1.3 Creating Images with Ultrasound
Ultrasound imaging is based on the principle of wave reflection, which has several applications. Sonar technology is used to measure the depth of water or detect flaws in materials. Radar technology is used to control air traffic. Numerous animals such as bats, dolphins and whales use ultrasound for the purpose of orientation and to catch their prey.
To generate an image it is necessary to send as well as receive an ultrasound wave.
This is achieved by piezoelectric crystals. Piezoelectricity is the ability of certain materials (such as ceramics) to generate electricity in response to strain or pressure. When an ultrasound wave hits a piezoelectric crystal, the pressure of the wave sparks an electrical impulse (the receiving function). This impulse can be used to generate an image. Conversely, when piezoelectric crystals are "connected to an alternating electric current" they deform, resonate and thereby emit ultrasound waves (the sending function). Thus piezoelectric crystals are used to send as well as receive ultrasound waves in medical sonography.
Medical Ultrasound
In echocardiography we use frequencies between 2 and 10 MHz.
The frequency of the ultrasound wave is determined by the frequency of the alternating current. Imaging frequency is an important factor because it determines the penetration (ability to pass through tissue) and resolution of the image. Modern transducers allow us to change the imaging frequency (within a certain range).
| Definitions | |
|---|---|
| Resolution | A measure of the extent of detail on an image |
| Penetration depth | A measure of how deeply the ultrasound wave is able to penetrate a certain material |
Ultrasound waves can be emitted as rapid repetitive pulses. Both, pulse duration and the pulse repetition period or frequency are important characteristics. They influence frame rate and image resolution. Higher ultrasound frequencies permit shorter pulses and a shorter pulse repetition period. This leads to higher frame rates and higher image resolution.
Ultrasound Pulse
The higher the US frequency, the shorter is the pulse duration. This permits a shorter pulse repetition frequency.
The higher the pulse repetition frequency, the higher is the frame rate and the greater the image resolution.