SPI flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Soft tissue propagation speed

Ultrasound travels through average soft tissue at approximately 1540 meters per second, or 1.54 millimeters per microsecond. This assumed constant underlies all depth and distance calculations.

Wavelength equation

Wavelength equals propagation speed divided by frequency. In soft tissue, wavelength in millimeters equals 1.54 divided by frequency in megahertz, so higher frequency gives a shorter wavelength.

Acoustic impedance

Impedance is tissue density multiplied by propagation speed, expressed in rayleighs. A large impedance mismatch at an interface causes strong reflection and little transmission.

Axial resolution equation

Axial resolution equals spatial pulse length divided by two, or wavelength times cycles per pulse divided by two. Shorter pulses and higher frequencies improve axial resolution.

Piezoelectric effect

Certain crystals such as lead zirconate titanate convert applied voltage into mechanical vibration and convert returning vibration back into voltage, letting one element both transmit and receive.

Curie temperature

The temperature above which a piezoelectric crystal loses its polarization and stops producing ultrasound. Heating a transducer above this point permanently reduces its efficiency.

Matching layer thickness

A layer one-quarter wavelength thick placed at the transducer face. It provides intermediate impedance between crystal and skin, reducing reflection and increasing transmitted intensity.

Backing (damping) material

Material behind the crystal that absorbs rearward sound and stops crystal ringing. It shortens the pulse, broadens bandwidth, and improves axial resolution at the cost of sensitivity.

Frame rate equation

Frame rate equals pulse repetition frequency divided by the number of pulses per frame. Deeper imaging, wider sectors, and higher line density all lower the frame rate.

Harmonic imaging

The transducer transmits at the fundamental frequency and receives echoes at twice that frequency. Harmonics arise within tissue and improve lateral resolution while reducing side-lobe clutter.

Doppler shift equation

The Doppler shift equals twice the transmitted frequency times flow velocity times the cosine of the angle, divided by propagation speed. The shift is maximal at zero degrees.

Nyquist limit

The highest Doppler shift that can be displayed without aliasing, equal to half the pulse repetition frequency. Shifts above this value wrap to the opposite side of the display.

Doppler aliasing

A wraparound error on spectral or color Doppler occurring when the Doppler shift exceeds the Nyquist limit, so peak velocities appear on the opposite channel or as a color reversal.

Continuous wave Doppler

Two separate crystals transmit and receive continuously, so there is no pulse and no aliasing. It detects the highest velocities but has no depth discrimination along the beam.

Resistive index

Peak systolic velocity minus end-diastolic velocity, divided by peak systolic velocity. Values near zero indicate low-resistance flow; values approaching one indicate high-resistance flow.

Bernoulli equation

The pressure gradient across a stenosis is approximately four times the square of the peak velocity in meters per second, allowing severity to be estimated from a Doppler measurement.

Continuity equation

Volume flow equals cross-sectional area multiplied by velocity. Because flow is constant along a vessel, velocity increases wherever the cross-sectional area narrows.

Power Doppler

Displays the amplitude of the Doppler signal rather than its frequency shift. It is angle independent, does not alias, and detects slow flow, but shows no direction or velocity.

Wall filter

A high-pass filter that removes strong, low-frequency Doppler signals originating from moving vessel walls and cardiac motion. Excessive filtering can eliminate genuine slow-flow signals.

Tissue-mimicking phantom

A test object with known attenuation, propagation speed, and target positions, used to measure resolution, distance accuracy, and sensitivity. Its cyst-like and solid targets simulate tissue contrast.

Dead zone

The near-field distance from the transducer face to the first resolvable echo. It reflects ring-down and pulse duration, and a long dead zone hides shallow structures.

Distance accuracy tolerance

Measured distances on a phantom should agree with true values within about two percent or two millimeters, whichever is greater, confirming correct depth calibration of the system.

Uniformity test

Evaluates whether echo brightness is consistent across the whole image at a fixed depth. Bright or dark vertical bands reveal time-gain compensation errors or failed transducer elements.

Anechoic cyst contrast test

Measures the ability to detect and display a fluid-filled target against background tissue. Loss of contrast resolution appears as fill-in of the cyst interior.

Transducer element dropout

Failure of individual transducer elements produces vertical bands of missing or reduced echoes in the image, most easily detected with a uniformity phantom.

Thermal index

An estimate of temperature rise from absorbed ultrasound, calculated as output power divided by the power needed to raise tissue one degree Celsius. Subscripts denote soft tissue, bone, or cranial.

Mechanical index

Peak rarefactional pressure in megapascals divided by the square root of frequency in megahertz. Higher values increase the likelihood of cavitation, especially when gas bodies are present.

Inertial cavitation

Bubble collapse during the compression phase of an ultrasound wave, producing high local temperatures and shock waves. The threshold falls sharply when contrast microbubbles are present.

Reverberation artifact

Equally spaced false echoes produced when sound bounces repeatedly between two strong reflectors, such as the transducer face and a needle or a layer of fat.

Acoustic shadowing

Reduced echoes deep to a highly attenuating or strongly reflecting structure such as bone, calculus, or gas, appearing as a dark band below the object.

Mirror image artifact

A duplicate of a structure appearing beyond a strong specular reflector, such as the diaphragm, because the reflector acts as a mirror and the extra path length is not recognized.

ALARA principle

As low as reasonably achievable: ultrasound exposure is kept to the minimum that yields diagnostic information, using the lowest acceptable output and the shortest scanning time.