Methoden 2 Hfst 7.1 - US (3).pptx

# Principles of ultrasound imaging Ultrasound imaging is a non-invasive, widely accessible, and real-time medical imaging technique that utilizes sound waves to visualize internal body structures. ### 1.1 Sound wave propagation Sound waves are longitudinal pressure waves that require a medium for propagation. They cannot travel in a vacuum, such as outer space. #### 1.1.1 Characteristics of sound waves * **Period ($T$)**: The time it takes for one complete cycle of a wave. * **Wavelength ($\lambda$)**: The spatial distance of one complete cycle of a wave. * **Frequency ($f$)**: The number of cycles per unit time, measured in Hertz (Hz). It is the inverse of the period: `$f = 1/T$`. * **Speed ($c$)**: The rate at which a wave travels through a medium. It is related to wavelength and frequency by: `$c = \lambda \times f$`. Waveforms can be described mathematically using sine or cosine functions. Waves with the same wavelength and amplitude can be shifted in time or space, which is known as a **phase difference**, often expressed in radians. **Interference** occurs when two or more waves combine. * **Constructive interference**: Waves combine to increase amplitude. * **Destructive interference**: Waves combine to decrease or cancel amplitude. Noise-canceling headphones utilize destructive interference. The speed of sound ($c$) in a medium is dependent on the medium's properties, primarily its density ($\rho$) and its elastic properties (represented by stiffness or bulk modulus). For example, the speed of sound in the human body (approximately 1540 m/s in soft tissue) is significantly higher than in air. > **Tip:** The relationship `$c = \lambda \times f$` highlights an inverse relationship between wavelength and frequency for a given medium, as the speed of sound ($c$) is constant in that medium. Higher frequencies correspond to shorter wavelengths. #### 1.1.2 Pressure and intensity Sound energy causes particle displacement and variations in local pressure within a medium. * **Pressure amplitude ($P$)**: The peak maximum or minimum deviation from the average pressure in the medium in the absence of a sound wave. The SI unit is Pascal (Pa). Diagnostic ultrasound typically involves peak pressure levels around 1.5 MPa, which is about 15 times atmospheric pressure. * **Acoustic intensity ($I$)**: The average power per unit area perpendicular to the direction of wave propagation. It is related to pressure amplitude by: `$I \propto P^2$`. Intensity levels for medical diagnostic ultrasound are typically measured in milliwatts per square centimeter (mW/cm$^2$). The FDA limits these levels to 750 mW/cm$^2$ for human medical use. A change of 10 on the decibel (dB) scale corresponds to a tenfold change in intensity, while a change of 20 dB corresponds to a hundredfold change. ### 1.2 Interactions between ultrasound waves and matter The interaction of ultrasound waves with tissues is governed by the acoustic properties of those tissues. #### 1.2.1 Acoustic impedance **Acoustic impedance ($Z$)** is a material property that quantifies the resistance a medium offers to sound wave propagation. It is defined as the product of the medium's density ($\rho$) and the speed of sound ($c$) in that medium: $$Z = \rho c$$ The SI unit for acoustic impedance is kg/(m$^2 \cdot$s), with 1 rayl being equal to 1 kg/(m$^2 \cdot$s). > **Tip:** Acoustic impedance differences between adjacent tissues are crucial for generating reflections (echoes). Materials with very different acoustic impedances (e.g., air and tissue, or bone and soft tissue) will produce strong reflections. #### 1.2.2 Reflection and refraction When an ultrasound wave encounters a boundary between two media with different acoustic impedances, part of the wave is reflected, and part is transmitted. * **Reflection**: The amount of reflection depends on the acoustic impedance mismatch between the two media. A larger difference results in a stronger reflection. * **Refraction**: The change in direction of a wave as it passes from one medium to another with a different speed of sound. This is governed by Snell's Law. If the wave strikes the boundary perpendicularly (normal incidence), there is no refraction. If the incidence is at an angle, refraction occurs. * For non-perpendicular incidence, the angle of reflection ($\theta_r$) equals the angle of incidence ($\theta_i$): `$\theta_i = \theta_r$`. * Echoes reflected away from the transducer cannot be detected. > **Example:** The interface between bone and soft tissue has a large acoustic impedance mismatch, resulting in strong reflection and significant attenuation of the ultrasound beam. This makes imaging through bone challenging. Similarly, air at the lung surface causes almost complete reflection. #### 1.2.3 Scattering Scattering occurs when ultrasound waves are deflected in various directions by small structures within a medium, typically those comparable to or smaller than the wavelength. * **Specular reflector**: A smooth surface (relative to the wavelength) that reflects the beam in a single direction. * **Scattering**: Occurs from rough surfaces or small structures, distributing the reflected energy in multiple directions. If the surface is rough relative to the wavelength, echoes are diffusely scattered. Higher frequencies (shorter wavelengths) are more likely to encounter surfaces that appear "rough," leading to diffuse scattering. The amplitude of an echo signal is influenced by: * The number and size of scatterers per unit volume. * The differences in acoustic impedance at the transitions of scatterers. * The ultrasound frequency. #### 1.2.4 Attenuation **Attenuation** is the gradual loss of ultrasound intensity as it propagates through a medium. It is caused by: * **Absorption**: Conversion of ultrasound energy into heat. * **Scattering**: Redirection of sound energy away from the main beam. * **Reflection**: Energy lost at interfaces. Materials like bone exhibit high attenuation, limiting the depth of penetration for ultrasound. Air also causes significant attenuation. Viscosity, a property indicating a fluid's resistance to deformation, also contributes to attenuation. ### 1.3 Principles of ultrasound image formation Ultrasound imaging relies on the principle of transmitting short pulses of ultrasound into the body and detecting the echoes that return