Chapter 17 Principles of Ultrasound PDF

Brief History of Ultrasound 1. Origins: ○ Developed in the 1960s using principles of sonar from WWII. ○ Technique is similar to the echo-location used by bats, whales, and dolphins. 2. Technique: ○ High-frequency sound waves are sent into tissues. ○ Echoes...

Brief History of Ultrasound 1. Origins: ○ Developed in the 1960s using principles of sonar from WWII. ○ Technique is similar to the echo-location used by bats, whales, and dolphins. 2. Technique: ○ High-frequency sound waves are sent into tissues. ○ Echoes from reflected sound waves are recorded to visualize soft tissue contrast. Introduction to Ultrasound Imaging 1. Definition: ○ Ultrasound: Sound waves with frequencies above the range of human hearing and their propagation in a medium (the process of transmitting signals or waves through a substance, or medium, to a specific point). ○ Medical diagnostic ultrasound is a modality (mode in which something exists) that uses ultrasound energy and the acoustic properties of the body to produce an image from stationary and moving tissues. 2. How It Works: ○ In ultrasound imaging a short pulse of mechanical energy is sent into tissues. ○ The pulse travels at the speed of sound (c), and changes in tissue properties cause echoes. ○ These echoes return to the source (surface) and provide information about the tissues. 3. Image Formation: ○ The process is repeated multiple times, with each sound pulse slightly changing direction. This helps gather data from different angles, allowing the creation of a detailed gray-scale tomographic image that represents the tissue structure in a specific area. Ultrasound Imaging uses high-frequency sound waves to create images of soft tissues. It works by sending sound pulses into the body and recording the echoes that return, which provide detailed information about tissue structure and motion. Key Notes on Ultrasound Imaging 1. Pulse and Echo Detection: ○ Transducer: Generates sound pulses, detects echoes, and directs the pulse along a linear path through the patient. 2. How Ultrasound Differs from CT and Nuclear Medicine: ○ Non-Ionizing: Ultrasound uses a non ionizing longitudinal wave (sound). ○ Reflection Mode: Records signals from reflected sound waves, unlike X-rays that rely on transmission through tissues.  3. Sound vs. Electromagnetic Waves: ○ Sound Waves: The speed of sound waves is not constant because it changes depending on the medium's properties (solid, liquid, or gas) to travel. Sound waves are mechanical waves. ○ Electromagnetic Waves: Do not require a medium and can travel through a vacuum (e.g., light, X-rays). Ultrasound Imaging uses sound waves instead of electromagnetic radiation. It creates images by sending sound pulses into the body and recording the echoes that bounce back, providing a safe and effective way to view soft tissues. Important Notes on Ultrasound and Sound Waves Ultrasound Waves are Not Photons: Ultrasound waves are mechanical waves, not electromagnetic waves (photons). Photons: Photons are energy parcels of electromagnetic energy. They can travel through vacuum and materials. Their speed in vacuum is constant. Sound Waves (Phonons): Sound waves, or phonons, are packets of mechanical energy. ○ They can only travel through a medium (solid, liquid, or gas), not a vacuum. Ultrasound Wave Generation: Ultrasound waves are created by mechanical means (vibrations or tension and compression) in certain materials. Ultrasound is a mechanical wave, while photons are energy particles of electromagnetic waves that can travel through a vacuum. Sound waves need a medium, while photons can travel in both a vacuum and material. Physics of Ultrasound 1. Generating and Detecting Ultrasound Piezoelectric Materials: Used in transducers to generate and detect ultrasound. Piezoelectric: From the Greek words "piezein" (to squeeze) and "piezo" (to push). Piezoelectric Effect: The ability of certain materials to produce an electric charge when mechanical stress is applied. 2. Piezoelectric Effect is Reversible Direct Piezoelectric Effect: Generates electricity when stress is applied. Reverse Piezoelectric Effect: Generates stress when an electric field is applied. Both effects happen in the same material. Piezoelectric Effect at the Molecular Level Mechanical Stress: When piezoelectric material is under stress, the positive and negative charge centers shift, creating an electrical field. Reversed Effect: When an external electrical field is applied, it either stretches or compresses the piezoelectric material. Examples of Piezoelectric Materials: Zinc Oxide (ZnO) Gallium Nitride (GaN) Quartz (SiO2) Bone Some Proteins The Ultrasound Transducer Transducer: A device that converts one form of energy into another. Piezoelectric Materials: Used in transducers to produce and detect ultrasound. ○ Convert Electrical Energy: Into mechanical energy to produce sound (ultrasound waves). ○ Convert Mechanical Energy: Of returning echoes into electrical energy to detect the signal. Function in Ultrasound Imaging: Both Source and Detector: The transducer sends and receives ultrasound waves. Frequency Determination: The frequency (𝑓) of ultrasound is based on the transducer's crystal thickness (𝑑). ○ Thickness (𝑑): Approximately