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Destructive vs constructive interference - ️️Constructive interference occurs when two waves overlap at the same location at the same point in time ( in phase) and the new combined wave has an amplitude greater than the original two waves. Destructive interference occurs when the waves are o...

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  • October 29, 2024
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  • 2024/2025
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SPI Physics Board Review
Destructive vs constructive interference - ✔️✔️Constructive interference occurs when
two waves overlap at the same location at the same point in time ( in phase) and the
new combined wave has an amplitude greater than the original two waves.
Destructive interference occurs when the waves are out-of-phase and the amplitude of
the new wave is less than one of the original waves

What is dynamic frequency tuning? - ✔️✔️A pulse from a transducer contains a wide
range of frequencies (broad bandwidth), in some systems the high frequencies in the
pulse create the shallow parts of the image while the low frequencies create the deeper
part. This is different than dynamic aperture in which the number of array elements
receiving the reflections are varied with depth

Intensity: what is it? Equation? Units? Determined by? Changed by sonographer?
Typical values? - ✔️✔️Intensity is the concentration of energy in a sound beam.
Intensity depends on the power & cross-sectional area of the beam.
Equation: intensity = power (watts)/beam area (cm squared)
Units: watts/cm squared; (watts is from power, cm squared is from beam area).
Determined by: sound source
Changed by sonographer: yes (since power can be changed by sonographer then
intensity can also be changed by the operator).
Typical values: 0.001 - 100 watts/cm squared

What time of relationship are period and frequency? What are their equations? What
are their complimentary units? - ✔️✔️Period and frequency are reciprocals, having an
inverse relationship (as one goes up the other goes down);
period (sec)=1/freq (Hz)
Freq (Hz) = 1/period (sec)
Freq (Hz) x period (sec) = 1

Complimentary units: sec & Hz; microseconds and MHz


Metric notations: giga, mega, kilo, hecto, deca, deci, centi, milli, micro, and nano -
✔️✔️10 to the... Giga 9, mega 6, kilo 3, hecto 2, deca 1, deci -1, centi -2, milli -3, micro
-6, nano -9

Complimentary metric units: what goes with giga? Mega? Hecto? Deca? -
✔️✔️Giga+nano
Mega+micro
Hecto+centi
Deca+deci

,Sounds must travel though a (blank) it cant travel through a (blank) - ✔️✔️Medium,
vacuum

Sounds is a (blank) wave - ✔️✔️Mechanical, longitudinal

What are the three acoustic variables? What are their units? - ✔️✔️1. Pressure:
force/area, pascals
2. Density: mass/volume, kg/cm3
3. Distance: cm, feet, miles

Transverse versus longitudinal wave - ✔️✔️In a Transverse wave the particles move
perpendicular to the direction of the wave (think ocean waves).
In longitudinal waves the particles move in the same direction as the wave (bunched up
slinky).

Compressions versus rarefactions - ✔️✔️Compressions are areas of increased
pressure or density whereas rarefactions are areas of decreased pressure or density

Acoustic Parameters: what do these do? What are the 7 with ultrasound? -
✔️✔️Acoustic parameters describe the features of a particular wave (whereas acoustic
variables determine which waves are sound waves). The 7 acoustic variables we are
concerned about with ultrasound are: period, frequency, amplitude, power, intensity,
speed and wavelength

Acoustic parameters: Period
Definition, units, determined by source or medium, can sonographer change it,
relationships - ✔️✔️The time required to complete a single cycle.
Units: seconds, microseconds, hours, any unit of time.
Determined by: sound source
Changed by sonographer: no
Relationships: period and frequency are reciprocals (inverse relationship); when one
goes up the other goes down.

