Biomedical imaging X-rays L4

  1. What are the differences between the voltage and current characteristics of Potential difference sources and heat sources for tungsten? What two circuits are required as a result?
    • Potential difference source: High voltage low current
    • Tungsten heating source: Low voltage high current

    Need a high and low voltage transformer
  2. What kind of circuit is used to supply DC to xray tube?
  3. Explain how the medium frequency generator works
    • An AC current is full wave rectified and smoothed
    • A DC chopper chops the signal into a high frequency 5-100kHz
    • An inverter transforms the signal into AC
    • The signal is fed into a primary step up transformer which steps the voltage to kV (advantage being that the increase in voltage from a step up transformer is proportional to the frequency of the current fed into it so a smaller step-up transformer is required.
    • This is rectified and smoothed to provide the tube voltage.
  4. What is the definition of ripple?
    • Percentage difference between the maximum and minimum kVp values.
    • 100 x (Vmax-Vmin) / Vmax
  5. How does computed radiography work?
    • Imaging plate: X-rays produce electrons trapped in phosphor
    • Electromagnetic energy stored until processing

    • Plate reader: Cassette is placed into a reader
    • Laser beam releases stored energy
    • Light emissions are read by a photodiode scanning the imaging plate.

    • Data output: Digital image 
    • Film
    • Image Upload 1
  6. How does the imaging plate work in computed radiography in 3 steps? What are they made of?
    • 1. X-ray excites electron to conduction band and into a trap
    • 2. laser light gives electron energy to escape trap
    • 3. Electron falls to lower energy, emitting light which is detected
    • Image Upload 2
    • Photostimulable phosphor plates (PSP) are made of Barium fluorohalide doped with Europium (Eu)
  7. How does a plate reader work?
    • 1. Plate is translated through reader
    • 2. Laser light is emitted onto plate
    • 3. Electrons are stimulated, light released
    • 4. Photons guided through light guide
    • 5. Photons are detected by PMT
    • 6. Signals are amplified and processed.
    • Image Upload 3
  8. What decreases spatial resolution in the imaging plate?
    • Larger laser spot sizes
    • Spread of photostimulated luminescence
    • Decay lag of the PSL during read out
    • Image Upload 4
  9. What is contrast resolution of PSP plate affected by?
    Bit depth
  10. What determines spatial resolution for PSP?
    The laser scan speed and PSP translation speed
  11. What tradeoff exists when looking at composition and thickness of the PSP?
    Trade-off between Resolution and sensitivity due to light scattering
  12. What is the dynamic range
    dunno, something about this thoImage Upload 5
  13. Why do Computed radiography (CR) systems requre a higher exposure than film/screens
    PSP (photostimulable phosphor plates) have worse absorption efficiency than Gd2O2S
  14. What is detective quantum efficiency? What is it for CR compared to film screen
    The efficiency of an imaging system's transfer from input to output as percentage of signal to noise ratios.

    Depends on the efficiency of the detector and the noise associated with each process.

    Indication of the efficiency in detecting low contrast objects

    Although lower dose is required for CR, all the additional processing adds a lot of noise so DQE of CR is similar to film-screen
  15. Why does dose creep happen in CR?
    With CR, doses can creep up becasue theres no visual feedback on exposure level

    With film/screen, it was obvious when exposure level was incorrect due to the optical density of the film
  16. How does one report exposure in CR?
    A numerical exposure indicator value is reported
  17. What is histogram equalisation for?
    • For ensuring full use of the dynamic range
    • Image Upload 6
  18. What greyscale processing algorithms are there and what do they do?
    • Contrast processing: Enhances conspicuity of desirable features.
    • Frequency processing: Enhances/diminishes features of specific spatial freq.
  19. Where are digital images stored for X-rays
    • PACS which stands for
    • Picture
    • Archiving
    • Communication
    • System
    • It links all digital imaging modalities, the radiology info system, storage servers, printers, and reporting workstations
    • It allows for some sweet soft copy reporting and allows radiologist to compare against older images/images from other modalities
  20. Advantages of PACS?
    • Availability:
    • Images available soon with a reader - no film processing
    • Images can be transmitted via pacs and viewed simultaneously at different locations

    • Image quality
    • Wider dynamic range than film screen
    • Image processing, windowing or filtering is possible
    • Can manipulate contrast/image quality to reduce dose.
  21. What are the disadvantages of digital imaging
    • Cost: Pacs system and calibrated monitors are $$$
    • Dose creep: No visual feedback on over-exposure
    • Image quality: Lower resolution compared to film screen.
  22. What the heck is fluoroscopy? What are its applications?
    • An imaging modality that uses x-rays to obtain real-time videos
    • Image Upload 7
    • angiography/cardiology
  23. What is interventional radiology? What are its applications
    • Using fluoroscopic imaging to guid small instruments such as catheters through blood vessels or other pathways in the body.
    • Image Upload 8Main applications are angiography and cardiology
  24. What are the main design components of an image intensifier
    • Input fluorescent screen (CsI)
    • Photocathode
    • Electrostatic focusing lens
    • Accelerating anode
    • Output fluorescent screen
    • TV camera

    Image Upload 9
  25. What allows an image intensifier to do its job
    Thin input screens reduce sensitivity to x-rays

    • Thick input screens lower resolution as the light photons diverge
    • Caesium iodide (CsI) is used since it grows in needles; aiding total internal reflection, giving high efficiency and a lower drop in resolution
    • Image Upload 10
  26. How do you do a low dose for little time in fluoroscopy?
    • Ramp up kV and reduce mA
    • Lower dose but lower contrast
  27. How do you do a high resolution for a short time in fluoroscopy?
    • Ramp up mA and reduce kV
    • Higher dose but higher contrast
  28. What filtration technique can one use in fluoroscopy?
    • Add 0.1mm Cu
    • Reduces patient skin dose by cutting out lower energy photons
  29. How can one magnify in xrays
    • Virtual magnification: can just increase the size when you dont need extra detail
    • Actual magnification: Only use if you actually need the detail cos patient dose rises.
Card Set
Biomedical imaging X-rays L4