Lesson 2 Flashcards
What is B0?
Main magnetic field, it is fixed
What is B1
RF field, it is oscillating. It is rotating in the xy transverse plane, around the z axis
What is the role of the RF coil?
It applis an RF magnetic field B1, rotating at the larmor frequency. It produces a torque that rotates M towards the x axis. If pulse is applied for log enough, B1 will eventually move M by 90 degress onto the y axis
(Q)If appropriate energy is applied and more spins are excited to antiparallel state what happens?
What is the equation for the appropriate energy
Spins gain coherance, and become in phase.
(For higher magnetic field scanner, you have to put in more energy in to do this)
E=hfL
How do we detect M (magnetisation)? (C)
Use a receiver coil of wire which gets a current generated in response to magnetic field, the coil has to be perpendicular to M. M is rotating about B0 at the lamor frequency. This produces a rotating magentic field which can be picked up by placing the wire close to this. (Get positive and negative flux in signal as M is close/far from coil)
What is meant by phase/frequency?
Frequency is rate of oscillations
Phase is the relationship between the peaks/troughs in a given amount of time (are they together or not)
What jhappens after 90 degree flip that occurs when B1 is turned on?
System returns to equilibrium by:
1. Regrowth of Mz magnetisation along z axis (longitudinal relaxation) - this takes longer
2. Transverse relaxation, decay of magnetisation vector Mxy in the transverse plane
These occur at different rates
What is T1
Spin lattice relaxation/ longitudinal relaxation
Mz realigns with |Bo in a time
What does T1 depend on?
It ideally has oscillating magentic field around at Lf, in order to release energy (which it needed to get excited). Spins lose their magentic energy to the surroundings (lattice). This process is stimulated emission in QM.
Fat is a good molecule for this, which is why fat is bright on T1.
What is the equation which shows T1 (Mz recovery by T1 relaxation)?
Mz (t) = M0 [1-e^t/T1]
What is T2?
Spin-spin relaxation, when there is loss of phase coherence as spins fan out. This is produced by local magnetic fields produced by neighbouring spins. Magentisation in transverse plane Mxy decays at exponential rate defined by spin-spin relaxation rate.
What is the equation used to exhibit T2 decay rate?
Mxy (t) = M0 [e^-t/T2]
What happens to energy in T1 and T2 relaxation?
Net E transfe between protons and their surroundings, spin - lattice.
In T2 there is exchange of E between spins, spin -> spin, not net E loss from the whole system
What is quicker, T1 or T2?
T2»T1
What is the most efficient environment for T2? (i.e longer T2, takes longer time to decay)
Water - liquid material molecules are rapidly tumbling and magnetic effect of neighbour molecules may be cancelled out on time scale of MR measurement. Longer T2, MRI can be performed
