G16.4427 Practical MRI 1 - PowerPoint Presentation

Slide1

G16.4427 Practical MRI 1

Receive ArraysSlide2

Receive Arrays Are Critical in MRI

Advantages

SNR

Speed (parallel MRI)Volumetric coverageImage qualitySimplicity

DisadvantagesCostComplexityData load…

How many elements do we need?Slide3

Benefits for Parallel Imaging

Max acceleration = # of detector coils

Need more coils to go faster!

Intrinsic SNR lossNeed more coils for multi-dimensional acceleration and volumetric coverage!Noise amplifications (geometry factor)Need more coils for improved encoding capabilities!Slide4

SNR at Depth

Number of elements

SNR

Body noise

dominated

Coil noise

dominated

more coils are better up to a certain point !Slide5

128-Element Cardiac Array

Front

BackSlide6

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?Slide7

Do not get scared: each element of a coil array is a surface coil designed to receive the signal from the nuclear spins

Let’s start by reviewing

some principles of receive-only surface coil designSlide8

Transmit Detuning

During RF excitation, receive coils must be transparent so B

1

+ is not distortedLimiting 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 Slide9

Transmit Detuning

During RF excitation, receive coils must be transparent so B

1

+ is not distortedLimiting 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 highTotal coil impedance must be switched from low during receive to high during transmissionPassive detuningActive detuning Slide10

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 Slide11

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 TrapSlide12

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 featureIf the transmit field not strong enough diodes will not be fully switchedPassive traps cannot be monitored independently to identify potentially dangerous situations (e.g. diodes burn out)Slide13

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 amplifier’s 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 resistanceSlide14

Active Detuning Schematic

DC + RFSlide15

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 μV

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 dBSlide16

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 B0RF and gradient field compatibilityGround plane as small and thin as possible to avoid shielding effects and eddy currentsVery high dynamic rangeMust work with very small to large input signalsAccurate complex gain reproducibilityAid in decoupling of resonant loops in arrayMust be protected against transmit power and excessive heatingSlide17

Power Matching

The goal is maximum power extraction from signal source (i.e. no reflected power)

Maximum power forSlide18

Noise Matching

The goal is maximum signal-to-noise ratio (SNR) at the preamp output

equivalent noise sources

Ideal noise-free preampSlide19

Noise Factor and Other Quantities

“Noise Factor”

S

= signal powerN = noise poweren = input referred spectral noise voltage density [V ·Hz-1/2]

in = 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/HzSlide20

Noise Matching Condition

For a bandwidth

Δf

(assuming no correlation between en and in:

minimum noise factor for Slide21

Noise Figure vs.

ρ

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

sSlide22

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 fromSlide23

Equivalent Circuit For Coupling

inductive coupling

resistive coupling

capacitive couplingSlide24

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”Slide25

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 Slide26

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”Slide27

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 SNRQuestion: in what conditions it is zero?Slide28

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 SNRIs zero in lossless mediaSome geometrical coil configurations can be found where resistive coupling is zeroSlide29

Geometric Decoupling

Standard method between nearest neighbors

Coil overlapped at a distance for which mutual inductance become zero

Only parasitic capacitance and mutual resistanceHas the advantage of being broadbandThere are some limitations:Cannot be extended beyond three coils or between non-neighboring coilsNon optimal for parallel imaging spatial encodingIncrease noise correlationSlide30

Coil Overlapping in Parallel Imaging

Intrinsic Noise

Final Noise

g-factor

Baseline SNR and

g

-factor are empirically optimized

for target image planes and accelerationsSlide31

Geometric Decoupling Example

w

~ 1 / (LC)

1/2single surfacecoilSlide32

Geometric Decoupling Example

lightly coupled

coilsSlide33

Geometric Decoupling Example

strongly coupled

coilsSlide34

Geometric Decoupling Example

critical overlapSlide35

Geometric Decoupling Example

Single Coil

Lightly Coupled

Strongly Coupled

Critical OverlapSlide36

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 1With geometric decoupling we set M12 = 0, with preamp decoupling we set I2 = 0Slide37

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

Ω

The preamp must see a 50

Ω

source:

R

0

= 50

Ω

The preamp must be noise matched:

r

s

=

r

n

≈ 1

kΩ

Step-up network (series resonance) that create a short-equivalent and impedance transformation to achieve 50

Ω

match Slide38

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 techniqueSlide39

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 techniqueSlide40

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 RFCalculating the correlation between data in different channelsWe’ll see more in lecture 15Slide41

Cabling and Safety Issues

Cabling and related grounding are critical parts of any array

Poor cabling can create:

additional coupling between channelsB1+ distortionsHeating hazards due to currents flowing on ground conductors during transmissionProper cable routing is the first step to avoid these problems (e.g. route cables along regions of low electric fields) Slide42

Cabling and Safety Issues

Cabling and related grounding are critical parts of any array

Poor cabling can create:

additional coupling between channelsB1+ distortionsHeating hazards due to currents flowing on ground conductors during transmissionProper 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 cablesSlide43

Essential Principles of Array Design

Coil arrays designed for parallel MRI need:

Good baseline SNR

Effective encoding capabilitiesGeneral requirements apply:Decoupling of signal and noise between elementsGood match circuitryGood preamplifiers behaviorSpatial encoding capabilities are controlled by tailoring the shape and distribution of coil sensitivities to maximize feasible accelerationSlide44

Any questions?Slide45

Acknowledgments

The slides relative to the geometric decoupling example are courtesy of Dr. Graham WigginsSlide46

See you next week!