from interfaces within the tissues. #### 1.3.1 The transducer The **transducer** is the key component that both generates and receives ultrasound waves. It contains a **piezoelectric crystal** that exhibits piezoelectric properties. * **Transmission (sending pulse)**: When an electrical voltage is applied across the piezoelectric crystal, it deforms mechanically, producing mechanical vibrations that generate ultrasound waves (electrical energy to mechanical energy). * **Reception (detecting echo)**: When returning ultrasound waves strike the piezoelectric crystal, they cause it to vibrate, inducing an electrical voltage (mechanical energy to electrical energy). > **Tip:** The piezoelectric crystal is the heart of the transducer. It acts as both a transmitter and a receiver. The transducer often includes: * **Backing layer**: Absorbs backward vibrations to prevent them from interfering with the outgoing pulse and to shorten the pulse duration. * **Matching layer**: Reduces the acoustic impedance mismatch between the transducer crystal and the body, thereby improving the efficiency of sound transmission into the tissue and echo reception. #### 1.3.2 Pulse-echo principle 1. The transducer emits a short pulse of ultrasound. 2. The pulse travels into the body. 3. When the pulse encounters an interface between tissues with different acoustic properties, some of the ultrasound energy is reflected back as an echo. 4. The transducer detects the returning echoes. 5. The time it takes for the echo to return is measured. Since the speed of sound in tissue is known (approximately 1540 m/s), this time can be converted into a depth measurement: $$ \text{Depth} = \frac{\text{Speed of sound} \times \text{Time}}{2} $$ The division by 2 accounts for the round-trip travel time (to the interface and back). 6. The strength (amplitude) of the returning echo provides information about the nature of the reflecting interface. 7. By emitting multiple pulses and processing the received echoes, a 2D image is constructed. #### 1.3.3 Image modes Several display modes are used to visualize ultrasound data: * **A-mode (Amplitude mode)**: Displays the amplitude of returning echoes along a single line as a function of depth. The horizontal axis represents depth, and the vertical axis represents echo amplitude. This was one of the earliest modes. * **B-mode (Brightness mode)**: The most common mode for diagnostic imaging. The amplitude of the echo is represented by the brightness of a dot on the screen. Stronger echoes produce brighter dots. A 2D image is formed by compiling many A-mode lines that sweep across a sector or a rectangular field of view. * **M-mode (Motion mode)**: Displays the movement of structures over time. A single B-mode line is displayed, and subsequent lines generated over time are placed side-by-side. This creates a 2D display where one axis represents depth and the other represents time, allowing visualization of the motion of structures along that line (e.g., heart valve motion). #### 1.3.4 Real-time imaging Modern ultrasound systems can generate images in real-time, allowing dynamic visualization of moving structures. A complete 2D image (frame) is composed of numerous individual scan lines (often over 100). * **Frame rate**: The number of complete 2D images (frames) displayed per second. A higher frame rate provides a smoother and more accurate representation of motion. * **Field of view (FOV)**: The sector angle or width of the anatomical area being imaged. * **Image depth ($D$)**: The maximum depth from which echoes are being displayed. * **Pulse repetition frequency (PRF)**: The number of ultrasound pulses emitted by the transducer per second. The maximum PRF is limited by the time it takes for echoes from the deepest desired structures to return. * **Line density (LD)**: The number of scan lines within the field of view. Higher line density can improve image quality but may reduce the frame rate. There is an inherent trade-off between image depth, frame rate, and line density. To increase the frame rate, one might need to decrease the image depth, reduce the number of scan lines, or decrease the field of view. ### 1.4 Spatial resolution Spatial resolution refers to the ability to distinguish between two closely spaced objects. In ultrasound, it is described in three dimensions: * **Axial resolution**: The ability to distinguish two objects lying one behind the other along the direction of the ultrasound beam. It is primarily determined by the spatial pulse length (SPL) of the transmitted pulse. Shorter pulses (higher frequency, shorter wavelength) lead to better axial resolution. * **Lateral resolution**: The ability to distinguish two objects lying side-by-side, perpendicular to the ultrasound beam, within the same scan line. It is determined by the width of the ultrasound beam. The beam is typically narrowest in the focal zone. * **Elevational resolution**: The ability to distinguish two objects lying one above the other, perpendicular to both the beam direction and the scan plane. It is determined by the thickness (height) of the transducer element. > **Tip:** Higher ultrasound frequencies generally lead to better spatial resolution (both axial and lateral) but also result in greater attenuation and reduced penetration depth. The choice of frequency is application-dependent. * **Applications and frequencies**: * Deep structures (e.g., abdomen): 3.5 - 5 MHz * Superficial structures (e.g., thyroid, breast): 7.5 - 10 MHz * Intravascular imaging: 30 - 40 MHz ### 1.5 Doppler echocardiography Doppler echocardiography utilizes the Doppler effect to assess the velocity of blood flow. The Doppler effect describes the change in frequency of a wave in relation to an observer moving relative to the wave source. * **Doppler shift**: The difference between the transmitted frequency and the received frequency due to the motion of red blood cells. This shift is proportional to the velocity of the blood flow and the cosine of the angle between the ultrasound beam and the direction of flow. $$ f_d = \frac{2 f_0 v \cos \theta}{c} $$ where $f_d$ is the Doppler shift frequency, $f_0$ is the transmitted frequency, $v$ is the velocity of the reflector (blood), $c$ is the speed of sound, and $\theta$ is the angle between the transducer beam and the direction of motion. > **Caution:** Accurate velocity