-n
- there are also different materials that require n²
independent equations
- this is solved by taking more projections at different
angles

MED 107- MEDICAL PHYSICS II (108 decks)

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