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Patent US4287770 - Method for the manufacture of ultrasonic transducers - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign in<nobr>Advanced Patent Search</nobr>PatentsIn a focusing ultrasonic transducer with a recessed active surface, it is desirable that, given values of the focal width and of the focal distance prescribable as desired, an optimum lateral resolution with the smallest possible influence of side lobes ensues. This is achieved by means of manufacture...http://www.google.com/patents/US4287770?utm_source=gb-gplus-sharePatent US4287770 - Method for the manufacture of ultrasonic transducersAdvanced Patent SearchPublication numberUS4287770 APublication typeGrantApplication numberUS 06/105,643Publication dateSep 8, 1981Filing dateDec 20, 1979Priority dateDec 20, 1978Also published asDE2855143B1, DE2855143C2, EP0012938A2, EP0012938A3Publication number06105643, 105643, US 4287770 A, US 4287770A, US-A-4287770, US4287770 A, US4287770AInventorsArthur WeynsOriginal AssigneeSiemens AktiengesellschaftExport CitationBiBTeX, EndNote, RefManPatent Citations (2), Non-Patent Citations (2), Referenced by (12), Classifications (8) External Links: USPTO, USPTO Assignment, EspacenetMethod for the manufacture of ultrasonic transducersUS 4287770 AAbstract In a focusing ultrasonic transducer with a recessed active surface, it is desirable that, given values of the focal width and of the focal distance prescribable as desired, an optimum lateral resolution with the smallest possible influence of side lobes ensues. This is achieved by means of manufacture in the following steps:
(a) first, for focal width (FB) or focal interval (FA), the outer dimension (2aO) is determined as a function of the radius of curvature (R) of the transducer or the radius of curvature (R) is determined as a function of the outer dimension (2aO);
(b) then the magnitude of outer dimension or, respectively, radius of curvature not yet determined is determined for the other of the two focal magnitudes (for example, FA); and
(c) the maximum value of the angle of aperture of the sound radiation field behind the focus deriving in that manner is limited by means of a suitable selection of the size (2ai) of the recess in the active surface of the transducer such that the relationship between the angle of aperture and side lobes becomes optimum.
I claim as my invention: 1. A method for the manufacture of ultrasonic transducers with at least one ultrasonic transducer whose active surface exhibits recesses and whose sound radiation field is to be prescribable in terms of focal distance, focal width and angle of aperture in connection with the focus and ultrasonic frequency, characterized by the following steps:(a) for the two focal magnitudes of focal width (FB) and focal distance (FA) to be prescribed, there is first determined for one of the two focal magnitudes (for example, FB) either its outer dimension (2aO) as a function of the radius of curvature (R) of the ultrasonic transducer (1; 5) or its radius of curvature (R) as a function of the outer dimension (2aO); (b) given the outer dimension determined in that manner or the radius of curvature determined in that manner, the magnitude radius of curvature or, respectively, outer dimension not yet determined for the other of the two prescribable focal magnitudes (for example, FA) is determined. (c) finally, the maximum value of the angle of aperture of the sound radiation field behind the focus deriving for the outer dimensiom (2aO) and radius of curvature (R) according to steps (a) and (b) is limited by means of the introduction of a recess (2: 8, 9) in the active surface of the ultrasonic transducer (1; 5) of such size (2ai) that the optimum relationship between the angle of aperture and side lobes of the sound radiation field ensue. PG,39 2. A method according to claim 1, characterized in that, when one of the two focal magnitudes is given, the outer dimension (2aO) of the ultrasonic transducer (1; 5) is determined as a function of a standardized radius of curvature RN (RN =R/[aO 2 /λ] standardized to the relationship aO 2 /λ, with aO as half the outer dimension of the ultrasonic transducer and λ as the ultrasonic wavelength.
3. A method according to claim 2, characterized in that, given an outer dimension (2aO) determined in such manner as a function of RN, the standardized radius of curvature (RN) as well as the actual value 2aO and from these, finally, the actual radius of curvature (R) is determined for the other prescribed focal magnitude.
4. An ultrasonic transducer manufactured according to the method of claim 1, characterized in that a transducer part (1; 5) determined according to steps (a) and (b) of claim 1 as to outer dimension (2aO) and radius of curvature (R) of the focusing has recess means (2; 8; 9) for effective removal of a portion of the transducer part.
5. An ultrasonic transducer manufactured according to the method of claim 1, characterized in that a transducer part (1; 5) determined according to steps (a) and (b) of claim 1 as to outer dimension (2aO) and radius of curvature (R) is electrically subdivided into individual radiation surfaces (6) and is effectively provided with a recess by means of the electrical disconnection of individual surface parts (8) in the active surface.
