Ultrasound Physics Registry Review

Download Review

Frequently Asked Questions on Ultrasound Physics

1. What is the fundamental nature of sound waves used in ultrasound imaging, and how are they classified? Ultrasound waves are mechanical, longitudinal waves, meaning they travel through a medium by the vibration of molecules parallel to the direction of wave propagation. They are classified based on their frequency, which is the number of cycles per second (Hertz or Hz). Infrasound is below 20 Hz, audible sound ranges from 20 Hz to 20,000 Hz, and ultrasound is above 20,000 Hz. Diagnostic ultrasound typically uses frequencies between 2 and 20 MHz.
2. How do frequency, wavelength, and propagation speed relate to each other, and how do they influence image quality and penetration depth? Wavelength and frequency are inversely related; as frequency increases, wavelength decreases, and vice-versa. The wavelength is directly related to the propagation speed of sound in the medium. A faster propagation speed means a longer wavelength. Importantly, the propagation speed is ONLY determined by the properties of the medium itself, and is NOT affected by frequency or wavelength. In soft tissue, propagation speed is always 1540 m/s. In ultrasound imaging, higher frequencies provide better axial resolution (ability to distinguish closely spaced objects along the axis of the sound beam) but reduced penetration depth, because higher frequencies attenuate faster within the tissue. Lower frequencies offer greater penetration but at the cost of reduced axial resolution.
3. What is impedance, how does it affect reflection, and why is understanding reflection critical in ultrasound imaging? Impedance is a measure of a medium's resistance to the passage of sound waves and is determined by the density and propagation speed of the medium. A reflection only occurs when there is a difference in impedance between two mediums, called an impedance mismatch. The larger the impedance mismatch, the greater the reflection of sound energy at the boundary and a greater reflection means less sound energy is transmitted through the boundary. Understanding reflection is crucial because it is the primary source of signals that create ultrasound images. In soft tissue, only a tiny percentage of sound is reflected, with the vast majority of sound being transmitted. But with interfaces such as soft tissue and air or lung there are large reflections.
4. How does attenuation of ultrasound waves occur, and what are its practical implications for image quality and depth? Attenuation is the weakening of the ultrasound beam as it travels through tissue. This occurs through three main processes: absorption (conversion of sound energy into heat), reflection (sound energy bouncing back at impedance boundaries) and scattering (redirection of sound in multiple directions). Attenuation is directly related to frequency; higher frequency waves attenuate more quickly and therefore have reduced penetration. For instance, a 4 MHz wave will attenuate 2 dB/cm, while an 8 MHz wave will attenuate 4 dB/cm. This means that to image deep structures lower frequencies are chosen to achieve adequate penetration, even at the cost of resolution.
5. What are the main types of ultrasound transducers, and how do they achieve focusing and steering of the ultrasound beam? There are two main categories of transducers: single-element and array. Single-element transducers use a single crystal to generate the ultrasound beam. Focusing in these transducers is typically achieved via a lens. Array transducers (linear, curvilinear, and phased) utilize multiple crystal elements that are electronically controlled to shape and steer the beam. In particular, phased array transducers use time delays to steer the beam without moving the transducer, offering a wider field of view. Beam focusing with arrays is achieved via electronic time delays. Different types of transducers are chosen based on the imaging depth and required field of view of the particular exam.
6. What is the pulse-echo principle, and what are the different display modes (A-mode, B-mode, and M-mode) and how are they used? The pulse-echo principle is based on emitting short pulses of ultrasound and detecting the time it takes for the echoes to return from tissue interfaces. The machine uses this time to determine the location of the reflectors. A-mode displays reflections as spikes, representing the echo's amplitude. B-mode converts A-mode data into dots of varying brightness, creating a grayscale image. M-mode displays the motion of structures over time, often used in cardiac imaging. The range equation allows the determination of reflector distance via travel time and the 13 microsecond rule states that for every 13µs of travel time a reflector is 1cm deeper.
7. What is the Doppler effect in ultrasound, and how is it used to assess blood flow, and what are common artifacts? The Doppler effect is the change in frequency of a sound wave when reflected from a moving object, such as blood. If the reflector is moving toward the sound source the returned wave will have a higher frequency, whereas if the reflector is moving away the returned wave will have a lower frequency. This principle is used to determine blood flow velocity and direction (towards or away from the probe). The Doppler shift is affected by velocity, frequency, and angle of insonation. Aliasing, which occurs when the Doppler shift exceeds the Nyquist limit, is a common artifact that causes misrepresentation of flow and can be addressed by increasing the PRF or scale of the Doppler. In PW doppler the use of spectral analysis can further be used to visualize and measure velocities in one specific area of flow.
8. What are the key safety principles and quality assurance (QA) procedures in ultrasound practice, and what is a typical setup? The principle of ALARA (As Low As Reasonably Achievable) guides ultrasound practice; meaning settings and acoustic exposure are kept to a minimum. There is a Mechanical Index (MI), which measures the likelihood of cavitation, and a Thermal Index (TI), which measures the potential for tissue heating. QA programs are designed to ensure equipment meets performance standards and the quality of images is maintained. Performance testing involves using phantoms with specific characteristics and structures such as the AIUM’s 100mm test object or a tissue equivalent phantom. Typical machine setups include a pulser which controls output power, a beamformer which controls the electronic elements, and a receiver which amplifies weak echoes. Pre-processing such as gain, time gain compensation, dynamic range and image compounding improve the image prior to display on a monitor and post-processing adjusts the pixel values before displaying them on the screen.

