Biomedical imaging X-rays L4

• Potential difference source: High voltage low current
• Tungsten heating source: Low voltage high current

Need a high and low voltage transformer

Rectifiers

• 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.

• Percentage difference between the maximum and minimum kVp values.
• 100 x (V_max-V_min) / V_max

• 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
• 

• 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)

• 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.
• 

• Larger laser spot sizes
• Spread of photostimulated luminescence
• Decay lag of the PSL during read out
• 

Bit depth

The laser scan speed and PSP translation speed

Trade-off between Resolution and sensitivity due to light scattering

dunno, something about this tho

PSP (photostimulable phosphor plates) have worse absorption efficiency than Gd₂O₂S

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

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

A numerical exposure indicator value is reported

• For ensuring full use of the dynamic range
• 

• Contrast processing: Enhances conspicuity of desirable features.
• Frequency processing: Enhances/diminishes features of specific spatial freq.

• 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

• 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.

• Cost: Pacs system and calibrated monitors are $$$
• Dose creep: No visual feedback on over-exposure
• Image quality: Lower resolution compared to film screen.

• An imaging modality that uses x-rays to obtain real-time videos
• 
• angiography/cardiology

• 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

• Input fluorescent screen (CsI)
• Photocathode
• Electrostatic focusing lens
• Accelerating anode
• Output fluorescent screen
• TV camera

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
• 

• Ramp up kV and reduce mA
• Lower dose but lower contrast

• Ramp up mA and reduce kV
• Higher dose but higher contrast

• Add 0.1mm Cu
• Reduces patient skin dose by cutting out lower energy photons

• 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.