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US Basic Physics Principles Flashcards

1
Q

Each object the sound travels through or

(an intervening substance, as air, through which a force acts or an effect is produced)

A

MEDIUM

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2
Q

The molecules vibrate in the same direction as the sound travels (e.x. Ultrasound)

A

LONGITUDINAL WAVES

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3
Q

The molecules vibrate at 90 degrees to the direction of energy travel (e.x. water waves)

A

TRANSVERSE WAVES

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4
Q

Concentration of force

(units: lb/sq inch, Pascals Pa)

A

PRESSURE

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5
Q

The concentration of matter (mass per unit volume)
(units: kg/cubic cm)

As density increases, speed decreases (inversely related)

Because the material is heavier and has more inertia. Its more difficult to start and stop the sound moving.

A

DENSITY

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6
Q

Measure of particle motion

(units: cm, feet, miles)

A

DISTANCE

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7
Q

The distance of one cycle

Determined by the source and the medium
Wavelength = propag speed (mm/ms)
frequency (MHz)
Units: mm or meter (any unit of length)

A

WAVELENGTH

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8
Q

The length of time it takes to complete one single cycle of sound or to one complete single wavelength
( Units: seconds, msec, hours – all units of time;
Determined by: sound source)

A

PERIOD

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9
Q

The number of wave crests passing a point in a single second
(Unit: Hertz = cycles per second)

A

FREQUENCY

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10
Q

The speed at which sound wave moves through the medium
C = frequency x wavelength
Units: meters/sec or mm/sec
Determined by the medium only
Average speed of sound through the body is 1540 m/s (NEED TO KNOW FOR REGISTRY)

A

PROPAGATION SPEED OR VELOCITY

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11
Q

The resistance of material to compression

The main factor in determining propagation speed

The harder the material, the less compressible it is

Harder media have higher sound speeds

A

STIFFNESS
(Compressibility is the opposite of stiffness)

Stiffness overrides density in determining sound speed

EXAMPLES:

Bones are very stiff but also very porous so have a low density
So has a high speed

Solids - highest speed
More molecules per area
Stiffer than liquids or gasses

Liquids - medium speed
More molecules than gas per area
Stiffer than gasses
Fat has lowest speed of the solids

Gasses lowest speed
Smallest number of molecules
(NEED TO KNOW FOR REGISTRY)

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12
Q

Contains additional frequencies that are even and odd multiples of the original frequency

A

HARMONICS

As the waveform becomes less sinusoidal, the harmonics get stronger

This is function programmed into the machine

Harmonics improve sonographic image quality

Harmonics do not affect the transmit frequency

Received frequency is twice that of the transmit frequency

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13
Q

A pulse is a collection of cycles that travel together

Continuous wave US cannot form images
Must have listening time to form images

The only thing we can change is the listening time between the pulsed cycles

We do this when we change “depth”

Deeper depth, more listening time (have to wait for original pulses to return before sending out new ones!)

Shallower depth, shorter listening times

The pulses themselves never change

A

PULSED ULTRASOUND

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14
Q

Time from the start of one pulse to the end of the pulse

ONLY the time that the pulses are on or transmitting

The time it takes for one pulse to occur

 =   # of cycles in the pulse x period
Or     # of cycles in pulse / frequency

Units: seconds; microseconds

Determined by: the source (transducer) only

Usually 2-3 cycles for imaging; 5-20 cycles for Doppler

A

PULSE DURATION

Shorter pulses generally create higher quality images

Short pulses have: Few cycles (less sending out)

Each individual cycle has a short period (time)

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15
Q

Percentage of time that the system is transmitting a pulse (or ‘on’ / not listening)

Units – No units (unitless)

The words: factor, coefficient, Index – tell you that there are no units

= Pulse duration (sec) / PRP (sec) x 100

Determined by the sound source

Continuous wave sends out 100% of the time so duty factor for continuous wave is 1.0 (always on!)

Can be changed by the sonographer ……..How?

