MRI Physics 3:Hardware

Douglas C. NollBiomedical EngineeringUniversity of Michigan

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MRI Hardware

• Recall the three magnetic fields– B0

– B1

– Gx, Gy, Gz

• Also, we need pulse control and data acquisition systems

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Major MRI Components

RF Coil

Gradient Coil

Main magnet

RF Screen Room

RF Amp

GradientAmps

Receiver Sampler

Real-TimeController

Operator’sComputer

Display &Console

Disks

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RF Screen Room• Encloses the MRI device• Shields MRI scanner from

electromagnetic noise from computers, radio stations, etc.– MRI signals are weak compared to noise

sources– Any wires going in/out must be filtered and

grounded so they don’t introduce noise• These rooms can also incorporate

magnetic shielding to contain the magnetic field.

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The Main Field

• B0 range: 0.2 T to 7 T

• Higher fields: – Higher SNR – Slightly longer T1’s, – Poorer RF homogeneity– Higher power deposition– Higher cost

• Typically superconducting

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The Main Field• Safety issues:

– Attraction of ferrous objects– Attraction of metallic implants (e.g. cochlear

implants, neurostimulators, pacemakers, and poorly designed/manufactured stents, screws, pins, aneurysm clips, etc.)

– Attraction of foreign metal objects (metal in eyes, shapnel, ingested ferrous objects).

– Affects magnetic switches in pacemakers– Some reports of dizziness, light flashes, unusual

tastes, etc. at very high magnetic fields.– The FDA has classified 7 T and under as a “non-

significant risk.”

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RF Fields• RF coils must generate a rotating (or

oscillating) magnetic field– Ideally uniform application of B1

• RF coils also detect precessing magnetization– Ideally sensitive mainly to tissue of interest– Uniformity is of secondary importance

• Three main types:– Volume Coils– Surface Coils– Arrays Coils (which are basically arrays of surface

coils)

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RF Coils

BirdcageCoil Saddle Coils Surface Coil

Array Coil

Volume Coils

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RF Coils

• Typical Coils– Head Coil (volume birdcage coil)– Body Coil (volume, mostly transmit only)– Surface Coils (e.g. occipital coils for visual

system studies, mostly receive only)• Improved SNR vs. volume coils

– Head Array (array of surface coils, receive only) • The idea here is that one can receive the benefits

of surface coils, but over a larger volume. • Also allows use of parallel MRI technology.

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RF Fields

• Safety issues:– RF heating of the body (FDA limits our

power deposition) – Focal RF heating around metals (glasses,

jewelry, implants, some tattoos, etc.) can lead to burns

– Focal heating near implanted devices e.g. cochlear implants, neurostimulators, pacemakers, and poorly designed/manufactured stents, screws, pins, aneurysm clips, etc.)

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Gradient Fields

• High power amplifiers generate magnetic fields in the same direction as B0, but with variations along x, y and z.

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Gradientsx

z

y

x

z

y

x

z

y

x-gradient (Gx)

y-gradient (Gy)

z-gradient (Gz)

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Gradient Coil Designs

B(z)

z

x

B(x)

Z Gradient X,Y Gradient

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Gradient Fields

• Safety issues:– Rapid changes of fields can lead to

peripheral nerve stimulation (e.g. like a twitching in the back). FDA guidelines state that stimulation cannot be “painful.”

– The gradients are also responsible for the substantial acoustic noise. FDA says it can’t exceed 99 dBA with hearing protection in place.

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Additional Slides

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Typical fMRI Protocol

• Scout Acquisition– Where am I?

• T1-weighted images – To align structure to fMRI maps

• T2-weighted images – To rule out pathology

• fMRI – T2*-weighted, dynamic – EPI or spiral

• High-resolution 3D T1-weighted images – For segmentation of anatomy