Ultrasound Imaging and Pulse Parameters

Study Notes

Ultrasound uses echo display modes, including A-mode (amplitude), B-mode (brightness), and M-mode (motion)
Different transducer types exist, such as linear array and phased array.

Ultrasound pulses are typically 2-3 wavelengths long.
The length of the pulse is called the spatial pulse length (SPL). SPL = nλ, where n is the number of cycles.
Pulse duration (PD) is the time period of the pulse. PD = nTp, where Tp is the pulse period.
After the pulse, there's a delay before the next pulse to receive signals.
Pulse repetition period (PRP) is the time interval between transmission pulses.
Pulse repetition frequency (PRF) is the frequency of repeating pulses. PRF = 1/PRP.

Duty factor (DF) is the percentage of time an ultrasound system spends transmitting a pulse in a transmit-receive cycle.
DF = Pulse duration (PD) / Pulse Repetition Period (PRP)
Higher DF means more time is spent transmitting.

Image depth (Dmax) is the maximum distance ultrasound can travel, reflect, and return to the transducer in one pulse cycle.
The transducer sends pulses and waits for echoes before sending the next pulse.
Image depth decreases with increasing pulse repetition frequency (PRF).
Higher frequency pulses result in a shallower image depth.

Spatial resolution is the smallest distance between two objects that can be distinguished in an ultrasound image.
Two types of resolution are axial and lateral.
Axial resolution is measured along the ultrasound beam.
Lateral resolution is measured perpendicular to the beam.
Axial resolution depends on spatial pulse length (SPL), and is improved by higher frequencies.
Lateral resolution depends on beam width and transducer aperture; best resolution is in the focal region. Improved by higher frequency.

Doppler imaging uses the Doppler effect to measure blood flow velocity.
The Doppler effect is the change in sound frequency when a source (blood) moves relative to a detector.
Higher frequency is detected when the source approaches the detector.
Lower frequency is detected when the source moves away from the detector.
Doppler ultrasound is used to analyze frequency shifts to measure blood velocity.