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Medical ultrasound terms

Encyclopedia of technical terms used in medical diagnostic ultrasound

Ultrasonography (Sonography)

A medical imaging technique used to see inside a living organism through the use of high frequency sound waves. Images are created when sound waves enter and travel through the body and are reflected back to the transducer for interpretation by a computer.

Ultrasound

Sound is a series of alternating waves of compression and rarefaction and ultrasound consists of sound at frequencies that are greater than 20,000 cycles per second (20,000 Hertz) and are above human hearing levels.

Ultrasound frequency

1 cycle=1 Hertz/1 second. Ultrasound frequency determines image quality. lower frequencies allow the soundwaves to penetrate further into the tissue, (2.5-5.0MHz) but you lose resolution/detail. Higher frequencies don't penetrate as far but offer increased resolution. (7.5-12MHz) The choice of frequency is a compromise between better resolution or better penetration.

Harmonic imaging

This is a method of improving image quality and contrast resolution in tissues viewed in B-mode (called tissue harmonic imaging in this case) and when used with Doppler ultrasound it is used to improve the sensitivity of blood flow detection in small vessels. Harmonic imaging works when the ultrasound transducer transmits at one frequency and then recieves at a different frequency twice the original frequency.

DICOM

DICOM stands for "Digital imaging & Communications in Medicine" and is a format for passing information between various pieces of digital medical equipment. It is the industry standard format for transferring images and movies from an ultrasound machine to a PACS network, storage device or computer.

CINE loop

A continuous series of images stored digitally as a sequence of individual frames. A cine loop is the continuous display of a set of images or frames that convey the effect of motion.

Hyperechoic

Increased brightness (eg: prostate, spleen, mesenteric fat).

Isoechoic

Similar or same brightness.

Hypoechoic

Decreased brightness (eg: cysts, bladder, fluid filled structures) The sound waves travel through the object and are not reflected by a surface.

Anechoic

No echoes, black.

Sagittal

Scan plane that is longitudinal, length wise down the patients' body.

Transverse

Scan plan at 90 degrees to sagittal. Think of slicing the body like a loaf of bread.

Ultrasound transducers / probes

Descriptions of each of the major ultrasound transducer types

Ultrasound transducer (ultrasound probe)

A transducer contains a piezoelectric crystal that will oscillate when an electric voltage is applied to it. When an electric current is applied to these crystals, they change shape rapidly. The rapid shape changes, or vibrations, of the crystals produce sound waves that travel outward. Conversely, when sound or pressure waves hit the crystals, they emit electrical currents. Therefore, the same crystals can be used to send and receive sound waves. When the wave travels through a medium and is reflected back to the transducer it will cause the crystal to oscillate and this will be converted to an electrical signal. Then an analog to digital conversion occurs and the signal is displayed on a computer screen. The transducer sends sound waves 1% of the time and receives waves 99% of the time.

Megahertz range of a transducer / probe

Ultrasound probes have ranges of 1 mhz to 15 mhz. Most older probes were set to only a single mhz value while most newer probes are able to scan at several fixed megahertz values allowing a single probe to do a wider range of scans. The lower the mhz frequency of the probe the deeper its penetration and conversely the higher the frequency the more shallow the depth of the scan.

Linear array transducer / probe

A probe that produces a rectangular image and has a non-curved scan head.

Convex array transducer / probe

A probe that produces an axe head shaped image and has a curved scan head.

TEE (transesophageal) transducer / probe

A probe meant to be put down the throat of a patient. It is usually a small convex scan head on the end of a short cable with controls at the end that allow the probe to be twisted and turned to different viewing angles remotely.

Pedoff transducer / probe

A non-imaging probe used for testing. Also known as a blind cardiac probe.

Ultrasound imaging modes

Basic explanation of the various imaging modes used in ultrasound imaging

B-mode (Brightness mode)

Brightness mode is the most basic greyscale imaging mode. It consists of a two dimensional image where the brightness of an area is stronger in places where the echo returns most strongly.

M-mode (Motion mode)

M-Mode is based upon B-mode but is used to track motion, such as the beating of a heart. Short movies are taken in M-mode and can usually be exported to a computer or PACS system to be viewed.

