Ultrasound Physics & Instrumentation – Comprehensive Review

Flashcards cover definitions, physics principles, historical milestones, machine components, wave parameters, beam behavior, resolution types, imaging modes, and safety/quality concepts essential for an ultrasound physics exam.

What is ultrasonography?

A diagnostic procedure that uses high-frequency sound waves (ultrasound) to create real-time images of internal body structures.

Why is ultrasound considered safer than X-ray or CT in pregnancy?

It uses non-ionizing sound waves rather than ionizing radiation.

Define sound in physics.

A mechanical, longitudinal energy wave that travels in a straight line through a medium by cycles of compression and rarefaction.

What is compression in a sound wave?

The region of high density and pressure within the wave.

What is rarefaction in a sound wave?

The region of low density and pressure within the wave.

Define ultrasound.

Sound with a frequency exceeding 20,000 Hz (20 kHz), above the upper limit of human hearing.

Typical diagnostic ultrasound frequency range?

2 MHz – 15 MHz.

Which material in most medical transducers converts electricity to sound and vice versa?

Lead zirconate titanate (PZT) piezoelectric crystal.

State the piezoelectric effect.

Certain crystals generate an electric charge when mechanically deformed and deform when an electric field is applied (reverse effect).

Relationship between crystal thickness and operating frequency?

Thicker crystal → lower resonant frequency; thinner crystal → higher frequency.

What is the bandwidth of a transducer?

The range between the highest and lowest frequencies it emits.

Who first studied bat echolocation, laying groundwork for ultrasound physics?

Lazzaro Spallanzani, 1794.

Who postulated the Doppler effect?

Christian Doppler in 1842.

Experiment that first applied the Doppler effect to sound waves?

C.H.D. Buys Ballot’s 1845 train-horn experiment.

Inventor of the Galton whistle and its significance?

Francis Galton, 1876 – produced ultrasonic frequencies above human hearing.

Discoverers of piezoelectricity?

Pierre and Jacques Curie, 1877.

Purpose of Paul Langevin’s 1915 hydrophone?

Detect submerged submarines; basis of modern sonar.

First successful echocardiogram was performed by?

Inge Edler and Hellmuth Hertz, 1953.

Who is called the 'Father of Obstetric Ultrasound'?

Dr. Ian Donald (1958).

Developers of pulsed Doppler ultrasound (PDU)?

Don Baker, Dennis Watkins, and John Reid, 1966.

Who captured the first 3D fetal images in 1986?

Kazunori Baba, University of Tokyo.

Main component that sends and receives ultrasound in a scanner?

The transducer (probe).

Function of the acoustic lens on a transducer?

Focuses/compresses sound waves to improve resolution.

Why is ultrasonic gel used?

Eliminates air between skin and transducer, reducing impedance mismatch.

Role of the matching layer on a probe face?

Minimizes acoustic impedance difference between crystal and tissue to transmit energy efficiently.

Purpose of the backing (damping) material?

Shortens pulse duration, broadens bandwidth, improves axial resolution, but reduces sensitivity.

What does the tuning coil do in a probe?

Offsets the crystal’s capacitance, removing residual electrical charges for cleaner signals.

Define propagation speed (c).

Speed at which sound travels through a medium; depends on stiffness and density.

Mathematical relation for propagation speed?

c = √(stiffness / density) or c = k / ρ (simplified).

Typical speed of sound in soft tissue?

1540 m/s (1.54 mm/µs).

Give the formula for wavelength (λ).

λ = c / f.

Define acoustic impedance (Z).

Resistance a medium offers to sound, Z = density (ρ) × speed (c).

What happens at a boundary with large impedance mismatch?

Most sound energy is reflected; little is transmitted.

Describe attenuation.

Loss of sound amplitude and intensity with depth due to absorption, reflection, refraction, and scattering.

Attenuation coefficient relationship with frequency?

Directly proportional; higher frequency → more attenuation, less penetration.

Define specular reflection.

Reflection from a smooth, large interface; angle-dependent, mirror-like.

Define scattering in ultrasound.

Redirection of sound from small or rough interfaces; makes tissue texture visible.

Explain refraction.

Change in the direction of a sound wave when it crosses a boundary at an oblique angle with different propagation speeds.

What is diffraction?

Bending/spreading of waves as they pass an opening or obstacle; greater with longer wavelengths.

Difference between continuous wave and pulse wave ultrasound?

Continuous wave transmits constantly (no imaging); pulse wave sends short bursts with listening intervals (used for imaging).

Define Pulse Repetition Frequency (PRF).

Number of ultrasound pulses emitted per second (Hz); inversely related to imaging depth.

Define Pulse Repetition Period (PRP).

Time from start of one pulse to start of the next (includes pulse + listening time); directly related to depth.

Relationship between PRF and PRP?

They are reciprocals (PRF = 1/PRP).

Define Pulse Duration (PD).

Actual time the pulse is ‘ON’; PD = number of cycles × period.

Define Duty Factor (DF).

Fraction of time the transducer is actively transmitting (PD/PRP); <1 % in imaging.

Define Spatial Pulse Length (SPL).

Physical length of one pulse; SPL = number of cycles × wavelength.

How is axial resolution calculated?

Axial Resolution = 0.5 × SPL.

How do you improve axial resolution?

Shorten SPL by using higher frequency and more damping.

What determines lateral resolution?

Beam width—narrower beam gives better lateral resolution.

What is elevational (slice-thickness) resolution?

Ability to distinguish structures in the plane perpendicular to imaging plane (third dimension).

