MRI Physics (Phys 352A) Manus J. Donahue: [email protected] Department of Radiology, Neurology, Physics, and Psychiatry Office: Vanderbilt University Institute of Imaging Science (VUIIS) AAA-3115 Web site: http://www.mc.vanderbilt.edu/root/vumc.php?site=fmri&doc=41377

Special thanks to Karla Miller Oxford University MRI Physics Graduate Course

Looking Inside the Body (c. 1632)

Rembrandt

Looking Inside the Body (2013)

Can see whole body in a few minutes Tissue structure, function, blood flow, chemistry, etc. Major developments in physics + diagnosis + physiology

7 Tesla MRI

Brief History of Magnetic Resonance Imaging (MRI)

MRI Physics •  A bit about me:

–  Faculty member in Dept. of Radiology, but also Physics, Neurology and Psychiatry •  My work focuses on developing and implementing new MRI methodologies for understanding brain function in health and disease

–  Post-doctoral fellow: Clinical Neurology –  Graduate school: Biophysics –  Undergraduate: Physics and Philosophy

•  MRI transcends many disciplines (e.g., Physics, Biophysics, Medicine, Neurology, etc.)

–  This course is designed to not just allow you to understand basic MRI methods, but to understand how MRI can be used to address biologically relevant questions

MRI Physics Schedule •  •  •  •  •  •  •  • 

Introduction Math review Image formation Pulse sequences Image contrast Hardware Fast imaging Review + Exam 1

•  •  •  •  •  • 

Structural clinical imaging Functional MRI Chemical imaging (MRS/CEST) Diffusion imaging Emerging sequences and high field (i.e., > 3 Tesla) Clinical MRI in cerebrovascular disease, neurological disorders, dementia, and outside the brain Projects Review + Exam 2

•  • 

MRI Physics Principles

Applications

Guest Lecturers

Jay Moore Pulses + coils

Chuck Nockowski Hardware + tour

Seth Smith Chemical imaging + MRI outside the body

Expected Background •  Course will focus on understanding MRI principles from a users perspective, therefore extensive knowledge of mathematics is not required –  I will review most of what you need to know

•  I am much more concerned that you understand how images are generated, where contrast comes from, and how new contrast can be obtained rather than that you can calculate Fourier transforms, spin density matrices, coherence pathways, etc. by hand •  Expected background: Introductory college calculus and physics (mechanics and electricity and magnetism) –  Knowledge of quantum mechanics, differential equations, and Fourier transforms is helpful but not required –  We will also cover clinical and neuroscience applications. Therefore knowledge of basic human physiology is helpful but not required

Grading •  •  •  • 

Homework (2/mo): 25% Midterm Exam: 25% Final Exam: 25% Project/Presentation: 25%

Exams will be based heavily on homework and lectures i.e., if you keep up with homework you will do well on exams Project/presentation: focus on an advanced topic of interest and present a written report (10-20 pgs/double spaced) and 15 min class presentation. Undergrads: Project grade may replace single exam (if lower) Grads: Project grade may not replace single exam

Project/Presentation •  Pick a special topic of interest to you:

–  Clinical (e.g., cancer, stroke, Alzheimer’s disease) –  Technical (e.g., parallel imaging, k-space, novel acquisition) –  Hardware (e.g., coils, gradients, radiofrequency transmission) NOTE: if you are actively working on an imaging research project (Ph.D., etc.) you must choose something different from your thesis topic! I must approve all topics: just email or talk to me.

Prepare written report 10-20 pgs; double-spaced; font=12 pt; margins=1’’ Present summary to class 15 min (~10 slides)

Office Hours / Expectations •  Office hours: After class or by appointment –  [email protected] –  VUIIS; 3rd Floor, AAA-3115

•  Expectations: -  I want everyone to do well in this class -  e.g., learn a lot and get an A -  Exams/grading designed not to be tricky, but to ensure knowledge of covered material. -  If you come to class and do the work, you should be happy with your grade

MRI Physics •  Why take a course in Magnetic Resonance Imaging (MRI)?

Unlike other imaging modalities (e.g., X-ray, PET, SPECT, CT, etc.) whose contrast depends on injected material and hardware, MRI contrast depends more broadly on changes to sequence parameters: - Many different aspects (e.g., structure, function, etc.) can be visualized if you understand basic principles!

