Understanding Ultrasound Physics chapter 10

Axial Resolution

Measures the ability of a system to display two structures that are very close together when the structures are PARALLEL to the sound beam's main axis.

Resolution

The ability to create accurate images

Axial Resolution

What answers the question, what is the minimum distance that two structures positioned front to back or parallel to the sound beam can be apart and still produce two distinct echoes on an ultrasound image.

distance (mm)

What are the units of axial resoution?

spatial pulse length, pulse duration

What is axial resolution is related to:

medium and sound source

spatial pulse length is determined by ?

Axial resolution

Shorter pulses improve what kind of resolution?

longitudinal, range, radial, depth resolution

What are synonyms for axial resolution?

No, because the spatial pulse length for a transducer is fixed.

Is axial resolution adjustable? Why or why not?

0.1-1.0 mm

What are the typical values for axial resolution?

shorter pulses and improved image accuracy.

In regards to axial resolution, lower numerical values indicate what?

lower

Image quality is better when axial resolution has a ______ numerical value.

axial resolution (mm) = spatial pulse length/2

What is the mathematical formula for Axial resolution?

axial resolution (mm) (0.77 x cycles in pulse)/frequency (MHz)

What is the formula for axial resolution in soft tissue?

1. less ringing 2. higher frequency

In what two ways is a short pulse created?

shorter spl, shorter pulse duration, higher frequencies (shorter wavelength) fewer cycles per pulse (less ringing), lower numerical values.

Better axial resolution is associated with:

greater

If there are more cycles in a pulse, the numerical value of range resolution is______.

degrades

If a new pulsed transducer has many more cycles in its pulse, the image accuracy _________.

C

Which of the following transducers has the poorest axial resolution?

a. 1.7 MHz and 4 cycles/pulse

b. 2.6 MHz and 3 cycles/ pulse

c. 1.7 MHz and 5 cycles/pulse

d. 2.6 MHz and 2 cycles/pulse

2.25, axial resolution is 1/2 the spatial pulse lenth

In Soft Tissue, a 3 cycle, 1 MHz pulse has a pulse length equal to 4.5 mm what is the axial resolution?

The 5 MHz transducer is more likely to create an image with better axial resolution. The Higher frequency transducer creates a shorter pulse and thus has a lower numerical value of axial res.

Two different transducers create sound pulses. One transducer is labeled 5 MHz and the other, 3 MHz. Which transducer is more likely to create a more accurate image with respect to axial resolution? Which transducer probably has a lower numerical value of axial resolution?

3 cycle pulse is more likely to create an image with better axial resolution. Less ringing, or fewer cycles in a pulse, generally implies shorter pulses and improved axial resolution.

Two different transducers create pulses. both transducers create sound with a freq. of 4MHz. One transducer creates a pulse that comprises 6 cycles and the other, 3 cycles. Which transducer is more likely to create a more accurate image with respect to axial resolution? Which transducer has a lower numerical value of axial resolution?

Lateral Resolution

What is the ability to distinctly identify two structures that are very close together when they are side by side, or perpendicular to the sound beam's main axis?

lateral resolution

What answers the question, what is the minimum distance that two structures, positioned side by side, can be apart and still produce two distinct echoes on an ultrasound image?

distance (mm or cm)

What are the units for lateral resolution?

The width of the sound beam

What is lateral resolution determined by?

narrow

Which beams have better resolution: narrow or wide beams?

Depth

Beam diameter varies with?

Depth

Lateral resolution changes with?

LATA, Lateral, angular, transverse, azimuthal

What are synonyms of Lateral resolution?

focus or end of the near zone.

Lateral resolution is best at the:

lateral resolution = beam diameter

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

Axial

Which is better clinically? Axial or Lateral resolution?

Only one reflection is observed.

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

Both axial and lateral resolution

Higher frequency transducers improve what?

Because shorter pulses are associated with higher frequency sound.

Why are higher freq. transducers better for Axial Resolution?

It is improved in the far field only because high freq. pulses diverge less in the far field than low frequency pulses. Higher frequency sound beams are narrower than lower frequency beams.

Why are higher freq. transducers better for Lateral resolution?

Focusing

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

External focusing, internal focusing, phased array focusing

What are the three methods of focusing?

External focusing

What type of focusing uses a lens?

Internal focusing

What type of focusing uses a curved active element?

phased array focusing

What type of focusing uses the electronics of the ultrasound system?

Fixed focusing

What type of focusing includes both external and internal techniques?

In front of the PZT, a fixed focusing technique for external focusing.

A lens is placed where on the transducer with external focusing? What type of focusing is this?

Concentrates the sound energy into a narrower or tighter beam. Internal focusing.

What does a curved PZT do to the beam? What type of focusing is this?

Phased array

With what type of transducer allows the sonographer to adjust the focusing characteristics of a beam?

1. Beam diameter in the near field and the focal zone narrows. 2. The focus is moved closer to the transducer (zone length is reduced). 3. The beam diameter beyond the focal zone widens. 4. Size of the focal zone is reduced.

What are the four distinct modifications that happen when a beam is focused?

resolution

The ability to distinguish two structures lying close together is called?

axial, longitudinal, range, radial, or depth resolution.

The ability to distinguish two structures lying close together front to back is callled?

lateral, angular, transverse, or azimuthal resolution

The ability to distinguish two structures lying close together side by side is called _____.

distance, mm.

Axial resolution and lateral resolution are both measured with units of____.

increases.

When the number of cycles in a pulse increases while the frequency remains the same, the numerical value of the range resolution increases, decreases, remains the same?

Degrades

When the number of cycles in a pulse increases (more ringing) while the frequency remains the same, the image quality improves, degrades, remains the same.

Higher frequency

What frequency transducers have the best range resolution?

Longitudinal, Axial, range, radial, depth

Name the 4 synonyms for axial resolution.

lateral, angular, transverse, and azimuthal.

Name the 3 synonyms for lateral resolution.

4mm, this value is one half the pulse length.

The length of a pulse is 8mm. what is the minimum distance between two reflectors, positioned one on front of the other, that still produces two echoes on our image?