Medical Physics: Diagnostics

Question 1

Describe the use of a medical tracer to diagnose the function of organs.

Answer 1

• Produces Gamma rays as the range is insufficient for Alpha and Beta radiation for it to leave the body.
• Small half life (6 Hours) so activity has fallen after scan is taken.
• The m (after -99) means metastable, this is the state of Technetium before it emits gamma radiation. This is lost once the gamma has been emitted.
• The radioisotope is combined with chemical compound to target specific organs in the body.

Question 2

Describe the main components of a gamma camera.

Answer 2

• Collimator: only lets vertically travelling Gamma rays through, absorbing photons in all other directions.
• Scintillator crystal: Converts gamma rays into flashes of (green) visible light.
• Photomultiplier tubes: Converts visible light photons into electrons.
• Computer: Electron signals are processed by computer.
• Display: Image is forced onto screen where the output of each photomultiplier tube corresponds to a pixel on screen.

Question 3

Describe the principals of Positron Emission Tomography.

Answer 3

• Tracer undergoes Beta plus decay.
• The positron immediately annihilates with an electron after production.
• Two gamma photons are emitted at 180o to each other in order to conserve angular momentum.
• The gamma photons are detected are detected by gamma detectors surrounding the patient, determining the exact position of the positron emission from the time delay between each photo hitting their respective detector.
• A composite image is produced from all pairs of photons.

Question 4

Outline the principles of magnetic resonance.

Answer 4

• Nuclei of hydrogen have spin.
• In strong magnetic field, nuclei precess about the direction of the field with a frequency known as the Larmor frequency.
• Radio wave pulse at Larmor frequency causes resonance and nuclei precess at high-energy state.
• After the pulse has ceased, the nuclei relax, emitting an RF signal.

Question 5

Describe the main components of an MRI scanner.

Answer 5

• Superconducting magnet: produces external magnetic fields needed to alight the protons (up to 2 Tesla).
• RF transmission coil: Transmits RF pulses into the body
• RF receiver coil: Detects signal emitted from relaxing protons.
• Gradient coil: Produce an additional magnetic field that varies across the patient’s body. Coils are arranged to alter the magnitude of the magnetic flux across the length, depth and width of the patient. Ensures that Larmor frequency of nuclei will vary slightly between different parts of the body. Means that only a small volume of the body will have exactly the right field value for resonance, thus the computer can precisely locate the source of RF signal in body and construct image.
• Computer: Controls gradient coils and RF pulses, stores and analyses the received date: producing images.

Question 6

Outline use of MRI to obtain diagnostic information.

Answer 6

• Patient subject top large magnetic field and calibrated non-uniform field.
• RF pulse at Larmor frequency transmitted to patient.- RF emissions from patient are detected and processed.
• Hydrogen nuclei within patient have Larmor frequency dependent on magnetic field strength.
• So that location (and concentration) of hydrogen nuclei can be detected.
• Total image built up by varying the non-uniform field to give a specific field strength at different positions within the patient.

Question 7

Advantages and disadvantages of MRI.

Answer 7

• Advantages:
  
  • It does not use ionising radiation.
  • Patient has no after-effects.
  • Gives better soft tissue contrast than X-ray/CAT scan.
  • A 3D image is produced.
• Disadvantages:
  
  • It is expensive/time consuming.
  • Patients cannot have metal objects e.g. pacemaker.

Question 8

Describe the need for non-invasive techniques.

Answer 8

Patients body does not have to be cut open, which could be problematic: causing
infections/scars or excessive bleeding for patients suffering from haemophilia.

Question 9

Explain what is meant by the Doppler effect.

Answer 9

• The reflected signal has a slightly different frequency and wavelength to the transmitted one.
• Waves reflected by blood moving towards the transmitter are ‘squashed up’ causing a smaller wavelength and greater frequency, the opposite is true for blood moving away from transmitter.