MRI Flashcards
What is the Lamor Equation and what does it tell us?
Precession frequency = gyromagnetic ratio x field strength (in Tesla)
Describes the precession frequency of a nuclear magnetic moment and resonant frequency of a nucleus, and relates these aspects to the magnetic field strength.
Basically says the precession frequency gets higher as the field strength increases.
What does an RF pulse do?
- Decreases the longitudinal magnetization
2. Causes the protons to synch up and precess in-phase (which establishes a transverse magnetization).
When can you measure signal in MRI?
When it is NOT in the longitudinal direction.
What is T1?
After you knock the protons down with an RF pulse, they will grow back up to normal size (magnitude will re-orient in the direction of Bo). The time it takes for this to happen is T1 - “longitudinal relaxation”.
Plot of time vs longitudinal magnetization creates the T1 curve - returns to 100% over time. T1 is the time at which longitudinal magnetization is 63% of its final value. Greater field strength = longer T1, b/c net magnetization is greater in a larger field
Sometimes called “spin-lattice relaxation” b/c energy from the RF pulse is handed over to the surrounding lattice.
The “1” looks like a thermometer - T1 relaxation involves the exchange of thermal energy.
Short T1 = bright.
Stronger magnet makes T1 longer - more energy in stronger field - takes longer to hand over to lattice
What is another name for T1?
Sometimes called “spin-lattice relaxation” b/c energy from the RF pulse is handed over to the surrounding lattice.
The “1” looks like a thermometer - T1 relaxation involves the exchange of thermal energy.
What is the definition of T1?
Time at which longitudinal magnetization is 63% of its final value.
Each tissue has different T1 and greater the field strength the longer the T1 (b/c net magnetization is greater in a larger field).
Short T1 = bright.
Is T1 different in a stronger magnet?
Stronger magnet makes T1 longer.
Protons in stronger field have more energy (precess faster), takes longer to hand that over to the lattice.
What is T2?
RF pulse causes the protons to synch up and precess in phase (establishes the transverse magnetization) - will slowly fall out of synch - T2 transverse relaxation.
Time at which the signal has decayed to 37% of its original value of transverse magnetization (63% of its decayed.
Plotting the time vs transverse magnetization creates the T2 curve - downward ski slope - T2 is shorter than T1 - less time to go down a hill than up it.
Also called “spin-spin relaxation”
What is the definition of T2?
Time at which the signal has decayed to 37% of its original value of transverse magnetization - 63% has decayed.
What is the relationship of time of T1 vs T2?
T2 is shorter - faster to go downhill than up it.
What causes protons to lose their transverse sync (T2 relaxation)?
- Inhomogeneities in the external field.
- Inhomogeneities in the local magnetic field - w/in the actual tissues and tissue spin interactions.
Pure things take longer to decay their transverse magnetization and are therefore bright (the opposite is true of impure liquids).
Difference between the T2 of pure vs impure liquids?
Pure things (water) take longer to decay their transverse magnetization - therefore bright (opposite of impure liquids).
What is T2*?
The signal of T2 decays faster than predicted based on tissue spin interactions alone. Math assumes the main external field is absolutely homogeneous, it’s not.
Heterogeneous field creates additional interaction which further speeds decay.
T2* decay is always faster than T2.
What is TR?
Time to repetition - time between initiation of two successive RF pulses.
What is FID?
Free Induction Decay
Give an RF pulse and the protons sync up - start getting a signal. Signal becomes less and less as times goes on - decay via T2* (random + fixed causes).
Will be created with by an RF pulse with any flip angle (90 degree as in spin echo or less than 90 degree as in gradient echo).
Won’t get with an 180 b/c it only inverts the longitudinal magnetization and doesn’t generate a transfer component.
How do you fix T2*?
Wait until half way through T2 decay - hit it with a 180 degree pulse and spin it all the way around - restart the process. This will:
- clear out those inhomogeneities in the field making T2* turn into T2
- Will create an “echo”
What is an “echo”?
The signal tails off, hit it with a 180 pulse and it will come back to refocus = “the echo” - the signal peaks in uniformity at the tip of the echo.
Great time to collect a nice “clean” signal
When do you deliver the 180 pulse?
The 180 is given at the 1/2 T.E. - Time to echo.
What is TE?
Time to echo
Give the 180 pulse at 1/2 TE.
Short TR and Short TE =
T1
Maximize the longitudinal contrast and minimize the transverse contrast - the difference between the T1 and T2 curves
More gap between the lines in the T1 curve early- more closely together early in the T2 curve
Long TR and Long TE =
T2
Maximize the transverse contrast (T2) and minimize the longitudinal contrast (T1).
Longer the TE = greater the T2 effects
Longer TE =
Greater the T2 effects
Increasing ___ increases the T2 effects?
TE
What is a Short TR in Spin Echo?
250-700 ms