Main Principles

Ultrasound is the designation for mechanical waves with fluctuation (frequency) over an audible range. Ultrasound is characterized by:
- Intensity, given by pressure amplitude
- Frequency, measured in Hertz (Hz). Audible sound is between 20 and 20,000 Hz. In a typical medical ultrasound, a frequency of about 2-10 MHz is used. In intravascular imaging (imaging from the inside of blood vessels), a frequency of up to 30 MHz is used. 
- Wavelength, hereinafter represented with "lambda"

The speed of sound (c) is an important feature of the medium in which sound propagates. It is 340 m/s in air and about 1500 m/s in water. Tissue is similar to water, and have a speed of sound of around c = 1540 m/s. Since wavelength "lambda" = c/f, the wavelength for medical ultrasound lands between 0.15 mm and 0.75 mm. To compare, the wavelength for audible sound lies in the range of 2 cm to 20 m. Ultrasound of the body can be used for imaging from a distance of up to 20 cm. Within this scale, ultrasound possesses features which remind one more of the features of visible light rather than those typically associated with audible sound:
- The sound waves travel in a straight beam which can be focused and deflected. 
- A sound wave that meets a medium which has significantly different properties from soft tissue will in all cases become fully reflective. Differences in the product density and sound are the determining factors. 
- A big reflection occurs when meeting bone or air. This means that one cannot obtain an image if there is a part of a lung lying in front of the heart. It is also difficult to obtain images of the stomach and intestines as they often contain air. Thus, it is important to have enough ultrasound gel on the probe to secure good contact between the probe and the skin. 

Ultrasound is generated with the help of piezoelectric crystals with electrodes on each side. This is called a transducer or probe (see figure 4). 

Figure 4: Image of an ultrasound probe for cardiology. The cable comes out from the top left and is connected to the ultrasound machine. The front of the probe is about 20 x 15mm and it is divided into a few individual rod elements. In practice, there are between 48 and 128 such elements.

The crystal possesses a feature where it compresses and expands in tact with pressurized electrical voltage. This movement is transferred to the tissue. In comparison, mechanical impact from the outside generates electrical charge. Figure 4: Image of an ultrasound probe for cardiology. The cable comes out from the top left and is connected to the ultrasound machine. The front of the probe measures about 20 x 15 mm and is divided into a few individual rod elements. In pactice, there are between 48 and 128 such elements such that the probe can be used as both the transmitter and receiver through quick changes between transmitting and receiving mode. Imaging is based on puls-echo measurement. It is the same principle used in radar and sonar locating. A short pulse wave is sendt from the probe, transplants through the medium and gives a reflection pulse back to the probe each time it meets a transition to a different tissue. The time it takes from when the pulse wave is sent until the echo is received by the probe gives a measure of distance to the reflecting area.