The design, fabrication, and evaluation of two dimensional array transducers for real-time volumetric imaging are described. The transducers we have previously described operated at frequencies below 3 MHz and were unwieldy to the operator because of the interconnect schemes used in connecting to the transducer handle. Several new transducers have been developed using new connection technology. A 40 x 40 = 1,600 element, 3.5 MHz array was fabricated with 256 transmit and 256 receive elements. A 60 x 60 = 3,600 element 5.0 MHz array was constructed with 248 transmit and 256 receive elements. An 80 x 80 = 6,400 element, 2.5 MHz array was fabricated with 256 transmit and 208receive elements. 2-D transducer arrays were also developed for volumetric scanning in an intra cardiac catheter, a 10 x 10 = 100 element 5.0 MHz forward-looking array and an 11 x 13 = 143 element 5.0 MHz side-scanning array. The-6dB fractional bandwidths for the different arrays varied from 50% to 63%, and the 50 omega insertion loss for all the transducers was about-64 dB. The transducers were used to generate real-time volumetric images in phantoms and in vivo using the Duke University real time volumetric imaging system, which is capable of generating multiple planes at any desired angle and depth within the pyramidal volume.
Two-dimensional arrays are necessary for a variety of ultrasonic imaging techniques, including elevation focusing, 2-D phase aberration correction, and real time volumetric imaging. In order to reduce system cost and complexity, sparse 2-D arrays have been considered with element geometries selected ad hoc, by algorithm, or by random process. Two random sparse array geometries and a sparse array with a Mills cross receive pattern were simulated and compared to a fully sampled aperture with the same overall dimensions. The sparse arrays were designed to the constraints of the Duke University real time volumetric imaging system, which employs a wide transmit beam and receive mode parallel processing to increase image frame rate. Depth-of-field comparisons were made from simulated on-axis and off-axis beamplots at ranges from 30 to 160 mm for both coaxial and offset transmit and receive beams. A random array with Gaussian distribution of transmitters and uniform distribution of receivers was found to have better resolution and depth-of-field than both a Mills cross array and a random array with uniform distribution of both transmit and receive elements. The Gaussian random array was constructed and experimental system response measurements were made at several ranges. Comparisons of B-scan images of a tissue mimicking phantom show improvement in resolution and depth-of-field consistent with simulation results.
The development of 2-D array transducers has received much recent interest. Unfortunately, fabrication of high density 2-D arrays is difficult due to the large number of electrical interconnections which must be made to the back side of the elements. A typical array operating at 2.2 MHz may have 256 or more connections within a 16.4 mm circular footprint. Interconnection becomes even more challenging as operating frequencies increase. To solve this problem, we have developed a multilayer flexible (MLF) circuit interconnect consisting of a polyimide dielectric with inter-laminar vias routing signals vertically and etched metal traces routing signals horizontally. A transducer is fabricated from an MLF by bonding a PZT chip to its surface and dicing the chip into individual elements, with the saw kerf extending partially into the top polyimide layer to ensure physical and electrical isolation of the elements. The KLM model was used to compare the performance of an MLF 2-D array to a conventional hand wired 2-D array. MLF and wire guide transducers were fabricated, each with 256 active elements, 0.4 mm interelement spacing, and 2.2 MHz center frequency. Vector impedance, pulse length, bandwidth, angular response, and cross-coupling were found to be comparable in both types of arrays. Using the MLF, however, fabrication time was reduced dramatically. More importantly, MLF technology may be used to increase 2-D array connection density beyond the limitations of current of hand wired fabrication techniques. Thus MLF circuits provide a means for the interconnection of current and future high frequency 2-D arrays.
Two-dimensional arrays are necessary for a variety of ultrasonic imaging techniques, including elevation focusing, 2-D phase aberration correction, and real time volumetric imaging. In order to reduce system cost and complexity, sparse 2-D arrays have been considered with element geometries selected ad hoc, by algorithm, or by random process. Two random sparse array geometries and a sparse array with a Mills cross receive pattern were simulated and compared to a fully sampled aperture with the same overall dimensions. The sparse arrays were designed to the constraints of the Duke University real time volumetric imaging system, which employs a wide transmit beam and receive mode parallel processing to increase image frame rate. Depth-of-field comparisons were made from simulated on-axis and off-axis beamplots at ranges from 30 to 160 mm for both coaxial and offset transmit and receive beams. A random array with Gaussian distribution of transmitters and uniform distribution of receivers was found to have better resolution and depth-of-field than both a Mills cross array and a random array with uniform distribution of both transmit and receive elements. The Gaussian random array was constructed and experimental system response measurements were made at several ranges. Comparisons of B-scan images of a tissue mimicking phantom show improvement in resolution and depth-of-field consistent with simulation results.
The Duke University real time volumetric imaging system employs receive mode parallel processing to increase image frame rate. This technique has an associated reduction in image resolution and contrast due to the requirement of a wide transmit beam. Since B-mode imaging can be accomplished in real time without this constraint. we propose a multiplexed 2-D array which has 2 sets of elements, one designed for volumetric imaging and one designed for B-mode imaging with improved resolution. Tbe interconnection of this array is challenging due to the large number of elements. We have developed a new technique for 2-D array interconnection which employs a thin multilayer flexible circuit The flex circuit element performance was comparable to hand wired 2-D arrays with greatly reduced fabrication time, regardless of the number of connections. METHODSVolumetric imaging can be performed in real time using receive mode parallel processing to increase image frame rate [ll . This requires a two-dimensional (2-D) array with transmit beam width sufficient to allow 4 x 4 parallel receive beams to be acquired simultaneously. The wide transmit beam is achieved by concentrating transmit elements in the center of the aperture. Unfortunately, this degrades lateral resolution and image contrast When the system is used for B-mode imaging, a real time frame mte is achieved without receive mode parallel processing, making the wide transmit beam and its associated loss in resolution and contrast unnecessary. Therefore, we propose a multiplexed sparse 2-D array which has 2 sets of elements, one with a wide transmit beam for volumetric imaging and one with a narrow transmit beam for B-mode imaging. Selection of the aperture would depend on the system imaging mode selected by the clinician.The multiplexed array element geometry is shown in Fig. 1. We employ sparse random 2-D arrays to improve resolution with a limited number of system channels [2]. The aperture for volumetric imaging is shown in Fig. 1(A) and the B-mode imaging aperture is shown in Fig 1(B). Each set of elements includes 192 transmit elements shown in black and 64 receive elements shown in white. Since the transmitters of both apertures 0-7803-3615-1/96/$5.00 0 1996 JEEE are highly sampled in the center of the array, it was necessary to share transmit elements between both apertures. In this design, 84 transmit elements are common to both apertmx resulting in a total of 428 active elements in the array. A B Fq. 1. Multiplexed apertures for (A) volumetric imaging and (B) B-mode imaging Multiplexing the 2-D apertures is accomplished using a relaxor ferroelectric material IBaTie -(Pb(Mgln Ntruj)o3-F'bTiO3))] (Lockheed-Martin, Baltimore MD)which has biased controlled sensitivity. The circuit diagram in Fig. 2 is used to select the aperture by applying a DC bias field of 0.75 W/m to activate elements while allowing passage of transient signals. We have previously reported a multiplexed 3 x 32 element 1.5-D relaxor ferroelectric transducer with expanding elevation aperture which...
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