Journal of Medical Engineering & Technology
ISSN: 0309-1902 (Print) 1464-522X (Online) Journal homepage: http://www.tandfonline.com/loi/ijmt20
Focusing of an ultrasonic field upon transmission using an annular transducer G. M. Grimshaw To cite this article: G. M. Grimshaw (1978) Focusing of an ultrasonic field upon transmission using an annular transducer, Journal of Medical Engineering & Technology, 2:2, 77-78, DOI: 10.3109/03091907809161763 To link to this article: http://dx.doi.org/10.3109/03091907809161763
Published online: 09 Jul 2009.
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Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=ijmt20 Download by: [University of California, San Diego]
Date: 09 April 2016, At: 16:05
Focusing of an ultrasonic field upon transmission using an 1 annular transducer G. M. Grimshaw, BSc., M S c .
Downloaded by [University of California, San Diego] at 16:05 09 April 2016
Sheffield University and Area Health Authority (Teaching), Medical Physics Depariment, Westori Park Hospital, Sheffield, UK.
With the advent of improved electronics in clinical B-scan systems, the limiting factor in many imaging situations is the width of the ultrasonic field. This technical note reports a simple method by which an ultrasonic beam may be focused along the axis of the beam, using a transducer which is suitable for interfacing t o a conventional ultrasound scanner. A transducer consisting of two concentric annuli and a small central disc is used in rather the same way as a zone lens. The annuli are fired in a carefully controlled time sequence to simulate the emission of a spherical wave. Careful choice of the time relationships will create an area of constructive interference as a focus at the desired point along the axis of the ultrasonic beam. One of the requirements for the transducer is that its elements should be electrically and mechanically isolated. Previous workers [1,2] have constructed this type of transducer by cutting each element from a different crystal blank and packing the interelement space with acoustic absorbers. F o r the transducer used in this system, elements are created on a single crystal blank of PZTSA by depositing the high voltage electrodes in the pattern required, without any mechanical isolation of the elements. A common low voltage electrode is shared by all the elements. This method overcomes one of the main problems of previous transducers in that the sensitivity of each element, provided the areas are the same,
i
i
TABLE 1 The path delays, calculated in ns, using a velocity of 1540 ms-I, required to produce a converging spherical wave. Focal Point 4 crn
8 crn
Outer Annulus . . . . . . . . . 0 Middle Annulus . . . . . . . . . . 136 Central Disc . . . . . . . . . 324
12 crn
Outer Annulus ......... 0 Middle Annulus . . . . . . . . . 91.4 Inner Annulus . . . . . . . . . 216.4
is identical. Figure l a shows the physical dimensions of each element which all have a fundamental frequency of 2.45 MHz. Figure l b shows the output from each element excited separately and plotted in a water tank at the crystal face using a hydrophone. Figure 2 shows the way in which a focused ultrasonic field can be produced on transmission. The timing relationships for the firing sequences were devised to give foci a t 4 cm, 8 cm and 12 cm. These positions were chosen arbitrarily as being in the middle of the three ranges which are of interest in obstetric work. One disadvantage of this system is that only one focal point can be created per transmission cycle and, therefore, only that part of the field which is around the focus should be viewed upon reception of the ultrasonic echoes. Excitation of the middle annulus and central disc occurs after excitation of the outer annulus, with the delays ranging from 91 t o 642 nsec (Table 1). The fact of these delays imposes several restrictions in the choice of the firing mechanism. The maximum pulse repetition frequency of high voltage transmitters is restricted t o approximately 1 kHz due t o the time needed t o accumulate the necessary charge. Delay lines could be used in conjunction with one transmitter but this would prove both inflexible and expensive. The solution adopted was t o provide a transmitter for each element capable of being fired upon reception of an externally generated trigger signal. The required time delays can, therefore, be generated accurately and flexibly in low voltage circuitry. The external trigger pulses are generated by applying a ramp voltage and a carefully stabilised reference voltage to the two
2
I
Diameter in mm I
',
-
Body . . . . . . . . Outer . . . . . . . . . Middle. . . . . . . . . Centre . . . . . . . . .
Time Delay (ns) Outer Annulus . . . . . . . . . 0 Middle Annulus . . . . . . . . . 269.5 Central Disc . . . . . . . . . 642
Outer
Inner
26 19.6 15.8 11
-
16.2 11.4
-5dB
1
II
L i
'I 2
CENTRAL DISC
I
\ , _ __ ANNULUS . ER
-
Figure Ib. The acoustic output of the transducer measured at the front f a c e when each element is excited separately. Volume 2 No. 2 March 1978
77
Crimshaw-Focusing of an ultrasonic field upon transmission
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ind nputs will of provide a comparator. the external The comparator trigger pulse, output when willthe go ramp high, .caches the reference voltage. The ramp and reference voltages ire all generated from the same reference voltage diode, and he outer annulus, to which all other time delays are relative, is ired in a like manner, so that any ramp or reference voltage nstability will not affect the timing relationships. As an indication of the effects of phasing the firing sequences, Cigure 3 shows the beam profiles of the field focused at I cm compared with the beam profiles obtained at the same lepth when all elements of the transducer are fired in coniunction. Focusing an ultrasound field upon transmission seems a .eliable and inexpensive method of reducing the lateral beam width of an ultrasonic field. It is suggested that when used in :onjunction with a system which focuses upon reception of he ultrasonic pulse, a system with lateral resolution com)arable to range resolution may be crealed.