15. An ultrasonic transducer according to claim 4, characterized in that an ultrasonic transducer with said recess means in the active surface and having a focal width FB =f (R, aO) and a focal distance FA =g (R, aO) is dimensioned as follows:(a) the outer dimension amounts to aO =F1 (R, FB) (b) the radius of curvature given this outer dimension amounts to R=G1 (FA, F1 (R, FB)) (c) given these values, the recess means has an inner dimension of such size 2ai at which the relationship between the angle of aperture and side lobes of the sound radiation field is optimum. 16. An ultrasonic transducer system comprising ultrasonic transducer means for pulse excitation having an active surface with a lateral extent 2ao with respect to a lateral axis normal to a propagation axis thereof and with a geometric curvature along said lateral axis exhibiting substantially a radius of curvature R,said ultrasonic transducer means having an effective recess along the lateral axis thereof with a lateral dimension 2ai such that the ratio ao /ai is substantially a measure of the relative lateral extent of the operating portion of said active surface along said lateral axis at each side of said propagation axis, said ultrasonic transducer means being operated with pulses of substantially an ultrasonic wavelength λ and exhibiting substantially a focal distance FA along the propagation axis and substantially a three decibel focal beam width FB with respect to the lateral axis such that FA /ao 2 /λ is less than about 0.37 and FB /ao is less than about 0.20, and such as to exhibit a standardized radius of curvature RN defined as to the ratio of radius of curvature R to the quantity ao 2 /λ, said standardized radius of curvature RN being of a value less than about six-tenths, and ao /ai being of a value in the range from about two to about six. 17. An ultrasonic transducer system according to claim 16, characterized in that the ultrasonic transducer means has an effective recess with lateral dimensions such that the ratio ao /ai is in a range between about two and about four.
18. An ultrasonic transducer system according to claim 17 with the exact value of the ratio ao /ai being such as to substantially minimize the side lobes in the radiated ultrasonic field.
BACKGROUND OF THE INVENTION The invention relates to a method for the manufacture of ultrasonic transducers, with at least one ultrasonic transducer configuration whose active surface exhibits recesses and whose sound radiation field is to be prescribable in terms of focal distance, focal width and angle of aperture in connection with the focus and ultrasonic frequency.
SUMMARY OF THE INVENTION The object of the present invention is to design ultrasonic transducers such that, given any desired prescribable values of the focal width and of the focal distance, optimum lateral resolution ensues with the smallest possible influence of side lobes.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 comprising FIGS. 1A and 1B show a compact ring transducer with mechanical focusing, manufactured according to the method according to the invention;
DETAILED DESCRIPTION FIG. 1 shows a ring transducer 1 consisting, for example, of piezoceramic material. In the present case, the ring transducer 1 exhibits a through-hole 2 as a mechanical recess. A recess in a contact layer 3 only can just as well be present instead of a through-hole. The ring transducer 1 provided with the recess 2 is mechanically bent with the radius of curvature R. Of course, instead of this mechanical curvature, a planar transducer with electronic curvature or a combined form consisting of both can be selected. In the two latter cases, the compact transducer form part must be subdivided into individual segments 4 for generating the delay times required for the electronic curvature (electronic focusing). This state of affairs, for example, is indicated in FIG. 2
First, for the use case respectively desired, the focal width FB and the focal interval FA are prescribed at their optimum values. The focal width FB, however, like the focal interval FA, is a function of the radius of curvature R of the focusing and of the outer dimension 2aO of the transducer. Therewith, there ensues the equation for the focal width (with reference to a specific number of oscillations within the excitation and/or echo pulse):
FB =f(R, aO)                                     (1)
The same is also true for the focal distance FA with the equation
FA =g(R, aO)                                     (2)
By means of conversion to aO, there follows from (1)
aO =F1 (R, FB)                              (3)
R=F2 (aO, FB)                               (4)
R=G1 (FA, F1 (R, FB))                  (5)
aO =G2 (FA, F2 (aO, FB))     (6)
For reasons of simplification, the determination of (3) first as a function of a standardized radius RN is to be recommended, said standardized radius being defined as follows:
RN =R/(ao 2 /&#955;)                       (7)
In this case, given complete frequency independence, the relative focal width FB /ao and the standardized focal distance
FA /(ao 2 /&#955;)                         (8)
are automatically determined from RN. See the second figure at page 220 of my article entitled "Radiation field calculations of pulsed ultrasonic transducers Part 2: spherical disc- and ring-shaped transducers", published in Ultrasonics, September 1980, pages 219 through 223. Thus, the correct dimension for the outer dimension 2ao is also automatically fixed as a function of RN. Then, however, the required value RN can be determined from the comparison of the prescribed focal distance FA to the focal distance FA obtained as a function of RN given a prescribed operating frequency. Therewith, however, the outer dimension 2ao is also determined, so that the actual radius of curvature R can now be obtained from the magnitudes RN and 2ao. The maximum value of the angle of aperture of the sound radiation field behind the focus deriving with the fixed values for the outer dimension and radius of curvature can now be limited in a simple manner by means of adjusting the size 2ai of the recess in the active surface of the ultrasonic transducer in such manner that an optimum relationship between the angle of aperture and side lobes of the sound radiation field which corresponds to the conception of the device ensues. A more direct method is also possible, using the relations:
FB /ao =h(RN, ai /ao)             (9)
FA /ao 2 /&#955;=i(RN, ai /ao) (10)
(see the ninth and tenth figures at page 222 of the above referenced paper). For a given FB and FA, a set of (RN, ai /ao, frequency)--values belonging together is found. Assuming again a given number of cycles within the pulse, the best (RN, ai /ao, frequency--combination can be obtained using the relation:
ai /ao =m(&#952;)                               (11)
(see the following Table A) and the conditions: a given frequency range, θ as well as the side lobes as small as possible (in formation about the side lobes follows from the measured or numerically calculated field patterns of the spherical annular transducers.
