Magnetic Resonance Imaging II Flashcards
What is a proton and what are its properties?
Charged body
Intrinsic spin - dipole moment
Rigid body - precession
Larmor frequency
What is electromagnetism?
Proton is a charged body Quantised, intrinsic spin angular momentum Classical interpretation… …proton spins on its own axis Angular momentum of magnitude L (neutron also has intrinsic spin) Current loop Looks like a magnet… …strength… m=yL Dipole – moment
What is precession?
Precession is in a plane orthogonal to plane of couple
Rotating direction of spinning mass affects direction of precession
The greater the applied torque, the faster the precession
What are linear mechanics?
Position vector r Force F Momentum p p = mr F = mr F = p Force = rate of change of linear momentum
What are rotational mechanics?
Torque = rate of change of angular momentum Position vector r Torque G Angular Momentum J G = r x F J = r x p G = J
What is the simplistic classical MR model?
Proton: Charged body, Rigid body, Intrinsic spin
Applied B-field: Precession, Radiated RF
Detection: Tuned RF sense coils
Proton density signal
What is phase coherence?
Many protons
RF signal emitted from each
Complete phase cancellation
No net RF power detected
How is phase coherence explored?
Apply B-field
Spins attempt to align
Small alignment -> magnetization
Aligning torque -> precession
Why does asymmetry in precessional plane garantees signal?
imperfect phase cancellation
Some phase coherence->detectable signal
What is Larmor frequency?
w=yB
How can you utilise asymmetry of magnetism?
Drive z-magnetization into the precessi onal plane
Use precession…
Resonance
B-component of incident RF absorbed by sample
What is relaxation?
A function of asymmetry in precessional plane
Asymmetry relaxes to return to equilibrium condition
Two independent relaxations
Transverse (spin-spin) – T2 (fast)
Longitudinal (spin-lattice) – T1 (slow)
Exponential in nature
