Richard E. Davidsen

Progress in Two-Dimensional Arrays for Real-Time Volumetric Imaging

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 × 40 = 1,600 element, 3.5 MHz array was fabricated with 256 transmit and 256 receive elements. A 60 × 60 = 3,600 element 5.0 MHz array was constructed with 248 transmit and 256 receive elements. An 80 × 80 = 6,400 element, 2.5 MHz array was fabricated with 256 transmit and 208 receive elements. 2-D transducer arrays were also developed for volumetric scanning in an intracardiac catheter, a 10 × 10 = 100 element 5.0 MHz forward-looking array and an 11 × 13 = 143 element 5.0 MHz side-scanning array. The −6 dB fractional bandwidths for the different arrays varied from 50% to 63%, and the 50 Ω 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 Random Arrays for Real Time Volumetric Imaging

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.

Two-dimensional arrays for medical ultrasound using multilayer flexible circuit interconnection

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.

Update on 2-D array transducers for medical ultrasound

l 1/2 -D and 2-D arrays offer a myriad of new imaging modalities and benefits when compared to the linear array. However, with added benefits come many problems and challenges and l 1/2 -D and 2-D arrays are no exception. The authors give possible solutions to a number of these challenges. The increase in transducer channels needed in a 1 1/2 -D and 2-D array can be reduced using a sparse periodic or sparse random array. The complexity of the fabrication is overcome using a multilayer flexible connector designed and fabricated using microelectronic techniques. The low SNR of 1 1/2 -D and 2-D arrays can be circumvented with the application of multi-layer ceramic elements to optimize the SNR given a specific transmit and receive configuration. In addition, optoelectronic transmitters allow for the reduction in size and increase in flexibility of the transducer cable because of the use of fiber optics. With the reduction in the number of channels, improvement in transducer fabrication, and increase in transducer SNR, l 1/2 -D and 2-D arrays will be accepted as viable replacements for the linear arrays of today.

Application of acoustic radiation force in ophthalmic ultrasound

The vitreous body of the eye is a collagen gel supported by hyaluronic acid (HA) molecules. As one ages, HA diffuses out of the eye, eventually leading to collapse of the vitreous body. Resultant changes in the material and structural properties of the vitreous are associated with a variety of sight threatening conditions, including retinal detachment. Unfortunately these changes are often difficult to quantify or counteract other than through invasive surgery. The authors are developing non-invasive techniques which utilize ultrasonic radiation force to diagnose changes in the vitreous and to treat retinal detachments which result in part from degradation of the vitreous. The authors are currently developing a novel technique, known as Kinetic Acoustic Retinal Evaluation (KARE), which would non-invasively generate and image vitreal motion as a means of diagnosis and localization of a variety of vitreo-retinal disorders. KARE utilizes acoustic radiation force to generate small, localized displacements of the vitreous body. These displacements are quantified using either Doppler or speckle tracking algorithms. The authors present encouraging simulation and experimental results using excised porcine eyes. They also present simulation results which predict the risks of acoustic heating associated with this technique. They are also working to develop a two step method for non-invasive repair of retinal detachments. In the first step, a new technique known as Acoustic Retinal Manipulation (ARM) is applied to move the detached retina into contact with the retinal pigment epithelium. Next, standard techniques such as laser photocoagulation or cryopexy are used to form a lasting bond between the retina and underlying tissues. ARM utilizes both direct acoustic radiation force and acoustic streaming to displace the retina. The authors present initial experimental results acquired using retina mimicking material in a water tank and excised porcine eyes. They also present simulation results which predict the risks of acoustic heating associated with ARM.

A multiplexed two-dimensional array for real time volumetric and B-mode imaging

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, the authors 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. The interconnection of this array is challenging due to the large number of elements. The authors 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.

A two-dimensional array for B-mode and volumetric imaging with multiplexed electrostrictive elements

A 2:1 multiplexed 2-D array has been developed that has a sparse element pattern designed for real time volumetric imaging and an alternate element pattern designed for B-mode imaging. For volumetric imaging, a small aperture was used to provide a wide transmit beam, allowing multiple beams to be received simultaneously. For B-mode imaging, a larger aperture with a more narrow transmit beam was used to improve image quality. Sparse random element patterns were evaluated by beamplot and cyst image simulations. Using the alternate element pattern for B-mode imaging, simulated cyst contrast was improved by 28%. The multiplexed transducer was fabricated using an electrostrictive material in which array elements were activated and deactivated by a dc bias field. The transducer had a 3.4 MHz center frequency with 46% bandwidth, which was consistent with KLM simulations. The high dielectric constant of the electrostrictive material resulted in an element clamped capacitance of 14.3 pF versus 2 pF for a convention PZT element. An output off isolation of −35 dB was measured for transmit and −61 dB for receive. The array was integrated with the volumetric scanner and used to make real time images of a cyst phantom. The images showed improved cyst contrast using the alternate aperture for B-mode imaging.

Advances in two dimensional arrays for real time volumetric imaging

The authors have previously described prototype two-dimensional arrays used for real time volumetric imaging (1991, 1992, 1995). However, these arrays primarily operated at frequencies below 3.0 MHz, have been unwieldy to the operator due to the interconnect schemes used, and have taken up to 6 weeks to construct. The authors have recently constructed transducers operating at higher frequencies, and having a smaller interconnect geometry. These transducers include a 40/spl times/40=1600 element array operating at 3.5 MHz and a 60/spl times/60=3600 element array operating at 5.0 MHz for cardiac applications. Each transducer includes 256 transmit and 256 receive elements. The authors also constructed an 80/spl times/80=6400 element array operating at 2.5 MHz on a muti-layer polyimide connector for obstetric applications. This transducer has 256 transmit and 208 receive elements. Using a flex circuit interconnect, the authors reduced the size in the transducer assembly by a factor of four compared to their previous method of pin and socket interconnects. The transducer/handle assembly is integrated with the Duke University real time volumetric imaging system capable of generating multiple planes at any desired angle and depth within the pyramidal volume.

Relaxor ferroelectric materials in two-dimensional transducer arrays

Relaxor ferroelectric materials are characterized by high dielectric permittivities and bias controllable piezoelectric response, making them attractive for ultrasonic imaging applications. We propose using relaxor ferroelectrics to multiplex a 2-D array for volumetric and B-mode imaging. Piezoelectric properties of 2-D relaxor elements were measured and used for modeling relaxor ferroelectric transducers. A variable aperture 96 element 1.5-D array was fabricated using a relaxor material and integrated with a commercial B-mode scanner to investigate its multiplexing capability.

Experimental results from an electrostrictive multiplexed 2-D array

A 2:1 multiplexed two-dimensional array has been developed which has a sparse element pattern designed for real time volumetric imaging and a second element pattern designed for conventional B-mode imaging. For volumetric imaging a small aperture was used to provide a wide transmit beam, allowing multiple beams to be received simultaneously. A larger aperture with a more narrow transmit beam was used for B-mode imaging to improve image quality when multiple receive beams were not required. The multiplexed transducer was fabricated using an electrostrictive relaxor ferroelectric material in which array elements were activated and deactivated by a DC bias field.