SPI key terms

The property of certain crystals and ceramics to change shape when an alternating voltage is applied, producing sound waves, and to generate a voltage when returning echoes deform them.

The resistance of a medium to sound travel, calculated as density multiplied by propagation speed. Large differences in impedance between two tissues cause strong reflections at their boundary.

The loss of sound energy as a beam travels through tissue, caused by absorption, reflection, and scattering. It increases with depth and with higher transducer frequency.

How fast sound travels through a medium, determined by its stiffness and density. In soft tissue it averages about 1,540 meters per second, while in bone and air it differs greatly.

The physical length of one pulse measured from its front to its back, equal to the number of cycles multiplied by the wavelength. Shorter pulses give better axial resolution.

The ability to show two reflectors lying along the beam path as separate echoes. It equals half the spatial pulse length and improves with higher frequency and fewer cycles per pulse.

The ability to distinguish two reflectors lying side by side, perpendicular to the beam. It is best at the focal zone and worsens with greater depth or a wider beam.

The region of the sound beam where it is narrowest and image detail is sharpest. Its position and length are set by the element's curvature or by electronic focusing.

The number of ultrasound pulses emitted per second. Higher values allow faster frame rates but reduce the maximum depth that can be imaged before range ambiguity occurs.

The fraction of time the transducer is actively transmitting compared with the total pulse repetition period. Continuous-wave Doppler has a duty factor of 1.0; imaging modes are far lower.

The ratio, usually stated in decibels, between the strongest and weakest signals a system can handle or display. Choosing it appropriately affects image contrast and displayed echo brightness.

A control that increases amplification of echoes returning from greater depths to offset attenuation, so that similar reflectors appear with similar brightness across the image.

The number of complete images produced per second. It falls when imaging depth, line density, or the number of focal zones is increased.

A technique that listens for echoes at twice the transmitted frequency, generated as sound travels through tissue. It improves contrast and reduces haze without the need for contrast agents.

The number of binary bits used to store each echo sample's amplitude, which determines how many shades of gray the displayed image can potentially show.

The change in frequency between transmitted sound and echoes returning from moving red blood cells. The shift is proportional to blood velocity and to the cosine of the Doppler angle.

The angle between the ultrasound beam and the direction of blood flow. Shifts are converted to accurate velocities only when this angle is 60 degrees or less.

A mode using separate elements to transmit and receive without pausing, so it detects flow at any depth along the beam but cannot localize where that flow occurs.

A mode that transmits short pulses and listens only during a chosen time window, allowing flow velocity to be measured at a specific depth selected by the operator.

The highest Doppler shift that can be displayed without ambiguity, equal to half the pulse repetition frequency. Shifts above it appear on the wrong side of the baseline.

The wrap-around of a Doppler waveform or color map when the shift exceeds the Nyquist limit, showing peak velocities on the opposite side of the baseline or as a color reversal.

A high-pass filter that removes strong, low-frequency Doppler shifts coming from slow-moving vessel walls and surrounding tissue so that blood flow signals stand out more clearly.

A mode that displays mean flow velocity across a region of interest as a color map, typically red toward and blue away from the transducer, with brightness indicating speed.

A mode that displays the strength of Doppler signals rather than their frequency shift, making it more sensitive to slow flow but unable to show direction or velocity.

The small gate in pulsed-wave Doppler, positioned along the beam, from which returning echoes are accepted. Its length sets the depth range over which flow is measured.

A test object made of material that mimics soft tissue attenuation and speed, containing targets of known position and size, used to check system performance over time.

A system's ability to detect weak echoes from deep structures. It is tested by finding the greatest depth at which echoes remain visible at a fixed output setting.

The region immediately in front of the transducer where echoes cannot be resolved because the pulse has not yet ended, so very superficial structures may be missed.

The evenness of brightness across an image made while scanning a uniform test object. Uneven areas point to element failures, focusing problems, or gain errors.

How closely a depth measured on the display matches the true depth of a target, checked using phantom pins placed at known separations.

Scheduled inspection, cleaning, and testing of scanners and transducers to catch problems early and keep image quality and safety within accepted limits.

As Low As Reasonably Achievable: the principle of using the lowest acoustic output and the shortest exposure time that still yields a diagnostic result, limiting patient exposure.

An estimate of the temperature rise tissue may experience from absorbed sound, calculated as acoustic output power divided by the power needed to raise tissue temperature by one degree Celsius.

An indicator of the likelihood of cavitation, calculated as the peak negative pressure in megapascals divided by the square root of the frequency in megahertz.

The formation, oscillation, or collapse of gas bubbles in tissue or contrast agents when exposed to sound, which can damage nearby cells at high intensities.

Spatial peak temporal average intensity: the highest intensity within the beam averaged over time. It is the value most closely linked to thermal bioeffects.

Multiple false echoes appearing behind a strong reflector at evenly spaced depths, caused by sound bouncing back and forth between the reflector and the transducer.

A darker region behind a structure that reflects or absorbs most of the sound, such as bone or a gallstone, because little energy reaches the tissue beyond it.

Increased brightness behind a fluid-filled structure that weakens sound less than the surrounding tissue, so tissues deeper than the fluid return stronger echoes.

A duplicate of a real structure appearing on the far side of a strong reflector, created when sound bounces off the reflector before reaching the true object.