half the wavelength of the ultrasound. Properties of Ultrasound Frequency: Ultrasound waves have frequencies much higher than the human hearing range. ○ Human Hearing Range: 15 to 20 kHz ○ Medical Ultrasound Range: 2.5 to 40 MHz Image Construction: Ultrasound images are created by calculating the time it takes for the ultrasound beam to travel from the transducer to the reflecting surface and back. Speed of Sound in Tissue: Assumed to be 1540 m/s or 1.54 mm/attenuation second. Depth Calculation: Using the known speed of sound and the time taken for the round trip, the depth of the reflecting surface can be calculated. Frequency, f and Wavelength, λ Compression and Rarefaction: These are events in the medium that are described as a sine wave. ○ The sine wave has: Frequency (f): The number of wave cycles per second. Wavelength (λ): The distance between two consecutive points of the wave. Speed of Sound (v₀): ○ The speed at which sound travels through a medium. ○ It is medium-dependent. ○ The faster sound propagates (travels) in a medium that is stiffer (more resistant to deformation) and has a smaller molecular mass. Propagation speeds increase from gasses to liquids to solids. ○ For example, sound travels faster in solids than in liquids, and faster in liquids than in gases. Typical speeds in soft tissue (a mixture of connective tissue and fat) range from 1480 m/s to 1568 m/s.  When ultrasound travels from one medium to another, its speed changes. This causes a change in the wavelength of the ultrasound wave. Bulk Modulus (ℬ) Bulk modulus (ℬ) measures how stiff a medium is and its resistance to compression. The SI unit of ℬ is Pascal (Pa). Typical values for ℬ: ○ Fat: 2.0 GPa ○ Soft tissue: 2.2 GPa ○ Bone: 25 GPa 1 GPa = 10⁹ Pa. Sound Characteristics Particle Speed (v): ○ It refers to the speed of the material particles as they move back and forth with the sound pressure. ○ A typical particle speed is 35 mm/s. Acoustic Pressure (p) Acoustic pressure is caused by pressure changes in the material due to sound energy. A typical value is 0.06 MPa = 60 kPa. Imaging pressures can be 10 times higher, and Doppler pressures can be 25 times higher, which is important for safety. Amplitude (A) Amplitude is the maximum height of the wave, occurring at the compression peak. Reduced power results in a reduced amplitude. Power (P) Power is the rate at which the sound energy is transferred. Intensity (I) Intensity or acoustic intensity measures how much sound power passes through a unit area. Typical intensities for medical ultrasound: Imaging: 0.01−1 mW/mm^2 Doppler flow imaging: 0.01−0.3 mW/mm^2  Relationship Between Intensity, Pressure, and Amplitude Maximum intensity I occurs at maximum pressure p Maximum amplitude A coincides (occur) with the compression peak (maximum p). Decibel (dB) Scale Decibel scale measures comparative sound intensity. Used to express the gain or loss in sound intensity between two waves which is I1 and I2. Key Points: Positive dB: Indicates gain in intensity. Negative dB: Indicates loss in intensity. Logarithmic scale makes it easier to compare large variations in intensity. Pressure Level (dB) Definition: Pressure level is a way to measure sound intensity changes using decibels (dB). Key Points: 20 log formula applies to pressure changes. 10 log formula applies to intensity changes. Negative values show how much weaker the echo is compared to the original sound. 5. Interaction of Ultrasound with Matter Ultrasound Interaction: are determined by the acoustic (sound waves) properties of the medium (material it passes through). Attenuation: As ultrasound travels, its energy or intensity decreases (weakens). It is measured by how much is reduced per unit length and frequency. Types of Interactions in Medium Reflection: Some ultrasound bounces back from boundaries. Refraction (or Transmission): Ultrasound changes direction when moving between different media. The ultrasound beam bends. Scattering: Ultrasound spreads out in various directions due to small structures. Absorption: Some energy is absorbed as heat in the medium. Reflection fraction (R) and transmission fraction (T) depend on the acoustic impedance (Z) of the two materials. Z = Material’s resistance to sound. Acoustic Impedance (Z) Definition: Acoustic impedance (Z) measures how sound travels through a material. It depends on: 1. Density (ρ): How heavy the material is. 2. Speed of Sound (vs): How fast sound moves in the material. Formula: Z=ρ⋅vs Material-Specific: Each material has a unique acoustic impedance, which is constant for that material. How Acoustic Impedance Effects Sound 1. Stiffness and Flexibility: ○ In a simplistic way, the impedance can be linked to the stiffness and flexibility of a medium. ○ Consider springs with different stiffness attached to each other.Small differences in stiffness: Sound passes through easily. ○ Large differences in stiffness: portion of sound is reflected. 