My spin on MRI: The basics of MRI physics and image formation

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)