DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

[0025] A particular embodiment of the invention will be explained with reference to the Figures.

[0026] FIG. 1 a 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 FIG. 1 b . A lens of a material with acoustic velocity different from that of the load material, can also be used for the pre-focusing.

[0027] FIG. 2 a shows a a cross section in the elevation direction of plane annular array, depicting the cross section of a set of elements 201 , 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 ˜απ/2, where α˜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 α, where α=1.5 can in many cases be tolerable. For the steered focus F _z at 204 in FIG. 2 a we see that the phase error Δφ _k (Z) across element #k is, when approximating the wave front over the element by a plane wave (plane wave approximation) 1 $\begin{array}{cc}\Delta L_k\left(z\right)=\frac{b_ka_k}{F_z}{\mathrm{\Delta \phi }}_k\left(z\right)=2\pi \frac{\Delta L_k\left(z\right)}{\lambda }=2\pi \frac{b_ka_k}{\lambda F_z}& \left(1\right)\end{array}$

[0028] where λ is the ultrasound wave length, a _k is the radius of the element center, and b _k is the element width. We hence see that as the radius a _k of the element center increases, the element width b _k must be reduced to maintain the phase error below the acceptable limit. We note that for the area for rings is 2πa _k b _k which implies that the phase error is the same with equal area elements. We also note that as the steered focus F _z is reduced, the phase error increases, which limits the maximal a _k to be used at low ranges with a given b _k .

[0029] 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 FIG. 2 b , or using a lens as shown in FIG. 3 b , or a combination of both. Which of these methods that are preferred, depend on the actual situation.

[0030] The phase error across each element is then zero for waves originating from the fixed focus F, and increases as the steered focus F _z at 206 in FIG. 2 b is moved inwards or outwards from F. With reference to FIG. 2 b we see that the phase error in this case can in the plane wave approximation be obtained as 2 $\begin{array}{cc}{\mathrm{\Delta \phi }}_k\left(z\right)=\frac{2\pi }{\lambda }\left(\frac{1}{F_z}-\frac{1}{F}\right)b_ka_k& \left(2\right)\end{array}$

[0031] 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 _k one must reduce the aperture (i.e. maximal a _k ) as F _z is both increased or reduced from F to maintain Δφ _k (z) below acceptable limits.

[0032] The diameter of the beam focus, can be expressed as 3 $\begin{array}{cc}{d}_F\left(z\right)=2\frac{\lambda }{D_k}F_z& \left(3\right)\end{array}$

[0033] where D _k =2a _k +b _k =d _k +b _k is the diameter of the active aperture with k elements electronically focused at the depth F _z . 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 12 dB drop of the field amplitude from the axial value. We note that d _F (z)˜F _z , which implies that for fixed active aperture diameter D _k 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 b _k is reduced with increasing k proportional to 1/a _k to satisfy a limit on Δφ _k (Z) in Eq.(2), which makes the outer elements very narrow and increases the number of elements.

[0034] 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 FIG. 3 a . In this particular embodiment, a central group of elements 301 with total aperture diameter Do participates in the active aperture over the whole steered focusing range of the array, i.e. from a steered near focus F _n at 302 to a steered far focus F _f at 303 . This group of elements has a common pre-focus F ₀ at 304 , preferably selected so that the phase error is the same at the far focus F _f and the near focus F _n . With the plane wave approximation, this gives a pre-focus 4 $\begin{array}{cc}{F}_0=\frac{2F_nF_f}{F_n+F_f}& \left(4\right)\end{array}$

[0035] 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 b _k ˜1/a _k , the area A _k =2πa _k b _k of the annular elements are independent of a _k . 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.

[0036] The focus F _z is steered electronically outwards from F _n by adding delays to the signals of the individual elements in the group according to well known methods. The focal diameter increases with F _z according to Eq.(3) with D _k =D ₀ , and indicated by the lines 307 in FIG. 3 a . When the focal diameter exceeds a selected limit d _F1 indicated by the lines 308 , a new group of elements 305 is added to the active aperture at a depth F _n1 at 306 . The new group of elements participates in the active aperture from F _n1 to F _f , and is given a pre-focus F ₁ at 309 in this range, preferably so that the phase error across each element is minimized for F _z in the range from F _n1 to F _f . With the plane wave approximation, this gives a fixed focus of 5 $\begin{array}{cc}{F}_1=\frac{2F_{\mathrm{n1}}F_f}{F_{\mathrm{n1}}+F_f}& \left(5\right)\end{array}$

[0037] This increase in active aperture diameter to D ₁ produces a reduction in the focal diameter below the limit d _F1 , as indicated by the lines 307 that describes Eq.(3).

[0038] The focus F _z is furthered steered electronically outwards from F _n1 by adding delays to the signals for all the elements that participates in the apertures, and the focal diameter further increases with F _z according to Eq.(3) with the new active aperture diameter D _k =D ₁ . At a depth F _n2 at 310 the focal diameter again passes a selected limit d _F1 where the procedure is repeated so that a new group of elements 311 is added to the active aperture so that one gets a diameter of the active aperture of D ₂ for F _z >F _n2 . The new element group 311 is pre-focused to a depth F ₂ at 312 , preferably so that the phase error across these elements is minimized over the whole range of the steered focus from F _n2 to F _f where the element group 311 participates in the active aperture.

[0039] Hence, the general procedure can be summarized so that for a given active aperture diameter D _m-1 , the focal diameter increases with the focal depth according to Eq.(3) with D _k =D _m-1 , and at the depth F _nm where the focal diameter exceeds a selected limit d _F1 , the aperture is increased with a new group of elements to participate in the active aperture from F _nm to F _f , 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 _nm to F _f 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 6 $\begin{array}{cc}{F}_m=\frac{2F_{nm}F_f}{F_{nm}+F_f}& \left(6\right)\end{array}$

[0040] 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 _k becomes impractically 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.

[0041] We have in this description used a fixed limit d _F1 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 _F1 =d _Fm to satisfy other design requirements, like a weakly expanding maximal focus to reduce the total number of elements.

[0042] 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 d _Fm . 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. απ/2 where α˜1) for the steered focus at the outer limits, i.e. at F _nm and F _f . 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.

[0043] The pre-focusing of the elements can be obtained by individual curving of the array elements, as shown in FIG. 3 a , or by a multiple focused lens system as in FIG. 3 b . This Figure shows a plane annular array where the elements 320 , 321 participate in the active aperture from F _n to F _f and are pre-focused with the lens 322 to a depth F ₀ at 323 , while the element 324 participates in the active aperture from F _n1 to F _f and is pre-focused by the lens 325 to a depth F ₁ at 326 , and the element 327 participates in the active aperture from F _n2 to F _f and is pre-focused by the lens 328 to a depth F ₂ at 329 .

[0044] 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 FIG. 3 c which provides the same reduction in phase error across the elements as the lens system 322 , 325 , 328 of FIG. 3 b . 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 time delays of the element signals to compensate for reductions in lens thickness, or offset positioning of the elements as shown in FIG. 3 d . The positioning of the elements as in FIG. 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 F _n to F _f .

[0045] 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.

[0046] An example of an array that allows for such phase aberration correction, is the r-θ array shown in FIG. 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.

[0047] 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.