Intravascular ultrasound (IVUS) imaging is by far the most favorable imaging modality for coronary artery evaluation. IVUS transducer design and fabrication, a key technology for intravascular ultrasound imaging, has a significant impact on the performance of the imaging results. Herein, a 35-MHz side-looking IVUS transducer probe was developed. With a small aperture of 0.40 mm × 0.40 mm, the transducer exhibited a very wide -6 dB bandwidth of 85% and a very low insertion loss of -12 dB. Further, the in vitro IVUS imaging of a porcine coronary artery was performed to clearly display the vessel wall structure while the corresponding color-coded graph was constructed successfully to distinguish necrotic core and fibrous plaque via image processing. The results demonstrated that the imaging performance of the optimized design transducer performs favorably

Jiang, Lixin

Li, Xiaobing

Li, Xinlun

Tian, Junting

Wang, Feifei

Wu, Zheng

Zhao, Xiangyong

Zhou, Jiewen

English

Electronics Science Technology and Application (E-Journal)

-46- Electronics Science Technology and ApplicationPreparation and Imaging of Intravascular High-FrequencyTransducerXinlun Li1, Xiaobing Li1,*, Zheng Wu2, Feifei Wang2, Lixin Jiang3, JuntingTian1, Jiewen Zhou1, Xiangyong Zhao21. School of Medical Instrument and Food Engineering, University ofShanghai for Science and Technology, Shanghai 200093, China.2. Key Laboratory of Optoelectronic Material and Device, Department ofPhysics, Shanghai Normal University, Shanghai 200234, China.3. Ultrasonography Department. Shanghai Renji Hospital, School ofMedicine, Shanghai Jiao Tong University, Shanghai 200025, China.*Authors to whom correspondence should be addressedAbstract: Intravascular ultrasound (IVUS) imaging is by far the most favorable imaging modality forcoronary artery evaluation. IVUS transducer design and fabrication, a key technology for intravascularultrasound imaging, has a significant impact on the performance of the imaging results. Herein, a 35-MHzside-looking IVUS transducer probe was developed. With a small aperture of 0.40 mm × 0.40 mm, thetransducer exhibited a very wide -6 dB bandwidth of 85% and a very low insertion loss of -12 dB. Further,the in vitro IVUS imaging of a porcine coronary artery was performed to clearly display the vessel wallstructure while the corresponding color-coded graph was constructed successfully to distinguish necroticcore and fibrous plaque via image processing. The results demonstrated that the imaging performance ofthe optimized design transducer performs favorably.Keywords: High-Frequency Ultrasound Transducer; Intravascular Ultrasound Imaging1. IntroductionCardiovascular disease is the number one cause of death worldwide. According to data provided by theWorld Health Organization (WHO), in 2016, approximately 17.9 million people died from cardiovasculardisease, accounting for 31% of global deaths. [1] It has been shown that atherosclerosis is an importantcause of coronary syndromes. [2] Currently, the gold standard for the diagnosis of atherosclerosis is invasivecoronary angiography (CAG). [3] But the CAG technique is limited to a rough assessment of luminalstenosis and does not provide information on the characteristics of the vessel wall and plaque. [4-5] Incontrast, intravascular ultrasound (IVUS) can effectively enable early prediction, diagnosis, intervention,and management of vulnerable plaques and is so far the most favorable imaging modality for coronaryVolume 9 Issue 4 -47-artery assessment. [6-7]As the core component of IVUS, the intravascular ultrasound transducer is a high-grade medicalultrasound transducer, and many domestic and foreign medical device companies have invested their effortsin research and development. Current commercial IVUS systems have a spatial resolution of 70-200 µm inaxial direction and 200-400 µm in lateral direction, which are insufficient for accurate diagnosis. [8-10] Therehave been many studies reported on the fabrication of IVUS high-frequency transducers. Li et al. [11]fabricated an 80 MHz IVUS transducer using a 30 µm-thick PMN-PT free-standing film that illustrated a-6dB bandwidth of 65%, and axial and lateral resolutions of 35 µm and 176 µm, respectively. Ma et al [12]used a 3-layer matching layer to increase the bandwidth of the transducer, which had a center frequency of45 MHz and achieved a -6dB bandwidth of 61%. Zhu et al. developed a 52-MHz transducer with a BW of61.5% using a Li-doped potassium sodium niobate (K,Na)NbO3 (KNN) thick film. [13] However, thepreparation process of some composite materials is more complicated and difficult to commercialize. [14-15]Although increasing the frequency of the transducer can improve the resolution, it will have a poorpenetration depth. It has become an urgent need to improve the overall performance of the transducer bybalancing the frequency and resolution of the transducer.In this paper, a center frequency 35-MHz ultrasound transducer is designed, and an optimizedprepared technique is used to improve the performance of the transducer. And a data acquisition platform isdesigned based on the transducer measurement. The transducer