chapter 10: axial and lateral resolution

resolution

the ability to create accurate images

axial resolution

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

how are the reflectors positioned in axial resolution?

they are positioned one in front of the other

what is axial resolution related to?

• spatial pulse length

• pulse duration

what is axial resolution determined by?

spatial pulse length

mnemonic for axial resolution:

• Longitudinal

• Axial

• Range

• Radial

• Depth

is axial resolution adjustable?

no

how is a short pulse created?

• less ringing

• higher frequency

better axial resolution is associated with:

• short SPL

• shorter pulse duration

• higher frequencies (shorter wavelength)

• fewer cycles per pulse (less ringing)

• lower numerical values

does axial resolution change with depth?

no, it is the same at all depths

lateral resolution

the ability to distinctly identify 2 structures that are very close together when they are side by side or perpendicular to the sound beam’s main axis

what is lateral resolution determined by?

the width of the sound beam

mnemonic for lateral resolution:

• Lateral

• Angular

• Transverse

• Azimuthal

better lateral resolution is associated with:

• narrowest beam

• best at focus

does lateral resolution change with depth?

yes

lateral resolution in the near field is best with:

smallest diameter crystal

lateral resolution in the far field is best with:

largest diameter crystal and highest frequency (least divergence)

which type of resolution id better in clinical imaging systems?

axial resolution because ultrasound pulses are shorter than they are wide

when 2 structures are closer together than the beam’s diameter what appears on the image?

only 1 reflection is seen on the image

focusing

concentrates the sound energy into a narrower beam which improves lateral resolution

what are the 3 methods of focusing?

• external focusing - with a lens

• internal focusing - with a curved active element

• phased array focusing - with the electronics of the ultrasound system

fixed focusing/conventional focusing/mechanical focusing

• includes both external and internal techniques

• the focal depth and the extent of focusing are determined when the transducer is fabricated and it cannot be changed

external focusing

• a lens is placed in front of the PZT

• fixed focusing technique

• as the arc of the lens becomes more prominent, the degree of focusing increases and the beam narrows in the focal zone

internal focusing

• a curved PZT concentrates the sound energy into a narrower or tighter beam

• as the curvature of the PZT becomes more pronounced, the degree of focusing increases

• most common form of fixed focusing

• no lens used

phased array focusing

• electronic focusing

• adjustable

• the system’s electronics focus the sound beam

• only used on multi-element transducers

effects of focusing:

• beam diameter in the near field and focal zone is reduced

• focal depth is shallower

• beam diameter in the far zone increases

• focal zone is smaller