High-Yield Notes for Ultrasound Physics (Sample Chapter – Ultrasound Instrumentation)

I. Fundamentals of Ultrasound Instrumentation: The Image Creation Pathway

The Core Principle: The Piezoelectric Effect

The transducer, the hand-held component of the ultrasound machine, serves as the primary interface with the patient. Its ability to convert electrical energy into mechanical (sound) energy and vice versa is based on the fundamental principle of the piezoelectric effect.¹ Piezoelectric materials, typically lead zirconate titanate (PbZT) crystals, exhibit this property: when an electric field is applied, they mechanically deform and vibrate, generating sound waves. Conversely, when mechanical pressure from returning acoustic echoes is applied, they generate an electric current.¹

The resonant frequency of the ultrasound wave generated is inversely related to the thickness of the piezoelectric element. A thicker element produces a lower frequency oscillation, while a thinner element produces a higher frequency oscillation. Specifically, the element thickness is designed to be equal to half the desired wavelength.⁴ Modern transducers typically consist of multiple piezoelectric elements, ranging from 128 to 512, arranged in linear or curvilinear arrays, allowing for sophisticated beam manipulation.⁴

Overview of the Ultrasound System

The circuit of the sequential flow of signals and control within a diagnostic ultrasound system. It begins with the Master Synchronizer initiating the process, guiding the generation of an electrical pulse, its transformation into an acoustic wave by the transducer, the reception and processing of echoes, and culminating in the visual display and archiving of the diagnostic image. Each component signifies a distinct stage where specific physical principles are applied to transform raw data into clinically meaningful information.

Table 1: Summary of Ultrasound System Components and their Primary Functions

Component Name	Primary Function	Key Physical Principle/Action
Master Synchronizer	Orchestrates timing and interaction of all system components	Precise temporal control, system integration
Pulser	Generates high-voltage electrical pulses for transducer excitation	Electrical pulse generation, energy control
Beam Former	Shapes, steers, and focuses the ultrasound beam during transmission and reception	Electronic phasing and delay, spatial distribution of pressure field
T/R Switch	Protects receiver from high transmit voltages; routes echoes to receiver	High-voltage switching, circuit isolation
Transducer	Converts electrical pulses to sound waves (transmit) and echoes to electrical signals (receive)	Piezoelectric effect, acoustic-electric conversion
Receiver	Processes and conditions weak electrical echo signals	Amplification, compensation, compression, demodulation, rejection
Image Processor	Converts analog signals to digital, reconstructs, enhances, and prepares images for display/storage	Analog-to-digital conversion, digital signal processing, scan conversion, digital-to-analog conversion
Display	Presents the processed ultrasound images visually	Visual rendering of grayscale/color data
Archive	Stores images and data for later review, analysis, or transfer	Digital data storage, image management

Detailed Explanation of System Components and Image Formation (Step-by-Step)

A. Master Synchronizer: The System’s Conductor

The Master Synchronizer functions as the central timing and control unit, orchestrating the precise sequence and interaction of all components within the ultrasound machine.⁸ It ensures that the pulser fires, the beam former shapes the pulse, the T/R switch activates, the transducer transmits, and the receiver listens—all with exquisite temporal accuracy. This synchronization is paramount for the entire ultrasound system to function as a cohesive, integrated unit.⁸

The precise temporal control exerted by the Master Synchronizer is a prerequisite for both accurate spatial mapping and high temporal resolution. While this component does not directly generate or process the ultrasound signal, its role in maintaining and organizing the proper timing and interaction of all other components is absolutely critical for image accuracy. The speed of sound in tissue is constant, approximately 1540 m/s ⁵, meaning accurate depth measurement (calculated as distance = speed × time / 2) fundamentally depends on the precise timing of echo arrival. If the synchronizer’s timing is even slightly off, the calculated distances would be incorrect, leading to misrepresentation of anatomical structures in the image. Furthermore, the rapid pulse-echo cycles, where the transducer spends over 99% of its time listening for returning waves ⁶, demand perfect coordination to prevent transmit pulses from interfering with received echoes. Such interference would severely degrade temporal resolution and introduce significant artifacts. This highlights that the Master Synchronizer, though seemingly a background control unit, is as vital as the signal path components, forming the foundational element for the system’s ability to produce accurate, real-time images.

B. Pulser: Generating the Ultrasound Pulse

The pulser is responsible for generating short, high-amplitude electrical voltage pulses of controlled energy.¹⁰ These electrical pulses are then delivered to the transducer elements, causing them to vibrate and produce ultrasonic waves.

