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Ultrasound Physics – High-Yield Summary (POCUS/ICU/Fellowship)

This chapter reviews the fundamental physics that explains how ultrasound creates images, why different probes are used, how to optimize image quality, recognize artifacts, and interpret Doppler studies.

1. Ultrasound Basics

Ultrasound consists of longitudinal sound waves with frequencies >20 kHz. Clinical ultrasound typically uses 2–15 MHz.

Every ultrasound wave has four important properties:

  • Amplitude = wave height (energy)
  • Frequency (Hz) = cycles/sec
  • Wavelength (λ) = distance between waves
  • Propagation velocity (c) = speed of sound

Relationship:

c = f × λ

Soft tissue assumes a propagation speed of:

1540 m/s

This value is built into all ultrasound machines.

2. Speed of Sound in Different Media

Approximate propagation velocities:

MediumSpeed (m/s)
Air331
Fat1430
Blood1570
Muscle1575
Liver1580
Bone3000–5000

Important:

  • Bone = very fast
  • Air = very slow
  • Machines assume 1540 m/s, so interfaces can create artifacts.

3. Frequency vs Resolution vs Penetration

The single highest-yield ultrasound concept.

Higher frequency

✔ Short wavelength

✔ Better resolution

✔ More attenuation

✔ Poor penetration

Best for:

  • Vascular access
  • Pleura
  • Thyroid
  • Soft tissue

Lower frequency

✔ Long wavelength

✔ Less resolution

✔ Better penetration

Best for:

  • Abdomen
  • Heart
  • FAST exam

Think:

High frequency = High detail

Low frequency = Long distance

4. Attenuation

Attenuation means:

Loss of ultrasound energy while traveling through tissue

Caused by:

  • absorption (most important)
  • reflection
  • scattering

Higher frequency → greater attenuation.

Fluid attenuates very little.

Bone and muscle attenuate substantially.

Gain compensates for attenuation by amplifying returning echoes.

5. Reflection

Reflection occurs whenever ultrasound crosses tissues with different acoustic impedances.

Greater impedance difference →

More reflection →

Brighter image

Examples:

Bone

Pleura

Diaphragm

Specular reflection

Smooth surfaces

Examples:

  • Bone
  • Pleura
  • Diaphragm

Produces strong bright echoes.

Diffuse reflection

Irregular surfaces

Examples:

  • Muscle
  • Liver

Produces shades of gray.

6. Refraction

Occurs when ultrasound crosses an interface at an angle.

The beam bends.

Can produce:

  • misplaced structures
  • duplicated structures
  • poor visualization

If angle exceeds the critical angle →

Total internal reflection.

7. Acoustic Impedance

Definition:

Resistance of tissue to ultrasound transmission.

Formula:

Z = density × propagation velocity

Large impedance differences produce strong reflections.

Examples:

Air ↔ tissue

Bone ↔ tissue

These appear very bright and limit visualization beyond them.

8. Echogenicity

Anechoic

Black

No echoes

Examples:

  • Blood
  • Urine
  • Cysts

Hypoechoic

Dark gray

Examples:

  • Muscle
  • Fat

Hyperechoic

Bright white

Examples:

  • Bone
  • Fascia
  • Calcifications

Homogeneous

Uniform appearance

Example:

Liver

Heterogeneous

Mixed appearance

Example:

Muscle

9. Safety

Diagnostic ultrasound is generally considered safe.

Important principle:

ALARA

As Low As Reasonably Achievable

Especially for:

  • fetus
  • eye
  • lung

Infection prevention:

Sterile gel for procedures.

Clean probes appropriately.

10. Piezoelectric Effect

Piezoelectric crystals:

Electrical energy →

Mechanical vibration →

Ultrasound wave

Returning echoes:

Mechanical vibration →

Electrical signal

Same crystals both transmit and receive.

11. Imaging Modes

B-mode

Brightness mode

Standard 2D grayscale image.

Most common mode.

M-mode

Motion mode

Measures movement over time.

Examples:

  • Lung sliding
  • Heart valve motion
  • MAPSE

12. Image Quality

Temporal resolution

Ability to detect motion.

Depends on:

Frame rate.

Important for echocardiography.

Spatial resolution

Ability to distinguish two nearby objects.

Two types:

Axial

Lateral

Higher frequency improves axial resolution.

Focus

Set focus at the depth of interest.

Gain

Brightness adjustment.

Too low →

Dark image.

Too high →

Snowstorm image.

