Topic 9: x-ray detectors Flashcards
(19 cards)
What do image receptors determine?
- Contrast
- Noise level
- Spatial resolution
- Patients radiation dose
Advantages of film
- Good spatial resolution
- Good sensitivity
- Wide availability
- Low price
- Permanency for records
Structure of film
- Film base = transparent polyester/acetate
- Emulsion coating = suspension of silver bromide + gelatin = captures image
Describe how film works
- When film exposed = photons interact with grains = sensitized =latent image
- Sensitized grains retained + blackened by developing = regions turn opaque
- Unsensitized grains = removed = transparent base
- Forms radiographic image
How to increase sensitivity of film
- Film-screen combinations
- X-ray photons hit intensifying screen = emit visible photons
- Visible photons = hit + expose film emulsion
- Phospors in screen = absorb X-ray + re-emit energy as visible light
- Improve detection = X-ray film made with emulsion on both sides
- Entire system = encased in light-tight casette
Problems with film
- Only magnified no other enhancements to image quality
- Limited ability to distinguish subtle variations in X-ray absorption = restricts image contrast
How is film perceived?
- Extent of transparenncy + opaqueness = measure using optical density
Describe how a photomultiplier works
- Scintillator detector = uses crystal = absorbs X-ray = re-emits energy as visible light photons = enter photomultiplier tube
- Photons hit photocathode = converts light > photoelectrons via photoelectric effect
- Too few e- to produce electric signal
- Photoelectrons accelerated large voltage towards +ve electrode = dynode = overcome problem
- Collison e- + dynode = frees more e-
- Accelerated to next dynode = multiplying signal = large electrical signal produced from original X-ray
What are the indirect systems?
1) Computed radiography
2) Indirect digital radiography
- X-rays absorbed > light > electrical signal
What are direct systems?
1) Direct digital radiography
- No intermediate stage
- X-ray directly > electrical signal
Describe CR
- Most commom
- Produces digital radiographic images = imaging plates
- Similar to film screen
- Doesn’t need modifications to equipment
- Exposure > latent image > scanned laser beam > digitized in CR reader > sent to computer display
What is the resolution for CR?
- 100-200 μm
- spatial resolution = lower than fine/detailed film screen technology
Describe indirect digital radiography
1) Based on thin film transistor tech
2) Based on charged coupled device
- Both used = phosphors/scintillators = produce light when exposed to X-rays
- Difference is how light detected + converted
What is the resolution for IDR?
- 100-200 μm
Describe CCD array
- IDR consider placement of CCD array = sensitive to X-rays = interaction cause false signal
- Cannot be positioned = scintillator crystal
- Achieved by = fiber optic tapers + mirror/optical lens arrangement
Describe DDR
- Similar to ionisation chambers
- Incident radiation > sensitive volume > e- removed from orbits
- Creates +/- ions to carry charge > 1 electrode to next > respective electrodes create current
- Image acquisition system converts X-rays > electrical signal directly
Describe image intensifier detectors
- X-ray > input phosphor > light > amplified + focused onto output phosphor = intensified image
- Good sensitivity + real-time imaging
- Lower spatial resolution than flat panel detector
- Use = fluoroscopy systems = real-time imaging during procedures = angio + interventional radiology + surgery
Describe flat panel detectors
- Directly convert X-ray photons > electrical signals
- High spatial resolution + contrast = detailed + high quality images
- Use = digital radioscopy + fluoroscopy + cone beam computed tomography + mammography
- Used in static + dynamic imaging > diagnostic + interventional procedures
Compare CR + DR
CR:
- Greatest versatility
- Can be used with unmodified equipment
- Different cassete sizes
DR:
- Faster
- Better resolution
