g16.4427 practical mri 1 – 16 th april 2015 g16.4427 practical mri 1 transmit arrays
TRANSCRIPT
G16.4427 Practical MRI 1 – 16th April 2015
G16.4427 Practical MRI 1
Transmit Arrays
G16.4427 Practical MRI 1 – 16th April 2015
Outline
• Paper Review
• RF power amplifiers
• Dual-tuned coils
G16.4427 Practical MRI 1 – 16th April 2015
Paper Review
D.K. Sodickson, 2-26-09
Sidebar: Component coil combinations in arrays
?
D.K. Sodickson, 2-26-09
= * + * + * + *
Optimal combination:
Sum of squares combination:
=( * + * + * + * )1/2
Sidebar: Component coil combinations in arrays
D.K. Sodickson, 2-26-09
Component coil combinations and signal-to-noise ratio
Sum
Coil #2
Coil #3
Coil #4
Unfiltered
Coil #1
Filtered
Matched filter effect:
D.K. Sodickson, 2-26-09
Generalized quadrature effect:
Component coil combinations and signal-to-noise ratio
D.K. Sodickson, 2-26-09
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
G16.4427 Practical MRI 1 – 16th April 2015
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
G16.4427 Practical MRI 1 – 16th April 2015
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
G16.4427 Practical MRI 1 – 16th April 2015
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
G16.4427 Practical MRI 1 – 16th April 2015
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
G16.4427 Practical MRI 1 – 16th April 2015
Actual RF Pulse Parameters
G16.4427 Practical MRI 1 – 16th April 2015
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
G16.4427 Practical MRI 1 – 16th April 2015
Pulse Pre-Shoot, Post Pulse Backswing
Pulse pre-shot
Post pulse backswing
G16.4427 Practical MRI 1 – 16th April 2015
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
G16.4427 Practical MRI 1 – 16th April 2015
Pulse Transition Duration
Risetime
Fall time
G16.4427 Practical MRI 1 – 16th April 2015
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%
G16.4427 Practical MRI 1 – 16th April 2015
Overshoot, Rising/Falling EdgeRisingpulse overshoot Pulse
falling edge
Falling pulse overshoot
Pulse rising edge
G16.4427 Practical MRI 1 – 16th April 2015
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%• 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
G16.4427 Practical MRI 1 – 16th April 2015
Pulse Overshoot Ringing/Decay Time
Pulse overshoot ringing/decay time
G16.4427 Practical MRI 1 – 16th April 2015
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%• 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
G16.4427 Practical MRI 1 – 16th April 2015
Pulse Tilt (Positive/Negative)
Pulse tilt
G16.4427 Practical MRI 1 – 16th April 2015
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
G16.4427 Practical MRI 1 – 16th April 2015
Pulse Overshoot Ringing/Decay Time
Blanked noise voltage
Un-Blanked noise voltage
G16.4427 Practical MRI 1 – 16th April 2015
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
G16.4427 Practical MRI 1 – 16th April 2015
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)
G16.4427 Practical MRI 1 – 16th April 2015
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
G16.4427 Practical MRI 1 – 16th April 2015
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
G16.4427 Practical MRI 1 – 16th April 2015
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
G16.4427 Practical MRI 1 – 16th April 2015
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
G16.4427 Practical MRI 1 – 16th April 2015
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
G16.4427 Practical MRI 1 – 16th April 2015
Any questions?
G16.4427 Practical MRI 1 – 16th April 2015
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
G16.4427 Practical MRI 1 – 16th April 2015
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
G16.4427 Practical MRI 1 – 16th April 2015
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
G16.4427 Practical MRI 1 – 16th April 2015
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
G16.4427 Practical MRI 1 – 16th April 2015
Any questions?
G16.4427 Practical MRI 1 – 16th April 2015
See you next week!