g16.4427 practical mri 1 – 16 th april 2015 g16.4427 practical mri 1 transmit arrays

G16.4427 Practical MRI 1 – 16th April 2015

G16.4427 Practical MRI 1

Transmit Arrays


Outline

• Paper Review

• RF power amplifiers

• Dual-tuned coils


Paper Review

D.K. Sodickson, 2-26-09

Sidebar: Component coil combinations in arrays

?


= * + * + * + *

Optimal combination:

Sum of squares combination:

=( * + * + * + * )1/2

Sidebar: Component coil combinations in arrays


Component coil combinations and signal-to-noise ratio

Sum

Coil #2

Coil #3

Coil #4

Unfiltered

Coil #1

Filtered

Matched filter effect:


Generalized quadrature effect:

Component coil combinations and signal-to-noise ratio


SENSE as a generalized optimal coil combination

1inverse 1 1

1no acceleration

1

H H

H

H

S S Ψ S S Ψ

S Ψ

S Ψ S

Matched filtercombination

Noisedecorrelation


RF Power Amplifiers

• RF Power Amplifiers (RFPAs) are a vital sub-system of any NMR spectrometer or MRI scanner– The sole purpose is the amplification of the RF pulse

• Many power amplifiers may be required to achieve higher power output levels– Divider/combiner networks sum multiple stages

• If the RFPA was ideal, the output would be an exact replica of the input waveform with greater amplitude– Conventional RFPA are not ideal and distort the signal


RFPA ArchitecturePre-Driver and Driver are low power amplifier stages that raise the power level of the input signal from mW to a level high enough to drive the high power PA sections

Directional coupler separates out proportional samples of forward and reflected power for internal/external power monitoring and fault detection

The DC power supply converts AC line voltages into DC voltages that are suitable to operate the Pre-Driver, Driver, Power Amplifiers and microcontroller

The microcontroller is a micro-computer that continuously runs a fixed program loop that monitors several vital operating parameters (e.g. DC voltages, currents, pulse width, etc.) If there is a risk of damage, it will put the system into fault mode


Actual RF Pulse

It takes 4 parameters to define an ideal RF pulse. What are they?

It takes 19 parameters to characterize a pulse that has been through a non ideal amplifier


Actual RF Pulse

It takes 4 parameters to define an ideal RF pulse:• Amplitude• Frequency• Pulse width• Duty factor

It takes 19 parameters to characterize a pulse that has been through a non ideal amplifier


Actual RF Pulse Parameters


RFPA Specifications: Time Domain• We want very high RF power pulses with precise fidelity

only for short periods of time– Maximum pulse width (during which the RFPA can put out

maximum power) is 20-300 ms for MRI– Average power requirements (duty factor) ~10-15% maximum

• Pulse pre-shoot, post pulse backswing– Distortion occurs after an RFPA has been un-blanked (or RF pulse

is terminated)– Appears as half or more cycles of a low frequency signal

superimposed on the un-blanked noise voltage– Low frequency, so it will be filtered out by the transmit coil


Pulse Pre-Shoot, Post Pulse Backswing

Pulse pre-shot

Post pulse backswing


RFPA Specifications: Time Domain• Very high RF power pulses with precise fidelity for short

periods of time– Maximum pulse width (during which the RFPA can put out maximum

power) is 20-300 ms for MRI– Average power requirements (duty factor) ~10-15% maximum

• Pulse pre-shoot, post pulse backswing– Distorsion occurs after an RFPA has been un-blanked (or RF pulse is

terminated)– Appears as half or more cycles of a low frequency signal

superimposed on the un-blanked noise voltage– As low frequency, it will be filtered out by the Tx coil

• Rise, fall time (transition duration)– Time to transition from 10% to 90% of the voltage waveform– Specification for MRI: 250 nsec to 10 μsec


Pulse Transition Duration

Risetime

Fall time


RFPA Specifications: Time Domain• Overshoot, rising/falling edge

– Distortion occurs from inductively stored energy within the RFPAs circuitry (transition from zero to full power in ~100 ns voltage spike due to large current changes in inductors get superimposed on the RF pulse)

– Specification for MRI: < 13%


Overshoot, Rising/Falling EdgeRisingpulse overshoot Pulse

falling edge

Falling pulse overshoot

Pulse rising edge




– Specification for MRI: < 13%• Pulse overshoot ringing/decay time

– Energy being fly between inductive and capacitive circuits in the RFPA generates a lower frequency dumped sinusoidal wave that is imposed on the RF pulse after the rise time and modulates its amplitude

– Specification: time for the amplitude modulation to drop to less than 5% of peak RF pulse amplitude < 5 μsec


Pulse Overshoot Ringing/Decay Time

Pulse overshoot ringing/decay time




– Specification for MRI: < 13%• Pulse overshoot ringing/decay time

– Energy being fly between inductive and capacitive circuits in the RFPA generates a lower frequency dumped sinusoidal wave that is imposed on the RF pulse after the rise time and modulates its amplitude

– Specification: time for the amplitude modulation to drop to less than 5% of peak RF pulse amplitude < 5 μsec

• Pulse tilt (positive or negative)– Gain change due to temperature increase in “on” RF transistors– Specification for MRI: < 8% over 20 ms rectangular pulse


Pulse Tilt (Positive/Negative)

Pulse tilt


RFPA Specifications: Time Domain• Long term amplitude/phase stability

– Ideally would amplify every pulse exactly the same way– Changes in environment (e.g. temperature) can alter RFPAs– Specifications for MRI: amplitude < 0.2 dB, phase < 3 degrees over

24 hours at constant temperature• Phase error over-pulse

– Occurs as a phase shift across the duration of a rectangular pulse in cases when the pulse tilt is substantial

– Specification: < 5 degrees across a 10 msec pulse width• Un-Blanking, Blanking propagation delay

– To reduce electronic noise during signal acquisition, the output stage of an RFPA are shut off

– Delay measures the ability of an RFPA to rapidly turn on/off– Specification for MRI: 2 μsec


Pulse Overshoot Ringing/Decay Time

Blanked noise voltage

Un-Blanked noise voltage


RFPA Specifications: Frequency Domain• Generic frequency domain specifications

– The bandwidth is the range of frequencies for which the RFPA complies with output power, linearity, etc.

