First Year Definitions Flashcards
Speed class definition.
Refers to the speed at which the detail of the images are produced. 100 speed class is used with lower and upper limbs due to the need for increased detail of bones and joints. However, using the speed class requires 4x higher mAs. With 400 speed class the CR reader amplifies the signal by 4x but the SNR (signal noise ratio also increases) ratio also increases which is not ideal. The 400 speed class is used with anatomy that doesn’t require much detail which is also complying with the need for ALARA.
AEC chambers definition.
Table, grid, AEC, detectors and cassette. There are 3 AEC cells. Each cell is made up of 6x4 cm radiolucent plates x2. The plates are 1 cm separated by dielectric air. The cell is connected to a power supply then to a variable resistor then a capacitor which is connected to a switch. This switching system has a n ordinary timer or an auto timer. There is also a guard timer that tends to turn off after an exposure of 5s. The amount of current that flows though the cell is determined by the amount of ionisation occurring. Once the capacitor is charged to the preset level it triggers the switching system to terminate the exposure. Limitation is the AEC are that centring, collimating and chamber selection must be precise. The indicator zones are often the centre point. Secondly, the selection of the chamber needs to be under the good part of the anatomy. Thirdly, the AEC cannot distinguish between scatter the primary beam, therefore lead strips need to be used.
Anode heel effect definition.
The intensity of radiation emitted at the cathode end is greater than at the anode end. This is because of the greater attenuation at the anode end due to its angle as photons need to travel through more anode material before exiting. Greatest difference in intensity is on a short distance on a large surface area and depends on the angle of the anode.
Phosphostimulable plate
Storage phosphor plate doped with europium creating electron traps. Exposure of the phosphor plate results in electrons travelling to the trap. This has a linear relationship. The latent image is read using a red laser projecting on a polygonal rotating mirror that cause the electrons to lose energy in the form of cyan light and the electrons return to their resting positions. The cyan light is collected by an optical system which is then sent to a photomultiplier tube which converts the light to an analog signal. Adc converts the analog to a digital image. PSp exposed to a bright light and all electrons fall from the trap and the latent image is wiped out.
Exposure index definition
EI is the amount of exposure received on average by the IR. It is dependent on mAs, total detection area radiated and beam attenuation. The EI is indicative of the image quality. The appropriate ranges of EI is provided by the systems manufacturers. The EI has a linear relationship with mAs I.e. As the mAs doubles so does the EI. The limitation with EI is that when a metal is placed on the cassette along with the anatomy there is complete beam attenuation in that region. As a result the EI drops indicating an insufficient exposure to the anatomy, this may not necessarily be true. Finally, collimating contributes to the change in EI as decreasing or increasing the irradiated field will consequently increase or decrease the EI respectively. Flipping the cassette does produce an image but a diagnostically useless image.