By changing depth or listening time

A

DUTY FACTOR

High PRF have high duty factor because they are pulsing often

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16
Q

Number of pulses that occur in a single second

Similar to frequency (number of events occurring per second)

Units: Hz or per second

US transducers usually emit a few thousand pulses per second

Determined by: Sound source

Can be changed by the sonographer

A

PULSE REPETITION FREQUENCY

As imaging depth increases, PRF decreases (inverse relationship)

As sonographer adjusts depth setting, they change the PRF

Shallow image – high PRF

Deep image – low PRF (takes longer for pulse to travel back to the transducer)

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17
Q

Time from the start of one pulse to the start of the next

Includes both the time that the pulse is on and the time that it is off (dead time)

Determined by: sound source

Units: Seconds, microseconds

Can be changed by the sonographer

We change only the listening time, never the pulse duration itself

A

PULSE REPETITION PERIOD

PRP decreases as PRF increases because more pulses occur in one second

PRP and PRF are reciprocals

Inverse (one goes up; other goes down)

PRP(sec) X PRF(Hz) = 1

PRP = 1/PRF

PRF = 1/PRP

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18
Q

The distance a pulse occupies in space

DISTANCE from the start of the pulse to the end of the pulse

Units: meter, mm, any unit of distance

=   # of cycles in the pulse x  	wavelength

Determined by the sound source and the medium

Cannot be changed by the sonographer

Decreases with increasing frequency (not with increasing PRF, but increasing frequency)

Shorter pulse lengths give better sonographic image quality

A

SPATIAL PULSE LENGTH

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19
Q

These fundamental properties never change, regardless if using pulsed wave or continuous wave US

A

FREQUENCY, PERIOD, WAVELENGTH, PROPAGATION SPEED

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20
Q

Weakening of a sound wave as it travels through media

Determined by frequency of sound and the distance it travels (as each increases, attenuation increases – directly related)

Consists of – Absorption, Reflection, Scattering, (mostly absorption - 80%)

Decrease in amplitude and intensity as sound beam travels

Units: Decibels (dB)
Named after Alexander Graham Bell (telephone)

Bel is the logarithmic ratio of the relative power in two acoustic beams

Decibel is 1/10th of a Bel

A

ATTENUATION

The farther the sound beam travels, the more attenuation that occurs

In soft tissue, the greater the frequency used, the greater the attenuation

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21
Q

Relative, not absolute units

Two intensities or amplitudes are required for computation of decibels

A

DECIBELS

Decibels involve the use of mathematical logarithms

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22
Q

The 2nd intensity is smaller than the original intensity

Example: the intensity of the sound that returns to the probe after it has traveled through the body is lower than the sound we originally sent out

A

NEGATIVE DECIBELS

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23
Q

The 2nd value measured is larger than the original value measured

Example: When we turn up the gain, there is a several decibel increase in the measured amplification.

A

POSITIVE DECIBELS

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24
Q

ATTENUATION COEFFICIENT

A

Attenuation is reduction in intensity as wave travels through a medium

Attenuation coefficients are numerical values that express how different materials will attenuate the sound beam per unit path length

Attenuation that occurs with each centimeter the sound travels

Expressed in dB/cm

Total attenuation (dB) =Atten coeff (dB/cm) x distance in cm

Multiply the coeff x the distance traveled

The coefficient itself does not change as path length changes (the coefficient is dB/cm)

However total attenuation changes with path length
Overall attenuation increases when frequency, path length, or attenuation coefficient increases

(Coefficient = ½ transducer frequency)

25
Q

ATTENUATION COEFFICIENT IN SOFT TISSUE

A

In soft tissue, the atten coeff and frequency are directly related.

The average attenuation coefficient in soft tissue is ½ the frequency in megahertz

As frequency increases, attenuation increases

26
Q

The distance the sound beam penetrates into the tissue when its intensity is reduced to half the original value

Represents a 3 dB decrease in intensity (- 3 dB intensity has decreased by ½ )

The higher the frequency, the shallower the depth it occurs – why?

A

HALF-VALUE LAYER

27
Q

The fraction of the original intensity remaining at the end of sound travel/after attenuation

A

INTENSITY RATIO

28
Q

Conversion of sound to heat

Transfer of energy from the sound beam to the tissue

Directly proportional to frequency

Higher frequency; higher absorption, more attenuation

A

ABSORPTION

29
Q

Just like you need a reflection from a mirror to see yourself, we need reflections from the body to make our images.