TABLE A______________________________________ao /ai A       &#8734; 5       4       3       2______________________________________ 0.32    39      39      34      33.5    230.40     26      26      25      23.5    210.53     25      24      23.5    23      21______________________________________
FIGS. 3A and 3B show a practical exemplary embodiment for a ring transducer according to FIG. 1 which, for example, for the operating frequency of f=3.5 MHz, exhibits an outer diameter 2ao =40 mm and an inner recess with a diameter of 2ai =13 mm. The radius of curvature is set at R=242 mm. The diagrams of FIGS. 3C, 3D and 3F show the plots of the lateral resolution La (mm) as a function of penetration depth Te (cm) of the ultrasonic energy in the examination area resulting with this transducer according to the invention for the -6dB lobe, the -20db lobe and the -40dB lobe of the sound radiation field, respectively. It can be clearly seen that the plot of lateral resolution for the embodiment of FIGS. 3A and 3B as indicated at I in FIGS. 3C-3E has a significantly better characteristic in the interesting penetration depth range from approximately 0 through 13 cm than the characteristics indicated at II and III in FIGS. 3C-3E showing the lateral resolution as a function of penetration depth of such ultrasonic transducers as were manufactured according to traditional methods.
FIG. 4 shows an alternative example to FIGS. 1 and 2. According to this alternative example, the ultrasonic transducer can be constructed in the form of an array 5 comprising a multitude of individual elements 6 (for example, between sixty and one hundred sixty elements) arranged next to one another. In such ultrasonic transducers in array form, the line-wise continuation of the ultrasonic beam for the construction of an ultrasonic image occurs by means of stepping the activation of the individual elements 6 in groups. In the array of FIG. 4, for example, such groups are referenced with 7. The determination of the inactive surface can ensue electronically or mechanically. In the exemplary embodiment of FIG. 4, there is a combined form. Thus, for example, each group 7 to be stepped is provided with an inner recess 8 by means of electrical disconnection of individual elements within the group. This recess 8 respectively extends at right angles to the longitudinal direction of the array. A further recess 9 which is not electronically but, rather, mechanically laid out in the present case, extends in the longitudinal direction of the array. The dimensions 2a01, 2ai1 and 2a02, 2ai2 of recesses 8 and 9 are determined according to step (c) of the inventive method. In the exemplary embodiment of FIG. 4, the group 7 of individual elements, in conjunction with the electric recess 8, determines the radius of curvature of the focusing according to the invention in the z, x plane of the indicated system of coordinates. The focusing in the z, y plane is predetermined according to the invention by means of the indicated mechanical curvature of the array individual elements in conjunction with the mechanical recess 9. Of course, an electronic focusing can be undertaken instead of the mechanical curvature for the purpose of focusing. In this case, the array need only be constructed matrix-like in its individual elements in accord with the exemplary embodiment of FIG. 2.
______________________________________Section    Section Heading______________________________________I          Introduction (page one)IV         Annular Transducer (page nine- page      twelve)V          Spherical Transducer (page twelve-      page sixteen)VI         Spherical Annular Transducer (page      sixteen- page eighteen)VII        Conclusion (page eighteen)FigureDesignation      Figure DescriptionTwelve     Ultrasonic pressure pattern for      plane annular transducer, ai =      ao /2; ao = 7.5 mm, frequency 2      MHz, nc = 2 - modulated      Gaussian pulse, (page nine)Fourteen   Influence of inner radius, ai, on      normalized -3 dB focal distance,      df, and on relative -3 dB focal      width, wf, for plane annular      transducer; ao = 7.5 mm, frequen-      cy 2 MHz, nc = 2 - modulated      Gaussian pulse. (page ten)Sixteen    Influence of normalized radius of      curvature, A, on relative -3 dB      focal width, wf, for spherical      disk shaped radiator; radius a =      7.5 mm, frequency 2 MHz, nc = 2 -      modulated Gaussian pulse -      Kossoff's approach. (page twelve)Seventeen  Influence of normalized radius of      curvature, A, on normalized -3 dB      focal distance, df, for spherical      disk-shaped radiator; radius a =      7.5 mm, frequency 2 MHz, nc = 2 -      modulated Gaussian pulse -      Kossoff's approach. (page twelve)Eighteen   Influence of normalized radius of      curvature, A, on -3 dB and -10 dB      angle of aperture, &#952;, for      spherical disk-shaped radiator;      radius a = 7.5 mm, frequency 2      MHz, nc = 2 - modulated Gaussian      pulse - Kossoff's approach.      (page fourteen)Twenty-one Ultrasonic pressure pattern for      spherical disk-shaped transducer,      A = 0.32; radius a = 7.5 mm,      frequency 2 MHz, nc = 2 - modula-      ted Gaussian pulse. (page four-      teen)Twenty-four      Influence of normalized radius of      curvature, A, on normalized -3 dB      focal distance, df, and relative      -3dB focal width, wf, for spheri-      cal annular transducer, ai = ao /2;      ao = 7.5 mm, frequency 2 MHz, nc      = 2 - modulated Gaussian pulse.      (page sixteen)Twenty-five      Influence of inner radius, ai, on      relative -3 dB focal width, wf,      for spherical annular transducer;      ao = 7.5 mm, frequency 2 MHz, nc      = 2 - modulated Gaussian pulse.      (page sixteen)Twenty-six Influence of inner radius, ai, on      normalized -3 dB focal distance,      df, for spherical annular trans-      ducer; ao = 7.5 mm, frequency 2      MHz, nc = 2 - modulated Gaussian      pulse. (page sixteen)Twenty-seven      Influence of transducer shape; -3      dB pressure contour for plane cir-      cular, plane annular (ai = ao /2)      spherical (A = 0.53) and spherical      annular (A = 0.53, ai = ao /2)      transducer; ao = 7.5 mm, frequency      2 MHz, nc = 2 - modulated Gaussian      pulse. (page seventeen)______________________________________
APPENDIX A The following technical paper authored by the inventor is incorporated as part of the Detailed Description of the present specification:
