G16.4427 Practical MRI 1 9 th April 2015 Receive Arrays Are
Critical in MRI Advantages SNR Speed (parallel MRI) Volumetric
coverage Image quality Simplicity Advantages SNR Speed (parallel
MRI) Volumetric coverage Image quality Simplicity Disadvantages
Cost Complexity Data load Disadvantages Cost Complexity Data load
How many elements do we need?
Slide 3
G16.4427 Practical MRI 1 9 th April 2015 Benefits for Parallel
Imaging Max acceleration = # of detector coils Need more coils to
go faster! Intrinsic SNR loss Need more coils for multi-dimensional
acceleration and volumetric coverage! Noise amplifications
(geometry factor) Need more coils for improved encoding
capabilities!
Slide 4
G16.4427 Practical MRI 1 9 th April 2015 SNR at Depth Number of
elements SNR Body noise dominated Body noise dominated Coil noise
dominated Coil noise dominated more coils are better up to a
certain point !
Slide 5
G16.4427 Practical MRI 1 9 th April 2015 128-Element Cardiac
Array Front Back
Slide 6
G16.4427 Practical MRI 1 9 th April 2015 Coil Design Challenges
What is the minimum practical coil size? What is the optimal number
of elements? What is the best geometrical arrangement? How do we
decouple the elements? What is the best cable layout?
Slide 7
G16.4427 Practical MRI 1 9 th April 2015 Do not get scared:
each element of a coil array is a surface coil designed to receive
the signal from the nuclear spins Lets start by reviewing some
principles of receive-only surface coil design
Slide 8
G16.4427 Practical MRI 1 9 th April 2015 Transmit Detuning
During RF excitation, receive coils must be transparent so B 1 + is
not distorted Limiting the currents on the coil induced by the
transmit field to negligible levels by ensuring that the total
impedance of the coil loop is very high
Slide 9
G16.4427 Practical MRI 1 9 th April 2015 Transmit Detuning
During RF excitation, receive coils must be transparent so B 1 + is
not distorted Limiting the currents on the coil induced by the
transmit field to negligible levels by ensuring that the total
impedance of the coil loop is very high Total coil impedance must
be switched from low during receive to high during transmission
Passive detuning Active detuning
Slide 10
G16.4427 Practical MRI 1 9 th April 2015 Passive Detuning Use a
pair of crossed high-speed diodes Diodes act as a switch that
connects a parallel resonant trap to the coil thus opening the
circuit Surface Loop Coil
Slide 11
G16.4427 Practical MRI 1 9 th April 2015 Passive Detuning Use a
pair of crossed high-speed diodes Diodes act as a switch that
connects a parallel resonant trap to the coil thus opening the
circuit High-Z Trap
Slide 12
G16.4427 Practical MRI 1 9 th April 2015 Passive Detuning Use a
pair of crossed high-speed diodes Diodes act as a switch that
connects a parallel resonant trap to the coil thus opening the
circuit Used mostly as redundant safety feature If the transmit
field not strong enough diodes will not be fully switched Passive
traps cannot be monitored independently to identify potentially
dangerous situations (e.g. diodes burn out)
Slide 13
G16.4427 Practical MRI 1 9 th April 2015 Active Detuning
Required bringing an external DC bias voltage to diodes on the coil
The additional logic signal required to switch the coil between
transmit and receive states is supplied either on a dedicated line
or using the RF power amplifiers un-blank signal The switching
devices most often used today are PIN diodes, which can control
large RF currents with a small DC current and low RF
resistance
Slide 14
G16.4427 Practical MRI 1 9 th April 2015 Active Detuning
Schematic DC + RF
Slide 15
G16.4427 Practical MRI 1 9 th April 2015 Preamplifiers One of
the key hardware elements in an RF coil from a standpoint of SNR
performance The induced voltage (i.e. signal) in a coil is very
small, typically on the order of a few V This small signal is
amplified to a few mV by a preamplifier with gain ~30 dB (i.e. 1000
times) The industry standard preamplifier has noise figure less
than 0.5 dB
Slide 16
