A flexible annular-array imaging platform for micro-ultrasound

Abstract

Micro-ultrasound is an invaluable imaging tool for many clinical and preclinical applications requiring high resolution (approximately several tens of micrometers). Imaging systems for micro-ultrasound, including single-element imaging systems and linear-array imaging systems, have been developed extensively in recent years. Single-element systems are cheaper, but linear-array systems give much better image quality at a higher expense. Annular-array-based systems provide a third alternative, striking a balance between image quality and expense. This paper presents the development of a novel programmable and real-time annular-array imaging platform for micro-ultrasound. It supports multi-channel dynamic beamforming techniques for large-depth-of-field imaging. The major image processing algorithms were achieved by a novel field-programmable gate array technology for high speed and flexibility. Real-time imaging was achieved by fast processing algorithms and high-speed data transfer interface. The platform utilizes a printed circuit board scheme incorporating state-of-the-art electronics for compactness and cost effectiveness. Extensive tests including hardware, algorithms, wire phantom, and tissue mimicking phantom measurements were conducted to demonstrate good performance of the platform. The calculated contrast-to-noise ratio (CNR) of the tissue phantom measurements were higher than 1.2 in the range of 3.8 to 8.7 mm imaging depth. The platform supported more than 25 images per second for real-time image acquisition. The depth-of-field had about 2.5-fold improvement compared to single-element transducer imaging.

Fig. 1

The block diagram of the real-time annular-array imaging platform for micro-ultrasound.

Fig. 2

Principle of the (a) transmit beamformer and (b) receive beamformer in the proposed annular-array imaging platform.

Fig. 3

The simulated fractional delay waveform. The solid black line is the original ultrasonic echo signal with 35 MHz center frequency and 200 MHz sampling frequency. It was delayed by 9 times with steps of 0.1× the sampling interval.

Fig. 4

The algorithms implemented in the field-programmable gate array (FPGA) for real-time annular-array imaging.

Fig. 5

Block diagram of the digital beamformer in the field-programmable gate array (FPGA) for real-time annular-array imaging.

Fig. 6

Annular-array transducer with 8 elements using an interdigital bonded 1–3 composite.

Fig. 7

Photograph of prototype for annular-array imaging platform. (a) 8-channel pulse generator based on electronics and eight-layer printed circuit board (PCB), (b) 8-channel digital imaging receiver based on electronics and 10-layer PCB.

Fig. 8

Gain measurements in the 8-channel imaging receiver.

Fig. 9

Comparison of wire phantom images using (a) single-element transducer imaging and (b) annular-array imaging.

Fig. 10

Comparison of (a) axial resolution and (b) lateral resolution between single-element imaging and annular-array imaging.

Fig. 11

Tissue-mimicking phantom imaging using different systems. (a) Phantom image using single-element transducer and one channel electronics in the annular-array platform, (b) the same phantom image using Vevo 770 40-MHz transducer, (c) beamforming at 5-mm focal point using the annular-array platform, (d) beamforming at 7-mm focal point using the annular-array platform, and (e) phantom image with 4 focal zones using the annular-array platform.