Radiation Field Calculations of Pulsed Ultrasonic Transducers Plane Circular, Square, Annular and Spherical Transducers A. Weyns *
I. Introduction The image quality of medical diagnostic ultrasonic equipment strongly depends on the characteristics of the nearfield as well as the farfield region of the transducer used. So for quality judgement and optimalisation of the ultrasonic radiator, a detailed picture of the whole acoustic field in front of the radiator, showing the nearfield interferences, the focussing effect and angle of aperture, is necessary. Beam patterns of harmonic-excited radiators have been published by Zemanek [1], Lockwood and Willette [2]. In most diagnostic ultrasonic equipment, however, the pulse-echo method is applied. Transient analysis of acoustic fields have been reported, by many investigators [4-15], but they have made their calculation only for so few points of the near- and farfield, that a detailed beam pattern cannot be reconstructed.
The sonic waves are assumed to propagate themselves in a homogeneous, lossless water medium of infinite extent. Only a thickness mode vibration with all parts of the radiator vibrating at the same amplitude is considered. The radiator is regarded as a rigid piston, surrounded by an infinite baffle. For plane radiators, FIG. 1 shows the coordinates of the system used. The position of the observation point P is a function of r, i.e. the radial distance from the center of the transducer, and of α, the angle between the piston axis and the radial vector.
z(r', &#964;)=zo e-k.spsp.2.sup.&#964;.spsp.2 �e-j&#969;.sbsp.o.sup.&#964;,               (1)
where k is a constant, which defines the width of the Gaussian envelope at a level of -10 dB below the peak. ##EQU1## Using u (τ)=δ/δτ, the pressure can be evaluated numerically is a way similar to that of Zemanek [1] using the well-known general equation of the pressure given by Morse [3]. The following transducer shapes are studied: plane circular, plane square, plane annular, spherical and spherical annular.
II Plane Circular Transducer According to the ultrasonic imaging system, especially to the brightness modulation, the maximum amplitude of the pressure wave in each point must be considered. Thus for each point of a 3360 point-, two-dimensional grid in the XZ-plane, the maximum amplitude of this pressure wave, pm =max|p(t)| is determined. For the graph of the radiation field, the maximum pm -value, (pm)max, of all points of the grid having the same Z-coordinate was ascertained and the pm values concerned were normalized in dB to this maximum value: ##EQU2## For the numerical integration, the surface of the transducer was divided into (λ/4)2 elements. The results obtained in this way were equal in accuracy within 1% of results using elements of half the size.
Z=z/(a2 /&#955;)                                    (4)
X=x/(a2 /&#955;)                                    (5)
with a radius of the transducer λ wavelength of the modulation wave
First, the influence of the number of cycles within the -10 dB pulse width, nc, is examined. FIG. 3 shows the sonic field of a continuously excited circular piston for a/λ=10 as published by Zemanek [1]. In FIGS. 4, 5 and 6, nc is reduced to 24, 2 and 1 respectively. A comparison of these four normalized sonic fields proves: the farfield, i.e. the range beyond the focus, is independent of nc. The influence of nc on the nearfield, however, is very significant. Compared to the harmonic wave propagation, for nc =24, only in the range 0<Z<0.5, does the nearfield pattern show some simplification, i.e. the number of points in which interference maxima and minima occur diminishes and the side lobes are reduced in size and number. This simplification progresses slowly at first, reducing nc from 24 to 6, then becomes considerable from nc =5 up to nc =1. At nc =1, the minimum on the Z-axis, which exists at Z=0.55 for nc =2 (FIG. 5), shifts to Z=0.65 and is reduced to a value of =2.4 dB. Therefore as shown in FIG. 6, no fine structure within the graph is recorded. In the focal range, a reduction of nc diminishes the focussing effect, i.e. the relative focal width wf (focal beam width divided by the radius a) and the normalized focal distance df (normalization factor a2 /λ) increase. As for the simplification of the nearfield structure this reduction of the focussing effect essentially happens for nc ≦5 as shown in FIG. 7. Thus, the transition from "continuous wave" to "pulse" beam pattern practically occurs in the range 0.5≦nc ≦5. For nc =0.5, some focussing effect is still found due to the wave cycles within the Gaussian pulse having an amplitude more than 10 dB below the peak value. The results agree very well with the data published by Beaver [3]. Especially for his Type IV pulse, which also contains about two cycles within the -10 dB width of the half-sine envelope, the agreement is excellent. Also for the beam pattern, corresponding to the one-cycle Type II pulse with a square envelope in [3], the similarity is quite good.