measurements using Doppler require the angle $\theta$ to be known. Large angles ($> 60^\circ$) lead to small Doppler shifts and significant errors in velocity estimation due to small errors in angle measurement. * **Doppler spectrum**: A graphical representation of the distribution of blood flow velocities over time. Normal flow patterns have characteristic spectral waveforms related to the hemodynamics of the vessel. Abnormal or turbulent flow produces disturbed spectra. * **Quantification parameters**: * **Pulsatility Index (PI)**: `$( \text{max velocity} - \text{min velocity} ) / \text{mean velocity}$`. Measures variations in velocity over the cardiac cycle. * **Resistive Index (RI)**: `$( \text{max velocity} - \text{min velocity} ) / \text{max velocity}$`. Measures resistance to flow in arteries. ### 1.6 Measurement capabilities Ultrasound is used for routine measurements of distance, area, and volume. The accuracy of distance measurements relies on the precise timing of pulse-echo detection and the known speed of sound in soft tissue (approximately 1540 m/s). ### 1.7 Contrast agents Ultrasound contrast agents are microbubbles (typically 1-10 $\mu$m in diameter) composed of gas encapsulated in a stabilizing shell. * They have a very low acoustic impedance compared to surrounding tissue. * This large impedance mismatch significantly enhances reflections, improving contrast in vascular structures. * They are injected intravenously and circulate through the bloodstream, allowing visualization of blood flow and perfusion in organs like the liver. ### 1.8 Biological effects Ultrasound can have biological effects, primarily thermal and mechanical. * **Thermal effects**: Ultrasound energy is absorbed by tissues, leading to localized heating. The **Thermal Index (TI)** is a parameter that estimates the potential for tissue heating. * **Mechanical effects**: Primarily related to **cavitation**, the formation and collapse of gas bubbles within tissues due to pressure fluctuations. The **Mechanical Index (MI)** is a measure of the likelihood of cavitation. At diagnostic imaging intensity levels and typical exposure durations, ultrasound is considered a very safe imaging modality. Significant biological effects are generally observed only at much higher intensity levels or longer exposure times, such as those used in therapeutic ultrasound applications (e.g., lithotripsy). ### 1.9 Advantages and disadvantages of ultrasound imaging **Advantages:** * Generally non-invasive. * Widely available and relatively inexpensive. * No ionizing radiation. * Real-time imaging capability. * Considered a very safe technique. **Disadvantages:** * Image acquisition and interpretation are highly operator-dependent. * Air and bone significantly impede ultrasound penetration and visualization of underlying structures. * Imaging of deep structures can be challenging. * Resolution and contrast may be lower compared to modalities like CT and MRI. --- # Ultrasound equipment and data acquisition This section details the essential hardware of ultrasound imaging, focusing on transducers and their mechanisms, alongside the principles of acquiring ultrasound data, including pulsed modes, pulse repetition frequency, and spatial resolution. ### 2.1 Fundamentals of sound propagation Sound is a longitudinal wave, a pressure wave, that requires a medium for propagation. Its characteristics include: * **Period ($T$)**: The time for one complete cycle of a wave. * **Wavelength ($\lambda$)**: The spatial distance over which a wave's shape repeats. * **Frequency ($f$)**: The number of cycles per unit time, measured in Hertz (Hz). It is inversely related to the period: $f = 1/T$. * **Velocity ($c$)**: The speed at which a wave travels through a medium. It is related to wavelength and frequency by: $c = \lambda f$. These wave properties can be described mathematically using sinusoidal functions. Waves with the same wavelength and amplitude can be phase-shifted in time or space, with the difference expressed in radians. Interference between waves can be constructive (amplitudes add) or destructive (amplitudes cancel), with noise-canceling headphones utilizing destructive interference. The speed of sound in a medium is dependent on its properties. While sound travels at approximately 343 meters per second in air at room temperature, it travels faster in denser media like water (approximately 1480 meters per second) and human soft tissue (approximately 1540 meters per second). ### 2.2 Pressure and intensity of sound waves Sound energy causes particle displacement and variations in local pressure within a medium. * **Pressure amplitude ($P$)**: The maximum deviation from the average pressure in the medium in the absence of a sound wave. The SI unit is Pascal (Pa), where $1 \text{ Pa} = 1 \text{ N/m}^2$. Diagnostic ultrasound waves typically have peak pressure levels around $1.5 \text{ MPa}$, which is approximately 15 times atmospheric pressure. * **Acoustic intensity ($I$)**: The average energy per unit time per unit area, perpendicular to the direction of propagation. Intensity is proportional to the square of the pressure amplitude: $I \propto P^2$. Medical diagnostic ultrasound intensities are typically in the range of milliwatts per square centimeter ($mW/cm^2$). The US Food and Drug Administration (FDA) limits diagnostic ultrasound intensity to $750 mW/cm^2$. A change of 10 decibels (dB) on the dB scale corresponds to a tenfold increase in intensity, while a change of 20 dB corresponds to a hundredfold increase. ### 2.3 Ultrasound interactions with matter The interactions of ultrasound waves with materials are governed by their acoustic properties. * **Acoustic impedance ($Z$)**: A material property defined as the product of its density ($\rho$) and the speed of sound ($c$) within it: $Z = \rho c$. The SI unit is rayl, where $1 \text{ rayl} = 1 \text{ kg/(m}^2\text{s)}$. Acoustic impedance indicates the resistance a medium offers to sound wave propagation. Air has very low acoustic impedance, while bone has high acoustic impedance. When an ultrasound wave encounters a boundary between two media with different acoustic impedances, reflection and transmission occur. The amplitude of the reflected echo depends on the difference in acoustic impedance. * **Reflection**: Occurs at interfaces. If the boundary is smooth and the incidence