Wave Relationships
Acoustic Parameters
Attenuation & Impedance
Pulsed-Wave Concepts
Image Artifacts
100
Define "reciprocal relationship" in the context of two acoustic variables
What is two variables are inversely related such that their product equals 1 (or a constant).
100
List the five acoustic parameters determined by the source for continuous wave ultrasound (CW) named in the powerpoint.
Period, Frequency, Amplitude (adjustable), Power (adjustable), Intensity (adjustable)
100
Define attenuation and name its principal unit.
What is weakening (reduction) of sound intensity as it propagates through a medium. Unit: decibels (dB) — expressed as negative dB change?
100
Define pulse duration (PD) and list the units used.
Pulse Duration (PD): time for one pulse to occur (transmit time, start to end).
Units: microseconds (μs)
100
What is an artifact?
What is error in imaging causing echoes to appear without a corresponding anatomical structure
200
If propagation speed in medium A doubles while density remains constant, describe qualitatively how impedance changes and the likely effect on reflection at an interface with medium
What is propagation speed doubles and density is constant, impedance z = density × speed will double. A larger impedance difference increases intensity reflected (greater reflection).
200
Define amplitude in ultrasound and list two units used to report it.
maximum variation in an acoustic variable
Units: Pascals (Pa), dB (decibels)
200
What is the attenuation coefficient formula for soft tissue (dB/cm)? Compute the attenuation coefficient for a 4 MHz transducer.
Atten Coef (dB/cm) = 0.5 × frequency (MHz).
What is 4 MHz: Atten Coef = 0.5 × 4 = 2.0 dB/cm?
200
Provide the formula for spatial pulse length (SPL) and calculate SPL for a three-cycle pulse with wavelength 0.41 mm.
SPL (mm) = Number of cycles in pulse × Wavelength (mm)
λ = 0.41 mm
SPL = 3 × 0.41 mm = 1.23 mm.
200
Name this artifact?
What is mirror image artifact?
300
Explain the difference between in-phase and out-of-phase waves and state how each produces constructive or destructive interference.
In-phase: peaks/troughs align; when two in‑phase waves meet they add (constructive interference) producing larger amplitude. Out‑of‑phase: peaks of one align with troughs of another; they subtract (destructive interference), possibly canceling if amplitudes equal (complete destructive interference).
300
Compute wavelength in soft tissue (propagation speed 1.54 mm/μs) for a 2 MHz wave.
C = 1.54 mm/μs
f = 2 MHz
λ = 1.54 mm/μs ÷ 2 MHz = 1.54 / 2 mm
0.77 mm
300
Explain Rayleigh scattering’s dependence on frequency; if frequency doubles, by what factor does Rayleigh scattering change?
What is rayleigh scattering being proportional to frequency doubling (×2), Rayleigh scattering increases by 2^4 = 16 times?
300
Define pulse repetition period (PRP) and its relation to PRF. If PRF = 5 kHz, what is PRP in microsecond?
time from the beginning of one pulse to the beginning of the next (transmit + receive time).
PRF = 1 / PRP
=1/5kHz
5000 Hz → PRP = 1 / 5000 s = 0.0002 s = 0.2 ms = 200 μs
300
Differentiate specular reflection from diffuse reflection (backscatter) with two distinguishing points each.
Specular Reflection -- Occurs at a large smooth boundary (diaphragm or pericardium) with a small wavelength beam; Angle of incidence equals the angle of reflection Returns back to the transducer when hitting at 90 degrees
Diffuse Reflection -- Occurs at a large rough boundary with a large wavelength beam; disorganized and random, return to the transducer, but with a lower strength than a specular reflection
400
Give the mathematical relationship between period and frequency. Calculate the period for a 3.5 MHz transducer and show units.
Relationship: Period (T) = 1 / Frequency (f).
For 3.5 MHz: T = 1 / 3.5 MHz = 0.2857 μs (0.286 μs) (rounded).
400
Which of the following would show the highest intensity values?
What is spatial peak temporal peak? SPTP
400
Give the impedance formula and compute impedance for a medium with density 1000 kg/m³ and propagation speed 1540 m/s. Include units.
What is 1000 × 1540 = 1,540,000 rayls = 1.54 × 10^6 rayls (expressed as 1.54 MRayl). Units: rayls
Impedance formula: z (rayls) = density (kg/m^3) × propagation speed (m/s).
400
Explain duty factor and compute the duty factor when PD = 2 μs and PRP = 250 μs (show steps; give decimal and percentage).
DF = 2 μs / 250 μs = 0.008 = 0.8%
Duty factor (DF) = Pulse duration / Pulse repetition period
PD = 2 μs
PRP = 250 μs
400
Explain these imaging characteristics: Hyperechoic, Hypoechoic, Anechoic, Isoechoic?
Hyperechoic -- portions are brighter than surrounding tissues, or tissues that appear bright than normal
Hypoechoic -- portions are not as bright as surrounding tissues, or tissues that appear less bright than normal
Anechoic -- without echoes; echo free
Isoechoic -- describes structures with equal echo brightness
500
Wavelength depends on what two factors?
What is frequency and propagation speed?
500
Compare spatial peak temporal average (SPTA) and spatial average temporal peak (SATP): which is more relevant to thermal bioeffects and why?
What is SPTA = Spatial Peak, Temporal Average — used to assess thermal bioeffects. SATP (spatial average, temporal peak) represents spatially averaged peak-in-time intensity.
500
Using the intensity reflection coefficient (IRC) simplified formula from the doc, compute IRC% when Z1 = 1 MRayl and Z2 = 2 MRayl. Show formula and numeric answer.
What is 11.1?
IRC% =[ Z2 – Z1/Z2+Z1]^2*100%
Z1=1M/Rayl
Z2=2 M/Rayl
1/3^2=1/9
500
Using the range equation for soft tissue, determine the go–return time for a reflector at 5 cm depth (use the 13 μs per cm rule) and show how depth relates to time.
5 cm × 13 μs/cm = 65 μs
Range / go–return time using 13 μs/cm rule: In soft tissue, 13 μs of go–return time corresponds to 1 cm depth (round trip).
500
Describe refraction (split image) conditions and state Snell’s Law as given; then explain qualitatively what happens to the transmission angle when medium 2 has a faster propagation speed than medium 1.
Refraction requires oblique incidence and different propagation speeds in the two media (no refraction for normal incidence).
Snell’s Law: sin(transmission angle) = (propagation speed 2 / propagation speed 1) × sin(incident angle)
different speeds + oblique angle = bending; faster second medium → bend away from normal; slower second medium → bend toward normal.