LED-based photoacoustic imaging system: why it achieves the same signal to noise ratio as solid-state-laser-based system: a review

Photoacoustic imaging is expected to be a next-generation diagnostic modality. However, systems using a solid state laser (SSL) are expensive, large in size and poor in operability of probes. In addition, protective goggles are required because of laser light. Therefore, we have adopted the LED technology and improved the signal-to-noise-ratio (SNR) of the LED-based system, which had been 1 / 2.3 million of the SSL, to the same level with four innovative technologies. These innovative technologies include a) High power and high density LED array technology: Use of high power LED chips with luminous efficiency comparable to laser diode, high density mounting of LED chips on Aluminum base and compact design, b) Giant and ultra-short-pulse drive circuit technology: High speed on-and-off by low-resistance Metal Oxide Semiconductor Field Effect Transistor (MOSFET), and position separation of high-voltage drive circuit from ultrasonic probe (USP) by series connection of LEDs, c) Optical pulse generation technology optimum for frequency response characteristic of USP, d) Noise reduction technology for faint signals using ultra-amplification: Minimization of quantization noise of Analog-digital-converter (ADC) by wide band ultra-amplification of 86 dB, and noise reduction by averaging of <100 times. Using these technologies, we have developed an LED-based photoacoustic imaging system. To use the system, we have discovered the mechanism of the absorption of pulsed light converted into photoacoustic signal detection is a "linear system" by frequency response characteristic analysis using an ideal point source phantom, and clarified the ultra-amplification over 80 dB and the SNR over 4 are required for real-time imaging using a biological phantom. Furthermore human in-vivo real-time functional imaging using dual-wavelength of both 820nm and 940nm has showed that the LED-based system can be used clinically.