3-D information can be acquired by steering the ultrasound beam electronically in both the azimuth and elevation directions using 2-D matrix transducer arrays. Such arrays were first introduced in the early 1990’s. Since no moving parts are involved, they can achieve real-time 3-D images at a frame rate of more than approximately 20 Hz.
High volume rate 3-D ultrasound imaging can be considerably beneficial for echocardiography, enabling detailed anatomical assessment of cardiac pathology. In principle, the concept is simple, but in practice it presents a formidable technical challenge. To achieve high quality images and steering to a desirable angle, 2-D matrix array must be designed with a pitch in the range of half of the wavelength of operating frequency.
It means that for a 3 MHz transducer operating in water, a typical pitch of 250 μm is required. Such a small pitch forces the elements to become smaller in a 2-D matrix, which in fact results in lowered capacitance and hence an increased electrical impedance mismatch between the element and the cable connecting it to the ultrasound scanner. This calls for preamplifiers and matching circuits in the probe handle.
On the other hand, to obtain the same spatial resolution as in 2-D imaging, the number of elements along each lateral dimension must be equal to that of a 1-D array. Yet, even a small 1-D array with 128 elements would translate into 128 × 128 = 16,384 elements in a 2-D matrix array. From a transducer fabrication viewpoint, the combination of small pitch and large number of elements generates few construction difficulties, particularly for interconnections and ground electrode distribution to each element. Nonetheless, the sheer number of wires results in an impractically large cable from the transducer to the scanner.
An N×N element 2-D array can be operated utilizing only 2N connections, when a row–column or cross-electrode addressing scheme is used. This is contrary to the N2 connections needed, when conventionally addressing the elements. In general, a row–column-addressed (RCA) array is a 2-D matrix array, which is addressed via its row- and column indices.
Effectively, it consists of two 1-D arrays arranged orthogonal to each other. As an example, for a 256+256 RCA array, a 2-D matrix array of equivalent size would have 65,536 elements, over a factor of 7 more than the current state-of-the-art X6-1 PureWave xMATRIX probe from Phillips (Eindhoven, Netherlands) having 9,212 elements. This exhibits the potential of having very large RCA 2-D arrays with low channel count and real-time capabilities.
It has been demonstrated in several studies that row–column technology is a realistic alternative to the state-of-the-art matrix probes, especially as a low-cost alternative. However, one major issue with the RCA arrays is that they can only emit acoustic energy directly below the array and in a cross-shape to the sides. Therefore, imaging can only be done in a rectilinear region in front of the array.
For applications such as cardiac imaging, it is relevant to have a probe with a small foot-print capable of phased array imaging, such that the heart can be visualized through the ribs. True volumetric phased array imaging is possible with RCA arrays, provided that the array is double curved to spread the energy during transmit. However, manufacturing curved transducer elements is challenging for both capacitive micromachined ultrasonic transducer (CMUT) and piezoelectric transducer (PZT) technologies.
Another approach to spread the acoustic energy is by using a double curved diverging acoustic lens on top of the RCA array. Using a lens makes it easier to fabricate curved arrays, as it is not needed to manufacture curved elements, and also making a lens is a well-tested technology. An in-depth study of the possibilities in this approach is therefore the main goal of this study.