SPI Basics III

incidence angle-

Angle of incidence

angle at which the sound beam encounters a boundary. Either normal or oblique. Determines the response of the sound pulse

Normal Incidence

sound beam encounters a boundary at 90 degrees.

Acronym for normal incidence

PORNN

perpendicular, orthogonal, right angle, ninety degrees

What MUST occur with normal incidence

for reflection to occur at a boundary, there must be a difference in acoustic impedance between interfaces

Z1 and Z2 meanings with normal incidence

Z1 is the medium that is closer to the transducer. Z2 is the medium further away

What determines a small vs large reflection w normal incidence

a small mismatch in impedance =small reflection vice versa

What happens with a normal incidence if the impedance between two media are identical?

all sound will continue to be transmitted

What is transmission

when there is no impedance mismatch between media Z1=Z2

Incident intensity

(starting intensity) intensity of the sound wave at the instant prior to striking a boundary

reflected intensity

portion of the incident intensity that, after striking a boundary, changes direction and returns back from where it came

transmitted intensity

portion of the incident intensity that, after striking a boundary, continues on in the same general direction that it was originally traveling

Incident intensity equation

reflected intensity + transmitted intensity (conservation of energy) exists at a boudnary

what is a coefficient

a percentage

Intensity reflection coefficient (IRC)

the percentage of the US intensity that bounces back when the sound strikes a boundary

Intensity transmission coefficient (ITC)

the percentage of the incident intensity that, after striking a boundary, continues on in the same general direction that it was originally travelingUn

units of IRC and ITC

none they are percentages

a greater percentage results in what

a reflection at a boundary between media which DIFFER in acoustic impedance

Percentage of reflection in biologic media between soft tissue: soft tissue

<1%

Percentage of reflection in biologic media between soft tissue: bone or calcific structure

~50%

Percentage of reflection in biologic media between soft tissue: air or gaseous structure

~99%

When IRC and ITC are added the result is

100%

IRC + ITC= 100

With normal incidence: whatever isn’t reflected is

transmitted or vice versa

Intensity reflection coefficient equation

(Z2-Z1/Z2+Z1)²

Oblique incidence

the incident sound beam encounters a boundary at an angle other than 90 degrees (non-perpendicular)

Transmission and reflection with oblique incidence

may or may not occur there are no simple rules to predict if so “i don’t know”

2 types of oblique incidence angles

acute and obtuse (oblique incidence is NON RELIANT ON IMPEDANCE)

When impedance between interfaces are equal with oblique incidence

reflection may or may not occur

What do we know with oblique incidence

the law of conservation of energy. The angle of incidence is = to the angle of reflection

Incident intensity equation for oblique

transmitted + reflected= incident intensity

Will the acoustic reflection return back to the transducer with oblique?

NO. sound is reflected in a different direction EQUAL and OPPOSITE to direction of the incident beam

Transmission with oblique incidence

can travel straight ahead or change direction.

What is refraction

a bend or change in direction as sound travels from one medium into another. occurs during transmission

Refraction requires both

oblique incidence and different propagation speeds

Refraction cannot occur with

normal incidence or with identical propagation speeds

Snell’s law

describes the physics of refraction

Equation for snell’s law

sin(transmission angle)/sin (incident angle)=propagation speed2/propagation speed 1

When the speed in medium 1 is faster than the speed in medium 2

angle of incidence>angle of transmission (angle of transmission is less)

go-return time, time of flight or round trip time

the time needed for a pulse to travel to and from the transducer and the reflector

since the average speed of sound in soft tissue is 1.54km/sec the time of flight and distance that sound travels in the body are

directly related

when one reflector is twice as deep as another the pulse’s time of light is

increased by. a factor of 2

the 13 microsecond rule

in soft tissue every 13us of go return time means the reflector is 1cm deeper in the body

time of flight-reflector depth-total distance traveled

13us- 1cm- 2cm

what if the time of flight is 26us

reflector depth=2cm and distance=4cm

speed=

distance/time

what is a range equation

how an ultrasound systems place anatomical structures at the appropriate depth (depth on right side of screen in cm)

range EQUATION

d= ct/2

c=speed of sound in soft tissue

t=round trip time

A short time PRP is required between pulse production and reception with

shallow reflectors

A long time PRP is required between pulse production and reception with reflectors

at greater depths

whenever you see the word range you know its referring to

depth