Magnetic Resonance Imaging

Joseph P. Hornak, Ph.D.

Basic FT Imaging Techniques

Multislice Imaging

Imaging of multiple tomographic slices in one TR period.

Timing diagram for a single-slice 90-FID imaging sequence.

Notice the unused time in the TR period.
Can weuse this time to excite other slices?

How do we excite other slices?

Timing diagram for a multi-slice 90-FID imaging sequence
using n₁, n₂, and n₃.

Oblique Imaging

Imaging of tomographic planes lying between the XY, XZ, and YZ planes.
Oblique imaging performed by applying linear combinations of the X, Y, and Z magnetic field gradients so as to produce a slice selection gradient that is perpendicular to the imaged plane, a phase encoding gradient which is along one edge of the imaged plane, and a frequency encoding gradient which is along the remaining edge of the image.

Example:
Image a slice lying along X axis, passing between Z and Y axes, and making an angle of 30^o to Y axis & 60^o with the Z axis.

Slice Selection Gradient G_z = G_s Sin 60^o

G_y = -G_s Cos 60^o

Phase Encoding Gradient G_z = G Sin 30^o

G_y = G Cos 30^o

Frequency Encoding Gradient

G_x = G_f

Timing Diagram

Spin-Echo Imaging

Timing Diagram

Advantage: introduces T₂ dependence

Inversion Recovery Imaging

Timing Diagram

Note: S = 0 when TI = T₁ ln2.

Gradient Recalled Echo Imaging

When T₁ is long, TR must also be long to maximize signal.

If angle q < 90^o, M_z will recover to an equilibrium more rapidly, but there will be less signal since the signal proportional to Sinq.

With this sequence, we trade off signal for imaging time. In some instances, several images can be collected and averaged together and make up for the lost signal.

Timing Diagram

10^o < q < 90^o
The sequence is repeated every TR seconds.
The TR period could be as short as tens of milliseconds.

Copyright © 2000 J.P. Hornak.
All Rights Reserved.