Physics Quiz 1
What are some other terms used to describe "off" time?
"Off" time is also called "listening" or "receiving" time.
What two words are used to describe the type of wave created by sound energy?
1) Mechanical (things physically bumping into each other) 2) Longitudinal (this bumping motion is in same direction as wave motion)
2. The audible range for sound frequency is ______ to __________. Anything higher than this is called _______ and anything lower than this is called _______.
20; 20,000; ultrasound; infrasound
8. A sector image with a curved top is produced by which type of transducer?
A convex (or curved-linear) array transducer produces modified (curved-top) sector images
9. A sector image with a flat top is produced by which type of transducer?
A vector array (or hybrid) transducer produces modified (flat-top) sector images
Define the Absorption.
Absorption is the conversion of sound energy into heat as it is absorbed by the tissue
10. What type of transducer produces a "ring"-like image that goes around in a complete circle?
An annular array transducer produces circular (ring-like) images
What determines the attenuation coefficient? What is the "rule of thumb" for calculating the attenuation coefficient for ultrasound in soft tissue?
Attenuation coefficient is determined by frequency and impedance (which is determined by the medium). In soft tissue, the attenuation coefficient is roughly one half the operating frequency (and change MHz to dB/cm) Att. Coef. = 1/2 x fo
Define attenuation. What units are used to describe attenuation?
Attenuation is the weakening of sound intensity as it travels through a medium. Attenuation is measured in decibels (dB)
So, what is the speed of sound in soft tissue (give it using two different sets of units)?
C = 1,540 m/s or C = 1.54 mm/us
Define the following, and give the symbol and typical unit used for each:
Cycle (no symbol or unit for this one): one complete pair (compression and rarefraction) Frequency: number of cycles per second (f), measured in MegaHertz (MHz) Period: amount of time it takes to complete one cycle (T), measured in microseconds (us) Wavelength: physical length of one cycle (λ), measured in millimeters (mm) Propagation Speed: speed of the sound wave (c), measured in millimeters per microsecond (mm/us)
what factors determine the attenuation and how you describe their relation with attenuation?
Depth and frequency; both factors are directly related with attenuation.
11. Which type of ultrasound process is used to detect direction and velocities of blood flow in the body?
Doppler ultrasound is used for this.
True or false: Each pulse of ultrasound creates only one echo.
False -- each pulse continues into the tissue, generating many echoes
What is the formula for fractional bandwidth? What is the formula for Q-Factor? How are these two things related?
Fractional BW = BW/ fund. freq Q-factor=fund.freq/BW Fractional bandwidth and Q-factor are inversely related to each other.
Assuming there is no loss due to absorption at the exact point of measurement (absorption requires distance), what must I.R.C. and I.T.C. add up to? What must Ir and It add up to?
I.R.C and I.T.C must add up 100%, and Ir plus It must equal Ii.
What is the formula to calculate I.R.C. if you know the impedances of the two media?
IRC=[z2-z1/z2+z1]^2
How is impedance calculated (give the formula) for any given medium? What symbol is used to denote impedance?
Impedance = propagation speed x density We use the sample z.
With increasing depth, does the listening time in each pulse repetition get longer or shorter? Why might this be true? What does this do to the duty factor?
It gets longer because the machine needs more time for the echo to bounce back to the transducer if the sound wave is travelling further (deeper). The P.D. stays the same, but the overall P.R.P gets longer (due to the lengthened "off" time); therefore, the D.F. gets smaller.
What is the "Rule of 13 μs" and how was this determined?
It takes 13 microseconds (us) for a pulse to go 1 cm and back (a total travel distance of 20 mm). So, based on this, we can estimate depth by the "rule of 13's": By dividing round trip time by 13us, we can calculate the depth of scanned organ
Does increasing the ultrasound frequency increase or decrease the S.P.L.? The P.D.? Is image resolution enhanced or degraded by doing this?
S.P.L is decreased (because the wavelength is made shorter), and so is P.D. The resolution is enhanced with short pulses.
We can find the pulse duration by multiplying the number of cycles per pulse (n) times the period (T) of the ultrasound. What formula should we use to find the spatial pulse length (S.P.L.)?
SPL = lambda x n
Based on the above formula, would you expect to see more reflection (higher I.R.C.) when the impedances are very similar, or very different? If they're different, does it matter which one (medium 1 or medium 2) has the greater impedance?
The I.R.C will be greater (most of the sound intensity will be reflected) when the impedances are very different. If the impedances are the same, there will be no reflection Because the entire quantity is squared, it doesn't matter which gets subtracted from which.
What is the Intensity Reflection Coefficient? How is it defined in terms of the intensity of the beam coming toward the boundary (= the incident beam)?
The Intensity Reflection Coefficient (I.R.C) is the fraction of intensity that is reflected back at the boundary of two media. It can be expressed as the Intensity of Reflection (Ir ) divided by the Intensity of Incidence ( Ii)
What is the Intensity Transmission Coefficient? How is it defined?
The Intensity Transmission Coefficient (I.T.C) is the fraction of intensity that is transmitted past the boundary of two media. Its sort of the opposite of the I.R.C. It can be expressed as the Intensity of Transmission (It ) divided by the Intensity of Incidence (Ii ).
In order for refraction (the bending of a sound beam) to occur at a border, what two things must be true?
The angle of incidence must be oblique (not perpendicular to the border), and the propagation speeds of the two media must be different.
Which angle always equals the angle of incidence? Which angle may be larger or smaller than the angle of incidence? What does this depend on? Describe the relationship. Based on the diagram above, what do we know about the propagation speed of Tissue 2 compared to that of Tissue 1?