Ultrasound Physics Registry Review: Study Guide

Quiz

1. Explain the difference between frequency and wavelength, and how they are related to each other.
2. Describe the effect of increasing the gain on the brightness of an ultrasound image and how it differs from adjusting output power.
3. What is the significance of the 1.54 mm/µs propagation speed in soft tissue, and what factors influence propagation speed in a medium?
4. Explain the relationship between impedance mismatch and reflection.
5. Describe the three main ways in which sound attenuates as it travels through tissue and include an example.
6. Define axial and lateral resolution, and identify the primary factors that influence each type.
7. Explain the importance of backing material in transducer construction and its effect on the pulse.
8. Describe the principle of the Doppler effect and its use in ultrasound imaging.
9. What causes aliasing in Doppler imaging, and how can it be reduced or eliminated?
10. Explain the significance of the ALARA principle in ultrasound scanning and how the Mechanical and Thermal Indices are measured.

Quiz Answer Key

1. Frequency refers to the number of cycles per second, while wavelength is the length of one cycle. Frequency and wavelength are inversely related; as frequency increases, wavelength decreases, and vice versa.
2. Increasing gain amplifies the received echo signals, making the overall image brighter. In contrast, adjusting output power affects the strength of the transmitted ultrasound pulse, directly impacting the amount of energy initially sent into the body.
3. 1.54 mm/µs is the average speed of sound in soft tissue and is essential for calculating the depth of structures. Propagation speed is dependent on the medium’s stiffness and density, with stiffness (bulk modulus) having a greater influence.
4. Reflection occurs at an interface when there is an impedance mismatch, or difference in acoustic impedance between two tissues. A larger impedance mismatch results in greater reflection, while no mismatch leads to 100% transmission.
5. Sound attenuates through absorption, where energy is converted into heat; reflection, which occurs at tissue interfaces; and scattering, which happens with small interfaces that cause sound to scatter in multiple directions. For example, bone or air results in great reflection, which is a form of attenuation.
6. Axial resolution is the ability to distinguish between two objects vertically along the axis of the beam and is half of the spatial pulse length (SPL). Lateral resolution is the ability to differentiate two objects horizontally and is primarily determined by beam width.
7. Backing material dampens the crystal vibrations, shortening the pulse and improving axial resolution. It stops the crystal from ringing.
8. The Doppler effect is a change in frequency when there is relative motion between a sound source and a reflector. It allows us to measure blood flow direction and velocity based on the frequency shifts.
9. Aliasing occurs when the Doppler shift exceeds the Nyquist limit, which is half the pulse repetition frequency (PRF). It can be reduced by increasing the PRF, adjusting the baseline, lowering frequency, or increasing the Doppler angle.
10. The ALARA principle emphasizes keeping power settings as low as reasonably achievable to minimize potential bioeffects. The Mechanical Index (MI) measures the likelihood of cavitation, while the Thermal Index (TI) measures the risk of tissue heating.

Essay Questions

1. Discuss the trade-offs involved in optimizing spatial and temporal resolution, and how these considerations impact image quality in different clinical scenarios.
2. Explain the process of tissue harmonic imaging, including the advantages it offers over fundamental frequency imaging and any limitations.
3. Describe the role of the pulser, beam former, receiver, and image processor in the ultrasound system, and analyze the sequence and function of these components in creating a diagnostic image.
4. Compare and contrast the principles, applications, and limitations of Pulsed Wave and Continuous Wave Doppler, including a discussion of how each modality displays the acquired information.
5. Outline the basic principles of hemodynamics, including the relationships between pressure, resistance, flow, and velocity, and explain how ultrasound is used to assess these factors in diagnosing vascular disease.

