Pulsed Wave (PW) Ultrasound

Flashcards covering the characteristics, measurements, and principles of Pulsed Wave Ultrasound, including intensity ratios and the echo-ranging principle.

Pulse

A short emission of sound consisting of a few cycles, separated in time by periods of no emission (gaps/listening time).

Echo-Ranging Principle

The basis for most ultrasonic scanning modes used to calculate distance between structures and the probe using the formula $\text{Speed} = \frac{\text{distance}}{\text{time}}$.

Range Equation

The formula used to determine depth of a reflector based on round trip time: $d(mm) = \frac{c(mm/\mu s) \times t(\mu s)}{2}$.

Pulse Repetition Frequency (PRF)

The number of pulses per second, measured in $kHz$. In DMU, it typically ranges from $4-15\,kHz$ for imaging and $5-30\,kHz$ for Doppler.

Pulse Repetition Period (PRP)

The time from the beginning of one pulse to the beginning of the next pulse, measured in milliseconds ($ms$).

Range Ambiguity

A phenomenon where information from a deep structure appears closer to the transducer than it really is because a subsequent pulse was sent before all echoes from the previous pulse returned.

Pulse Duration (PD)

The time it takes for a single pulse to occur, calculated as $PD(\mu s) = T \times \text{\# of cycles pulse}$.

Duty Factor (DF)

The percent or fraction of time that the sound is on, calculated as $DF = \frac{PD(\mu s)}{PRP(\mu s)}$ or $DF = PD(\mu s) \times PRF(kHz)$. For CW, $DF = 1$; for PW, $DF < 1$.

Bandwidth (BW)

The range of frequencies contained in a pulse, measured in $MHz$. Shorter pulses result in wider/broader bandwidths.

Quality Factor (QF)

A measure of the purity of the beam or how close it is to the operating frequency, calculated as the reciprocal of fractional bandwidth.

Spatial Pulse Length (SPL)

The distance covered by a single pulse, calculated as $SPL(mm) = \lambda(mm) \times \text{\# of cycles}$.

Amplitude (A)

The difference between the maximum value and the equilibrium value of an acoustic variable such as pressure, density, or temperature.

Power (P)

The rate at which work is done, measured in watts ($W$) and milliwatts ($mW$).

Intensity (I)

The rate energy passes through a unit area, calculated as $I = \frac{P}{\text{Area}}$. It is also proportional to the square of the amplitude ($I \propto A^2$).

Spatial Peak (SP)

The greatest intensity in a pulse with reference to space, typically found at the center of the beam.

Spatial Average (SA)

The average intensity across the pulse in reference to space.

Beam Uniformity Ratio (BUR)

A ratio showing how uniform the intensity is in space, calculated as $BUR = \frac{SP}{SA}$. It is always greater than 1.

Temporal Peak (TP)

The greatest intensity measured as the ultrasound pulse passes by a given point.

Pulse Average (PA)

The average of all intensities measured specifically during the time the pulse is passing by.

Temporal Average (TA)

The average intensity measured over the entire pulse repetition period, including the 'listening' time.

SPTA (Spatial Peak/Temporal Average)

The intensity measurement most used in DMU because it relates well to thermal interactions within tissue. The FDA limit is $I_{SPTA} < 720\,mW/cm^2$.

Decibels (dB)

Relative units of measure for changes in sound intensity, found using a logarithm of the ratio between two powers, intensities, or amplitudes.

Attenuation

The weakening of sound strength as it propagates through a medium.

Gain

An increase in the strength of the sound wave.

Time Gain Compensation (TGC)

Controls that allow sonographers to adjust brightness based on echo received time to compensate for the effects of attenuation.

Intensity Ratio (Positive and Negative dB)

A change of $-3\,dB$ represents a reduction by a factor of 2 ($50\%$); a change of $-10\,dB$ represents a reduction by a factor of 10 ($90\%$); a change of $-6\,dB$ in amplitude represents a reduction by a factor of 2.