Understanding the Pulse Shapes in Medical Ultrasound: Square Waves vs. Sine Waves

When exploring medical ultrasound technology, one common question often arises: What is the shape of the waveform generated from the pulser? Is it a square pulse or closely resembling a sine wave pulse? The answer, as we will discuss, is more nuanced and depends on several factors including the type of pulser, the frequency of the transducer pulse, and the repetition frequency of the ultrasound pulses.

The Types of Pulse Shapes

The pulse shape in medical ultrasound systems can vary depending on the design and specific requirements of the imaging device. Typically, the pulsing waveform can be described as either a square wave or a peak-shaped pulse. As illustrated in [1], there are variations within these basic shapes. These differences are primarily due to the nature of the transducer and the electronic components involved in pulse generation.

[1] Chao C Pertijs MA: Integrated transceivers for medical ultrasound imaging devices: A review. IEEE Open Journal of the Solid-State Circuits Society. 2021 104–114.

Frequency Determination

Understanding the pulse shape is closely tied to the frequency of the transducer pulse, which is influenced by several key factors:

Transducer Element Material: The choice of material for the transducer element is crucial. Different materials have varying sound propagation speeds, which in turn affect the frequency of the pulse. Sound Propagation Speed: The speed at which sound travels through the transducer material also impacts the pulse frequency. Transducer Element Thickness: The thickness of the transducer element is another critical factor that influences the frequency of the generated pulse.

However, it is essential to distinguish between the transducer pulse frequency and the repetition frequency of the ultrasound pulses. The repetition frequency refers to the number of times an ultrasound pulse is emitted per second, which is influenced by the depth of the structures to be visualized.

Repetition Frequency (PRF)

The repetition frequency (PRF) is a vital parameter in medical ultrasound. This frequency indicates how often the system can transmit and receive an ultrasound pulse. A lower PRF is often chosen for deeper structures, as it allows more time for the ultrasound pulse to reach the target and return to the transducer.

During the imaging process, the ultrasound transducer acts both as a transmitter and a receiver. After each pulse has been generated, the system must wait for the returning signal. This waiting period is crucial for accurate imaging.

Multielement Transducers

Most modern medical ultrasound machines utilize multielement transducers, which can generate pulses in multiple directions. By changing the timing between electronic pulses, the transducer can steer and focus the ultrasound beam. This capability is essential for acquiring detailed images of complex internal structures.

When an ultrasound pulse returns to the transducer, it generates an electrical signal that is recorded as a voltage. The electronics that filter and process these signals are referred to as beam formers. These beam formers reconstruct the multiple received ultrasound pulses into a coherent image, representing the view of the tissue as the ultrasound pulses interact with it.

Conclusion

In conclusion, the shape of the waveform generated in medical ultrasound is a result of multiple factors, including the transducer material, sound propagation speed, and pulse repetition frequency. Understanding these factors is crucial for optimizing the performance of ultrasound imaging devices and ensuring accurate and reliable medical imaging.

[1]: Chao C Pertijs MA: Integrated transceivers for medical ultrasound imaging devices: A review. IEEE Open Journal of the Solid-State Circuits Society. 2021 104–114.