ECSE4963 Introduction to Subsurface Sensing and Imaging Systems

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ECSE-4963Introduction to Subsurface Sensing and
Imaging Systems

• Lecture 17 MRI
• Kai Thomenius1 Badri Roysam2
• 1Chief Technologist, Imaging technologies,
• General Electric Global Research Center
• 2Professor, Rensselaer Polytechnic Institute

Center for Sub-Surface Imaging Sensing
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Last Lecture

• Introduced yet another form of SSI, GPR.
• Identified its nature as a pulse-echo or
  reflection mode modality.
• Reviewed modeling concepts for land mine
  detection
• Reviewed results from several applications.
• Today Introduction to MRI

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Fundamentals of MRIModule 1 Quick Overview

• From the notes of Charles Dumoulin, PhD
• December 17, 2009

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Magnetic Resonance ImagingOverview

• What is Magnetic Resonance Imaging?
• What are the intrinsic parameters?
• Are there biohazard and safety issues?
• What are some of the clinical applications?

5
Features of Imaging Modalities
Source

• X-ray
• Source/detector geometry
• Projective and computed tomography
• Ultrasound
• Source/detector at same location
• Real time 2D imaging
• Real time 3D/4D imaging
• Magnetic Resonance
• Source of signal from within body
• 2D and 3D imaging

Detector
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Intrinsic Parameters of Other Imaging Modalities

• X-ray
• electron density
• Attenuation
• Ultrasound
• variations in tissue compressibility density
• velocity
• Positron Emission Tomography (PET)
• concentration of radio-labeled metabolites

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Intrinsic Parameters of Magnetic Resonance

• Nuclear spin density
• Motion on the molecular scale
• T1
• T2
• Motion on the microscopic scale
• diffusion
• perfusion
• Macroscopic motion (velocity, acceleration etc.)
• Chemical composition
• Chemical exchange
• Temperature
• Mechanical and magnetic properties of tissue

8
Costs of ImagingInstrumentation

• X-ray
• Portable units inexpensive (lt 100K)
• CT scanners (500K - 1.2 million)
• Angiography suites (700K - 1.7 million)
• Ultrasound
• 20K-150K
• Magnetic Resonance
• 500K - 2 million

Cost
Ultrasound X-ray CT MR
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Safety and Biohazards of Imaging Modalities

• X-ray
• Ionizing radiation
• Morbidity associated with contrast agents
• Ultrasound
• No safety or biohazard problems
• Magnetic Resonance
• No biohazards
• Safety is an issue

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Magnet Safety
The whopping strength of the magnet makes safety
essential. Things fly Even big things!
Source www.howstuffworks.com
Source http//www.simplyphysics.com/ flying_objec
ts.html
Perhaps more dangerous are small items, screw
drivers, scissors, etc.
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Subject Safety

• Anyone going near the magnet subjects, staff
  and visitors must be thoroughly screened
• Subjects must have no metal in their bodies
• pacemaker
• aneurysm clips
• metal implants (e.g., cochlear implants)
• intrauterine devices (IUDs)
• some dental work (fillings okay)
• Subjects must remove metal from their bodies
• jewelry, watch, piercings
• coins, etc.
• wallet
• any metal that may distort the field (e.g.,
  underwire bra)
• Subjects must be given ear plugs (acoustic noise
  can reach 120 dB)

This subject was wearing a hair band with a 2 mm
copper clamp. Left with hair band. Right
without. Source Jorge Jovicich
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Clinical Applications of Magnetic Resonance
Imaging

• Brain
• Stroke
• Cancer
• MS
• Parkinsons, Alzheimer's etc.
• Spine
• Spinal cord
• Disks
• Abdomen

• Vascular
• Neuro
• Peripheral
• Joints
• Knees
• Shoulder

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Emerging Applications of Magnetic Resonance
Imaging

• Cardiac
• Myocardial function
• Cardiac dynamics
• Coronary artery disease
• Interventional
• Image guided Biopsy
• Minimally invasive surgery
• Vascular interventions
• Functional MRI

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New Technologies for Magnetic Resonance Imaging

• New magnet designs
• Higher fields
• Open geometry
• Faster and more powerful gradient subsystems
• MR-compatible devices

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Conventional Cylindrical Magnet
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0.7 Tesla Open Magnet
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MR imaging
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MR imaging
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MR thermal imaging
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Contrast Agents in MRI
Contrast Enhanced MR Angiogram
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fMRI image of the brain during finger tapping
Right Hand
Left Hand
Image courtesy of University of Melbourne
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MR imaging
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Future of Magnetic Resonance Imaging

• MR will continue to grow at a rapid rate,
  particularly outside the field of radiology.
• MR will displace many diagnostic methods in use
  today.
• The cost of MR will continue to drop.

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What happens in an MRI Scan?