is normal (perpendicular), the angle of reflection equals the angle of incidence. For non-normal incidence, reflection follows the law of reflection, where the angle of reflection ($\theta_r$) equals the angle of incidence ($\theta_i$). * **Refraction**: Occurs when ultrasound passes from one medium to another with a different speed of sound. The angle of refraction is governed by Snell's Law. * **Scattering**: Occurs when ultrasound waves interact with objects or interfaces smaller than or comparable to the wavelength. Diffuse scattering happens with rough surfaces, sending echoes in multiple directions, reducing the signal returning to the transducer. Specular reflectors are smooth surfaces that reflect ultrasound away from the transducer. * **Attenuation**: The reduction in ultrasound intensity as it travels through a medium, caused by absorption and scattering. Tissues like bone exhibit high attenuation, making imaging through them difficult. Air also causes significant attenuation. Viscosity, a measure of a fluid's resistance to deformation, contributes to attenuation. The basis of ultrasound imaging relies on the differences in acoustic impedance and speed of sound between tissues. A sound pulse is emitted, and the returning echoes are detected and converted into electrical signals. ### 2.4 Ultrasound equipment: The transducer The transducer is the core component that both generates and detects ultrasound waves. It converts electrical energy into mechanical energy to produce sound waves and vice versa. * **Piezoelectric crystal**: The heart of the transducer. These crystals deform under an applied electric field, generating ultrasound waves (transmitter function). Conversely, when struck by returning sound waves, they deform and induce an electric field, which can be measured (receiver function). Common piezoelectric materials include lead zirconate titanate (PZT) and polyvinylidene fluoride (PVDF). * **Backing material**: Placed behind the piezoelectric crystal to absorb excessive vibrations, improving pulse quality. * **Matching layer**: Situated between the piezoelectric crystal and the body to minimize reflection at the interface due to large acoustic impedance mismatches. #### 2.4.1 Transducer arrays Transducers can be composed of multiple piezoelectric elements arranged in arrays: * **Linear array**: Elements are arranged in a straight line. They can be activated individually or in groups to steer the ultrasound beam linearly. * **Phased array**: Elements are arranged in a pattern that allows for electronic steering of the ultrasound beam by introducing small time delays (phase differences) in the activation of individual elements. This allows for focusing the beam and creating a focal zone at a specific depth. The ultrasound beam has distinct regions: the **near field** (Fresnel zone) and the **far field** (Fraunhofer zone). Beam steering allows for scanning a sector of tissue with a single transducer. ### 2.5 Ultrasound data acquisition Data acquisition involves the process of generating and receiving ultrasound pulses. * **Pulsed mode**: Ultrasound is transmitted as short pulses of high voltage, followed by a listening period of low voltage to detect returning echoes. * **A-mode (Amplitude mode)**: Displays the amplitude of the received echo as a function of depth (or time) along a single line. It represents the strength of the returning signal. * **B-mode (Brightness mode)**: Converts the amplitude of the detected echo into a brightness level on a display, creating a 2D cross-sectional image. Brighter pixels indicate stronger echoes. * **M-mode (Motion mode)**: Tracks the movement of structures over time along a single A-line. This is useful for visualizing dynamic processes, such as heart valve motion. #### 2.5.1 Pulse repetition frequency (PRF) and related parameters Several parameters govern the pulsed nature of ultrasound imaging: * **Pulse repetition frequency (PRF)**: The number of ultrasound pulses transmitted per second. Typical values range from 2 to 7 kHz. The PRF is determined by the desired imaging depth. $$PRF = \frac{1}{PRP}$$ * **Pulse repetition period (PRP)**: The time interval between the start of consecutive transmitted pulses. It is the reciprocal of the PRF. $$PRP = \frac{1}{PRF}$$ * **Pulse duration**: The time during which a single ultrasound pulse is transmitted. It is determined by the number of cycles in the pulse and the transducer's central frequency. * **Duty cycle**: The fraction of time the transducer is actively transmitting a pulse. For real-time imaging, this is typically very low, around 0.2-0.4%. $$ \text{Duty cycle} = \frac{\text{Pulse duration}}{PRP} $$ **Tip:** The maximum PRF is limited by the time it takes for echoes from the deepest desired structures to return. If a new pulse is transmitted before the echoes from the farthest point are received, range ambiguity can occur, where echoes from a second pulse are mistaken for those from a nearer structure. The maximum imaging depth is related to the PRP and the speed of sound: $$ \text{Maximum Depth} = \frac{c \times PRP}{2} $$ The division by 2 accounts for the round trip of the ultrasound pulse. **Important Distinction:** Ultrasound transducer frequency is typically in the megahertz (MHz) range, while PRF is in the kilohertz (kHz) range. Similarly, the period of an individual ultrasound wave cycle is in the microsecond ($\mu$s) range, while the PRP is in the millisecond (ms) range. #### 2.5.2 Spatial resolution Spatial resolution refers to the ability to distinguish between two closely spaced objects. There are three main types: * **Axial resolution**: The ability to distinguish objects separated along the direction of the ultrasound beam. It is primarily determined by the spatial pulse length (SPL), which is directly related to the pulse duration and the speed of sound: $SPL = c \times \text{Pulse duration}$. Shorter pulses lead to better axial resolution. * **Lateral resolution**: The ability to distinguish objects separated perpendicular to the ultrasound beam, within the plane of the beam. This is determined by the beam width. Better lateral resolution is achieved with a narrower beam. * **Elevational resolution**: The ability to distinguish objects separated perpendicular to the imaging plane. This is influenced by the height of the transducer elements. Higher ultrasound