David Martin Mills

Reconfigurable arrays for portable ultrasound

A collaborative effort is aimed at the development of reconfigurable array technology. The goal is to enable innovative medical ultrasound imagers ideally suited for portable ultrasound and applications such as remote emergency medicine and combat casualty care. Success depends on developing several technologies, the first of which is capacitive micromachined ultrasound transducers (cMUTs). The monolithic nature of cMUTs facilitates close connection with microelectronics. Thus a second technology under development is a switch matrix application specific integrated circuit (ASIC) that will enable the changing of interconnect between cMUT cells. The reconfigurable array concept arises from this ability to dynamically combine cMUT cells to form ideal apertures for a given imaging target (e.g. annular and phased apertures of various ring widths) and to move these apertures across the reconfigurable array plane [1-4]. Two central hypotheses are being tested: (1) the reconfigurable array can acquire acoustic pulse-echo data in a manner equivalent or superior to today’s 1D piezoceramic arrays (2) reconfigurable array technology will enable highly portable ultrasound platforms. Keywords-reconfigurable; annular; array; cMUT; ASIC; switch matrix; dynamic; phased; real-time

Multi-layered PZT/polymer composites to increase signal-to-noise ratio and resolution for medical ultrasound transducers. II. Thick film technology

For pt.I, see ibid., vol.46, no.4, p.961-71 (1999). Increasing transducer bandwidth and signal-to-noise ratio (SNR) is fundamental to improving the quality of medical ultrasound images. In previous work, we have proposed the use of multi-layer 1-3 PZT/polymer composites to increase both, but have encountered significant fabrication challenges. Thus, we have developed a multi-layer composite hybrid array that will not require post alignment. Starting from a 2-MHz, three-layer PZT-5H, thick film transducer designed for 1.5-D arrays, cuts are made only through the top layer and back-filled with epoxy, forming a composite layer on top of two ceramic layers. Finite element (PZFlex) simulations show that for a 2-MHz phased-array element with a single matching layer, the three-layer hybrid structure increases the pulse echo SNR by 11 dB versus a single layer PZT element and improves -6 dB pulse echo fractional bandwidth by a factor of 1.4. Composite hybrid arrays fabricated in our laboratory showed an improvement in SNR of 6 to 11 dB over a PZT control and an increase in -6 dB bandwidth by a factor of 1.1. Images from a phased-array scanner confirmed these improvements.

4-D spatiotemporal analysis of ultrasound contrast agent dispersion for prostate cancer localization: a feasibility study

Currently, nonradical treatment for prostate cancer is hampered by the lack of reliable diagnostics. Contrastultrasound dispersion imaging (CUDI) has recently shown great potential as a prostate cancer imaging technique. CUDI estimates the local dispersion of intravenously injected contrast agents, imaged by transrectal dynamic contrast-enhanced ultrasound (DCE-US), to detect angiogenic processes related to tumor growth. The best CUDI results have so far been obtained by similarity analysis of the contrast kinetics in neighboring pixels. To date, CUDI has been investigated in 2-D only. In this paper, an implementation of 3-D CUDI based on spatiotemporal similarity analysis of 4-D DCE-US is described. Different from 2-D methods, 3-D CUDI permits analysis of the entire prostate using a single injection of contrast agent. To perform 3-D CUDI, a new strategy was designed to estimate the similarity in the contrast kinetics at each voxel, and data processing steps were adjusted to the characteristics of 4-D DCE-US images. The technical feasibility of 4-D DCE-US in 3-D CUDI was assessed and confirmed. Additionally, in a preliminary validation in two patients, dispersion maps by 3-D CUDI were quantitatively compared with those by 2-D CUDI and with 12-core systematic biopsies with promising results.

5F-2 Packaging and Design of Reconfigurable Arrays for Volumetric Imaging

Recent advances in capacitive micromachined ultrasound transducers (cMUTs), piezoceramic acoustic stack design, and transducer-to-electronics integration are enabling the fabrication of highly integrated reconfigurable ultrasound arrays. Reconfigurability in this context refers to the ability to reorganize the array elements in any configuration deemed desirable. Attractive configurations are an annular array, which can be translated electronically along a 2D array surface, or a phased array whose element orientation and pitch can be varied to optimize an image [1-6]. Further, annular array configurations make possible 3D-stacked miniaturized systems by reducing channel count, system size, and power consumption. In addition to such benefits, the annular array also provides dynamic axisymmetric focusing for excellent image quality. Several methods of integration have been explored to realize stacked transducer arrays with low channel count. Key components of the design include advanced interconnect such as through silicon vias in cMUTs and z-axis backing stacks, switch matrix application specific integrated circuits (ASICs), and high density multi-layer flex.

