Ultrasound Physics

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💻 B-mode (2D) Image Controls with Physics

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| Gain | Increases or decreases the overall brightness of the image. | Amplifies returning echoes (post-processing). Does not affect the strength of the initial ultrasound pulse. | Higher gain brightens all echoes (including noise); too high = washed out image. Too low = dark image, may miss important structures. |

| TGC (Time Gain Compensation) | Adjusts brightness at specific depths. | Compensates for attenuation of sound waves as they travel deeper (selective amplification of returning echoes). | Makes deep structures appear as bright as superficial ones. Prevents over-brightening shallow tissue and under-brightening deep areas. |

| Depth | Sets how deep the system listens for echoes. | Controls listening time (pulse-echo time) and adjusts how much tissue is displayed. | Increasing depth shows more anatomy but reduces frame rate and resolution. Shallower depth improves detail and image rate. |

| Sector Width | Adjusts the angular width of the scan field. | Affects the number of scan lines: more lines = wider field but lower frame rate. | Narrowing sector improves temporal resolution (higher frame rate), useful for moving targets like fetal heart. |

| Power (Acoustic Output Power) | Adjusts energy of transmitted ultrasound pulse. | Increases intensity of ultrasound beam (pre-processing). Affects mechanical and thermal index (MI/TI). | Higher power increases signal strength but also patient exposure. Use ALARA principle. |

| Dynamic Range | Number of grey shades used to display echoes. | Range of echo amplitudes processed into greyscale; controlled via logarithmic compression. | High dynamic range = soft, more detailed image; low range = high contrast but loss of subtle differences. |

| Harmonics | Uses echoes at twice the transmitted frequency (second harmonic). | Based on non-linear propagation of sound in tissue—stronger echoes come from center of beam, reducing artefacts. | Improves lateral resolution and reduces clutter; good for imaging cysts or obese patients. |

| Compound Imaging | Combines multiple scan angles into one image. | Spatial compounding reduces artefacts by averaging data from different angles. | Reduces speckle noise and improves border definition, but may reduce frame rate. |

| Zoom | Magnifies a region of interest. | Read zoom = post-processing (no added detail); write zoom = rescans area with higher line density (pre-processing). | Write zoom improves resolution in region of interest, useful for small structures.

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🌊 Doppler Controls with Physics

| Doppler Gain | Adjusts brightness of flow signals. | Amplifies returning Doppler-shifted echoes. | Too much gain = noise and artefacts; too little = weak signal. Optimal gain shows clear waveforms without overpainting. |

| Angle Correction Cursor | Aligns the Doppler beam with blood flow. | Doppler shift depends on cosine of angle between beam and flow. Accurate velocity = angle ≤60°. | Correct alignment ensures accurate flow velocity; poor angle = underestimated or incorrect readings. |

| Scale | Sets the velocity range displayed (PRF adjustment). | Based on Nyquist limit = PRF ÷ 2. High velocities need higher PRF to avoid aliasing. | Too low = aliasing (colour wraps); too high = loss of slow flow. Adjust to flow type (e.g., artery vs vein). |

| Pulse Repetition Frequency (PRF) | Number of pulses sent per second. | Determines maximum measurable velocity before aliasing. Related to sampling depth. | High PRF for fast flow (arteries), low PRF for slow flow (veins). Deep structures limit PRF due to longer listening time. |

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## 🌀 Fundamental Physical Terms

| Term | Definition | Relevance to Ultrasound |

|----------|----------------|------------------------------|

| Acoustic Impedance (Z) | The resistance of a medium to sound wave propagation: Z = density × speed of sound. | Determines how much sound is reflected at tissue boundaries. Greater difference = more reflection. |

| Acoustic Impedance Mismatch | Difference in impedance between two tissues. | Large mismatch (e.g., soft tissue to bone/air) = strong reflection and poor transmission, producing bright echoes or shadowing. |

| Speed of Sound | The rate at which sound travels through a medium (in soft tissue: ~**1540 m/s**). | Affects distance calculations; assumed constant in most machines. Variations can cause artefacts. |

| Stiffness | Resistance of a medium to deformation. | More stiffness = faster sound speed. Bone is stiff and fast; fat is soft and slow. |

| Density | Mass per unit volume of a tissue. | Affects acoustic impedance and sound speed. Important in reflection and transmission. |

