Chapter 16: Diagnostic Methods In Medicine Flashcards
Why are medical tracers gamma emitters
- Least ionising
2. Most penetrating so can be detected outside body
How are medical tracers used
Patient injected with radioisotope; radiation is detected outside body using a gamma camera
Why do medical tracers have a short half life
- Gives out radiation quickly so only a small amount is required to produce an image
- Reduces exposure to ionising radiation
What’s a radiopharmaceutical
Particular radioactive isotopes are combined with other elements so a substance is produced which will be taken up by tissue of interest
Cancerous cells absorb large amounts so can clearly be seen on a trace
Name 4 radioisotopes and their uses
fluorine-18: bone imaging
technetium-99m: bone growth/blood circulation(lungs/brain/liver)/function of heart/liver
iodine-123: function of thyroid/kidney
xenon-133: function of lungs
Draw a diagram of a gamma camera
See page 240
Describe the main components of a gamma camera
- Collimator: Gamma photons that are parallel travel to scintillator (~10%)
- Scintillator: Gamma ph converted to many 1000s of vis. light photons
- Photomultiplier tubes: electrical pulse/electrons produced from vis. light photons
- Computer: signals from photomultiplier tubes used to produce image where 1 pixel = output from 1 tube
Draw a diagram of a photomultiplier tube
See page 241
Describe how a photomultiplier tube works
- Each vis. light photon from scintillator releases a single electron from cathode by photoelectric effect
- Electrons accelerated to next electrode where more electrons are released
Uses of a gamma camera
- Bone scan - static study
2. Kidney scan - dynamic study
Describe the principles of positron emission tomography
- Beta+ emitting tracer injected into patient (e.g. fluorine-18)
- Positron annihilates with an electron inside the patient
- This produces 2 gamma photons travelling in opp. directions to each other
- The patient is surrounded by a ring of gamma detectors which compare the times the pair arrive at the detector
- Delay time indicates location
- 3D image created by computer
Uses of PET scans
- Showing up cancerous tissue
2. Bodily functions e.g. brain activity
Outline the principles of magnetic resonance imaging
- Protons/nuclei have spin -> behave like tiny magnets
- P/N precess about magnetic field provided by STRONG electromagnet
- Transmitting coils provide pulses of radio waves with freq. equal to Lamor freq./frequency of precession
- P/N absorb radiowaves causing them to resonate and flip into a higher energy state
- When radiowaves turned off, P/N flip back into lower energy state and emit radio photons
- Radio waves picked up by receiving coils and the signal is processed by a computer -> 3D image
- Relaxation time depends on surrounding tissue
- Gradient coils alter mag. flux density through body so location of P/N can be determined
Write down equation for Larmor frequency
See page 245
Draw an MRI scanner diagram
See page 246
What does relaxation time depend on for biological materials
Water content
- Water/watery tissue = few seconds
- Fatty tissues = few hundred milliseconds
- Cancerous tissues = intermediate
Describe the main components of an MRI scanner
- Large superconducting magnet: produces external mag. field up to 2T; cooled to 2.4K using liquid helium
- Radio freq. transmitting coil
- Radio freq. receiving coil
- Longitudinal gradient coils: vary mag. field so only 1 section @ resonant freq. so can precisely locate source of radio signal
- Computer: controls gradient coils and RF pulses; stores and analyses received data
Advantages of MRI
- Does not use ionising radiation which causes a hazard to patients/staff
- No moving mechanisms
- Patient feels nothing during a scan and there’s no after effects
- Gives better soft tissue contrast than a CAT scan
- Computer images can show any section through patient or a 3D image
Disadvantages of MRI
- Patient with metallic object cannot be scanned e.g. surgical pins/pacemakers
- Does not show bone as clearly as CAT scan
- Confined space -> difficult for patients with claustrophobia
Describe how image intensifier works
Intensifier used or X-rays would pass through film
Converts 1 Xph to many vis phs
Therefore the patient is exposed to a lower amount of ionising radiation
Describe how contrast media are used
Iodine/Barium are used as the contrast material
Have a large attenuation coefficient/Z number to improve image contrast
They are ingested/injected into the body
Scan shows outline of soft tissue