frequencies lead to shorter wavelengths and thus potentially better spatial resolution, but also increased attenuation and reduced penetration depth. The choice of frequency is application-dependent: lower frequencies (e.g., 3.5-5 MHz) are used for deeper structures (like abdominal imaging), while higher frequencies (e.g., 7.5-10 MHz) are used for superficial structures (thyroid, breast). Very high frequencies (30-40 MHz) are used for specialized applications like intravascular imaging. #### 2.5.3 Real-time ultrasound imaging Real-time imaging creates a sequence of 2D images (frames) rapidly. * **Frame**: A 2D image composed of numerous A-lines. * **Field of view (FOV)**: The angular sector covered by the ultrasound beam. * **Imaging depth ($D$)**: The maximum depth from which echoes are displayed. * **Frame rate**: The number of frames displayed per second. It is inversely related to the time required to acquire a single frame. $$ \text{Time per frame} = N \times T_{\text{line}}(D) $$ where $N$ is the number of A-lines and $T_{\text{line}}(D)$ is the time to acquire one A-line to depth $D$. * **Line density (LD)**: The number of A-lines per unit angle, determined by $N$ and FOV. To increase the frame rate, one can reduce the imaging depth, decrease the number of A-lines ($N$), or reduce the FOV, often involving a trade-off in image quality or coverage. ### 2.6 Doppler ultrasonography Doppler ultrasonography utilizes the Doppler effect to measure the velocity of moving reflectors, most commonly blood flow. When ultrasound waves reflect off moving objects, their frequency is shifted. * **Doppler shift frequency ($\Delta f$)**: The difference between the transmitted and received ultrasound frequency. It is dependent on the speed of sound ($c$), the velocity of the reflector ($v$), the transmitted frequency ($f_0$), and the angle ($\theta$) between the transducer beam and the direction of motion: $$ \Delta f = \frac{2 f_0 v \cos \theta}{c} $$ * **Angle dependence**: The accuracy of velocity estimation is highly dependent on the angle of insonation. Large angles (greater than 60 degrees) lead to small Doppler shifts and significant errors in velocity calculations. The Doppler spectrum displays the distribution of velocities within the sample volume over time. Normal blood flow has a characteristic spectral waveform, while disturbed or turbulent flow exhibits disrupted spectra, which can correlate with pathological conditions. Parameters like Pulsatility Index (PI) and Resistive Index (RI) are used to quantify flow characteristics. ### 2.7 Measurement capabilities and contrast agents Ultrasound systems can perform routine measurements of distance, area, and volume. The speed of sound in soft tissue is approximately $1540 \text{ m/s}$ (with about 1% accuracy), allowing for precise calibration by timing pulse-echo travel. * **Contrast agents**: Microbubbles (1-10 $\mu$m in diameter) composed of gas (e.g., air, nitrogen) encapsulated in a shell (e.g., albumin). They are much smaller than the ultrasound wavelength and act as strong scatterers due to their large acoustic impedance difference from tissue. They enhance image contrast, particularly in vascular structures, by highlighting blood flow. ### 2.8 Biological effects Ultrasound can induce biological effects through thermal and mechanical mechanisms. * **Thermal effects**: Result from the absorption of ultrasound energy and its conversion into heat. The **Thermal Index (TI)** quantifies the potential for tissue heating. * **Mechanical effects**: Primarily cavitation, which involves the formation and collapse of microscopic gas bubbles due to negative pressure variations. The **Mechanical Index (MI)** measures the likelihood of cavitation. At diagnostic imaging levels, ultrasound intensities and exposure durations are kept low, making it a very safe imaging modality. Macroscopic damage (e.g., cell rupture) and microscopic damage (e.g., chromosomal breaks) have been observed at much higher intensity levels than those used in diagnostics. ### 2.9 Advantages and disadvantages of ultrasound imaging **Advantages:** * Generally non-invasive. * Widely available and relatively inexpensive. * No ionizing radiation. * Provides real-time imaging. * Considered a very safe technique. **Disadvantages:** * Image acquisition and interpretation are highly operator-dependent. * Air and bone significantly impede visibility of underlying structures. * Imaging deep structures can be challenging. * Resolution and contrast may be lower compared to CT and MRI. --- # Advanced ultrasound applications and biological effects This topic delves into specialized ultrasound applications and examines the biological impacts of ultrasound, emphasizing safety in diagnostic imaging. ### 3.1 Doppler ultrasonography Doppler ultrasonography is a crucial application that measures the velocity of moving structures, most notably blood flow, by detecting the frequency shift between the emitted and reflected ultrasound waves. This phenomenon is analogous to the change in pitch of a siren as an ambulance approaches or recedes. #### 3.1.1 The Doppler effect and frequency shift The Doppler frequency shift ($f_d$) is the difference between the received frequency ($f_r$) and the transmitted frequency ($f_t$). It is determined by the speed of the ultrasound wave ($c$), the velocity of the moving object ($v$), the transmitted frequency ($f_t$), and the angle ($\theta$) between the transducer beam and the direction of motion: $$ f_d = f_r - f_t = \frac{2 v f_t \cos(\theta)}{c} $$ * **Tip:** The cosine term indicates that the Doppler shift is maximal when the ultrasound beam is parallel to the direction of motion ($\cos(0^\circ) = 1$) and zero when the beam is perpendicular ($\cos(90^\circ) = 0$). This angle dependence is critical for accurate velocity estimation. #### 3.1.2 Estimating velocity By measuring the Doppler frequency shift, the velocity of the moving object can be calculated. However, the accuracy of this velocity estimation is highly dependent on the angle between the ultrasound beam and the direction of movement. $$ v = \frac{f_d c}{2 f_t \cos(\theta)} $$ * **Tip:** Large angles (greater than 60 degrees) result in small Doppler frequency shifts, making velocity estimations very sensitive to small errors in angle measurement. #### 3.1.3 Doppler spectrum and quantification