This glossary is a contribution made by Ursula Muller MD, Department of Diagnostic Imaging, Hospital Regional Español, Bahia Blanca, Argentina
1- Ultrasound: diagnostic imaging method that uses the mechanical energy of ultrasound waves (US) and the acoustic properties of matter. It requires a cyclic sound pressure with a frequency greater than the upper limit of human hearing which is 20 kilohertz (20,000 hertz). Synonyms: ultrasonography, sonography.
2 - Frequency : the number of cycles per second. The frequency is measured in cycles per second or Hertz units.
3- Propagation speed: the speed at which sound travels through a medium (aprox. 1,540 m / sec for soft tissues. The rate may vary but can be slower in fat and air and faster in fluids.
4- Echo: the representation in the screen of the reflection or bouncing of the ultrasonic beam to the transducer.
5- Hyperechoic: the most intense reflections or echoes reflected in white.
6- Hypoechoic: weaker reflections observed in various shades of gray.
7- Anechoic: no reflections echoes that are seen in black.
8- Piezoelectric effect: linear electromechanical interaction between the mechanical and the electrical state in crystalline or ceramic materials with no inversion symmetry experiment deformation.
9- Ultrasonic pulses: when the piezoelectric crystals in the probe are stimulated they emit pulses of US, so that the transducer emits no US continuously but generates cycles of US by way of pulses.
10-Pulse Repetition Frequency (PRF): is the number of times that the piezoelectric crystals are excited by the second. The PRF depends on the depth of the image and varies between 1000 and 10.000 KHz.
11- Attenuation: the progressive loss of US energy beam as it passes through tissues. The attenuation depends on the absorption and dispersion. Liquids are considered non attenuators; however, bone and air are potent attenuators.
12- Absorption: transforming mechanical energy into heat.
13- Dispersal: is the deviation from the direction of propagation of energy.
14-Acoustic impedance (Z): is the resistance of the medium to the propagation of the sound waves. The solids have a high impedance and transmit fluids better.
15- Interface: the boundary between two media of different impedances.
16- Modes of Ultrasound: representation of the image in:
-Mode A (amplitude modulation).
-Mode M (motion) is used infrequently.
-Mode B (brightness mode): often not used in dermatology.
-Real-time image (Mode B dynamic) Doppler
17- Doppler Mode: uses changes in the frequency of sound produced by the moving blood flow.
18-Color Doppler: is the change in the frequency received from a fixed receiver in relation to a moving point source (red blood cells.) This principle applied to ultrasound waves allows us to know flow rate of a particular vessel.
Ultrasound emitted with a certain frequency (Fe) from a transducer to a column of moving blood particles will be scattered and reflected with a different frequency. The frequency difference between emitted and reflected is called Doppler frequency (Fd). As a result, Fd is proportional to the speed of blood flow.
Color Doppler is essentially the computer system built into the ultrasound machine. The color assigned units, depend on the speed and direction of blood flow. By convention, color red is assigned for flow toward the transducer and blue for those going away
19- Transducer or probe: a device attached to the ultrasound machine that converts electrical signals into mechanical motion and vice versa. They can present variable frequency ranges:
-High Frequency (7.5 -20 MHz) maximum resolution with little penetrating power.
-Low frequency (3.5 MHz). Lower resolving power with high penetration
Different Probes can be used:
- Extracavitary: directly applied over the skin surface
- Intracavitary: within a corporal cavity
19- Gain: it is the acoustic feature in the ultrasound machine that allows us to compensate for the attenuation suffered by the sound waves when passing through the different tissues
20- Focus: the level of depth where we want to get the most of the detailed information of our study.
21-Posterior acoustic enhancement : occurs when the ultrasound pass through a medium without interfaces inside and goes to a solid medium (echogenic). It is almost a unique feature of fluid or cystic images within solid structures.
22-Posterior acoustic shadowing: occurs when the ultrasound strikes a very solid interface and can not pass through. It is very typical of calcium deposits such as gallstones, kidney stones and muscle calcifications.
23-Reverberance: occurs when the ultrasound beam strikes an interface between two media of very different acoustic impedance, such as between a solid echogenic and gas in the digestive tract or between solid and bone or oily fluid within a solid structure.
24-Comet-tail artefact: occurs when the ultrasound beam strikes a close and very echogenic interface appearing behind this interface a series of linear echoes. It is seen in bladder wall adenomyoma, highly echogenic foreign bodies and small air bubbles within the solid medium.
25-Power Doppler: is a newer technology that is up to 5 times more sensitive in detecting blood flow color Doppler and can therefore detect slow blood flow vessels.
26-Peak systolic velocity: every spectrum has a rising wave, the height of it indicates the maximum speed of blood flow in the arterial segment studied.