BMED-4962/ECSE-4962 Introduction to Subsurface Sensing and Imaging Systems

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BMED-4962/ECSE-4962 Introduction to Subsurface
Sensing and Imaging Systems

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

Center for Sub-Surface Imaging Sensing
2
Recap

• Last time we discussed
• Proton spins
• B0 and RF fields
• Flips of the magnetic moment
• Spin relaxation, T1 and T2 times
• System block diagram
• Today
• More on the various coils in MRI
• Spatial localization of NMR responses

3
A Typical MRI System
Host Computer
Patient- Fred Blogs
Sequence- Fast Spin Echo
TR 2000 msec
TE eff 100 msec
Nex 1
Axial NP FC
ACQUISITION
OPERATOR CONSOLE
MEMORY
Array Processor
ADC
P
U
NETWORK
L
S
LASERCAM
E
T/R Switch
RECEIVER
ARCHIVE
C
O
N
TRANSMITTER
T
R
X GRADIENT
O
Magnet
L
AMPLIFIER
L
Gradient
E
Y GRADIENT
RF Coil
R
AMPLIFIER
Coils
Z GRADIENT
AMPLIFIER
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Review Individual Nuclei Precess about the
Applied Field
n
Bo
The precession frequency is given by the
Larmor Equation
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The Larmor Equation

• ? ? B0
• ? is the Larmor frequency
• B0 is the strength of the external magnetic field
• ? is the gyromagnetic ratio and depends on the
  nucleus

1H has the highest gyromagnetic ratio of any
nuclear species and is highly abundant in the
human body
http//lcni.uoregon.edu/downloads/science_teachers
_2005/Mri_rough_guide.ppt
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Review 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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The Gradient Subsystem

• Gradients provide spatial information. They
  must
• Be linear over the field-of-view
• Be accurately controlled
• Gradients must move rapidly i.e.
• Rise to their required value quickly
• Settle at the value as soon as possible
• Switching magnetic fields induce eddy currents in
  any conductor they penetrate
• Eddy currents oppose the field causing them
• Faraday's law of induction
• Therefore these currents must be minimized
• Compensation circuitry
• and/or active shielding

8
A Field Gradient Makes the Larmor Frequency
Depend upon Position
1.500 T
1.501 T
B0
63,872,000 Hz
63.861,000 Hz
Z
Gradient in Z
B(Z)
B
G



Z
o
Z
g
B
n

p
2
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Pulse bandwidth and slice thickness
Frequency (Hz)
Gradient, e.g. 10 G/cm 4,260 Hz/cm
Narrow pulse excites fat slice
Position (cm)
http//vision.psych.umn.edu/caolman/courses/Fall2
006/Lectures/Lecture2.ppt
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Pulse bandwidth and slice thickness
Frequency (Hz)
Gradient, e.g. 10 G/cm 4,260 Hz/cm
Strong gradient decreases slice thickness
Position (cm)
http//vision.psych.umn.edu/caolman/courses/Fall2
006/Lectures/Lecture2.ppt
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Pulse bandwidth and slice thickness
Frequency (Hz)
Gradient, e.g. 10 G/cm 4,260 Hz/cm
Center frequency determines slice position
Position (cm)
http//vision.psych.umn.edu/caolman/courses/Fall2
006/Lectures/Lecture2.ppt
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Typical Gradient Coil Designs
Current
Current

Magnetic Field
Y
?Bz ?z
Z
Z - axis
X
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Typical Gradient Coil Designs
Current
Current
Current
Current
Usable Gradient Volume
?Bz ?y
Y - axis
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Typical Gradient Coil Designs
Current
Current
Current
Current
?Bz ?y
Y - axis
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Gradient Coil Designs
Fingerprint Gradient Coils
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The Gradient Subsystem

• Two important parameters define the performance
  of a gradient subsystem
• Slew rate
• Maximum Gradient Strength
• Slew rate
• Determined by the voltage applied to the gradient
  coil
• Can be as high as 500 Volts
• Maximum Gradient Strength
• Determined by the current applied to the gradient
  coil
• Can be as high as 400 Amps
• Higher power subsystems give
• Faster Imaging (e.g. Gradient echo, Fast Spin
  Echo, EPI..)
• Fewer Artifacts (e.g. Better flow compensation )
• Diffusion weighted imaging

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Additional Info on Gradients
See Reference below for additional info on phase
and frequency encoding.
Source www.lancs.ac.uk/depts/physics/
teaching/py336/MRI-short-version.doc
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Slew Rate is an important parameter
Gmax risetime
Slew Rate
Gmax
Gradient Amplitude mTesla/meter or Gauss/cm
risetime
0
Time (ms)
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For high-end systems, gradient performance is
limited by physiological stimulation
SR-20 SR-77 SR-120 SR-150
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8
Bi-polar pulse Uni-polar pulse
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Gradient (G/cm)
4
PNST(mean)
66 PNST(mean)
2
66 PNST(mean)
0
Peripheral Nerve Stimulation Threshold
0
5
10
15
20
25
30
35
Slew Rate (G/cm/ms)
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Radiofrequency (RF) Coils

• RF coils act as
• Transmitter, i.e. apply the excitation
• Receiver i.e. detect the induced signal
• Transmitter coils must
• Provide a strong uniform field for a short period
• Give a uniform excitation (i.e. are usually
  volume coils)
• Receiver coils must
• Detect the weak NMR signal from the region of
  interest
• Are therefore often surface or local coils
• One coil can act as transmitter and receiver e.g.
  head body coils but frequently they are
  different
• Multiple receiver coils can be combined to form a
  phased array