2. Reflection and Transmission: ○ At a boundary between two materials with very different Z values, more sound reflects, and less passes through. ○ Example: Soft tissue and air-filled lung—almost all ultrasound reflects due to a big difference in Z. Importance in Imaging Acoustic impedance differences help create ultrasound images using the pulse-echo technique. Reflection of sound at boundaries helps map tissue structures. This explains why Z is crucial for sound interaction and imaging! Reflection in Ultrasound Reflection occurs when the ultrasound beam interacts with a boundary between two materials with different acoustic impedances. There are two types: 1. Specular or (Mirror) Reflection Definition: Happens when the ultrasound beam hits a smooth boundary between two media (tissue) with different impedances. Key Features: ○ It forms the basis of ultrasound imaging. ○ Surface irregularities are much smaller than the sound wavelength. ○ Minimal scattering occurs—most of the sound reflects in a single direction. Why Specular Reflection is Important Helps capture clear echoes to create images of internal structures. The smooth boundary ensures strong, focused reflections essential for accurate imaging. Reflection in Ultrasound What Happens During Reflection? Definition: When an ultrasound wave hits a surface perpendicular to it, part of the wave reflects back. Key Points: ○ Change in speed: The wave slows down or speeds up when moving between different materials. ○ Wavelength: The reflected wave's wavelength stays the same and remains in phase with the incoming wave. Reflection Depends on Acoustic Impedance (Z)  Reflection is stronger when the difference in acoustic impedance between two materials is large. Examples: ○ Large reflection occurs between soft tissue and air or soft tissue and bone (like a tennis ball hitting a wall). ○ Small reflection occurs between different soft tissues (like a tennis ball hitting a net), creating weak echoes. Why It Matters Reflection enables ultrasound to capture echoes and form images, even from small tissue differences. That is why ultrasound gel is used to have the beam propagate into the body. The impedance of the gel should be similar to or slightly lower in fat. 2.Non-Specular Reflection (Scattering) Definition: Scattering occurs when an ultrasound beam hits a boundary with irregularities or structures similar in size to the wavelength of the ultrasound. Causes: It also happens when the beam passes through a medium with small particles (e.g., blood corpuscles) that differ in impedance from their surroundings. Key Points: 1. Formation of Scatter Cone: ○ The scattered echoes from the incident beam form a cone about the reflection axis. ○ The cone's angle depends on: The wavelength of the ultrasound. The surface roughness. 2. Effect of Surface and Wavelength: ○ Rough surfaces and shorter wavelengths create wider scatter angles. 3. Imaging Benefits: ○ Ultrasound scatter helps in imaging of curved surfaces and boundaries which are not perpendicular to the ultrasound beam. ○ Scatter intensity decreases significantly with frequency, following a power of 4 f^4. 4. Usefulness: ○ While the scatter echoes has a smaller intensity than the reflected ultrasound, it can contribute useful information on tissue characteristics inside an organ., Non-specular reflection, or scattering, happens when ultrasound waves interact with rough or uneven surfaces or small particles, producing scattered echoes that form a cone. This helps in imaging non-flat surfaces and provides details about internal tissues. Speckle Definition: Speckle is the noisy, textured background seen in ultrasound images. Cause: It results from constructive interference of scattered ultrasound waves from different tissue sites. Key Point: Speckle is caused by micro-scatterers but carries no useful information about them. Transmission Definition: Transmission refers to the part of the ultrasound beam that is not reflected and continues to pass through as a transmission beam. Refraction in Ultrasound Definition: Refraction occurs when a transmitted ultrasound wave travels at a different speed, causing the beam to deviate from its original path. Key Points: ○ The effect of refraction is minor in diagnostic imaging because soft tissues have similar sound speeds. ○ Acoustic lenses in transducers deliberately use refraction to focus the ultrasound beam. Absorption and Attenuation in Ultrasound Attenuation: A decrease in the intensity of the ultrasound beam as it travels through a medium. Causes of Attenuation: 1. Absorption: Tissue absorbs ultrasound energy (accounts for ~80% of power loss). 2. Beam divergence: Loss due to reflection or scattering. Absorption Coefficient (α)  Describes how much ultrasound is attenuated in soft tissue. Rule of thumb: ○ α = 0.5 (dB/cm)/MHz ○ Multiply the ultrasound frequency (in MHz) by 0.5 dB/cm to find the attenuation coefficient: Attenuation Coefficient, α (dB/cm)=f (MHz)×0.5 (dB/cm/MHz) Attenuation Coefficient (α) and Half-Value Thickness (HVT) Attenuation Coefficient (α) In tissue, ultrasound absorption increases linearly with frequency from 0.2 to 100 MHz. Examples: ○ At 2 MHz: α2MHz=2×0.5 (dB/cm/MHz)=2×α1MHz ○ At 10 MHz: α10MHz=10×0.5 (dB/cm/MHz)=10×α1MHz Intensity Half-Value Thickness (HVT) HVT: The tissue thickness where the ultrasound intensity drops by 50% or decreases by 3 dB. At 2 MHz, HVT = 6/2=3 cm At 10 MHz, HVT = 6/10=0.6 cm Key Points: Higher frequency = higher attenuation coefficient = smaller HVT. HVT ensures positive values, so we use 3 dB instead of -3 dB. Formulas: Question- Problem