Ch 10 - Axial and Lateral Resolution

What are the typical values for axial resolution?

0.1-1.0 mm

What are the AKAs for lateral resolution?

1. Angular 2. Transverse 3. Azimuthal (LATA)

What are the three methods of focusing?

1. External focusing= with a lens 2. Internal focusing= with a curved active element 3. Phased array focusing= with the electronics of u/s system.

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

1. Higher frequencies improve both axial and lateral resolution. 2. Axial resolution is improved in the entire image because shorter pulses are associated with high frequency sound. 3. Lateral resolution is improved in the far field only because high frequency pulses diverge less in far field than low frequency pulses.

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

1. Lower numerical values= short pulses 2. Shorter pulses= more accurate images or better image quality.

Better axial resolution is associated with?

1. Shorter pulse duration 2. Shorter SPL 3. Higher frequencies (shorter wavelength) 4. fewer cycles per pulse ( less ringing) 5. lower numerical value

Two different transducers create pulses. Both transducers create sound with a frequency of 4 MHz. 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?

1. The 3-cycle pulse transducer 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. 2. The 3-cycle pulse has a lower numerical value of axial resolution. Lower numbers mean improved image accuracy.

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?

1. The 5 MHz transducer is more likely to create an image with better axial resolution. 2. The higher frequency transducer creates a shorter pulse which has a lower numerical value of axial resolution (Lower numbers= improved image quality).

What happens to beam when it is focused?

1. The beam diameter in the near field and the focal zone narrows. 2. The focus is moved closer to the transducer (the near zone length is reduced. 3. The beam diameter beyond the focal zone widens. Therefore, focusing improves lateral resolution in the near and focal zones, and degrades lateral resolution beyond the focal zone. 4. The size of the focal zone is reduced.

Determinants of Sound Beams:

1. The characteristics of frequency- continuous waves is determined by the frequency of electrical signal from u/s system. 2. The characteristics of frequency- pulsed waves is determined by thickness of ceramic and speed of sound in ceramic. 3. The characteristics of focal length is determined by diameter of ceramic and frequency of sound. 4. The characteristics of beam divergence is determined by diameter of ceramic and frequency of sound. 5. The characteristics of lateral resolution is determined by beam width.

What allows some transducers to have better axial resolution than others?

1. less ringing 2. higher frequency

How do you calculate axial resolution?

1. spatial pulse length (mm)/ 2 2. wavelength (mm) x # of cycles in pulse/2 3. in soft tissue: 0.77 x # cycles in pulse/ frequency (MHz)

The diameter of a disc-shaped, unfocused piezoelectric crystal is 1.2 cm. The near zone length is 8 cm. What is the best estimate for the lateral resolution at a depth of 16 cm? A. 0.6 cm B. 1.2 cm C. 8 cm D. 16 cm

1.2 cm sound beams produced by a disc-shaped crystal have a profile where the beam tapers to a minimum diameter at the focus and then starts to diverge.

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

1.7 MHz and 5 cycles/pulse. This is longest pulse and it has the lowest frequency and most ringing (more cycles/pulse).

In soft tissue 3 cycles. 1 MHz pulse has a pulse length equal to 4.5 mm. What is the axial resolution? A. 3 mm B. 1 mm C. 2.25 mm D. 1.54 mm

2.25 mm. Axial resolution is one half spatial pulse length (4.5 mm/2= 2.25 mm).

Which of the following transducers has the best 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

2.6 MHz and 2 cycles/pulse. This is the shortest pulse and has the highest frequency and least ringing (fewest cycles/pulse).

The diameter of a disc-shaped, unfocused piezoelectric crystal is 1 cm. What is the best estimate for the minimum lateral resolution? A. 1 mm B. 5 mm C. 1 cm D. 5 cm

5mm It will be hourglass shaped. The beam tapers to a minimum diameter at the focus.

Phased array means adjustable or multiple focusing or both? A. Adjustable B. Multiple focusing C. Both

Both

Two u/s systems produce acoustic pulses. One pulse is 0.4 milliseconds in duration and the other is 0.2 milliseconds. Which pulse will most likely provide the best lateral resolution? A. 0.4 milliseconds pulse B. 0.2 milliseconds pulse C. they are the same D. cannot be determined

Cannot be determined The duration of a pulse is unrelated to lateral resolution. Pulse duration affects axial (longitudinal) resolution. It is not possible to answer the question with the information provided.

