1 / 12

Spinning Nucleus Produces Magnetic Moment

. p. Spinning Nucleus Produces Magnetic Moment. A moving electric charge produces a magnetic field. An atomic nucleus can be thought of as a spinning charged body, which acts like a tiny magnet. nuclear magnetic moment  =  p p = angular momentum I = nuclear spin (quantum number)

norah
Download Presentation

Spinning Nucleus Produces Magnetic Moment

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. p Spinning Nucleus Produces Magnetic Moment • Amovingelectriccharge produces a magnetic field. • An atomic nucleus can be thought of as a spinning charged body, • which acts like a tiny magnet. • nuclear magnetic moment =p • p = angular momentum • I = nuclear spin (quantum number) • = gyromagnetic ratio •  is collinear with p • Normally, the direction that these tiny magnets point in is randomly distributed.

  2. Bo  Macroscopic Alignment with B-field • A spinning nucleus placed within a large external magnetic field (B0) will align with the external field. M = MBo I = 1/2 case cM: magnetic susceptibility For protons: cM~10-6

  3. Precession at “Resonance” Frequency • The magnetic field exerts a torque on the spinning proton, causing it to precess, similar to a spinning top. • The magnetic moment precesses around the applied field at a rate proportional to the applied static field: the Larmor frequency. • Bo = 1 Tesla • H-1 : 42.58 MHz (o/2) • Na-23 : 11.26 MHz • P-31 : 17.24 MHz • The Lamor frequency for conventional MRI • lies in the radio frequency range.

  4. Bo a B1 Excitation = Tip Magnetization into Transverse Plane • An additional magnetic field B1, perpendicular to the static field B0, can be added to tip the spins into the transverse plane. • B1 is most efficient when its frequency matches the Lamor frequency: resonance condition. Z X Y Rotation frequency B1 Flip angle  = g B1 t M

  5. Bo Mz a B1 Mx,y Relaxation  T1 and T2 “Relaxation” = Return to equilibrium magnetization Longitudinal Transverse

  6. Mxy Mz T1 Recovery and T2 Decay T1 Recovery T2 Decay M0 exp(-t/T2) M0 [1-exp(-t/T1)] time time • T1 and T2 are independent processes, T2≤T1 • Transverse magnetization, Mxy, is the detected signal T2 = T1 T2 = 0.5T1 T2 = 0.25T1

  7. B0 T1 T2

  8. T2 is Dephasing of Transverse Signal Spins precess in XY plane about B0. Variation in B0 causes faster and slower precession rates. z Bo B0 B0 Mxy y x z T1 recovery of Mz Bo MZ MRI signal is “net” vector Mxy Mxy y x

  9. Terminology • T1 is the time constant of Mz to return to equilibrium. • T2 and T2* are time constants of loss of Mxy • T2 signal loss is “entropic” -- it cannot be recovered. • T2* signal loss is reversable (sometimes) with a spin-echo. • TR, Repetition Time • Tissue with shorter T1 recovers Mz faster. • TE, Echo Time (signal acquisition time) • Tissue with shorter T2 (or T2*) loses Mxy faster.

  10. The Spin Echo 90o pulse spins dephase 180o pulse spins re-align spin echo • Spin echo refocuses dephasing from static field inhomogeneity, i.e T2*. • T2 dephasing is not refocused. • Gradient echo creates an artificial, gradient-induced echo. • No refocusing of T2 or T2*.

  11. Contrast • Contrast: Difference in signal intensity • Spatial contrast (e.g. tissue types) • Temporal contrast (changing properties, T1 or T2) • BOLD is a T2* (or T2) contrast T1 Contrast T2 Contrast T1 Contrast T2 Contrast TR (s) TE (ms)

  12. 300 200 100 0 0 1 2 3 4 5 CSF GM WM T1 Contrast Example White Matter Gray Matter CSF TR (s) WM GM CSF

More Related