Acoustic Parameters: Frequency
Definition, units, determined by source or medium, can sonographer change it, typical
values, relationships, what does freq affect? - ✔️✔️In DMU the frequency of a wave is
the number of cycles of an acoustic variable that occur in one second.
Units: per second, 1/second = Hertz (Hertz is per second, 1,000/sec = kilohertz;
1,000,000/sec=megahertz).
Determined by sound source.
Changed by sonographer: no
Typical values 2 to 15 MHz
Relationships: frequency and period are inversely related.
Frequency affects penetration and axial resolution (image quality)

,Ultrasound is a wave with a frequency greater than (blank) Hertz - ✔️✔️20,000 Hz,
(20kHz)

Audible sound is between (blank) and (blank) Hz - ✔️✔️20 and 20,000 Hz

Infrasound is sound with frequncies less than (blank) Hz - ✔️✔️Less than 20 Hz


What are the 3 bigness parameters which describe a sound beam's strength? -
✔️✔️Amplitude, Power, Intensityl

Amplitude: what is it? Typical units? Determined by? Changed by sonographer? -
✔️✔️Amplitude is the difference between the average value and the maximum value of
an acoustic variable.
Units: the units of the acoustic variables - pascals (pressure), grams per cm cubed
(density), and particle motion (units of distance); amplitude is usually expressed in
decibels.
Determined by: sound source
Changed by sonographer: yes

What is peak-to-peak amplitude and how does it compare the amplitude? -
✔️✔️Amplitude is the difference between the average value and the maximum vale of
an acoustic variable. Peak-to-peak amplitude is the difference between the maximum
and minimum values of an acoustic variable. Amplitude is half of the peak-to-peak
amplitude.

Amplitude (blanks) as sound propagates through the body - ✔️✔️Decreases

Power: what is it? Units? Determined by? Changed by sonographer? - ✔️✔️Power is
the rate that work is performed, or the rate of energy transfer.
Units: Watts
Determined by: source
Changed by sonographer: Yes

What is the relationship between power & amplitude? Equation &
Example: what happens to power if amplitude is tripled? Halved? Doubled? -
✔️✔️Power is proportional to the wave's amplitude squared.
Power=amplitude squared
Example: if amplitude is tripled, power is increased by a factor of 9; if amplitude is
halved, the power js decreased by a factor of 4 (quartered); if amplitude is doubled, the
power is increased by a factor of 4.

, How are intensity and power related? What happens to intensity if power is doubled?
Quartered? - ✔️✔️Intensity and power are proportional, or directly related. If one goes
up, so does the other.
If power is doubled, intensity doubles.
If power is quartered, intensity is quartered.

How are intensity and amplitude related? Explain why. What happens to intensity if
amplitude is doubled? Quartered? - ✔️✔️Intensiry is proportional to amplitude squared.
We know this because intensity is proportional to power, and power is equal to
amplitude squared; therefore intensity is equal to amplitude squared.
If amplitude is doubled, the intensity is increased by a factor of four.
If amplitude is quartered, then intensity is reduced by a factor of 16

Wavelength: what is it? Units? Determined by? Changed by sonographer? Typical
values? Equation - ✔️✔️Wavelength is the length or distance of a single cycle.
Units: meters, mm, any unit of length.
Changed by sonographer: no (wavelength influences axial resolution-image quality).
Typical values: 0.1-0.8 mm (in soft tissue)
Equation: wavelength (mm)=propagation speed (mm/microsecond)/frequency (MHz)
Lambda = c/F

What is the propagation speed (c) in soft tissue? - ✔️✔️1.54 mm/microsecond
1540 m/second
1.54 km/second

Higher frequencies mean (blank) wavelengths; lower frequencies mean (blank)
wavelengths - ✔️✔️Higher frequencies mean shorter wavelengths.
Lower frequencies mean longer wavelengths.

Rule for wavelength: in soft tissue, divide 1.54 mm/microsecond by (blank) - ✔️✔️Rule
for wavelength: in soft tissue, divide 1.54 mm/microsecond by frequency in MHz

Propagation Speed: what is it? Units? Determined by? Changed by sonographer?
Typical values? Equation? - ✔️✔️Propagation speed is the rate that sound travels
through a medium. Aka velocity or speed.
Units: meters per second, millimeters per microsecond.
Determined by: Medium only (density and stiffness).
Changed by sonographer: No
Typical values: 1.54 km/sec; 1540 m/s; 1.54 mm/microsecond
Equation: speed (m/s)=freq (Hz) x wavelength (meters)

Put these in order from slowest to gastest propagation speeds: bone, air, fat, soft tissue
- ✔️✔️Slowest in gas, fastest in solid so:
Slowest to fastest:
Air, fat, soft tissue, bone

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