basicI.C-001
Sound is what kind of wave?
→ Mechanical, longitudinal wave. Particles vibrate along the direction of propagation. Cannot travel through a vacuum.
clozeI.C-002
Ultrasound is defined as any sound wave with a frequency greater than 20,000 Hz (20 kHz).
basicI.C-003
Which parameter of a sound wave is determined by BOTH the source and the medium?
→ Wavelength (λ). Period and frequency are set by the source; propagation speed is set by the medium; wavelength depends on both.
clozeI.C-004
Average speed of sound in soft tissue is 1540 m/s (= 1.54 mm/μs).
basicI.C-005
What two properties of the medium determine propagation speed?
→ Density and stiffness. Speed increases with stiffness and decreases with density.
basicI.C-006
Rank propagation speed: bone, air, fat, soft tissue, lung.
→ Bone (2000–4000) > soft tissue (1540) > fat (1450) > lung (300–1200) > air (330 m/s).
clozeI.C-007
In soft tissue, wavelength (mm) = 1.54 / frequency (MHz).
basicI.C-008
Does propagation speed depend on the sound wave's frequency?
→ No. All frequencies travel at the same speed through a given medium. Speed depends only on the medium.
basicI.C-009
High-frequency transducers give what trade-off in image quality?
→ Better axial resolution but less depth of penetration. Low-frequency transducers give the opposite.
clozeI.C-010
Intensity is proportional to amplitude², so doubling the amplitude quadruples the intensity.
basicI.C-011
How many dB corresponds to a doubling (or halving) of signal amplitude?
→ 6 dB. (20 · log 2 ≈ 6.)
basicI.C-012
How many dB corresponds to a 100× difference in amplitude?
→ 40 dB. (20 · log 100 = 40.)
clozeI.C-013
Acoustic impedance Z = tissue density × propagation velocity (ρ × c).
basicI.C-014
What determines the amount of ultrasound reflected at a tissue interface?
→ The difference in acoustic impedance between the two tissues, and the angle of incidence (best reflection when beam is perpendicular).
basicI.C-015
What causes ultrasound 'dropout'?
→ The beam is parallel (not perpendicular) to the tissue interface, so little or no signal reflects back to the transducer.
basicI.C-016
What phenomenon produces speckle in the image?
→ Scattering — radiation of ultrasound in multiple directions from structures smaller than the wavelength (e.g., red blood cells).
basicI.C-017
What is the major source of ultrasound information used to build a 2-D image?
→ Backscatter (a diffuse form of reflection). Because it redirects sound in many directions, some energy always returns to the transducer.
basicI.C-018
What is refraction and what artifact does it cause?
→ Deflection of the ultrasound beam at an interface between tissues of different acoustic impedance. Causes double-image (edge-duplication) artifacts.
basicI.C-019
What is the most common mechanism of ultrasound attenuation in tissue?
→ Absorption. Attenuation also depends on transducer frequency, the tissue's attenuation coefficient, and distance from the transducer.
basicI.C-020
What is Time Gain Compensation (TGC)?
→ An adjustment that amplifies returning signals more with increasing depth, compensating for attenuation. Default preset: less gain near-field, more gain far-field.
clozeI.C-021
A typical clinical ultrasound pulse contains 2–4 cycles.
basicI.C-022
Can imaging be performed with continuous-wave (CW) ultrasound?
→ No. Anatomic imaging requires pulsed ultrasound (the system must alternate transmit and listen). CW is used for Doppler.
clozeI.C-023
Pulse duration (μs) = number of cycles × period. Typical clinical value 0.5–3 μs.
clozeI.C-024
Spatial pulse length (mm) = number of cycles × wavelength. Determines axial resolution.
basicI.C-025
What determines the pulse repetition period (PRP) and PRF?
→ Imaging depth. Deeper imaging → longer PRP and lower PRF. The sonographer changes only the listening time, not the pulse duration.
basicI.C-026
Are PRP and PRF related to transducer frequency?
→ No. PRP/PRF are set by imaging depth, not by transducer frequency. They are reciprocals of each other.
clozeI.C-027
Round-trip time for a pulse in soft tissue = 13 μs per cm of depth.
clozeI.C-028
For imaging depth d (cm) in soft tissue: PRP (μs) = 13 × d and maximum PRF (Hz) = 77,000 / d.
basicI.C-029
What is duty factor?
→ The fraction (or %) of time the transducer is transmitting sound. Unitless. Typically < 1% in imaging. CW = 100%; a system that is off = 0%.
basicI.C-030
What does a larger transducer aperture do to the beam?
→ Produces a more focused (narrower) beam. A smaller aperture allows better angulation.
basicI.C-031
Which parameter of a pulse is most improved by using a shorter pulse (short SPL)?
→ Axial resolution. Shorter SPL → better ability to resolve two closely spaced structures along the beam axis.
basicI.C-032
What two categories of bioeffects can ultrasound cause?
→ Thermal (heating from absorption) and non-thermal / mechanical (cavitation — bubble formation and collapse).
basicI.C-033
What safety principle governs ultrasound exposure?
→ ALARA — As Low As Reasonably Achievable. Use the lowest power and shortest exposure needed for diagnosis.
clozeI.C-034
Increasing the transmitted power increases the amplitude of reflected US signals and can produce bioeffects at excess levels.
basicI.C-035
What is 'axial' resolution and what determines it?
→ Ability to distinguish two reflectors along the beam axis (in the direction of beam travel). Determined by SPATIAL PULSE LENGTH — shorter SPL = better axial resolution. Achieved with higher-frequency transducer or fewer cycles per pulse.
basicI.C-036
What is 'lateral' resolution and what determines it?
→ Ability to distinguish two reflectors perpendicular to the beam axis. Determined by beam width, which is narrowest in the focal zone. Larger aperture and dynamic focusing improve lateral resolution.
basicI.C-037
State the range of typical ultrasound frequencies used for cardiac imaging.
→ 2–5 MHz for adult TTE (better penetration for deeper structures). 5–10 MHz for TEE and pediatric echo (closer targets, better resolution). Higher frequencies used for vascular imaging (7–15 MHz).
basicI.C-038
How does duty factor differ between imaging and CW Doppler?
→ Imaging (pulsed): duty factor < 1% (mostly listening). CW Doppler: duty factor = 100% (continuous transmission). CW cannot form anatomic images because it never stops transmitting.
basicI.C-039
How does frequency affect penetration and axial resolution?
→ HIGHER frequency: BETTER axial resolution but WORSE penetration (more attenuation). LOWER frequency: WORSE axial resolution but BETTER penetration. Trade-off is fundamental to transducer selection.
basicI.C-040
What is the piezoelectric effect?
→ A property of certain crystals (PZT) that converts mechanical stress (pressure waves) to electrical voltage AND converts electrical voltage to mechanical vibration. This is how ultrasound transducers both send and receive sound.
basicI.C-041
State the average acoustic impedance of soft tissue and its formula.
→ Z = ρ × c (density × propagation velocity). Soft tissue Z ~1.63 × 10⁶ kg/(m²·s). Reflection at an interface depends on the DIFFERENCE in Z between two tissues.
basicI.C-042
What safety metric considers cumulative ultrasound exposure to a patient?
→ The 'Output Display Standard' shows MI (mechanical index) and TI (thermal index) in real time on the machine. MI = peak rarefactional pressure / √(f). TI = ratio of transmitted acoustic power to power needed to raise tissue temperature by 1°C.
