Transcript MRI Imaging - Cornell University

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.
Hydrogen
90o RF
Relaxes and
Pulse
Realigns in
Applied
Magnetic field
Bo
H
However, hydrogen does not simply pivot back
into alignment.
It precesses!
Physics Review
Precession – Gyroscope Example
Animation by Dr. Michael R. Gallis
Penn State Schuylkill
Creative Commons Lisence
Precession of Hydrogen
Atoms
Hydrogen are knocked out of alignment with a radio frequency pulse and
process until they are again aligned with the magnetic field.
Larmor frequency (ω) is
proportional to magnetic field
strength:
ω=γB
Movement is analogous to gyroscope movement.
Animation from http://www.e-mri.org
Physics Review
Lenz’s Law
Increasing B through coil = CW Current
“Right Hand Rule”
If no change in B
NO CURRENT
S
N
Decreasing B through coil = CCW 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.
Pickup Coil
Current

Time
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
Coming up next: How do we tell hydrogen along each axis apart?
Images: http://www.magnet.fsu.edu/education/tutorials/magnetacademy/mri/page5.html
Gradient Slice Selection
Bapplied
wL= γBo
wL : Lamor Freq.
H
H
H
H
γ : Gyration Const.
Bo : Mag. Field
H
H
H
H
Applied
Perpendicular to
desired plane.
H
H
H
H
Spin speed
represents
processional
frequency.
H
H
H
H
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.
Bapplied
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Lets select this
slice
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.
Current

Time
B
B
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Next,
one
slice
on
First select
resolve
Turn off
the
gradient!
first based
dimension
Apply
a gradient along
another axis.
precessional
frequency.
with an applied
gradient.
Notice the precession is “out of sync”
Phase Encoding
Animation from http://www.e-mri.org
Resolving the Third Dimension
Frequency Encoding
Review of Spatial Resolution:
1. Apply slice selection gradient and
choose a slice based on precession
frequency.
H
H
H
H
B
H
H
H
H
Consider plane your image!
2. Apply and turn off phase encoding
gradient. This gets hydrogen in the xaxis out of sync.
H
H
H
H
3. Apply a third gradient, now we can
distinguish hydrogen in the y-axis based
on the precessional speeds.
H
H
H
H
We have now resolved all three
dimensions!
But now what do we do with all this info….
Slice plane
y
z
x
Fourier’s Transform


The pick up coil receives many different frequency oscillations.
Use Fourier’s Transform to process the data.
1.5
1
Transform
Time [s]
Signal Strength
Signal Strength
1.5
1
4
0.25
Freqency [Hz]
-1
-1.5
0.5
f = 1/T = ¼ = .25
f = 1/T = ½ = .5
f = 1/T = 1/1 = 1
1.0
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:
Fourier
Signal Strength
Current
Signal Strength
1
1
Time [s]
4
+
1
Time [s]
4
=
Time [s]
4
-1
-1
1
1
1
0.25
0.25
0.25
0.25
Frequency [Hz]
-1
1
Frequency [Hz]
0.25
Frequency [Hz]
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.
Signal Strength

1
Time
[s]
1
1
4
+
1
4
1
=
4
1

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-Space
A 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-Space
General 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 ModificationsOpen 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 ModificationsFunctional 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/
Thank you for viewing the presentation
Please post comments or suggestions on
the “feedback” section of the project
website.
http://www.simpsonstrivia.com.ar/simpsons-photos/wallpapers/homer-simpson-wallpaper-brain-1024.jpg