Range and Axis flashcards

Time of flight in ultrasound

The time between pulse creation and echo reception.

Speed of sound in soft tissue

1.54 mm/μs (or 1540 m/s) used in ultrasound systems.

Reflector depth formula

Depth = (1.54 mm/μs × go-return time) / 2.

13 microsecond rule

For every 13 μs of go-return time, the reflector is 1 cm deep.

Pulse Repetition Period (PRP)

The time from the start of one pulse to the start of the next.

PRP and imaging depth relationship

PRP (μs) = Imaging depth (cm) × 13.

Pulse Repetition Frequency (PRF) formula

PRF = 77,000 / imaging depth (cm).

Axial resolution

The ability to distinguish two reflectors that are close together along the beam’s path.

Measurement of axial resolution

Measured in units of distance (mm).

Axial resolution and SPL formula

Axial Resolution = SPL / 2.

Factors that improve axial resolution

Shorter SPL, shorter pulse duration, higher frequency, less ringing.

LAARD acronym purpose

Represents names for axial resolution: Longitudinal, Axial, Range, Radial, Depth.

Axial resolution using wavelength and cycles

Axial Resolution = (Wavelength × # of cycles) / 2.

Axial resolution using frequency and cycles

Axial Resolution = (0.77 × # of cycles) / frequency (MHz).

Transducer function in ultrasound

Converts electrical energy to sound and vice versa.

Piezoelectricity

The ability of a material to generate voltage when deformed.

PZT and its importance

Lead zirconate titanate, a man-made piezoelectric crystal used in ultrasound probes.

Effect of heating PZT above Curie point

It becomes depolarized and loses piezoelectric properties.

Matching layer function in transducer

Matches impedance between PZT and skin to improve sound transmission.

Thickness of the matching layer

0.25 × wavelength.

Thickness of the PZT crystal

0.5 × wavelength.

Role of backing (damping) material

Reduces ringing and shortens pulse duration.

Drawbacks of backing material

Decreased sensitivity, wide bandwidth, low Q-factor.

Q-factor definition

A unitless number representing the ratio of main frequency to bandwidth.

Q-factor of imaging probes

Low Q-factor (wide bandwidth).

Definition of continuous wave (CW) probe

A continuous electrical signal drives the PZT.

Frequency determination in a CW probe

Frequency = electrical signal frequency.

Frequency determination in a pulsed wave probe

Determined by speed of sound in PZT and its thickness.

Effect on frequency if PZT is thinner

Frequency increases.