Upload
harvey
View
76
Download
9
Tags:
Embed Size (px)
DESCRIPTION
My spin on MRI: The basics of MRI physics and image formation. Jonathan Dyke, Ph.D. Assistant Research Professor of Physics in Radiology Citigroup Biomedical Imaging Center Weill Cornell Medical College Sackler Institute for Developmental Psychobiology Summer Lecture Series - PowerPoint PPT Presentation
Citation preview
Now not all nuclei are “MRI active”..Which of the following could produce an MRI image?
1H11C13N18F19F31P
Only those with an odd number of protons and neutrons.
•Which isotopes at the right are radioactive?
The MRI signal is generated by receiving radiofrequency
photons that return to their lower energy state.
•Does an MRI scanner produce radiation?
E lec tron
P ro ton
A hydrogen atom (whether bound in water or lipid) acts as a small magnet due to the spinning
of the positively charged _______.proton
Protons from what compounds comprise an MRI signal?
What percentage of your body is composed of water?
What percentage of your body is composed of fat?
A) 40%-50%, B) 50%-60%, C) 60%-70%, D) 70%-80%
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 MRIWhat 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
• Implants and foreign bodies • Projectile or missile effect • Radio frequency energy • Peripheral nerve stimulation (PNS) • Acoustic noise • Cryogens • Contrast agents • Pregnancy • Claustrophobia and discomfort
-“Cheap” Earrings
- Tattoo Ink
> Rock concert @ the gardens.- “Quench”
- Nephrogenic Systemic Fibrosis- No X-rays/Gd crosses placenta.
How does resonance come into play in MRI?
•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.
A typical tuning fork produces a frequency of 400 Hertz,while a scan from Sackler was actually resonating at
127, 503, 172 Hertz.
Larmor Equation: =Precessional Frequency
= Gyromagnetic Ratio=Magnetic Field Strength
(42.57 MHz/Tesla * 3.0 Tesla = 127.5 MHz)
What field strength does my favoriteFM 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.
X
Y
Z
Coil
3.0 Tesla GE MRI Scanner
“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 thechanging 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)
H2O (4.7ppm)
Lipids CH2 (1.3ppm)
Lipids CH3 (0.9ppm)
Typical NMR signal after Fourier transformation.Can you identify the peaks? How about concentration?
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?
20 dB30 dB40 dB50 dB60 dB70 dB80 dB
Ticking watchQuiet whisperRefrigerator humRainfallSewing machineWashing machineAlarm clock (two feet away)
85 dB95 dB100 dB105 dB110 dB120 dB130 dB
Average trafficMRIBlow dryer, subway trainPower mower, chainsawScreaming childRock concert, thunderclapJackhammer, jet plane (100 feet away)
How loud is loud?
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?
Back to the Larmor equation..
Magnetic Field Strength
Posi
tion
i.e. 1 Gauss will increase the frequency by 4.3kHz. Typical gradient strengths are 2-5 Gauss/cm.
What would the frequency difference bebetween two objects that are separated by 3cm?
B
= 42.57E6 Hz/Tesla
B = Gz * z = 0.01 T/m * 0.03 m
= 12,771 Hertz
Conventional 3-Axis MRI Gradient Coil Diagram
Slice Selection1st step is to excite a single slice instead of all space!
Frequency
To excite a thickness z use: GZ
To excite off axis use: where = GZ
How thin a slice could an MRI scanner produce?
i.e. Could we perform in-vivo pathology scans?
Slice Selection
General Electric Spin Echo Pulse Sequence Diagram
180°
TE/2
Rewinder
Readout
Slice Select Gradients
TE/2
90°
Rewinder
Phase Encode
Read
Phase
Slice
TR
Explaining the spin echo pulse sequence
Ready,Set, Go!!
Gun startsWith 90 deg pulse.
Courtesy: Siemens
Runners fan out with ability
Gun fires again reversing direction of
race.[180 deg pulse]
The runners thenreach the finish line
at the same timeTE.
General Electric Spin Echo Pulse Sequence Diagram
180°
TE/2
Rewinder
Readout
Slice Select Gradients
TE/2
90°
Rewinder
Phase Encode
Read
Phase
Slice
TR
Now that we have selectively excited a specific slicein 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.
x = FOVx/Nx = 1/(/ Gx x)
e.g. A standard brain scan uses a 24 cm FOVand a 512x512 matrix size on our 3T magnet.
This gives an in-plane resolution of 0.47mm/pixel.
RBW = Nx / x = 1 /T
e.g. A 15.63kHz RBW and Gx = 0.3 G/cm wouldthen 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 of the signal ~ position on the y-axis.
y = FOVy/Npe = =1/(2 / Gyr y)
The phase of a signal is given by: t
To acquire the next line in “k-space”, an additional
phase (Gyy) 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” domainare transformed into the MRI “frequency”
domain.
K-space vs. Image Space
FT
http://www.leedscmr.org/images/mritoy.jpg
FT
http://www.radinfonet.com/cme/mistretta/traveler1.htm#part1
k-space Contribution to Image Properties
Center = contrast
Periphery = resolution
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(B0 =0.3 eV vs. ECT~ 25keV
CT T1 T2
T1WGM=950msWM=600ms
T2WGM=100msWM=80ms
Image Weighting in MRI – * Learning Point *
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 agentsMR guided focused ultrasound, Multinuclear imaging Susceptibility weighted imaging (SWI)