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Advanced Biomedical Imaging

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Title: PowerPoint Presentation Author: Mr. Neo Last modified by: pc Created Date: 9/17/2007 6:11:55 PM Document presentation format: On-screen Show – PowerPoint PPT presentation

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Title: Advanced Biomedical Imaging


1
  • Advanced Biomedical Imaging
  • Lecture 4
  • Dr. Azza Helal
  • A. Prof. of Medical PhysicsFaculty of
    MedicineAlexandria University

2
Radiographic Image Quality
  • Image quality parameters (photographic
    properties)
  • Density
  • Contrast
  • Resolution (Image
    details)
  • Signals / Noise (Grainy
    image)
  • Lack of artifacts

3
1. Density
  • Overall blackening of the image.
  • If the image is too dark, it has too much
    density, overexposed.
  • If the image is too light, it lacks density,
  • underexposed.

4
2. Contrast
  • Difference between densities of adjacent
    structures
  • Controlled by energy of the beam.
  • µa1/E3
  • Depends on tissue and type of film
  • High contrast is too black white.
  • Low contrast is too gray.

5
Contrast Density
  • Contrast density are controlled by exposure
    factors and film processing.

6
X ray control
7
X-ray Exposure Factors
  • Settings controlled by the operator used to
    produce the radiograph.
  • Will influence the amount of patient exposure
  • Type of x-ray machine and screen film combination
    will impact the exposure factors.

8
Exposure factors
  • Increasing voltage (kV) increases maximal energy
    of x-rays produced, ? penetration of beam, film
    exposure, making the film darker, ? pt dose ?
    exp time but ? contrast. (obese pt)
  • Increasing mAs increases the amount of x-rays
    produced, exposure, and signals, making the film
    darker, ? exp time.
  • Film dose is a Kv4 mAS.

9
  • A small focal spot produces sharper images.
  • A large focal spot can tolerate more heat but
    increase image unsharpness. (see lecture 3).
  • Exp time ? by ? Kv large focal spot (?blurring)

10
mAs
  • Product of mA times the exposure time
  • 100 mA _at_ .10 seconds 10 mAs
  • 200 mA _at_ 0.05 seconds 10 mAs
  • As long as the product of the mA and time are the
    same, the exposure should be the same if the Ma
    and timer calibration is accurate.

11
  • mAs determines the quantity of x-ray photons.
  • mAs establishes the density on the image.
  • Density is the blackness of the image.
  • It takes a change of 25 to 30 in mAs to make a
    significant change in the density of the image.

12
Change the mAs and Change the Density of the Image
  • The center image has the mAs decreased by 25.
    Density is decreased.
  • Image on the right has the mAs increased by 25
    above the image on the left. Density is
    increased.

13
Wrong mAs
This image is overexposed for the patella. Too
much mAs was used density increased.
  • This image is under exposed.
  • Not enough mAs was used density reduced

14
30-50 Rule
  • Relatively large changes in mAs are needed to
    change the density of the image.
  • It takes a change of 20 to 30 to make any change
    in the density of the image.
  • It needs a change by 50 to make a significant
    change.
  • This is the 30-50 Rule

15
Changing mAs
  • As mAs increased
  • the density of the image changes from under
    exposed to over exposed.

16
Over exposed
  • This was taken with 60 mAs.
  • To correct the density of the image, 30 mAs would
    be used.
  • So mAs reduced by 50.

17
Under exposed
  • This image was taken at 30 mAs.
  • To darken the image, 60 mAs would be used.
  • To correct the density of the films double the
    mAs.

18
Grid Ratio and Frequency
  • The higher the ratio and frequency, the more
    radiation is needed to produce the image.
  • Low grid ratio is limited to low kVp. A 51 grid
    is limited to below 90 kVp.
  • High ratio grids can be used up to 125 kVp.

19
Grid Ratio and Frequency
  • Low frequency grids have prominent grid lines on
    image.
  • They are used in true Buckies.
  • The grid moves during the exposure to blur the
    lines.
  • High frequency grids produce almost invisible
    lines and do not need to move during the exposure.

20
Grid Ratio and Technique
  • For imaging of lumbar spine without the grid
  • 8 mAs _at_ 74 kVp used
  • 15-16 mAs _at_ 74 kVp used to produce the image with
    the 51 grid. (doubling mAs)
  • The conversion factor would be 2 for adjusting
    the mAs for the 51 grid

21
Grid Ratio and Technique
  • The 51 grid required doubling the mAs.
  • The conversion factor is determined by dividing
    the new mAs by the old mAs.
  • What would be the conversion factor for the 101
    grid? Old mAs 8 New mAs
    40

22
No Grid Used
  • Lumbar Spine needs the use of a grid.
    why?
  • There is too much scatter radiation due to the
    size of the body part
  • Exposure factors needed to be adjusted to
    visualize this area.

23
Low Ratio Frequency Grid
  • For the A-P Lumbar Spine, a 51 with 80 grid
    lines will remove much of the scatter radiation.
  • Going from Non-Grid to a 51 ratio requires
    doubling the mAs.
  • Grid lines are very noticeable.

24
High Ratio Frequency Grid
  • A 101 grid with 100 lines will provide excellent
    clean-up of scatter radiation.
  • Grid lines are nearly invisible.
  • But, requires 5 times more radiation.

25
Grid Cut-off
  • When high frequency and ratio focused grids are
    used,
  • the lead strips must be perfectly aligned with
    the primary beam.
  • A minor misalignment will result in the grid
    removing primary radiation.
  • Misalignment of more than 2 will result in grid
    cutoff.

26
Grid Cut-off
  • Is the density of the image of both knees the
    same?
  • This is an example of grid cut-off.
  • Some of the primary beam is being removed by the
    grid.

27
Grid Cut-off causes
  • If grid lines are not parallel to primary
    radiation due to
  • tube being too close
  • too far from the grid,

28
  • If the tube is angled against the grid lines.
  • If grid is not perpendicular to the beam (most
    common problem).

29
  • If grid is backwards, only center of beam will
    pass though the grid.
  • So proper alignment must be maintained.

30
Air-gap Vs Grids
  • Using air gap not grid,
  • Radiation exposure reduced five folds
  • With no loss of image quality.

31
3. Resolution
  • Resolution (image details) Minimum distance
    between two points that can be recognized as
    being separate.

32
4. Noise
  • Noise (grainy image) variation in no of x ray
    photons absorbed

33
5. Lack of artifacts
  • Artifact is the appearance of signal in an
    image location not representative of actual
    properties of object.

34
Questions
  • What are main parameters that control image
    quality?
  • Enumerate causes of grid cut-off?
  • Define contrast, density and artifact?

35
Thank you
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