MRI Imaging

By: Scott Hayes

MRI measures the movement of hydrogen atoms: Why hydrogen atoms?

Hydrogen is abundant in the water molecules in human tissue. The nuclei of Hydrogen can act like a “compass needle” in a magnetic

field. Some types of tissues prevent hydrogen from spinning as freely. For

example, bone is more restrictive to movement than fat. These differences can be detected to distinguish tissue type.

http://www.cs.sfu.ca/~stella/papers/blairthesis/main/_4056_figure87.gif

RF Pulse knocks H out of alignment

Hydrogen are knocked out of alignment with a radio frequency (RF) pulse and relax back into alignment with magnetic field.

HBo

90o RF Pulse

Applied

Hydrogen Relaxes and Realigns in

Magnetic field

However, hydrogen does not simply pivot back

into alignment. It precesses!

Physics ReviewPrecession – Gyroscope Example

Animation by Dr. Michael R. GallisPenn State Schuylkill

Creative Commons Lisence

Precession of Hydrogen Atoms

Larmor frequency (ω) is proportional to magnetic field strength:

ω = γ B

Animation from http://www.e-mri.org

Hydrogen are knocked out of alignment with a radio frequency pulse and process until they are again aligned with the magnetic field.

Movement is analogous to gyroscope movement.

Physics ReviewLenz’s Law

NS

Increasing B through coil = CW Current

“Right Hand Rule”

Decreasing B through coil = CCW Current

If no change in B

NO CURRENT

Why is Lenz’s Law important to MRI?

Each Hydrogen acts like a mini-magnet. Procession of Hydrogen can produce a measurable electric

current along a pickup coil. As hydrogen precesses, current oscillates until hydrogen aligns

with magnetic field.

Cur

rent

Time

Pickup Coil

Hydrogen is relaxing back into alignment of

magnetic field.

MRI Basic Layout

The magnetic field of an MRI machine is typically 3 Tesla!

The Earth’s magnetic field is less that 30 microtesla (0.00003 Ts).

http://www.magnet.fsu.edu/education/tutorials/magnetacademy/mri/index.html

MRI Machinery

Images: http://www.magnet.fsu.edu/education/tutorials/magnetacademy/mri/page5.html

Coming up next: How do we tell hydrogen along each axis apart?

Gradient Slice Selection

H H H H

H H H H

H H H H

H H H H

BappliedwL= γBo

wL : Lamor Freq.

γ : Gyration Const.

Bo : Mag. Field

Applied Perpendicular to desired plane.

Spin speed represents processional frequency.

Why is a Slice Selection Gradient used?

Magnetic Field applied perpendicular to desired slice, because we can now “focus” on a layer with a specific processional frequency. Hydrogen atoms to

either side of desired layer are either too fast or too slow.

H

H

H

H

Bapplied

H

H

H

H

H

H

H

H

H

H

H

H

Lets select this slice

H

H

H

H

H

H

H

H

H

H

H

H

Phase Encoding – Resolves image in second

dimension. Apply a magnetic gradient, but

only briefly. Goal: Get hydrogen atoms out of

sync with each other so they can be distinguished along another axis.

H

H

H

H

H

H

H

H

H H

H H

H H

H H

B

First resolve the first dimension with an applied gradient.

Next, select one slice based on precessional frequency.Apply a gradient along another axis.

Turn off gradient!

Notice the precession is “out of sync”

B

Cur

rent

Time

Phase Encoding

Animation from http://www.e-mri.org

Resolving the Third DimensionFrequency Encoding

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

x

y

z

Slice plane

Review of Spatial Resolution:

1. Apply slice selection gradient and choose a slice based on precession frequency.

Consider plane your image!

2. Apply and turn off phase encoding gradient. This gets hydrogen in the x-axis out of sync.

3. Apply a third gradient, now we can distinguish hydrogen in the y-axis based on the precessional speeds.

We have now resolved all three dimensions!

But now what do we do with all this info….

B

Fourier’s Transform The pick up coil receives many different frequency oscillations. Use Fourier’s Transform to process the data.

Time [s]Sig

nal S

tren

gth 1

4

Freqency [Hz]

Sig

nal S

tren

gth

1

0.25

f = 1/T = ¼ = .25

1.5

0.5

1.5

-1

-1.5f = 1/T = ½ = .5

f = 1/T = 1/1 = 1

1.0

Transform

Fourier Transform (cont.)The pickup coil does not distinguish between the input of each hydrogen.

They are all read together, and constructively and destructively interfere.

Fourier’s allows us to determine which frequencies are along the axis.

For instance, if there are two hydrogen at different frequencies along an axis:

Sig

nal

Str

engt

h

Time [s]

1

-14

Time [s]

1

-14

Time [s]

1

-1

4+ =Current

Fourier

Frequency [Hz]

1

0.25Sig

nal

Str

engt

h

0.25

Frequency [Hz]

1

0.25 1

0.25

Frequency [Hz]

1

0.25

1

Image formation

Animation from http://e-mri.org

2D Fourier Transform

Recall that the second axis is resolved with a phase encoding gradient.

These hydrogen have the same frequency, but interfere with each other due to phase shift.

Sig

nal

Str

engt

h 1

-1

4Time [s]

1

-1

4

1

-1

4+ =

A 1D Fourier Transform cannot distinguish between shifted phases.

But if we take the Fourier Transform again, orthogonal to the first access the phase encoding gradient can be distinguished!

The resulting data is known as a K-Space.

K-SpaceA 2D Fourier transform is

conducted by performing two Fourier transforms orthogonal to

each other.

This yields a “K-Space”

An example is seen on the right.

The “K-Space” undergoes an Inverse Fourier Transform.

Following this mathematical step, we finally have an image.

http://www.revisemri.com/tutorials/what_is_k_space/what_is_k_space_files/fullscreen.htm

K-SpaceGeneral spatial information is

concentrated towards the center of “K-Space”

In the figure to the right we see an image formed taking only the

Inverse Fourier Transform of the center of the K-Space.

As seen on the right, the peripheral regions of the K-Space encode for

the edges of the image.

http://www.revisemri.com/tutorials/what_is_k_space/what_is_k_space_files/fullscreen.htm

Why does an MRI machine make so much noise?

When gradients are applied, the strong magnetic field causes the coils to stretch.

Examples of sounds resulting from standard pulse sequence – Link

Pulses can be “tuned” (Wilson 2001)

Just for fun…some MRI music.

http://www.adarngooddog.com/Man_Covering_His_Ears%20cartoon.gif

MRI Modifications-Open MRI

Claustrophobic patients can’t tolerate the confined enclosure of an MRI machine.

Even mildly claustrophobic patients have trouble due to the very loud noise produced by the machine.

Open MRI machine works the same way, but with a weaker magnetic field and less resolution.

New 1 Tesla open MRI machines offer adequate resolution A standard MRI machine has a 3

Ts magnetic field.

A 1 Tesla open MRI machine manufactured by Phillips.

MRI Modifications-Functional MRI (fMRI)

Hemoglobin has different magnetic properties when bound to oxygen, that can be distinguished by fMRI.

Areas of brain activity have a surge of oxygenated blood.

fMRI can identify areas of the brain with high oxyhemoglobin content, which correlates to areas of heightened brain activity.

http://www.unmc.edu/dept/alliedhealth/rste/ctmri/

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