|Title of Invention||
"AN ULTRASOUND ARRAY TRANSDUCER"
|Abstract||The present invention relates to an ultrasound array transducer for electronic depth steering of symmetric focus from a near focus Fn to a far focus Ff includes elements that are divided into k groups with different fixed prefocusing. The central group participates in beam forming from Fn to Ff, the next outer group in beam forming from Fn1>Fn to Ff, and the kth outer group in beam forming from Fnk>Fnk-1 to Ff. The fixed focus for the kth group is selected at Fk between Fnk and Ff. In this manner, beam formation close to Fn is performed only by the central group. By steering the focus outward from Fn, the focal diameter increases and, at a depth where the focal diameter with subsequent increases in diameter as the focus is further steered toward Ff. In the same manner, the kth group of elements is included in beam formation for steered foci deeper than Fnk, presenting a growing aperture that enables maintenance of the diameter below limits with a low total number of elements and avoids impractically small widths of the annular elements. The elements may also be subdivided in the angular direction, allowing for phase aberration correction.|
|Full Text|| 1. Field of the invention
The present invention is directed to technology and design of ultrasound transducer, arrays with symmetric electronic steering of the focus for ultrasound imaging, particularly both two-dimensional and three-dimensional medical ultrasound imaging.
2. Background of the invention
Ultrasound array transducers are used in ultrasound imaging for. electronic direction steering and focusing of theultrasound beam. The commonly used arrays have a linear arrangement of the elements for two-dimensional scanning of the beam. The linear phased arrays, for example, produce a sector scanning of the beam centered at the array, while the linear or curvilinear switched arrays provides a wider image field at the transducer.
A problem with the linear arrangement of the elements, is that the beam.focus can be electronically steered only within the two-dimensional (2D) scan plane, what is referred to as the azimuth direction. The beam.focus in the direction normal to the 2D scan plane, what is referred to as ihe elevation direction, must with these arrays be set to a fixed depth.
In many practical situations one makes a 2D ultrasound image where the variation of the object is limited transverse to the 2D scan plane (i.e. in the elevation direction). Such examples are short and long axis imaging of the heart, imaging of the fetal trunk and head, amongst other. In such cases there is limited need for electronic steering of the elevation focus. On the other hand, imaging of objects with short dimension in the elevation direction, like vessels, cysts, a fetal heart, etc., is greatly improved when the beam has an electronically steered focus both in the elevation and the azimuth directions. Electronic steering of both the elevation and azimuth focus is also important for three-dimensional (3D) imaging where the object can be viewed from.any perspective (direction) that favors optimal focusing with minimal resolution in all directions.
Electronic steering of the focus in the elevation direction can be obtained by dividing the linear array elements into sub elements in the elevation direction. A particular, solution to such steering of the elevation focus is given in US Pat No 5,922,962. However, to obtain full symmetric steering of the azimuth and elevation foci, a large number of elements is required with this solution, complicating the cabling and drive electronics for this array. Also, the elements of this array becomes small, increasing the electrical impedance of the elements that increases noise and cable losses, which
further limits the maximal frequency that can be used with such arrays for. a given depth, and consequently the resolution obtainable with these arrays at a given depth.
Another, well known method to obtain an electronically steered symmetric focus is to use an array of concentric annular elements, the so-called annular, array. Such an array is usually pre-focused mechanically to a depth F, either by curving the array or by a lens, or by a combination of the two, The focus, F, is then steered electronically from.a near focus Ftt F by adding delays to the element signals before they are added, according to well known principles. The beam will then be optimally focused symmetrically around the beam.axis, i.e. equally focused in the azimuth and the elevation directions, with fewer and larger elements than with the 2D arrays described above. This gives lower electric impedance of the elements, reducing noise and cable losses with improved sensitivity compared to the 2D arrays. For mechanical scanning of the beam direction, the annular array is immersed in a fluid inside a dome. The array itself is therefore not pushed against the skin as the linear arrays, and can hence be made with a lighter weight backing than the linear arrays., for example a plastic foam. This reduces the backing losses which further improves the sensitivity of the annular arrays above the linear 2D arrays. The improved sensitivity of the annular array hence allows the use of higher ultrasound frequencies, which further improves the image resolution above (he linear 2D arrays.