4-D ICE: A 2-D Array Transducer With Integrated ASIC in a 10-Fr Catheter for Real-Time 3-D Intracardiac Echocardiography

We developed a 2.5×6.6 mm2 2-D array transducer with integrated transmit/receive application-specific integrated circuit (ASIC) for real-time 3-D intracardiac echocardiography (4-D ICE) applications. The ASIC and transducer design were optimized so that the high-voltage transmit, low-voltage time-gain control and preamp, subaperture beamformer, and digital control circuits for each transducer element all fit within the 0.019-mm2 area of the element. The transducer assembly was deployed in a 10-Fr (3.3-mm diameter) catheter, integrated with a GE Vivid E9 ultrasound imaging system, and evaluated in three preclinical studies. The 2-D image quality and imaging modes were comparable to commercial 2-D ICE catheters. The 4-D field of view was at least 90° × 60° × 8 cm and could be imaged at 30 vol/s, sufficient to visualize cardiac anatomy and other diagnostic and therapy catheters. 4-D ICE should significantly reduce X-ray fluoroscopy use and dose during electrophysiology ablation procedures. 4-D ICE may be able to replace transesophageal echocardiography (TEE), and the associated risks and costs of general anesthesia, for guidance of some structural heart procedures.We developed a 2.5 x 6.6 mm 2D array transducer with integrated transmit/receive ASIC for 4D ICE (real-time 3D IntraCardiac Echocardiography) applications. The ASIC and transducer design were optimized so that the high voltage transmit, low-voltage TGC (time-gain control) and preamp, subaperture beamformer, and digital control circuits for each transducer element all fit within the 0.019 mm2 area of the element. The transducer assembly was deployed in a 10 Fr (3.3 mm diameter) catheter, integrated with a GE Vivid1 E9 ultrasound imaging system, and evaluated in three pre-clinical studies. 2D image quality and imaging modes were comparable to commercial 2D ICE catheters. The 4D field of view was at least 90° x 60° x 8 cm and could be imaged at 30 volumes/sec, sufficient to visualize cardiac anatomy and other diagnostic and therapy catheters. 4D ICE should significantly reduce X-ray fluoroscopy use and dose during electrophysiology (EP) ablation procedures. 4D ICE may be able to replace trans-esophageal echocardiography (TEE), and the associated risks and costs of general anesthesia, for guidance of some structural heart procedures.

Automated catheter detection in volumetric ultrasound

Ultrasound guided catheter insertion is a common procedure in current clinical practice, but it requires a skilled ultrasound practitioner to correctly acquire the images so that the catheter and tip are well visualized. Automated detection of the catheter location in ultrasound images can help in procedural guidance as well as surveillance of catheter position post placement, especially when the ultrasound imaging is being performed by a non-sonographer. Accurate and fast localization of the catheter is a very challenging task because of the poor observability of the catheter in ultrasound images. In this paper, we present a novel algorithm for fast catheter detection in 3D ultrasound images. We begin by generating a catheter-likelihood map using a physical model of the catheter. For fast detection, we perform a greedy optimization, where the likelihood map is projected onto a single image plane using a variation on the maximum-intensity-projection approach. The highest likelihood curves on this plane then help us to determine the curved planes on which the catheter may be located. The potential catheter locations are then detected on these curved planes. We finally apply some post-processing to enable robust detection of the catheter. We illustrate the performance of our proposed method through quantitative comparisons with expert annotations as well as qualitative results on 26 images obtained from 7 subjects (of which 15 are with and 11 are without a catheter).

Finite element comparison of single crystal vs. multi-layer composite arrays for medical ultrasound

Finite element (PZFlex; Weidlinger Assoc., New York, NY and Los Altos, CA) simulations predict that for a 2-MHz phased array element with a single matching layer, the three-layer hybrid structure increases the pulse echo signal-to-noise ratio (SNR) by 16 dB over that from a single layer PZT element and -6 dB pulse echo fractional bandwidth from 58% for the PZT element to 75% for the hybrid element. Analogous finite element method (FEM) simulations of single crystal material [lead zinc niobate (PZN)-8% lead titanate (PT)] showed increased SNR by only 3.1 dB, but a -6 dB bandwidth of 108%.

