6.5 Medical Imaging Flashcards
how do you produce x-rays? what are the two ways?
- when fast-moving electrons decelerate rapidly by smashing into a metal target, as the electron interacts with the electric field around the nucleus, an X-ray photon is emitted and by the principle of conservation of energy the kinetic energy lost by the electron is equal to energy of the photon emitted
- when a fast-moving electron ejects an inner electron in an atom of the target metal, another electron from a higher energy level moves to occupy the vacancy created, and this transition to a lower energy level releases an X-ray photon of a specific wavelength corresponding to the difference in energy levels
what is an x-ray tube?
x-ray tubes are electrical circuit with a cathode (where electrons are emitted) and an anode (the target metal) as well as a vacuum and a filament
how do x-ray tubes produce x rays?
- electrons are emitted from a filament, which is heated by an electric current
- the high p.d accelerates electrons between the filament (cathode) and the positive anode, which is a metal with a high melting point like tungsten
- the electrons would loose energy by colliding with any gas molecules on their path towards the positive anode, so the tube is evacuated
- if the electrons have enough energy when they hit the anode, some x-rays will be emitted
what is the simple way of saying how x-rays are produced?
x-rays are produced by bombarding tungsten with high energy electrons
what is x-ray attenuation?
the gradual decrease in intensity of x rays when they pass through matter (e.g a person’s body)
what is the attenuation coefficient?
the attenuation (or absorption) coefficient is a constant used to calculate how the intensity of x-rays decreases as they pass through a material
what are the four main ways x-rays can interact with matter?
- simple scattering
- the photoelectric effect
- the Compton effect
- pair production
what is the relationship of attenuation intensity?
exponential, when x-rays pass through matter, they are absorbed and scattered, the intensity of the x-ray beam decreases (attenuates) exponentially with distance from the surface, according to the material’s attenuation coefficient
what is the equation for x-ray attenuation?
I = Io x e^-μx
where I = final intensity
Io = initial intensity
μ = attenuation coefficient of the medium through which the x rays are passing
x = the distance through the medium (or the thickness of the medium if you like)
what is simple scattering of x-rays? how does it work?
- when low energy x-rays encounter the electrons in an atoms the energy of the x ray photon is not sufficient to cause ionisation
- the photon is scattered (deflected so that its direction is changed) but there is no change in energy and no absorption of the photon
- this is simple scattering and causes ‘noise’ (blurry bits) in the image due to the arrival at the detector of scattered x-rays from several angles as well a from the main beam
what is the photoelectric effect way x-rays are attenuated?
- in the same way UV radiation is able to release electrons from a metal x-rays are capable o causing emission of free electrons from atoms
- because the energy of the x-ray photon is so high the work function is so small in comparison it can be ignored when x-rays interact with an atom by the photoelectric effect, the emitted photoelectrons have a maximum kinetic energy equal to the photon energy of the x-rays
what is the definition for Compton scattering?
Compton scattering is the effect whereby an x-ray is deflected by interaction with an orbital electron has a longer wavelength than its initial wavelength, the electron is ejected from the atom at high speed
wha is Compton scattering? how does it work?
this is when a photon with a high enough energy knocks an electron out of an atom, which causes the photon to loose energy and be scattered, the x-ray photon and electron are scattered in different directions due to the conservation of momentum - the GREATER THE DEFLECTION, the GREATER THE LOSS OF ENERGY OF THE PHOTON and hence the BIGGER ITS WAVELENGTH BECOMES
what is pair production in terms of x-ray attenuation?
when a beam of high freq. x-rays causes an x-ray photon to interact with the nucleus of an atom and the x-ray photon vanishes, spontaneously producing a positron and an electron (pair production occurs)
do bones absorb x-rays better than flesh?
yes, film remains white, where the x-rays pass through the film turns black (soft tissue)
what is a contrast material used for?
to see detail of different types of soft tissue which have similar attenuation coefficients, a contrast media such as barium or iodine that as a large attenuation coefficient, this is due to barium and iodine having a high atomic number
what is CAT? (computerised axial tomography)
it is a process using multiple x-ray scans to produce images of ‘slices’ through the body in one plane, in order to produce a 3D image
why are CAT scans (also known as CT scans) better than traditional 2D x-ray scans?
- CAT allows for image reconstruction in 3D as it uses multiple cross sections
- provides a very accurate picture of the position of the internal organs without being obscured by other structures in the body
- particularly sensitive to different densities, give much better contrast of soft tissue than x-rays could, for example to detect cancerous tumours
- the digital technology allows the doctor to rotate the image and view the patient’ body from a number of angle without having to take more images of the patient which would expose them to more potentially harmful x-rays
how do you produce a CAT scan image?
- to produce a 3D image, many x-ray scans must be taken from different angles to get different views of the same organ
- a thin, fan-shaped x-ray beam is produced which has a very little thickness os only irradiated only a very thin slice of the patient at any one time
- having passed thought the patient, the x-rays are detected by a stationary ring of detectors
- the x-ray source is rotated around the patient and after one revolution, both it and the detectors have moved up about a cm, which means that on the next revolution it looks at the next slice of the body
- a computer then processes this and produces a high quality 3D image that can be rotated, zoomed in and seen from different angles
what is a tracer?
a tracer is a radioactive substance either ingested by, or injected into a patient, it emits gamma photons to be detected by a gamma camera