6.5 Medical Physics (X Rays) Flashcards
How are X-rays produced?
When charged particles are rapidly decelerated (or accelerated) and their kinetic energy is transformed into high frequency photons
X rays and gamma rays have overlapping frequency spectra. What might we see as a result of this?
X rays and gamma rays with the same frequency, and therefore equal wavelength and energy.
What is the only difference between x-rays and gamma rays that have identical frequency?
Gamma rays are produced from radioactive decay
X-rays are produced by Bremsstrahlung or braking radiation
What is Bremsstrahlung (braking radiation)?
When radiation is given off by charged particles due to their acceleration.
What can x-rays used in medical imaging be referred to as?
Soft X-rays (as they usually have lower energy than gamma rays)
How do X-ray tubes produce X-rays?
By acccelerating electrons in a high voltage electric field, then rapidly decelerating them via collision with hard metal anode.
Where are electrons first emitted from in the process of producing x-rays?
A heater or filament (at the negative electrode) via thermionic emission
What is thermionic emission?
The process by which electrons are emitted from a heated source (such as a high resistance coiled wire).
Why are electrons emitted into a vacuum tube during the process of x-ray emission?
To prevent electrons from colliding with air molecules before they have acquired enough energy to emit x-rays
What is the maximum voltage used in the electric field accelerating the electrons during the process of x-ray emission?
200kV
What is the maximum gain of kinetic energy of electrons during the process of x-ray emission? (where the maximum voltage is 200kV)
200keV
(Using W=QV)
How does the motion of an electron change when it collides with the hard metal anode during the process of x-ray emission?
It decelerates rapidly
During x ray emission, how does the energy of an electron change once they collide with the hard metal anode?
Some of their kinetic energy is emitted as X-rays. The rest is lost to thermal energy in the anode.
During x ray emission, there is a risk of the anode overheating. What are two ways to prevent this?
- Rotate the anode. This way, a new area is constantly exposed to the electron beam.
- Cool it with a circulating water supply.
X rays are emitted in every direction from the anode in an X-Ray tube. How would we turn this into a collimated beam?
Encase the vacuum tube in lead shielding, except in one spot, the window, so that the X-rays emerge in one place outside the tube.
We can direct the beam into a collimator to further collimate the beam.
What is a collimator?
A series of straight and parallel metal tubes. Any rays which aren’t parallel to the tubes are absorbed.
Why is a collimated beam of x rays much more useful?
It can be directed at specific parts of a patient.
[Breaking Radiation Intensity Profile] There are a few sharp lines of characteristic radiation which is not due to decelerating electrons. What are they caused by?
Incident electrons knocking out bound low energy level electrons in the anode atoms. Higher energy electrons will then transition down to the unoccupied shell, and their excess energy will be emitted as radiation.
X rays can ionise matter. How can we minimise the damage to living tissue?
It must be exposed to only low intensity beams for short periods of time.
What is attenuation?
The gradual decrease in energy/intensity of X rays as they pass through matter.
Does attenuation change depending on the material?
Yes. For example, bone attenuates X rays to a greater extent than soft tissues.
In what way does the intensity of a collimated beam of X-rays decrease?
Exponentially.
State equation for attenuation of an X-Ray beam
I = I₀ e^-μx
I : Attenuated intensity after passing through medium
I₀: Initial Intensity
μ: Attenuation Coefficient
x: Thickness of medium
What is another name for the attenuation coefficient?
Absorption coefficient
