Principles of Echo Ultrasound

Question 1

Sound Wave Definition

Answer 1

Mechanical longitudinal energy waves

Produce alternate rarefactions and compressions of any physical medium through which they pass

Question 2

4 Acoustic Variables

Answer 2

Pressure
Density
Temperature
Particle Motion (Distance)

Question 3

Diagnostic Imaging Frequencies

Answer 3

Between 1 and 20 MHz

Question 4

Propagation speed in soft tissue

Answer 4

1540 m/sec or 1.54 mm/us

Question 5

Frequency Definition

Answer 5

Cycles per second (Hz)

Question 6

Wavelength Definition

Answer 6

Distance of one cycle (mm)

Question 7

Amplitude

Answer 7

Highest acoustic variable (pressure) - average

can be expressed in dB

Question 8

Changes in amplitude, power, intensity can be described as what?

Answer 8

Decibels (dB)
Amplitude Db = 20 log (amp final/amp initial)
Power/Intensity Db= 10 log (power final/power initial)

Question 9

What determines impedance?

Answer 9

density and propagation speed of the tissue

Question 10

Amount of ultrasound reflected at a boundary is dependent on the relative changes in ___________ between two structures.

Answer 10

Acoustic Impedance (Z)

Question 11

What degree angle provides optimal return to the transducer (AKA best for 2D)?

Answer 11

90 Degrees

Question 12

What type of reflection occurs with smooth tissue boundaries with lateral dimensions greater than the wavelength?

Answer 12

Specular or mirror like reflections

Question 13

What type of reflection occurs with rough surfaces or boundaries with lateral dimensions greater than the wavelength?

Answer 13

Backscatter reflection

can bring in tissue texture and non perpendicular structures

Question 14

Boundaries with lateral dimensions less than the wavelength result in what?

Answer 14

scattering of the ultrasound signal randomly in multiple directions

Question 15

What is rayleigh scattering?

Answer 15

scattering that dissipates ultrasound equally in all directions
example: RBCs

Question 16

Attenuation is dependent on what?

Answer 16

Frequency
Low frequency attenuates less
Maximal depth of penetration is roughly 200 wavelengths

Question 17

Piezoelectric Crystals

Answer 17

Material that changes shape and vibrates when a current is applied to it (allows transmission of US) and when signal returns it changes shape and gives off electrical signal to computer (reception of US)

Question 18

What is time of flight?

Answer 18

Allows for the depth of the structure to be calculated (13 us = 1 cm depth in soft tissue)

Question 19

How to improve axial resolution?

Answer 19

Shorter Pulses

• damping material shortens pulses (reduces ringing)
• higher frequencies shortens pulses

Question 20

Bandwidth definition

Answer 20

The range of frequencies present within the pulse

Question 21

What type of transducer beam allows for electronic focusing?

Answer 21

Phased array

Question 22

US energy can disperse laterally to beam creating what?

Answer 22

side lobe artifact in mechanical probes or

grating lobe artifact in phased array probes

Question 23

Transducer frequencies in Adults, Peds/Adolescent, and Peds/Infants?

Answer 23

Adults 1-3 MHz
Peds/Adolescents 3-5 MHz
Peds/Infants 5-7 MHz

Question 24

Longitudinal Resolution

Answer 24

AKA Axial Resolution
Ability to see two structures front to back
Determined mostly by pulse length

Question 25

Lateral Resolution

Answer 25

Ability to see two structures side to side

Determined mostly by beam width

Question 26

Elevational Resolution

Answer 26

Ability to see two structures top to bottom

Determined by beam height

Question 27

Temporal Resolution

Answer 27

Determine position of a structure at an instant in time; generally a tradeoff between line density and frame rate

Question 28

Desired frame rate for cardiac imaging

Answer 28

> 30 Hz

Question 29

How to improve aliasing?

Answer 29

• increase scale
• decrease depth
• use high PRF (pulses per second)
• reduce frequency
• shift baseline
• use CW