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, azimuthal

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

What type of depth is optimal for lateral resolution

shallow depth. Wider reflections at depths further away from the focus exhibit poor lateral resolution.

In the near field, how does it effect axial and lateral resolution the best

axial: shortest pulse

lateral: smallest diameter crystal

In far filed, best with

axial: shortest pulse

lateral: largest diameter and highest frequency (least divergence)

focusing alters the beam in what 3 ways

1- narrower “waist” in the US beam

2- shallower focus (decrease focal depth)

3-smaller focal zone

2 types of focusing

1- fixed: aka conventional or mechanical

2- adjustable by electronics (phased array)

3 specific methods of focusing

lens, curved PZT crystal, electronic focusing

Lens

external focusing (acoustic lens is external) single element transducers, refraction

Type of curvature for lens: steeper vs flatter

very steep curves= shallow, narrow focus

flatter curves= deeper, wider focus

Is the lens fixed or adjustable?

FIXED, cannot be changed

Curved PZT crystal

internal focusing. can focus sound from a single PZT crystal.

Is a curved PZT crystal fixed or adjustable

Fixed, cannot be changed

2 other words for fixed

mechanical or conventional

What does phased array mean

adjustable or multi focusing

Electronic focusing

adjustable, provides dynamic, variable focusing or multi-focusing. Modern transducers, multi-ELEMENT transducers. Electronic time delays

Electronic focusing is fixed or adjsutable?

adjustable, phased array, multi focusing (BETTER lateral resolution)

What type of lateral resolution do fixed focus transducers have

poorest since the focal depth is fixed and can’t be adjusted (single crystal transducers are always fixed focus)

How to decrease SPL

-using a backing material in a transducer limits the #of cycles in a pulse

-increasing transducer frequency decreases wavelength

Poor lateral resolution looks like what

two reflectors positioned closer together than the width or diameter of the beam. Suboptimal because it’s important to differentiate between adjacent structures if there are truly two reflectors present

How to concentrate the beam

acoustic energy can be concentrated to narrow the beam, achieved by focusing