Pulsed Ultrasound Techniques

Pulsed Waves

• Pulsed waves differ from continuous waves. Continuous waves produce a constant signal, while pulsed waves emit short bursts of acoustic energy.
• Diagnostic ultrasound uses pulsed waves to create images
• Pulsed wave parameters are different from other types of waves
• Pulsed waves can be characterized using several new parameters.

Describing Pulsed Waves

• Seven parameters describe continuous waves, but pulsed waves need additional parameters
• The seven parameters of continuous waves are not sufficient to describe pulsed waves, new terms are required.

Pulsed Ultrasound

• Continuous wave sound creates no anatomic images in diagnostic ultrasound - Pulsed ultrasound creates images
• Imaging systems use short bursts (pulses) of acoustic energy

What is Pulsed Sound?

• A pulsed ultrasound is a collection of cycles that creates a pulse - It contains a beginning and an end - A pulse is a single unit of energy

Pulsed Ultrasound Has 2 Components

• Transmit time: the transducer is sending a pulse
• Receive time: the transducer is not sending a pulse, but waiting for reflections
• The machine has two phases: emit a pulse and receive a pulse.

A Pulse is a Pulse...

• The characteristics of a pulse are determined by the source (machine/transducer), not the user
• The characteristics of the pulse signal can't be changed once it has been sent

Parameters Describe Pulsed Sound

• Five parameters describe the characteristics of pulsed sounds - Pulse duration, spatial pulse length, pulse repetition period, pulse repetition frequency and duty factor.

Pulse Duration

• The time from the start of a pulse to the end
• The pulse duration is determined by the ultrasound source.

Spatial Pulse Length

• The length or distance a pulse occupies in space
• Directly proportional to the wavelength, longer wavelength means longer spatial pulse length.

Pulse Repetition Period (PRP)

• PRP is the time between the start of one pulse a the start of the next pulse
• The time between the start of one pulse and the start of the next - can be adjusted by the user

Pulse Repetition Period & Depth

• Increasing depth increases the time it takes for the sound beam to return.
• Increased depth correlates directly with incresed repetition period

PRP is Directly Related to Depth

• Increasing depth increases PRP
• Increasing listening time increases PRP

Pulse Repetition Period (PRP) & Depth / Directly Related

• Increasing depth increases the time taken for a sound beam to return
• PRP is directly related to depth and indirectly proportional to the frequency

When You Decrease Depth...

• Decreasing depth results in a closer view (zoomed in) and a shorter time, also more frequent pulses

Shallow Imaging

• Deeper imaging requires longer listening times
• Shallower imaging requires less listening time

Deep Imaging

• Deeper imaging requires more listening time
• Shallower imaging requires less listening time

Pulse Repetition Frequency (PRF)

• PRF is the number of pulses emitted each second
• The number of pulses an ultrasound system transmits into the body each second - determined by the ultrasound machine.

Pulse Repetition Frequency (PRF)

• More shallow (shallower) scanning will require a higher PRF (pulse repetition frequency rate). Deeper (deeper) scanning will require a lower PRF and longer listening time.

Pulse Repetition Frequency (PRF) & Depth / Inversely Related

• PRF is the number of pulses transmitted into a body each second.
• PRF is inversely related to depth

Relationship of PRP & PRF

• PRP and PRF are reciprocals. PRP (time) multiplied by PRF (frequency) equals 1
• They have an inverse relationship

Duty Factor

• Duty factor is the percentage or fraction of time during which the ultrasound machine produces a pulse or transmits sound.
• The duty cycle is equal to the pulse duration divided by the pulse repetition period and then multiplied by 100
• Clinically, the imaging of duty factors is in the range of 0.2% to 0.5%.

Duty Factor

• The percentage or fraction of a period in which a device sends a pulse - the amount of time a pulse is sent compared to the total time

Duty Factor

• Duty factor is the percentage of time the system produces a pulse - High duty factors mean pulses are frequently produced.

Duty Factor

• A duty factor is one of the factors that make up pulsed signals - Duty factor is unrelated to frequency - The duty factor is higher for shallower imagining, and lower for deeper imagines.

Questions

• What is the duty factor?: The duty factor of a wave is the ratio of pulse duration to pulse repetition period multiplied by 100 - Calculated as pulse duration divided by pulse repetition period.

Duty Factor

• Depth change does not change the Duty Factor
• Duty factor is determined by the ultrasound machine's source

Duty Factor

• Duty factor is inversely related to depth
• Shallower imaging requires a higher duty factor (more pulses) - Deeper imaging requires a lower duty factor (fewer pulses).