Two u/s systems have a near zone lengths of 8 cm. At the focus, System S's lateral resolution is 3.0 mm, whereas System C's is 5 mm. Which system is most likely to appropriately display two small body structures that lie, one in front of the other, at depths 8.6 and 9 mm? A. System C B. System S C. both have comparable quality D. cannot be determined

Cannot be determined There is insufficient information to answer the question. The information provided gives insight into lateral resolution of the systems. Depends on axial resolution which is not provided with the information given.

Which of the following techniques is internal focusing? A. lens B. curved crystal C. electronic

Curved crystal Focusing with a curved PZT crystal is known as internal focusing.

If a new pulsed transducer has many more cycles in its pulse, the image accuracy? A. Improves B. Degrades C. Remains the same

Degrades (more ringing)

True or false? Shorter wavelengths are characteristics of lower frequency sound.

False

True or false? As the curvature of the PZT, becomes more pronounced, the degree of focusing decreases.

False because as it becomes more pronounced, the degree of focusing increases.

True or false? Fixed Focusing techniques include only external techniques.

False because it includes both external and internal techniques.

True or False? At the end of the near zone, the beam diameter is twice the transducer diameter.

False because the beam diameter is one half the transducer diameter.

True or False? Internal and external focusing with multiple active elements.

False because they are used with single active elements.

Concentrates the sound energy into a narrower beam which improves lateral resolution.

Focusing

If there are more cycles in a pulse, the numerical value of range resolution is? A. Greater B. Lesser C. Same

Greater

High/ Low? frequency transducers generally have best range resolution.

High

Which of the following explains the difference in sound beam shape created by a disc-shaped crystal from that of a small chip of PZT? A. Huygens' Principle B. Snell's Law C. Bernoulli's Law D. Murphy's Law

Huygens' Principle explains the difference between the beam shape from tiny chips of PZT and disc crystals of a larger dimension.

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

In clinical situations, axial resolution is better than lateral resolution because u/s pulses are shorter than they are wide and the numerical value for axial resolution is less than the value for lateral resolution.

Electronic Focusing: Phased Array

In phased array transducer, the system's electronics focus the sound beam.

Huygen's Principle is based on A. refraction B. constructive indices C. transverse waves D. interference

Interference Huygens' Principle explains the difference between the beam shape from tiny chips of PZT and disc crystals of a larger dimension. The hourglass-shaped beam is caused by the interference of spherical waves.

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 are the AKAs for axial resolution?

Longitudinal Axial Range Radial Depth (LARRD)

All of the following occur with focusing except: A. lower intensity at the focus B. shorter near zone length C. more compact focal length D. smaller diameter beam diameter at the end of the near zone

Lower intensity at the focus Focusing does not lower the beam's intensity at the focus. With focusing, the near zone is shallower, the focal zone is smaller, and the beam is narrower at the focus.

Can fixed focusing be changed?

No

Is axial resolution adjustable?

No because SPL is fixed.

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

Only one reflection is observed on the image.

The focus of an u/s beam is the location where the A. beam is the broadest B. optimum transverse resolution is C. frequency is the highest D. finest depth resolution is obtained

Optimum transverse resolution is The focus of an u/s beam is the location where the beam is the most narrow which provides the optimal transverse or later resolution.

Which of the following focusing techniques is different from the others?

Phased focusing is achieved with the electronics of an u/s system. Phased focusing is adjustable. Focusing with either a lens or curved crystal is fixed.

What is axial resolution determined by?

Related to spatial pulse length. .Recall that spatial pulse length is determined by sound source and medium.

Ability to create accurate images.

Resolution

Two u/s systems have near zone lengths of 8 cm. At the focus, System G's lateral resolution is 3.0 mm, whereas system P's is 5.0 mm. Which system will produce higher quality pictures at their foci? A. System P B. System G C. both will be a similar quality

System G It has a narrower beam than System P at the end of the near field (which is the location of the foci), System G will produce higher quality images at that depth.

Two u/s systems have near zone lengths of 8 cm. At their foci, System Q's lateral resolution is 3.0 mm and System H's is 5.0 mm. Which system will correctly display two small structures at a depth of 8 cm? the objects are side by side and are 0.4 cm apart. A. System H B. System Q C. both produce similar quality pictures

System Q At a depth of 8 cm, System Q will produce a more accurate picture of two structures that are 0.4 cm apart. A system's lateral resolution is approximated by the beam diameter at that depth. System Q's beam diameter is 3.0 mm at its focus and will display this pair of reflectors accurately. System H's beam, with a diameter of 5 mm. is too wide to display this pair of reflectors as distinct and separate.