Control functions associated with the pulser circuit include:

• Pulse Energy (Voltage): The pulser controls the voltage applied to the transducer, typically ranging from 100 to 800 volts.¹⁰ A greater voltage produced by the pulser directly correlates with a higher amplitude and intensity of the generated ultrasound pulses.⁹ Higher intensity pulses generally penetrate deeper into tissues, as they carry more acoustic energy.
• Pulse Length (Damping): This control determines the duration for which the electrical pulse is applied to the transducer.¹⁰ A shorter pulse length, often achieved through damping materials within the transducer, is desirable for better axial resolution. This is because a shorter pulse allows for finer distinction between closely spaced reflectors along the beam axis.⁴
• Pulse Repetition Period (PRP) and Pulse Repetition Frequency (PRF): The pulser dictates the time interval between successive voltage spikes, known as the pulse repetition period (PRP).⁸ The inverse of PRP is the PRF, which represents the number of ultrasound pulses launched per second.¹² A higher PRF allows for faster frame rates, thereby improving temporal resolution. However, a very high PRF can lead to range ambiguity artifacts if echoes from a previous pulse are still returning when the next pulse is transmitted.

The pulser can be designed to generate single electrical spikes for pulsed wave transducers or numerous electrical spikes for phased array transducers.⁸ In continuous wave (CW) Doppler applications, the pulser generates a constant electrical signal in the form of a sine wave, resulting in continuous sound wave emission.⁶ Pulser sections typically feature very low output impedance to efficiently drive the transducer elements, ensuring maximum power transfer.¹⁰

The pulser’s controls—voltage/energy and pulse length—directly influence the characteristics of the transmitted ultrasound wave. Higher voltage leads to greater intensity and thus deeper penetration. However, medical ultrasound operates under strict safety guidelines that limit peak amplitude to prevent bioeffects such as cavitation and tissue heating.¹³ This imposes a direct constraint on the pulser’s output. While longer pulse durations can increase total energy for better signal-to-noise ratio (SNR) and penetration ⁵, they inherently degrade axial resolution ¹³ unless advanced techniques like coded excitation are employed. This forces a critical design and operational compromise: maximizing one desirable characteristic, such as penetration, often necessitates a trade-off with another, like resolution, or requires complex compensation mechanisms. This demonstrates that the pulser is not merely a simple signal generator; its settings embody the fundamental physical and safety considerations inherent in ultrasound imaging.

C. Beam Former: Shaping the Sound Beam

The beam former is a sophisticated electronic component that precisely controls the electrical signals sent to, and received from, each individual element within the transducer array.¹⁴ Its primary role is to determine the shape, size, position, and direction of the interrogating ultrasound beam, effectively shaping the spatial distribution of the acoustic pressure field.¹⁵

During the transmission phase, the beam former generates precisely timed and phased electrical signals for each transducer element. By applying specific time delays (phase coefficients) to the excitation pulses across the array, it can electronically steer and focus the ultrasound beam to a desired depth and direction.⁵ This phased excitation allows for maximizing the pressure field at a specific location, thereby improving signal strength and signal-to-noise ratio (SNR).¹⁵ In the reception phase, the beam former receives the individual echo sequences from each transducer element. It then applies reverse time delays and sums these signals coherently to combine them into a single, focused echo sequence.¹⁴ This process, known as dynamic receive focusing, ensures that echoes from the focal zone are optimally weighted and combined.

The beam former’s control parameters, as indicated in the diagram, include:

• Mode: This selects the imaging mode, such as B-mode for 2D anatomical imaging, M-mode for motion over time, or various Doppler modes for blood flow analysis.
• Depth: Adjusts the overall imaging depth, which influences the transmit focal depth and the extent of dynamic receive focusing. Increasing depth generally reduces image resolution and frame rate.¹²
• Steer: Electronically steers the ultrasound beam, allowing for different angles of insonation without physically moving the transducer. This capability is fundamental for techniques like spatial compounding.⁵
• Focus: Electronically controls the focal zone, the region where the ultrasound beam is narrowest and lateral resolution is maximized.¹⁸ Placing the focus at the depth of interest maximizes spatial resolution in that specific region.¹⁸

Beam forming techniques are central to optimizing spatial resolution (axial, lateral, and elevational), temporal resolution (frame rate), contrast, and penetration depth.¹⁵ A larger array aperture, defined by the number, size, and distribution of elements, allows for higher pressure fields and improved SNR.¹⁵ The electronic manipulation of individual transducer elements by applying precise phase coefficients and timing is the cornerstone of modern ultrasound’s versatility. This digital control, in contrast to older mechanical scanning methods, allows for dynamic, real-time adjustments to beam characteristics (steering, focusing, shaping) that are fundamental to achieving high image quality and implementing advanced techniques. For instance, spatial compounding directly relies on the beam former’s ability to steer the beam from multiple angles.¹⁶ This highlights that the beam former is the technological core that translates raw electrical pulses into sophisticated, manipulable acoustic beams, enabling not only higher frame rates and better resolution control but also the feasibility of complex multi-angle and multi-frequency processing.

