Chapter 27 Flashcards
How does an X-ray tube work
Consists of a high voltage supply which accelerates electron and emits them through a filament by thermionic emission into a vacuum, which hits a target metal which has an anode charge to attract the electrons.
The kinetic energy of the electrons is turned into X-rays and heat(99%)
The anode is shaped so that X rays are all in one direction through a window
What is attenuation?
The decrease in intensity or energy of an X-ray beam as it passes through matter
Attenuation mechanisms
Simple scattering
X-rays with energy 1-20KeV do not have enough energy to remove electrons so they just reflect
Photoelectric effect
X-rays with energy less than 100kEV are absorbed by electrons and are enough to free the electron
Compton effect
X-rays with energy 0.5-5MeV eject an electron from the atom but are then scattered to a less energetic X-ray
Pair production
X-rays with energy greater than 1.02MeV can pass the electric field and interact with the nucleus of an atom, producing an electron positron piar in opposite directions
Contrast medium
soft tissues have poor attenustion coefficients so contrast medium is needed with high AC too absorb X-rays.
AC is proportional to atomic number cubed
Iodine is used to identify regions of poor or good blood flow, areas with high attenuation indicate healthy blood flow
Barium is used to image digestive systems
CAT scan with advantages and disadvantages
Computerised Axial Topography
A fan shaped Xray transmitter transmits X-rays into a ring of detectors. This can rotate about the person and move about their length and produce a a 3d image using software on a display by combing several 2D X-rays
advantages:
Super indepth and can provide size,location of abnormalities very accurately. Can distiniguish between similar tissues with similar ACs
disadvantges: requires uset to be extremely still
prolonged exposure to radiation can cause cancer
Medical tracers
Compounds where radioisotope are combined with elements to monitor function of target organs
Two radioisotopes
Fluoride-18 and Technetium-m99 both have short half lifes to reduce exposure to radiation over a long time, and they both produce gamma radiation which is least ionising
Fluoride-18 is produced with a particle accelerator. It is used in sodium fluroide for the rapid bone uptake
TC-m99 is used with sodium and oxygen in the compound NaTcO4 to target the brain
Function of a gamma camera
Gamma photons leave the body.
Collimator consists of long series of metal tubes which absorb an X-ray that are at angle to them.
Scintillitor can produce thousands of visible light photons from a single gamma photon
Photons enter photomultiplier tubes where they are converted to a voltage which calculates the impact positions of the electrons on the scintillator
Difference between xray and gamma imaging
Gamma cameras can show functions and processes of the body rather than just anatomy
Fluroide-18 decay and how and why is it produced in the lab
Beta plus decay into oxygen-18, atomic number 8 producing a positron and a neutrino as well as gamma photon
It has a short half life so has to be produced onsite with a particle accelerator, by accelerating a proton into oxygen-18 producing f-18 and a neutron in the process
How does a PET scanner work
Patient is injected with fluorodexyglucose, which decays by producing a positron which by annihilation creates two directionally opposed gamma photons. Their recorded times are used to calculate their relative posistions.
The rate of emission from a certain position can be calculated to determine uptake and produce a 3d model
Advantages and Disadvantages of PET scan
It is non invasive and non surgical which can produce images of the brain and detect diseas like Alzheimers early
It is expensive, requires the set up of a medical tracer production facility in the hospital as well as a radiation shield
How does ultrasound tranducer work
Alternating current about a piezoelectric material causes sound energy to be produced. Frequency of AC is chosen to make the crystal oscillate
When the transducer recieves ultrasound it causes the piezo electric material to slightly vibrate producing an emf about it which is detected by computer
acoustic impedance, dopppler imaging, impedance matching
