MRI Physics 3:
Hardware
Douglas C. Noll
Biomedical Engineering
University of Michigan
MRI Hardware
• Recall the three magnetic fields
– B0
– B1
– Gx, Gy, Gz
• Also, we need pulse control and data
acquisition systems
Noll
Major MRI Components
RF Screen Room
Main magnet
Gradient Coil
RF Coil
Receiver
Sampler
RF Amp
Real-Time
Controller
Gradient
Amps
Display &
Console
Operator’s
Computer
Disks
Noll
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.
Noll
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
Noll
The Main Field
•
Safety issues:
–
–
–
–
–
–
Noll
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 “nonsignificant risk.”
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)
Noll
RF Coils
Volume Coils
Birdcage
Coil
Saddle Coils
Array Coil
Noll
Surface Coil
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.
Noll
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.)
Noll
Gradient Fields
• High power amplifiers generate
magnetic fields in the same direction as
B0, but with variations along x, y and z.
Noll
z
x
Gradients
x-gradient (G )
y
x
z
x
y
y-gradient (Gy)
z
x
y
z-gradient (Gz)
Noll
Gradient Coil Designs
z
B(z)
B(x)
x
Z Gradient
Noll
X,Y Gradient
Gradient Fields
•
Safety issues:
–
–
Noll
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.
Additional Slides
Noll
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
Noll