• Power gain– Specification: maximum peak power when maximum

output power is required• Gain flatness

– The wider the bandwidth the harder is to maintain constant power gain flatness at key frequencies

– Specifications: broadband = ± 3 dB, nuclei centered = ± 0.2 dB at ± 500 kHz


RFPA Specifications: Frequency Domain• Harmonic content

– Practical RFPAs are not perfectly linear output frequency spectrums also at integer multiples of the input frequency

– Mostly filtered out by the transmit coil– Specification: even/odd order harmonics = -20 db/-12 dB

• Spurious RF output emissions (oscillation)– Erratic frequency components that the RFPA puts out (e.g.

DC feed that couples RF power from output to input)– Specification: < -50 dBc

• Input VSWR– How close the input impedance is to an ideal 50 Ω resistor– Specification: < 2:1 (perfect match = 1:1)


RFPA Specifications: Frequency Domain• Output noise (blanked)

– To minimize electronic noise, the bias of the transistors of the final stages of power amplification are shut off (there will still be some tolerable noise output)

– Specification: -20 dB over thermal noise• Noise Figure

– In applications where RFPA is transmitting at one frequency and RF receivers are listening at another, the less NF an RFPA has, the less will interfere with this second frequency

– Specification: < -10 dB


RFPA Specifications: Power Domain

• The input power to the RFPA is swept across a range of power levels (usually 30-40 db)– E.g.: if an RFPA is driven to full power at 0 dBm input

(i.e. 1 mW), the unit will be tested for input -40 to 0 dBm to check for phase linearity and gain

• RF power output– 1T-3T: 0.5-2 kW extremities (legs and arms), 4-8 kW

head, up to 35 kW whole body– Higher field strengths: 10-20 kW is common– Multi-channel: 4 kW (3T) and 1 kW (7T) per channel


RFPA Specifications: Power Domain• Gain linearity

– Defined in terms of dynamic range (from maximum specified output power level to some dB down from such level)

– Specification: ± 1 dB gain variation over 40 dB dynamic range• Phase linearity

– Although it takes few nanoseconds for the signal to go from input to output of the RFPA, there is a propagation delay

– In ideal case the phase shift is constant across the dynamic range– Phase non-linearity is a due to parasitic junction capacitance

present in all types of RF power transistors (change with output power)

– Specification: ± 7.5 degrees phase variation over 40 dB dynamic range


Gain Linearity

In the non-ideal case, the transfer function changes over the dynamic range of the amplifier power levels will be amplified by different power gain factors

Pulse sequences can contain RF waveforms that have precisely proportioned amplitude ratios, which can change dramatically in case of severe deviation from ideal gain linearity


Troubleshooting

Excessive gain non-linearity Slice profile distortion

Excessive phase non-linearity Slice profile distortion

Excessive rise/fall time Slice profile distortion

Gain instability Image ghosting/shading

Phase instability Image ghosting

Excessive pulse overshoot Slice profile distortion

Spurious oscillation Image artifacts/streaking

Low power output Inability to achieve desired flip angle

Amplifier Performance Anomaly Symptom


Any questions?


Dual-Tuned Coils• A major problem of implementing multinuclear MRI is the

construction of a probe capable of operating at more than one frequency

• To make a single-tuned coil resonant, we normally add a tuning circuit (a capacitor in series with the coil):

In order to multiple tune a coil, we need to make the reactance curve of the tuning network cross the anti-reactance curve of the coil more than once


Double Resonant Circuit• A useful tuning network consists of a parallel LC trap in

series with the tuning capacitor network– The reactance, as a function of frequency, will begin capacitive,

then pass through a pole (trap resonant frequency) and then become capacitive again

The reactance curve crosses the anti-reactance curve of the coil twice two resonances are established


Matching• Normally a reactive element is added in parallel to the series

tuned network so that the input impedance to the entire network is real and equal to the generator impedance

• For dual-tuned coil, we can use a parallel LC matching network– At the low frequency C is large enough so that we may consider only the

inductor and adjust its value for proper matching– The capacitor can then be tuned so that the parallel combination of C and

L has the required reactance for matching at the higher frequency


Example: Dual-Tuned Birdcage at 1.5 T• The fourth harmonic of the sodium frequency is very close to

the proton frequency at 1.5 T (67.8 MHz vs. 64 MHz)– It is challenging to decouple the two channels in a birdcage– Modified inductive coupling circuits (with baluns) are used to provide

better decoupling

• The trap circuit method is used to obtain identical current distributions for both resonance frequencies– Same B1 field distribution


Any questions?


See you next week!

g16.4427 practical mri 1 – 16 th april 2015 g16.4427 practical mri 1 transmit arrays

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