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.
PET scans
A radiotracer is injected into a patient, this radiotracer is used by the body releasing a positron. This positron collides with an electron in the body releasing two identical gamma rays travelling in opposite directions. These gamma rays are detected by a ring of gamma cameras. The time between the two gamma rays being detected allows an estimate of the location of the tracer. This is repeated to get an image.
Define Ultrasound
Longitudinal sound waves with a frequency greater than the human hearing range (> 20kHz). It is used for finding the boundary between two medias.
What is the Piezoelectric Effect?
A piezoelectric material generates a p.d when it is contracted or expanded and will contract or expand when a p.d is applied.
Transducers
Piezoelectric crystal that produce ultrasound waves in a brief pulse and detect ultrasound waves after they generated pulse has been reflected. Smaller the wavelength the more detailed the image.
A-Scan
A single transducer emits a signal and detects that signal, it is used to determine the distance from the device to the point of reflection.
B-Scans
Multiple A-scans from different angles that are stitched together to make a 2D image. The transducer is moved across the patients skin.
Acoustic Impedance
The product of a sound wave’s density and the speed of sound in the medium
Z = pc
What happens when an ultrasound hits the boundary of two medias?
A fraction of the wave’s energy/intensity is reflected and the rest is transmitted. The amount is dependent on the acoustic impedance of each medium.
Iᵣ/Iₒ = (Z₂-Z₁)²/(Z₂+Z₁)²
Z₁ Impedance of 1st Medium
Z₂ Impedance of 2nd Medium
Iᵣ Reflected Intensity
Iₒ Original Intensity
Doppler Effect
There is a change in frequency due of a wave when it is reflected or emitted by a moving source.
Explain Doppler Imaging
- Ultrasound waves are sent into a blood vessel.
- The iron in the blood cells reflects the waves back to the transducer and the frequency is shifted depending on the direction and speed of the cells.
Δf/f = 2vcosθ / c
f is frequency
v is speed of blood
θ is the angle between the probe and blood flow
c is the speed of ultrasound in blood
