C27 Medical Imaging Flashcards
What are noninvasive procedures?
Noninvasive procedures do not involve tools that break the skin or enter into the body.
When are x-rays produced?
when charged particles are rapidly decelerated (or accelerated) and their kinetic energy is transformed into high frequency photons of electromagnetic radiation.
X-rays are produced by Bremsstrahlung or braking radiation (when radiation is given off by charged particles due to their acceleration).
What’s the difference between x-rays and gamma rays?
X-rays and gamma rays have overlapping frequency spectra. Only distinction is their history.
How are gamma rays produced?
produced via radioactive decay or during particle collisions with a mass defect e.g. electron-positron annihilation or nuclear fission of uranium-235.
How are X-ray tubes used to produce X-rays?
They 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.
Explain the process of how x-rays are formed through using X-ray tubes:
-Electrons are first emitted from a heater or filament (cathode or negative electrode) into a vacuum tube via thermionic emission, (the process by which electrons emitted from a heated source, in this case a high resistance coiled wire).
-The vacuum tube is needed to prevent the electrons from colliding with air molecules before they have acquired enough energy to emit X-rays. An 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 .
-Upon collision, the electrons decelerate rapidly and some of their kinetic energy (~1%) is emitted as X-rays, (rest is lost to thermal energy in anode).
-To prevent the anode from overheating, the anode is either rotated so a new area is constantly exposed to the electron beam or it is cooled with circulating water supply.
-X-rays are emitted in every direction from the anode. However, a straight and parallel or collimated beam much more useful, can be directed specific parts patient’s anatomy, minimise exposure. Therefore, the vacuum tube is encased in a material which is thinner in one place than the rest, the window, so that X-rays only emerge in one place outside the tube.
-Beam then directed into series of straight and parallel metal tubes (known collimator) that further collimate beam by absorbing any rays, not parallel to the axis of the tubes.
X-ray Spectra
(Braking Radiation produces what?)
Braking radiation produces a broad range X-ray wavelengths with a hump-shaped intensity profile.
-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. Photons produced via this process have specific wavelengths and so the number of X-rays at these energies will be greater and so the intensity here will be higher.
X-rays, Ionising Radiation:
-X-rays can ionise matter (cause electrons be emitted from atoms e.g. photoelectric effect).
-In living cells, X-rays ionise DNA and other tissues causing damage to the organism and possible harmful mutations to genes.
-To minimise damage, living tissue must be exposed to only low intensity beams for short periods (exposure times).
-The ability of X-rays to destroy cells can be utilised in the treatment of cancer (radiotherapy).
What are some limits to a conventional x-ray image?
-A conventional X-ray image is a useful diagnostic tool.
-Cheap and quick, however only provides a two-dimensional image and cannot distinguish overlapping bones or different soft tissues.
What are some advantages of using a CAT scan, over a conventional x-ray scan?
-A CAT scan, effective way of examining the internal three-dimensional structure of patient using X-ray imaging. The CAT scanner records large no. of 2D X-ray images then assembles them into 3D image with help of computer software.
-The resolution of image is greater than the conventional X-ray and CAT scan can distinguish between differing soft tissues.
Disadvantage of using CAT scans:
CAT scans take significantly longer time, expose the patient, far greater dose of ionising radiation.
How do CAT scans work?
-CAT scanners contain an X-ray tube that generates a fan-shaped beam, (directed onto patient whilst lying on their back).
-A ring of electronic detectors opposite detect the X-ray beam intensity. This info then converted into electrical signals and processed to reconstruct the tissues that the beam has passed through.
-X-ray tube and detectors can then rotate about patient and move up and down, create full 3D image of patient’s body when all images of each slice, stitched together. Image can then be displayed on a computer monitor and analysed.
The current in the x-ray machine is …
(Equation)
the x-ray machine is the rate of flow of electrons
I = N x q
Where I is current in amps.
N= no. particles flowing each sec.
q is the charge.
KE= eV (for electron)
Absorption Mechanisms:
What are the Four important mechanisms by which X-rays are absorbed and beams are attenuated (having been reduced in force, effect, or value).
Simple Scattering
Photoelectric Effect
Compton Effect
Pair Production
Simple Scattering:
X-rays of energy 1-20 keV will reflect off layers of atoms or mols in the material as they do not have enough energy to undergo more complex processes.
