This lesson was derived

from pages 15 through 21

in the textbook:

Lesson 04:

Resolution and Attenuation

This lesson contains 21 slides

plus 18 multiple-choice

questions.

Resolution and

Attenuation

GOOD

GOOD POOR

Axial and lateral resolution, which are categories of spatial resolution are

parameters of an ultrasound imaging system that characterizes its ability to

detect closely spaced interfaces and to display the echoes from those

interfaces as distinct and separate objects. The better the resolution, the

greater the clarity, or sharpness, of an ultrasound image.

Interfaces not closely spaced

Closely spaced

RESOLUTION

Closely spaced

GOOD

GOOD POOR

Of the many parameters affecting the axial and lateral resolution of an

ultrasound system, the most important is the operating frequency of the

ultrasound transducer. Transducers that operate at higher frequencies provide

improved resolution. The trade-off when using higher frequencies is reduced

penetration.

Interfaces not closely spaced

Closely spaced

RESOLUTION

Closely spaced

AXIAL RESOLUTION

SCANNED STRUCTURE DISPLAYED IMAGE

Axial resolution (also called longitudinal, range or depth resolution) is the

minimum required reflector separation along the direction of propagation

required to produce separate reflections.

AXIAL RESOLUTION

The axial resolution has a value equal to one-half the spatial pulse length.

Good axial resolution is achieved with short spatial pulse lengths. Short spatial

pulse lengths are the result of higher frequency, highly damped transducers.

SPATIAL PULSE LENGTH AXIAL RESOLUTION

4 mm 2 mm

3 mm 1.5 mm

2 mm 1 mm

LATERAL RESOLUTION

Lateral resolution (also called azimuthal resolution) is the minimum reflector

separation perpendicular to the direction of propagation required to produce

separate reflections.

SCANNED STRUCTURE DISPLAYED IMAGE

LATERAL RESOLUTION

Good lateral resolution is achieved with narrow acoustic beams. A narrow

acoustic beam is the result of a long near field (Fresnel zone) and a small angle

of divergence in the far field (Fraunhofer zone). The lateral resolution has a

value equal to the beam-width.

Most transducers in use today incorporate arrays rather than single-elements.

However, understanding the basic concepts of single-element transducers

should provide a basis for a better understanding of transducer arrays.

BEAM-WIDTH

LATERAL RESOLUTION

The beam-width, or beam diameter, from a non-focused, circular piezoelectric

element narrows as it propagates through the near field to a dimension equal to

half the diameter, or aperture size, of the piezoelectric element. The beam-

width then widens (diverges) as it propagates through the far field. At a

distance from the piezoelectric element equal to twice the length of the near

field, the beam-width has a dimension equal to the element’s aperture size.

The lateral resolution at given depth has a value equal to the beam-width at

that depth.

BEAM-WIDTH

LATERAL RESOLUTION

BEAM-WIDTH

BEAM-WIDTH LATERAL RESOLUTION

4 mm 4 mm

3 mm 3 mm

2 mm 2 mm

LATERAL RESOLUTION

The angle of beam divergence decreases in the far field and the length of the

near field increases if a higher frequency transducer is used. Additionally, the

angle of beam divergence decreases in the far field and the length of the near

field increases if a transducer with a larger aperture size.

Focusing can be used to further improve the lateral resolution. Focusing

creates a beam pattern that converges within the near field to produce a very

small cross section. The reduced cross section results in an intensity increase,

which is proportional to the square of the reduction. However, beyond the focal

zone, beam divergence is greater than from a non-focused transducer.

LATERAL RESOLUTION

Focal zones from single-element transducers are fixed during the

manufacturing process and cannot be changed. Various “fixed-focus” methods

are used including curved piezoelectric elements and external acoustic lenses.

Fixed focusing is often called “mechanical” focusing.

LATERAL RESOLUTION

Although the focus of a single-element transducer cannot be changed,

transducer arrays, produce sound beams with dynamic (multiple) transmit focal

zones at all imaging depths along the two-dimensional scanning plane. The

use of a dynamic aperture minimizes variations in beam width.

LATERAL RESOLUTION

HIGH-FREQUENCY TRANSDUCERS

BETTER RESOLUTION

GREATER ATTENUATION

POORER PENETRATION

LOW-FREQUENCY TRANSDUCERS

POORER RESOLUTION

LESS ATTENUATION

BETTER PENETRATION

Although improved resolution is obtained when high frequency transducers are

used, there is greater attenuation, which is the reduction of sound energy as it

passes through most materials. Many factors contribute to attenuation,

including reflection and beam divergence. However, the major cause of

attenuation is absorption, which is the result of heat conversion due to friction

created by vibrating tissue.

RESOLUTION vs. PENETRATION

(in tissue)

a = - 0.5 dB per cm per MHz

ATTENUATION COEFFICIENT

Attenuation in human soft tissue averages 0.5 dB per centimeter for each

megahertz. A loss totaling 3 dB is equivalent to a loss of one-half of the sound

energy. A 3 dB loss is present at the half intensity depth (H.I.D).

Attenuation is very high in bone with high and low frequencies. Conversely,

attenuation is low in fluids (e.g., blood) and very low in water, even when high

frequencies are used. Regardless of the medium, penetration is inversely

proportional to the transducer’s frequency.

In human soft tissue (assuming attenuation of -0.5 dB per cm per MHz), the

H.I.D. (in cm) may be determined by dividing 6 by the transducer frequency

(in MHz).