Example: Brain Tumor FLAIR

T1

1/CBV

Total Tumor

Unhealthy brain Necrosis/tumor Border

Segmented Tumor T1+Contrast

GBM

AO

LG

GBM: Glioblastoma multiforme; AO: Anaplastic oligodendroglioma; LG: Low-grade

Why Learn MRI Physics Knowledge of MRI physics will help you: •  Address a relevant biological question –  e.g., How can I measure tissue volume, blood flow, glutamate, etc.?

•  Identify the relevant parameters –  Bandwidth, resolution, echo time (TE), etc.?

•  Evaluate data quality –  Measure distortion and signal-to-noise ratio

MRI Physics: All About “Spin” H1

•  Protons (e.g., hydrogen): consist of charged particles with spin –  Spin is a quantum mechanical, intrinsic property. Classical analogue: rotation about an axis –  Charge + spin = magnetic moment –  Source of (most) MRI signal: protons in water

•  What do we know about magnetic objects? •  What to we know about rotating objects?

Principles of MRI Compass

Magnetically sensitive pointer

Gyroscope

Spinning wheel + disk

Compass Oscillation •  Lars G. Hanson –  http://www.youtube.com/watch? v=1OrPCNVSA4o

Magnetic Resonance •  Think about the compass –  Compass needle oscillates about magnetic field before stopping –  This oscillation has a well-defined frequency (“resonance frequency”)

Magnetization Excitation and Relaxation •  Excitation: An additional magnetic field (B1) can deflect the compass needle –  This deflection can be maximized by choosing the new field to be the same as the resonance frequency

•  Relaxation: After B1 field is removed, the magnetic oscillations decay with a welldefined time constant

Polarization •  What direction does a compass point? –  In absence of magnetic field, compass needle is randomly oriented

•  In presence of magnetic field (e.g., Earth), needle has slight tendency to align with magnetic field.

Spin •  Magnetization + Spin –  Think about gyroscope –  Gravity tilts a spinning object –  Because of spin, the axis precesses instead of tilting –  Spin + Gravity = Precession

What About Protons? •  Can we use what we know about: Magnets  

Spinning  Objects  

•  To understand Hydrogen proton in water molecule?

H1

Yes: Protons are Tiny Magnets

H1

Magnetic Field (B0) = 0 Tesla

•  With no magnetic field, the spin associated with hydrogen nuclei are randomly oriented

Protons in Magnetic Field H1

H1

H1 H1

H1

H1

Magnetic Field (B0) > 0 Tesla H1

M

H1

•  With magnetic field, spins along (slightly) with the field, creating a net magnetic moment (M)

Coordinate System z

B0

M y

x

•  The direction of the main field (B0) defines the coordinate system –  Longitudinal axis: Parallel to B0 –  Transverse axis: Perpendicular to B0

•  MRI experiments involve reorienting M relative to B0 and studying behavior of M

Magnetic Resonance H1

ω = γB0

ω  = Larmor frequency γ = gyromagnetic ratio B0 = static field strength

•  MRI and NMR detects magnetization (M)

–  Usually we detect protons on water –  In principle can detect other elements with spin (13C, 19F, 31P, etc.)

•  Magnetization has characteristic resonance frequency:

–  Larmor frequency (ω) –  For water protons, ω is in (or just above) radiofrequency (RF) range: 64 - 128 MHz

MR Excitation z

B0

B0

M

M

y x

z y

B1

x

•  Excitation: Additional field (B1) tips magnetization (M) away from main field (B0) if this new field is applied at resonance frequency (ω)

Relaxation: Turn Off B1 Field B0

z

B0 M

x

y

z M

B1

y

x

•  Relaxation: Excited magnetization reverts to orientation before B1 introduced –  Described by time constant T1 (longitudinal plane) –  Described by time constant T2 (transverse plane)

Magnetization Precesses about Main Magnetic Field (B0) B0

Movie  courtesy  of  William  Overall  

What Happens After We Turn Off the RF Excitation Pulse? B0

Mz returns to alignment with main magnetic field - T1 describes this time Movie  courtesy  of  William  Overall  