G16.4427 Practical MRI 1 9 th April 2015 Requirements for MR
Applications Static magnetic field compatibility Preamps are in an
extremely strong and homogeneous static magnetic field No ferrites
or iron, Cu-only coaxial cables, no magnetic distortion of B 0 RF
and gradient field compatibility Ground plane as small and thin as
possible to avoid shielding effects and eddy currents Very high
dynamic range Must work with very small to large input signals
Accurate complex gain reproducibility Aid in decoupling of resonant
loops in array Must be protected against transmit power and
excessive heating
Slide 17
G16.4427 Practical MRI 1 9 th April 2015 Power Matching The
goal is maximum power extraction from signal source (i.e. no
reflected power) Maximum power for
Slide 18
G16.4427 Practical MRI 1 9 th April 2015 Noise Matching The
goal is maximum signal-to-noise ratio (SNR) at the preamp output
equivalent noise sources Ideal noise-free preamp
Slide 19
G16.4427 Practical MRI 1 9 th April 2015 Noise Factor and Other
Quantities Noise Factor S = signal power N = noise power e n =
input referred spectral noise voltage density [V Hz -1/2 ] i n =
input referred spectral noise current density [A Hz -1/2 ] =
thermal noise voltage density of source resistor [V Hz -1/2 ] noise
input resistance of the preamplifiers in Ohms spectral noise power
density of the preamplifiers in W/Hz
Slide 20
G16.4427 Practical MRI 1 9 th April 2015 Noise Matching
Condition For a bandwidth f (assuming no correlation between e n
and i n : minimum noise factor for
Slide 21
G16.4427 Practical MRI 1 9 th April 2015 Noise Figure vs. n
noise matching for: for power matching was If we have a good
transistor with a small p n, even if we do not meet exactly the
minimum, the noise figure is still ~F min the smaller the noise
figure of a preamp (i.e. the smaller p n ), the wider the allowed
range of source impedance r s
Slide 22
G16.4427 Practical MRI 1 9 th April 2015 Array Coupling
Creating an array is not as simple as putting together a number of
surface coil elements Coupling reduces the spatial uniqueness of
the signal acquired from the coils due to signal crosstalk and
introduces correlation in the noise between channels
Electromagnetically, coupling can be divided into three categories
based on the fields that it originates from
Slide 23
G16.4427 Practical MRI 1 9 th April 2015 Equivalent Circuit For
Coupling inductive coupling resistive coupling capacitive
coupling
Slide 24
G16.4427 Practical MRI 1 9 th April 2015 Inductive Coupling Due
to the direct interaction of coil loops through magnetic fields
produced by currents that are flowing on the conductors The
equivalent circuit is a mutual inductance (M), or transformer, and
leads to changes in the frequency response of the elements and
degrade their sensitivity magnetic coupling coefficient
Slide 25
G16.4427 Practical MRI 1 9 th April 2015 Electric (Capacitive)
Coupling Electric coupling is due to the direct interaction of coil
loops through (conservative) electric fields due to charges on the
coils (Coulomb fields), which is equivalent to a mutual capacitance
between the coils This parasitic capacitance is more relevant at
higher frequencies (smaller reactance) and can be enhanced by
body/phantom permittivity, therefore making it sensitive to
positioning, patient size, etc. It can also be introduced or
controlled to compensate for inductive coupling
Slide 26
G16.4427 Practical MRI 1 9 th April 2015 Resistive Coupling Due
to the indirect interaction of coil loops through currents
supported by the finite conductivity of the body or phantom on
which the array is placed Appears as a mutual resistance term in
the equivalent circuit: mutual resistance
Slide 27
G16.4427 Practical MRI 1 9 th April 2015 Mutual Resistance
Determines the lowest achievable coupling (i.e. by eliminating the
reactive components) Cannot be eliminated by any decoupling method
Is associated with intrinsic noise correlation that influences
image reconstruction and SNR Question: in what conditions it is
zero?
