Axial and Lateral Resolution & Focusing

What is resolution

the ability to image accurately (ACCURACY not merely quality)

Axial resolution

distinguish two structures close to each front to back, parallel to or along the beam’s axis

Acronym and synonym for Axial resolution

LARRD

Longitudinal, axial, range, radial, depth

Units of Axial resolution and its typical value

mm, cm all units of distance (smaller the better)

.05-.5mm

What type of pulses provide better LARRD resolution

short pulses (smaller distance)

Can LARRD be adjusted

no, a new transducer is needed to change LARRD resolution

What does a short pulse mean

short spatial pulse length or short pulse duration (the backing material allows have few cycles per pulse)

Equation for Axial Resolution

LARRD mm= SPL/2

Box car analogy

Short length of boxcars and fewer cars in train is good axial. High frequency=shorter wavelength

LESS RINGING (fewer cycles in pulse) and HIGH FREQUENCY (each car in train is shorter)

As frequency increases what happens to the numerical value of LARRD

it decreases, meaning that high frequency transducers improve LARRD resolution and create more accurate images (smaller number is better)

Recipe for the best axial resolution

highest frequency transducer with lowest # of cycles (shorter pulses)

Lateral Resolution

minimum distance that two structures are separated by side to side or perpendicular to sound beam

Synonym and acronym for lateral resolution

LATA

lateral, angular, transverse, azimuthalIU

Units for LATA

mm, all units of length (smaller number creates a more accurate image)

Equation for LATA

LATA=beam diameter (width)

What adjusts LATA

depth. Beam diamter varies with depth, LATA also varies with depth

Where is lateral resolution the best

at the focus or near zone length (focal depth) because its the narrowest

What is point spread artifact

two side by side structures are closer together than the beam width, only 1 wide reflection is see on image

LATA vs LARRD

LATA is usually not as good as LARRD because sound pulses are wider than they are short

How does high frequency sound affect both LATA and LARRD

LARRD= frequency improves resolution everywhere in image

LATA= frequency improves resolution only in the far field

Beam shape and divergence

in the far field higher frequency