The fewer, number of elements of die annular array compared to the 2D array, allows the use of wider apertures, which further reduces the focal diameter, and hence improves the lateral resolution. With very wide aperture annular arrays, however, the outer elements can become quite narrow when steering of the focus over a large range is required. This can introduce complex vibration modes of the elements, reducing the efficiency of the elements. Further, narrow elements complicate the manufacturing and increase the total number of elements in the array which complicates electrical connections to the moving array.
The present invention presents a solution to this problem.with annular arrays by acoustically pre-focusing the annular elements at different depths, where a core group of elements are pre-focused to participate in the active aperture for the whole image range. Outer elements that are pre-focused at deeper ranges are then included to the active aperture at deeper ranges so that the angular expansion of the focal diameter with depth is reduced by the increased aperture size. The invention hence allows the full use of the advantages of the annular arrays: 1) A symmetrical focus that is steered electronically within the actual image range, 2) fewer and larger elements with the annular array with lower impedance backing gives high sensitivity that allows for the use of high frequencies with improved resolution, and 3) the lower number of elements simplifies the front end electronics.
3. Summary of the drawings
Figure 1 shows an example annular array where Figure la shows a front view of the array with
depiction of the radiating surface and coordinate system.for. the description, and Figure Ib shows a side view that illustrates a curved focusing of the array.
Figure 2 shows an illustration to calculation of the phase error across the elements from a point
source in the steered focus, where Figure 2a illustrates calculations for a plane array, while Figure 2b illustrates calculations for a focused array,
Figure 3 illustrates a method of selecting the pre-focuses of the elements to obtain an expanding
aperture that limits angular expansion of the steered focus with depth while using maximal width of the elements, where Figure 3a illustrates the basic principles with pre-focusing obtained by curving of the elements, Figure 3b illustrates pre-focusing obtained by lenses, Figure 3c illustrates pre-focusing obtained by thin lenses, and Figure 3d illustrates pre-focusing by curved elements with offset positions.
Figure 4 illustrates how the same principle of multiple pre-focusing can be applied to an expanding aperture annular array with added angular division of the elements.
4. Detailed description of the invention
A particular embodiment of the invention will be explained with reference to the Figures.
Figure la shows a schematic front view of an example of a typical prior art annular array, where the coordinate x denotes the azimuth direction which is the 2D scan plane direction, the coordinate y denotes the elevation direction, and the coordinate z denotes the depth. In this example, the elements are composed of a center disc 101 with two concentric annuli 102 and 103. By shaping the array as a spherical shell with center at a depth F, the array is pre-focused to this depth, as illustrated in Figure Ib. A lens of a material with acoustic velocity different from that of the load material, can also be used for the pre-focusing.
Figure 2a shows a a cross section in the elevation direction of plane annular array, depicting the cross section of a set of elements 20 1 , 202, and 203 . A requirement for adequate participation of an element in the formation of a focused aperture, is that the phase error across the element of a spherical wave from a point source in the steered focus, is less than a certain limit, typically ~ o.it/2, where a ~ 1 . The degradation of the beam with increasing phase error is continuous, so that there is not a sharp limit on the acceptable value of a, where a = 1.5 can in many cases be tolerable. For the steered focus Fz at 204 in Figure 2a we see that the phase error Afc(z) across element #k is, when approximating the wave front over the element by a plane wave (plane wave approximation)
0) where A, is the ultrasound wave length, atis the radius of the element center, and bk.is the element
width . We hence see that as the radius atof the element center, increases, the element width b^ must be reduced to maintain the phase error below the acceptable limit. We note that for the_areji_for rings is 27tatbt which implies that the phase error is the same with equal area elements. We also note that
as the steered focus Fz is reduced, the phase error, increases, which limits the maximal ak:to be used at low ranges with a given bk.
