Ultrasound Scanning has become a very common and reliable method of diagnosing health conditions in patients. This method is considered safer than x-rays. Therefore it is commonly used for pregnancy scanning and scanning most of the parts of the human body. The basic technology used in Ultrasound Scanning is similar to the technique used by bats, ships, and fishermen for communication. The fundamental idea is based on the fact that a sound wave, when hit on an object, bounces back or reverberates. An image is created based on the intensity of the returning echo. The calculation is based on the time taken by the wave to return by measuring the distance, mass, nature, and stability of the object hit.
The best aspect of ultrasound scan is that there is no hazardous ionizing radiation exposure, and it is a real-time use. The scanned image can be seen on a computer screen while scanning is done. Ultrasound scanning is usually done to either examine the internal organs or blood vessels. The images can be 3-D or 4-D, wherein the 3-D image will be in motion. Doppler Ultrasound assesses the blood flowing through the arteries and veins in the arms, legs, neck, and abdomen. The three types of Doppler scans are Color Doppler, Power Doppler, and Spectral Doppler scans (General Ultrasound Imaging. 2008.)
How it works
The ultrasound machine has a handheld probe used on the body where scanning has to be done. This probe sends and receives the high-frequency sound waves over a water-based gel, which is applied to the skin. The probe contains many transmitters set lengthwise in a straight line. A short pulse of ultrasound is produced when five of the transmitters fire concurrently. This short pulse moves away from the probe in a thin line. The same transmitters then receive the echo and measure the same. The process is repeated along the length of the probe. The distance is measured based on the time taken for the echo assuming sound travels at a constant velocity of 1540 m/s. This length of the echo is represented by the brightness on the screen display. A single pulse’s path is considered a beam. The thickness of the beam decides the lateral resolution.
The duration of the pulse determines the axial resolution. Usually, the highest frequency pulses are used for shorter pulses and faster responses. The bright appearance of leathery composition such as tendons and of boundaries between different layers of tissues becomes possible due to Specular reflection. This happens due to the difference in the surface met by the sound wave. The difference depends on the thickness and elasticity of the tissue. Scattering is responsible for the texture in the image of each layer (Physics, instrumentation, and basic technique).
The echo can be a reflection or refraction depending on the surface where the sound wave hits before echoing. To be more technical, the probe is called a transducer. It uses a stream of piezoelectric components (called an aperture) to transmit a sound pulse into the body and to receive the echoes returning from scattering structures within. There’ll be a time delay in returning echo due to the distance traveled and/or density of the surface contacted. The returning stream is used to form 2-D or 3-D images, which are used for analysis. The method of course-plotting and focusing these audio pulses is known as beamforming.
There are 3 Cartesian coordinate systems in ultrasound, which helps in differentiating very close scatterers. The axial or range coordinate determines the direction of the sound wave produced. The axial and the lateral dimensions determine the tomographic plane or slice of the image. The impulse response of the system is three-dimensional to give the exact image of the area scanned. This ability is known as the Point Spread Function (PSF). The axial dimension is measured by the center frequency and bandwidth of the signal sent, and the lateral and height dimensions are measured by the aperture and element values, and the beamforming applied. These three dimensions are called the resolution volume. The spatial impulse for each beam is calculated separately and then combined to get an overlaid image. The sound in ultrasound is a physical longitudinal wave (A brief introduction to ultrasound).
There are 2 types of ultrasound scanning systems – A-Mode and B-Mode. A-Scan or Amplitude Scan typically draws a graph based on the characteristics of the pulse generated and its echo. In B-mode or Brightness–mode, the ultrasound system represents the result as variations in brightness. Another form of scan getting popular is the Doppler scan. Under this scan, the flow of blood through the arteries and veins is scanned.
Advantages and Disadvantages of Ultrasound Scanning
The advantages of Ultrasound scanning are that it is non-invasive; fast, causes less discomfort and less expensive; is relatively harmless; and is best suited for examining soft tissues. The disadvantages are that they cannot be used on bone; the images may not be very clear, and air or gas present inside the body can make the image unreliable (The Principles of Medical Ultrasound). Due to the advantages it has, it is widely used in scanning abdomen, heart and other blood vessels, kidney, liver, gallbladder, spleen, pancreas, uterus, ovaries, thyroid and parathyroid glands, scrotum and during pregnancy. With more advancements in technology, new frontiers of usage is being developed.
Reference
A brief introduction to ultrasound. 2008. Web.
General Ultrasound Imaging. 2008. Web.
Physics, instrumentation and basic technique. 2008. Web.
The Principles of Medical Ultrasound. 2008. Web.