- Email: [email protected]
An acoustic injection test object for colour flow imaging systems
Copyright
ELSEVIER
l
Ultrasound in Med. & Biol.. Vol. 24. No. I, pp. lhl--161. 19% 0 1998 World Federation for Ultrasound in Medicme & Biology Printed in the USA. All rights reserved KrJ-5629/9X ~19.llO + .oo
PI1 SO301-5629(97)00214-7
Technical Note AN ACOUSTIC
INJECTION TEST OBJECT IMAGING SYSTEMS
FOR COLOUR
FLOW
S. F. LI, P. R. HOSKINS, T. ANDERSON and W. N. MCDICKEN Department of Medical Physics and Medical Engineering, Royal Infirmary, University of Edinburgh, Edinburgh EH3 9YW UK (Received
30 December
1996; in jinal
,form13
August
1997)
Abstract-There are few test objects suitable for colour-flow ultrasound scanners. An acoustic injection device is described that enables the production of a 2-D region of colour on a colour-flow image. The device involves detection of the transmitted ultrasound pulse from the scanner, followed by the emission of a synthesised echo that consists of a radiofrequency burst modulated by an audiofrequency signal in such a way that, on reception by the ultrasound scanner, it is interpreted as a signal arising from a region of flow. The depth of the colour region may be controlled by adjustment of the length of the synthesised echo. The audiofrequency content may be altered as desired, enabling examination of the relationship between the displayed colour and the signal spectral content. 0 1998 World Federation for Ultrasound in Medicine & Biology. Key Words: Acoustic injection,
Colour
flow, Doppler
ultrasound,
Test object
Typically, coupling of the test device transducersto the transducerof the ultrasound scannerunder test is accomplished via a perspex block. A double sidebandsystem was originally described by Evans et al. (1989). However, audio signals that simulated forward and reverse flow componentscould not be adequately injected using this device; consequently, the simulated Doppler spectrum displayed was symmetrical about the zero line. A single sidebandinjection system able to produce either a forward or a reverse flow component has been described (Wallace et al. 1993). Colour imagesof flow in arteries and veins display a 2-D region of colour. The test objects described above all give satisfactory Doppler signals when using realtime spectral display. When used with a colour scanner, a string phantom produces only a single line of colour. Flow or belt phantoms provide images with regions of colour. Our experience of using a single sidebandacoustic injection system with a colour-flow scanneris that a single line of colour is produced at the interface between the perspex block and the transducer used for transmission of the pulse. At a slightly greater depth, a second colour image is displayed consisting of several lines of colour, which it is assumedis at a depth determined by reverberation of ultrasound within the perspex block. The presenceof severallines of colour is taken to be due to reverberation within the transmit transducer.
INTRODUCTION The available test devices that have been described for Doppler ultrasound systems include moving-target devices, such as flow phantoms (Holdsworth et al. 1991; Hoskins et al. 1989; Law et al. 1987), string phantoms (Walker et al. 1982; Russellet al. 1993) and belt phantoms in which the moving target is a volume of semisolid scatterer (Rickey et al. 1992; Rickey and Fenster 1996). A second category involves the use of electronically synthesisedDoppler signals.These electronics-baseddevices have the potential to offer significant advantages over mechanical moving-target devices and flow phantom systems, such as stability, freedom from artefacts, portability etc. Electronics-baseddevices produce signals that may be injected directly into the Doppler receiver (Bastos and Fish 1991; Sheldon and Duggen 1987) or, alternatively, the transmitted ultrasound pulse from a scanner may be captured by a receiving transducer, an acoustic test signal admixed with the captured pulse, and the composite signal transmitted back to the scanner. This latter technique is termed “acoustic reinjection.” Dr. Li’s current address is Telemedicine project, 23 Chalmers Street, Royal Infirmary, Edinburgh EH3 9YW UK. Address correspondence to: P. R. Hoskins, The University of Edinburgh, Royal Infirmary, Dept. of Medical Physics & Medical Engineering, 1 Lauriston Place. Edinburgh EH3 9YW UK. E-mail: [email protected] 161
Volume 24, Number I. 1998
Ultrasound in Medicine and Biology
162
Perspex block -t
Test object electronics
Fig. 1. Arrangement
of the transducers and the coupling block.
In this paper, an electronic injection device is described that is capable of producing a region of colour on the display of a colour-flow scanner.