Automated bone and joint-region segmentation in volumetric ultrasound

Ultrasound (US) is being increasingly used in the assessment of joints for disorders such as rheumatoid arthritis (RA). Pathologies related to RA in joints, manifest themselves commonly as changes in the bone (e.g. erosions) and the region enclosed by the joint-capsule (e.g. synovitis). Automated tools for detecting and segmenting such structures would help significantly towards objective and quantitative assessment of RA in joints. In this paper we present a novel method for automatic extraction of the 3D bone surface and segmentation of the joint-capsule region from volumetric (3D) scans of commonly affected joints such as the metacar-pophalangeal (MCP) and metatarsophalangeal (MTP) joints. We improve and extend the cost function proposed in [1] to 3D such that it is more robust to loss of acoustic intensity due to the surface normals facing away from the incident acoustic beam. The extracted bone coupled with a simple anatomical model of the MCP joint provides a coarse localization of the joint-capsule region. A probabilistic speckle model is then used to iteratively refine the capsule segmentation. We illustrate the performance of proposed methods through quantitative comparisons with expert annotations as well as qualitative results on over 30 scans obtained from 11 subjects.

Toward Quantitative Assessment of Rheumatoid Arthritis Using Volumetric Ultrasound

Goal: Rheumatoid arthritis (RA) is characterized by inflammation within the joint space as well as erosion or destruction of the bone surface. We believe that volumetric (3-D) ultrasound imaging of the joints in conjunction with automated image-analysis tools for segmenting and quantifying the regions of interest can lead to improved RA assessment. Methods: In this paper, we describe our proposed algorithms for segmenting 1) the 3-D bone surface and 2) the 3-D joint capsule region. We improve and extend previous 2-D bone extraction methods to 3-D and make our algorithm more robust to the intensity loss due to surface normals facing away from incident acoustic beams. The extracted bone surfaces coupled with a joint-specific anatomical model are used to initialize a coarse localization of the joint capsule region. The joint capsule segmentation is refined iteratively utilizing a probabilistic speckle model. Results: We apply our methods on 51 volumes from 8 subjects, and validate segmentation results with expert annotations. We also provide the quantitative comparison of our bone detection with magnetic resonance imaging. These automated methods have achieved average sensitivity/precision rates of 94%/93% for bone surface detection, and 87%/83% for joint capsule segmentation. Segmentations of normal and inflamed joints are compared to demonstrate the potential of using proposed tools to assess RA pathology at the joint level. Conclusion: The proposed image-analysis methods showed encouraging results as compared to expert annotations. Significance: These computer-assisted tools can be used to help visualize 3-D anatomy in joints and help develop quantitative measurements toward RA assessment.

Packaging and modular assembly of large-area and fine-pitch 2-D ultrasonic transducer arrays

A promising transducer architecture for largearea arrays employs 2-D capacitive micromachined ultrasound transducer (CMUT) devices with backside trench-frame pillar interconnects. Reconfigurable array (RA) application-specified integrated circuits (ASICs) can provide efficient interfacing between these high-element-count transducer arrays and standard ultrasound systems. Standard electronic assembly techniques such as flip-chip and ball grid array (BGA) attachment, along with organic laminate substrate carriers, can be leveraged to create large-area arrays composed of tiled modules of CMUT chips and interface ASICs. A large-scale, fully populated and integrated 2-D CMUT array with 32 by 192 elements was developed and demonstrates the feasibility of these techniques to yield future large-area arrays. This study demonstrates a flexible and reliable integration approach by successfully combining a simple under-bump metallization (UBM) process and a stacked CMUT/interposer/ASIC module architecture. The results show high shear strength of the UBM (26.5 g for 70-μm balls), high interconnect yield, and excellent CMUT resonance uniformity (s = 0.02 MHz). A multi-row linear array was constructed using the new CMUT/interposer/ASIC process using acoustically active trench-frame CMUT devices and mechanical/ nonfunctional Si backside ASICs. Imaging results with the completed probe assembly demonstrate a functioning device based on the modular assembly architecture.