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MRI Hardware: magnet,
gradient, and RF coils
Hsiao-Wen Chung (鍾孝文), Ph.D., Professor
Dept. Electrical Engineering, National Taiwan Univ.
Dept. Radiology, Tri-Service General Hospital
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The Main Magnet
• What does MRI need ?
– A strong magnet, with sufficient
size to put a person in
• Permanent made of iron ?
• Electromagnet using currents ?
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MRI using Iron-core Permanent Magnet
As large as 0.3 Tesla has been reported
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Permanent MRI with Rectangular Bore
Hitachi System
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C-shaped Permanent Magnet MRI
Siemens Magnetom Open
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Pros and Cons
• No electricity required
• Magnetic field restricted within the
iron core (least fringing field)
• Iron-core magnet too heavy (30 tons !)
for the floor
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Another Type of Permanent Magnet
Open space MRI (reduced volume and weight)
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Open MRI with Reduced Weight Design
Toshiba MRI System
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Resistive Electromagnet MRI
As large as 0.3 Tesla has been reported
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Electromagnet MRI Photo
Bruker Electromagnet MRI
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Pros and Cons
• Somewhat lighter and cheaper (perhaps
the only advantages)
• Strong electromagnet needs huge
electrical power: Expensive electricity bills,
strong fringing field, poor stability
• Maximal current limited in copper wires
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Iron-core Electromagnet MRI
Less iron than permanent magnet MRI
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To me, it looks similar to this …
Toshiba System
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An Example (Still Not Too Light)
Early Siemens open system
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Combine the Advantages
• Iron-core electromagnet : Few
• Electromagnet without electricity
– Somewhat lighter with no electricity
bills from the magnet
– Superconducting electromagnet :
currently the dominant type for MRI
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Structure of Superconducting Magnet
As large as 8 Tesla has been reported
liquid helium
liquid nitrogen
vacuum
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Superconducting Magnet
• Disappearance of resistance of metal/alloy
under very low temperature
• Wire immersed in liquid helium (boiling
point –269 0C)
• Vacuum for thermal isolation
• Liquid nitrogen buffer (boiling point –1960)
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Photo for Superconducting Magnet
Before decorating packaging
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After Beautiful Packaging …
GE Signa Horizon Siemens Magnetom
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Double-donut MRI
Open design for simultaneous surgery
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Other Open MRI (Fonar)
Patient standing Table can be rotated
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Symptom may be Gesture Dependent
Lying (recumbent) Standing (weight bearing)
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I Can Image Whatever Position !
Useful for musculoskeletal examinations
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Back to Supercond MRI
• Strong field with good stability
• No electricity required once charged up
• Lighter than permanent magnet (6~7 tons)
• Fringing field is also strong
• High price for the instrument and cryogen
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Improved Performance
• Magnetic shielding
– Iron placed outside to restrict the
fringing field (passive)
– Outer coil with reversed current to
cancel the fringing field (active)
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Schematic Passive Shielding
~50% reduction per cm of iron plate
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Passive Shielding
~50% reduction per cm of iron plate
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Schematic Active Shielding
Inner field gets partially cancelled too
Major coil (e.g., 2.5T)
Reversed current (e.g., 1.0T)
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Improved Performance
• Field homogeneity : shim coils
– Individual single wire loops added
to the main magnet
– Independent current sources
– Locally adjusting the field strength
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Shim Coil
Independent power for current adjustment
Power
sources
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Why Supercon Popular ?
• Strong magnetic field = large signal
= good images
– Signal at 0.3 T ~ 20% at 1.5 T
• Good stability for the magnetic field
• The market needs push a great deal
of research and developments
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Spatial Encoding Hardware
• Gradient coils
– Gradient = locally varying
magnetic field
• Still the principle of electromagnet
• One in each of the x, y, z directions
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Locally Varying Magnetic Field
The Maxwell pair
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The z Gradient
Local field along z, variation also along z
Bo
z
y
x
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Photo of z Gradient Coil
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The More Precise z Gradient
Schematic z gradient
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Photo of MRI z Gradient Coil
z gradient coil
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Direction of the Gradient
• Local field always along z
• “Direction of the variation”
• “x gradient” means that the magnetic
field along z direction varies as a
function of x
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y Gradient
Local field along z, variation along y
Bo
z
y
x
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y Gradient Coil
Golay-type gradient coil (four saddles)
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x Gradient
Local field along z, variation along x
z
y
x
Bo
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x Gradient Coil
Golay coil (y gradient rotated for 900)
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Photo for x or y Gradient Coil
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More Precise x(y) Gradient Coil
Fold as a cylinder Photo of the real object
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Combing the x, y, z Gradinet Coils
z gradient
x gradient
y gradient
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Winding of Gradient Coil
• Just like a solenoid electromagnet
• Coil resists changes in current
– Lenz’s Law
• Gradient change is slower than
alteration of the driving current !
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Inductance & Gradient Rise Time
Gradient
(less winding)
rise time
rise time
Gradient
(more winding)
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The Gradient Echo Sequence
t
t
t
t
RF
Gs
Gp
Gr
Changes are not immediate
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Rise Time
• Certainly related to gradient strength
• 0.5 ~ 1.0 msec easily achievable
• Not a severe issue, unless for very
fast imaging
• Will be mentioned in future classes
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Other Gradient Criteria
• 3-axis ability
• Eddy current
• Linearity
• Duty cycle
• Acoustic noise, coil cooling, torque
balance ...