• 1) Put subject in big magnetic field (leave him
  there)
• 2) Transmit radio waves into subject about 3
  ms
• 3) Turn off radio wave transmitter
• 4) Receive radio waves re-transmitted by subject
• Manipulate re-transmission with magnetic fields
  during this readout interval 10-100 ms MRI
  is not a snapshot
• 5) Store measured radio wave data vs. time
• Now go back to 2) to get some more data
• 6) Process raw data to reconstruct images
• 7) Allow subject to leave scanner (this is
  optional)

Source Robert Coxs web slides
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Fundamentals of MRIModule 2 Basic Physics

• From the notes of Charles Dumoulin and several
  other sources
• Robert Coxs web site http//afni.nimh.nih.gov/a
  fni/doc/edu/mri/
• defiant.ssc.uwo.ca/Jody_web/fMRI4Dummies/
  pdfs_and_ppt/Basic_MR_Physics.ppt

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Sources of Atomic Magnetism
-

• At the atomic level magnetism arises from three
  sources
• Electronic Orbital Momentum
• Electron Spin
• Nuclear Spin

Proton Spin
Neutron Spin
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Spinning nuclei generate magnetic fields
m
N
S
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Nuclei Interact with a Magnetic Field
With a magnetic field
With no magnetic field

Nuclei in random
Nuclei align to the applied
orientations
field
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Individual Nuclei Precess about the Applied Field
n
Bo
The precession frequency is given by the
Larmor Equation
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Individual Nuclei Precess about the Applied Field
n
Bo
Note Unlike a gyroscope, the angle
of precession is quantized.
The precession frequency is given by
the Larmor equation-
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Some nuclei align with the magnetic field and
some align against it
n
n
Bo
MI 1/2
MI -1/2
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Some nuclei align with the magnetic field and
some align against it
n
n
Bo
There are a few more nuclei aligned with the
field than aligned against it.
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In reality, there are many nuclear spins in a
sample
n
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The Rotating Frame of Reference
The Rotating Frame
The Laboratory Frame
Z?
Z
n
Y?
Y
X?
X
Bo
n
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In the rotating frame of reference the net
magnetization can be represented by a single
vector, Mz
Net Magnetization
The Rotating Frame
Z?
Z?
Mz
Y?
Y?
X?
X?
Bo
n
n
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Energy absorption a classical physics view
Rotating Frame of Reference
Z?
Z?
Mz
Y?
Y?
X?
X?
Mxy
B0
B1
Before
After
An RF pulse applied at the Larmor frequency
creates a magnetic field in the rotating frame of
reference
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RF Excitation

• Excite Radio Frequency (RF) field
• transmission coil apply magnetic field along B1
  (perpendicular to B0) for 3 ms
• oscillating field at Larmor frequency
• frequencies in range of radio transmissions
• B1 is small 1/10,000 T
• tips M to transverse plane spirals down
• analogies guitar string (Noll), swing (Cox)
• final angle between B0 and B1 is the flip angle

Transverse magnetization
Source Robert Coxs web slides
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Relaxation and Receiving

• Receive Radio Frequency Field
• receiving coil measure net magnetization (M)
• readout interval (10-100 ms)
• relaxation after RF field turned on and off,
  magnetization returns to normal
• longitudinal magnetization? ? T1 signal
  recovers
• transverse magnetization? ? T2 signal decays

Source Robert Coxs web slides
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T1 and TR

• T1 recovery of longitudinal (B0) magnetization
• used in anatomical images
• 500-1000 msec (longer with bigger B0)
• TR (repetition time) time to wait after
  excitation before sampling T1

Source Mark Cohens web slides
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Summary

• We have discussed
• Relation of MRI with respect to other imaging
  modalities.
• MRI images
• Nuclear spin
• Larmor frequency
• Next time
• MR Imaging physics
• How do we make images with the spins?

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Acknowledgments

• Thanks to Dr. Charles Dumoulin of GE Global
  Research for the introductory slides.
• We have borrowed stuff from several sites on the
  web with excellent intros to MRI
• http//www.bme.umich.edu/dnoll/primer2.pdf
• http//www.erads.com/mrimod.htm
• http//www.cis.rit.edu/htbooks/mri/mri-main.htm
• http//airto.loni.ucla.edu/BMCweb/BMC_BIOS/MarkCoh
  en/abstracts/Basic/BasicMRPhysics.html

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Instructor Contact Information

• Badri Roysam
• Professor of Electrical, Computer, Systems
  Engineering
• Office JEC 7010
• Rensselaer Polytechnic Institute
• 110, 8th Street, Troy, New York 12180
• Phone [phone]
• Fax [phone]
• Email roysam_at_ecse.rpi.edu
• Website http//www.rpi.edu/roysab
• NetMeeting ID (for off-campus students)
  128.113.61.80
• Secretary Betty Lawson, JEC 7012, (518) 276
  8525, lawsob_at_.rpi.edu

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Instructor Contact Information

• Kai E Thomenius
• Chief Technologist, Ultrasound Biomedical
• Office KW-C300A
• GE Global Research
• Imaging Technologies
• Niskayuna, New York 12309
• Phone [phone]
• Fax [phone]
• Email thomeniu_at_crd.ge.com, thomenius_at_ecse.rpi.edu
  
• Secretary Betty Lawson, JEC 7012, (518) 276
  8525, lawsob_at_.rpi.edu