Abstract
Understanding the basic physics of ultrasound is essential for acute care physicians. Medical ultrasound machines generate and receive ultrasound waves. Brightness mode (B mode) is the basic mode that is usually used. Ultrasound waves are emitted from piezoelectric crystals of the ultrasound transducer. Depending on the acoustic impedance of different materials, which depends on their density, different grades of white and black images are produced. There are different methods that can control the quality of ultrasound waves including timing of ultrasound wave emission, frequency of waves, and size and curvature of the surface of the transducer. The received ultrasound signal can be amplified by increasing the gain. The operator should know sonographic artifacts which may distort the studied structures or even show unreal ones. The most common artifacts include shadow and enhancement artifacts, edge artifact, mirror artifact and reverberation artifact.
Keywords: Artifacts, physics, ultrasound, waves
INTRODUCTION
Understanding the basic physics of ultrasound is essential for acute care physicians who perform point-of-care ultrasound to make accurate critical decisions. Ultrasound is made up of mechanical waves that can transmit through different materials like fluids, soft tissues and solids. It has a frequency higher than the upper human auditory limit of 20 KHz.[1] Ultrasound frequency is defined as the number of ultrasound waves per second, and medical ultrasound machines use waves with a frequency ranging between 2 and 15 MHz.[2] The velocity of ultrasound in a specific medium equals the frequency of ultrasound multiplied by its wave length.[1]
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BASIC PHYSICS
Medical ultrasound machines generate ultrasound waves and receive the reflected echoes. Brightness mode (B mode) is the basic mode that is usually used.[2] The B mode gives a two dimensional (2D) black and white image that depends on the anatomical site of the slice. The body can be imaged in different planes depending on the position of the probe. These thin slices are of less than 1 mm each and can be sagittal, coronal, transverse, or oblique. Sound waves are emitted from piezoelectric crystals from the ultrasound transducer. Piezoelectric crystals are fabricated from material that changes electrical signals to mechanical vibrations and changes mechanical vibrations to electrical signals.[2] As ultrasound waves pass through various body tissues, they are reflected back to the transducer creating an image on the ultrasound screen.[3] Acoustic impedance is defined as the resistance for propagation of ultrasound waves. This varies according to the density of the material ultrasound passes through. When the material is more solid, then the particles are denser and sonongraphic waves will reflect more [Figure 1].[4] Fluid transmits more sound waves than solid material. So less ultrasound waves will reflect back from fluids. This produces an an echogenic “black” image. Stones and bones reflect more sound waves than fluid and produce “white” bright images. Since ultrasound waves cannot transmit through stones, a black acoustic shadow will be present behind them. Air is a strong ultrasound beam reflector making it difficult to visualize structures behind it.[5