---4Jnfocussed -Focussed
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~EFERENCES
1) Guibarra, E. J., Wells, P. N. T. and Evans, K. T. (1976).
IERE Conference Proceedings NO.3. Applications of Ultra-
sonics in Medicine. 21 Burckhardt, C. B., Grandchamp, P. A. and Hoffman, H. (1974). IEEE Trans. Son. Ultrason. SU-21. 1 .
2cm
1
0
1
2cm
Figure 3. Comparing the beam profiles at 4 cm, 8 cm and 16 cm depth in the ultrasonic field for the transducer elementsfired in SynchrOnY (unfocused) and using time relationships to give a focal point at 8 cm.
Chemical Engineering in medicine in North America P. A. F. White Tontinuedfrom page 68 While we have primarily been concerned with research ictivities as that seems to be the area in which we received he most input, we did attempt to obtain data about the :mployment of chemical engineers in the biomedical engineerng industry in the USA. The result is given in Table 1. The nformation was obtained by searching for all firms who night employ chemical engineers in this lield, from the USA wblication ‘Medical and Health Care Marketplace Guide’, ind sending a questionnaire to ?. More detailed information i o m the questionnaires, including the names and addresses If the respondents along with their specific fields of research, he number of chemical engineers involved in their work, and he titles of their publications, is available from the author. The technical directors of those firms. Unfortunately, the 78
response was much less satisfactory. Of 49 questionnaires seit out, only 11 firms replied. These 1 1 h s , however, use 26 chemical engineers, so it is likely that some 100 chemical engineers are employed in biomedical industries in the USA. We have only been able to discover three so employed in Great Britain. It is clear that chemical engineering skills of many types are being used in the medical field in the USA, and that they are working in connection with many medical interests. It is hoped that it will not be long before a similar, substantial interaction between the two professions becomes as widespread in the UK and elsewhere. The author would be pleased to endeavour to make contacts between the two professions where an interest appears to exist. Journal of Medical Engineering and TechnologY
ISSN: 0309-1902 (Print) 1464-522X (Online) Journal homepage: http://www.tandfonline.com/loi/ijmt20
Focusing of an ultrasonic field upon transmission using an annular transducer G. M. Grimshaw To cite this article: G. M. Grimshaw (1978) Focusing of an ultrasonic field upon transmission using an annular transducer, Journal of Medical Engineering & Technology, 2:2, 77-78, DOI: 10.3109/03091907809161763 To link to this article: http://dx.doi.org/10.3109/03091907809161763
Published online: 09 Jul 2009.
Submit your article to this journal
View related articles
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=ijmt20 Download by: [University of California, San Diego]
Date: 09 April 2016, At: 16:05
Focusing of an ultrasonic field upon transmission using an 1 annular transducer G. M. Grimshaw, BSc., M S c .
Downloaded by [University of California, San Diego] at 16:05 09 April 2016
Sheffield University and Area Health Authority (Teaching), Medical Physics Depariment, Westori Park Hospital, Sheffield, UK.
With the advent of improved electronics in clinical B-scan systems, the limiting factor in many imaging situations is the width of the ultrasonic field. This technical note reports a simple method by which an ultrasonic beam may be focused along the axis of the beam, using a transducer which is suitable for interfacing t o a conventional ultrasound scanner. A transducer consisting of two concentric annuli and a small central disc is used in rather the same way as a zone lens. The annuli are fired in a carefully controlled time sequence to simulate the emission of a spherical wave. Careful choice of the time relationships will create an area of constructive interference as a focus at the desired point along the axis of the ultrasonic beam. One of the requirements for the transducer is that its elements should be electrically and mechanically isolated. Previous workers [1,2] have constructed this type of transducer by cutting each element from a different crystal blank and packing the interelement space with acoustic absorbers. F o r the transducer used in this system, elements are created on a single crystal blank of PZTSA by depositing the high voltage electrodes in the pattern required, without any mechanical isolation of the elements. A common low voltage electrode is shared by all the elements. This method overcomes one of the main problems of previous transducers in that the sensitivity of each element, provided the areas are the same,
i
i
TABLE 1 The path delays, calculated in ns, using a velocity of 1540 ms-I, required to produce a converging spherical wave. Focal Point 4 crn
8 crn
Outer Annulus . . . . . . . . . 0 Middle Annulus . . . . . . . . . . 136 Central Disc . . . . . . . . . 324
12 crn
Outer Annulus ......... 0 Middle Annulus . . . . . . . . . 91.4 Inner Annulus . . . . . . . . . 216.4
is identical. Figure l a shows the physical dimensions of each element which all have a fundamental frequency of 2.45 MHz. Figure l b shows the output from each element excited separately and plotted in a water tank at the crystal face using a hydrophone. Figure 2 shows the way in which a focused ultrasonic field can be produced on transmission. The timing relationships for the firing sequences were devised to give foci a t 4 cm, 8 cm and 12 cm. These positions were chosen arbitrarily as being in the middle of the three ranges which are of interest in obstetric work. One disadvantage of this system is that only one focal point can be created per transmission cycle and, therefore, only that part of the field which is around the focus should be viewed upon reception of the ultrasonic echoes. Excitation of the middle annulus and central disc occurs after excitation of the outer annulus, with the delays ranging from 91 t o 642 nsec (Table 1). The fact of these delays imposes several restrictions in the choice of the firing mechanism. The maximum pulse repetition frequency of high voltage transmitters is restricted t o approximately 1 kHz due t o the time needed t o accumulate the necessary charge. Delay lines could be used in conjunction with one transmitter but this would prove both inflexible and expensive. The solution adopted was t o provide a transmitter for each element capable of being fired upon reception of an externally generated trigger signal. The required time delays can, therefore, be generated accurately and flexibly in low voltage circuitry. The external trigger pulses are generated by applying a ramp voltage and a carefully stabilised reference voltage to the two
2
I
Diameter in mm I
',
-
Body . . . . . . . . Outer . . . . . . . . . Middle. . . . . . . . . Centre . . . . . . . . .
