7/25/2019

1

Basic MR Physics and MR Safety

Juliet Varghese, PhDResearch Scientist, Dorothy M. Davis Heart and Lung Research Institute

Foundations of CMR Level 1 Workshop, Columbus, Ohio

July 13, 2019

MRI: What is in the name?

• MRI: Magnetic Resonance Imaging

‒ Magnetic: What do protons do in a magnetic field?

• Align and Precess

‒ Resonance: What does magnetic nuclei do when the right energy is

applied?

• Resonance and Relaxation

‒ Imaging: How to resolve spatially MR signals from different voxels in the

body?

• Spatial Encoding

• How can MRI be performed safely?

Magnetic Resonance Imaging

• Non-ionizing radiation, hydrogen in the body is the source of signal

• Absorption and Emission of energy in the radiofrequency spectrum

• Image acquisition in any plane, ~1 mm resolution, good soft-tissue contrast

7/25/2019

2

• Magnetic

‒ Nuclei with odd total number of protons + neutrons possess a magnetic moment

• 1H, 31P, 23Na, 13C

MR Physics

Hydrogen atom1 proton + 1 electron

Nucleus

+-

Electron +m

Hydrogen protonSometimes referred to as “a spin”

Magnetic

field directionN

S • A very strong magnet is needed to induce even a weak magnetization in tissue.

• Normal Conditions (No external field)• Random orientation of spins

• No net magnetization

MRI in a Nutshell

Cardiovascular MRI : physical principles to practical protocols / Vivian S. Lee ; Lippincott Williams & Wilkins, c2006

Z

X

Y

B0

Main Magnetic Field (B0)

• Uniform magnetic field inside the cylinder

• Protons in body align along two directions

‒ Parallel and antiparallel to B0

‒ More protons aligned parallel to B0

(lower energy state)

• Equilibrium or net magnetization in tissue

along B0

• Greater B0, greater M0

Creating Net Magnetization

Z

X

Y

B0

External magnetic field

B0

Net Magnetization

=M0

7/25/2019

3

Spin Precession

• Under the influence of main magnetic field (B0)

‒ Spinning protons begin to precess

‒ Frequency of precession = Larmor frequency

‒ Larmor frequency ∝ strength of external field (B0)

• The phenomenon of precession makes magnetization measurable

𝒇𝑳 =𝜸

𝟐𝝅𝑩𝟎

Problem:

• At equilibrium state, Mz = M0; Mxy = 0

• Difficult to measure Mz directly

‒ Mz very weak compared to B0

Detecting and Measuring an MRI signal

Z

X

Y

B0

M0

Solution

• Excite nuclei by applying an RF

energy

• To generate RF:

• Time varying electric current passed

through a coil

• Electric field converted to oscillating

magnetic field (B1 << B0)

• Very short duration

• RF pulse

RF Pulse

7/25/2019

4

• Applied at Larmor/Precession frequency

• Protons absorb energy – Magnetic resonance

• Rotate the net magnetization away from B0 direction, towards X-Y plane

RF Pulse

y

x

z

B1

Bo

M

RF Pulse OFF

• Return to equilibrium

• Two processes occurring at the same time

• T1 Relaxation

• T2 Relaxation

• Tissue-specific time constantshttps://casemed.case.edu/clerkships/neurology/Web%20Neurorad/MRI%20Basics.htm

https://mri-q.com/bloch-equations.html

• Spin-lattice or Longitudinal Relaxation

• Regrowth of Mz towards M0

• Transfer of thermal energy between excited nuclei and surrounding atomic lattice

‒ Efficient transfer – shorter T1

‒ Depends on molecular motion and size, water has long T1

• Time required for Mz to regain 63% of its equilibrium value

T1 Relaxation

Mo

time

Mz

0.63Mo

B1 field off

T1

)1()( 1/

0

Tt

z eMtM −−=

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 500 1000 1500 2000 2500 3000

Time (ms)

Mz

bloodfatmyocardium

0.63Mo

Mo

e = 2.71

7/25/2019

5

• Spin-spin or transverse relaxation

• Transverse magnetization decays back to zero

• Interaction between magnetic field and individual protons

• Time required for 63% of the initial magnetization to dissipate

• Effective transverse relaxation time (T2*) –static inhomogeneities in local magnetic field

T2 Relaxation

Mxy

0.63Mo

Mo

0.37Mo

B1 field off

T2time

2/

0)(Tt

xy eMtM −=

T2*

Ridgway. JCMR 2010 12:71

• Emission of RF energy can be measured before equilibrium is obtained

• Change of Mxy detected by a coil

• Free induction decay

• For cardiac imaging, a separate RF receiver coil is used to maximize signal detection

Signal Detection

M

z

B1

y

x

Detector coil

https://www.siemens-healthineers.com/magnetic-resonance-imaging/options-and-

upgrades/coils/body-18

Ridgway. JCMR 2010 12:71

Spatial Encoding

Cardiovascular MRI : physical principles to practical protocols / Vivian S. Lee ; Lippincott Williams & Wilkins, c2006

7/25/2019

6

• 3 pairs of coils with equal and opposite polarity

Gradient Magnetic Field

X

Y

https://casemed.case.edu/clerkships/neurology/Web%20Neurorad/MRI%20Basics.htm

Z X Y

ZCoronalSagittalAxial

Z

X

Y

B0

• Gradient Magnetic Field

‒ Selective Excitation - Define slice for imaging and excite the protons in that slice