The Doppler spectrum represents the amplitude of the received Doppler signal as a function of frequency (and thus velocity). Normal blood flow exhibits a characteristic spectral waveform influenced by the hemodynamic properties of specific blood vessels. Deviations from this normal pattern, such as disturbed or turbulent flow, can indicate disease processes. Quantifiable parameters derived from the Doppler spectrum include: * **Pulsatility Index (PI):** This measures the difference between the peak systolic and minimum diastolic velocities, normalized by the mean velocity. It reflects the pulsatility of blood flow. $$ \text{PI} = \frac{v_{\text{max}} - v_{\text{min}}}{v_{\text{mean}}} $$ * **Resistive Index (RI):** This measures the difference between peak systolic and minimum diastolic velocities, normalized by the peak systolic velocity. It provides an indication of the resistance in the artery. $$ \text{RI} = \frac{v_{\text{max}} - v_{\text{min}}}{v_{\text{max}}} $$ ### 3.2 Contrast-enhanced ultrasound (CEUS) Contrast-enhanced ultrasound utilizes microbubbles, typically 1 to 10 micrometers in diameter, injected intravenously to enhance the visualization of blood flow and vascular structures. These microbubbles are filled with gases like air or nitrogen and are encapsulated by substances such as albumin. #### 3.2.1 Mechanism of contrast enhancement The microbubbles possess acoustic impedance values significantly different from surrounding tissues, leading to strong reflections of ultrasound waves. Because their size is small relative to the ultrasound wavelength, they act as point sources, scattering the ultrasound in all directions. These contrast agents are designed to remain stable in circulation for a sufficient duration to reach target vascular areas. * **Tip:** The key to CEUS is the large difference in acoustic impedance between the microbubbles and the biological tissue, which generates a strong echo signal, thus increasing the contrast in the ultrasound image. #### 3.2.2 Applications of CEUS CEUS is valuable for: * **Characterizing liver lesions:** It can differentiate between various types of liver tumors and other abnormalities by observing their vascular patterns during different phases of contrast enhancement (e.g., arterial, venous, late phases). For instance, a neoplastic thrombus in a blood vessel may show contrast enhancement during the arterial phase and minimal washout in the late phase, aiding in its diagnosis. ### 3.3 Biological effects of ultrasound Ultrasound, while generally considered safe for diagnostic imaging, can exert biological effects through thermal and mechanical mechanisms. #### 3.3.1 Thermal effects Biological tissues absorb ultrasound energy, which is converted into heat. The extent of tissue heating depends on the rate of heat deposition by the ultrasound beam and the body's ability to dissipate this heat. * **Thermal Index (TI):** This parameter quantifies the potential for thermal effects. It is calculated as the ratio of the acoustic power produced by the transducer to the power required to raise the temperature of the tissue in the beam by 1 degree Celsius. Higher TI values indicate a greater risk of thermal damage. #### 3.3.2 Mechanical effects Mechanical effects are primarily associated with cavitation, which is the formation and subsequent collapse of microscopic gas bubbles within the tissues. * **Cavitation:** Under negative pressure cycles of the ultrasound wave, microbubbles can form. When the pressure increases, these bubbles can collapse violently, potentially causing tissue damage. * **Mechanical Index (MI):** This index is a measure of the likelihood of cavitation occurring due to an ultrasound beam. Higher MI values suggest a greater risk of mechanical bioeffects. #### 3.3.3 Safety considerations in diagnostic imaging While significant biological effects (such as cell lysis or chromosome damage) have been demonstrated at high ultrasound intensities and prolonged exposure times, diagnostic ultrasound operates at much lower intensity levels. * **Tip:** The safety of diagnostic ultrasound imaging relies on maintaining intensity levels below those known to cause significant harm and limiting the duration of the examination. Regulatory bodies often set limits on acoustic power output for diagnostic devices. #### 3.3.4 Comparison to high-intensity applications The tissue-damaging effects observed in applications like shock wave lithotripsy (used to break kidney stones) occur at significantly higher ultrasound intensities than those used in routine diagnostic imaging. ### 3.4 Measurement and quantification in ultrasound Beyond imaging, ultrasound is routinely used for making precise measurements. * **Distance measurements:** The time taken for an ultrasound pulse to travel to a structure and return as an echo is directly related to the depth of that structure. The speed of sound in soft tissues is approximately 1540 meters per second (m/s) with a variation of about 15 m/s. This relatively constant speed allows for accurate distance calculations. $$ \text{Depth} = \frac{c \times t}{2} $$ where $c$ is the speed of sound and $t$ is the round-trip time of the pulse. * **Calibration:** The precise timing of pulse transmission and echo reception allows for straightforward calibration of ultrasound instruments. This enables accurate measurements of anatomical structures, such as fetal femur length or abdominal circumference. * **Surface and volume measurements:** By acquiring multiple depth measurements or using specialized software, ultrasound can also be used to estimate surface areas and volumes of organs or lesions. ### 3.5 Contrast agents Contrast agents in ultrasound are typically microbubbles (1-10 micrometers in diameter) composed of a gas core (like air or nitrogen) encased in a stabilizing shell (e.g., albumin). Their primary function is to enhance the echogenicity of blood or specific tissues due to the significant difference in acoustic impedance between the microbubble and the surrounding tissue. They scatter ultrasound effectively, acting as point sources of reflection. These agents are designed to remain stable in the bloodstream for sufficient time to reach the area of interest, thereby improving image contrast. * **Example:** In liver imaging, contrast-enhanced ultrasound can help distinguish between a neoplastic thrombus (a tumor within a blood clot) and a benign thrombus by observing how the contrast agent is taken up and retained within the lesion over time. A neoplastic thrombus might show enhancement during the arterial phase of contrast administration and less washout in later phases compared to a non-neoplastic clot. --- # Measurements and advantages/disadvantages Ultrasound imaging is a versatile and widely used diagnostic technique that, in addition to producing images, is capable of performing various measurements, and offers distinct advantages and disadvantages compared to other imaging modalities. ### 4.1 Measurements using ultrasound Ultrasound is utilized for quantitative measurements, including distance, area, and volume, which are routinely performed in clinical settings. #### 4.1.1 Distance measurements The principle behind distance measurement relies on the time it takes for an ultrasound pulse to travel to a structure and for its echo to return to the transducer. * **Princ:** The time-of-flight of an ultrasound pulse is directly related to the distance traveled. Since the speed of sound in soft tissue is relatively constant and well-defined, this time can be converted into a distance. * **Sound speed in soft tissue:** The average speed of sound in soft tissue is approximately $1540 \pm 15$ meters per second. This value is remarkably consistent, allowing for accurate calibration of ultrasound instruments. * **Calibration:** The instrument's accuracy can be easily calibrated by precisely timing the transmission of a pulse and the reception of its echo. * **Applications:** Distance measurements are crucial for various clinical applications, such as determining the length of fetal long bones (e.g., femur length) or measuring the abdominal circumference of a fetus. #### 4.1.2 Area and volume measurements Once distances can be accurately measured, it becomes possible to calculate areas and volumes of anatomical structures. * **Methodology:** By tracing the boundaries of a structure in multiple planes or by using specific measurement tools within the ultrasound software, areas and volumes can be estimated. * **Clinical relevance:** These measurements are vital for assessing the size of organs, quantifying the volume of fluid collections, or monitoring the growth of tumors. #### 4.1.3 Contrast agents and measurements Contrast agents enhance ultrasound imaging, which can indirectly aid in more accurate measurements, especially in vascular structures. * **Mechanism:** Injected microbubbles, typically 1 to 10 micrometers in diameter, are used as contrast agents. These bubbles are small relative to the ultrasound wavelength and act as point sources, reflecting ultrasound in all directions. * **Contrast enhancement:** The large difference in acoustic impedance between the microbubbles and surrounding tissue significantly increases the contrast in the image, making it easier to delineate structures like blood vessels. * **Application in diagnosis:** For instance, in a case of suspected hepatic neoplasm, contrast-enhanced ultrasound can differentiate between a benign thrombus and a tumor based on how the contrast agent is taken up and washed out over time, which aids in diagnostic accuracy that can inform subsequent measurements. ### 4.2 Advantages and disadvantages of ultrasound imaging Ultrasound imaging possesses a unique set of strengths and weaknesses when compared to other medical imaging modalities. #### 4.2.1 Advantages * **Non-invasiveness:** In most applications, ultrasound is a non-invasive technique, meaning it does not require any surgical procedures or insertion of instruments into the body, making it well-tolerated by patients. * **Availability and cost-effectiveness:** Ultrasound machines are widely available in healthcare settings and are generally less expensive to purchase and maintain compared to modalities like CT or MRI scanners. * **No ionizing radiation:** Unlike X-rays and CT scans, ultrasound does not use ionizing radiation, making it a safer option for repeated examinations, particularly for sensitive populations such as pregnant women and children. * **Real-time imaging:** Ultrasound provides real-time visualization of anatomical structures and their motion. This capability is invaluable for dynamic assessments, such as observing fetal movement, cardiac valve function, or blood flow. * **Safety:** When used within established safety guidelines, diagnostic ultrasound is considered a very safe imaging technique with no known long-term adverse biological effects. #### 4.2.2 Disadvantages * **Operator dependency:** The quality of ultrasound images and the accuracy of measurements are highly dependent on the skill and experience of the sonographer (the operator). Proper acquisition and interpretation require significant training. * **Limitations due to air and bone:** Ultrasound waves are significantly attenuated (weakened) by air and bone. This means that structures deep to air-filled lungs or bone, such as the brain or abdominal organs when obscured by bowel gas, can be difficult or impossible to visualize clearly. * **Penetration depth limitations:** Higher ultrasound frequencies, which provide better resolution, have limited penetration depth. Conversely, lower frequencies can penetrate deeper but offer poorer resolution. This means visualizing very deep structures can be challenging. * **Resolution and contrast compared to other modalities:** While improving, ultrasound generally offers lower spatial resolution and contrast compared to CT and MRI. This can make it more difficult to discern fine details or subtle tissue differences in certain cases. > **Tip:** Always consider the limitations imposed by air and bone when interpreting ultrasound images. If visualization is poor due to these factors, alternative imaging modalities may be more appropriate. > **Tip:** The safety of ultrasound is primarily governed by intensity and duration of exposure. Diagnostic ultrasound typically uses low intensities for relatively short periods, making it a safe modality. High intensity levels are reserved for therapeutic applications where specific biological effects are desired. --- ## Common mistakes to avoid - Review all topics thoroughly before exams - Pay attention to formulas and key definitions - Practice with examples provided in each section - Don't memorize without