Define temporal resolution.

Ability to accurately depict motion; quantified by frame rate (frames per second).

Three ways to increase temporal resolution?

Increase PRF (shallower depth), decrease number of focal zones, or reduce sector width/line density.

Near zone length (NZL) formula?

NZL = (r²) / λ, where r = probe radius.

How does increasing frequency affect near zone and divergence?

Extends near zone, decreases far-field divergence.

Effect of increasing transducer diameter (aperture)?

Extends near zone and reduces divergence (narrower beam).

Name two focusing methods.

Mechanical (fixed lens/curved crystal) and electronic (phased timing delays).

Trade-off of beam focusing?

Improves lateral resolution at focal zone but increases divergence beyond focus.

Define side lobes/grating lobes.

Unwanted off-axis energy that can create artifacts and degrade lateral resolution, especially in phased arrays.

Mechanical scanning characteristics?

Motor steers element/mirror; fixed focus & frequency; inexpensive but fragile; image built line by line.

Define a linear sequential array.

Rectangular probe where elements fire sequentially to create a rectangular image; good for vascular/small parts.

Typical frequency range of linear probes?

5–13 MHz (high resolution, shallow penetration).

Define curvilinear (convex) array.

Curved element row; sequential firing; produces sector/trapezoid image; 1–8 MHz for abdominal/OB scans.

Why are phased array probes preferred for cardiac imaging?

Small footprint fits between ribs, electronic steering focuses/steers beam for rapid sector images (2–8 MHz).

Annular array key feature?

Concentric ring elements mechanically steered; excellent 2D focus; cone-shaped beam.

What is real-time scanning (frame formation)?

Sequential placement of multiple scan lines across a plane to form a 2D frame displayed many times per second.

Define A-mode display.

Amplitude spikes vs. depth; used primarily in ophthalmology for length measurements.

Define B-mode display.

Brightness-modulated dots where brightness reflects echo amplitude, producing 2D grayscale images.

Define M-mode display.

Motion mode; records dynamic movement of structures along a single scan line over time (e.g., cardiac valves).

State the range equation used for depth calculation.

Distance (cm) = (c × round-trip time) / 2; with c ≈ 1.54 mm/µs, 13 µs round trip ≈ 1 cm depth.

What is the 13-microsecond rule?

Sound travels 1 cm to a reflector and back in 13 µs in soft tissue.

What does the pulser control?

Amplitude (output power), PRF, and PRP of the transmitted pulses.

Advantage of increasing output power?

Improves signal-to-noise ratio and penetration but raises patient exposure.

Purpose of coded excitation.

Uses encoded pulse sequences to improve SNR, penetration, multiple focal zones, contrast, and speckle reduction.

What is tissue harmonic imaging (THI)?

Uses harmonic frequencies generated within tissue (not transmitted) to improve lateral resolution, remove near-field artifacts and side lobes.

Why are harmonic beams narrower than fundamental beams?

Higher frequency → shorter wavelength → narrower beam, enhancing lateral resolution.

List five factors that influence image quality settings.

Field of view, number of scan lines, line density, penetration depth, output power.

Formula for intensity (I).

Intensity = Power / Area (W/cm²).

Relationship between power and amplitude?

Power ∝ Amplitude².

What units measure acoustic impedance?

Rayls (kg/m²·s).

Approximate acoustic impedance of soft tissue?

~1.65 × 10⁶ Rayls (liver).

Which interface reflects >99 % of sound energy?

Soft tissue–air interface (large impedance mismatch).

Give two ways to lengthen the near field zone.

Increase transducer frequency or increase transducer aperture (diameter).

Define Huygens’ Principle in ultrasound.

Each point on a vibrating surface acts as a source of secondary wavelets; the superposition forms the overall beam.

Why does higher frequency improve axial resolution?

Shorter wavelength → shorter SPL → ability to distinguish reflectors closer together along the beam path.

What degrades axial resolution?

Long SPL, low frequency, or excessive ringing of the crystal (poor damping).

Contrast resolution can be improved by?

Increasing bits per pixel, adjusting compression/post-processing curves, changing overall contrast/brightness.

What is frame rate’s mathematical relation to PRF and lines per frame?

Frame Rate = PRF / Lines per Frame.

Describe frequency compounding.

Averaging images acquired at multiple sub-band frequencies to reduce speckle and improve contrast resolution.

Function of the master synchronizer.

Coordinates timing of system components to ensure echoes are correctly processed and displayed.

What artifact suppression benefit does THI offer?

Eliminates near-field noise, reverberations, and grating-lobe artifacts because these weak beams don’t generate harmonics.

Define sensitivity in ultrasound transducers.

Ability to detect and display weak echo signals.

How does backing material affect sensitivity?

Damping shortens pulses (better resolution) but decreases sensitivity by reducing vibration amplitude.

State the divergence angle formula for an unfocused circular aperture.

sin α = 1.22 λ / D, where D = aperture diameter.

When does diffraction become negligible?

When the wavelength is significantly smaller than the obstacle/opening.

Main disadvantage of mechanical scanners?

Moving parts are prone to wear and may break, causing image failure.

Why are curvilinear probes suited for obstetrics?

Sector-shaped field of view with low-frequency (1–8 MHz) waves gives deeper penetration to image fetus.

What is acoustic absorption?

Conversion of ultrasound energy into heat within the tissue.

How do you minimize patient exposure while keeping image quality?

Use lowest feasible output power and shortest scanning time consistent with diagnostic need.