Treating the influence of the transducer radius a and the frequency f, the same effects on the nearfield structure as those for harmonic wave excitation are found, i.e. a decrease of a/λ causes a reduction of the nearfield complexity and a reduction of the side lobes. However, the smaller nc is, the smaller these effects become.
In the focal range, Table I proves that, for the same nc -value, the normalized focal distance as well as the relative focal width are independent of the radiator size. Up to nc ≧6, the same holds for a variation of the frequency as shown in Table II. This is understandable as up to nc =6, the beam pattern practically behaves in the same way as with harmonic excitation where the same effects are found. For smaller values of nc, a reduction of the frequency causes a decrease of df as well as an increase of wf, both of about 5%. Thus the influence of the frequency of the normalized radiation field is less pronounced than that of nc.
Finally, Table II proves that the -10 dB pulse width itself does not directly influence the acoustic field. For instance, a same -10 dB pulse width of 1 μs, corresponding to two cycles at 2 MHz and only one cycle at 1 MHz, results in a different nearfield structure and a different focal range.
As a consequence, for plane circular radiators, the number of cycles nc turns out to be a more important factor for the normalized radiation field than the modulation frequency and the radius a. Moreover, a comparison with the results of Beaver [4] proves that the form also of the envelope pulse does not have a great influence on the radiation field.
III Plane Square Transducer In order to compare the beam patterns of square radiators with those of the circular transducers described above, the areas of both radiator types are taken to be equal. In analogy to the case of a circular piston, d2 /λ (d is half the side of the square) was used as normalization factor. Further, for a square piston, it is important to note that there are two main intersections to be considered; one along a line of gravity and another along the diagonal.
Concerning the acoustical field in the plane along a line of gravity, FIGS. 8 and 9 show a simplification of the nearfield compared to that of the circular radiator, independent of nc. This agrees with the on-axis intensity variations for a continuously vibrating square radiator as published by Marini and Rivenez [16]. On the other hand, the number and magnitude of the side lobes increase. The converse applies to the wave pattern in a plane along a diagonal: the nearfield is more complicated, whereas the number and magnitude of the side lobes decrease compared to the circular transducer as can be seen in FIGS. 8 and 9. Considering the sonic fields in the two main sections of the square, Table III shows that the focussing effect is more pronounced in the diagonal section. This is understandable in consequence of the greater possibility of interference in this plane.
In order to compare the normalized focal distance of circular to square transducers, it is more convenient to take the same normalization factor a2 /λ in both cases. Here, a is the radius of a circular radiator having the same area as the square transducer with half the side d. The values within the brackets in Table III correspond to this normalization. They prove that the focal distance for a square piston is always larger than that of a circular piston, but the difference diminishes as nc decreases. A comparison of the relative focal width using the same reference value, i.e. a instead of d, demonstrates that the focal width for the square transducer is also larger, but the dependence of the focal width on nc is less pronounced than with the circular radiator.
Concerning the dependence of the wave patterns on the number of cycles nc, the same holds as with the circular transducer: a reduction of nc causes a simplification of the nearfield structure and a decrease in the focussing effect. The latter especially occurs for nc ≦5 as shown in FIG. 10.
As for the circular transducer, a decrease of d/λ causes a reduction of the nearfield complexity and a reduction of the side lobes. Dealing with the focal range, Table IV proves that the normalized focal distance as well as the relative focal width are independent of the radiator size. Up to nc ≧8, the same holds for a variation of the frequency as shown in Table V. For smaller values of nc, a reduction of the frequency causes a decrease of df as well as an increase of wf. But, contrarily to the circular transducer, this increase of wf is less pronounced: about 1% instead of 5%.
In consequence, the number of cycles, nc, turns out to be the most important parameter for the normalized radiation field of the plane square transducer as well.
IV Annular Transducer To facilitate the comparison with results given in paragraph II, the outer radius of the annular radiator (ao) is assumed to be equal to the radius a of the plane, circular, disk-shaped transducer already examined.
The following influences are studied: shape of ring, nc and position of the recess in the transducer.