‒ Spatial Encoding – Identify location of protons for reconstruction of MRI image

• MRI directly measures data in the frequency domain

• Inverse 2D Fourier Transform - Decode spatial position

Spatial Encoding and k-space Formation

K-space

Image

Space

Full dataset Central k-space Outer k-space

IFFT

Decoding the MRI Image

7/25/2019

7

MR Safety

• 1 in 10 people undergo MR imaging in the USA annually

• More than 10,000 systems across the country

• Millions of MRI scan have been performed safely

• Failure to comply with recommended safety procedures can result in

‒ Serious injury to patients and personnel

‒ Serious damage to MR equipment

MR Safety

https://science.howstuffworks.com/mri.htm

• MR systems have a very POWERFUL MAGNET

‒ 30,000 times stronger than the earth’s magnetic field

‒ 200 times stronger than a fridge magnet

‒ 1.5 to 3 times stronger than a junkyard lifting magnet

• The Magnet is ALWAYS ON

MR Safety

https://mrimetaldetector.com/blog/

7/25/2019

8

• Ferromagnetic objects become projectiles

‒ Paper clips, Hair pins, Pens, Scissors

(> 40 mph)

‒ Most clinical equipment are NOT MR

Safe

‒ People must be screened and items

have to be tested

• Pre-procedure screening

• Hand held magnet

• Ferromagnetic detection systems

• Verify presence of implants,

shrapnel, machine-shop debris

MR Safety

Zone 1Zone 2

Zone 3

Zone 4

Other serious incidents

• 2009 - Gun flew into magnet from police officer’s holster

• 2018, MA – MRI-safe clothes hamper was switched with regular one, flew into magnet and fractured patients facial bone

Adverse MR Incidents

Fatal incidents from projectiles

• 2001, NY – child struck in the head by oxygen cylinder

• 2018, India – patient’s relative struck by oxygen cylinder

MR Safety Labeling

MR Unsafe MR Conditional MR Safe

• Follow information on label

• Maybe safe in the room but not the bore

• Risk/benefit decision to scan the patient

• No safety hazard in MR environment

• Non-metallic

• Patients identified with MR safe devices can be scanned

Safety posters: FDA https://www.fda.gov/media/101205/download

• Items NOT allowed in scanner room

• Do NOT scan patients identified with MR Unsafe implants and devices

7/25/2019

9

Implant Safety

Safety posters: FDA https://www.fda.gov/media/101205/download

Before During After

• Check device label

• Document

• Consult

• Pre-scan procedure

• Follow guidelines

for device

• Constant

monitoring

• Assess patient

• Post-scan device

check/reprogramming

Fatal incident, 1992 - Aneurysm clip wrongly identified as MR safe

http://www.mrisafety.com

Check with your institution’s designated MR safety officer

Heating/Burn Safety

Screen and Verify

Avoid street clothing

Proper positioning

Proper cable routing

Normal mode,

whenever possible

Keep your eyes and

ears on the patient

• Most common MRI related injury

• RF energy can be deposited in the body as heat

Safety posters: FDA https://www.fda.gov/media/101205/download

MR Safe Electrodes

http://www.magmedix.com/mri-

ecg-electrode-cleartrace-

2.html

Bertrand et al. 2017

• Physiological and Biological effects‒ Tissue heating

‒ Peripheral nerve stimulation – tingling/twitching

• Acoustic Effects‒ Increased anxiety / Communication trouble

‒ Temporary and permanent hearing loss

• Tattoos/Permanent makeup/Piercings‒ Heating, Local edema and swelling

‒ Image artifacts

• Contrast Agent Safety‒ Nephrogenic systemic fibrosis

‒ Gadolinium retention

Other MR Safety Concerns

https://www.universalme

dicalinc.com/mri-safe-

paired-foam-earplugs-

37db.html

https://www.allmri.com

/product_info.php?lan

guage=en&info=p505_

earmuffs-for-mri.html

7/25/2019

10

• Completion of MR screening form

• Confirm safety of implanted devices

• Check if inpatients have temporary devices

• Stop scan if unreported object is discovered during scanning

A Safe MR exam

https://www.ismrm.org/mr-safety-links/mr-safety-resources-page/

Question twice!

• Any personnel with reason to enter MR suite must be screened and trained

• Train staff on protocols for emergency in MRI

‒ Medical

• Remove patient from scan room, Close door

‒ Fire, Flood

• Emergency Power OFF

‒ Life-threatening event / Uncontrolled fire

• Quench

MR Safety Training

Emergency POWER OFF

QUENCH

https://mri.fsu.edu/for-researchers/safety-manual/

• MR Physics

• https://www.mriquestions.com/index.html

• MR Safety

• http://www.mrisafety.com

• https://www.ismrm.org/mr-safety-links/mr-safety-resources-page/

• ACR Guidance Document on MR Safe Practices: 2013. JMRI 37:501-530

• Gilk and Kanal. 2013. JMRI 37:531-543

• https://www.fda.gov/radiation-emitting-products/mri-magnetic-resonance-imaging/mri-safety-posters

Resources

7/25/2019

11

WITH GREAT POWER COMES GREAT RESPONSIBILITY