Digital Radiography Flashcards
Digital radiography uses what type of systems
- Direct detectors
2. Cassetteless system
What is the organizational approach to digital radiography
- Capture element
- Coupling element
- Collection element
Capture element
Where the x-ray is captured
Coupling element
That which transfers the x-ray generated signal to the collection element.
Coupling means attaching
Three types of collection elements
Photodiode
Charge-Coupled Device
Thin Film Transistor
2 light sensitive devices that collect photons
Photodiode
Charge-coupled device
Charge sensitive device that collects electrons
Thin film transistor
CCD are used in
Military use
Astronomy
Digital photography
What is a charge-coupled device
Silicone-based semi conductor that converts visible light photons into electrons.
A highly sensitive light sensitive solid-state device.
3 advantages of charge coupled device
Sensitivity—- ability of CCD to detect and respond to low levels of visible light which promotes lower dose.
dynamic range- ability of CCD to respond to a wide range of light intensity,
Size is very small so it is well-suited for DR applications
Charge couple devices have linear radiation response therefore
Image contrast is unrelated to x-ray exposure.
At low x-ray exposure CCD response is greater than filmscreen. Resulting in lower patient dose.
Describe indirect DR Cesium Iodide/CCD
Charge coupled devices can be tiled to receive the light from an area x-ray beam as it interacts with a scintillation phosphor such as cesium iodide.
What is the benefit of cesium iodide
It has a high photoelectric capture; so x-ray interaction is high and patient dose is lowered.
What are the steps from x-ray beam in cesium iodide CCD indirect DR
X-ray interacts with cesium iodine a scintillation phosphor which gives off light and interacts with fiber optic bundles and Is sent down to charge couple device array.
What is the challenge with cesium Iodide CCD indirect DR.
Creating a seamless Image at the end of each CCD in the tiled arrangement (The cross areas don’t capture image)
Solution is interpolation of pixel values at each tile interface (The lines get airbrushed out by interpolation)
Cesium Iodide/amorphous Silicon (indirect DR)
An indirect DR process by which x-rays are converted first to light then to an electric signal.
Cesium Iodide/amorphous Silicon (indirect DR)
Uses cesium iodide scintillation phosphor to?
Capture x-ray
Cesium iodide is coated over an active matrix array of amorphous silicon photodiodes.
Cesium Iodide/amorphous Silicon (indirect DR)
From X-ray
X-rays interact with cesium iodine and produce light.
The light interacts with the amorphous silicon to produce a signal.
The signal is stored in the thin film transistors until readout.
Cesium Iodide/amorphous Silicon (indirect DR)
Image receptor
Image receptor is made into individual pixels with each having a light sensitive face of cesium iodide with a capacitor and a thin film transistor embedded.
Challenge of Cesium Iodide/amorphous Silicon (indirect DR)
A large portion of the face of the pixel is covered by electronic components that are not sensitive to light.
20% does not contribute to the image.
Cesium Iodide/amorphous Silicon (indirect DR)
The percentage of pixel face sensitivity to x-ray is known as the
Fill factor
Currently this is 80%
Dilemma for indirect DR
As pixel size decreases the spatial resolution increases but at the expense of patient dose. This because as pixel size decreases, the fill factor decreases and x-ray intensity must be increased to maintain signal strength.
One solution for Dilemma of indirect DR
One solution is to use gadolinium Oxysulfide instead of Cesium Iodide which can be made thicker without compromising spatial resolution.
Direct Capture DR
Amorphous Selenium
No phosphor coating is used.