The dependence of beam pattern on the ring shape is examined by comparing the disk to the annular form and by changing the inner radius on the ring (ai). The wave pattern of annular transducers having an inner radius equal to half the outer radius for "continuous" and "pulse" excitation accordingly can be seen. in FIGS. 11 and 12. A comparison of these figures with FIGS. 4 or 5 respectively proves that an annular shape causes the following effects:
The greater the inner radius of the ring, ai, is, the more pronounced are these effects. FIGS. 13 and 14 prove that only from ai ≧0.25 ao on, does the influence of the ring shape on the focussing effect become noticable. A comparison of both figures demonstrates a similar dependence of df and wf on ai for "continuous" as well as for "pulse" excitation. For "continuous" excitation, increasing ai from zero to 0.5 ao, wf and df decrease slowly at first, up to ai =0.25 ao for about 15% for the former and 5% for the latter respectively, then rapidly, at ai =0.5 ao approx. 50% or 30% as before. As follows from these percentages, the relative reduction of wf is always more than that of df. For a "pulse" excitation with nc =2 the decrease of wf is less pronounced, approx. 40% at ai =0.5 ao, whereas the reduction of df (35%) is nearly the same. Much more dependent on the number of cycles, nc, are the side lobes and the fine structure within the nearfield. A comparison of FIGS. 11 and 12 proves the considerable decrease of the -10 dB and -20 dB side lobes in size, as well as the simplification of the nearfield at the transition from "continuous" to "pulse" excitation. Nevertheless, even with these small values of nc, the dimensions of the side lobes and the complexity of the nearfield structure are more pronounced for a ring- than for a disk-shaped radiator.
Another interesting aspect is the position of the recess in the transducer. Instead of an annular radiator, which is a circular transducer with a hole in the center, a circular piston with a wedge-shaped recess of equal area at its top is considered. From FIG. 15 and Table VI, it is clear that this recess at the top of the radiator has less influence on the radiation field than a centrally positioned recess does. Compared to the wave pattern of the disk-shaped transducer (FIG. 5), the change of the nearfield structure and the focal range depends on the area and depth of the wedge-shaped recess. Table VI shows: a wedge-shaped recess influences wf always less than an annular shape. On the other hand, the reduction in df strongly depends on the depth and the area of the wedge extending from approx. 0% for a depth of ao /5 and an area of 1/25 of a disk-shaped radiator up to 34% for a depth of ao /2 and an area of 1/4 of a disk. As indicated, the decrease in d.sub. f is here also less than with a ring. Finally, an application of a recess at the border results in an asymmetrical sonic field and a shifting of the acoustical axis to the opposite side of the transducer.
V Spherical Transducer For the spherical radiators considered, the secondary diffraction, i.e. the fact that waves radiated from any part of the surface are diffracted by other parts of this surface, will be neglected because the surface S is taken to be much larger than the wavelength. Here, the coordinate system of O'Neil is used [17]. For possibility of comparison with previous acoustic fields, the same transducer radius a and the same normalization factor a2 /λ, are assumed.
Concerning the influence of the normalized radius of curvature A, FIGS. 16 and 17 illustrate the dependence of wf or df respectively on A, as results from numerical calculations. The form of the graphs for wf and df proves that the smaller A is, the greater the reduction rate of both parameters will be; this means: a decrease of the radius of curvature causes an increase of the focussing effect which is the more important the smaller the radius of curvature. For instance as FIG. 17 illustrates, reducing A from ∞ to 2.0 results in a decrease of df of approx. 30%, whereas a supplementary reduction to 1.0 or 0.5 augments this percentage to 48% or 63% respectively. FIG. 16 shows that the same holds for wf but here, the corresponding reductions are about 2% higher. According to the slope of the graphs in FIG. 16, the increase of the focussing effect occurs for A≦0.5 especially. But even for a relatively high value of A, 1.0 or 2.0, the increase of the focussing effect is still considerable: approx. 50% or approx. 30% respectively. Another interesting feature attained by a geometrically focussed radiator: as contrasted with a plane-shaped piston, an increase in the focussing effect by reduction of A causes the focus of the different pressure contours (-3 dB, -6 dB, -10 dB) to approximate each other. Thus for small values of A, the focal distances of -3 dB and -10 dB are about the same as shown in FIG. 21.
Furthermore, in FIGS. 16 and 17, a comparison with the values obtained by the Kossoff formula for curved radiators [20] is made. For df as well as wf, the values obtained by Kossoff are always larger than the numerically calculated values. Considering df, the deviation decreases for small and great A-values whereas for wf, the difference diminishes only for a decreasing A. Dealing with the different pressure contours, Kossoff's approach results in the same df -value for all of them.
On the other hand, A does influence the angle of aperture of the beam pattern as well. As the -3 dB contours beyond the focal range are not always straight lines, as can be seen in FIGS. 19, 20 and 21, the angle of aperture is ascertained as the angle between the radiator axis and the straight line through the -3 dB focus and the point at which twice the focal width is attained. Thus the formula for θ is:
&#952;=arc tan [wf /(d2f -df)]             (6)
with d2f, the normalized distance on the piston or Z-axis where twice the focal width is attained.
