Chapter 27 - Medical Imaging Flashcards
How do X-ray tubes produce X-Rays?
· Electrons are accelerated in a high voltage electric field then rapidly decelerating by colliding with a hard target metal anode.
· Electrons are emitted into a vacuum via thermionic emission.
· 1% of electrons produce an x-ray the rest are converted to thermal energy, there for the anode is cooled by a circulating water system.
· To direct the x-rays the tube is incased in a material that is thinner is a small window so only x-rays are emitted from there.
Spectra of X rays produced by Bremsstrahlung (Braking Radiation / X-Ray Tubes)
Bremsstrahlung radiation produces a broad range of X-ray wavelengths. A graph looks a bit like a normal distribution with the peak pushed left, except with 2 sharp peaks.
These peaks are cause by incident electrons knocking out low energy electrons in the anode atoms, causing high energy electrons to transition down and their energy to be emitted as well.
Ionisation ability of X-rays
X-rays have high energies so they can ionise matter, these means they can mutate organic cells. The exposure of X-rays should be minimised to prevent damage to cells.
X-ray Attenuation
Attenuation is a gradual decrease in intensity of the X-ray beams as they ionise matter.
I = Iₒe^(-μx)
I is intensity
μ is attenuation coefficient
x is the thickness of the material
Simple Scattering
X-rays of energy 1-20 keV will reflect off layers of atoms in the material as they don’t have enough energy.
Photoelectric Effect
X-rays of energy less than 100 keV can be absorbed by electrons in a material as they have the same energy as the ionisation energy of the atom. Releasing a photoelectron.
Compton Effect
X-rays of 0.5 to 5 MeV lose only a fraction of their energy to electrons in the absorbing material. The scattered x-ray will have less energy and a Compton electron will be emitted in the opposite direction to conserve momentum.
Pair Production
X-rays with energy greater than 1.02 MeV can pass through the electric field of an atom and if it does it will produce an electron positron pair.
X-ray Imaging
X-rays are directed at an area of a patient’s body and pass through the bone and soft tissue. Since the bone has a higher attenuation coefficient, it absorbs more X-rays than soft tissue. A photographic film is placed behind the patient, the area where the bone is wont darken as much. A digital detector is more commonly used.
Contrast Media
Have a high attenuation coefficient and are heavy atoms. Make X-rays images clearer as they absorb a lot of X-rays. Barium -56 and Iodine-53
Relationship between μ and proton number
μ α Z^3
CAT Scans
Computerised Axial Tomography
A 3D image produce by multiple 2D x-rays.
An X-ray tube generates a fan-shaped beam onto a patient with a ring of detectors around them. The tube and detects rotate around the patient and up and down their body to make a 3D image of their whole body.
Compare CAT scans and regular X-ray images
CAT scans give a better resolution of image and having a 3D image is easier to assess. But CAT scans take significantly longer and there is much more exposure.
Medical Tracer
A compound made of a radioactive isotope and specific element that collects in a particular location in the body. They use radioisotopes with a short half-life to reduce exposure. So they have to make them on site.
Gamma Camera
Gamma Cameras detect gamma photons emitted by medical tracers.
What is the General Structure of a Gamma Camera?
- Collimator which only allows photons travelling a certain direction through.
- Scintillation crystal which emits many visible light photons for every incident high-energy photon.
- Photocathode which produces an electron an electron for incident visible photon.
- Photomultiplier tube which amplifies the signal.
- Computer which detects the signal and displays the image on a screen.
