Correction of Partial Volume Effects in Arterial Spin Labeling MRI
My spin on MRI: The basics of MRI physics and image formation.
description
Transcript of My spin on MRI: The basics of MRI physics and image formation.
![Page 1: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/1.jpg)
![Page 2: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/2.jpg)
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?
![Page 3: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/3.jpg)
The MRI signal is generated by receiving radiofrequency
photons that return to their lower energy state.
•Does an MRI scanner produce radiation?
![Page 4: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/4.jpg)
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
![Page 5: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/5.jpg)
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%+
![Page 6: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/6.jpg)
Vs.
![Page 7: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/7.jpg)
Typical Magnetic Field Map of a Clinical 3T MRIWhat effects will be felt by a pacemaker, credit cards, earrings, IPAD or cell phone?
![Page 8: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/8.jpg)
The MRI scanner is always on!!A magnetic field is present 24/7!!
![Page 9: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/9.jpg)
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.
![Page 10: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/10.jpg)
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.
![Page 11: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/11.jpg)
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.
![Page 12: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/12.jpg)
X
Y
Z
Coil
3.0 Tesla GE MRI Scanner
![Page 13: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/13.jpg)
“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?
![Page 14: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/14.jpg)
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.
![Page 15: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/15.jpg)
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)
![Page 16: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/16.jpg)
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?
![Page 17: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/17.jpg)
Where were all of these metabolic peaks hiding?
What price is paid in detecting these signals?
![Page 18: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/18.jpg)
Damadian’s Design for a Clinical MRI Scanner - 1974
![Page 19: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/19.jpg)
Basic MRI Hardware Block Diagram
How many of you have had an MRI? What’s it like?
![Page 20: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/20.jpg)
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.
![Page 21: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/21.jpg)
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.
![Page 22: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/22.jpg)
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
![Page 23: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/23.jpg)
Conventional 3-Axis MRI Gradient Coil Diagram
![Page 24: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/24.jpg)
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
![Page 25: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/25.jpg)
How thin a slice could an MRI scanner produce?
i.e. Could we perform in-vivo pathology scans?
Slice Selection
![Page 26: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/26.jpg)
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
![Page 27: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/27.jpg)
Explaining the spin echo pulse sequence
Ready,Set, Go!!
Gun startsWith 90 deg pulse.
Courtesy: Siemens
![Page 28: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/28.jpg)
Runners fan out with ability
![Page 29: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/29.jpg)
Gun fires again reversing direction of
race.[180 deg pulse]
![Page 30: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/30.jpg)
The runners thenreach the finish line
at the same timeTE.
![Page 31: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/31.jpg)
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
![Page 32: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/32.jpg)
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?
![Page 33: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/33.jpg)
•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”.
![Page 34: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/34.jpg)
•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.
![Page 35: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/35.jpg)
Why is a Fourier Transform used?
Application of pulses in the “time” domainare transformed into the MRI “frequency”
domain.
![Page 36: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/36.jpg)
K-space vs. Image Space
FT
http://www.leedscmr.org/images/mritoy.jpg
FT
![Page 37: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/37.jpg)
http://www.radinfonet.com/cme/mistretta/traveler1.htm#part1
k-space Contribution to Image Properties
Center = contrast
Periphery = resolution
![Page 38: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/38.jpg)
Voila’ - Spin Echo Images
![Page 39: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/39.jpg)
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
![Page 40: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/40.jpg)
T1WGM=950msWM=600ms
T2WGM=100msWM=80ms
Image Weighting in MRI – * Learning Point *
![Page 41: My spin on MRI: The basics of MRI physics and image formation.](https://reader033.fdocuments.in/reader033/viewer/2022061616/56813ff5550346895dab1779/html5/thumbnails/41.jpg)
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)