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MR Radio Frequency (RF) Coils

• Transmit coils can require a lot of power
• Power requirements scale as the square of the
  magnetic field strength
• Larger coils require more power
• At 1.5 Tesla
• 2 KW amplifier used for the head coil
• 16-20 KW amplifier used for the body coil
• Receiver coils must be very sensitive
• The MR signal induces ?Volts in the coil
• The best MR coils are well tuned and matched to
  the load
• The MR scanner is placed inside an RF screen room
• Keeps the RF excitation pulse away from the
  surrounding environment
• Prevents image artifacts due to interfering
  signals (e.g. radios, computers etc.)

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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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RF Coils Transmit
Net magnetization
http//lcni.uoregon.edu/downloads/science_teachers
_2005/Mri_rough_guide.ppt
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RF Coils on ReceiveFree induction decay
Details of the relaxation depend on the local
environment. We can exploit these differences to
emphasize different types of contrast in images.
http//lcni.uoregon.edu/downloads/science_teachers
_2005/Mri_rough_guide.ppt
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MR Signals are Complex.
but a simple loop can only detect one
component of the field
Laboratory Frame of Reference
Z
Y
Mxy
X
I
I sin(?t)
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MR Signals are Complex.
but a simple loop can only detect one
component of the field and the orientation
of the coil is critical!
Laboratory Frame of Reference
Z
Y
Mxy
X
I
I sin(?t)
I 0
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MR Signals are Complex
Laboratory Frame of Reference
Z
Two orthogonal coils detect the same signal
with a 90 degree phase shift.
Y
Mxy
X
I
I
Q cos(?t)
I sin(?t)
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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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RF Coil Pickup
z
transverse magnetization
y
x
coil oriented to intercept precessing magnetizatio
n
Voltage
Time
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RF Coil Pickup with Time
Transverse Magnetization
Fluid
White Matter
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168
Time (ms)
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Quadrature Coils
Q
Quad Hybrid
I
Birdcage coil

• Even though the birdcage coil is a single
  structure, it has two ports that can detect both
  components of the MR signal.

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Quadrature Coils

• Birdcage coils are frequently used for whole-body
  excitation and for head transmit/receive.

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Quadrature coils detect both orthogonal components

• Each coil detects the same signal, but different
  noise.
• The signals can be added, but the noise has a
  Gaussian distribution and will partially cancel.
• Combining the two signals will give a sqrt(2)
  increase in the signal-to-noise ratio.

Apply ?/2 phase shift and add
Signal is 2x as large
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Phased Array Coils

• Multiple Array of Coils embedded into the table
• Neurovascular Array attached.
• Software switching of array combinations
• Anterior Signal acquired with a flex coil Blanket
  or formidable array (size of Torso array)

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Surface Coils

• Volume coils such as the body coil have excellent
  coverage, but low SNR.
• Small surface coils have limited coverage, but
  higher SNR.
• Volume and surface coils can be used together
• Use volume coil for excitation to obtain uniform
• excitation of all spins within the imaging volume
• Use small surface coil to maximize the SNR over
  the volume of interest

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Fundamentals of MRIImage Quality
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MRI always involves trade-offs
Speed
Image Quality
Resolution
Signal-to-Noise
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The Signal-to-Noise Ratio (SNR) is a Key Factor
in Image Quality
Intrinsic Signal-to-noise is determined by

• Magnetic field strength
• SNR increases with Field strength
• Data acquisition time
• Bandwidth
• Signal averaging
• System engineering
• Quality of RF coils
• System noise figure
• Screen room quality

NOTE In a properly functioning
scanner the noise contribution
from system is negligible compared
to the noise from the patient.
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SNR is roughly proportional to field strength
SNR
Acquisition time
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Image Quality with 1.5 T 3 T Scanners
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How much Signal?

• Number of protons energized is directly
  proportional to field strength (e.g. 1.5T yields
  more signal than 0.5T)
• Number of protons per pixel is related to amount
  of tissue in the pixel
• Signal strength is related to the number of
  protons/pixel

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SNR is proportional to voxel volume

• Signal is proportional to voxel volume
• Noise is independent of voxel volume
• MR noise has a Gaussian distribution
• Therefore SNR is proportional to voxel volume

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SNR is proportional to acquisition time

• Acquisition time depends on
• Number of acquisitions
• Acquisition bandwidth
• Signal Averaging (NEX)

SNR
Acquisition time
NOTE Acquisition time is the total time
period during which data is sampled.
It is independent of TR and TE.
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Acknowledgments

• Thanks to Dr. Charles Dumoulin of GE Global
  Research for the introductory slides.
• http//www.erads.com/mrimod.htm
• http//rad.usuhs.mil/rad/handouts/fletcher/fletche
  r/sld025.htm
• There are numerous sites on the web with
  excellent intros to MRI

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Lecture 17 Homework

• In the lectures on MRI, we have briefly discussed
  pulse sequencing techniques.
• Using the usual Internet search tools,
• Identify three common pulse sequences in MRI
• Identify associated clinical applications

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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 Laraine Michaelides, JEC 7012, (518)
  276 8525, michal_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 Laraine Michaelides, JEC 7012, (518)
  276 8525, michal_at_rpi.edu