D. T/R Switch: Transmit/Receive Gate

The T/R (Transmit/Receive) switch is a high-voltage, differential switch designed to protect the highly sensitive, low-noise receiver circuitry from the extremely powerful electrical pulses generated by the pulser during transmission.¹⁹ It acts as a rapid gate, allowing high-voltage transmit pulses to reach the transducer, and then quickly switching to allow only the much weaker returning echoes to pass from the transducer to the receiver during the listening phase.

Typically, a T/R switch operates as a normally closed switch with very low resistance, such as 15Ω, allowing small signals (echoes) to pass.¹⁹ However, when the voltage drop across its two terminals exceeds a nominal threshold, for example, ±130V, the device rapidly turns off, becoming high impedance. This effectively isolates the receiver from the high transmit voltage.¹⁹ The amplitude difference between the transmitted pulse and the received echo can be enormous, often several orders of magnitude. Without adequate “off-isolation,” the high-voltage transmit pulses would overwhelm and potentially cause permanent damage to the delicate, low-voltage transistors and amplifiers in the receiver circuitry.²⁰ This switch ensures the integrity, longevity, and reliable operation of the entire ultrasound system.

The T/R switch’s primary function as a protective mechanism is a fundamental engineering necessity in any system that must simultaneously handle both high-power transmission and extremely sensitive low-power reception using the same physical interface, the transducer. Without a robust and fast-acting protective mechanism, the sensitive receiver would be instantly damaged or saturated, rendering the system inoperable. This underscores that practical system design in medical imaging involves not only optimizing signal generation and reception but also implementing critical protective mechanisms. The T/R switch ensures the system’s reliability, operational lifespan, and ultimately, its clinical viability by safeguarding expensive and delicate components from destructive energy levels, representing a crucial balance between power and sensitivity.

E. Transducer: The Heart of the System

The ultrasound transducer is a complex assembly comprising several key components that facilitate the conversion of electrical energy to acoustic energy and vice versa:

• Piezoelectric Element(s): These are the active elements, typically made of lead zirconate titanate (PbZT), which convert electrical energy into acoustic energy during transmission and acoustic energy back into electrical energy during reception.¹ Their thickness is precisely engineered to resonate at a specific frequency, equal to half the desired wavelength.⁴
• Electrodes: Positive and ground electrodes are coated on the faces of the piezoelectric element to provide electrical connection for signal transmission and reception.⁴
• Damping (Backing) Block: Adhered to the back of the crystal, this material absorbs ultrasound energy directed backward and attenuates stray signals. Crucially, it dampens the resonant vibrations in the element, which creates a shorter spatial pulse length. A shorter pulse length is vital for achieving better axial resolution, allowing for finer distinction between closely spaced reflectors, and for providing a broader bandwidth for receiving echoes.⁴
• Matching Layer: An interface positioned between the transducer element and the patient’s tissue. It consists of one or multiple layers of material with acoustic impedances intermediate between the transducer material and soft tissue.⁴ Each layer is typically one-quarter wavelength thick. This design minimizes reflection at the interface, allowing for close to 100% transmission of ultrasound from the element into the tissues, thereby maximizing signal strength.
• Housing: Provides electrical insulation and protection for the internal components, typically including a plastic case, metal shield, and acoustic insulator.⁴

The conversion process within the transducer involves:

• Transmission: Electrical pulses from the pulser excite the piezoelectric elements, causing them to vibrate rapidly and generate high-frequency ultrasound waves that propagate into the body.¹
• Reception: When these ultrasound waves encounter tissue interfaces, they reflect (echo) back to the transducer. The returning pressure waves cause the piezoelectric elements to vibrate, converting this mechanical energy back into electrical signals.¹ The transducer spends over 99% of its operational time “listening” for these echoes.⁶

Different transducer types are designed for specific clinical applications, optimizing the trade-offs between resolution and penetration:

• Linear Array Transducers: Crystals are embedded in a straight line, producing parallel ultrasound beams and a rectangular image.⁴ They offer excellent resolution in the near field and are ideal for superficial structures, vascular access, and peripheral nerve blocks.²¹
• Phased Array Transducers: All elements work together, and the beam is electronically steered and focused by applying precise time delays to the excitation of individual elements.⁴ They require a small acoustic window and are widely used in cardiovascular scanning where rib gaps limit access.⁵
• Curved Array (Convex) Transducers: Elements form a curved line, producing a sector-shaped image with a wider field of view at depth.⁵ They are suitable for abdominal and obstetric imaging where a larger scan area is needed from a limited acoustic window.

