The Basics of MRI Physics and Image Formation

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Now not all nuclei are “MRI active”..
Which of the following could produce an MRI image?
Only those with an odd number of protons and neutrons.
1H
11C
13N
18F
19F
31P
•Which isotopes at the right are radioactive?
•Does an MRI scanner produce radiation?
The MRI signal is generated by receiving radiofrequency
photons that return to their lower energy state.
A hydrogen atom (whether bound in water or lipid) acts
as a small magnet due to the spinning
proton
of the positively charged _______.
Proton
Electron
Protons from what compounds comprise an MRI signal?
What percentage of your body is composed of water?
A) 40%-50%, B) 50%-60%, C) 60%-70%, D) 70%-80%
What percentage of your body is composed of fat?
Description
Women
Men
Essential fat
10–12%
2–4%
Athletes
14–20%
6–13%
Fitness
21–24%
14–17%
Acceptable
25–31%
18–25%
Overweight
32-41%
26-37%
Obese
42%+
38%+
Vs.
Typical Magnetic Field Map of a Clinical 3T MRI
What effects will be felt by a pacemaker,
credit cards, earrings, IPAD or cell phone?
The MRI scanner is always on!!
A magnetic field is present 24/7!!
MRI Safety
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•
•
Implants and foreign bodies -“Cheap” Earrings
Projectile or missile effect
Radio frequency energy - Tattoo Ink
Peripheral nerve stimulation (PNS)
> Rock concert @ the gardens.
Acoustic noise
- “Quench”
Cryogens
Contrast agents - Nephrogenic Systemic Fibrosis
Pregnancy
- No X-rays/Gd crosses placenta.
Claustrophobia and discomfort
How does resonance come into play in MRI?
A typical tuning fork produces a frequency of 400 Hertz,
while a scan from Sackler was actually resonating at
127, 503, 172 Hertz.
•A tuning fork produces sound waves at a single
frequency that may be detected by objects that are of
lengths related to multiples of the wavelength.
Larmor Equation: w=gB
w=Precessional Frequency
g= Gyromagnetic Ratio
B=Magnetic Field Strength
(42.57 MHz/Tesla * 3.0 Tesla = 127.5 MHz)
What field strength does my favorite
FM Classic Rock station transmit at?
•Radio waves are transmitted at an angle of 90˚
into the body at the Larmor frequency.
• This imparts energy to the nuclei to achieve “resonance”
The additional energy in turn rotates the nuclei
out of alignment with the main field.
3.0 Tesla GE MRI Scanner
Y
Coil
X
Z
“Magneto”
Faraday’s Law of Induction states that a voltage is
created by a changing magnetic flux. (1831)
Was it easier back then to get a law named after you?
How do the motion of these two objects differ?
“Rotation” vs. “Precession”
•It is the precession of the nuclei that creates the
changing magnetic field needed to produce a signal.
What kind of signal is actually received by the scanner?
•The frequency & phase information in time from the
Free Induction Decay “FID” are transformed
into the frequency domain. (NMR 1946)
•A Fourier series can represent any function
as a sum of sines and cosines. (1822)
Typical NMR signal after Fourier transformation.
Can you identify the peaks? How about concentration?
H2O (4.7ppm)
Lipids CH2 (1.3ppm)
Lipids CH3 (0.9ppm)
Where were all of these metabolic peaks hiding?
What price is paid in detecting these signals?
Damadian’s Design for a Clinical MRI Scanner - 1974
Basic MRI Hardware Block Diagram
How many of you have had an MRI? What’s it like?
How loud is loud?
20 dB
30 dB
40 dB
50 dB
60 dB
70 dB
80 dB
Ticking watch
Quiet whisper
Refrigerator hum
Rainfall
Sewing machine
Washing machine
Alarm clock (two feet away)
85 dB
95 dB
100 dB
105 dB
110 dB
120 dB
130 dB
Average traffic
MRI
Blow dryer, subway train
Power mower, chainsaw
Screaming child
Rock concert, thunderclap
Jackhammer, jet plane (100 feet away)
Fast imaging sequences such as EPI/Spiral used in
functional neuroimaging (fMRI) can play upwards of
100+ decibels inside the bore of the scanner.
