Compact GHz Ultrasonic Micro-Imager for Cells and Tissues

Optical microscopy is a powerful tool for studying cells and tissues, and it is arguably the most well- known type of microscopy due to its ubiquity in labs and schools. Ultrasonic imaging, while not as common as optical microscopy, has also been a valuable imaging modality for visualizing tissues and organs in real-time and in a non-invasive way. The majority of medical diagnostic processes use ultrasonic pulses in the 3–10 MHz range, which typically have a lateral resolution of 0.15 to 5 mm [1, 2]. However, these devices are bulky systems that limit their mobility and are extremely expensive. Here, we report on a compact, portable ultrasonic imager that can operate in the GHz frequency range with a spatial resolution of 50 μm and can show real-time images at a frame rate of up to 20 fps. We demonstrate the scanning of chicken tissues, showing contrast between fat and muscle cells, as well as the visualization of individual onion cells. Using ultrasound instead of light also allows for imaging in the dark and provides additional information about the mechanical properties of tissues, in addition to the visualization aspect. The imaging system, which, to our knowledge, is the first “ultrasonic camera” of its kind, consists of an array of 128x128, 50x50μm Aluminum Nitride (AlN) transducers. The necessary circuitry for transmit and receive for each transducer is integrated within the pixel using a 130 nm CMOS process. The overall packaging of the GHz ultrasonic imager is compact and handheld, with a silicon surface as the sensing region (Figure 1A). More details on the circuitry and fabrication of the GHz imager, with another application of imaging nematodes, are described in our previous works [3,4]. Ultrasonic pulses generated by the transmitting pixels are transmitted and reflected off the backside of the silicon. The silicon layer opposite to the CMOS/transducer side becomes the imaging surface where samples can be placed (Figure 1B). Thin slices of approximately 100 to 200 μm thick tissues, consisting of fat and muscles, were extracted from chicken thighs (commercially purchased from a supermarket) using a biopsy needle. The sample was placed on the silicon surface for imaging. To demonstrate the ability of the imager to visualize individual cells, a thin membrane of onion cells was also imaged. A linear stage was used to gently press the layer for better contact of the cells with the chip. The transducer was operated at a frequency of 1.853 GHz, and the imaging was performed at an acquisition rate of 6.5 fps. The background air echo was subtracted from the acquired image to reduce baseline noise, resulting in each scan being faster than 160ms. The ultrasonic image of the biopsy slice shows areas with higher reflected ultrasonic echoes separating fat tissues from muscles (Figure 2A). As fat cells have lower acoustic impedance than muscles, they absorb less ultrasound, causing higher reflected echo magnitude to be received by the transducers [5]. The ultrasonic images of onion cells also reveal clear contrast between the cell walls and individual cells, which appear as compartments (Figure 2B). Similarly, the length and width of individual cells extracted from ultrasonic images (n=18) were 338 ± 33 μm and 85 ± 12 μm, respectively. For the same cells, the dimensions obtained from optical images were 324 ± 21 μm and 74 ± 6 μm (paired t-test p-value > 0.05 for optical vs. ultrasonic), which shows minimal variation in spatial data measurements between the two imaging modalities. A) The imager unit with the silicon surface as the sensing region. B) Schematic showing pressing of thin sample (tissue slice, cell layers) against the silicon surface for ultrasonic imaging. A) Ultrasonic image of tissue slice consisting of fat and muscle layers. The fat layers have greater reflected echo compared to muscle cells due to lower ultrasound absorption. B) Ultrasonic and optical image of individual onion cells.