Image detectors - computed and direct radiography Flashcards
What is the main difference between computed and direct radiography?
Computed radiography uses a cassette which needs to be processed seperately and direct radiography uses captures the image directly onto a flat pannel detector.
What are the 3 elements that allow an image to be detected.
capture, coupling and collection element
In CR, what material is the screen made from and how are they aligned?
Made up of a photostimulable phosphor material (barium fluoride). They mostly appear to be white as they scatter light. They can be arranged randomly, or in a linear filaments (like a bunch of needles stacked together).
How does CR collect an image?
- The x-rays interact witht with the PSP on the surface of the plate. The electrons get excited and move to the conduction band. As they return to the ground state, they release energy in the form of light.
- This is captured by the plate, which is fed through a reader and a processing unit.
- The brightness of the light is converted into digital values for each pixel and stored in the computer memory as an image.
What is the light produced on the cr plate known as?
photostimulable luminescence
What is an image artefact?
A false visual feature on a medical image that acts as or obscures a tissue.
List 3 benefits of a CR plate.
- Has a lead backing - which reduces back scatter
- Cost effecient as it can be reused up to 10,000 times
- It does not require a dark room to process the images
What are the various types of direct radiography with direct conversions?
SPR and flat pannel detector
What is SPR and how does it work?
Scanned projection radiography.
In this, the patient is driven through a CT gantry (which is fixed and does not rotate). Therefore, the beam needs to be collimated to reduce patient dose.
What is the most commonly used direct radiography method with direct conversions?
A flat pannel detector.
It consists of various digital elements in which there is a build up of charge during x-ray exposure. When the exposure has ended, the charge is held in place by a capacitator in each element. These charges are then read.
How is the charge generated in a flat pannel detector?
When the x-ray interacts, the electrons in the valence band gain energy and travel to the conduction band. Here they become mobile due to the free space and create charge, which is collected and stored by the capacitor.
What are the types of indirect conversions in direct radiography?
Scintillator CCD & CMOS, Scint. TFT
What is the main disadvantage of indirect conversion direct radiography?
They have a spreading peak in the scintillator material which leads to low resolution, whereas direct conversions have a sharp peak.
How does a Scintillator CCD produce an image?
CCD - charge coupled device
When there is an exposure, the x-rays interact with the scintillator phosphor (ceasium iodide) and produces light.
The coupling element is a a fibre optic bundle that transmit the light to either the CCD array or CMOS.
What is CMOS and how is it different from CCD?
CMOS - complementary metal oxide semiconductor.
Where cmos has amplifiers in each pixel to amplify charges and move them through wires, ccd moves charge from one pixel to the other. They travel down the array and along the row to the end where they are read.