The angle of reflection will always be equal to the angle of incidence. The angle of transmission may be larger or smaller, depending on the propagation speeds of the two media. The difference in angles and is roughly proportional to the difference in propagation speeds. Therefore, if c2 (propagation speed of medium 2) is twice as fast as c1, then will be approx. twice . This appears to be the case in the above diagram.
What is the range of frequencies the human ear can hear (on average)? What do we call sound whose frequency is below this range? What about sound whose frequency is above this range?
The audible range is 20 Hz - 20,000 Hz (=20 kHz); frequencies below this are infrasound, while frequencies above this are ultrasound.
what determine the reflection at a border between different media.
The difference between their impedances determines how much ultrasound is reflected back from that border.
With very short pulses of sound, other frequencies are generated along with the central (main) frequency. What do we call the entire range of frequencies generated? Do we like for this range to be small or large?
This range of frequencies is called bandwidth, and we prefer it to be large rather than small. The bandwidth can never be greater than the central frequency itself.
What do we call the ratio of "on" time to total pulse repetition time? Express this as a formula. Generally speaking, what does this ratio tend to be with diagnostic sonography?
This ratio is called duty factor (D.F) DF = PD/PRP D.F. is usually about (0.1% or 0.001)
How can we calculate the total amount of attenuation that occurs over a travel distance?
Total attenuation = attenuation coefficient x total distance traveled
Typically, how many cycles occur in each pulse with grayscale ultrasound? What symbol is used to denote this number?
Usually there are 2, 3, or 4 cycles in a pulse, and we call this number n.
Based on the above formula, how can we describe the relationship between wavelength and propagation speed? How about the relationship between wavelength and frequency?
Wavelength is directly related to propagation speed, but it is inversely related to frequency.
Give the formula which describes the relationship between frequency and period: How do we describe this type of relationship?
f=1/t or T=1/f This is called an inverse (or reciprocal, or indirect) relationship.
Give the formula which describes the relationship among intensity, power, and beam area. What units are used to describe intensity?
intensity = power / beam area Intensity is measured in mW/cm^2
Give the formula which describes the relationship among wavelength, propagation speed, and frequency:
lambda=c/f
We measure changes in intensity as ratios, and use a special type of scale to do this. This kind of scale is called logarithmic, and the unit we use is the decibel (dB).
logarithmic; decibel (dB)
3. To create images, the ultrasound must be _______, not continuous.
pulsed
6. When these (answer to #5 above) are all parallel, the resulting image is called _______. Which type of transducer produces such images?
rectangular; a linear transducer produces rectangular images
When sound travels in a given medium, it strikes the surface of another medium and bounces back in some other direction, this phenomenon is called the reflection of sound.
reflection
5. The string of echo information sent back from each single pulse is called a _____ _____.
scan line
1. Using echoes of ultrasound to create anatomic images is called
sonography
7. When these lines all radiate out from a single starting point, the resulting image is called _____ _____. Which type of transducer produces such images?
true sector; a phased array transducer produces true sector images
How can the relationship between the above two answers be expressed? [Hint: similar to #5]
PRF = 1/PRP ; PRP = 1/PRF This is an inverse relationship
What units are used to describe power? How is power related to amplitude? If you doubled the amplitude, what would happen to the power?
Power is measured in milliwatts (mW). Power is directly related to the square of the amplitude. Power (alpha sign) (Amp)^2 If we double the amplitude the power will quadruple.
The number of pulse repetitions that occur each second is called what?
Pulse repetition frequency (P.R.F) The number of complete pulses (both "on"-time and "off"-time) that occur each second.
The time it takes for one pulse repetition to occur is called what?
Pulse repetition period (P.R.P) The total amount of time it takes from the beginning of one pulse to the beginning of the next pulse (includes both "on"-time and "off-time).
When we graph a sound wave, what do we call the "peaks" (points of maximum amplitude)? What do we call the "valleys" (points of minimum amplitude)?
The peaks are known as compressions, and the valleys are known as rarefactions.
What determines the propagation speed of sound?
The propagation speed is determined entirely by the medium through which it travels.
The Range Equation is used to determine the depth of a reflector. Explain how this works. Why is there a ½ in this equation?
The range eq is as follows: depth = c x 1/2 x total travel time In this eq, c= 1.54 mm/us and total travel time is the time from the pulse being sent to its echo being received (total round-trip time, in us). The ½ is there because we're only interested in the distance of a one-way trip (actual depth). This would give us depth in mm.
What do we call the time it takes for just the sound wave portion ("on" time) of one pulse repetition to occur?
This is the pulse duration (P.D)
In the following diagram, label the angle of incidence, the angle of reflection, and the angle of transmission. What symbol is used to denote an angle?
The symbol θ ("theta") is used to denote an angle.
The type of oblique incidence described above depends on a very smooth, regular boundary. What is this type of reflection called? Is it typical of tissues in the human body? What kind of reflection usually occurs here? Compare the two types of reflections to a flashlight reflecting off of two different types of surfaces.
The type of reflection described above is called specular. It doesn't occur with tissues in the human body. Instead, we see scattered reflection here. Specular reflection is like light reflecting off a mirror; we can see an image of the thing being reflected. Scattered reflection is like light reflecting off a white wall. We can see brightness, but not a true reflection of the object.
When we draw a picture of a sound wave, what does the vertical axis (amplitude) usually represent? What unit is used for this?
The vertical axis usually represents pressure, and this is measured in megapascals (MPa).
What do we call things which change as sound energy moves through a medium? Give four examples of these:
These are acoustic variables, and they are: pressure, density, particle vibration, temperature.
According to the above formula, how are intensity and beam area related?
They are inversely related; if beam area decreases, intensity increases.
The amount of attenuation (in dB) that occurs with every 1 cm of travel is called what?
This is called the attenuation coefficient.