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Yayın CMUT-based volumetric ultrasonic imaging array design for forward looking ICE and IVUS applications(SPIE-Int Soc Optical Engineering, 2013) Tekeş, Coşkun; Zahorian, Jaime S.; Xu, Toby; Rashid, Muhammad Wasequr; Satır, Sarp; Gürün, Gökçe; Karaman, Mustafa; Hasler, Jennifer Olson; Değertekin, Fahrettin LeventDesigning a mechanically flexible catheter based volumetric ultrasonic imaging device for intravascular and intracardiac imaging is challenging due to small transducer area and limited number of cables. With a few parallel channels, synthetic phased array processing is necessary to acquire data from a large number of transducer elements. This increases the data collection time and hence reduces frame rate and causes artifacts due to tissue-transducer motion. Some of these drawbacks can be resolved by different array designs offered by CMUT-on-CMOS approach. We recently implemented a 2.1-mm diameter single chip 10 MHz dual ring CMUT-on-CMOS array for forward looking ICE with 64-transmit and 56-receive elements along with associated electronics. These volumetric arrays have the small element size required by high operating frequencies and achieve sub mm resolution, but the system would be susceptible to motion artifacts. To enable real time imaging with high SNR, we designed novel arrays consisting of multiple defocused annular rings for transmit aperture and a single ring receive array. The annular transmit rings are utilized to act as a high power element by focusing to a virtual ring shaped line behind the aperture. In this case, image reconstruction is performed by only receive beamforming, reducing total required firing steps from 896 to 14 with a trade-off in image resolution. The SNR of system is improved more than 5 dB for the same frequency and frame rate as compared to the dual ring array, which can be utilized to achieve the same resolution by increasing the operating frequency.Yayın Monolithic CMUT-on-CMOS integration for intravascular ultrasound applications(IEEE-INST Electrical Electronics Engineers Inc, 2011-12) Zahorian, Jaime S.; Hochman, Michael; Xu, Toby; Satır, Sarp; Gürün, Gökçe; Karaman, Mustafa; Değertekin, Fahrettin LeventOne of the most important promises of capacitive micromachined ultrasonic transducer (CMUT) technology is integration with electronics. This approach is required to minimize the parasitic capacitances in the receive mode, especially in catheter-based volumetric imaging arrays, for which the elements must be small. Furthermore, optimization of the available silicon area and minimized number of connections occurs when the CMUTs are fabricated directly above the associated electronics. Here, we describe successful fabrication and performance evaluation of CMUT arrays for intravascular imaging on custom-designed CMOS receiver electronics from a commercial IC foundry. The CMUT-on-CMOS process starts with surface isolation and mechanical planarization of the CMOS electronics to reduce topography. The rest of the CMUT fabrication is achieved by modifying a low-temperature micromachining process through the addition of a single mask and developing a dry etching step to produce sloped sidewalls for simple and reliable CMUT-to-CMOS interconnection. This CMUT-to-CMOS interconnect method reduced the parasitic capacitance by a factor of 200 when compared with a standard wire-bonding method. Characterization experiments indicate that the CMUT-on-CMOS elements are uniform in frequency response and are similar to CMUTs simultaneously fabricated on standard silicon wafers without electronics integration. Experiments on a 1.6-mm-diameter dual-ring CMUT array with a center frequency of 15 MHz show that both the CMUTs and the integrated CMOS electronics are fully functional. The SNR measurements indicate that the performance is adequate for imaging chronic total occlusions located 1 cm from the CMUT array.Yayın A tunable analog delay element for high-frequency dynamic beamforming(IEEE, 2009) Gürün, Gökçe; Şişman, Alper; Zahorian, Jaime S.; Satır, Sarp; Karaman, Mustafa; Hasler, Paul E.; Değertekin, Fahrettin LeventImplementing beamforming for high frequency arrays is challenging because of the accurate delay requirements at high frequencies. High frequency digital beamforming is not suitable for catheter based applications as a large number of cables is required between the array and the external beamformer. A possible solution is to perform analog beamforming on an integrated circuit adjacent or monolithically integrated to the imaging array. In this study, we introduce an improved voltage in voltage out low pass filter as an analog delay cell for high frequency dynamic beamformers. This circuit can generate three times more delay with a given bandwidth when compared to conventional low pass filters. Delay of the circuit is tunable and the gain of the cell is inherently very close to unity. The proposed delay cell operates single ended and therefore is more suitable for CMUT operation which generates single ended output. We designed a test beamformer for a 30MHz, equal area, annular array with 100% bandwidth using the proposed delay cell and the unit-delay focusing architecture. Required delays are implemented using a delay line made up of improved delay elements with tunable delays. To demonstrate functionality we designed and fabricated a custom front-end IC in a 0.5µm standard CMOS process. The IC chip consists of 8 transimpedance amplifiers, voltage-to-current converters, the analog dynamic beamformer, and two buffers. We present results of preliminary imaging experiments that demonstrate the focusing capability.Yayın An annular CMUT array beamforming system for high-frequency side looking IVUS imaging(2010) Satır, Sarp; Gürün, Gökçe; Zahorian, Jaime S.; Karaman, Mustafa; Hasler, Paul E.; Değertekin, Fahrettin LeventA CMUT annular array system for Side-Looking Intravascular Ultrasound (SL-IVUS) with fixed transmit and dynamic receive focusing capabilities has been developed. The system was experimentally characterized and validated through analytical models that simulate the beamformed transducer behavior. An 840 m diameter, 35MHz array was fabricated, characterized, and used in experiments. The array consists of curved 18m by 60m CMUT membranes that form 8 ring transducer elements with approximately equal areas. The beamforming system uses an IC chip consisting of 8 transimpedance amplifiers and delay elements for receive beamforming with adjustable delays between 2ns and 4ns, that are constant up to 50 MHz with close to unity gain. Transmit focusing is implemented with an FPGA controlled, high voltage pulser board that can generate adjustable electrical pulses with delays as small as 2ns. The system is characterized by measuring the radiation patterns of individual CMUT annular array elements as well as the unfocused and fixed transmit focused arrays. The results show predicted behavior including acoustic crosstalk effects at certain frequencies. For transmit-receive beamforming characterization, a 25m gold wire was imaged using 4 beamformed transmit elements and 4 beamformed receive elements with different delay values. The results show improved lateral resolution and lower side lobes with proper beamforming.
