Axial and Lateral Resolution chapter 10 Flashcards Preview

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Flashcards in Axial and Lateral Resolution chapter 10 Deck (41):
0

What is resolution?

the ability to create accurate images

1

What is axial resolution?

one measure of detail found in an image

measures the ability of a system to display two structures that are very close together when the structures are PARALLEL TO THE SOUND BEAMS main axis

the reflectors are positioned one in front of the other

2

What are the units of axial resolution?

measured in mm or any other unit of distance

3

what is axial resolution related to?

spatial pulse length

determined by both the sound source and the medium

4

what improves axial resolution?

shorter pulses

5

in a particular medium short duration pulses also have _______

short length

6

What are the synonyms for axial resolution?

longitudinal

range

radial

depth resolution

7

is axial resolution adjustable?

no...spatial pulse length for a transducer is fixed.

8

what are typical values for axial resolution?

0.1 to 1.0 mm

lower numerical values indicate shorter pulses and improved image accuracy

9

what is LARRD

all the synonyms for axial resolution

Longitudinal

Axial

Range

Radial

Depth

10

What is the relationship between the numerical value of the axial resolution and the image quality?

Lower numerical values of axial resolution indicate shorter pulses

shorter pulses create more accurate images

image quality is better when axial resolution has a lower numerical value

11

What is the formula for axial resolution?

axial resolution (mm) = Spatial pulse length (mm)/ 2

or

axial res (mm) = wavelength (mm)x #of cycles in pulse/2

12

what is the formula for axial resolution in soft tissue?

axial res (mm) = 0.77 x # cycles in pulse/frequency MHz

13

How are short pulses created?

less ringing

higher frequency

14

What is less ringing?

a pulse is short if there are few cycles in the pulse

dampen the crystal after it has been excited by an electrical signal from the system.

a pulse is short if each cycle has a short wavelength

shorter wavelengths are characteristic of higher frequency sound

15

pulses made of higher frequency cycles have _______axial resoultion

superior

16

Better axial resolution is associated with....

shorter spatial pulse length

shorter pulse duration

higher frequencies (shorter wavelength)

fewer cycles per pulse (less ringing)

lower numerical values

17

Axial resolution is _________ the spatial pulse length

one half

18

What is lateral resolution?

the ability to distinctly identify two structures that are very close together when they are SIDE BY SIDE or PERPENDICULAR to the SOUND BEAM main axis

19

What are the units of lateral resoultion?

mm cm or any distance

smaller numbers are preferred since they indicate more accurate images

20

What is lateral resolution determined by?

the WIDTH OF THE SOUND BEAM

narrower beams have better resoultion

beam diameter varies with depth

later resolution changes with depth

21

what are the synonyms for lateral resolution?

LATA

Lateral

Angular

Transverse

Azimuthal

22

What location is lateral resolution the BEST at? Near zone, focal zone, focus, or far zone?

at the focus where the beam is the narrowest

lateral res is GOOD within the focal zone

23

What is the mathematical relationship that describes lateral resolution and beam diameter?

lateral resolution (mm) = beam diameter (mm)

24

Which type of resolution is better in clinical imaging systems: lateral or axial resolution?

axial

ultrasound pulses are shorter than they are wide

therefore the numerical value for axial res is less than the value for lateral res

25

what appears on an image when two reflectors positioned perpendicular to the sound beam are closer to each other than the beam width?

When two side-by-side reflectors are closer to each other than the width of the beam, only one reflection is observed on the image.

26

With regard to resolution, what are the advantages of using a high frequency transducer?

higher frequencies improve both axial and lateral resolution

axial: improved in the entire image because shorter pulses are assoc with high frequency sound

Lateral: is improved in the far field only because high frequency pluses diverge less in the far field than low frequency pulses (higher frequency sound beams are narrower!)

27

Axial resolution facts:

orientation: front-to-back parallel to beam

Mnemonic: LAARD

Best With: shortest pulse, highest frequency and fewest cycles

does it change: same at all depths, does not change

In Near field best with: shortest pulse

In the far field, best with: shortest pulse

28

Lateral resolution facts:

orientation: side-by-side perpendicular to beam

Mnemonic: LATA

Determined by: beam width

Best with: narrowest beam

Does it change: changes with depth, best at focus

In the near field, best with: smallest diameter crystal

In the far field, best with: largest diameter and highest frequency (least divergence)

29

What is focusing?

concentrates the sound energy into a narrower beam and thus improves lateral resolution

30

What are the three methods of focusing?

External - with a lens

Internal - with a curved active element

Phased Array - with the electronics of the US system

31

what kind of transducer is phased array focusing reserved for?

array transducers ( those with multiple active elements)

32

What is fixed focusing?

aka: conventional or mechanical focusing

both external and internal techniques

33

with fixed focusing what is determined when the transducer is fabricated and cannot be changed?

focal depth

extent of focusing

34

With external focusing, what is placed in front of the piezoelectric material?

a lens

35

When you use a lens for fixed focusing technique, what happens to the lens arc?

becomes more prominent

the degree of focusing increases

beam narrows in the focal zone

36

With internal focusing a curved piezoelectric crystal does what?

concentrates the sound energy into a narrower or tighter beam

37

what happens as the curvature of the PZT becomes more pronounced?

the degree of focusing increases

***internal focusing is the most common form of fixed focusing***

38

With phased array focusing what can the sonographer do?

adjust the focusing characteristics of a beam

only can be used on multi-element transducers

more versatile

39

What does phased array mean?

adjustable or multiple focusing

40

What happens to a beam when it is focused

a sound beam undergoes four distinct modifications

1. the beam diameter in the near field and focal zone narrows

2. the focus is moved closer to the transducer (near zone length is reduced)

3. the beam diameter beyond the focal zone widens (focusing improves lateral res in near and focal zones)

4. the size of the focal zone is reduced (lateral res is better where the beam is narrow)