A conventional ultrasound imaging system comprises an array of ultrasonic transducers which are used to transmit an ultrasound beam and to receive the reflected beam from the object being studied. For ultrasound imaging, a one-dimensional array typically comprises a multiplicity of transducers arranged in a line and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducers can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. Multiple firings may be used to acquire data along the same scan line. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. By changing the time delay and amplitude of the applied voltages, the beam with its focal point can be moved in a plane to scan the object.
The same principles apply when the transducer array is employed to receive the reflected sound (receiver mode). The voltages produced at the receiving transducers are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting a separate time delay (and/or phase shift) and gain to the signal from each receiving transducer.
A phased-array ultrasound transducer consists of an array of small piezoelectric elements, with an independent electrical connection to each element. In most conventional transducers the elements are arranged in a single row, spaced at a fine pitch (one-half to one acoustic wavelength on center). As used herein, the term "1D"transducer array refers to a single-row transducer array having an elevation aperture which is fixed and an elevation focus which is at a fixed range. Electronic circuitry coupled to the elements uses time delays and possibly phase rotations to control the transmitted and received signals and form ultrasound beams which are steered and focused throughout the imaging plane. For some ultrasound systems and probes, the number of transducer elements in the probe exceeds the number of channels of beamformer electronics in the system. In these situations an electronic multiplexer is used to dynamically connect the available channels to different (typically contiguous) subsets of the transducer elements during different portions of the image formation process.
Various types of multi-row transducer arrays, including so-called "1.25D", "1.5D", "1.75D" and "2D" arrays, have been developed to improve upon the limited elevation performance of 1D arrays. As used herein, these terms have the following meanings: 1.25D) elevation aperture is variable, but focusing remains static; 1.5D) elevation aperture, shading, and focusing are dynamically variable, but symmetric about the centerline of the array; 1.75D) elevation geometry and control are similar to 1.5D, but without the symmetry constraint; and 2D) elevation geometry and performance are comparable to azimuth, with full electronic apodization, focusing and steering. The elevation aperture of a 1.25D probe increases with range, but the elevation focusing of that aperture is static and determined principally by a mechanical lens with a fixed focus (or foci). 1.25D probes can provide substantially better near-and far-field slice thickness performance than 1D probes, and require no additional system beamformer channels. 1.5D probes use additional beamformer channels to provide dynamic focusing and apodization in elevation. 1.5D probes can provide detail resolution comparable to, and contrast resolution substantially better than, 1.25D probes, particularly in the mid-and far-field. 1.75D probes, with independent control of the beamforming time delays for all elements in the aperture, allow the beamformer to adaptively compensate for inhomogeneous propagation velocities in the body (or nonuniformities in the imaging system or transducer). In addition to such adaptive beamforming or phase aberration correction, 1.75D probes may also support limited beam steering in the elevation direction.
By providing at least apodization (1.25D) and possibly dynamic beamforming (1.5D), phase aberration control (1.75D), or full 2D beam steering, multi-row transducer arrays significantly improve upon the limited elevation performance of 1D probes. However, as the number of elements in the transducer increases, the number of channels in the beamformer does not keep pace, and the function of the multiplexer becomes increasingly important.
U.S. Pat. No. 5,520,187, issued to Snyder, the disclosure of which is incorporated by reference herein, does not discuss multi-row transducer arrays, but describes a flexible multiplexer which supports different multiplexer states for systems with different numbers of beamformer channels. The multiplexer states can be reprogrammed by the ultrasound imaging system, e.g., via a serial interface. These features are advantageously used in multi-row array multiplexers such as those disclosed hereinbelow.
U.S. Pat. No. 5,329,930, issued Jul. 19, 1994 to Thomas and Harsh and assigned to the instant assignee, the disclosure of which is incorporated by reference herein, discloses a method of synthetic aperture imaging, whereby a finite number of system beamformer channels are coupled via a multiplexer to successive subsets of a large number of transducer elements. For each desired image vector and for each subset of the transducer elements, an acoustic beam is transmitted, received and summed coherently with the acoustic data from the other subsets of the transducer elements. In this way an N-channel beamformer can achieve most of the resolution and signal-to-noise performance available from an (M.times.N)-element transducer, albeit at the cost of M transmit-receive cycles per resulting image vector (hence the name 1-for-M or 1:M imaging). Thomas and Harsh discuss criteria for the multiplexer but do not disclose any specific design for it. The multiplexer disclosed hereinbelow satisfies the criteria of U.S. Pat. No. 5,329,930 and is designed to support 1:M beamforming of 1.5D and 1.75D transducer arrays.
