mri hardware: magnet, gradient, and rf coils · mri hardware: magnet, gradient, and rf coils...

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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