To be able to increase the width of the elements while the phase error is less than a limit, the array can be pre-focused to a depth F, either by curving of the array as a spherical shell with center at F at 205 in Figure 2b, or using a lens as shown in Figure 3b, or a combination of both. Which of these methods that are preferred, depend on the actual situation.
The phase error across each element is then zero for waves originating from the fixed focus F, and increases as the steered focus Fz at 206 in Figure 2b is moved inwards or outwards from F. With reference to Figure 2b we see that the phase error in this case can in the plane wave approximation be obtained as
We see that also for this array with constant curvature, equal area elements gives the same phase error across each element. We also note that for a given b one must reduce the aperture (i.e. maximal a^) as F2 is both increased or reduced from F to maintain AJJZ) below acceptable limits.
The diameter of the beam focus, can be expressed as
where Dt= 2ai^bj0= dt+btis the diameter of the active aperture withk elements electronically focused at the depth Fz. As the field has a smooth drop in amplitude from the axial value, Eq.(3) is only an approximate assessment of the focal diameter. It corresponds for the circular aperture with uniform excitation to approximately 12dB drop of the field amplitude from the axial value. We note that dp(z) ~ F2, which implies that for fixed active aperture diameter Dt. the beam has a fixed angular expansion with depth, One hence wants to increase the active aperture with depth to avoid that the focal diameter expands without limit, for example by increasing the number of participating elements with depth. With the same fixed focus of all participating elements, this requires that bjcis reduced with increasing k proportional to l/a^to satisfy a limit on Afyifz) in Eq.(2), which makes the outer elements very narrow and increases the number of elements.
The invention provides a solution to this problem.by dividing the annular elements into groups of neighboring elements, where each group has a different pre-focus obtained by mechanical curving of the elements, or a lens, or a combination of both. The depth of a group's pre-focus increases with the group's distance from the array center. An example of such an embodiment of the invention is given in Figure 3a. In this particular embodiment, a central group of elements 301 with total aperture diameter D0 participates in the active aperture over the whole steered focusing range of the array, i.e. from a steered near focus Fn at 302 to a steered far focus Ff at 303. This group of elements has a common pre-focus F0 at 304, preferably selected so that the phase error is the same at the far focus Ff and the near focus Fn. With the plane wave approximation, this gives a pre-focus
This pre-focus also gives the minimal phase error for. the participating elements over, the whole focusing range. Reducing the width of the elements as bt- I/a, the area A±~ 271% of the annular. elements are independent of a^. Hence, equal area annular elements gives the same phase error for all elements in the group, and as the area is constant, the electric impedance is similar for all the
elements in the group.
The focus Fz is steered electronically outwards from Fa.by adding delays to the signals of the individual elements in the group according to well known methods. The focal diameter increases with Fz according to Eq.(3) with Dfc= D0, and indicated by the lines 307 in Figure 3a. When the focal diameter exceeds a selected limit dpi indicated by the lines 308, anew group of elements 305 is added to the active aperture at a depth FDJ at 306. The new group of elements participates in the active aperture from.Fai to Ff> and is given a pre-focus FI at 309 in this range, preferably so that the phase error, across each element is minimized for Fz in the range from.Fai to Ff. With the plane wave approximation, this gives a fixed focus of
This increase, in active aperture diameter to Dj produces a reduction in the focal diameter below the limit d^, as indicated by the lines 307 that describes Eq.(3).
The focus Fz is furthered steered electronically outwards from F^j by adding delays to the signals for all the elements that participates in the apertures, and the focal diameter further increases with Fz according to Eq.(3) with the new active aperture diameter. D^— Dj. At a depth F^ at 3 1 0 the focal diameter again passes a selected limit dpi where the procedure is repeated so that a new group of elements 3 1 1 is added to the active aperture so that one gets a diameter of the active aperture of D2 for Fz > F^. The new element group 311 is pre-focused to a depth F2 at 312, preferably so that the phase error across these elements is minimized over the whole range of the steered focus from F^ to Ff where the element group 311 participates in the active aperture.