Glossary

• Acoustic Impedance (Z): A measure of a material's resistance to sound propagation; determined by the density and propagation speed of the medium.
• Aliasing: An artifact in Doppler imaging where high-velocity flow is incorrectly displayed due to under-sampling, usually when the Doppler shift exceeds the Nyquist limit.
• ALARA: Acronym for “As Low As Reasonably Achievable,” a principle emphasizing the minimization of ultrasound exposure to reduce bioeffects.
• Amplitude: The height of a pressure wave, related to the intensity of the sound; measured in megapascals (MPa).
• Attenuation: The weakening of sound as it travels through a medium, resulting from absorption, reflection, and scattering.
• Axial Resolution: The ability to distinguish between two objects that are close together along the axis of the beam; determined by half of the spatial pulse length.
• Bandwidth: The range of frequencies a transducer can generate and detect; a wider bandwidth is associated with shorter pulses.
• Beam Former: Electronic component of the ultrasound machine responsible for generating, controlling, and manipulating the characteristics of the ultrasound beam.
• Compression: An area of high pressure in a longitudinal sound wave, created by the molecules being closer together.
• Continuous Wave Doppler (CW): A mode of Doppler that uses two crystals to continuously transmit and receive signals to detect flow, allowing measurements of very high velocities but lacking depth resolution.
• Decibel (dB): A unit used to express the ratio of two power or intensity levels; often used to describe changes in signal amplitude.
• Duty Factor: The fraction of time the ultrasound machine is actively transmitting pulses; typically very low in pulsed ultrasound.
• Doppler Effect: The change in frequency of a sound wave when there is relative motion between the source and the reflector.
• Frequency: The number of cycles per second of a sound wave; measured in hertz (Hz).
• Gain: Amplification of the received echoes to compensate for signal loss, making the image brighter; does not affect transmitted sound.
• Harmonic Imaging: A technique that uses the multiple frequencies produced in response to the transmitted frequency to reduce artifacts and improve image quality.
• Huygens' Principle: The concept that each point on a wavefront acts as a source of secondary wavelets; used to understand beam formation.
• Intensity: The power per unit area of a sound beam; measured in watts per square centimeter (W/cm²).
• Lateral Resolution: The ability to distinguish between two objects that are close together perpendicular to the axis of the beam; determined by beam width.
• Mechanical Index (MI): A measure of the likelihood of cavitation, the formation of microbubbles that can cause bioeffects.
• Nyquist Limit: The maximum Doppler shift frequency that can be accurately detected, equal to half the pulse repetition frequency.
• Output Power: The setting that controls the strength of the transmitted ultrasound pulse.
• Piezoelectric Effect: The ability of certain materials to generate an electrical charge when subjected to pressure or mechanical stress and vice-versa.
• Propagation Speed: The speed at which sound travels through a medium, determined solely by the medium's properties; 1540 m/s in soft tissue.
• Pulse Repetition Frequency (PRF): The number of ultrasound pulses emitted per second.
• Pulse Repetition Period (PRP): The time from the start of one pulse to the start of the next pulse.
• Pulsed Wave Doppler (PW): A mode of Doppler that emits pulses of ultrasound and uses range gating to sample flow from a specific location.
• Quality Factor (Q): A measure of the purity of the resonant frequency of a transducer; inversely related to bandwidth.
• Rarefaction: An area of low pressure in a longitudinal sound wave, created by the molecules being further apart.
• Rayleigh Scattering: When interfaces are smaller than a wavelength and reflections are scattered equally in all directions.
• Reflection: The bouncing back of sound at an interface due to an impedance mismatch between two tissues.
• Refraction: The bending of a sound beam when it passes from one medium to another with a different propagation speed.
• Resolution: The ability of an imaging system to distinguish between two closely spaced objects (either spatially or temporally).
• Scattering: Redirection of sound waves in multiple directions by small interfaces.
• Spatial Pulse Length (SPL): The length or distance of one pulse in space; determined by the wavelength and number of cycles in the pulse.
• Spectral Broadening: The filling-in of the spectral window due to turbulent flow patterns, indicating a range of velocities at a specific point.
• Temporal Resolution: The ability to distinguish between two separate events in time; determined by frame rate.
• Thermal Index (TI): A measure of the likelihood of tissue heating during ultrasound exposure.
• Time Gain Compensation (TGC): A control that adjusts the gain at different depths to compensate for attenuation, ensuring uniform brightness on the image.
• Transducer: A device that converts one form of energy to another; specifically in ultrasound, converts electrical energy into mechanical (sound) energy and vice-versa.
• Wavelength: The length of one complete cycle of a sound wave; distance between two compressions.