The fundamental principle of ultrasound imaging is the Pulse-Echo Method. The distance (D) to a reflector is calculated based on the known speed of sound in tissue (c ≈ 1540 m/s) and the time (t) it takes for the ultrasound pulse to travel to the reflector and its echo to return to the transducer (D = ct/2).⁵ The transducer’s physical construction directly dictates its acoustic properties. The piezoelectric element’s thickness determines its resonant frequency, which in turn dictates the fundamental frequency of the ultrasound wave.⁴ The damping block is crucial for shortening the pulse, directly impacting axial resolution.⁴ The matching layer ensures efficient energy transfer into the body.⁴ This entire design directly influences the trade-off between resolution and penetration ³: higher frequencies (thinner elements, more damping) offer better resolution but less penetration, and vice-versa. This demonstrates that the transducer is far from a passive component; it is an active determinant of image quality and clinical applicability. Its intricate design embodies core physics principles that govern the fundamental compromises in ultrasound imaging, making understanding this relationship vital for students to appreciate why different transducers are used for different exams and how their physical properties dictate image characteristics.

F. Receiver: Processing the Echoes

Once the transducer converts the returning acoustic echoes into weak electrical signals, these signals are sent to the receiver. The receiver’s primary role is to process and condition these radio-frequency (RF) signals, preparing them for subsequent image formation.²¹ Modern systems can have 256-512 active receive channels, offering significant processing flexibility.⁸

The receiver performs several key processing steps:

• Amplification: This is the initial step, where the very weak electrical signals from the transducer are strengthened.¹ The amount of amplification is controlled by the “gain” knob.¹ Linear amplification applies equal strengthening to all echoes.²¹ Proper gain adjustment is crucial to avoid images that are too dark (under-gained) or too bright (over-gained).¹⁸
• Compensation (Time Gain Compensation – TGC): Ultrasound waves attenuate (lose energy) as they travel deeper into tissue.²³ Compensation selectively amplifies signals based on the depth from which they originated, providing more amplification for deeper echoes and less for superficial ones.²¹ This process ensures that structures of similar echogenicity appear with uniform brightness throughout the image depth, overcoming the effects of attenuation.²¹ TGC is often adjustable by the user, allowing for fine-tuning of brightness at different depths.²¹
• Compression: The dynamic range of the received echoes (the ratio of the strongest to the weakest signals) is extremely wide, for example, 120 dB from the transducer ²⁵, far exceeding the display’s capability (e.g., 20-30 dB ²⁵) or the human eye’s ability to discern shades of gray.²⁵ Compression reduces this dynamic range by applying logarithmic amplification, meaning weaker signals are amplified more than stronger signals.²¹ This process allows all relevant information to be displayed without saturation or loss of detail, preserving the relative ranking of signal strengths.²⁵
• Demodulation (Envelope Detection): This step converts the oscillating radio-frequency (RF) signal into a simpler, smoother form that represents the amplitude envelope of the echo.⁸ It essentially extracts the “information” about echo strength from the high-frequency carrier wave, making it easier for subsequent processing.
• Rejection: This function eliminates very low-amplitude electrical signals that are below a user-defined threshold.⁸ Its purpose is to remove electronic noise and low-level acoustic clutter from the image, thereby improving clarity and potentially spatial and contrast resolution.²³ However, caution is advised as aggressive rejection can inadvertently remove diagnostically relevant low-amplitude tissue echoes.²³

The raw echoes returning to the transducer vary enormously in strength due to tissue attenuation and the nature of reflections. Without amplification, signals from deeper structures would be too weak to be detected. Without compensation (TGC), the image would be unnaturally dark at depth, obscuring vital diagnostic information.²¹ Crucially, without compression, the vast dynamic range of received signals (120 dB) cannot be effectively mapped to the limited dynamic range of the human eye or the display (20-30 dB). This would result in either saturation of bright signals or loss of detail in weak signals, rendering the image diagnostically useless. The receiver’s processes, therefore, are not merely arbitrary steps but are essential signal conditioning operations that transform raw, physically captured data into a format that is both perceptible and diagnostically meaningful to the human observer. This stage is where the system actively manages the inherent physical challenges of sound propagation and reflection to produce a visually coherent representation of anatomical structures.

