18-12-2012, 01:17 PM
PULSE SEQUENCES IN MRI
PULSE SEQUENCES.pptx (Size: 302.7 KB / Downloads: 22)
Types of pulse sequence in MRI
Spin echo pulse sequence
Multislice technique
Multiecho technique
Inversion recovery pulse sequence
SPIN ECHO
The Spin Echo imaging technique has the advantage that it is not as sensitive to static inhomogeneity of the magnet and inhomogeneity caused by magnetic susceptibility of patient tissue.
The dephasing of the nuclear dipoles after 90° phase is due to two quantities.
a) spin spin relaxation time. (T2)
b) H- field inhomogenity.
It starts with a 90° pulse , M will precess at about 'x‘ & 'y' axis at larmor frequency.
But at end of 90° pulse, dephasing starts due to inhomogeneity of the field.
some precess faster & some are slower.
Fast dipoles appear to move in +y direction & slow dipoles appear to move in -y direction. They therefore, appear to move in opp. direction.
After time τ,180° pulse is applied.
180° pulse inverts the magnetisation component.
Multislice imaging:
It is a way to acquire all the slices at one time, called “multislice imaging”.
This tech takes advantages of long waiting time required by the TR interval. if the TR interval is 2000ms,& TE is 90ms there is about 1900ms of free time between end of echo and the next 90° pulse.
It is this long wait, imposed by the long TR, that allows multislice imaging.
Multiecho technique:
A technique called “ multiecho imaging” allows one to obtain two or three images with different TE times during the same time interval required for one image.
Fig shows simplified diagram of a multiecho acquisition using a TR of 2000ms, and TE of 30,60,and 90ms.
Notice that the amplitude of echo decreases with lengthening TE time. this is effect of continued T2 dephasing. Because the early echo yields the strongest signal ,the early echo will have the best signal to noise ratio.
Inversion recovery technique:
Creation of heavily T1-weighted images without a dominant contribution from fat (e.g. brain, liver and musculoskeletal imaging).
Suppression of selected tissues (e.g. orbital fat, liver screening, fatty tumours , CSF).
The basic IR pulse sequence consists of a 180° “inversion pulse” inserted before whatever sequence you usually choose for a particular contrast. The standard 90° RF excitation pulse of your sequence follows the inversion pulse after an inversion time TI.
First, application of the 180° RF pulse inverts the macroscopic magnetization. During the inversion time, the macroscopic magnetization shrinks along the negative Z axis, eventually passes through Z=0 and regrows along the positive Z axis toward thermal equilibrium. Before the macroscopic magnetization is fully relaxed, the 90° RF pulse flips the partially relaxed longitudinal magnetization into the transverse plane in order to measure the signal induced in an RF coil.