Slide 28
G16.4427 Practical MRI 1 9 th April 2015 Mutual Resistance
Determines the lowest achievable coupling (i.e. by eliminating the
reactive components) Cannot be eliminated by any decoupling method
Is associated with intrinsic noise correlation that influences
image reconstruction and SNR Is zero in lossless media Some
geometrical coil configurations can be found where resistive
coupling is zero
Slide 29
G16.4427 Practical MRI 1 9 th April 2015 Geometric Decoupling
Standard method between nearest neighbors Coil overlapped at a
distance for which mutual inductance become zero Only parasitic
capacitance and mutual resistance Has the advantage of being
broadband There are some limitations: Cannot be extended beyond
three coils or between non-neighboring coils Non optimal for
parallel imaging spatial encoding Increase noise correlation
Slide 30
G16.4427 Practical MRI 1 9 th April 2015 Coil Overlapping in
Parallel Imaging Intrinsic Noise Final Noise g-factor Baseline SNR
and g-factor are empirically optimized for target image planes and
accelerations
Slide 31
G16.4427 Practical MRI 1 9 th April 2015 Geometric Decoupling
Example w ~ 1 / (LC) 1/2 single surface coil
Slide 32
G16.4427 Practical MRI 1 9 th April 2015 Geometric Decoupling
Example lightly coupled coils
Slide 33
G16.4427 Practical MRI 1 9 th April 2015 Geometric Decoupling
Example strongly coupled coils
Slide 34
G16.4427 Practical MRI 1 9 th April 2015 Geometric Decoupling
Example critical overlap
Slide 35
G16.4427 Practical MRI 1 9 th April 2015 Geometric Decoupling
Example Single CoilLightly CoupledStrongly CoupledCritical
Overlap
Slide 36
G16.4427 Practical MRI 1 9 th April 2015 Preamplifier
Decoupling It has been the enabling technology for many- element
receive arrays It prevents currents from flowing around the coil,
so signal cannot couple inductively By tuning and matching we
minimize the noise associated with coil 1 With geometric decoupling
we set M 12 = 0, with preamp decoupling we set I 2 = 0
Slide 37
G16.4427 Practical MRI 1 9 th April 2015 Three Design Goals
First transistor of the preamp with equivalent noise input
resistance r n The coil must see almost a short: r n 5 The preamp
must see a 50 source: R 0 = 50 The preamp must be noise matched: r
s = r n 1 k Step-up network (series resonance) that create a
short-equivalent and impedance transformation to achieve 50
match
Slide 38
G16.4427 Practical MRI 1 9 th April 2015 Reactive Decoupling If
the coupling matrix is known it is possible to design networks of
capacitors and inductors that introduce couplings that are equal
but opposite to those present between the coils Used in Tx/Rx
arrays where preamp decoupling is not feasible. Question: why?
Limitations: Changes is coupling with time, position, loading are
not easily accommodated generally a narrowband technique
Slide 39
G16.4427 Practical MRI 1 9 th April 2015 Reactive Decoupling If
the coupling matrix is known it is possible to design networks of
capacitors and inductors that introduce couplings that are equal
but opposite to those present between the coils Used in Tx/Rx
arrays where preamp decoupling is not feasible. Question: why?
Limitations: Changes is coupling with time, position, loading are
not easily accommodated generally a narrowband technique
Slide 40
G16.4427 Practical MRI 1 9 th April 2015 Noise Correlation
Measurements Measurement of noise correlation is required for
optimal-SNR image combination and is also a commonly used measure
of coil coupling It is performed by: acquiring a sufficient number
of noise samples with the array connected to the MR system and no
RF Calculating the correlation between data in different channels
Well see more in lecture 15
Slide 41
G16.4427 Practical MRI 1 9 th April 2015 Cabling and Safety
Issues Cabling and related grounding are critical parts of any
array Poor cabling can create: additional coupling between channels
B 1 + distortions Heating hazards due to currents flowing on ground
conductors during transmission Proper cable routing is the first
step to avoid these problems (e.g. route cables along regions of
low electric fields)
Slide 42
G16.4427 Practical MRI 1 9 th April 2015 Cabling and Safety
Issues Cabling and related grounding are critical parts of any
array Poor cabling can create: additional coupling between channels
B 1 + distortions Heating hazards due to currents flowing on ground
conductors during transmission Proper cable routing is the first
step to avoid these problems (e.g. route cables along regions of
low electric fields) Cable traps near the coils and/or baluns along
cables are used to block shield currents that would flow outside of
the shields of the coaxial cables
Slide 43
G16.4427 Practical MRI 1 9 th April 2015 Essential Principles
of Array Design Coil arrays designed for parallel MRI need: Good
baseline SNR Effective encoding capabilities General requirements
apply: Decoupling of signal and noise between elements Good match
circuitry Good preamplifiers behavior Spatial encoding capabilities
are controlled by tailoring the shape and distribution of coil
sensitivities to maximize feasible acceleration
Slide 44
G16.4427 Practical MRI 1 9 th April 2015 Any questions?
Slide 45
G16.4427 Practical MRI 1 9 th April 2015 Acknowledgments The
slides relative to the geometric decoupling example are courtesy of
Dr. Graham Wiggins
Slide 46
G16.4427 Practical MRI 1 9 th April 2015 See you next
week!