MATERIALS
AND METHODS
A rectangular perspex block 3 cm thick is usedas a coupling device between the transducer of the scanner under evaluation and the transducer of the injection device. Figure 1 illustrates the relative positioning of the transducerson opposite sidesof the block. To date, only scannerswith linear array probes have been tested. In the reinjection devices described above, the thickness of the displayed colour line is determined by the duration of the transmitted ultrasound pulse. To produce a region of colour, it is necessaryto increase the duration of the signal received by the colour-flow scanner. In autocorrelation colour-flow systems, movement in any area is detected by changesin the phase of the echoesreceived from that area, without reference to the transmitted signal frequency. The frequency of the synthesisedecho must, however, be within the passbandof the ultrasound system. In the device of this paper, the transmit frequency was set equal to the nominal ultrasoundprobe Doppler transmit frequency, as indicated in the manufacturer’s user manual. The device described here is based on an independent sideband modulator (ISBM), which mixes a radiofrequency (RF) signal and an audiofrequency (AF) signal to produce independent upper and lower sidebands (Frerking and Groshong, 1987). This signal, suitably gated, is returned to the ultrasoundmachine where the upper and lower sidebands are interpreted as forward and reverse flow Doppler signals for decoding and display.
A simplified block diagram of the acoustic injection system is illustrated in Fig. 2. The digital control unit controls the overall operation of the system and determines the size and position of the colour block on the scannerimage. The actions of the digital control unit are synchronisedwith the scannerby meansof the transmit pulse detector, which detects the arrival of the transmitted ultrasonic pulse from the scannerat the transducer of the injection device. The RF quadrature oscillator and AF quadrature oscillator generatethe signalsfor input to the ISBM. After amplification, the output from the ISBM is fed, via the gate under control of the digital control unit, to a final amplifier driving the injection device transducer. An external audio signal may be used as the modulation source, but must first be passedthrough a phaseshift network to provide the necessaryquadrature signalsfor the ISBM. Colour-flow imageswere acquired using an Acuson 128 XP/lO scannerwith a L7384 linear array (715MHz) and a L558 linear array (Y3.5 MHz).
RESULTS Figure 3a, b, c showscolour imagesobtained using synthesisedechoeswith a single frequency audio signal. In eachcase,a region of colour of uniform hue is shown. At the highest frequency, seenin Fig. 3c, the frequency is beyond the aliasing limit and the colour is displayed as blue. Figure 4 showsthat Doppler waveforms can also be reproduced by the device. In the case shown, Doppler waveforms from a flow phantom that were previously recorded on audio tape are usedas the audio input to the test device.
Acoustic injection for colour flow 0 S. F. Ll
From Transducer
Signal Generator -
163
ET AL.
Modulator
Amplifier
1
I
Fig. 2. Block diagramof the test device.
DISCUSSION AND CONCLUSION The device works well for the production of a 2-D region of colour on the colour-flow image. The device, in its current form, is well suited for the performance of a number of tests. Injection of single audio frequencies would allow the calibration of the colour scale,including an assessment of the minimum detectable frequency and, hence, of the level of the clutter filter. Control over the audio signal intensity is possible, and a protocol for assessmentof colour sensitivity could be developed using this feature. By reducing the colour box to its minimum size, it is possible to investigate the registration
between the location of the colour image and of the spectral Doppler range gate. The display of colour-flow signalsis dependenton a numberof factors:the strengthof the Dopplersignalfrom the desiredmovingtarget(usuallyblood),the strengthof thesignal from stationaryand slowly moving tissue,plus the effectsof machinenoise. In addition, true colour-flow imagesfrom bloodhave a specklepattern.It shouldbe possibleto develop the currenttestdeviceto simulatethe signalfrom tissueandto producea specklepattern.Sucha device would enableinvestigationsto be performedon the affect of eachof theseeffects on the displayedcolour image.
Fig. 3. Colour images obtained using a single audiofrequency tone burst: (a) 1 kHz equivalent to 0.154 ms ‘: (b) 3 kHz equivalent to 0.462 rns- ‘; (c) 4 kHz equivalent to 0.616 ms ~ ’ : it is noted that the colour image is aliased.
