resolutions

resolution

the ability to create accurate images

what are the 4 types of resolution?

• lateral

• axial

• elevational

• temporal

lateral resolution

• measures the ability of a system to display 2 structures that are very close together perpendicular to the sound beam’s main axis

• 2 structures are positioned side-to-side

• best at the focus where the beam is narrow

synonyms for lateral resolution

• Lateral

• Angular

• Transverse

• Azimuthal

axial resolution

• measures the ability of a system to display 2 structures that are very close together parallel to the sound beam’s main axis

• 2 structures positioned front-to-back

synonyms for axial resolution

• Longitudinal

• Axial

• Range

• Radial

• Depth

short pulses are created in two ways:

• less ringing

• higher frequency

better axial resolution is associated with:

• shorter SPL

• shorter PD

• higher frequencies (shorter wavelength)

• fewer cycles per pulse (less ringing)

• lower numerical values

axial and lateral resolution are both improved by

high frequency sound

elevational resolution (slice thickness resolution)

• measured in a direction above-to-below the imaging plane

• measured in the 3rd dimension

• the width of the beam is perpendicular to the imaging plane (beam thickness)

slice thickness artifact (section thickness artifact, partial volume artifact)

• the imaging place is 3D and flares out like the bell of a trumpet

• determined by the thickness of the imaging plane

• images that are above or below the imaging plane may appear in the image

what shaped elements have the best elevational resolution?

disc shaped elements

what shaped elements have poor elevational resolution?

rectangular shaped elements

1.5 dimensional array

has added PZT crystals in the up and down direction to improve elevational resolution

temporal resolution

• refers to “accuracy in time”

• describes the ability to precisely position moving structures from instant to instant

• determined by frame rate

frame rate

• the ultrasound system’s ability to create numerous frames each second

• most important parameter associated with the system’s ability to image in real time like a “movie”

frame rate is determined by the

• speed of sound in the medium

• depth of imaging

what are the 2 setting the sonographer can control that will affect the frame rate?

• imaging depth

• number of pulses in each picture

two factors that affect the number of pulses needed to create an image are:

• number of focal points

• sector size

single focus imaging

only 1 sound pulse is transmitted down each scan line

multi-focus imagining

more pulses are transmitted down each scan line

sector size (field of view)

more pulses are needed to create an image with a wider sector width

line density

the spacing between sound beams that can be altered by ultrasound systems

closely spaced sound beams represent what type of line density?

increased line density

wider spaced sound beams represent what type of line desity?

decreased line density

focusing

concentrates sound energy into a narrower beam and improves lateral resolution

what are the 3 methods of focusing?

• external focusing

• internal focusing

• phased array focusing

external focusing

• single element transducer (fixed focus)

• uses a lens to focus

internal focusing

• single element transducer (fixed focus)

• curved active element used to focus

phased array focusing

• multiple active elements (adjustable focal zone)

• electronics of the ultrasound system used to focus

a beam undergoes these 4 modifications when it is focused:

• beam diameter narrows in the near field and focal zone

• focus is moved closer to the transducer

• more beam divergence in the far field

• the size of the focal zone is reduced