MIRA2015 Lecture Notes - Lecture 4: Free Induction Decay, Spin Echo, Larmor Precession

• School: Curtin University
• Department: Medical Imaging, Radiology & Radiography
• Course: MIRA2015
• Professor: Cat Kealley

Document Summary

So far, fid arises from 90 pulse, but fid created by rf pulse of any flip angle because some components of longitudinal magnetisation is always tipped into transverse plane. Fid = sine wave oscillating at larmor frequency damped by exponential decay (relaxation) Gradient echo: manipulation of fid signal by applying external dephasing gradient field across tissue. Step 2: process reversed, rephrasing gradient (same strength, opposite polarity) applied, reversing/undoing phase scramble. A small gradient echo has been generated note: rephrasing gradient only refocused spins scrambled by the dephasing gradient itself. T2*: decay of transverse magnetisation caused by a combination of spin-spin relaxation and magnetic field inhomogeneity. T2* relaxation is seen only with gradient-echo (gre) imaging because transverse relaxation caused by magnetic field inhomogeneity is eliminated by the 180 pulse at spin-echo imaging. Spins precessing rapidly and accumulating phase due to location in a stronger portion of the gradient. More slowly precessing spins in a weaker part of gradient.