Diagnostic Methods (CH16) Flashcards
How are medical tracers used to diagnose the function of organs?
They are bound to a substance used by the body.
It goes where that substance would normally go.
Tracer injected into body, absorbed by organ.
The radiation emitted is detected and thus located using beta detector or gamma camera.
How does a gamma camera work?
The emitted gamma photons pass through a collimator: parallel lead tubes so only one direction of photon passes.
The photons then strike a scintillator (NaI crystal), producing photons of visible light.
A single photon of visible light produces an electron on the first dynode, and the number of electrons increases with more dynodes.
So at the last electrode/dynode there is an electrical pulse and the computer can show the location.
Describe the principles of PET
The patient is injected with an e+ emitting radiotracer (F-18) attached to glucose, for eg.
The radiotracer will be taken up by most active parts (of brain).
The positron collides with an electron, emitting pairs of gamma rays which are detected and so location can be determined.
Outline the principles of magnetic resonance
Nuclei spin about an axis so they have N&S poles.
They will line up parallel & anti parallel to external magnetic field. Anti-parallel is a high energy state.
Their angular frequency of precession is the Larmor Frequency, prop. to B.
Radio waves at this frequency(/2pi) are used to flip the proton into a higher energy state by resonance.
The radio waves stop, protons relax and release EM energy and the rate of relaxation shows what molecule SURROUND THE PROTONS.
How does an MRI scanner work?
A cooled (4.2K using liquid helium) superconducting magnet provides an external magnetic field up to 2T.
There are radio frequency transmitter and detector coils.
There are gradient coils that ensure only a small volume of body resonates so the source of relaxation signal can be located.
A computer controls gradient coils and RF pulses and produces an image from the relaxation time.
What are the advantages of MRI?
Safe as no ionising radiation
It shows good soft tissue contrast.
Any cross/section or a 3D image can be shown.
What are the disadvantages of MRI?
Pacemakers or metal objects cannot be scanned as they become hot.
The machines are very expensive (millions) and take a long time (up to an hour) for a scan.
What is meant by the Doppler Effect?
The wavelength/frequency of a wave changes when there is a relative velocity between the source and detector.
How can the Doppler Effect be used to determine the speed of blood?
Ultrasound transducer emits and detects ultrasound.
Transducer placed at angle to artery.
Ultrasound is reflected by blood.
The change in frequency is related to the speed of the blood.