| Ultrasound Frequency | Number of sound wave cycles per second (measured in MHz). | Higher frequency = better resolution but less penetration. Lower frequency = deeper penetration but lower resolution. |

| Wavelength (λ) | Distance between two consecutive wave peaks. λ = speed ÷ frequency. | Shorter wavelengths (high frequency) improve resolution. Longer wavelengths penetrate deeper. |

| Spatial Pulse Length (SPL) | Length of an ultrasound pulse in space: SPL = # cycles × wavelength. | Short SPL improves axial resolution. Long SPL reduces image detail. |

| Attenuation | Gradual weakening of the ultrasound signal as it travels through tissue. | Caused by absorption, scattering, and reflection. Limits image depth. |

| Absorption | Conversion of sound energy into heat. | Main cause of attenuation in soft tissue. Higher in bone; contributes to thermal bioeffects. |

| Reflection | Echoes that return to the transducer at tissue boundaries. | Essential for image formation; strength depends on impedance mismatch and angle. |

| Refraction | Bending of sound waves at an interface due to change in speed. | Can displace structures or duplicate them (artefact). Occurs at oblique angles and velocity change. |

| Scattering | Redirection of sound in many directions by small structures. | Causes speckle; helps create texture but may reduce clarity. |

| Specular Reflection | Reflection off a smooth surface (e.g., diaphragm). | Produces strong echoes when beam strikes at 90°; poor return at oblique angles. |

| Diffuse Reflection | Echoes from irregular surfaces. | Scattered echoes return from many angles; less angle-dependent than specular. |

| Speckle | Granular noise due to interference of scattered echoes. | Can reduce image quality; some systems use filters or compounding to reduce it. |

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## 🎯 Image Modes & Techniques

| Term | Definition | Relevance |

|----------|----------------|---------------|

| B-mode (Brightness mode) | Standard 2D imaging; each pixel’s brightness reflects echo amplitude. | Most common mode; used for structural assessment. |

| M-mode (Motion mode) | Displays motion over time along a single scan line. | High temporal resolution; useful for cardiac and fetal heart motion. |

| Pulse-Wave Doppler | Measures velocity of blood at a specific depth using pulsed ultrasound. | Allows spectral display of flow; subject to aliasing at high velocities. |

| Colour Flow Doppler | Displays direction and velocity of flow in a region using colour overlay. | Red = flow towards probe; blue = away. Detects turbulence and vessel patency. |

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## 📏 Image Resolution Types

| Term | Definition | Relevance |

|----------|----------------|---------------|

| Spatial Resolution | Ability to distinguish two structures that are close together. | Includes axial and lateral resolution. Better resolution = sharper images. |

| Axial Resolution | Ability to distinguish two structures along the beam’s path. | Depends on SPL. Improved by using high frequency (shorter wavelength). Best resolution overall. |

| Lateral Resolution | Ability to distinguish structures side by side (perpendicular to beam). | Depends on beam width. Better with narrow focus and higher frequency. |

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## 🔁 PRF & Timing

| Term | Definition | Relevance |

|----------|----------------|---------------|

| Pulse Repetition Frequency (PRF) | Number of sound pulses emitted per second. | Affects Doppler scale and depth. High PRF = better for fast flow; low PRF = for slow flow/deep targets. |

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## 📡 Echolocation

| Term | Definition | Relevance |

|----------|----------------|---------------|

| Echolocation | Process of emitting sound pulses and analysing returning echoes to locate structures. | Basic principle of ultrasound imaging. Calculates depth using time-of-flight and assumed speed of sound. |

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⚠ Bioeffects & Safety Indices

| Index | Definition | Relevance & Acronyms |

| MI (Mechanical Index) | Indicates likelihood of cavitation (formation of gas bubbles). | Related to peak rarefaction pressure and frequency. High MI = risk of cavitation. Keep low in obstetrics. |

| TI (Thermal Index) | Predicts temperature rise due to absorption. | Safe scanning requires TI <1.5 in obstetrics. |

| TIS (Thermal Index – Soft Tissue) | For early pregnancy or superficial tissues. | Reflects heating in soft tissue with no bone in the beam path. |

| TIB (Thermal Index – Bone) | Used after 10 weeks gestation when fetal bone is present. | Reflects heating of bone close to focus. |

| TIC (Thermal Index – Cranial) | For transcranial imaging. | Reflects heating near bone in the skull. Used in neonatal head scans and brain studies. |