resolution performance was evaluated bywire phantom imaging. Finally, the transducer was tested with an ex vivo imaging of a porcine arterialvessel.2.Experimental2.1Design and Fabrication of High-frequency TransducerThe aperture of the transducer was minimized to better fit the IVUS catheter. According to the KLMmodel, the high-frequency transducer with two acoustic matching layers and one backing layer wasdesigned [16]. The materials and dimensions of matching layers and backing layer were optimized to providethe desired BW and sensitivity of the transducer. For the first matching layer, the acoustic impedance of Z1was controlled at 10.3 MRayls by mixing ZrO2 particles with epoxy (EPOTEK 301, Epoxy Technology Inc.Billerica, MA). The parylene (Z2=2.5MRayls) served as the second matching layer. The acoustic impedanceof the two matching layers gradually increases the external energy projection between the piezoelectricelement and the transmission medium. The PZT sample of 0.40 mm × 0.40 mm × 0.05 mm was embeddedin a steel tube with an inner diameter of 0.8 mm with a backing layer made of E-solder 3022 (Von RollIsola, New Haven, CT). The acoustic impedance of the backing layer (Zbacking) was 6.5 MRayls after curedat 60 oC for 5 h. Finally, the acoustic stack was carefully connected to a coaxial cable, and the resultanttransducer was poled under a DC electric field of 1.1 kV/mm for 30 min at room temperature.2.2 Performance Evaluation of High-frequency TransducerThe transducer performance was investigated using the pulse-echo method. A platform was designedand developed for IVUS transducer, which allowed for precise control of displacements along the x, y, andz directions and the rotation on the z-axis. The electric circuitry with a computer interface was designed-48- Electronics Science Technology and Applicationbased on the LabVIEW program (National Instruments, Austin, TX) to acquire ultrasound pulse-echosignals.A smooth steel block was used to reflect the ultrasound wave in deionized water. The distance betweenthe transducer and steel surface was 2 mm. A pulser/receiver (DPR500, JSR Ultrasonics, a division ofimaging Inc, Pittsford, NY, USA) with 5-kHz repetition rate and 50-Ω damping was used to drive thetransducer. The excitation voltage was set to ~100MV in a low-pass mode. Finally, the echo signals werereceived by a 100-MHz oscilloscope (Dsox1102a, Key Technology, United States), and the frequencyspectrum was obtained using the Fourier transformation. The IL of the transducer was calculated from theratio of the received echo and the transmitted pulse, using 2.2×10-4 dB/mm MHz2 to compensate theattenuation in the water [17]. The point spread function of the line model was used to evaluate the spatialresolution. A-line scan for a 6 µm-diameter tungsten wire was performed to determine the axial and lateralresolutions of the transducer.2.3 Signal Processing and ImagingThe data acquisition and signal analysis processes for IVUS imaging are shown in Fig. 1. Thetransducer emitted ultrasound waves and received echo signals during the rotation. The echo signals weresampled and quantified by the data acquisition card and then transmitted to the computer. In the subsequentsignal processing, the digital filtering process was performed using band-pass filtering to removeinterference noises, followed by amplifying and compensating echo signals. Then, the signals weredemodulated using the Hilbert transformation. Finally, the rectangular coordinates were transformed topolar coordinates to synthesize the cross-sectional images of blood vessels.Fig. 1 Schematic of signal acquisition and imaging.The signal acquisition device and platform are depicted in Fig. 2. The setup comprises the transducer,movable fixture, rotation setup, and electric circuits. The IVUS transducer was fixed at the front of therotating shaft, and the electrical signals were transmitted through a slip ring to the computer when thetransducer rotated along with a guidewire. The vessel sample was immersed in water and fixed at themovable fixture, which could shift in the x, y, and z directions independently to adjust the position precisely.Then, the IVUS transducer probe was placed into the intracavity vessel to conduct imaging experimentsVolume 9 Issue 4 -49-including the simultaneous processes of mechanical rotation, signal transmission and collection. Theradio-frequency data were digitized using a 12-bit data acquisition board (Gage Applied Technologies,Lockport, IL). Afterward, a series of signal processing was performed with the computer to produce atwo-dimensional (2D) grayscale image. Based on the grayscale images, the plaques were analyzed usingthe pattern recognition method [18]. The region of interest was detected and the pixels were classified usingfeature extraction followed by color-coding.Fig. 2 Setup for ultrasound signal collection and IVUS imaging.3. Results and analysis3.1 Performance measurement results of IVUS TransducerThe minimum size of the IVUS catheter for current medical use is 0.87 mm, and 0.8 mm was set asthe standard size in this experiment. Fig. 3(a) shows a high-frequency IVUS transducer probe. As thetransducer was placed at the tip of a steel needle with an inner diameter of 0.8 mm to ensure the stablemechanical rotation in a coronary vessel for ultrasound imaging, the piezoelectric element must be as smallas 0.4 mm × 0.4 mm, as depicted in Fig. 3(b). The diameter of the ultrasound beam is narrow enough toobtain a high lateral resolution.