What Happens After We Turn Off the RF Excitation Pulse? B0

Mx,y dephases in transverse plane T2 describes this time

Movie  courtesy  of  William  Overall  

Signal Detection B0

•  Changing magnetic field introduces a current in a wire •  Precessing magnetization detected with a coil tuned to the appropriate frequency •  Important: can only detect components in transverse (x-y) plane Movie  courtesy  of  William  Overall  

Magnetic Resonance Summary •  Polarization: In external magnetic field (B0), protons align to create net magnetization (M) •  Excitation: RF pulse tips magnetization away from B0 •  Precession: Excited magnetization rotates about B0 •  Detection: Magnetization induces a current in a correctly tuned coil close to the object •  Relaxation: Magnetization returns to alignment with B0, causing signal decay

Making an Image B0

G

ω = γB0 ω = γ(B0+G)

•  Need to differentiate protons at different locations •  Add a second, spatially varying magnetic field (G) –  Gradient field at each location is either parallel or antiparallel to B0 –  Therefore, the gradient field either adds or subtracts from the main magnetic field

Magnetic Gradients High Frequency Fast Precession

G

B0 Low Frequency Slow Precession

ω = γB0 ω = γ(B0+G)

•  Spins at each position have different frequency •  RF coil hears all of the spins at once •  Differentiate material at different positions by selectively listening to only a certain frequency

MRI Scanner Hardware GE

Philips

Siemens

Major Vendors (Humans): General Electric (American) Siemens (German) Philips (Dutch)

MRI Hardware: Three Important Coils Magnet (B0=1.5T, 3.0T, 7.0T) RF Coil

Gradient Coil

•  The MRI scanner consists of coils that generate the different fields: –  Main static field (B0) –  Transient field (gradients) –  Transmit field (RF)

Coils

Magnet

Larry Wald MGH

RF

Gradient

Flexibility of MRI: Structural Imaging 3.0 T (1.0 mm)

7.0 T (0.7 mm) Axial

Sagittal

Flexibility of MRI: Vascular Imaging •  Structural imaging: visualization of blood water in vessels

Flexibility of MRI: Visualization of the Vessel Wall and Plaque Circle of Willis •  Structural

Vessel Wall imaging: visualization of blood water in vessels

R

L

R

L

Flexibility of MRI: Functional Imaging 9  

0   Z-­‐sta4s4c  

Flexibility of MRI: Functional BOLD  Reac4vity  Scan  1  (Pre-­‐Surgery)   imaging in the clinic

BOLD  Reac4vity  Scan  2  (Six  months  post-­‐surgery)  

BOLD  Reac4vity  Scan  3  (Twelve  months  post-­‐surgery)  

0  

12   Z-­‐sta4s4c  

Flexibility of MRI: Measuring Functional Changes Based on Changes in Cerebral Blood Flow (CBF) FIG 1

A

CBFw

P R

(a)

RPI

L

(b)

(c)

Beyond Water Imaging: Interactions between Proteins, Iron and Water

Beyond Water Imaging: Interactions between Amide Protons and Water

Technical Issues Global Shim + order Dynamic 1st

Global Shim Only Slice 3

Slice 3

Slice 15

Saikat Sengupta

Design

Post-processing

Implementation

Improving Detection Strategies Design

spin-system H1 H3

density matrix H5

metabolite(k) =

basis selection initial conditions

H2

H4

p(k) = Σωj|ϕj > 300,000 papers using traditional BOLD fMRI –  Many more with structural imaging (MRI, CT, Xray, etc.)

•  The ability to use new methods to address new or old questions greatly increases research potential!

MRI Physics Schedule •  •  •  •  •  • 

Introduction Image formation Pulse sequences Hardware Image contrast Fast imaging

•  •  •  •  •  • 

Structural clinical imaging Functional MRI Chemical imaging (MRS/CEST) Diffusion imaging Emerging sequences Review

MRI Physics Principles

Applications

Office Hours / Expectations •  Office hours: After class or by appointment –  [email protected] –  VUIIS; 3rd Floor, AAA-3115

•  Expectations: -  I want everyone to do well in this class -  e.g., learn a lot and get an A -  Exams/grading designed not to be tricky, but to test knowledge of covered material. -  If you come to class and do the work, you should be happy with your grade.