Analog Cancellation of Unwanted Reflections for Enhanced Ultrasound Microscopy

Our scanning acoustic microscope uses coded excitation. The excitation signal consists of linear chirps of $1\ \mu\mathbf{s}$ duration. Parts of this signal are reflected from the internal structures of the lens, which causes high signal amplitudes in the recorded signal. The signals from the lens reflections can be 5 to 10 times larger than the signal from the sample. As coded excitation utilizes long modulated signals, the signals originating from the lens reflection will overlap with signals originating from the sample. The frequency encoding combined with the pulse compression method enables the reconstruction of the time resolution and allows the separation of the reflections. Pulse compression works properly only if the full duration and the full amplitude of the signal is captured. Therefore, the amplifier and the gain of the analog-to-digital converter (ADC) resolution needs to be adjusted to the amplitude of the strongest reflection present, i.e. the lens echoes. This results in quantization of the small-amplitude echoes from the sample. To overcome these limitations, we implemented an analog signal cancellation method. The signal of the lens reflections was first recorded. During measurements, the signal containing lens reflections was inverted, then reproduced by an arbitrary waveform generator, and finally combined with an analog signal combiner to the measurement signal from the microscope. The maximum signal amplitude was reduced by 80 %, which made it possible to change from the 1000 mV-range to the 200 mV-range of the ADC, thus improving the dynamic range. This reduced the quantisation noise and led to a 3 times lower noise-level in the image.