Magnetic Resonance Imaging

Joseph P. Hornak, Ph.D.

Advanced Imaging Techniques - I

Volume Imaging (3-D Imaging)

The acquisition of magnetic resonance data from a volume rather than a tomographic slice.

Single Slice Contiguous Slices

Timing Diagram

Imaging Time = TR x phase steps in dimension 1 x phase steps in dimension 2

Magnetic Resonance Angiography (MRA)

The imaging of flowing blood in the arteries and veins of the body.
MRA produces images of flowing blood and the intensity is (generally) proportional to the velocity of the flow.

Time-of-Flight Angiography
One method uses a spin-echo sequence where the slice selective 90^o and 180^o pulses have different frequencies.
Recall the flow artifact description.

Make these changes to the above.

Phase Contrast Angiography
Bipolar magnetic field gradient (G_BP) pulse.

Bipolar Gradient Pulse Shape
Positive

Negative

The area under first lobe must equal that of the second.

A bipolar gradient pulse has no net effect on stationary spins.
Phase accumulated from first pulse: f_A = 2p g x G_BP dt
Phase accumulated from first pulse: f_B = -2p g x G_BP dt
If G_BP of the two lobes are equal and the positions are equal during the two pulses the phase acquired from the A lobe equals that from the B lobe.

BPGP Stationary, Flowing, & Reference Spins
Positive

Negative

Subtracting
Vectors

Pulse Sequence

Examples:
Coronal projection of the flow in the head.

Projection through the brain.

Diffusion Imaging

Imaging of the diffusion coefficient.
Diffusion imaging can be performed in a manner identical to the phase contrast angiography sequence. The major difference is that the gradients must be increased in amplitude so as to image the much slower motions of molecular diffusion in the body.

Fast Spin-Echo Imaging

A multi-echo spin-echo sequence where different parts of k-space are recorded by different spin-echoes.

Echo Number k-space
First Central (lines 96-160)

Second 64-96 and 160-192
Third 32-64 and 192-224
Last 1-32 and 224-256

Chemical Shift Imaging (Fat Suppression)

The production of an image from just one chemical shift component in a sample.
Inversion Recovery Method

Saturation Method.

This technique works best when T₁ for suppressed component is long compared to the time between the saturation pulse and the spin-echo sequence.

Echo Planar Imaging

A rapid MRI technique capable of producing tomographic images at video rates.
The technique records an entire image in a single TR period.
Timing Diagram

Zoom into data acquisition region of the timing diagram.

k-space trajectory map

Functional MRI (fMRI)

Imaging that relates body function or thought to specific locations in the brain.
Images are recorded at video rates and show differences in blood oxygenation/flow in the brain.

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