How it Works
The ultrasound image is created by first transmitting sound waves into the body and then interpreting the intensity of the reflected echoes. This is achieved using a hand held probe which contacts the body via a water based gel. The data collected is then processed within the body of the scanner and displayed as a black and white image generally referred to as grey scale.
The physics and the technology involved in ultrasound imaging has a profound effect on how structures appear. The dynamic nature of ultrasound scanning makes understanding of the processes and physics of musculoskeletal ultrasound essential.
The probe contains a large number of transmitters (crystals) set in a line along its length. Typically up to five of these firing simultaneously generate a short pulse of ultrasound that travels in a narrow column away from the probe. The transmitters then act as receivers and record the intensity of the reflected sound. The process is repeated sequentially along the length of the probe. The time taken for an echo to return is used determine the distance from the probe and is calculated assuming that sound has a constant speed (1540m/s). The strength of the echoes returning from any point is represented by the brightness of that point on the screen.
The path that a single pulse passes along is described as the beam. The width of the beam determines the lateral resolution. The length of the pulse determines the axial resolution. Shorter pulses can be achieved using higher frequency, so the highest frequency practicable is generally used.
Different Types of Reflection
Two distinct patterns of reflection give rise to the echoes that make up an ultrasound image – specular reflection and scattering.
Specular reflection is responsible for the bright appearance of fibrous structures such as tendons and of boundaries between different tissues. It occurs when the sound wave meets a distinct surface (significantly larger than the wavelength of the ultrasound). The process that occurs is similar to when light passes from air to water on the surface of a lake. Some of the light travels in to the water, while some is reflected back. The amount of sound that is reflected at the boundary between two different tissues, such as fat and muscle, depends on how marked the difference is in their acoustic properties. Acoustic impedance, which is the measure of this, varies with the density and compressibility of the tissue.
Scattering occurs when an interface is equivalent to 1 wavelength in size, which leads to the reflected echoes being scattered in many directions. When the interface is smaller than the wavelength, then echoes scatter in all directions equally, these are known as Rayleigh scatterers. This can be seen in Figure 3, termed diffuse reflection.
Features of an Ultrasound Image
Recognising structures on ultrasound takes practice and a knowledge of the anatomy does help!. What follows is a brief description of some of the features that make up the image.
In almost all applications the top of the screen represents the probe and as you look further down the screen you are seeing progressively deeper tissues, starting with skin.
Typical Appearance of Normal tissue
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