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In this Article - Discover the technology and principles behind ultrasound machines. Learn how these devices use sound waves to create detailed images of the body.
Introduction -
Ultrasound machines are devices that use sound waves to create images of the inside of the body. Ultrasound machines are useful for diagnosing and monitoring various medical conditions. Diagnostic ultrasound is based on the pulse-echo principle: the ultrasound machine sends short pulses of sound waves into the area to be examined, and then receives and displays the echo signals that bounce back from different structures. The probe or transducer is a key part of the ultrasound machine. It converts electrical energy into sound energy and vice versa. The ultrasound machine processes the signals and displays the image on a screen.
A scan with an ultrasound machine involves sending out a series of sound wave pulses. We will go over some common calculations related to the pulse of a sound wave. Some ultrasound machines let you change some of the settings we will talk about, but others don’t. We think it’s important for you to know the basics of these settings.
Spatial Pulse Length (SPL) - Spatial pulse length is a term used in ultrasound imaging to describe how long a pulse of sound waves is in space. It is calculated by multiplying the number of cycles in a pulse by the wavelength of the sound waves. The wavelength is the distance between two peaks or troughs of a wave. The number of cycles is how many times the wave goes up and down in a pulse. A shorter spatial pulse length means a better image quality because it can distinguish between two objects that are close together. A longer spatial pulse length means a worse image quality because it can blur the objects together.
Pulse Repetition Frequency (PRF) - Pulse Repetition Frequency is how many times a sound wave pulse is sent and received by an ultrasound machine in one second. It is measured in hertz (Hz). PRF affects how good and fast the ultrasound image is. For example, a higher PRF can make the aliasing artifact less, which is when the colour or spectral Doppler shows the wrong way or speed of blood flow. A lower PRF can make the twinkling artifact more, which is when the ultrasound shows a bright spot behind a stone or a calcification. PRF also changes with the depth of the area to be examined and the number of lines per frame.
Listening Time - Listening time is how long the ultrasound machine waits for the sound wave pulse to come back after sending it. It is the time when the ultrasound machine is not sending any sound waves and is listening for the echoes from the area to be examined. Listening time changes with the depth of the area, because the sound waves take more time to go deeper and come back. The sonographer can change the listening time by changing the depth setting on the ultrasound machine. Listening time affects how many times the ultrasound machine sends and receives a sound wave pulse in one second. This is called the pulse repetition frequency.
Duty Factor - Duty Factor is how long the ultrasound machine is on and sending sound waves in a scan. It is the fraction of the pulse duration and the pulse repetition period. Pulse duration is the length of one sound wave pulse. The pulse repetition period is the time between one pulse and the next. The duty Factor is a percentage. For example, if the pulse duration is 5 microseconds and the pulse repetition period is 250 microseconds, the Duty Factor is 5/250 = 0.02 or 2%. This means the ultrasound machine is on for 2% of the time and off for 98% of the time.
The duty factor range in ultrasound is usually between 0.1% and 1%, which means that the ultrasound machine is mostly listening for echoes during a scan.