Flow Imaging Cardiac Ultrasound system

by Larry Miller PhD

www.linkedin.com/in/lrmillermiller@elect-design.com

US Patent 4,612,937 Ultrasound Diagnostic Apparatus,Inventor: Lawrence (Larry) R Miller

11/21/2014

Cardiac ultrasound technology existing at start of project

• Anatomical imaging ultrasound– Manually aimed and rotated for desired fan placement.

Produces 2D fan image– Observe heart wall motion, valve leaflet motion, etc.

• Display: 2D fan-shaped grayscale reflectivity image: Heart structures are reflective, blood shows minimally visible reflectivity.

• Doppler probe– Manually aimed in desired direction– Shows blood velocity as a function of depth (distance from

probe along the 1-dimensional beam)• Display: strip chart similar to a sonogram V=depth, H=time

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Project goal: Flow-imaging cardiac ultrasound

Requirements– Display real-time anatomical image of heart and

related structures as grayscale

– Superimpose image of blood flowing towards the probe in red and away from the probe in blue

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Example of Flow imaging ultrasoundMitral insufficiency

From http://www.ntnu.edu/isb/ultrasound/bloodflowNorwegian University of Science and Technology 41/21/2014

Flow imager pulse sequence• Ultrasound frequency f0

– 3.5 MHz. This is typical for cardiac ultrasound as it provides adequate penetration depth. λ = 0.43 mm.

• Pulse sequence– N pulses in a given direction them move to the next

direction– Frequency resolution increases with increasing N. N=5

chosen for first prototype

• Pulse repetition rate– A typical cardiac image will be 10 cm deep– Speed of sound in tissue and water c = 150,000 cm./sec. – Pulse rate thus Fpulse = 7500 pulses per second.

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Flow imager architecture

• Single element oscillating probe vs phased array probe– Acquiring Doppler signal requires multiple pulses in

the same direction before moving to the next direction

– Oscillating probe cannot perform move and stop, move and stop, … thus phased array probe needed• Added advantage: phased array probe allows dynamic

receive focus, providing greater effective depth of focus

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Doppler velocity detection parameters

• Max unambiguous blood velocity ±vmax

– Nyquist: max unambiguous Fd is Fpulse / 2 which is 3750 Hz

– Backscatter Doppler frequency Fd = 2 f0 v / c where v is blood velocity. v = Fd c / (2 f0 )

– Practical max. for 5 pulse sequence is 80% of Nyquist – vmax = 0.8 x 3750 x 150,000 / (2 x 3.5 x 106 ) = 64 cm. / sec.

• Interleaving– An interleaved by m pulse sequence will have vmax = (64 /

m) cm./sec. This mode is used for observing flows with lower peak velocities.

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Flow imager fan

• Fan width: 90 degrees

• Probe elements: – linear array of 32 elements– Lead zirconate titanate (PZT) (ATL Technologies)– dielectric constant 700– Longitudinal resonance thru thickness: 3.5 MHz– Backed by carbon loaded silicone:

reduces Q of longitudinal resonance to 3

• Probe Dimensions– 15 mm wide by 10 mm high

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Array patterns

Single element response

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Array patterns• Array response for four steering angles

Infinite focus case 101/21/2014

Blood velocity estimator• Requirements

– Should not respond to low velocities.– Should use a minimal number of pulse samples in order to

achieve high frame rate

• Vest(beam_direction,depth_bin) = abs ()2 – abs ()2

– c[i] are 5 fixed coefficients: 1-2i, -4+4i, 6, -4-4i, 1+2i– d[i] are 5 consecutive data points for 5 consecutive pulses along

the given beam direction in the given depth bin

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Blood velocity estimator

• Doppler frequency estimator response function

Nyquist band

Relation between velocity and Doppler frequency: v = Fd c / (2 f0 ) 121/21/2014

Electronic block diagram

32 piezoelectric element ultrasound probe

32 element driver/receiver boards

Receive signal combiner

A to D converter

Digital controller

Log amplifier & detector

Polar to rectangular scan converter

2 x D to A converter

Color NTSC formatter

Display

Digital Doppler processor

cable

Blood veloc.

Anatom.image

…

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Element Driver/Receive board

Probe element

Fine delay line0.05 μs/tap

Coarse delay line0.5 μs/tap

mux mux

Static RAM

Memory download and control from backplane

Combined analog receive signal(drive to backplane analog bus)

14

Outputs change during receive interval to implement dynamic focus

Matching network & Preamplifier

agc1

agc2 agc3 agc4

3 subsequent gain-controlled amplifier stages

Pulse generator

1/21/2014

Doppler processor electronics Doppler processor detail

I and Q demodulator and digitizer

Combined analog receive signal

3 MHz transmit oscillator output

Digital logic

Blood velocity signal15

From US Patent 4,612,937

1/21/2014

Matching network and preamplifier (simplified – parasitic snubbing resistors omitted)