Ultrasound
Fig.
in Medicine
4. Doppler
and Biology
waveforms
acquired
Despite the potential benefits of electronic injection devices, they continue to suffer from a number of limitations, including the difficulties of interfacing to curved transducers and problems associated with the complex beam-forming methods used in modern ultrasonic scanners. Further development work is, therefore, required before the technique can be widely applied in ultrasonic colour-imaging system characterisation. REFERENCES Bastos CC, Fish PJ. A Doppler signal simulator. Clin Phys Physiol Meas 1991;12:177-183. Evans JA, Price R, Luhana F. A novel testing device for Doppler ultrasound equipment. Phys Med Biol 1989:34:1701-1707. Frerking ME, Groshong RA. Single-sideband systems and circuits. In: Sabm WE, Schoenike EO, eds. Digital signal processing. New York: McGraw-Hill. 1987:247-29 1. Holdsworth DW, Rickey DW, Drangova M. Miller DJM. Fenster A.
Volume
from
24. Number
within
I. 199X
the colour
region
Computer controlled positrvc drsplacement pump for physiological flow simulation. Med Biol Eng Comput 1991;29:565-570. Hoskins PR, Anderson T, McDicken WN. A computer controlled flow phantom for generation of physiological Doppler waveforms, Phys Med Biol 1989;34:1709-1717. Law YF. Cobbold RSC. Johnston KW, Bascom PAJ. Computer controlled pulsatile pump for physiological fow simulation. Phy Med Biol 1987:34:1709-1717. Rickey DW, Fenster A. A Doppler ultrasound clutter phantom. Ultrasound Med Biol 1996;22:747-766. Rickey DW, Rankin R, Fenster A. Velocity evaluation phantom for colour flow and pulsed Doppler instruments. Ultrasound Med Biol 1992;18:479-494. Russell SV, McHugh D. Moreman BR. A programmable Doppler string test object. Phys Med Biol 1993:38: 1623-1630. Sheldon CD, Duggen TC. Low-cost Doppler signal simulator. Med Biol Eng Comput 1987:25:226-228. Wallace JJA, Martin K, Whittingham TA. An experimental singlesideband acoustical reinjection test method for Doppler systems. Physiol Meas 1993;14:479-484. Walker AR. Philips DJ, Powers JE. Evaluating Doppler devices using a moving string test target. J Clin Ultrasound 1982;10:25-30.
ELSEVIER
l
Ultrasound in Med. & Biol.. Vol. 24. No. I, pp. lhl--161. 19% 0 1998 World Federation for Ultrasound in Medicme & Biology Printed in the USA. All rights reserved KrJ-5629/9X ~19.llO + .oo
PI1 SO301-5629(97)00214-7
Technical Note AN ACOUSTIC
INJECTION TEST OBJECT IMAGING SYSTEMS
FOR COLOUR
FLOW
S. F. LI, P. R. HOSKINS, T. ANDERSON and W. N. MCDICKEN Department of Medical Physics and Medical Engineering, Royal Infirmary, University of Edinburgh, Edinburgh EH3 9YW UK (Received
30 December
1996; in jinal
,form13
August
1997)
Abstract-There are few test objects suitable for colour-flow ultrasound scanners. An acoustic injection device is described that enables the production of a 2-D region of colour on a colour-flow image. The device involves detection of the transmitted ultrasound pulse from the scanner, followed by the emission of a synthesised echo that consists of a radiofrequency burst modulated by an audiofrequency signal in such a way that, on reception by the ultrasound scanner, it is interpreted as a signal arising from a region of flow. The depth of the colour region may be controlled by adjustment of the length of the synthesised echo. The audiofrequency content may be altered as desired, enabling examination of the relationship between the displayed colour and the signal spectral content. 0 1998 World Federation for Ultrasound in Medicine & Biology. Key Words: Acoustic injection,
Colour
flow, Doppler
ultrasound,
Test object
Typically, coupling of the test device transducersto the transducerof the ultrasound scannerunder test is accomplished via a perspex block. A double sidebandsystem was originally described by Evans et al. (1989). However, audio signals that simulated forward and reverse flow componentscould not be adequately injected using this device; consequently, the simulated Doppler spectrum displayed was symmetrical about the zero line. A single sidebandinjection system able to produce either a forward or a reverse flow component has been described (Wallace et al. 1993). Colour imagesof flow in arteries and veins display a 2-D region of colour. The test objects described above all give satisfactory Doppler signals when using realtime spectral display. When used with a colour scanner, a string phantom produces only a single line of colour. Flow or belt phantoms provide images with regions of colour. Our experience of using a single sidebandacoustic injection system with a colour-flow scanneris that a single line of colour is produced at the interface between the perspex block and the transducer used for transmission of the pulse. At a slightly greater depth, a second colour image is displayed consisting of several lines of colour, which it is assumedis at a depth determined by reverberation of ultrasound within the perspex block. The presenceof severallines of colour is taken to be due to reverberation within the transmit transducer.