The influence of A on θ is shown in FIG. 18. Firstly, it is important to notice that the angle of aperture not only depends on A but also on the pressure contour considered, for instance -3 dB or -10 dB. However, the same holds for both contours: a reduction of A increases the angle of aperture especially from A≦0.5 on. The considerable increase of θ for the -3 dB pressure contour in this range is caused by the nonlinear beam pattern between the distances df and d2f. Thus for spherical radiators, the optimum value of A, combining a good focussing effect and a small angle of aperture lies within the range 0.4≦A≦0.6.
FIG. 18 also shows the values of θ obtained with the Kossoff formula. Here, a difference must be made between the -10 dB and -3 dB contours. For -10 dB, Kossoff's approach results in smaller θ values. In the range 0.5≦A≦0.9, for both methods of calculation, the same angle of aperture is found. Considering -3 dB, Kossoff's formula delivers higher values especially for 0.3≦A≦0.8, and about the same values for A>1.5.
VI Spherical Annular Transducer A combination of both foregoing transducer shapes, i.e. a geometrical focussed ring, causes a further increase in the focussing effect as shown in FIG. 24. A comparison of the spherical with the plane ring proves that the increase of the focussing effect (i.e. decrease of wf and df) becomes the more important the smaller A is. Assuming ai =ao /2, the decrease of 35% for df at A=1.07 rises to 65% at A=0.32. The same holds for the reduction of wf but now the percentages of decrease are about 5% greater than the corresponding values for df. Thus the spherical ring shape has a little more influence on wf than on df, which is important for ultrasonic imaging systems. Nevertheless, a comparison of the influence of the ring shape on plane transducers to that on spherical transducers, i.e. comparing FIG. 25 and 26 with FIG. 14, demonstrates that the ring structure does not increase the focussing effect for the spherical transducer as strongly as it does for the plane radiator. The smaller A, the more pronounced this effect is. For instance, at A=0.53 a decrease of ao /ai =√ to ao /ai =2 results in a reduction of wf by about 30% in opposition to 50% for the plane annular transducer (FIG. 25) and in a decrease of df by approx. 15% as opposed to 35% (FIG. 26). For A=2.0, however, the same reduction in ao /ai results in a decrease of wf by about 40% and a reduction in df by approx. 25%. A much more important feature of the geometrical focussed ring is the fact that this radiator shape reduces the negative characteristics of the beam patterns of spherical as well as of annular transducers, which were inherent in an increase of the focussing effect. For instance, for the plane annular radiator a reduction of wf by more than 40% as compared to the plane circular, disk-shaped radiator requires an inner radius of more than one half the outer radius and results in a considerable increase of the side lobes. For spherical disk-shaped transducers, a reduction of wf by more than 65% as compared to the plane circular disk-shaped radiator requires a value of A smaller than 0.5 and results in an important increase of the angle of aperture. θ, together with an approaching of the farfield interference structure. In the case of spherical ring-shaped radiators, a decrease of θ can be obtained by increasing the inner radius as shown in Table VII and in FIG. 27. As the sonic field of the spherical ring has larger side lobes than that of the spherical disk-shaped radius has, there exists an optimum for the magnitude of the inner radius which depends on the characteristics of the imaging system. Furthermore, FIG. 27 also demonstrates the considerable reduction of the side lobes using a spherical annular transducer by comparison to those of a plane ring shape.
In consequence, the use of an annular radiator can best be combined with a geometrically focussed shape. For spherical transducers, the combination with a ring shape should be used if wf - or df -values less than 0.20 or 0.37 respectively are demanded.
VII Conclusion The sonic field of pulse-excited radiators strongly depends on the number of cycles within the pulse. The transition from "continuous" to "pulse" excitation occurs from nc =6 on and shows the following features:
TABLE B______________________________________Figure Designation of            Corresponding Figurethe Technical Paper            Designation andQuoted Herein, (as            Location in theSubmitted for    Published PaperPublication)     Identified Above______________________________________Twelve           Ninth figure, Part 1,            page 187Fourteen         Tenth figure, Part 1,            Page 187Sixteen and Seventeenth            Second figure, views a            and b, Part 2,            page 220Eighteen         Third figure, Part 2,            page 221Twenty-one       Sixth figure, Part 2,            page 221Twenty-five      Ninth figure, Part 2,            page 222Twenty-six       Tenth figure, Part 2,            page 222Twenty-seven     Eleventh figure, Part            2, page 223______________________________________
Supplemental Discussion In order to assist in a correlation of the incorporated paper (partially found in the foregoing Appendix A) with the notation and description of the application FIGS. 1-3e, the following further discussion is presented.
______________________________________Detailed Description                    Corresponding                    Parameter ofTerminology in           IncorporatedSpecification   Symbol   Paper______________________________________Standardized Radius           RN  AFocal Distance  FA  df ao 2 / &#955;Focal Beam Width           FB  Wf ao______________________________________
Thus the term df in the incorporated paper corresponds to the normalized focal distance as given in equation (8) of the description of application FIGS. 1-3e.
