Echography is about emitting and receiving mechanical waves that we interpret in order to build an image of the environment which was sounded. The principle is simple, but the electronic behind to achieve that goal is more complex.
The central part of the components is the piezoelectric transducer. It’s the element which is able to convert electrical signal into mechanical wave and vice versa. Therefore, it can alternatively play the role of transmitter and receiver.
The electrical signal that we just mentionned above is a high voltage signal, around 100 V. We use a high voltage pulse generator to obtain it and send it to the transducer by order of an Arduino for instance.
The excited transducer will generate high frequency waves, which will be reflected by the different obstacles of their path. The converted electrical signal corresponding to the returned waves is at low voltage. To protect the circuit downstream from the high votage pulse and at the same time allow the low voltage signal to penetrate it, we use a switch. This eletronic component can manage the abrupt changes of high and low voltage signal. It can be intergrated or not into the pulser.
The low voltage and high frequency signal coming from the transducer has to be filtered in order to minimize noise and to be amplified to be able to handle it. That’s the signal conditionning. We use a Time Gain Compensation (TGC) to amplify the returned signal. Actually, the emitted signal is attenuated by the tissues it meets (function of the distance covered). This attenuation is exponentiel. In order to remedy to this phenomenon, we use a component that will give an exponentiel gain, the TGC.
To sample the high frequency signal obtained, we can use a Red Pitaya (acquisition card with ADC) or we can first use an envelop detector and sample the result of it. In fact, the initial frequency is around 5 Mhz. To sample that by respecting the Shannon criterium, we have to use a sampling frequency of at least 15 Mhz, which is important. The Red Pitaya can do it without any problem. Meanwhile, the envelop detector allows us to reduce the frequency and use a less efficient and expensive ADC.
Once the signal has been digitized, it can be treated by a computer to finally build an image.