Diagnosis Methods in Medicine Flashcards
Describe the use of medical tracers to diagnose the function of organs.
A radioactive isotope - usually technetium-99m is bound to a substance that is used by, or gravitates towards the area of the body you wish to examine. The isotope is either injected or ingested. The radiation emitted is then recorded and an image is produced.
Why is technetium-99m a common medical tracer?
It has a short half life (6hrs) and emits gamma-radiation, which is hardly ionising. It decays to a much more stable isotope.
Describe the main components of a gamma camera.
Lead shield stops radiation from other sources entering the camera.
Lead collimator has thousands of vertical holes that collimate the beam of gamma-rays - only gamma-rays parallel to the holes can pass through.
Sodium iodide crystal emits a flash of light whenever a gamma-ray hits it.
Photomultiplier tubes detect the flashes of light from the crystal and turns them into pulses of electricity.
Electronic circuit collects the signals and sends them to a computer for processing into an image.
Describe the principles of PET.
The patient is injected with a substance used by the body that is attached to a positron-emitting radio-tracer with a short half life, which is then given time to move through the body to the organs. Positrons emitted by the radioisotope collide with electrons in the organs, causing them to annihilate, emitting high-energy gamma-rays. Detectors in a circle around the body detect these gamma-rays and a computer builds a 3-dimensional image of the radioactivity in the body. Cells that are doing more work show up more clearly on the image.
Outline the principles of MRI.
Hydrogen nuclei in the body spin, but when a very strong external magnetic field is applied, the protons line up according to the field. They rotate slightly about this line - this is called precession. Radio frequency pulses equal to the Larmor frequency are and absorbed by the protons, causing them resonate and flip into a higher energy state. When the radio frequency waves are switched off, the protons gradually relax (back into the lower energy state) and emit a radio frequency wave, which is then detected as the rate of relaxation.
Describe the main components of an MRI scanner.
- A large superconducting magnet produces the external magnetic field to align the protons.
- A radio frequency coil transmits radio frequency pulses into the body.
- A radio frequency coil detect the signal emitted by the relaxing protons.
- A set of gradient coils produce an additional external magnetic field that ensures the Larmor frequency is different across the body - allowing only one part of the body to have the right field value for resonance.
- A computer controls the gradient coils and radio frequency pulses, and also stores and analyses the data to produce and display images.
Give three advantages of MRI scanning.
Any three from:
- No known side effects.
- An image can be made for any slice in any orientation of the body.
- High quality images can be obtained for soft tissue, such as the brain.
- Contrast can be weighted in order to investigate different situations.
Give three disadvantages of MRI scanning.
Any three from:
- The imaging of bones is very poor.
- People can suffer from claustrophobia in the scanner.
- Scans can be noisy and take a long time.
- Cannot be used on people with pacemakers.
- Scanners cost millions of pounds.
Outline how MRI scanning is used to obtain diagnostic information about internal organs.
Protons in different tissues have different relaxation times depending on what kind of molecules surround them.
Why do we need non-invasive techniques in diagnosis?
There is less risk of infection, and causes less trauma for the patient.
Explain what is meant by the Doppler effect.
Frequency and wavelength change when the source is moving, because they bunch up in front of it and stretch out behind it - the amount of shift depends on the speed.
How can the Doppler effect be used to calculate the speed of blood?
Ultrasound waves reflect off the blood, which shifts their frequency according to the Doppler effect. How much the waves are shifted depends on how fast the blood is moving, so by measuring the shift, you can measure the speed of the blood flow.
