Ultrasound Physics: Doppler and Spectral Analysis

Comprehensive vocabulary flashcards covering Doppler ultrasound principles, instrumentation, artifacts, and spectral analysis based on the lecture notes.

Doppler shift

The change in frequency of sound as a result of relative motion between the sound source and the receiver, also called Doppler frequency.

Demodulation

The process of extracting the low Doppler frequency from the transducer's much higher carrier frequency.

Positive Doppler Shift

A shift that occurs when blood cells move toward the transducer, resulting in a reflected frequency higher than the transmitted frequency.

Negative Doppler Shift

A shift that occurs when blood cells move away from the transducer, resulting in a reflected frequency lower than the transmitted frequency.

Speed

A purely magnitude measurement indicating the distance that a red blood cell moves in 1 second, reported in units of distance divided by time.

Velocity

A measurement defined by both a magnitude and a direction, often represented as an arrow.

Doppler Equation

$$\text{Doppler shift} = \frac{2 \times \text{velocity of blood} \times \text{transducer frequency} \times \text{cos}(\theta)}{\text{propagation speed}}$$

Cosine $\theta$

The mathematical factor that determines the percentage of true velocity measured when an angle exists between the sound beam and the direction of flow.

Bidirectional Doppler

A clinical Doppler device that distinguishes the direction of flow toward or away from the transducer, typically using phase quadrature.

Phase quadrature

A signal processing technique used for bidirectional Doppler to identify flow direction, also known as quadrature detection.

Continuous Wave Doppler

A technique requiring two crystals (one to transmit and one to receive) that can measure very high velocities without aliasing but suffers from range ambiguity.

Range ambiguity

The primary disadvantage of continuous wave Doppler where velocities from the entire region of beam overlap blend together.

Pulsed Wave Doppler

A technique using a single crystal that alternates between sending and receiving ultrasound to measure velocities at a specific location.

Range resolution

The ability of pulsed Doppler to select the exact location where velocities are measured, also called range specificity or freedom from range ambiguity artifact.

Sample volume

The small marker or gate positioned by the sonographer on a two-dimensional image to specify where pulsed Doppler measurements are taken.

Aliasing

The most common error in pulsed Doppler where high velocity flow in one direction is incorrectly displayed as traveling in the opposite direction.

Nyquist limit

The highest Doppler frequency or velocity that can be measured without the appearance of aliasing, mathematically defined as $\frac{\text{PRF}}{2}$.

Duplex imaging

The simultaneous performance of anatomic imaging and Doppler evaluation.

Color Flow Doppler

A two-dimensional pulsed ultrasound technique where velocity information is coded into colors and superimposed on a gray scale image.

Mean Velocity

The average velocity reported by color Doppler systems, in contrast to the peak velocities reported by spectral Doppler.

Velocity Mode

A color map dictionary that provides information on both flow direction and velocity magnitude.

Doppler packets

Multiple ultrasound pulses used in color Doppler to accurately determine blood velocities, also called an ensemble.

Power Doppler

A non-directional color Doppler mode that identifies the presence of a Doppler shift based on the amplitude of the reflected signal, also called energy mode or color angio.

Flash artifact

A burst of color in power Doppler images caused by the motion of the transducer, patient, or soft tissues.

Clutter

Low frequency Doppler shift artifacts on spectral displays created by slowly moving anatomy such as heart muscle or pulsating vessel walls.

Ghosting artifact

Low frequency Doppler shift artifacts arising from moving anatomy specifically when using color Doppler.

Wall filter

A high pass filter used to eliminate low frequency Doppler shifts around the baseline resulting from moving anatomy rather than moving blood cells.

Crosstalk

A special form of mirror image artifact occurring only with spectral Doppler where an identical spectrum appears both above and below the baseline.

Spectral analysis

A tool that breaks a complex Doppler signal into its basic building blocks to identify individual velocities.

Fast Fourier Transform (FFT)

A digital (computer-based) technique used to process both pulsed and continuous wave Doppler signals for spectral display.

Spectral window

The clear region between the baseline and the spectral trace typically seen in laminar flow.

Spectral broadening

The filling in of the spectral window associated with turbulent flow, representing a wider range of velocities.

Autocorrelation

A modern method of spectral analysis used specifically for color flow Doppler.

Baseline shift

A display adjustment that slides the baseline down to devote the entire velocity scale to one direction, used to eliminate the appearance of aliasing.

Home line

An imaginary transducer location reference used with steered Doppler boxes in rectangular images to determine the direction of flow.