13. Ultrasound Artifacts

Air artifact

Poor probe contact.

Prevent:

Gel.

Firm pressure.

Acoustic shadow

Occurs behind:

Bone

Air

Calcification

Metal

Little ultrasound passes through.

Posterior acoustic enhancement

Occurs behind fluid.

Image becomes brighter.

Helpful for:

Cysts

Bladder

Gallbladder

Reverberation

Repeated bouncing between reflective surfaces.

Examples:

Needles

Metal

Pleura

Mirror artifact

Highly reflective surfaces duplicate structures.

Classic:

Diaphragm.

14. Lung Ultrasound Artifacts

A-lines

Horizontal lines

Normal reverberation artifact.

Seen in:

Normal lung

Pneumothorax

B-lines

Vertical comet-tail artifact.

Erase A-lines.

Move with lung sliding.

Suggest:

Interstitial syndrome

Pulmonary edema

Fibrosis

ARDS

<3 B-lines at lung bases can be normal in elderly patients.

15. Doppler

Based on frequency shift.

Blood toward probe →

Positive shift

Higher frequency

Often red.

Blood away →

Negative shift

Lower frequency

Often blue.

Important:

Red ≠ artery

Blue ≠ vein

Color only represents direction relative to the probe.

Pulse-wave Doppler

Measures velocity at one location.

Can localize flow.

Continuous-wave Doppler

Measures highest velocities.

Cannot determine exact depth.

16. Probe Manipulation

Five essential movements:

Pressure

Slide

Fan (tilt)

Rotate

Rock (heel-toe)

Mastering these is critical for obtaining optimal windows.

Board-Style Questions

1. Increasing ultrasound frequency results in:

A. Increased penetration

B. Increased attenuation

C. Longer wavelength

D. Lower resolution

Answer: B

2. Best probe for a central line?

A. 2 MHz phased array

B. 3 MHz curvilinear

C. 12 MHz linear

D. Continuous-wave Doppler

Answer: C

3. Black fluid on ultrasound is called:

A. Hyperechoic

B. Hypoechoic

C. Anechoic

D. Heterogeneous

Answer: C

4. Posterior acoustic enhancement is most likely behind:

A. Bone

B. Air

C. Fluid-filled cyst

D. Calcification

Answer: C

5. Acoustic shadowing commonly occurs behind:

A. Blood

B. Urine

C. Bone

D. Liver

Answer: C

6. Which imaging mode evaluates movement over time?

A. A-mode

B. B-mode

C. M-mode

D. Doppler

Answer: C

7. Normal lung artifact:

A. B-lines

B. A-lines

C. Mirror artifact

D. Enhancement

Answer: B

8. B-lines suggest:

A. Pneumothorax

B. Interstitial edema

C. Pleural effusion only

D. COPD

Answer: B

9. Color Doppler red always indicates:

A. Artery

B. Oxygenated blood

C. Blood moving toward probe

D. High pressure

Answer: C

10. The equation relating sound velocity is:

A. V = IR

B. c = fλ

C. PV=nRT

D. Q=AV

Answer: B

High-Yield Flashcards

Q: Clinical ultrasound frequency range?
A: 2–15 MHz.

Q: Speed of sound assumed by ultrasound machines?
A: 1540 m/s.

Q: Formula relating velocity, frequency, and wavelength?
A: c = f × λ.

Q: Higher frequency causes what tradeoff?
A: Better resolution but poorer penetration.

Q: Main cause of attenuation?
A: Absorption.

Q: Tissue with greatest attenuation?
A: Bone.

Q: Black structures on ultrasound?
A: Anechoic.

Q: Bright structures?
A: Hyperechoic.

Q: Standard imaging mode?
A: B-mode.

Q: Motion mode?
A: M-mode.

Q: Doppler measures what?
A: Blood flow direction and velocity.

Q: Red on Doppler means?
A: Flow toward the probe (not necessarily an artery).

Q: Horizontal lung artifact?
A: A-lines.

Q: Vertical comet-tail artifact?
A: B-lines.

Q: Posterior enhancement occurs behind what?
A: Fluid.

Q: Shadow artifact occurs behind?
A: Bone, air, metal, calcification.

Q: Gain adjusts what?
A: Image brightness by amplifying returning echoes.

Q: Best focus setting?
A: At the depth of the target.

Q: Five probe maneuvers?
A: Pressure, slide, fan/tilt, rotate, rock.

Q: Safety principle for ultrasound exposure?
A: ALARA (As Low As Reasonably Achievable).

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