Medical Imaging Flashcards
How are X rays produced
X rays are produced when charged particles are rapidly decelerated causing their kinetic energy to transform into high frequency photons.
How does an x ray tube work
A vacuum contains 2 electrodes. A power supply creates a large pd between the electrodes. Electrons are themionically emitted from the cathode which is a heater. The electron accelerates towards the anode, and a photon is produced when the electron hits the target metal and decelerates. The target metal is shaped so that the photon is emitted in the desired direction through the window. X ray tube is lined with lead to shield radiographer from radiation.
What is a simple scatter
X ray photon is scattered elastically by an electron. 1-20keV
What is a photoelectric scatter
X ray photon is absorbed by electron causing it to be emitted. Less than 100keV
What is Compton scattering
X ray photon is scattered by an electron reducing its energy and causing the electron to be emitted from the atom. 0.5-5MeV. Can be used to kill cancer cells
What is pair production scattering
The x ray photon disappears to produce and electron-positron pair. Can be used to kill cancer cells. Greater than 1.02MeV
What is x ray attenuation
The decrease in the intensity of an x ray as it travels through matter
What is the attenuation coeffecient
How much a substance absorbs x rays
How is iodine used as a contrast medium
Iodine is injected into the blood stream so that blood vessels show up more clearly on the x ray image. Since soft tissues and blood have low absorption coefficients, iodine is required when wanting to view blood vessels.
How is barium used as a contrast medium
Barium sulfate is ingested by the patient in order to view digestive organs more clearly. The large atomic number means that there are more electrons for the photo electric effect to occur.
What is a CAT Scan
X ray tubes produce thin fan shaped beams, passing through the patient and the intensity of the attenuated x rays are recorded by the detector array, creating an image of the slice of the patient. This process is repeated as the X ray tube spirals the patient until a 2d image of every part of the patient is made. The slices can be viewed individually or manipulated by a computer to create a 3d image of the patient.
What are the advantages and disadvantages of a CAT scan
Its more expensive than a regular x ray. CAT scans are much more detailed as they can create 3d images. Can expose the patient to high level of radiation which may be harmful. Patient has to stay still or image will be blurry.
How is a gamma camera used
A radioisotope combined with elements that target the desired organ, called a medical tracer is injected into the patient. The medical tracer emits gamma photons at the desired organ. The gamma camera is placed at the desired area. Only gamma photons travelling parallel to collimator reach the scintillator. A gamma photon striking the scintillator produces thousands of photons of visible light. The photons of visible light travel through the light guide into the photomultiplier tubes, causing a single photon of visible light to convert into a voltage. The electrical signals creates an image on the computer from the impact positions of the photons on the scintillator.
What is a PET scan
A PET scan is when a positron emitting isotope is used as the medical tracer. The patient is injected with the medical tracer and as it decays, it produces a positron which annihilate electrons inside the patient. This produces 2 gamma photons travelling in opposite directions. The gamma photons are detected by the gamma detectors and the computer calculates the point of annihilation by the difference in the time of arrival at the detectors. Voltage signals from detectors are sent to computer which analyses them and produces an image.
What are the advantages and disadvantages of PET scans
Non invasive and help plan out complex surgeries. Medical tracers are expensive to produce.