HALF INTENSITY DEPTH

(in tissue)

H.I.D. = 6 divided by frequency

TRANSDUCER FREQUENCY ATTENUATION PENETRATION HALF INTENSITY DEPTH

Increase Increase Decrease Decrease

Decrease Decrease Increase Increase

ATTENUATION COEFFICIENTS IN TISSUE(based on - 0.5 dB per cm per MHz)

Frequency -dB per cm Half-Intensity-Depth

2 MHz 1 3 cm

2.25 MHz 1.125 2.67 cm

2.5 MHz 1.25 2.4 cm

3 MHz 1.5 2 cm

3.5 MHz 1.75 1.71 cm

4 MHz 2 1.5 cm

5 MHz 2.5 1.2 cm

7 MHz 3.5 0.86 cm

7.5 MHz 3.75 0.8 cm

10 MHz 5 0.6 cm

15 MHz 7.5 0.4 cm

2 MHz 2.25 MHz 2.5 MHz

TRANSDUCER FREQUENCIES

5 MHz 7 MHz 7.5 MHz

10 MHz 12 MHz 15 MHz

3 MHz 3.5 MHz 4 MHz

ADULT LIVER

2.5 MHz 4 MHzAlthough frequencies in the range of 3 MHz to 5 MHz are normally used for

general purpose imaging (abdominal, obstetrical, gynecologic, and cardiac), a

frequency of 2.25 MHz or 2.5 MHz may be needed for adequate sound

penetration of a larger than normal patient.

Although frequencies in the range of 3 MHz to 5 MHz are normally used for

general purpose imaging (abdominal, obstetrical, gynecologic, and cardiac), a

frequency of 2.25 MHz or 2.5 MHz may be needed for adequate sound

penetration of a larger than normal patient.

3.5 MHz 5 MHz

ADULT LIVER AND RIGHT KIDNEY

7.5 MHz

THYROIDBREAST VASCULAR OPHTHALMIC

7 MHz 12 MHz10 MHz

SMALL PARTS

A frequency of 5 MHz or higher may be used if it is not necessary to display

echoes from deeper structures. Frequencies above 5 MHz are typically used

for thyroid, breast, peripheral vascular, ophthalmic, endocavity,

musculoskeletal, intraoperative, and some general purpose pediatric studies.

High frequency ultrasound (above 20 MHz) is used for dermatology,

stomatology, and intravascular ultrasound.

Answers to the following

EIGHTEEN practice

questions were derived

from material in the

textbook:

Question 1

The minimum reflector separation required to produce

separate echoes is

the spatial resolution of the ultrasound system

a function of TGC

the dynamic range

the attenuation coefficient

the reflection coefficient

Page 15

Question 2

Axial resolution is affected by all of the following EXCEPT

frequency

damping

spatial pulse length

wavelength

focusing

Page 16

Question 3

Far field beam divergence can be reduced on a

single-element transducer by using

electronic focusing

a transducer with a smaller element diameter

a higher frequency transducer

a lower frequency transducer or a smaller

element diameter

adjustable focusing

Pages 17 and 18

Question 4

The area between the face of an unfocused single-element

transducer and the point where the beam starts to diverge

is the

fraunhofer zone

refraction zone

focal plane

far zone

near field

Page 17

Question 5

Assuming a fixed frequency, when the diameter of an

unfocused transducer is increased, the

far field divergence increases

penetration decreases

length of the near field increases

length of the near field decreases

attenuation increases

Page 18

Question 6

The fresnel zone is the

focal zone of a focused transducer

far zone

region where the near field changes to the far zone

distance from the face of a non-focused transducer

to the beginning of the far field

the fraunhofer zone

Page 17

Question 7

Assuming no losses due to attenuation, which reflector

provides the strongest echo?

A

B

C

D

E

Page 18

Question 8

A focused, curved, single piezoelectric element

can be used for CW Doppler

can be dynamically focused

produces a beam pattern that is determined during

manufacturing

can be electronically focused

can be used in a convex array transducer

Page 18

Question 9

Which of the following does NOT affect lateral resolution?

focusing

beam width

element diameter

frequency

Spatial pulse length

Pages 16 and 17

Question 10

Higher frequency transducers provide

improved lateral resolution

smaller Doppler shifts

improved axial resolution and reduced attenuation

increased penetration

poor axial resolution

Pages 15 and 19

Question 11

Which of the following does NOT contribute to attenuation?

beam divergence

scattering

absorption

reflection

constructive interference

Pages 4 and 19

Question 12

As the frequency of sound increases,

the amount of scatter is increased

the attenuation decreases

the amount of scatter decreases

the penetration increases

there will be a decrease in the number of specular reflectors

Page 19

Question 13

Which of the following transducers provides the maximum

penetration?

10.0 MHz

7.5 MHz

2.25 MHz

3.5 MHz

5.0 MHz

Pages 19 and 20

Question 14

Which transducer configuration is the best choice for

imaging superficial structures?

high frequency, far focus

high frequency, near focus

low frequency, far focus

low frequency, near focus

low frequency, mid focus

Page 21

Question 15

The average attenuation of ultrasound energy in the patient

is approximately

2.0 dB per cm per MHz

10.0 dB per cm per MHz

20.0 dB per cm per MHz

5.0 dB per cm per MHz

0.5 dB per cm per MHz

Page 19

Question 16

If sound from a 3 MHz transducer has 3 dB of attenuation

after traveling through 2 cm of tissue, what is the amount of

attenuation of sound from a 5 MHz transducer after

traveling through 1 cm of the same tissue?

5 dB

1 dB

2 dB

2.5 dB

3 dB

Page 19

Question 17

The half-value-layer or the half-intensity-depth

is the depth where the intensity is 50% of the originally

transmitted intensity

increases as the frequency of the transducer increases

is the thickness of the matching layer in an array

is the range of frequencies contained in an ultrasound

pulse

is the attenuation coefficient in tissue

Page 19

Question 18

Lateral resolution is determined mainly by

reflector size

beam diameter

pulse duration

bandwidth

spatial pulse length

Page 17

END OF LESSON 04