Studied by 1 person
How do X-ray tubes work?
use a high voltage power supply connected to a heater element which heats a filament at the cathode which releases electrons (thermionic emission, accelerated by p.d.), which hit a metal target (anode) and X-rays are emitted
In an X-ray tube, how much kinetic energy of electrons is converted to X-ray energy?
1%
What is thermionic emission?
when electrons are released from a material due to heat
What is Brehmsstrahlung radiation?
energy given off by electrons due to their acceleration
What are characteristic X-rays?
sharp lines on intensity-energy graph, caused by incident electrons knocking out bound low energy electrons in target metal - the photons produced have specific wavelengths
What is simple scattering in X-rays?
if an X-ray has energy lower than work function, it will hit an atom and be scattered, travelling in a different direction
What is the photoelectric effect in X-rays?
if an X-ray has medium energy, it releases an electron with some kinetic energy
What is the compton effect in X-rays?
if X-ray has high energy it removes the electron, and a lower energy photon from the leftover energy is scattered
What is pair production in X-rays?
if X-ray has VERY high energy it will interact with the nucleus of the atom, and an electron-positron pair is produced, which annihilate and produce a pair of lower energy photons
What are constrast media used for?
to tell differences between tissues - they absorb X-rays due to high number of protons as attenuation coefficient of a medium is proportional to Z3
What can be used as contrast media?
barium and iodine
What are the components of a CAT scan?
an X-ray tube rotates around the patient, X-rays are absorbed differently by different tissues, intensity is recorded by detector, images of slices of the patient are taken, X-ray tube moves along patient to produce 3D image
What are the advantages of a CAT scan over an X-ray image?
produces a 3D image, tissues are contrasted
What are some examples of medical tracers?
technetium-99m and fluorine-18
What are the properties of medical tracers?
general: short half lives (produced on site), high activities so it can be detected, non-toxic
technetium-99m: gamma-emitter so non-ionising, very penetrative, non-invasive, 6 hours half life
fluorine-18: beta emitter so not very ionising, 110 minutes half life
What are the components of a gamma camera?
collimator - filters photons travelling in one direction, scintillation crystal - emits many photons when one high energy photon is indicent upon it, photocathode - produces an electron for each visible photon, photomultiplier tube - each electron produces a cascade of electrons to amplify the signal, computer - detects electron signal positions, displayed on screen
How does a gamma camera work?
a medical tracer is inside the patient releasing high energy photons, the collimator filters photons at right angles, the scintillation crystals turns a high energy photon into many visible photons, photocathode turns one photon into one electron, photomultiplier tube creates more electrons for each signal, its position detected and displayed by a computer
How do PET scans work?
patient is surrounded by a ring of gamma cameras, positron emitting tracer is injected into body, positron annihilates with electron in the body and produces two gamma photons with the same energy in opposite directions, the delay time between the two photons is used to determine the location of the annihilation, a computer is connected to the detectors and is used to form an image
advantages of pet scans
produces real time images, non-invasive
What is ultrasound?
a longitudinal wave above 20kHz so above human hearing
What is the typical frequency of an ultrasound?
around 5MHz
What is the piezoelectric effect?
a piezoelectric material generates a voltage when it is contracted or expanded, or will contract and expand if a voltage is applied - which can produce ultrasound vibrations and a piezoelectric crystal absorbing ultrasound will produce an alternating voltage
once the ultrasound has been created the potential difference is turned off and the reflected signal is read
What is an ultrasound A-scan?
uses a single transducer to emit a signal and receive the reflected signal back, used to determine distances from the ultrasound device to the point of reflection by measuring the time delay and using the speed of sound in the medium to approximate the distance
What is an ultrasound B-scan?
moving the transducer over the patients skin produces measures of time intervals at each point, all stitched together to form a 2D image
What is acoustic impendence of a medium?
the product of the medium’s density and the speed of sound in that medium
What happens when ultrasound is reflected at a boundary?
ca fraction of the intensity is reflected, the rest is transmitted - this is dependent on the acoustic impendances of the media
What is the reflection coefficient?
the fraction of the intensity which is reflected at the boundary - Ir/I0
What is impendence matching?
use of a gel between the transducer and the skin which has a very similar impedance to skin to minimise reflected wave energy and maximise transmission of ultrasound
How can you determine the speed of blood in a patient?
send ultrasound waves into blood vessel, reflected waves have shifted in frequency, use equation to find speed of blood flow
How is the doppler effect used in ultrasound?
the frequency of a wave is changed when reflected by a moving source e.g. blood flow - use a transducer to send a frequency, measure change in frequency
What are the advantages of ultrasound?
non-ionising and non-invasive, quick and affordable
Note
Flashcard