A

REFLECTION

Reflections occur at boundaries between two different media.
These are also called acoustic interfaces.

REFLECTION occurs when sound energy strikes a boundary and some gets sent back to transducer. For reflection to occur, we MUST HAVE…..

  1. An acoustic interface between two media……and…
  2. The media must have different impedances for reflection to occur

The greater the impedance difference, the greater the reflection that occurs

30
Q

Material’s resistance to sound traveling through it (unit is Rayl)

A characteristic of the medium only

Each tissue has its own acoustic impedance

It is the acoustic resistance to sound traveling through the medium (represented by Z)

A

IMPEDANCE

Impedance (Z) = Density (kg/m3) x propagation speed (m/s)

If density or propagation speed increases, impedance increases

-When an US beam strikes a specular reflector (large flat smooth boundary [larger than wavelength]) perpendicularly……
The intensity of the reflected wave depends on the difference in acoustic impedance between the 2 media that form the interface

-The larger the difference in acoustic impedance at the interface…..
The greater the intensity of the sound reflected back toward the transducer

-Sound will not reflect off the interface between 2 media if their acoustic impedance is exactly the same

31
Q

Very smooth surfaced, mirror-like reflector

Reflector is very large compared to sound beam’s wavelength

Reflection is well-defined and regular

A

SPECULAR REFLECTOR

32
Q

TYPES OF REFLECTORS

A

Backscatter – scatter returning in the same general direction as the transducer
But sound is disorganized and random
Occurs when boundary has irregularities about the same size as the sound’s wavelength

Scattering Non-specular – the random redirection of sound waves in multiple directions
Produced when sound beam strikes a rough surface
Where surface irregularities are same size or smaller than the wavelength

33
Q

Attenuation process in which the beam interacts with interfaces smaller than the wavelength of the beam

Causes the US energy to be dispersed in multiple directions

Can also be caused by rough or irregular surfaces (called non-specular reflector)

Directly related to frequency; higher frequency sound scatters more (shorter wavelengths)

A

SCATTERING

Shows that scattering allows US imaging of tissue boundaries that are not perpendicular to the incident sound

Scattering also allows imaging of tissue parenchyma in addition to organ boundaries

34
Q

Sound scatters symmetrically in all directions

Not related to incidence angle

Directly proportional to the frequency

A

RAYLEIGH SCATTERING

If frequency is doubled, Rayleigh scattering is 16 times greater

35
Q

The angle between the incident sound beam and the interface between the 2 tissues

Perpendicular, Orthogonal, Right Angle, Ninety Degrees (PORN) – Occurs when the sound beam strikes a boundary between 2 media at exactly 90 degrees

A

ANGLE OF INCIDENCE

36
Q

Incoming sound strikes the boundary at 90 degrees

A

NORMAL INCIDENCE

37
Q

sound beam strikes at any angle other than 90 degree

A

OBLIQUE INCIDENCE

38
Q

Uncertain if Reflection will Occur!
Must Say we Don’t Know!

Some sound is transmitted; some reflected

A

REFLECTION WITH OBLIQUE INCIDENCE

39
Q

Reflection occurs if the 2 media at the boundary have different acoustic impedance

A

REFLECTION WITH NORMAL INCIDENCE

40
Q

A bending from the straight line path or a change in direction of a sound wave traveling from one medium to another (goes in same general direction

ASSOCIATED WITH TRANSMISSION

Must have:
1. Oblique Incidence
2. Media with different propagation
speeds

The amount of deflection off the straight line path is directly related to the change in acoustic velocity from one tissue to the other (the difference between the 2)

Slightly different sound speeds creates a small deviation

Greatly different sound speeds creates a substantial deviation

A

REFRACTION

41
Q

SNELL’S LAW

A

The physics of refraction are described by Snell’s Law

For two given media, the sine of the angle of incidence bears a constant relation to the sine of the angle of refraction

Equation states:
Sine (transmission angle) = Propagation Speed 2
Sine (incidence angle) Propagation Speed 1

