medical physics Flashcards
Explain how X-rays are produced in an X-ray tube.
Electrons are emitted from the cathode
These electrons are accelerated by a high potential difference toward the anode
When the electrons hit the anode, their kinetic energy is converted into X-ray photons and thermal energy.
The X-ray photons are emitted, while the anode is cooled to prevent melting.
Explain the four mechanisms of X-ray attenuation.
Simple scatter (1-20 keV): X-ray photon bounces off an electron without losing energy.
Photoelectric effect (<100 keV): X-ray photon is absorbed by an electron, which is ejected from the atom.
Compton scattering (0.5-5 MeV): X-ray photon ejects an electron and scatters with reduced energy.
Pair production (≥1.02 MeV): X-ray photon interacts with a nucleus, creating an electron-positron pair.
Explain why iodine and barium are used as contrast mediums in X-ray imaging.
Iodine and barium have high atomic numbers, which increases their attenuation coefficient (proportional to the cube of atomic number).
This makes soft tissues (e.g., blood vessels, digestive system) more visible in X-ray images by enhancing the contrast between tissues.
Explain how a CAT scan produces a 3D image.
The X-ray source and detector rotate around the patient, taking multiple X-ray images from different angles.
A thin X-ray beam is used to create cross-sectional “slices” of the body.
A computer assembles these slices into a 3D image, which can differentiate between tissues with similar attenuation coefficients.
Explain how a gamma camera works.
A radioactive tracer is injected into the patient, emitting gamma photons.
Gamma photons pass through a collimator (lead tubes), which filters out photons not traveling along the axis.
The photons hit a scintillator (sodium iodide crystal), producing visible light.
Photomultiplier tubes convert the light into electrical signals, which a computer processes to create an image.
Explain how a PET scan detects and locates positron emissions.
A medical tracer (e.g., F-18) emits positrons, which annihilate with electrons in the body.
This annihilation produces two gamma photons traveling in opposite directions.
A ring of gamma detectors records the photons and measures the time difference in their arrival.
A computer uses this data to determine the location of the annihilation and creates a 3D image.
Explain how ultrasound is produced and detected in medical imaging.
A piezoelectric crystal in the transducer vibrates when an alternating p.d. is applied, producing ultrasound waves.
The waves travel into the body and reflect at tissue boundaries.
The reflected waves return to the transducer, causing the crystal to vibrate and produce an electrical signal.
A computer processes the signal to create an image.
Explain the differences between A-scans and B-scans.
A-scan: Produces a one-dimensional image showing the time delay and intensity of reflected ultrasound pulses. Used for measuring distances (e.g., bone thickness).
B-scan: Produces a two-dimensional image by moving the transducer over the skin. Each reflected pulse corresponds to a dot on the screen, with brightness proportional to the intensity of the reflection.
Explain how Doppler imaging measures blood flow.
Ultrasound is directed at an angle toward a blood vessel.
The frequency of the reflected ultrasound changes due to the Doppler effect (higher frequency if blood flows toward the transducer, lower if it flows away).
The change in frequency is used to calculate the speed of blood flow.
Disadvantages of CAT Scans
High radiation exposure: Increases the risk of cancer or cell damage.
Expensive: Requires costly equipment and maintenance.
Time-consuming: Takes longer to perform and analyse compared to X-rays or ultrasound.
Explain why short half-life isotopes are used in medical tracers.
Short half-life isotopes decay quickly, ensuring high activity for imaging.
They minimize long-term radiation exposure to the patient, as they decay to safe levels shortly after the procedure.
Explain how the piezoelectric effect is used in ultrasound transducers.
An alternating p.d. is applied to a piezoelectric crystal, causing it to vibrate and produce ultrasound waves.
When ultrasound waves hit the crystal, they cause it to vibrate, generating an electrical signal.
This signal is processed to create an image.
Explain why coupling gel is used in ultrasound imaging.
Air pockets between the transducer and skin would cause significant reflection of
Coupling gel fills these gaps, matching the acoustic impedance of the skin and allowing more ultrasound to penetrate the body.
