Magnetic Resonance Imaging

Joseph P. Hornak, Ph.D.

Advanced Imaging Techniques - II

In Vivo Spectroscopy

Producing NMR spectra of living organisms.

Surface Coil Techniques
B₁ magnetic field near a surface coil.

Example of the intensity variation from surface coil using a high power RF pulse.

Spoiling B_o near the surface will give signal from a deeper region.

Sensitive Point Techniques

Slice Selective Techniques
Pulse Sequence

A second echo is recorded as the signal.
FT the echo to produces an NMR spectrum.

Spectroscopic Imaging (SI) Techniques
(Recording an NMR spectrum from each voxel in an image.)
Pulse Sequence

Chemical Contrast Agents

A substance that is introduced into the body to change the contrast between the tissues.
The contrast is changed by varying the T₁ and T₂ of the tissues.

Affinity Based Contrast Agent Application

Central Nervous System

Central Nervous System

Central Nervous System

feric ammonium citrate Stomach and
Upper Small Intestine

Lesions of the Liver

Central Nervous System

Physiology Activated Application

Ca

Magnetization Transfer Contrast

A method of increasing the contrast between tissues by physical rather than chemical means.
For this technique to be effective, there must be at least two spin systems in the imaged anatomy which are capable of exchanging energy between themselves and one of the systems must have a much shorter T₂ than the other system.

Pulse Sequence

The two spin systems could be protein and water.
The protein has a very short T₂ relative to the water T₂.

Applying the saturation pulse 1 kHz away from the center of these peaks could directly saturate the protein spin system and not the water.

Energy Reservoirs

Protein reservoir which is connected to a water reservoir

and unconnected water reservoir.

Energy placed in any one of the energy reservoirs will return to the lattice or surrounding molecules via spin-lattice relaxation.

Energy which is placed only in the protein reservoir by the frequency selective saturation pulse will influence the energy of the water spin system which is connected to it.

If a pulse sequence is used to probe the magnetization of the two water spin systems while there is still energy in the protein connected water system, the protein connected water system will produce image intensity as if a short TR was used. The unconnected water system will produce image intensity as if a long TR was used. There will now be contrast between the two types of water, even if the T₁ values for the two types of water are equivalent.

Variable Bandwidth Imaging

The amount of noise in an image is related to the sampling frequency of the FID or echo.
Since the sampling rate, f_s, is related to the field of view (FOV), the frequency encoding gradient, G_f, must be lowered proportionately to the sampling frequency in order to keep the FOV constant.

FOV = f_s / G_f

Timing Diagram - fast sampling frequency

Timing Diagram - slow sampling frequency

Disadvantages of a slower sampling frequency.
1. An increase in the chemical shift artifact.
2. A loss of contrast.
3. A restricted range of echo times, TE.
Advantage of a slower sampling frequency.
1. Better SNR.
Examples: 16 kHz bandwidth and 3 kHz bandwidth

16 kHz Bandwidth 3 kHz Bandwidth

As the sampling rate decreases, the sampling window increases.

T₁, T₂, & Images

T₂

T₁

Tissue Classification

Tissue classification, or image segmentation as it is also called, is the identification of tissues in a magnetic resonance image.

Example: Segmentation

Histogram & Linear Look-up Table

Segmentation of CSF and gray matter from other tissues by modifying the lookup table such that the green and blue components of a pixel are turned off for data values greater than 865.

This procedure will create red CSF and gray matter pixels.

Example: Segmentation with T₁, T₂, and r images of the human brain.

Images used to create a 3-D histogram

Segmented tissues

Example: Morphology or Texture-Based Segmentation

red = normal
green = regions which have deminished trabecular or osteoporotic
blue = cystic
light blue = sclerotic

Hyperpolarized Xe Imaging

Imaging of Xe gas prepared such that its magnetization is much greater than at equilibrium.
Rat Brain

Rat Brain (Rat breathing hyperpolarized ¹²⁹Xe for approximately 40 s.

(¹²⁹Xe image courtesy of S. Swanson.)

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