-50- Electronics Science Technology and ApplicationFig. 3 photos of (a) the developed IVUS catheter and (b) transducer probe.Fig. 4 displays the pulse-echo responses of the transducer. The frequency spectrum of the echo wasanalyzed using the Fourier transformation. The center frequency fc at -6 dB and the fractional BW weredetermined using the following (2) and (3) [18]:�� =��+��2(1)BW= ��−����× 100% (2)where fl and fu are the lower and upper frequencies at -6 dB. The center frequency of the transducerwas 37 MHz, and the -6 dB BW was 85%. The IL was -12 dB. The detailed performances of thehigh-frequency IVUS transducer are listed in Table 1 [17, 19, 20]. It was found that the transducer proposed inthis work exhibited much larger BW and higher sensitivity to achieve IVUS imaging, demonstrating that theimproved structure is a promising way for IVUS to obtain critical fine morphological features of someplaques.Fig. 4 Pulse-echo response of the BCHT ceramic transducer.Table 1 Performance of high-frequency ultrasound transducers and other reported intravascularultrasounds.Volume 9 Issue 4 -51-Material fc/MHz-6 dBBWIL/dBAxialresolution/μmLateralresolution/μmPZT -5H (this work) 35 85% -12 40 250PZT-5H [18] 40 50% -26 38 400PMN-PT [20] 35 54% - 34.5 392PMN-PT 1-3 composite [21] 34 72% 92 1353.2 Signal processing and imaging results of IVUS TransducerThe spatial resolution of ultrasound imaging is defined as the minimum distance between two adjacentfeatures that can be distinguished, which is a crucial factor for evaluating the capability of IVUS imaging.The high spatial resolution corresponds to the small distinguishable distance. The spatial resolution isdivided into axial and lateral resolutions, which can be estimated as follows [21]:axial 2 2ccRf BW BW  (3)#lateral #ccFR Ff  (4)where c is the speed of sound, F# is the ratio of the focal length to the aperture size of the ultrasoundtransducer, and λ is the wavelength of the ultrasound wave. Fig. 5 demonstrates the spatial resolutions of 40μm in the axial direction and 250 μm in the lateral direction. In the diagnosis of coronary therosclerosis, thethickness of the thin fibrous cap is considered as a reliable indicator [22]. Atherosclerotic plaques arevulnerable to rupture when the thickness of the fibrous cap is less than 65 μm. The IVUS transducer ispromising to detect such thin fibrous caps.In general, a vessel wall primarily consists of three layers: the inner tunica intima, muscular tunicamedia, and outer tunica adventitia. IVUS imaging is capable of providing morphological information suchas lumen area, size, distribution, composition, and so on. Typical B-mode imaging of a cross-sectionalporcine arterial vessel was acquired to verify the practical imaging capability of the lead-free IVUStransducer probe (Fig. 6(a)). With the image processing [17], the color-coded graph was obtained as shownin Fig. 6(b). The white arrow points to the high echo region and the reflection interface are concentrated,while the yellow arrow points to the normal region for low echo. With clearer boundary of blood vessels athigh resolution, the transducer sufficiently meet the needs for IVUS imaging in clinical diagnosis.-52- Electronics Science Technology and ApplicationFig. 5 (a) 6-μm tungsten wire phantom image acquired by the IVUS transducer, and the corresponding (b)axial resolution and (c) lateral resolution of the transducer.Fig. 6 (a) In vitro IVUS image of an in vitro porcine artery vessel, I = intima; M = media; A = adventitia.And (b) its color-coded image after feature extraction.4. ConclusionIVUS as a device to diagnose atherosclerotic plaques has been rapidly developed in clinicalapplications due to its safety and efficiency. However, the resolution of IVUS images currently used inclinical practice is low, and the acquisition of fine morphological features of some plaques is limited. Thispaper considers the requirements for the size and imaging resolution of the transducer for coronary imaging.The 35-MHz miniature high-frequency transducer with an effective aperture size of 0.4 mm × 0.4 mm wasdesigned and fabricated, which exhibited excellent performance on both BW (85%) and IL (-12 dB). TheIVUS imaging for a porcine artery was acquired by the developed lead-free transducer with a high axialresolution of 40 μm and a lateral resolution of 250 μm. The results show the improvement of intravascularimaging by optimizing the performance of the intravascular transducer.Volume 9 Issue 4 -53-References[1] World Health Organization. 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Preparation and imaging of intravascular high-frequency transducer

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Preparation and imaging of intravascular high-frequency transducer

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