G. Image Processor: From Raw Data to Visual Representation

The Image Processor is the “brain” of the ultrasound system, responsible for converting the conditioned electrical signals into a visual image. It performs several critical functions, transforming the raw echo data into a coherent and optimized display.

• Analog-to-Digital (A/D) Converter: The electrical signals received from the receiver are analog. Before any digital processing can occur, these analog signals must be converted into digital data.²⁴ The A/D converter samples the analog waveform and assigns a discrete numerical value (bit-depth) to each sampled point, representing the echo’s amplitude.²⁷ The resolution (number of bits) and sampling rate of the A/D converter are crucial for maintaining the dynamic fidelity of the ultrasound signal and avoiding image artifacts, ensuring good contrast and sharper images.²⁷
• Pre-Processing Functions: These are manipulations of the scan data that occur before the data is stored in the scan converter’s memory.¹⁶ Many functions performed by the receiver, such as amplification, compensation, compression, demodulation, and rejection, are considered preprocessing steps.²⁴ Other preprocessing functions include:

• Write Magnification (Write Zoom): This is a dynamic function that occurs while the ultrasound image is still live.¹⁶ The sonographer identifies a region of interest (ROI), and the system rescans only that ROI, writing new, denser data into the scan converter. This results in a magnified image where the number of scan lines and pixels within the ROI is greater than in the original image, leading to improved spatial resolution without degradation.¹⁶
• Pixel Interpolation / Fill-in Interpolation: In sector-shaped images, scan lines diverge at increasing depths, creating gaps in data between lines.¹⁶ Interpolation is a method of filling in these undetected data gaps by estimating pixel values based on adjacent known values.¹⁶ This process improves line density and spatial (detail) resolution, preventing the image from appearing as a series of disconnected scan lines with blank spaces.¹⁶

• Scan Converter (Memory): This is a memory device that converts the processed RF signals into a video display format composed of picture elements (pixels).²¹ The image plane is typically divided into a grid of equally sized pixels (e.g., 512 x 512, totaling 262,144 pixels).²¹ A digital number, proportional to the amplitude of the returning echo, is stored in each pixel, assigning a specific shade of gray (brightness) to that pixel.²¹ The scan converter effectively transforms the echo data from scan line format (time-of-flight and amplitude) into a standard video display format (x-y grid).
• Post-Processing Functions: These manipulations occur after the image data has been stored in the scan converter’s memory.¹⁶ They allow for image optimization without requiring new data acquisition. Examples include:

• Freeze Frame: Allows the user to pause the real-time image and browse through previous frames using a trackball or cursor.¹⁸
• Black/White Inversion: Inverts the grayscale display, which can be useful for generating negative images.²⁴
• Contrast Variation: Adjusts the overall contrast of the image, making it sharper or smoother.²⁴
• Frame Averaging (Persistence): Superimposes information from older images onto the most current frame.¹⁶ This improves image quality by smoothing the image and reducing noise, thereby increasing the signal-to-noise ratio.¹⁶ However, it reduces the frame rate and temporal resolution, making it less effective for rapidly moving structures.¹⁶
• Read Magnification (Read Zoom): A post-processing function that occurs while the image is frozen.¹⁸ The system “reads” and magnifies the existing pixel data. This results in a loss of image resolution as the image is magnified because the machine is simply enlarging pre-existing data, leading to more prominent pixels and blurriness.¹⁸ Write zoom is generally preferred over read zoom when possible.²⁸
• Edge Enhancement: A filtering technique that sharpens the image by identifying and increasing the contrast in the area immediately around sharp interfaces or boundaries between structures.¹⁶ This creates subtle bright and dark highlights, making boundaries more defined and images appear “crisper”.¹⁶ Advanced methods aim to enhance edges without amplifying speckle noise.³²
• Digital-to-Analog (D/A) Converter: After all digital processing, the image data needs to be converted back into an analog signal format that can be displayed on a conventional monitor.²⁴

The image processor is a critical nexus where raw echo data is transformed into a diagnostically useful image. The progression from analog-to-digital conversion, through various pre- and post-processing steps, and finally back to analog for display, represents a sophisticated manipulation of information. The ability to perform both preprocessing (affecting data acquisition) and post-processing (affecting display of stored data) provides significant flexibility for image optimization. This dual capability allows sonographers to refine image quality for specific diagnostic needs, either by influencing how the data is initially captured and organized (e.g., write zoom for better resolution) or by enhancing its visual presentation after acquisition (e.g., edge enhancement for clearer boundaries). This intricate interplay of digital signal processing ensures that the final image is not just a raw representation of echoes, but a highly refined visual map of internal anatomy.

