Describe the use of contrast media
- Iodine/Barium used as contrast media
- Injected or ingested into body
- High Z number, absorbs more X-Rays
- Used to give greater contrast
Describe the operation of a CAT scanner
- Fan shaped X-Rays rotate around the patient
- Detected by a ring of detectors
- Relative attenuation detected are processed and turned into a 3D image via computer
Describe the advantages of CAT scan compared to X-ray
Differences: x-ray is one directional/produces single image
CAT produces image from slices
Advantages: - CAT produces 3D images which can be rotated
- greater detail/contrast + soft tissues can be seen
Describe the use of medical tracers to diagnose the function of organs
- Ingested or injected
- Technitium99m used as a radioactive source
- Path can be followed by gamma detectors and image of organs can be observed
Describe the main components of a gamma camera
Collimator: allowed parallel gamma photons travel to the scintillator (narrow lead tubes makes image less blurred)
Scintillator: gamma ray photon produces many visible light photons
Photomultiplier: an electrical pulse produced from light photons
Computer: signals user to produce an image
Describe the principles of PET scanning
- Radioactive substance used (fluorine-18)
- Positron-electron annihilation
- Picked up by diametrically opposite detectors
- Computer produces 3D image
- Can reveal function of the brain
Describe the main components of an MRI scanner
Strong electromagnet Radio transmitting coils Radio receiving coils Gradient coils Computer
Outline the principle of MRI
- Protons have spin/act like tiny magnetic
- Protons precesses about the magnetic field provided by the strong electromagnet
- Transmitting coil provides pulses of radio waves of equal frequency to the Larmor frequency
- Protons absorb radio waves, resonate and flip to higher energy level
- When protons flip back to lower energy level they emit radio waves
- The relaxation time depends on the tissues
- Radio waves picked up by the receiving coils
- Gradient coils alter the magnetic flux density throughout the body
- Larmor frequency variety throughout the body
- Computer processes image
Describe advantages and disadvantages of MRI
Adv: not ionising and better soft tissue contrast
Disadv: heating effect on metal objects so people with metal or pacemakers can’t use it
Long time to do, have to sit still
Describe the need for non-invasive techniques
No cutting. Less risk of infection and less trauma
Describe the properties of ultrasound
Longitudinal vibration with frequency > 20kHz
Describe the piezoelectric effect
The application of a p.d across a material causes expansion/contraction and vice versa
Describe the principles of ultrasound scanning
- Pulses of ultrasound sent into body
- Use of gel for acoustic impedance matching
- Pulse reflected at boundaries of tissue
- Time delay used to determine depth/thickness
- Fraction reflected used to identify tissue
- Greater resolution if smaller wavelength used
Describe the use of X-Rays in imaging internal body structures including the use of image intensifiers and photomultipliers
- X-Rays would normally pass through film
Intensifier converts X-Ray photon to many visible light photons (which can be absorbed by film) - Photon releases electron that is accelerated onto fluorescent screen
Number of electrons increased/multiplied - Lower exposure/fewer X-Rays needed