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
what are the three types of relaxation
T1-TR
T2-TE
T2*-TE
what is quenching
Deliberate or accidental release of liquid helium removes magnetic field
Risk of frostbite, suffocation (due to replacement of oxygen) or trapping of personnel (due to increased ‘air’ pressure).
what has the slowest relaxation time
water
what has the fastest relaxation time
fat
in a T2 weight image what are the relaxtation times of water and fat
water - high
fat - low
in a T1 weight image what are the relaxtation times of water and fat
water - low
fat - high
effect of short tR
maximise T1 contrast
effect of long TR
maximise T2 contrast
effect of short TE
maxmises T2 contrast
effect of long TE
mamxise t2 contrast
what imaging is produced from a short TR and TE
T1W
what imaging is produced from a long TR and TE
T2W
use of DWI in MI
Acutely infarcted tissue can look normal on standard MRI
After 1 hour post cerebral artery occlusion changes have been seen on DWI but T1 & T2 retain normal appearance. T2 changes may take up to 12 hrs.
Approx stage of infarction can be identified – clinically significant
88-100% acute or subacute infarcts may be identified
use of perfusion scanning
Image demonstrates cardiac muscle perfusion
May also be used in the brain to evaluate salvageable cerebral tissue in case of stroke
why do you need a RF generator
The energy difference between the two states is very small-corresponds to a radio wave energy level