42
Q

2 Sound waves encounter / or interfere with one another

The net result is adding the 2 waves together

May be Constructive or Destructive

A

WAVE INTERFERENCE

43
Q

Peaks encounter peaks
Troughs encounter troughs
Stronger Wave Results

A

CONSTRUCTIVE INTERFERENCE

44
Q

Peaks encounter Troughs
Troughs encounter Peaks
Smaller Wave Results

A

DESTRUCTIVE INTERFERENCE

45
Q

To display echoes anatomically correctly on the US screen the US unit uses the time delay between pulse transmission and reception to determine the distance to each interface

Relationship between round trip pulse-travel time (sound to interface and echo coming back from it), propagation speed, and distance to a reflector

Distance to boundary =
go-return time (microsec) x speed
2

D =  ct    or    ½ CT    (c = prop 
   2				   speed)
D = distance to reflector
V = velocity or propagation speed
       (1540 m/s)
T = round trip travel time (time of flight)

The longer the round trip travel time, the further the distance from the transducer

A

RANGE EQUATION

46
Q

In soft tissue, every 13 microsec of go-return time means the reflector is 1 cm deeper in the body

A

13MICROSECOND RULE

47
Q

All three are measures of the strength of the sound wave

All are related to one another

All can be changed by the sonographer

All decrease as the sound wave travels through the body

A

AMPLITUDE-POWER-DENSITY

48
Q

Strength of the beam

The maximum variation in acoustic variable from its baseline state (when sound is moving through the area)

Units depend on the acoustic variable being affected

Can be expressed in dB units (decibels)

Can be changed by the operator (with intensity and power)

When considering attenuation, the new intensity being compared is always less than the initial intensity

A

AMPLITUDE

Amplitude is the difference between the average or equilibrium value and the maximum or

The amplitude of US waves emitted from the transducer depends on the electric stimulation sent to the transducer.

The larger the electric pulse the more intense the beam produced and the more particle displacement occurs in the medium

Positive dB are used to express amplifier gain of receiver

49
Q

Rate that work is performed; rate of energy transfer (WATTS)

Force that causes displacement

Units – Watts

IT is divided by a beam’s cross sectional area

Therefore IT is directly proportional to a wave’s amplitude squared

In a beam IT’s expressed in watts

Directly proportional to the wave amplitude squared

Whatever value or power amplitude increases, square IT and IT goes up that much more

A

POWER

50
Q

Concentration of energy in certain areas of the sound beam (used for bioeffects) (WATTS/CM2

IT depends on the power of the beam and the cross-sectional area of the beam

Concentration of energy in a sound beam

Equals the beam’s power divided by the beam’s cross sectional area

Directly proportional to power
If power is doubled, IT is doubled
If power is quartered, IT is quartered

Directly proportional to amplitude squared
If amplitude is doubled, IT goes up 4 times
If amplitude is quartered, IT reduces to 1/16

A

INTENSITY

51
Q

The highest intensity area or time of the sound beam (maximum value)

A

PEAK

52
Q

take peak, low, and medium intensities and average them together to get average intensity (mean value)

A

AVERAGE

53
Q

Space related variations

Related to space/areas of the beam

TERMINOLOGYSpatial intensity is greatest at the center of the beam and at the focus of the beam

Intensity diminishes as you near the edges of the beam

Maximum spatial intensity is called spatial peak (SP)

Spatial average (SA) is the AVERAGE intensity across the US beam (considerably less than spatial peak)

A

SPATIAL

54
Q

Time related variations

Temporal non-uniformity
Occurs with time (temporal)

In continuous wave, temporal peak and average are the same

In pulsed US, temporal peak occurs while the pulse is on

Temporal average includes the time that the pulse is off. Low in pulsed US.

A

TEMPORAL

55
Q

TERMINOLOGY

A 
SP = Spatial Peak
SA = Spatial Average
TP = Temporal Peak
TA = Temporal Average
56
Q

BEAM UNIFORMITY

A

In order of lowest to highest intensities or values
SATA Lowest intensities
SAPA
SATP
SPTA
SPPA
SPTP Highest intensities

57
Q

Average intensity calculated during the most intense half cycle

Similar in intensity to SPTP

A

Im

58
Q

Describes the distribution of the intensity of an ultrasound beam in space

= SP / SA (spatial peak divided by spatial average)

Unitless

1.0 is the minimum value

A

BUC (Beam Uniformity Coefficient)

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