LECTURE 1: X-RAY IMAGING SYSTEMS 1 Flashcards
Are imaging systems serial or parallel type?
Serial - if one part is not working, the rest of the machine won’t work
Can x-rays be deflected by electric or magnetic field?
NO! because they carry no charge (remember x-rays are photons)
What does “heterogenous” mean (energy of x-rays)
Means that x-rays can have a whole range of energy
What voltages are x-rays operated at?
25-150kVp - Think of what you do in labs!
What currents are x-rays operated at? (mA)
100-1200 mA [Note that it is not mAs]
Where is the High-Voltage Generator located and why?
Always close/inside the examination room - so voltage has less travelling distance, so less resistance
What does mAs and kVp control?
mAs is the number of x-rays, kVp controls their penetrability
AEC uses an ionisation chamber; T or F
T
What does APR stand for? What is it?
Anatomically Programmed Radiography - buttons with already set kVp and mAs for certain projections
What are the 3 main components of an x-ray system?
X-ray tube, operating console, high voltage generator
How many transformers do you often have in an x-ray tube?
3: autotransformer, step-up and step-down
What is an autotransformer?
Calculates how much step up or step down is needed
What is “V.eff”?
the voltage that would produce an effective current so that a DC current of the same magnitude would generate heat in a resistor at the same rate as the AC current
What is a “line voltage”?
the voltage supplied to the x-ray imaging room by the power company
Most imaging systems are designed to operate at what voltage?
220V
What is a Line Compensator?
It is a type of autotransformer! steps up or down the voltage supplied by the LINE - electricity from company
Why is a Line Compensator needed?
To compensate for the fluctuation of the VOLTAGE supplied by the LINE
What is a voltmeter, what does it do and where is it located?
Measures voltage - located BEFORE the autotransformer (line compensator). Tells us if there are enough windings on the primary coil
How does an autotransformer work?
It has a core, with windings around it, then has tabs at different windings where you can choose how many coils you want - step up or step down
The increase in voltage is directly related to the number of turns in the autotransformer- True or False?
True
What is the Autotransformer Law?
Vs/Vp = Ns/Np V = voltage N = windings
What is the difference between major and minor kVp selector?
Major and minor kVp selector: Taps on the autotransformer select major kV changes in increments of ten, or minor increments of one kV.
Does filament current have high or low voltage?
LOW voltage, HIGH current
The number of exposure is determined by the temperature of the filament. Why?
The hotter the filament, the more e- are ready (bullets) to be fired
What is “current”?
The number of e- passing per unit time
At what current do filaments operate at?
3-6 AMPERES
Where are exposure timers/timer circuit located in an x-ray circuit?
On the primary side
What does a timer circuit do?
They “make” or “break” the high voltage across the x-ray tube
Where is the mAs timer located - what does it do?
On the secondary side of the high voltage transformer - since actual TUBE current must be monitored
What are the two types of AEC?
Ionisation chamber and photodiode/photomultiplier
AEC - what is it set at?
1.5 times the expected exposure time
Difference between AEC (Photodiode+chamber) - Location?
Chamber = patient and IR Photo = behind IR
Difference between AEC (Photodiode+chamber) - Detection
Chamber = ionisation in the chamber creates a charge and this is measured Photodiode = turns photons into light and this is detected
