MRI Flashcards
benefits of MRI
No ionising radiation Uses magnetic fields and radiofrequency pulses Allows repeat studies Non-invasive Excellent soft tissue detail
negatives of MRI
Expensive hardware
Time consuming scans (cf CT secs v 10 x min)
Some participants cannot be scanned
cant scan everyone - metal implants, obese
what is the magnetic field measured in
TESLA (T)
what is the most common magentic field strength used in clinical use
1.5T
why do we use hydrogen
Has spin. Other nuclei like carbon and oxygen do not
Plentiful in body (density varies with tissue type)
Hydrogen + carbon = fat
Hydrogen + oxygen = water
what happens to hydrogen in a magnetic field
Two orientations: parallel or antiparallel
Systems prefer low energy to high energy, so:
The parallel orientation (the “right” way) is a low-energy state
The antiparallel orientation (the “wrong” way) is a high-energy state
why arent all protons the “right”wat in the parallel alignment
Because heat provides energy to move protons to the high-energy (antiparallel) state-and energy gap is very small (depending on magnet strength)
That’s why the magnetic field must be strong – to overcome the effect of heat and produce a significant net alignment of the protons
how do you know where the signal comes from
the RF energy you collect will have an energy that is position dependent
This means the signal you now collect will contain a ‘code’ that tells you where it originated.
This is the key to constructing an image which is basically a ‘signal map’.
typical scan sequence to get one image
5 minute sequence
what is the echo time
Time between putting signal in and picking it up
what is the repetition time
Time between putting each signal in
why do you keep chnaging the magnetic field across the body during the scan
gives equal distribution and good quality image
are more or less repetitions required if the resolution is higher
more reptitions = higher resolution
what is the benefit of tissues having different characteristics in the rate they lose energy
allows different types of MRI scan to be performed that result in different signal contrasts
what is relaxation
the rate energy is lost
