Pulsed Ultrasound Physics

Flashcards covering the parameters of pulsed ultrasound, including pulse duration, spatial pulse length, PRF, PRP, and duty factor based on the lecture transcript.

What are the five parameters needed to completely describe pulsed sound?

1. Pulse duration, 2. Spatial pulse length, 3. Pulse repetition period, 4. Pulse repetition frequency, 5. Duty factor.

What is the definition of a pulse of ultrasound?

A collection of cycles that travel together; it must have a beginning and an end and propagates as a single unit.

What are the two components of pulsed ultrasound?

1. Transmit, talking, or "on" time, and 2. Receive, listening, or "off" time.

What is the definition of Pulse Duration?

The actual time from the start of a pulse to the end of that pulse (a single transmit, talking, or "on" time).

What is the typical range of values for pulse duration in clinical diagnostic ultrasound?

$0.3$ to $2.0\,\mu s$ (microseconds).

How is Pulse Duration (PD) determined and is it adjustable?

Determined by the sound source only; it is NOT adjustable by the sonographer.

What are the two formulas used to calculate Pulse Duration?

\text{pulse duration (\mu s)} = \#\text{cycles} \times \text{period (\mu s)} and \text{pulse duration (\mu s)} = \frac{\#\text{cycles}}{\text{frequency (MHz)}}.

What characteristics create a pulse of long duration?

Many cycles in the pulse or individual cycles with long periods.

What type of pulse duration is more desirable in diagnostic imaging and why?

Shorter duration pulses are more desirable because they create images of greater accuracy.

What is the definition of Spatial Pulse Length (SPL)?

The distance that a pulse occupies in space from the start to the end of a pulse.

What determines Spatial Pulse Length and is it adjustable?

Determined by both the source and the medium; it is NOT adjustable.

What is the formula for Spatial Pulse Length?

$\text{spatial pulse length (mm)} = \#\text{cycles} \times \text{wavelength (mm)}$.

How does Pulse Duration differ from Spatial Pulse Length in terms of measurement?

Pulse duration is the time the pulse is "on" (measured in $\mu s$), while spatial pulse length is the distance of the pulse from end to end (measured in $mm$).

What is the definition of Pulse Repetition Period (PRP)?

The time from the start of one pulse to the start of the next pulse, which includes one pulse duration and one listening time.

How are Pulse Repetition Period and imaging depth related?

They are directly related; as depth of view increases, pulse repetition period increases.

Which component of Pulse Repetition Period can be changed by the sonographer?

The receive or "listening" time is changed when the sonographer adjusts the imaging depth.

What is the definition of Pulse Repetition Frequency (PRF)?

The number of pulses that an ultrasound system transmits into the body each second.

What is the relationship between Pulse Repetition Frequency and imaging depth?

They are inversely related; as depth of view increases, PRF decreases.

What is the reciprocal relationship between PRP and PRF?

$\text{PRF} \times \text{PRP} = 1$; a longer pulse repetition period results in a lower pulse repetition frequency.

What is the definition of Duty Factor (DF)?

The percentage or fraction of time that the system transmits a pulse.

What is the typical value range for duty factor in clinical anatomic imaging?

$0.002$ to $0.005$, or $0.2\%$ to $0.5\%Code$.

What is the mathematical formula for Duty Factor?

$\text{duty factor (\%)} = \frac{\text{pulse duration}}{\text{pulse rep. period}} \times 100$.

How does imaging depth affect the Duty Factor?

Duty factor is inversely related to imaging depth; it is higher at shallow depths and lower at greater depths.

What is the duty factor for continuous wave (CW) sound?

$1.0$ (or $100\%Code$) because the system is always transmitting.

Which parameters describe both pulsed and continuous waves?

Period, frequency, wavelength, and propagation speed.

What happens to the PRF and duty factor when a sonographer increases the imaging depth from $8\,cm$ to $16\,cm$?

The PRF decreases and the duty factor decreases.