Time Delay (ns) Outer Annulus . . . . . . . . . 0 Middle Annulus . . . . . . . . . 269.5 Central Disc . . . . . . . . . 642
Outer
Inner
26 19.6 15.8 11
-
16.2 11.4
-5dB
1
II
L i
'I 2
CENTRAL DISC
I
\ , _ __ ANNULUS . ER
-
Figure Ib. The acoustic output of the transducer measured at the front f a c e when each element is excited separately. Volume 2 No. 2 March 1978
77
Crimshaw-Focusing of an ultrasonic field upon transmission
I‘ I
I
r
IcDu.tC
1r.mdus.r
Downloaded by [University of California, San Diego] at 16:05 09 April 2016
I
*knp,h
N.
ind nputs will of provide a comparator. the external The comparator trigger pulse, output when willthe go ramp high, .caches the reference voltage. The ramp and reference voltages ire all generated from the same reference voltage diode, and he outer annulus, to which all other time delays are relative, is ired in a like manner, so that any ramp or reference voltage nstability will not affect the timing relationships. As an indication of the effects of phasing the firing sequences, Cigure 3 shows the beam profiles of the field focused at I cm compared with the beam profiles obtained at the same lepth when all elements of the transducer are fired in coniunction. Focusing an ultrasound field upon transmission seems a .eliable and inexpensive method of reducing the lateral beam width of an ultrasonic field. It is suggested that when used in :onjunction with a system which focuses upon reception of he ultrasonic pulse, a system with lateral resolution com)arable to range resolution may be crealed.
---4Jnfocussed -Focussed
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I
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\
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\
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,-
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\
/
/
‘\16cmDepth
/
,/
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~EFERENCES
1) Guibarra, E. J., Wells, P. N. T. and Evans, K. T. (1976).
IERE Conference Proceedings NO.3. Applications of Ultra-
sonics in Medicine. 21 Burckhardt, C. B., Grandchamp, P. A. and Hoffman, H. (1974). IEEE Trans. Son. Ultrason. SU-21. 1 .
2cm
1
0
1
2cm
Figure 3. Comparing the beam profiles at 4 cm, 8 cm and 16 cm depth in the ultrasonic field for the transducer elementsfired in SynchrOnY (unfocused) and using time relationships to give a focal point at 8 cm.
Chemical Engineering in medicine in North America P. A. F. White Tontinuedfrom page 68 While we have primarily been concerned with research ictivities as that seems to be the area in which we received he most input, we did attempt to obtain data about the :mployment of chemical engineers in the biomedical engineerng industry in the USA. The result is given in Table 1. The nformation was obtained by searching for all firms who night employ chemical engineers in this lield, from the USA wblication ‘Medical and Health Care Marketplace Guide’, ind sending a questionnaire to ?. More detailed information i o m the questionnaires, including the names and addresses If the respondents along with their specific fields of research, he number of chemical engineers involved in their work, and he titles of their publications, is available from the author. The technical directors of those firms. Unfortunately, the 78
response was much less satisfactory. Of 49 questionnaires seit out, only 11 firms replied. These 1 1 h s , however, use 26 chemical engineers, so it is likely that some 100 chemical engineers are employed in biomedical industries in the USA. We have only been able to discover three so employed in Great Britain. It is clear that chemical engineering skills of many types are being used in the medical field in the USA, and that they are working in connection with many medical interests. It is hoped that it will not be long before a similar, substantial interaction between the two professions becomes as widespread in the UK and elsewhere. The author would be pleased to endeavour to make contacts between the two professions where an interest appears to exist. Journal of Medical Engineering and TechnologY