Color Doppler (Color flow Doppler)

Color Doppler uses standard ultrasound methods to produce a picture of a blood vessel. In addition, a computer converts the Doppler sounds into colors that are overlaid on the image of the blood vessel and that represent the speed and direction of blood flow through the vessel.

Generally the color above the bar indicates the flow towards the transducer , while the color below the bar indicates the flow away from the transducer, the brighter the color, the faster the flow speed, whereas the darker the color, the slower the flow speed.

Color Doppler has poor temporal resolution/flow dynamics. (frame rate can be low when scanning deep)

Power Doppler

Power Doppler is also referred to as energy Doppler, amplitude Doppler and Doppler angiography. Power Doppler does not display flow direction or different velocities. Power mode is used to image blood flow by displaying the density of red blood cells, as opposed to their velocity. Power increases the sensitivity range of the soundwaves allowing even small flows of blood through organs to be tracked rather than the faster, more substantial flow in arteries and veins detected by Color Doppler.

Power Doppler is:

  1. sensitive to low flows,
  2. No directional information in some modes,
  3. Very poor temporal resolution,
  4. Susceptible to noise

Spectral Doppler

The transducer sends and receives sound waves that are amplified through a microphone. The sound waves bounce off solid objects, including blood cells. The movement of blood cells causes a change in pitch of the reflected sound waves (called the Doppler effect). If there is no blood flow, the pitch does not change. Information from the reflected sound waves can be processed by a computer to provide graphs or pictures that represent the flow of blood through the blood vessels.

Spectral Doppler:

  1. Examines flow at one site,
  2. Detailed analysis of distribution of flow,
  3. Good temporal resolution - can examine flow waveform,
  4. Allows calculations of velocity and indices

Spectral steered doppler

Produces a spectral doppler graph but the direction of the imaging can be steered to different locations rather than in only one direction. Spectral steered doppler can be used in both PW and CW Doppler.

Duplex Doppler

Duplex combines Spectral and Color Doppler images side by side on the screen for easy comparison.

Triplex Doppler

When using color flow imaging with pulsed wave Doppler, the color flow/B-mode image is frozen while the pulsed wave Doppler is activated. Recently, some manufacturers have produced concurrent color flow imaging and pulsed wave Doppler, sometimes referred to as triplex scanning.

PW Doppler (Pulsed wave)

Pulsed wave Doppler is used to provide analysis of the flow at specific sites in the vessel under investigation. PW produces a series of pulses used to study the motion of blood flow at a small region along a desired scan line. The x-axis of the graph represents time while the Y axis represent Doppler frequency shift. The shift can be converted into velocity and flow if an appropriate angle between the beam and blood flow is known Pulsed wave systems suffer from a fundamental limitation.

The time interval between sampling pulses must be sufficient for a pulse to make the return journey from the transducer to the reflector and back. If a second pulse is sent before the first is received, the receiver cannot discriminate between the reflected signal from both pulses and ambiguity in the range of the sample volume ensues. As the depth of investigation increases, the journey time of the pulse to and from the reflector is increased, reducing the pulse repetition frequency for unambiguous ranging. The result is that the maximum fd measurable decreases with depth.

When pulses are transmitted at a given sampling frequency (known as the pulse repetition frequency), the maximum Doppler frequency fd that can be measured unambiguously is half the pulse repetition frequency. If the blood velocity and beam/flow angle being measured combine to give a fd value greater than half of the pulse repetition frequency, ambiguity in the Doppler signal occurs. This ambiguity is known as aliasing. A similar effect is seen in films where wagon wheels can appear to be going backwards due to the low frame rate of the film causing misinterpretation of the movement of the wheel spokes.

CW Doppler (Continuous wave)

Continuous wave systems use continuous transmission and reception of ultrasound. Doppler signals are obtained from all vessels in the path of the ultrasound beam (until the ultrasound beam becomes sufficiently attenuated due to depth). Continuous wave Doppler ultrasound is unable to determine the specific location of velocities within the beam and cannot be used to produce color flow images. CW allows the operator to measure extremely high velocities. CW is not limited like PW with respect to the ability to measure high velocities as it is always sending and receiving signals, it does not have to wait for a signal to return before sending out another.