INTRODUCTION The available test devices that have been described for Doppler ultrasound systems include moving-target devices, such as flow phantoms (Holdsworth et al. 1991; Hoskins et al. 1989; Law et al. 1987), string phantoms (Walker et al. 1982; Russellet al. 1993) and belt phantoms in which the moving target is a volume of semisolid scatterer (Rickey et al. 1992; Rickey and Fenster 1996). A second category involves the use of electronically synthesisedDoppler signals.These electronics-baseddevices have the potential to offer significant advantages over mechanical moving-target devices and flow phantom systems, such as stability, freedom from artefacts, portability etc. Electronics-baseddevices produce signals that may be injected directly into the Doppler receiver (Bastos and Fish 1991; Sheldon and Duggen 1987) or, alternatively, the transmitted ultrasound pulse from a scanner may be captured by a receiving transducer, an acoustic test signal admixed with the captured pulse, and the composite signal transmitted back to the scanner. This latter technique is termed “acoustic reinjection.” Dr. Li’s current address is Telemedicine project, 23 Chalmers Street, Royal Infirmary, Edinburgh EH3 9YW UK. Address correspondence to: P. R. Hoskins, The University of Edinburgh, Royal Infirmary, Dept. of Medical Physics & Medical Engineering, 1 Lauriston Place. Edinburgh EH3 9YW UK. E-mail: [email protected] 161
Volume 24, Number I. 1998
Ultrasound in Medicine and Biology
162
Perspex block -t
Test object electronics
Fig. 1. Arrangement
of the transducers and the coupling block.
In this paper, an electronic injection device is described that is capable of producing a region of colour on the display of a colour-flow scanner.
MATERIALS
AND METHODS
A rectangular perspex block 3 cm thick is usedas a coupling device between the transducer of the scanner under evaluation and the transducer of the injection device. Figure 1 illustrates the relative positioning of the transducerson opposite sidesof the block. To date, only scannerswith linear array probes have been tested. In the reinjection devices described above, the thickness of the displayed colour line is determined by the duration of the transmitted ultrasound pulse. To produce a region of colour, it is necessaryto increase the duration of the signal received by the colour-flow scanner. In autocorrelation colour-flow systems, movement in any area is detected by changesin the phase of the echoesreceived from that area, without reference to the transmitted signal frequency. The frequency of the synthesisedecho must, however, be within the passbandof the ultrasound system. In the device of this paper, the transmit frequency was set equal to the nominal ultrasoundprobe Doppler transmit frequency, as indicated in the manufacturer’s user manual. The device described here is based on an independent sideband modulator (ISBM), which mixes a radiofrequency (RF) signal and an audiofrequency (AF) signal to produce independent upper and lower sidebands (Frerking and Groshong, 1987). This signal, suitably gated, is returned to the ultrasoundmachine where the upper and lower sidebands are interpreted as forward and reverse flow Doppler signals for decoding and display.
A simplified block diagram of the acoustic injection system is illustrated in Fig. 2. The digital control unit controls the overall operation of the system and determines the size and position of the colour block on the scannerimage. The actions of the digital control unit are synchronisedwith the scannerby meansof the transmit pulse detector, which detects the arrival of the transmitted ultrasonic pulse from the scannerat the transducer of the injection device. The RF quadrature oscillator and AF quadrature oscillator generatethe signalsfor input to the ISBM. After amplification, the output from the ISBM is fed, via the gate under control of the digital control unit, to a final amplifier driving the injection device transducer. An external audio signal may be used as the modulation source, but must first be passedthrough a phaseshift network to provide the necessaryquadrature signalsfor the ISBM. Colour-flow imageswere acquired using an Acuson 128 XP/lO scannerwith a L7384 linear array (715MHz) and a L558 linear array (Y3.5 MHz).
