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MR Equipment Review

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					MR Equipment Review
Terry M. Button, Ph.D.

Basic MR Hardware Components
• Primary magnetic field • Gradient • RF transmit/receive

Primary Magnet
• • • • Field Strength Magnet type Field orientation Performance criteria

Field Strength
• Typical field strengths
– 0.2 – 3.0 T

• “Bigger is better.”
– SNR improves with Bo – Spectroscopic resolution improves with Bo.

Magnet Types
• Permanent magnets • Resistive magnet • Super conducting magnets

Permanent Magnets
• Field strength is limited
– Saturation – Flux line divergence in air (homogeneity issue as well). – 0.2 T

• Massive (0.2 T/gap suitable for adult pelvis)
– 20 - 30 tons; iron – 5 ton; neodymium alloy

• Advantage
– Open configuration possible – Potential variety of easy-patient-access geometry

Resistive Magnets
• Current supply must be stable. • Current produces heat (50 kW at 0.15 T) through resistive turns:
BI Heat produced  I2 Water cooling is required; very high fields are not practical

Super Conducting Magnet
• Niobium/Titanium alloy
– Super conduction at 10o K – Maintained by liquid He (4 oK)
• Reliquefy He • 6 mo fill

– Quench; coils become resistive/He turns to gas
• Quench vent • Vent heater

• Very stable (0.001 G/hr) and uniform • Always on

Super conducting

Body coil
patient opening

Gradients Magnet: -cryogens -coils -shims

Field Orientation
Solenoid – large fringe field bare magnet passively shielded actively shielded

Vertical – little fringe field “open magnet” impacted by environment

Homogeneity
• Measured as the maximum deviation of the field over a specified volume in ppm. • Homogeneity
– controls resolution (several – gradient recall SNR (few ppm) – ability to perform spectroscopy (tenths of ppm)

Impact of Homogeneity on Resolution
• Minimum SE homogeneity requirements are determined by voxel size and gradient strength:
– 10 mT/m gradient = 425 Hz/mm – If inhomogeneity exceeds this => voxels misregister => BLUR! – At 1.5 T this is 7 ppm; at 0.3 T this is 33 ppm

Magnet Performance
• Typical homogeneity
– Closed high field: 0.5 ppm over 30 cm sphere – Open low field: 5 ppm over 30 cm sphere

• Methods to over come inhomogeneity
– Passive shim – pieces of iron – Active shim – resistive or super conducting – Gradient shim – for each patient

Gradients
• Provide the ability to localize MR signal • Factors:
– Linearity – Basic configuration – Performance

Gradient Structure
B

Bo-

Bo+0

Bo+

x

• Gradient should be linear with position.

Basic Gradient Configuration
Z gradient X gradient

Gradient is in the direction of Bo.
G Bo

x

Gradient Performance
• Gradient strength
– 10 mT/m support SE – 20-50 mT/m for functional imaging (EPI)

• Gradient slew rate
– Rise times of 0.6 ms (17T/m/s) support SE – 0.1 ms (200 T/m/s) for EPI

• Eddy current compensation
t ideal eddy current distorted

G Eddy current effects are minimized using preemphasis; over drive gradient just a pulse is turned on.

Imaging Coils
• Proper selection of imaging coils can greatly improve signal-to-noise ratio (SNR). • SNR improvement is due to filling factor. • Trade off is field-of-view.

Physics of a Tuned Circuit (A)

+++++++++++++

C
---------------

L

Physics of a Tuned Circuit -B

+ + + + + + + +

L
- - - - - - - -

Physics of a Tuned Circuit - C

+

+

C
-

L

Physics of a Tuned Circuit - D
+++++++++++++

C
---------------

L

---------------

+++++++++++++

+++++++++++++

---------------

Coil Resonance
• Coil resonant frequency:
fR = 1/[2(LC) 1/2 ]

• In MR, the coil resonant frequency is matched to the Larmor frequency:
fR = fL

• Coil Q is an important factor in SNR
I Q = fo /(FWHM)

fo

Coil Operation
• Coil must be tuned to resonance. • Coil must match the preamplifier input impedance (50 ) • Receive coils must be decoupled during transmit.

MR Coils Types
• • • • Linear coils Quadrature coils Phase array coils Sense coils

Linear Coils
• Loop/surface coils

• Saddle coils

• Helmholz pair

Linear Coil

Quadrature Coils
• Thought of as two orthogonal linear coils
M

• Each coil contributes to signal (90o out of phase) so SNR improves by 40%. • A common quad coil is a birdcage configuration:

Birdcage Coil

Phased Array
• Phased array is the use of several surface coils.
– Advantages:
• Good SNR • Large FOV

– Disadvantages:
• Coil coupling - “magic separation” geometry • Multiple receiver channels ($)

Two Separated Resonant Structures
No coupling

A

B

I

I

fo

fo

Two Close Resonant Structures
Couple

A

B

I

I

fo

Phased Array
Nearest neighbors do not interact using the proper magic geometry: (sep = 1.5 r)

A B

I

fo
I

Phased Array

Sense Imaging
• Surface coil arrays have the potential to provide superior image signal-to-noise ratio (SNR). • Availability of individual coil signals allows implementation of parallel imaging techniques.
– SMASH (SiMultaneous Acquisition of Spatial Harmonics). – SENSE (SENSitivity Encoding).

SENSE Considerations
• Rather than magic geometry, ultra highimpedance pre-amplifiers are used to decouple coil elements. • Provides greater flexibility in positioning coil elements (6-8 elements). • Ultimately allowing higher image signalto-noise-ratio (SNR).


				
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