H. Display: Visualizing the Image

The display is the screen that presents the ultrasound images generated from the scans.¹ It is the final visual output of the complex signal processing chain. Modern ultrasound systems often feature high-definition LED monitors with advanced display technologies, such as IPS (In-Plane Switching) technology, to provide outstanding image quality and detail.³³

The image displayed is typically a two-dimensional representation composed of bright dots, where the brightness of each dot (pixel) is determined by the amplitude of the returned echo signal.³⁴ This is commonly referred to as B-mode (Brightness mode) imaging.³⁴ The display’s dynamic range, typically narrower than that of the receiver or scan converter (e.g., 20-30 dB for display vs. 100-120 dB for receiver) ²⁵, necessitates the compression performed earlier in the processing chain to ensure all relevant information is visible. The quality of the display directly impacts the sonographer’s ability to discern subtle tissue differences and make accurate diagnoses.

I. Archive: Storing Diagnostic Information

The archive system is a digital device or network solution designed to store ultrasound images and associated patient data for later use, review, analysis, or transfer.¹ This is crucial for patient management, follow-up examinations, teaching, and research.

Modern archiving solutions often utilize Picture Archiving and Communication Systems (PACS) software and cloud storage, allowing for secure, efficient movement, viewing, and sharing of ultrasound images.³⁶ These systems support DICOM (Digital Imaging and Communications in Medicine) connectivity, the standard format for medical images, ensuring compatibility and seamless integration with existing healthcare workflows.³³ The ability to store and retrieve images quickly and securely is vital for maintaining smooth operations, boosting patient outcomes, and facilitating collaboration among physicians.³⁶ The archive also plays a role in image analysis, with integrated tools for zooms, pans, and measurements.³⁶

II. Advanced Imaging Features and Image Optimization

Modern ultrasound systems incorporate numerous advanced imaging features and optimization controls that go beyond the basic pulse-echo principle to enhance image quality, provide additional diagnostic information, or improve workflow. These features leverage sophisticated signal processing algorithms and beamforming techniques.

Magnification

Magnification in ultrasound allows for a closer look at a specific region of interest (ROI). There are two primary types:

• Write Magnification (Pre-processing Zoom): This occurs before the image data is stored in the scan converter’s memory, while the ultrasound machine is still in live imaging mode.¹⁶ When write zoom is activated, the system discards the existing data for the entire image and then rescans only the selected ROI with a higher density of scan lines and pixels.¹⁶ This means new, more detailed data is acquired for the zoomed area. The result is a magnified image where the spatial resolution is maintained or even improved, as more pixels are used to represent the same physical area, and the pixels themselves are the same size as in the original image.¹⁶ Write zoom is generally preferred for small structures as it results in less degradation or blurriness.¹⁸
• Read Magnification (Post-processing Zoom): This occurs after the image has been frozen and stored in memory.¹⁸ The ultrasound machine “reads” and magnifies the existing pixel data from the scan converter. Since no new data is acquired, the system simply enlarges the already present pixels.²⁸ This leads to a loss of image resolution, as the pixels become larger and more prominent, resulting in a more pixilated or blurry appearance.¹⁸ While useful for reviewing stored images, it is not ideal for obtaining diagnostic quality magnification.

Fill-in Interpolation

Fill-in interpolation is a preprocessing technique used to address the issue of diverging scan lines, particularly in sector-shaped images, where the space between scan lines increases with depth.¹⁶ Without interpolation, these images would appear as a series of distinct scan lines with blank data gaps between them.¹⁶ Interpolation algorithms estimate and fill in these missing data points by analyzing the values of adjacent, known pixels.¹⁶ This process effectively increases the line density and improves the spatial (detail) resolution, creating a smoother, more continuous image display.¹⁶ It helps to present a more complete representation of structures, especially at deeper depths where scan lines naturally spread further apart.

