Читать книгу The Physics and Technology of Diagnostic Ultrasound: A Practitioner's Guide - Robert Gill - Страница 28

When the ultrasound machine sends a pulse into the body, it travels along a defined pathway (referred to as the ultrasound "beam") and the echoes return to the probe along the same beam, as shown in Figure 3.6. The echoes therefore provide information to the machine about the tissues and structures that are within the beam.

Figure 3.6 The transmit pulse travels into the tissue along a well-defined path or "beam" and the echoes return to the probe along the same path. Thus the echoes provide information about the tissues lying along this path.

In order to build up an image, the machine must repeat the process of transmitting a pulse and receiving the echoes from the body numerous times, moving the beam so that it passes through a different area of tissue each time. (The movement of the beam is referred to as "scanning"; it will be discussed in more detail in chapter 4.) Thus the machine must produce a series of transmit pulses – usually 100 or more – to produce a single image.

Furthermore, the ultrasound machine must create each image in a fraction of a second so that a rapid sequence of images can be created and displayed in the form of a "real-time" (i.e. movie-like) image.

Clearly this means that the machine needs to send out transmit pulses as rapidly as possible. For example, if each image takes 100 transmit pulses to produce and we require an imaging rate of 20 frames per second (i.e. 20 images per second), it is easy to see that the machine must transmit a total of 2000 pulses each second.

The term used to describe the number of transmit pulses each second is the "Pulse Repetition Frequency" (abbreviated PRF).

For the machine's electronics, transmitting 2000 pulses per second is no problem. However, we will see in the next section that there is an important consideration (namely the depth of penetration of the ultrasound) that limits the number of pulses that can be transmitted each second.