The term Wf in the incorporated paper corresponds to the relative focal beam width FB /ao.
______________________________________Numerical Values inApplication FIGS. 3a-3e______________________________________ao /ai = 3             ao = 20 mmWf = .10     ai = 6.5 mmdf = .22     &#955; = .43 mmR = 242 mm        nc = 2RN = A = .26 frequency = 3.5 MHzFA = 205 mm  FB = 2.0 mm-3dB Angle of Aperture, &#952; (-3dB) = 57�-10dB Angle of Aperture, &#952; (-10dB) = 72�______________________________________
(i) relative focal beam width Wf less than 0.20
(ii) normalized focal distance df less than 0.37
(iii) standardized radius RN or A less than 0.6
(iv) ratio of outer radius to inner radius ao /ai in the range from 2 to 6
The optimum values for item (iv) above are as follows: The ratio ao /ai is optimally in the range from 2 to 4, the smaller the value nc the smaller the optimum value of ao /ai, so that the following approximate optimum values hold for nc =2, ao /ai =3; for nc =1, ao /ai =2.5; for nc =4, ao /ai =3.5. The exact optimum values depend on what percentage of side lobes is allowed in the imaging system of the equipment; the higher such percentage, the smaller the optimum value of ao /ai for a given value of nc. For the -40 dB contours, decreasing ao /ai results in an increase of Wf and df, so for good focusing, a value for ao /ai which is smaller than two is not advisable.
(i) FB =Wf �ao =2 mm
(ii) FA =df ao 2 /λ greater than 20 cm (for the case where a water coupling path is used)
(iii) nc =2, a frequency less than 4 MHz
(iv) -3 dB angle of aperture θ (-3 dB) and side lobes to be as small as possible
The relationships df =F (A, ao /ai) and Wf =G (A, ao /ai) are of the type shown in Figures twenty-six and twenty-five of the incorporated paper. With these relationships established, a set of values of A (RN), ao /ai and frequency which belong together are determined for fulfilling the above conditions. A compromise between decrease of angle of aperture θ (see Table A) and increase of side lobes is effected, by decreasing ao /ai, especially to a value less than three but not less than two, such a selection of ao /ai resulting in an optimum transducer configuration.
Patent CitationsCited PatentFiling datePublication dateApplicantTitleUS3666979 *Jun 17, 1970May 30, 1972Automation Ind IncFocused piezoelectric transducer and method of makingUS3936791 *Sep 13, 1973Feb 3, 1976The Commonwealth Of AustraliaLinear array ultrasonic transducer* Cited by examinerNon-Patent CitationsReference1 *C. B. Burckhardt et al., "A Real-Time B-Scanner with Improved Lateral Resolution," pp. 81-87 and 89, 1975.2 *G. Kossoff et al., "Ultrasonic Two-Dimensional Visualization for Medical Diagnosis," J. Acous. Soc. Am., vol. 44, No. 5, pp. 1310-1318, 1968.* Cited by examinerReferenced byCiting PatentFiling datePublication dateApplicantTitleUS4460841 *Feb 16, 1982Jul 17, 1984General Electric CompanyUltrasonic transducer shadingUS4550606 *Sep 28, 1982Nov 5, 1985Cornell Research Foundation, Inc.Ultrasonic transducer array with controlled excitation patternUS5080101 *Jun 19, 1989Jan 14, 1992Edap International, S.A.Method for examining and aiming treatment with untrasoundUS5080102 *Apr 21, 1989Jan 14, 1992Edap International, S.A.Examining, localizing and treatment with ultrasoundUS5111822 *May 16, 1989May 12, 1992Edap International, S.A.Piezoelectric articleUS5143073 *Jun 14, 1988Sep 1, 1992Edap International, S.A.Wave apparatus systemUS5150712 *Jan 9, 1991Sep 29, 1992Edap International, S.A.Apparatus for examining and localizing tumors using ultra sounds, comprising a device for localized hyperthermia treatmentUS5398551 *Jun 9, 1994Mar 21, 1995Ngk Insulators, Ltd.Ultrasonic testing method for ballsUS6658710 *Jun 25, 2001Dec 9, 2003Agilent Technologies, Inc.Method for fabricating an annular ring transducerUS7309947 *Jul 6, 2004Dec 18, 2007Kabushiki Kaisha ToshibaPiezoelectric transducer including a plurality of piezoelectric membersUS8727993Jan 18, 2007May 20, 2014General Electric CompanyApparatuses comprising catheter tips, including mechanically scanning ultrasound probe catheter tipUSRE33590 *Nov 22, 1988May 21, 1991Edap International, S.A.Method for examining, localizing and treating with ultrasound* Cited by examinerClassifications U.S. Classification73/632, 29/25.35, 310/334International ClassificationG10K11/32, H04R1/32, H04R17/00Cooperative ClassificationG10K11/32European ClassificationG10K11/32RotateOriginal ImageGoogle Home - Sitemap - USPTO Bulk Downloads - Privacy Policy - Terms of Service - About Google Patents - Send FeedbackData provided by IFI CLAIMS Patent Services©2012 Google