1. BME 560 Medical Imaging: X-ray, CT, and Nuclear Methods
X-ray Instrumentation Part 2

2. Today Anti-scatter devices X-ray screen-film systems
Other methods of X-ray detection

3. X-ray System Converts X-rays to light and records
Produces X-rays from electrical energy
Tailors X-ray spectrum
Source
Filter
Restrictor (Collimator)
Subject
Anti-scatter
Detector
Determines size and shape of beam
Selectively removes scattered photons

4. Total linear attenuation coefficient
At X-ray energies, most photons that interact in the patient are Compton-scattered.
Soft Tissue

5. X-ray Scatter Tube Object Grid Detector
Scattered radiation comes into detector from all directions.
Result is a relatively uniform background “fog” that reduces dynamic range of the detector available to image true signal.
Would like some way to reduce scattered radiation without blocking much direct radiation.

6. Anti-scatter Strategies
Collimation of the beam at the front end
Air gaps
Grids
Scanning Slits

7. Air Gap
Moving the patient away from the detector reduces the scatter reaching the detector.
Square-law
Solid angle
What price do we pay for this?

8. Anti-scatter Grids
Construct a device to collimate photons after they leave the patient.
Thin lead strips must be precisely aligned.
Performance depends on the grid ratio
What is the price paid for a high grid ratio?
Typical grid ratios: 5:1 to 16:1 (lower for mammography)

9. Anti-scatter Grids
A stationary grid will leave line artifacts in the image.
A Potter-Bucky diaphragm is a movable grid that basically blurs the grid lines during exposure.
The grid also blocks some primary radiation in the system.

10. Anti-scatter Grids
Tradeoff between scatter penetrating the grid and primary radiation detected

11. Anti-scatter Grids
Thickness of strips determines the likelihood of penetration.
Less scatter penetration = less primary radiation
Low-angle scatter may still get through.
Multiply-scattered photons may get through.

12. Anti-scatter Grids
Additional exposure is needed to maintain same detector exposure level when using grid.
Grid Conversion Factor
Typically 3 < GCF < 8
May avoid grid for small body parts or low energies.

13. Scanning Slits Moving source and collimator
Move source, collimator, and slit together.
Only takes one part of image at a time.
Very high scatter reduction
Slow
Moving slit
Stationary detector

14. X-ray Detectors
Film-Screen
Image Intensifiers
Panel Detectors

15. Film-Screen Detectors
Roentgen’s first X-rays exposed a photographic plate directly.
But photographic film has very low stopping power (a couple of percent).
To expose the film to its full dynamic range (contrast) would require high dose and most would be wasted.
Augment this with an intensifying screen that converts X-ray photons to visible light.

16. Screen-film System
Reflective Layer
Double emulsion film sandwiched between pair of intensifying screens
Phosphor particles (high Z) covert X-ray into light photons
Screen enhances contrast but lowers resolution
Engineering tradeoff: Phosphor thickness
Base
Film
Phosphor
Coating

17. Screen-film System Reflective layer reflects light back into the film
Base for mechanical support
Phosphor layer material choice:
More fluorescent than phosphorescent
High linear attenuation coefficient = stopping power
Conversion efficiency: total light energy per unit incident X-ray energy (usually 5 – 20%)
Energy dependent
Base
Film
Phosphor
Coating

18. Film
Very similar to photographic film; must be developed to fix the image
Two components:
Base: Plastic sheet, dimensionally stable (size and shape do not change under environmental and processing conditions)
Emulsion: Crystals of silver bromide suspended in gelatin substance; on one side (single-emulsion), or both sides (double-emulsion) of base.
Image is formed in the silver bromide crystals.

19. Screen-film System
Exposure to get a standard level of film density
The screen-film combination usually has a speed quoted
More sensitive (= fewer X-ray photons to result in a given image density) pairs have higher speed
At a particular energy!
Speed
Sensitivity (mR)
1200
0.1
800
0.16
400
0.32
200
0.64
100
1.28 50
2.56 25
5.0 12
10.0
From Sprawls

20. Radiographic Cassette
Ensures firm and uniform contact between intensifying screens and film sandwiched in between
Optical mirrors located outside screens to direct light towards film, maximize light conversion efficiency
Contains ID card and loaded only one way into X-ray machine
Image source: The Essential Physics of Medical Imaging

21. Film Density Density describes the overall blackness of the radiograph
Image source:

22. Screen-film System
Thicker screens result in higher sensitivity but increase image blur

23. Other Detection Schemes
Detection is a result of radiation interaction with matter. Radiation interaction results in emission of by products, e.g. electrons, electromagnetic radiation, that can be sensed by instrumentation and recorded by data acquisition systems
Gas-Filled Detectors
Scintillation Detectors
Flat-panel detectors
PSP plates
Solid State Detectors

24. Gas Filled Detectors
Radiation ionizes the gas. Charges freed by ionization produce a current.

25. Gas Filled Detectors
Radiation interacts with gas and ionizes its atoms
Freed electrons interact with gas and ionize more atoms - amplification
Ionization chamber: No amplification
Proportional counter: Amplification up to 106 times
Geiger-Muller counter: Very strong avalanche
Spatial sensitivity is lacking – Not used for imaging

26. Scintillation Detectors
Interaction of X-rays with some materials (CsI, cadmium zinc telluride - CZT) produces ‘scintillation’ or “flash of light”.
scintillator
Not capable of handling high photon flux.
Visible light
X-ray
Electrical pulse
photomulitplier
The pulse can tell you about the energy of the incident photon.

27. Photomultiplier Tube
Photomultiplier tube (PMT) converts light into electric current by photoelectric effect

28. Flat-panel Detectors Scintillator Light coupling
Light-sensitive digital detector (CCD array)
Varian Medical Systems

29. Photostimulable Phosphor Plates
PSP plates
X-rays excite electrons which are trapped in the material lattice.
Later, the plate is scanned by a laser in a “plate reader” which frees the electrons locally and digitizes the image.
The plate can be reused.
Plugs in to the film-screen cassette slot.

30. Solid State Detectors
They are compact semiconductors. Electrical conductivity of semiconductor is sensitive to impurities. The depletion layer is sensitive to radiation and electric current flow through, thus the measured current is a measure of radiation.

31. X-ray Image Detection Screen-film: Still in use
PSP Plates: Displacing screen-film in many applications
Flat-panel: Increasing use but expensive
Solid state: Still in development for X-ray
Scintillation detectors: Not fast enough for X-ray imaging, but still important research tools.
SPECT imaging
Gas counter: Not useful for imaging but used for active measurement of patient exposure.