Education / Basics of Ultrasound
Diagnostic ultrasound has been one of the most commonly used modalities in diagnostic medicine for many decades. It is a safe, cost effective tool for diagnostic procedures. Ultrasonography utilizes the echoes of high frequency sound waves to create images of soft tissue anatomy. The ultrasound probe transmits high frequency sound waves into the body. When these sound waves hit a boundary between acoustically different tissues (i.e. fluid and soft tissue) some of the waves are reflected back. Other waves travel further until they reach another boundary. The ultrasound machine then calculates the location and intensity of these reflections. The imager than displays a two dimensional image, incorporating the reflection intensities and distances.
The probe is the principle component of the scanner. The transducer both produces and receives the sound waves. Probes come in a variety of sizes and types. The EPISCAN uses a single element transducer driven by a small motor that transverses back and forth within the probe. Probes also come in a variety of frequencies. The frequency is the number of waves that are emitted per a period of time. This frequency is measured in hertz (Hz). The higher the frequency, the higher the image resolution is achieved. However, the higher the frequency the less penetration of tissue is achieved. The ideal situation therefore is to use the highest frequency possible to achieve penetration to the area of interest. Typical diagnostic ultrasound for fetal imaging is around 3.5 to 7 megahertz (MHz). The EPISCAN uses 20 MHz and greater central frequency probes. This makes our system suitable for imaging the skin and superficial soft tissue to a depth of approximately 2 cm.
The central processing unit is basically a computer that contains a microprocessor, memory, power supplies and a processing board that converts the waves into images. The keyboard and mouse are used to change settings, add notes and take measurements. There are several settings that can be changed to affect the image. The gain setting changes the overall amplification of the received signal. The gain is set to the point where tissue boundaries are best imaged in the area of interest. The amount of gain needed to get the most useful image will vary depending on the feature you are examining and its depth. Over amplification will result in obscuring structures you are trying to identify. Under amplification will result in a decrease in the definition between tissue types. The Time Gain Compensation (TGC) control compensates for the attenuation of sound waves. As ultrasound signals travel through tissue they become attenuated. The signals that travel further (i.e. from deeper tissue) are attenuated more than those that are reflected from the surface. The TGC amplifier allows you to compensate for this by applying a gain ramped with time to the received signal.
The scan depth represents the height of the image in millimeters. The lowest depth setting will display the most superficial tissue at its maximum resolution. The depth setting should be set at the lowest value to include the entire area of interest. The processed images are displayed onto the monitor. Different palettes can be used to maximize the information on the screen. Once displayed, items of interest can be labeled and measured.