Two systems are identical except for the frequency of the emitted pulse. Which system will have the deeper focus? A. the lower frequency system B. the higher frequency system C. both foci will be at same depth

The higher frequency system High frequency piezoelectric crystals tend to produce u/s beams with deeper foci. Frequency and focal depth are directly related.

Two systems are identical except for the diameter of the transducer's piezoelectric crystal. Which system has the deepest focus? A. the system with the smaller diameter crystal B. the one with the larger diameter crystal C. their foci will be at the same depth

The one with the larger diameter crystal Large diameter crystals tends to produced u/s beams with more distant foci. The larger the crystal diameter, the deeper the focus.

In which region of a sound beam is focusing most effective?

The region of a beam that is most affected by focusing is close to the end of the near zone and at the beginning of the far zone= The focal zone.

What is lateral resolution determined by?

The width of the sound beam.

Two ultrasound systems are identical except for the PRP of the emitted pulse. Which system will have the deeper focus? A.the system with the lower PRP B. the system with the higher PRP C. their foci will be at the same depth

Their foci will be at the same depth Changes in PRP do not affect the depth of the focus. The frequency of the sound and the piezoelectric crystal's diameter help to determine the focal depth.

True or False? At a depth of twice the near zone, the beam is as a wide as the transducer.

True

True or False? Electronic focusing may be used only on multi-element transducers, never on single crystal transducers.

True

True or False? Phased array technology is more versatile than fixed focusing techniques.

True

True or false? As the arch of the lens becomes more prominent, the degree of focusing increases and the beam narrows in focal zone.

True

True or false? Fixed focusing is a fixed-focusing technique.

True

True or false? Fixed focusing techniques are also called conventional or mechanical focusing.

True

True or false? One way to reduce ringing is to dampen the crystal after it has been excited by an electrical signal from the system.

True

True or false? Shorter pulses improve axial resolution.

True

True or false? When using the standard u/s imaging instrumentation, the lateral resolution has a higher numerical value than the axial resolution.

True because most imaging systems have better axial resolution than lateral resolution.

True or false. In comparison to other locations along the length of an u/s beam, focusing is generally ineffective in the far zone.

True.

True or false? Lateral resolution is best at the focus (at the end of near zone) where the beam is narrowest.

True. Lateral resolution is good within focal zone.

The lateral resolution of an u/s system primarily determined by the A. width of the sound pulse B. length of the u/s pulse C. duration of the sound pulse D. none of the above

Width of the sound pulse The lateral resolution achieved by an imaging system is approximately equal to the width of the u/s pulse.

Is phased array transducers adjustable?

Yes, the sonographer can adjust the focusing characteristics of a beam.

Internal focusing: Curvature of PZT

a curved piezoelectric crystal concentrates the sound energy into a narrower or tighter sound beam.

External Focusing: The lens

a lens is placed in front of the piezoelectric material.

Describes one measure of the detail found in an image.

axial resolution

It 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.

axial resolution

Measured in mm or any other unit of distance.

axial resolution

As sound travels deeply into the far zone, the beam diverges, or spreads out. Which of the following will result in a minimum beam divergence deep in the far zone? A. small diameter , high frequency B. high frequency, large diameter C. large diameter, low frequency D. low frequency, small diameter

high frequency, large diameter large diameter crystals of high frequency tend to produce sound beams that diverge less in the deep far zone.

All of the following terms describe the shape of a sound wave created by a tiny fragment of PZT except: A. spherical B. Huygens' wavelet C. V-shaped D. hourglass shaped

hourglass shaped small paces of the PZT create V-shaped waves. Choices A and B have the same meaning as V-shaped.

For a disc-shaped PZT crystal producing continuous sound, which design will create a beam with the deepest focus? A. large diameter, low frequency B. large diameter, high frequency C. small diameter, low frequency D. small diameter, high frequency

large diameter, high frequency It has the longest near zone (the deepest focus) will have a large diameter and a high frequency.

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

lateral resolution (mm)= beam diameter (mm)

What are units for lateral resolution?

mm, cm , or any unit of distance.

For a disc-shaped PZT crystal producing continuous sound, which design will create a beam with the shallowest focus? A. large diameter, low frequency B. large diameter, high frequency C. small diameter, low frequency D. small diameter, high frequency

small diameter, low frequency It has the shortest near zone (the most shallow focus) will have a small diameter and a low emitted frequency

What will lower the value of an u/s system's lateral resolution?

use an acoustic lens because to lower the numerical value of lateral resolution of an u/s system, the diameter of the beam must be reduced. A commonly used technique to decrease the diameter of the beam is to focus the beam with an acoustic lens.