Hence, the general procedure can be summarized so that for a given active aperture diameter Di, the focal diameter increases with the focal depth according to Eq.(3) with Dfc= Di, and at the depth Fnnj where the focal diameter exceeds a selected limit dpi, the aperture is increased with a new group of elements to participate in the active aperture from F^ to Ff, and pre-focused in this range, preferably so that the phase error across the new elements are minimized for the steered focus in the whole range F^ to Ff where the new group participates in the active aperture. The pre-focus is then with the plane wave approximation for the phase error, given as
The advantage of the multiple pre-focusing of groups of elements compared to a fixed pre-focus annular array, is that one can use larger area of the elements as the pre-focus is increased, because the elements participates to the active aperture for a shorter.range. This reduces the total number of elements and hinders that the element width b^-becomes unpractically narrow. The net result is hence a practical way to obtain so wide active aperture for the deep ranges that a low diameter of the steered focus is maintained as the focal depth increases.
We have in this description used a fixed limit djj of the focal diameter, where the active aperture is expanded with new elements. It is clear that in the general spirit of the invention, this limit can vary, say d& = dpni to satisfy other design requirements, like a weakly expanding maximal focus to reduce the total number of elements.
The procedure above is then applied for expanding the aperture with one or more new annular, elements when the focal diameter increases above a selected limit Asm. The pre-focus of the new elements is preferably chosen as in Eq.(6), and the width of the elements are chosen so that the phase error across the elements is kept below a limit (e.g. cm/2 where a ~ 1) for the steered focus at the outer limits, i.e. at F^ and Ff. We then recall that equal area of the elements in the group gives the same phase error across each element, and also the same electrical impedance for the elements. It is also convenient to use element areas for each new group that are a whole number, multiplied by the area of the elements in the first group. This makes a simple solution for matching of the transmitter and receiver amplifiers to the different element impedances in each group, by parallel coupling a number of equal transmitter and receiver amplifiers to each element, given by the fraction of the element area to the area of the central elements.
The pre-focusing of the elements can be obtained by individual curving of the array elements, as shown in Figure 3a, or by a multiple focused lens system as in Figure 3b. This Figure shows a plane annular array where the elements 320, 321 participate in the active aperture from.Fn. to Ff and are pre-focused with the lens 322 to a depth F0 at 323, while the element 324 participates in the active aperture from F^ to Ff and is pre-focused by the lens 325 to a depth FI at 326, and the element 327 participates in the active aperture from F^ to Ff and is pre-focused by the lens 328 to a depth F2 at 329.
Due to absorption and pulse reverberations in the lens, it is advantageous to make the lens as thin as possible. This is achieved by the lens system.330, 331, 332 of Figure 3c which provides the same reduction in phase error across the elements as the lens system 322, 325, 328 of Figure 3b. The important function of the lens or curving of the elements, is to minimize the phase error across each element for the range of steered foci where the elements participate in the active aperture. One can then adjust the individual rime delays of the element signals to compensate for reductions in lens thickness, or offset positioning of the elements as shown in Figure 3d. The positioning of the elements as in Figure 3 a gives the simplest manufacturing of a curved array, although some offset positioning of the elements gives lower maximal delays of the element signals for focusing in the whole range from Fn.to Ff.
In practical imaging, spatial variations in the acoustic properties of the tissue, such as the wave propagation velocity, reduces the focusing capabilities of an array below that what is theoretically possible with the design above. This phenomenon is often referred to as phase front aberrations, and can be corrected for by dividing the whole array into smaller elements, and filtering the signals from
each element before they are further, delayed and processed according to standard beam forming techniques. An approximate filtering of the element signals are obtained by delay and amplitude corrections of the signals.