B-Color

B-Color, or Brightness Color, is an image optimization technique that improves contrast resolution by converting grayscale information into various levels of color intensity.²³ In standard B-mode imaging, structures are depicted as points of variable brightness on a grayscale.³⁴ The human eye, however, can distinguish a greater number of different color hues than shades of gray.²³ By mapping different grayscale values to a color scale, B-Color can enhance the perception of subtle soft tissue differences and boundaries, making them more visually distinct.²³ It is important to note that B-Color does not change the underlying ultrasound information or the physical data acquired; rather, it changes how that information is presented to the observer, potentially improving the subjective appreciation of structures for diagnostic purposes.²³

Panoramic Imaging

Panoramic ultrasound is a technique that stitches multiple B-mode images together to create a single, elongated composite image with an increased field of view (FOV).³⁷ This is particularly useful for visualizing and measuring masses or objects that are larger than the typical FOV of a single ultrasound image.³⁷ To perform this technique, the transducer is moved across the desired area, and the system automatically or semi-automatically combines the sequential images into one continuous display.³⁷ The reliability of the produced image can vary and often relies on a specifically trained and experienced operator to ensure accurate stitching and minimize motion artifacts.³⁷ While not commonly used in all contexts, it provides a valuable tool for assessing large anatomical regions or extensive lesions.

Spatial Compounding

Spatial compounding is an advanced ultrasound technique that enhances image quality by utilizing information obtained from several different imaging angles to create a single composite image.¹⁶ In conventional sonography, tissue is insonated from a single direction.¹⁷ With spatial compounding, the ultrasound beam is electronically steered by the beam former to transmit and receive from multiple predetermined angles, typically within 20 degrees from the perpendicular.¹⁶ The frames acquired from these different angles are then averaged or overlapped to form a single, real-time image.¹⁶

The advantages of spatial compounding are significant: it reduces angle-dependent artifacts (such as posterior acoustic enhancement and shadowing), minimizes speckle noise (the granular appearance in images due to scattering from small reflectors), and improves the signal-to-noise ratio.¹⁶ This leads to improved visualization of tissue boundaries, better definition of nerve borders and fascia contours, and clearer imaging around bone.¹⁶ However, a primary limitation is the reduction in frame rate and temporal resolution, as multiple acquisitions are needed for each composite image.¹⁶ It also increases the likelihood of movement artifacts if the patient moves during the multiple acquisitions.³⁹

Persistence (Temporal Compounding)

Persistence, also known as temporal compounding or temporal averaging, is an image processing technique that continuously displays information from older images by superimposing a number of previous frames onto the most current frame.¹⁶ This process averages the frames obtained from the same angle or view.³¹

The primary benefit of persistence is an improvement in image quality during real-time acquisition. It creates a smoother image with reduced noise, thereby increasing the signal-to-noise ratio.¹⁶ This is particularly effective for imaging slow-moving structures, where the averaging of multiple frames can effectively suppress random noise while preserving true anatomical information.¹⁶ However, the limitation of persistence is that it reduces the frame rate, which in turn reduces temporal resolution.¹² This makes it less useful for rapidly moving structures, such as in cardiac imaging or fast blood flow, where motion blur can obscure important details.³⁰

Frequency Compounding

Frequency compounding is an advanced technique designed to reduce speckle noise and improve image quality by combining image information obtained from different transmission frequencies.⁴⁰ In conventional ultrasound, a single fixed frequency component is typically extracted from the received signals to form an image.⁴⁰ However, ultrasound images often contain speckle patterns, which are a form of noise resulting from constructive and destructive interference of scattered echoes within a resolution cell.⁴⁰

Frequency compounding addresses this by forming two or more image frames, each corresponding to different transmission frequencies or frequency sub-bands extracted from a wide-band signal.⁴⁰ These sub-band images contain uncorrelated speckles. By combining or averaging these images, either with or without weighting factors, the speckle noise level is significantly reduced, resulting in a smoother ultrasound image with clearer tissue contours.⁴⁰ This method can improve the signal-to-noise ratio (SNR) without compromising temporal resolution, unlike spatial compounding.⁴¹ Some advanced systems may even vary the steer angle based on the transmission frequency to further optimize the compound image.⁴⁰

Frequency Tuning

Frequency tuning refers to the ability of an ultrasound system to adjust the operating frequency of the transducer. Diagnostic ultrasound typically uses frequencies between 2 and 20 MHz.⁶ This control allows the sonographer to optimize the image resolution at the level of the object being evaluated.¹²

The choice of frequency involves a fundamental trade-off between penetration and resolution:

• Higher Frequencies: These waves have shorter wavelengths and offer higher spatial resolution, meaning finer details can be displayed.³ They are ideal for scanning superficial objects (in the near field), such as vascular structures or peripheral nerves, as they provide superior detail.³
• Lower Frequencies: These waves have longer wavelengths and are able to penetrate deeper into tissue.³ They are used when scanning deeper objects (in the far field, typically deeper than 5-6 cm) where greater penetration is required, albeit with poorer resolution.³

Adjusting the frequency setting allows the sonographer to select the highest possible frequency that still provides adequate penetration to visualize the anatomy of interest.¹² This dynamic adjustment is crucial for obtaining the best possible image quality across varying depths and anatomical targets.