So how do we get spatial information?
w=gB
Position
Back to the Larmor equation..
Magnetic Field Strength
i.e. 1 Gauss will increase the frequency by 4.3kHz.
Typical gradient strengths are 2-5 Gauss/cm.
What would the frequency difference be
between two objects that are separated by 3cm?
w=gB
g= 42.57E6 Hz/Tesla
B = Gz * z
= 0.01 T/m * 0.03 m
w = 12,771 Hertz
Conventional 3-Axis MRI Gradient Coil Diagram
Slice Selection
1st step is to excite a single slice instead of all space!
Frequency
To excite a thickness z use: DZ=Dw/gGZ
To excite off axis use: w+w0 where w= gGZ DZ
Slice Selection
How thin a slice could an MRI scanner produce?
i.e. Could we perform in-vivo pathology scans?
General Electric Spin Echo Pulse Sequence Diagram
Readout
Read
Rewinder
Phase Encode
Phase
Slice Select Gradients
Slice
TE/2
90°
TE/2
180°
TR
Rewinder
Explaining the spin echo pulse sequence
Ready,
Set,
Go!!
Gun starts
With 90 deg pulse.
Courtesy: Siemens
Runners fan out with ability
Gun fires again reversing
direction of race.
[180 deg pulse]
The runners then
reach the finish line
at the same time
TE.
General Electric Spin Echo Pulse Sequence Diagram
Readout
Read
Rewinder
Phase Encode
Phase
Slice Select Gradients
Slice
TE/2
90°
TE/2
180°
TR
Rewinder
Now that we have selectively excited a specific slice
in space, we then must localize a specific xy-plane.
With what pattern is MRI data generally acquired?
Why would you choose one over the other?
•Spatial encoding in x is called “Frequency Encoding”.
•The frequency of the signal ~ position on the x-axis.
dx = FOVx/Nx = 1/(g/2p Gx tx)
e.g. A standard brain scan uses a 24 cm FOV
and a 512x512 matrix size on our 3T magnet.
This gives an in-plane resolution of 0.47mm/pixel.
RBW = Nx /
tx = 1 /DT
e.g. A 15.63kHz RBW and Gx = 0.3 G/cm would
then apply the x-gradient for 32.8 ms to get
a single line of image “k-space”.
•Spatial encoding in y is called “Phase Encoding”.
•The phase f of the signal ~ position on the y-axis.
dy = FOVy/Npe = =1/(2 g/2p Gyr ty)
The phase of a signal is given by: f = w t
To acquire the next line in “k-space”, an additional
phase (gGyy) is applied for a time t.
This is repeated until the entire image space is covered.
•It is standard for the time to be fixed and the
gradient amplitude to increase/decrease.
Why is a Fourier Transform used?
Application of pulses in the “time” domain
are transformed into the MRI “frequency” domain.
K-space vs. Image Space
FT
FT
http://www.leedscmr.org/images/mritoy.jpg
k-space Contribution to Image Properties
Center = contrast
Periphery =
resolution
http://www.radinfonet.com/cme/mistretta/traveler1.htm#part1
Voila’ - Spin Echo Images
How does an MRI scanner differ from a CT scanner?
1)Radiation, 2) Soft-Tissue Contrast
The intensity on a CT scan is directly related to what?
How much energy does MRI impart?
EMRI=h(g/2p)B0 =0.3 meV vs. ECT~ 25keV
CT
T1
T2
Image Weighting in MRI – * Learning Point *
T1W
GM=950ms
WM=600ms
T2W
GM=100ms
WM=80ms
Summary:
Magnetic Resonance Imaging
• Soft Tissue Contrast (GM vs. WM, etc.)
• High Spatial Resolution ( 1 mm isotropic voxels)
• Oblique scanning options
Additional functionality:
Diffusion MRI, Magnetization Transfer MRI
Fluid attenuated inversion recovery (FLAIR)
Angiography, CSF Dynamics, Spectroscopy
Functional MRI, Interventional MRI, Contrast agents
MR guided focused ultrasound, Multinuclear imaging
Susceptibility weighted imaging (SWI)

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