medical imaging Flashcards
X-rays are produced when
charged particles are rapidly decelerated (or accelerated)
and their kinetic energy is transformed into high frequency photons of electromagnetic
radiation.
Gamma rays are
produced via
radioactive decay or during particle collisions with a mass defect e.g.
electron-positron annihilation or nuclear fission of uranium-235
X-rays are produced by
Bremsstrahlung or braking radiation which
is when radiation is given off by charged particles due to their acceleration. X-rays used
in medical imaging are often referred to as soft X-rays as they have energies generally
lower than gamma rays.
X-ray tubes produce X-rays by
accelerating electrons in a high-voltage electric field then
rapidly decelerating them via collisions with a hard metal anode (positive electrode) e.g.
tungsten. Electrons are first emitted from a heater or filament (cathode or negative
electrode) into a vacuum tube via thermionic emission. The vacuum tube is needed to prevent the electrons from
colliding with air molecules before they have acquired enough energy to emit X-rays. external power supply produces a potential difference between the cathode and the
anode of up to 200kV. Therefore, the electrons gain a kinetic energy of up to 200keV
(see Nuclear and Particle Physics 6.4). Upon collision, the electrons decelerate rapidly
and some of their kinetic energy (~1%) is emitted as X-rays.
collimator
that further
collimate the beam by absorbing any rays that are not parallel to the axis of the tubes.
Braking radiation
broad range X-ray wavelengths with a hump-shaped
intensity profile as seen below. However, there are also a few sharp lines of
characteristic radiation that are not due to decelerating electrons. These lines are
instead caused by incident electrons knocking out bound low energy level electrons in
the anode atoms. Higher energy electrons will then transition down to the unoccupied
shell and their excess energy will be emitted as radiation
X-ray Attenuation Mechanisms
Different materials attenuate X-rays to a different extent so
tissues can be contrasted by measuring the intensity of the attenuated beam once it has
passed through the patient. For example, bone attenuates X-rays to a greater extent
than flesh or other soft tissues. Therefore, when a limb is exposed to an X-ray beam the
X-rays that collide with bone are more likely to be absorbed and the beam shows more
attenuation directly behind the bone. If a photographic film is held behind the patient’s
limb then it will be blackened less if it is in the direct path of X-rays that passed through
bone. This is clearly seen as a white outline of the patient’s skeleton. Nowadays, digital
detectors are used as the images are easier to process, store and transfer.
The intensity of a collimated beam of X-rays (collimated so there is no intensity
decrease due to the spreading out of the beam) decreases exponentially.
𝐼= 𝐼 𝑒^−𝜇x
where 𝐼𝐼0 is the initial intensity before entering the medium, 𝐼𝐼 is the attenuated intensity
after passing through a thickness 𝑥𝑥 (m) of the medium and 𝜇𝜇 (m-1) is a property of the
material known as the attenuation or absorption coefficient
Simple Scattering:
X-rays of energy 1-20 keV will reflect off layers of atoms or
molecules in the material as they do not have enough energy to undergo more
complex processes.
Photoelectric Effect
X-rays of energy less than 100 keV can be absorbed by
electrons in the material as they have the same energy as the ionisation energies
of the atoms. When an X-ray is absorbed by an atom, a photoelectron is released
Compton Effect
X-rays of 0.5 to 5 MeV lose only a fraction of their energy to
electrons in the absorbing materials. This is due to an inelastic interaction
between the photon and the electron. The scattered X-ray photon will have less
energy than before, and so its wavelength will be greater. The Compton electron
will be scattered in a different direction as momentum must be conserved.
Pair Production
When X-ray energy is greater than 1.02 MeV passes through the
electric field of an atom it will spontaneously produce an electron-positron pair
via the mass-energy relation. The positron will then go on to collide with another
electron and annihilate producing photons. This process is not very important in
medical X-rays as the photon energies are usually not high enough to produce an
electron-positron pair.
Contrast media
high attenuation coefficient materials that have heavy atoms with a
large proton number and so a large number of electrons.hese materials, such as barium,, or iodine
Computerised Axial Tomography (CAT)
examining the
internal three-dimensional structure of a patient using X-ray imaging. The CAT scanner
records a large number of 2D X-ray images then assembles them into a 3D image with
the help of computer software. The resolution of the image is greater than the
conventional X-ray and the CAT scan can distinguish between differing soft tissues.
However, CAT scans take a significantly longer time and so expose the patient to a far
greater dose of ionising radiation.
CAT scanners contain an
X-ray tube that generates a fan-shaped beam. This is directed
onto the patient whilst lying on their back. A ring of electronic detectors opposite detect the X-ray beam intensity. This information is then converted into electrical signals and
processed to reconstruct the tissues that the beam has passed through. The X-ray tube
and the detectors can then rotate about the patient and move up and down their length
to create a full 3D image of the patient’s body when all images of each slice are stitched
together. The image can then be displayed on a computer monitor and analysed.
