X-ray imaging systems Flashcards
What are the components of an x-ray imaging system?
- x-ray tube
- operating console
- high voltage generator - which runs the tube
How is the radiation exposure stopped?
- Exposure time (mAs) in an x-ray tube is stopped by an electronic timer.
- AEC (automatic exposure control) is a device that measures the quantity of radiation. When the image receptor has recieved the required amount of radiation, the exposure is automatically terminated.
With the AEC, where does the radiation termination occur?
At the ion chambers, where the ion pairs are present.
Full form of DAP meter
Dose area product
SI unit of the dap meter
mGycm^2
What does the DAP meter measure?
measures the dose in a plane, i.e. the photons passing through the a field to the patient.
How many transformers are present in an xray generation circuit and what are their functions?
3 transformers
The primary transformer is for the mains input voltage and the secondary is the output voltage.
T1 = mains voltage
T2 = carries stepped up voltage
T3 = smaller transformer as it supplies the charge to the tube filament.
What is a high voltage generator?
A high voltage generator increases the output voltage from a step up transformer to the required kVp for xray production.
What are the components of an xray generator?
High voltage transformer, filament transformer and voltage rectifier.
What type of current do transformers use?
AC
SI unit of V and I used in primary and secondary coils
primary: V = volts, I = A
secondary: V = kilovolts, I = mA
What does voltage rectification refer to?
The process of converting the AC to DC to ensure that current is flowing in one direction.
What does a diode show?
A diode shows the direction of flow of conventional current (the arrow head direction).
What happens when voltage is unrectified?
When voltage remains unrectified, current may flow back towards the cathode.
How can unrectified voltage be fixed and which method is preferred?
Half-wave and full wave rectification, however, full wave rectification is preferred as it utilizes the power supply more effectively and results in less exposure time.
What is a half wave rectifier?
This process involves removing the negative voltage power supply, so that for the negative portion of AC waveform, no current is conducted.
Why is the half wave rectifier not beneficial?
As only half the wave is being used it is not an effective usage of power. By only using half the power, it requires double the exposure time.
What is full wave rectification?
Allows for the negative cycle to be inverted in that it rectifies the negative component of the input voltage and converts it to DC.
what type of power is half and full wave rectification?
single phase
What are the phases of power?
Single phase and Three phase
Describe single phase power
Has one distinct wave cycle and provides low energy, therefore offers low penetrability. Overall it provides less power - 2 pulses per 1/60s.
Describe three phase power
Has 3 wave forms that are superimposed. So there is nearly always a high and constant voltage applied across the xray tube. Provides more power - 6 pulses per 1/60s.
What is a high frequency generator?
They fully rectify AC and supply the tube with DC power. The frequency measured is very high.
Why is a high freq generator preferred?
It produces a nearly constant voltage waveform, so the image quality is improved at a lower patient dose. The exposure time is also short.
It is also smaller, cheaper and more efficient than high volt generator.
What is a voltage ripple?
the amount that the voltage varies between zero and the maximum value.
The less the voltage ripple?
the more constant voltage is supplied to the xrayt tube and the more xray quality and quantity.
Benefits of a rotating anode
- promotes cooling between exposures as the xray beams are dispensed across the surface.
- lasts longer
- heat dissipation enables longer scans.
How does the anode rotate?
Through an induction motor
What are the 2 components of an induction motor and how do they work?
Rotor and stator.
The stator has numerous windings with current in them that create magnetic fields. The induced magentic flux as a result of this further induces eddy currents in the rotor. These eddy currents induce their own magnetic fields. So the mag field of the stator and the mag field of the rotor work interact with each other which creating a torque, and the rotor turns.