V supply

Probe element equivalent circuit +V

bias

Cascode stage

Lp

Cp

RpLt

Cf (4 x parasitic cap. G to S) = 16 pFLt = 14.6 uH Cp = 10 pF Lp = 220 uHRp = 1.6 kOhm Cc = 134 pF

G

S

Voltage source proportional to ultrasound signal

G

S S

G

S

G

Preamp output to next stage

Zload

4 x 2N4416JFET

AGC in

Cc

16

-I bias

+I bias

Pulse in1/21/2014

Preamplifier Noise Figure

• Noise Figure at 3.5 MHz– NF = 20 log (Vt / Vs)

• A measure of noise added by the preamplifier• Vt = total noise voltage per at preamp output

• Vs = noise voltage contributed by source resistance Rp per at preamp output

– Vs = G = 4.4 nV per • G = preamplifier gain E/e

– Vt = • = 3 nanovolts per

» Each JFET has 6 nV per noise, so 4 averaged provides 3 nV per

– = 5.32 / 4.4 = 1.21– NF = 1.65 dB

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Probe and matching network frequency response

Frequency (MHz)

Relative response amplitude

Half power band: 3.0 MHz to 3.9 MHz

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Probe and preamp input network impulse response

Envelope full width half maximum = 0.88 microsecondsCorresponds to 0.66 mm depth range. Thus depth resolution is 0.66 mm

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Automatic Gain Control (AGC)

• Four successive AGC amplifier stages starting with preamp– Chart shows control voltage applied to each of these 4

stages, and the total AGC achieved

– AGC in voltageis linearly relatedto 10gain_in_dB/20

– AGC in voltages ramps are Generated from table Driving DAC

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Log amplifiersimplified schematic3 of 6 stages shown

21

-supply -supply

Output

-supply

+supply

Input

-supply

Current mirror

Detector stages

HF limiting amplifier stages(Bandpass filtering not shown)

-bias

R R RR/6

Detector zero reference

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Log amplifier stage transfer functions

22

• HF limiting amplifier stage transfer function• (exp (Ein/2) - exp(-Ein/2))/(exp (Ein/2) +

exp(-Ein/2))• E0 = kT/q ≈ 27 mV.• Ein =

stage input voltage / E0

• Stage output voltage = Gain * Eout * E0

• Gain = 10 dB

• Detector stage transfer function• Detector: Eout = log(1 + exp (Ein))• Ein = detector input voltage / E0

• Detector output voltage = Eout * E0

1/21/2014

Log Amplifier transfer functionfrom model

23

• Gain 20 dB per differential amplifier stage

• Gain 10 dB per differential amplifier stage

Input signal level in dB

Output signal level linear scale

Output signal level linear scale

• This gain per stage was used for prototype

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Dynamic apodization

• Contributions from elements at each end reduced for first 1 cm.– Reduces sensitivity to reflections from adjacent ribs– Method: agc for outer elements reduced over first 1 cm.

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Scan Converter10 cm. depth mode: 0.39 mm. per raster line

25

Raster lines

12

256

Raster lines 1 to 256 Scan convert on write

Raster lines 17 to 256Scan convert on read

128 beam directionsRaster scanout: 2048 pixels in 40 μsec 50 MHz output pixel clock1/21/2014

Scan Converter10 cm. depth mode: 0.39 mm. per raster line

26

Raster lines

12

256

Raster lines 1 to 256 Scan convert on write

Raster lines 17 to 256Scan convert on read

128 beam directionsRaster scanout: 2048 pixels in 40 μsec 50 MHz output pixel clock1/21/2014

Clinical tests of prototypes• Approvals and tests

– Preapproved by review board at all medical facilities where tested as investigational device exemption

– Substantially equivalent to Toshiba diagnostic ultrasound device. Same power levels, repetition rate, probe area, and general function.

– Tested by cardiologists at ten hospitals

• Results– Image rated very good and flow imaging worked well on

most patients. – No interference artifact (because of good isolation of

sensitive electronics from digital electronics)

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Appendix

• FDA output limits for diagnostic ultrasound – A spatial-peak temporal-average intensity (ISPTA) less than 720 mW/cm2. – The acoustic output depends on the output power, pulse repetition frequency,

and scanner operating mode (eg, B-mode, M-mode, pulsed, or color or power Doppler imaging).4,9

– J Ultrasound Med 2009; 28:139–150 141

• Power output of our prototype– Pulse width: 0.8 microseconds. Min rep period: 133 μsec – Pulse voltage: 10 volts. Minimum probe impedance: 1600 ohms.– Max instantaneous power generated: 32 x 102/1600 = 2.0 watts– Max temporal average power (at spatial peak) 2.0 x 0.8/133 = 12 mW– Probe active area: 1.6 cm2

– Max spatial peak temporal average power per cm < 7.6 mW• Probe carbon-loaded silicone backing absorbs some of the energy

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