An example of an array that allows for such phase aberration correction, is the r-9 array shown in Figure 4. To allow for larger elements and reduce the total number of elements it is then advantageous to use multiple pre-focusing of the elements, where all elements located at the same distance from the center, is typically given the same pre-focus.
Thus, while there have been shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the fonn.and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention.
It is also expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps'shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or. suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.
 US Pat no 5,922,962 My 13, 1999: "Sparce two-dimensional transducer array with
compound lens". Inventors: Sayed Omar Ishrak, Mehmet Sahlahi, Farhad Towfiq, Alan chi-Chung Tai, Ha Thanh Pham. Assignee: Diasonics Ultrasound, Inc., Santa Clara, Calif.
1. An ultrasound array transducer comprising annular, concentric transducer elements, for electronic steering of a symmetric focus (F) from a near focus (Fn) to a far Locus (Ff), by adding different delays to respective element signals, and where
- the annular array elements are divided into groups of neighboring elements with at least one element per group, where each group has a common fixed mechanical pre-focus different from the prefoci of other groups, and where the elements within each group have practically equal area,
- the central group of elements participates in an active aperture for the whole focal range from Fn to Ff with a fixed pre-focus F0 between Fn and Ff,
- and beyond a focus depth Fn1 where the focal diameter of the central group expands past a selected limit, the next outer group of elements is added to contribute to the active aperture from Fn1 to Ff, where the fixed pre-focus F1 of said next outer group is between Fn2 and Ff,
- and beyond each focus depth Fnm where the focal diameter expands past selected limits, a further next outer group of elements is added to contribute to the active aperture from Fnm to Ff, where the fixed pre-focus Fm of said further next outer group is between Fnm and Ff, characterized by
- the fixed focus of each group of elements being selected so that the phase error across the elements of that group is minimized for the whole focal depth range where the group participates to the total aperture,
- and the area of the elements in each group is selected so that the phase error across each element is below a defined limit for the focal depth range where the elements participate to the active aperture,
- so that for a maximal alloted phase error across each element and a given active aperture for a focal distance F, there will be a minimal number of elements necessary to generate the active aperture for focusing in the range from Fn to Ff.
2. An ultrasound array transducer as claimed in claim 1,
where transducer elements in group number m participate to the active aperture from focus depth from Fnm to Ff, and
- to minimize the phase error across each element for the focal depth range where
it participates to the total aperture, the fixed focus Fm of group m is selected as
- and the area Am of the elements within group m is chosen as approximately
- where λ is the ultrasound wave length and ΔΦm is the maximal allowable phase
error across the elements for the focal range they participate to the active aperture,
the deviation between the actual element area and Am being given by the
separation kerf between the elements and the plane wave approximation of the
3. An ultrasound transducer array as claimed in claim 1 or 2, where the pre-focusing of the elements is obtained by curving the elements.
4. An ultrasound transducer array as claimed in claim 1 or 2, where the pre-focusing of the elements is obtained by an acoustic lens assembly.
5. An ultrasound transducer array as claimed in claim 1 or 2, where the pre-focusing of the elements is obtained by a combination of curving the elements and an acoustic lens assembly.
6. An ultrasound transducer array as claimed in any one of claims 1-5, where the area of the elements of each group is selected as a. whole number times the area of the elements in the central group, and to match the variable impedance between
|Indian Patent Application Number||01137/DELNP/2003|
|PG Journal Number||42/2008|
|Date of Filing||22-Jul-2003|
|Name of Patentee||TERARECON INC.|
|Applicant Address||2955 CAMPUS DRIVE, SUITE 325 SAN MATEO,CALIFORNIA DRIVE CA 94402, U.S.A.|
|PCT International Classification Number||G10K 11/30|
|PCT International Application Number||PCT/NO02/00006|
|PCT International Filing date||2002-01-07|