6.5 Medical Imaging Flashcards
(30 cards)
What are the two ways in which x-rays are produced?
- When fast moving electrons decelerate by smashing into metal targets, the electron interacts with the metals nucleus, an x-ray is emitted due to the conservation of energy as the kinetic energy lost is equal to the energy of the photon emitted(which is the same energy as an x-ray)
- When a fast moving electron ejects an inner electron in the nucleus of a metal atom, an electron on the outer layer moves in to occupy this space, this transition to a lower energy level releases energy as an x-ray
Describe a modern day x-ray tube?
- Electrons are emitted from the filament, heated by the electric current
- The high P.d. accelerates electrons towards the positive anode, the tube is vaccumed so electrons don’t collide with gas molecules
- Electrons hit the anode and x-rays will be emitted
What is the equation for minimum wavelength of an x-ray with a fixed p.d. across it?
Describe this graph of intensity plotted against wavelength for a particular anode?
-There are no x-rays emitted below the minimum wavelength
-The sharp peaks show strong emission at particular wavelengths, this is due to the characteristics of the anode
What are the four ways X-rays can interact with matter?
1.Simple scattering
2.Photoelectric effect
3.Pair production
4. Compton effect
Describe the process of simple scattering?
-A low energy x-ray hits an electron in an atom but the energy of the x-ray is not enough to cause ionisation
-The x-ray photon is scattered (deflected) and there is no change in energy in the photon
What is the photoelectric effect? and are x-rays capable of producing the photoelectric effect?
-Phenomenon where light causes the emissions of electrons from a materials surface
-Yes x-rays are capable of producing the photoelectric effect
How can you use the photoelectric effect to measure the energy of x-rays?
-The energy of the x-ray photon is equal to the maximum kinetic energy of the photoelectrons emitted
What is Compton scattering?
-The effect where an x-ray is deflected by its interaction with an orbital electron, this causes it to have a longer wavelength as it loses energy, the orbital electron is also ejected with a small amount of kinetic energy so mass-energy is conserved.
-The larger the angle the x-ray photon is deflected the more energy it would have lost and the larger wavelength
What is Attenuation and the attenuation coefficiant?
-Attenuation: is the gradual decrease in intensity
-Attenuation coefficient: a constant used to calculate the intensity of x-rays which changes for different mediums
What is the equation for attenuation of intensity?
I-Intensity
I₀-initial intensity
x-distance
pie looking symbol- attenuation constant
How can you use x-rays to see different types of soft tissues?
-As soft tissues have similar attenuation constants a contrast material like barium can be used (which has a large attenuation constant) to highlight one of the soft tissue
How does a CAT scan produce an image?
- A thin fan shaped X-ray beam is produced so the x-ray only irradiates a small slice of the patient
- The x-ray source is rotated around the patient and x-rays are detected by a stationary ring of detectors
3.The CAT equipment moves down the patient and the process repeats
What are the advantages of CAT scans over an x-ray image?
-Picture isn’t obscured by other structures in the body
-More sensitive to changes in density so better contrast for soft tissue
-Can view patients body from a number of angles so don’t have to take more images and expose patient to harmful x-rays
What is a tracer and a gamma camera?
-Tracer: a radioactive substance that is injected or ingested and emits gamma photons
-Gamma camera: detects gamma photons emitted by a patient with a tracer and produces an image of the path of the tracer through the body
What things must be considered when putting a radioactive material into a patients body?
-Gamma sources must be used as alpha and beta particles are too ionising so would be absorbed by the body
-The patient will remain radioactive after the tracer is injected so people will be exposed to radioactivity is they stand close
-The half-life of the source must be long enough to carry out investigation but not long enough to cause damage to cells
-Source must not be toxic
-It must be possible to get radioactive material to intended part of body
What are the three major components of a gamma camera?
-Collimater: a device used to produce parralel sided beams of electromagnetic radiation
-Scintillator: a material(crystal) that produces photons of visible light when struck by a high energy photon
-Photomultiplier tube: photons produced by the scintillator are very faint they are converted into an electrical signal and amplified by this device
What is the use of a gamma camera?
-Can be used to diagnose diseases of the thyroid, liver, brain etc
How do you carry out a PET scan?
- Tracer is introduced into the bloodstream chemically combined with a biologically active molecule
- Patient and doctor wait until the tracer becomes concentrated in certain areas
3.The PET scanner detects gamma radiation emmited by the tracer, the tracer emits positrons which travel a short distance before interacting with an electron - This results in an electron-positron annihilation, two gamma rays are produced in opposite directions and detected.
- Computer analyses the timings and uses it to work out the point of origination and produces an image
What are the uses of a PET scan?
-As the tracer in PET scans are combined with a biological molecule they can be used to reveal information about biological processes e.g. blood flow
Compare PET scans with CAT scans?
-PET scans are more expensive than CAT scans
-PET scans take longer than CAT scans
-CAT scans are more damaging then PET scans
What are the principles of ultrasound scanning?
-Ultrasounds are partially reflected at boundaries e.g between air and skin
-Ultrasound must be pulsed into the body from the transducer otherwise the reflected echoes won’t be properly picked up
-The pulse must be sharp with only a few ultrasound waves to make the timings less difficult
What are the pros of ultrasound compared to other medical imaging techniques?
-No known dangers associated with ultrasound, whereas x-rays and gamma rays are ionising
-You can obtain real time images of soft tissue
-Inexpensive and portable
What is an ultrasound and a transducer?
-Ultrasound: longitudinal waves with a frequencies higher than 20,000Hz
-A transducer: a device which converts non-electrical signal (sound) into electrical signal