understanding the underlying concepts

| Term | Definition | |------|------------| | Longitudinal wave | A wave in which the particles of the medium move parallel to the direction of wave propagation. Sound waves are an example of longitudinal waves, also known as pressure waves. | | Wavelength | The spatial period of a wave, the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests or troughs. | | Frequency | The number of cycles or oscillations of a wave that occur per unit of time, typically measured in Hertz (Hz). It is the reciprocal of the period. | | Amplitude | The maximum displacement or distance moved by a point on a vibrating body or wave measured from its equilibrium position. In sound waves, this relates to the pressure variation. | | Phase difference | The difference in phase between two waves or oscillating systems. It describes how much one wave is shifted in time or space relative to another, often measured in radians. | | Constructive interference | The superposition of two or more waves that results in a wave of increased amplitude. This occurs when waves are in phase, and their crests and troughs align. | | Destructive interference | The superposition of two or more waves that results in a wave of decreased amplitude or cancellation. This occurs when waves are out of phase, and a crest aligns with a trough. | | Sound speed | The speed at which sound waves propagate through a medium. It is dependent on the properties of the medium, such as its density and elasticity. | | Pressure amplitude | The peak value of the variation in local pressure in a medium caused by a sound wave, relative to the average pressure in the absence of the wave. It is measured in Pascals (Pa). | | Acoustic intensity | The average power per unit area carried by a sound wave, measured perpendicular to the direction of propagation. It is proportional to the square of the pressure amplitude. | | Acoustic impedance | A measure of the opposition a medium presents to the propagation of sound waves. It is defined as the product of the medium's density ($ \rho $) and the speed of sound ($c$) in that medium ($ Z = \rho c $). | | Scattering | The process by which a wave or particle is deflected in many directions as a result of interactions with inhomogeneities in the medium or with particles. | | Specular reflector | A surface that is smooth relative to the wavelength of the incident wave, causing the wave to be reflected in a single direction according to the law of reflection. | | Refraction | The bending of a wave as it passes from one medium to another in which its speed is different. The angle of refraction depends on the angle of incidence and the refractive indices (related to sound speed) of the media. | | Attenuation | The gradual loss in intensity of a signal, wave, or other energy form as it travels through a medium. In ultrasound, this is caused by absorption and scattering. | | Transducer | A device that converts one form of energy into another. In ultrasound, it converts electrical energy into mechanical (sound) energy for transmission and mechanical energy back into electrical energy for detection. | | Piezoelectric crystal | A material that exhibits the piezoelectric effect, meaning it generates an electric charge in response to applied mechanical stress, and conversely, deforms mechanically when an electric field is applied. | | Phased array | A type of transducer that uses multiple small piezoelectric elements that can be fired with precise timing differences. This allows for electronic steering and focusing of the ultrasound beam. | | Near field (Fresnel zone) | The region close to the transducer where the ultrasound beam is complex and not well-focused, characterized by interference patterns. | | Far field (Fraunhofer zone) | The region farther from the transducer where the ultrasound beam becomes more collimated and its divergence can be described by diffraction patterns. | | A-mode | Amplitude mode, a display in ultrasound where the amplitude of the reflected echo is plotted as a function of depth along a single line. | | B-mode | Brightness mode, a display in ultrasound where the amplitude of the echo is represented by the brightness of a dot on a 2D screen, forming an image. | | M-mode | Motion mode, a display in ultrasound that shows the movement of structures over time along a single scan line, typically used for visualizing cardiac motion. | | Doppler effect | The change in frequency of a wave in relation to an observer who is moving relative to the wave source. In ultrasound, it's used to detect and quantify blood flow. | | Doppler spectrum | A graphical representation of the Doppler shift frequencies present in a received ultrasound signal, showing the distribution of velocities within the sample volume. | | Pulsatility Index (PI) | A quantitative measure derived from Doppler ultrasound, representing the variation in blood flow velocity over the cardiac cycle. It is calculated as ($ (max - min)/mean $) of the velocity waveform. | | Resistance Index (RI) | A quantitative measure derived from Doppler ultrasound, used to assess the resistance to blood flow in arteries. It is calculated as ($ (max - min)/max $) of the velocity waveform. | | Contrast agent | A substance introduced into the body to enhance the visibility of tissues or blood vessels in medical imaging. In ultrasound, these are typically microbubbles. | | Thermal effects | Biological effects caused by the absorption of ultrasound energy, leading to heating of tissues. The Thermal Index (TI) quantifies this potential. | | Mechanical effects | Biological effects caused by the physical forces of ultrasound waves, primarily through cavitation. The Mechanical Index (MI) quantifies the likelihood of these effects. | | Cavitation | The formation and collapse of microscopic bubbles in a liquid due to rapid pressure changes, which can generate localized shock waves and heat, potentially causing tissue damage. | | Spatial resolution | The ability of an imaging system to distinguish between two closely spaced objects. In ultrasound, it is divided into axial resolution (along the beam) and lateral resolution (perpendicular to the beam). | | Penetration depth | The maximum depth into tissue that ultrasound waves can travel and still provide useful diagnostic information. Higher frequencies have shallower penetration depths. |