RESULTS Figure 3a, b, c showscolour imagesobtained using synthesisedechoeswith a single frequency audio signal. In eachcase,a region of colour of uniform hue is shown. At the highest frequency, seenin Fig. 3c, the frequency is beyond the aliasing limit and the colour is displayed as blue. Figure 4 showsthat Doppler waveforms can also be reproduced by the device. In the case shown, Doppler waveforms from a flow phantom that were previously recorded on audio tape are usedas the audio input to the test device.
Acoustic injection for colour flow 0 S. F. Ll
From Transducer
Signal Generator -
163
ET AL.
Modulator
Amplifier
1
I
Fig. 2. Block diagramof the test device.
DISCUSSION AND CONCLUSION The device works well for the production of a 2-D region of colour on the colour-flow image. The device, in its current form, is well suited for the performance of a number of tests. Injection of single audio frequencies would allow the calibration of the colour scale,including an assessment of the minimum detectable frequency and, hence, of the level of the clutter filter. Control over the audio signal intensity is possible, and a protocol for assessmentof colour sensitivity could be developed using this feature. By reducing the colour box to its minimum size, it is possible to investigate the registration
between the location of the colour image and of the spectral Doppler range gate. The display of colour-flow signalsis dependenton a numberof factors:the strengthof the Dopplersignalfrom the desiredmovingtarget(usuallyblood),the strengthof thesignal from stationaryand slowly moving tissue,plus the effectsof machinenoise. In addition, true colour-flow imagesfrom bloodhave a specklepattern.It shouldbe possibleto develop the currenttestdeviceto simulatethe signalfrom tissueandto producea specklepattern.Sucha device would enableinvestigationsto be performedon the affect of eachof theseeffects on the displayedcolour image.
Fig. 3. Colour images obtained using a single audiofrequency tone burst: (a) 1 kHz equivalent to 0.154 ms ‘: (b) 3 kHz equivalent to 0.462 rns- ‘; (c) 4 kHz equivalent to 0.616 ms ~ ’ : it is noted that the colour image is aliased.
Ultrasound
Fig.
in Medicine
4. Doppler
and Biology
waveforms
acquired
Despite the potential benefits of electronic injection devices, they continue to suffer from a number of limitations, including the difficulties of interfacing to curved transducers and problems associated with the complex beam-forming methods used in modern ultrasonic scanners. Further development work is, therefore, required before the technique can be widely applied in ultrasonic colour-imaging system characterisation. REFERENCES Bastos CC, Fish PJ. A Doppler signal simulator. Clin Phys Physiol Meas 1991;12:177-183. Evans JA, Price R, Luhana F. A novel testing device for Doppler ultrasound equipment. Phys Med Biol 1989:34:1701-1707. Frerking ME, Groshong RA. Single-sideband systems and circuits. In: Sabm WE, Schoenike EO, eds. Digital signal processing. New York: McGraw-Hill. 1987:247-29 1. Holdsworth DW, Rickey DW, Drangova M. Miller DJM. Fenster A.
Volume
from
24. Number
within
I. 199X
the colour
region
Computer controlled positrvc drsplacement pump for physiological flow simulation. Med Biol Eng Comput 1991;29:565-570. Hoskins PR, Anderson T, McDicken WN. A computer controlled flow phantom for generation of physiological Doppler waveforms, Phys Med Biol 1989;34:1709-1717. Law YF. Cobbold RSC. Johnston KW, Bascom PAJ. Computer controlled pulsatile pump for physiological fow simulation. Phy Med Biol 1987:34:1709-1717. Rickey DW, Fenster A. A Doppler ultrasound clutter phantom. Ultrasound Med Biol 1996;22:747-766. Rickey DW, Rankin R, Fenster A. Velocity evaluation phantom for colour flow and pulsed Doppler instruments. Ultrasound Med Biol 1992;18:479-494. Russell SV, McHugh D. Moreman BR. A programmable Doppler string test object. Phys Med Biol 1993:38: 1623-1630. Sheldon CD, Duggen TC. Low-cost Doppler signal simulator. Med Biol Eng Comput 1987:25:226-228. Wallace JJA, Martin K, Whittingham TA. An experimental singlesideband acoustical reinjection test method for Doppler systems. Physiol Meas 1993;14:479-484. Walker AR. Philips DJ, Powers JE. Evaluating Doppler devices using a moving string test target. J Clin Ultrasound 1982;10:25-30.