Ultrasound imaging is based on pulse-echo method. The ultrasound transducer transmits an ultrasound pulse and the switch to listen mode, recording the echo reaches the transducer surface. The echoes from targets close to the transducer will return firstly, and later for the echoes from further targets. Since the ultrasound travels inside homogeneous medium at a constant velocity and along a straight line, and thus the distance can be easily calculated. Most targets are moving and thus the pulse has to be repeated in certain frequency to track the target.
In human tissue or room temperature, the sound velocity is around 1540m/s or 1.54mm/ms. Assuming the pulse is transmitted at time 0, and at time T, the echo from target arrives, then the distance from transducer surface to target D is calculated:
For example, if it takes 10ms to receive the echo, then the distance from the target to transducer surface is about 7.7mm in water.
When transmits an ultrasound pulse, the pulse has a time duration. If the pulse center frequency is 1MHz, then a single cycle of the carrier wave is 1ms. The pulse transmitted has to include at least one cycle since it is an alternative signal or energy. At certain frequency and amplitude, the longer the time duration of the pulse, the more energy of it, and thus it can transmit further before it die out due to the attenuation. If the pulse is too long, the echoes from targets that are close to each other may merged together. In this way, a short pulse is preferred to distinguish close target. It is obvious that with the same cycle number, higher frequency pulse will result in shorter pulse, and can detected finer target.
As mentioned above, pulse need to be repeatedly transmitted to track targets if it is moving. However, the next pulse can be sent out only when the echo from the furthest target has returned. Otherwise, the echo from the far target of the previous pulse and the echo from the near target of the recent pulse will come the same time and true target location cannot be determined. So if the maximal detection depth is D, and sound velocity is c, then the minimal time interval between two pulse is :
This time interval is also called pulse repetition period, and according f = 1/T is called pulse repetition frequency (PRF). The higher the PRF, the lower the maximal detection depth, and the faster the detectable moving target.
As soon as the ultrasound pulse energy leaves a piston transducer surface and moves forward, it starts to spreads in all the directions, but the main energy will confine in a disk shape when it is very close to the transducer surface. The diameter of the disk is the same as the transducer surface and the thickness of the disk is sound velocity multiply pulse duration. This disk spreads as it moving forward, and eventually will become a dome shape.
The definition of duty factor is the pulse duration over the pulse repetition period, or the pulse duration multiply PRF. For imaging ultrasound, energy duty factor is very low, and thus have very less averaged power.