Читать книгу The Physics and Technology of Diagnostic Ultrasound: A Practitioner's Guide - Robert Gill - Страница 30

We now come to the fundamental concept that underlies diagnostic ultrasound – the pulse echo principle.

Simply put, by carefully measuring the time between the transmission of the transmit pulse and the reception of a given echo, the ultrasound machine can calculate the distance between the probe and the structure that caused that echo. Consider the situation shown in Figure 3.7.

Figure 3.7 Geometry showing the round-path distance travelled by the ultrasound for an echo coming from a reflector at a depth d in the tissues.

The total "round-path" distance travelled by the ultrasound (from the probe to the reflector and then from the reflector back to the probe) is simply (2 × d). The time taken to travel this distance (and therefore the time delay between the transmit pulse and the echo) is calculated as:

Rearranging this equation allows us to calculate the depth of the reflector from the delay time as follows:

Thus we see that there is a simple proportional relationship between the arrival time of an echo (t) and the depth of the structure causing the echo (d).

For the ultrasound machine to use this relationship, it must assume a value for c. It assumes the average propagation speed in soft tissue (1540 m/sec).

As an example, consider a reflector at a depth of 1 cm. Using the above equation gives a delay time of 13 μsec. This is a useful number to remember – for every centimetre of depth the echo delay is 13 μsec. For a reflector at 15 cm depth, for instance, the delay time will be (15 × 13 μsec) = 195 μsec.