Medical Physics Flashcards
Two different ways X-rays are produced?
Rapid deceleration of electrons at anode, knocked electrons in target material emitting characteristic rays
Four ways X-rays are attenuated?
Simple scattering, photoelectric effect, compton effect, pair production
What is simple scattering?
<20keV, X-rays absorbed by electrons in all directions
What is the Compton effect?
0.5MeV-5MeV, X rays ionise electrons but leftover scattered
What is pair production?
X-rays interact with nucleus to form electron-positron pair
How to image soft tissue using X-rays?
Ingest barium sulphate/inject iodine whihc absorb X-rays strongly CONTRAST MEDIUMS
CT scan advantages
3D image produced, good differentiation between soft tissue
CT scan disadvantages
Exposure to ionising radiation, time consuming
What is a CT scan?
fan shaped beam irradiates thin slice of patient, rotated around body
Four stages of Gamma Camera?
Collimator, Scintillator, Photomultiplier tubes, Computer
What is a PET scan?
Inject B-plus emitter, positrons annihilate electrons, gamma emitted
PET scan advantages
non-invasive, can diagnose different types of cancers
PET scan disadvantages
expensive, ionising radiation
How to improve resolution of an ultrasound image?
Shorter wavelength
What is ultrasound?
A sound wave above the audible range >20kHz
What crystal is used in a transducer?
lead zirconate titanate (PZT)
What is the purpose of the backing material in a transducer?
Damps oscillations, prevents backwards propagation so no reflected waves
What is the pulse repetition frequency?
the frequency at which ultrasound pulses are emitted
What is the formula for ultrasound distances?
d = (s x t)/2
What is acoustic impedence?
The product of the speed of sound through a given
medium and the density of the medium.
What is an A scan?
Straight line of ultrasound, intensity shows boundary type, time delay shows thickness
What is a B scan?
Dots on screen build image, brightness proportional to intensity
What is a Doppler scan?
For blood flow, c is speed of sound in substance, v is velocity of blood, angle is measured from beam to blood
What effect does increasing velocity have?
Higher Doppler frequency
What effect does increasing angle have?
Lower Doppler frequency
Why is gel used for ultrasound?
There would be a high reflection at dry skin-air boundary due to large difference in acoustic impedence
What are the energy transfers when producing X-rays?
Electric potential to kinetic, kinetic to thermal mostly, some kinetic to X-rays
Why do we need to filter out low-energy X-rays?
They will be absorbed by the skin and could cause damage as they are ionising
What affects how X-rays are attenuated?
Z of medium
How does an X-ray scan work?
X-rays penetrate patient, attenuated differently, shadow effect, intensity proportional to darkness of film
What is the purpose of the intensifying layer?
photons hit atoms in layer, fluorescence occurs, emit visible light photons, so extra contrast; film is more sensitive to visible light
