Wave Properties
What is the relationship between frequency and period?
They are inversely related (Period = 1/Frequency).
What is the correct formula for Doppler shift?
Δf = (2 × v × f₀ × cosθ)/c.
What type of artifact results in a duplicate image across a reflector?
Mirror image artifact.
Which strategy improves signal-to-noise ratio?
Spatial compounding or averaging.
What simulates tissue heating for thermal index tests?
Absorbing tissue-equivalent phantoms.
The medium's stiffness (bulk modulus).
Transient (inertial) cavitation.
A Doppler shift of 3 kHz is obtained... what direction is flow?
Toward the transducer (positive shift).
What artifact occurs due to high-amplitude reflections?
Reverberation or comet tail artifact.
What process reduces dynamic range from 80 dB to 40 dB?
Compression.
What is true about cavitation thresholds?
Higher intensity and lower frequency increase risk.
What is the typical propagation speed in soft tissue?
1,540 m/s
What is the hydrostatic pressure at 10 cm depth?
+7.5 mmHg (0.77 mmHg/cm × 10 cm)
What occurs when sound changes speed and direction?
Refraction (Snell’s Law).
What does apodization do?
Reduces side lobes by varying crystal excitation.
What transducer type provides the widest near field?
Curved (convex) array.
What happens to SPL when frequency increases?
SPL decreases (shorter pulses)
What is the minimum number of crystals for CW Doppler?
Two crystals (one transmits, one receives)
What artifact occurs due to reflections outside the main beam?
Side lobes or grating lobes.
What is the relationship between frame rate and temporal resolution?
Higher frame rate = better temporal resolution.
In high-attenuation media, what is most affected?
Penetration depth and attenuation rate.
Explain how changes in both density and stiffness of a medium influence the propagation speed of ultrasound. Provide the formula and interpret the effect of each variable.
The speed of sound in a medium is determined by the formula:
Propagation Speed (c) = √(Stiffness / Density)
Stiffness (bulk modulus): If stiffness increases, the propagation speed increases.
Density: If density increases, the propagation speed decreases.
This means that sound travels fastest in media that are stiff and not very dense—for example, bone has high stiffness, which results in high propagation speed.
A sonographer detects aliasing during a pulsed Doppler exam. Explain two techniques that can be applied to reduce aliasing without changing the transducer. Include a rationale for each method.
Increase the PRF (Pulse Repetition Frequency): This raises the Nyquist limit, allowing the system to sample higher velocities without aliasing.
Shift the baseline: This does not eliminate aliasing but allows more of the aliased signal to appear above or below the baseline, improving interpretability.
Describe how increasing the number of focal zones affects lateral resolution and temporal resolution. Why does this trade-off matter in clinical imaging?
Increasing the number of focal zones improves lateral resolution because the beam is more precisely focused at multiple depths. However, this also reduces temporal resolution (frame rate) because the system must send multiple pulses per scan line, which takes more time to construct an image.
Compare the construction and beam characteristics of an annular array transducer and a linear phased array. How do their focusing and steering methods differ?
Annular Array: Comprised of concentric ring-shaped elements; uses mechanical steering and electronic focusing in both lateral and elevational planes.
Linear Phased Array: Has multiple small elements aligned in a straight line; uses electronic steering and focusing by applying phasing delays.
What does Ohm’s Law describe in ultrasound systems?
V = IR (Voltage = Current × Resistance).