D.G. Wildes et al. U.S. Pat. No. 5,897,501, filed on Feb. 20, 1998 and assigned to the instant assignee, discloses multiplexers for 1D transducer arrays and also discloses design rules and a physical design for a new class of multiplexers which allow flexible control of aperture shape and beamformer connectivity for multi-row transducer arrays. The connections between system channels and transducer elements obey the following design rules:
Rule I. The order and cycle length of the channel to element assignments is the same for all rows.
Rule II. The rows of the aperture are grouped in pairs. Channel assignments in one row of each pair are offset from the assignments in the other row by one-half the cycle length.
Rule III. Pairs of rows may also be grouped in quads. Channel assignments in one pair of each quad are offset from the assignments in the other pair by one-quarter of the cycle length.
Rule IV. If any element is coupled through switches to two channels, then the two channels coupled to that element are one-half the cycle length apart.
All of the 1D and 1.25D apertures shown in U.S. Pat. No. 5,897,501 have a separate system beamformer channel (or pair of channels) assigned to each column of transducer elements, so that the maximum width of the active aperture is limited by the number of system channels. Similarly, the 1.5D and 1.75D apertures have a separate system beamformer channel connected to each transducer element (or elevation-symmetric pair of elements, for the 1.5D arrays), so that the total area of the active aperture is limited by the number of system channels.
M.S. Seyed-Bolorforosh et al. U.S. Pat. No. 5,902,241, filed on Nov. 24, 1997 and assigned to the instant assignee, discloses an adaptive transducer array in which the element pitch is controlled by the imaging system depending on the mode of operation. Aperture size is increased by increasing the pitch of a row of transducer elements. In the case of a multi-row transducer array, the pitch can be increased in more than one row. A multiplicity of transducer elements are coupled to a multiplicity of beamformer channels by a multiplexing arrangement having multiple states. In one multiplexer state, successive transducer elements are respectively coupled to successive beamformer channels to produce an aperture having an element pitch equal to the distance separating the centerlines of two adjacent transducer elements (hereinafter "small pitch"). In another multiplexer state, selected transducer elements are respectively coupled to successive beamformer channels to produce an aperture having an increased element pitch equal to the small pitch multiplied by a factor of two or more. Three techniques to increase the aperture are disclosed. Methods of applying the invention in U.S. Pat. No. 5,902,241 to 1.25D, 1.5D and 2D arrays are discussed. The preferred method is to interconnect the adjacent elements in an array in order to form a larger active aperture by increasing the pitch. Alternatively, every other element in an array could be coupled to a respective beamformer channel to form a sparse array having a larger aperture. The last method is a combination of these two techniques wherein the active aperture is divided into a number of segments, each segment comprising a region with small pitch, a region with larger pitch obtained by shorting adjacent elements together, or a region with larger pitch obtained with sparse spacing of elements.
Ultrasound transducers are divided into fine-pitch arrays of elements so that an independent, electronically controlled time delay can be applied to the signal to or from each element. The algebraic sum of those signals forms an electronically steered and focused beam of ultrasound. If the transducer is a so-called linear or convex array and the beam of ultrasound is directed perpendicular to the face of the transducer without steering, then the time delays necessary for focusing are approximately: ##EQU1##
where r is the distance from the beam center to points on the face of the transducer, F is the focal length, and V.sub.s is the speed of sound. The resolution at the focus is: EQU .delta..apprxeq..lambda..function. (2)
where .lambda. is the wavelength of the ultrasound and .function. is the f-number of the array.
To maintain relatively uniform resolution over the image, the active aperture of the transducer array is increased proportional to the focal distance, maintaining a constant f-number, typically .function./1 to .function./2 in azimuth. The minimum f-number used is constrained by the desired depth of field and by the time-delay resolution of the beamformer. In the far-field of the image, however, the active aperture of the array is typically constrained by the number of channels in the system beamformer, and the f-number increases and resolution decreases with depth. A major decision in the design of a transducer array is how to trade-off between fine pitch, for broad directivity of individual elements and good control of time delays for low f-number beamforming in the near-field and mid-field, and coarse pitch, for maximum aperture, sensitivity and resolution in the far-field.