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Generation of Eddy Current
Current induced in (outer) cyrostat metal
Magnetic flux lines of the z gradient
z gradient coil
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Restricting the Outer Flux Lines
Gradient shielding to reduce eddy current
z gradient coil
Magnetic flux lines of the z gradient
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Radiofrequency Coils
• Radio-frequency (RF) coils
• Responsible for signal excitation
and detection near the Larmor
frequency
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For Excitation
• Needs to generate high-frequency rotating
magnetic fields
• Requirements
– High efficiency near Larmor frequency
– B1 should be perpendicular to Bo
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For Receiving
• Needs to receive high-frequency signals
effectively
• Requirements
– High efficiency near Larmor frequency
– Winding orientation should be
perpendicular to Bo
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Analogy of Radio Broadcasting
Similar although strictly not exactly the same
proton
MRI RF excitation
Radio broadcasting
and receiving
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The Resonant Circuit
• High efficiency around a specified
frequency range
• Covering human tissue of interest
• Inductance-capacitance resonant
circuits
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Principles of Resonant Circuits
Somewhat similar to simple pendulum
Inductor Capacitor
potential energy <--> kinetic energy
exchange at fixed frequency
electric field <--> magnetic field
energy exchange at fixed frequency
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Schematic Resonant Circuit
inductor capacitor
inductor (inherent value)
capacitor added
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Surface Coils in MRI
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Don’t Use Solenoid for RF Coil !
B1 has to be perpendicular to Bo !
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Solenoid: No Excitation Nor Receiving
B1 is exactly parallel with Bo !
z
y
x
Bo Bo
RF coil
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Solenoid RF in Permanent Magnet MRI
Bo perpendicular to human body axis
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Saddle Coil and Helmholtz Coil
Saddle coil Helmholtz pair
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Alderman-Grant Coil
Volume coverage
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Knee Coil
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Another Possibility for the Knee Coil
Slotted tube resonator
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Extension of Simple Resonant Circuit
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Birdcage coil
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Head Coil
“Struts” typical of birdcage type
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Image Comparison from Different Coils
Body coil Head coil 3-in surface coil
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Improving the RF Coil
• Circularly polarized coil
– or Quadrature coil (GE)
• Extra 40% gain in SNR compared
with linearly polarized coil
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Linearly & Circularly Polarized Coil
Linear polarized Circularly polarized
y
x
B1
y
x
B1
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CP Alderman-Grant Coil
Linear polarized Circularly polarized
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CP Birdcage Coil
Note two pairs of driving current entrance
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CP Head Coil
No visual difference from outer package
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Excitation & Receiving
• Excitation coil
– B1 preferably homogeneous
• Receiving coil
– wire close to human subject
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Comparison of Coil
• Volume coil
– Wide coverage, good B1 spatial
homogeneity
• Surface coil
– Can be placed near tissue of interest,
good receiving sensitivity
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Usage of RF Coils
• Large coverage :
– Use smallest volume coil for both
excitation and receiving
• Local area :
– Volume (body) coil for excitation,
surface coil for receiving
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Endorectal Coil
Soft wire in balloon, extended naturally after inflation
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Soft Foldable Coil
Wrist imaging Shoulder imaging
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Combining Advantages
• Large field-of-view (FOV)
• Strong signal, high-quality images
• Assemble many surface coils
– Surface coil phased array
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Phased Array Coil
• Ensemble of many surface coils
• Avoid interference among antennas
• Proper geometric arrangement to
eliminate mutual inductance
• Signals received separately and
simultaneously (multi-channel = $)
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Phased Array Coil
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Photo of Phased Array Coil
Spine phased array
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Images from Spine Phased Array
Large FOV from combining many images
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Other Phased Array Coils
8-channel head coil array
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Other Phased Array Coils
8-channel torso coil array
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Large-N Phased Array Coils
32 channels (aggressively continued R&D)
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Excitation Phased Array ?
• Yes, it is possible !
• But needs multi-channel matched RF
power amplifier
• Not used at 3T or lower
• Useful at >7T and large coverage (out of
scope for this semester)
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Other RF Coil Criteria
• B1 homogeneity
• Output power requirement
• Linearity of RF power amplifier
• Number of receiver channels
• Automatic arc detection ...
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RF Shielding
• Voltage detected in MRI ~ uV level
• Frequency ~ FM radio (everywhere)
• All MRI systems need RF shielding to
isolate radiofrequency waves from
outside of scanner room
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Copper RF Shielding for MRI Room
from Nelco
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Major MRI Hardware
Shim coil
RF coil
Magnet
Gradient coil
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The Entire MRI System Block Diagram
Grad Amp
Grad Amp
Grad Amp
RF Amp
RF Xcvr
Gx
Gy
Gz
RF
Coil T/R
Pulse
Controller
DAQ
Host
Computer
Array
Processor
Console,
Monitor,
Disk, ...
Magnet & Cryogen
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MRI Hardware: magnet,
gradient, and RF coils
Hsiao-Wen Chung (鍾孝文), Ph.D., Professor
Dept. Electrical Engineering, National Taiwan Univ.
Dept. Radiology, Tri-Service General Hospital