From key@physics.utoronto.ca Mon Apr 7 09:24:26 2008 Date: Mon, 07 Apr 2008 09:24:25 -0400 From: Tony Key To: jessica.mak@utoronto.ca Subject: Re: Relaxation times Hi Jessica - the return of the macroscopic spin to its normal value in the applied field takes place through two different mechanisms. The spin-spin interaction - between neighbouring spins - has associated time T2; the spin-lattice, with T2, comes about because of interaction with the lattice of the medium where the protons are located. The former happens faster - i.e. the xy components are reduced to zero faster as randomization of the precession takes place - than the former - which is a return of the z-component of the macroscopic spin to its full value under the action of the external applied field. The two things start together,immediately the rf field excitation is switched off, but the reduction of the x-y component happens more quickly than the reestablishment of the z component - the spin-spin interaction is stronger than the spin-lattice interaction. If you play the MRI movie flash diagram linked to the N&R page, you'll see it happening - the x-y vector goes to zero before the Mz has reached its full value. I'm not sure if I answered your excellent question - let me know if I need to try again! TonyK jessica.mak@utoronto.ca wrote: > Hi Prof. Key! > > Quick question: How is it that T1 and T2 take place simultaneously > (quoted from suppl. VI) When T1 is required to return Mz to M and then T2 > reduces Mxy using the M (formula in notes). Should T1 take place before > T2 then??? > > Thanks! > > Jessica > [ Part 2, Text/X-VCARD (charset: UTF-8 "Internet-standard ] [ Unicode") (Name: "key.vcf") 9 lines. ] [ Unable to print this part. ]