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
What kind of circuit is used to supply DC to xray tube?
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.
What is the definition of ripple?
- Percentage difference between the maximum and minimum kVp values.
- 100 x (Vmax-Vmin) / Vmax
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
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
- Photostimulable phosphor plates (PSP) are made of Barium fluorohalide doped with Europium (Eu)
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.
What decreases spatial resolution in the imaging plate?
- Larger laser spot sizes
- Spread of photostimulated luminescence
- Decay lag of the PSL during read out
What is contrast resolution of PSP plate affected by?
What determines spatial resolution for PSP?
The laser scan speed and PSP translation speed
What tradeoff exists when looking at composition and thickness of the PSP?
Trade-off between Resolution and sensitivity due to light scattering
What is the dynamic range
dunno, something about this tho
Why do Computed radiography (CR) systems requre a higher exposure than film/screens
PSP (photostimulable phosphor plates) have worse absorption efficiency than Gd2O2S
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
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
How does one report exposure in CR?
A numerical exposure indicator value is reported
What is histogram equalisation for?
- For ensuring full use of the dynamic range
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.
Where are digital images stored for X-rays
- PACS which stands for
- 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
Advantages of PACS?
- Images available soon with a reader - no film processing
- Images can be transmitted via pacs and viewed simultaneously at different locations
- Image qualityWider dynamic range than film screen
- Image processing, windowing or filtering is possible
- Can manipulate contrast/image quality to reduce dose.
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.
What the heck is fluoroscopy? What are its applications?
- An imaging modality that uses x-rays to obtain real-time videos
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.
- Main applications are angiography and cardiology
What are the main design components of an image intensifier
- Input fluorescent screen (CsI)
- Electrostatic focusing lens
- Accelerating anode
- Output fluorescent screen
- TV camera
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
How do you do a low dose for little time in fluoroscopy?
- Ramp up kV and reduce mA
- Lower dose but lower contrast
How do you do a high resolution for a short time in fluoroscopy?
- Ramp up mA and reduce kV
- Higher dose but higher contrast
What filtration technique can one use in fluoroscopy?
- Add 0.1mm Cu
- Reduces patient skin dose by cutting out lower energy photons
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.