Coded Excitation

Coded excitation is an advanced ultrasound imaging technique used to improve the signal-to-noise ratio (SNR) and increase penetration depth, particularly under low echo-signal-to-noise-ratio conditions.¹³ In conventional ultrasound, safety considerations limit the peak amplitude of the interrogating pulse to prevent cavitation and tissue damage from thermal heating.¹³ This can limit the total energy delivered, affecting penetration.

Coded excitation addresses this by increasing the total energy input into the system not by increasing peak amplitude, but by increasing the transmit pulse duration.¹³ Instead of a single short pulse, a longer, complex coded pulse (e.g., a Barker code or Golay code) is transmitted.¹³ Longer pulses would normally result in poorer axial resolution.¹³ However, the codes are designed such that a matched filter can be used upon reception to compress the received echo energy into a very short time duration, effectively restoring the axial resolution.¹³ This decoding process amplifies the echo SNR by a factor equal to the time-bandwidth product of the code without compromising spatial resolution or contrast.⁴² This allows for greater penetration while maintaining diagnostic image quality, making it valuable for applications such as vascular imaging, bone attenuation estimates, and elasticity imaging.⁴²

Edge Enhancement

Edge enhancement, also known as Sharpness or Enhance, is a post-processing filtering technique that aims to sharpen the image by identifying and enhancing the interfaces or boundaries between structures.¹⁶ This process increases image contrast in the area immediately around sharp edges, creating subtle bright and dark highlights on either side of these boundaries.¹⁶ The result is a “crisper” image with more defined borders, which can improve the visualization of anatomical structures and pathological conditions.²³

Traditional edge detection techniques can inadvertently enhance speckle noise, which is prevalent in ultrasound images, and tend to widen detected edges.³² Modern edge enhancement algorithms often employ a two-step approach: first, the image is segmented into processing blocks, and then only blocks that satisfy certain statistical constraints (indicating the presence of a true edge rather than just speckle) are selected for enhancement.³² Pixels in these selected blocks are then transformed via a sigmoidal function to enhance the edges, while blocks containing only speckles or reverberation artifacts are not processed.³² This targeted approach ensures that the desired edges are sharpened without substantially enhancing noise or widening the processed edges.

Elastography

Elastography is a non-invasive medical imaging technique that visualizes and quantifies the stiffness (or elasticity) of organs and other structures within the body.⁴³ It is particularly useful for detecting and assessing the severity of liver disease, guiding treatment decisions, and even potentially replacing liver biopsies.⁴⁴

The general principle of elastography involves:

1. Perturbation: Applying a mechanical force to the tissue. This can be a quasi-static (manual compression), harmonic (continuous low-frequency vibration), or transient (single pulse) mechanical source.⁴³ In ultrasound elastography, a transducer or a specialized driver sends painless low-frequency vibrations into the body.⁴⁴
2. Measurement of Mechanical Response: Ultrasound imaging then measures how quickly these vibrations or shear waves move through the organ.⁴³ Stiffer tissues transmit shear waves faster than softer tissues.
3. Inference of Biomechanical Properties: A computer uses this information to create a visual map (elastogram) showing the stiffness of the tissue.⁴³ Stiff tissue is often indicative of disease, such as fibrosis in the liver.⁴⁴

Elastography provides valuable diagnostic information that complements traditional B-mode imaging, as tissue stiffness can be an early indicator of pathology not visible on conventional grayscale images.

Cardiac Strain Imaging

Cardiac strain imaging is a non-invasive echocardiography (ultrasound) method used to evaluate the function of the heart muscle (myocardium) by measuring its deformation (change in shape and dimension) during the cardiac cycle.⁴⁵ This technique allows for the identification of subtle changes in heart function that may not be evident with other imaging methods, offering early detection of heart failure and aiding in treatment decisions.⁴⁵

One common method for cardiac strain imaging is Speckle Tracking Echocardiography.⁴⁵ This technique analyzes the motion of naturally occurring “speckle patterns” within the myocardium or blood when imaged by ultrasound.⁴⁵ These speckle patterns are unique acoustic signatures generated by the interference of ultrasound waves scattered from microscopic tissue structures. By tracking the movement of these patterns from frame to frame, the system can assess the deformation (strain) directly in the myocardium.⁴⁵ Cardiac strain imaging can display and measure the simultaneous function of different regions of the heart, revealing weakened or altered function in specific areas.⁴⁶ It is particularly useful for screening and monitoring patients receiving cardio-toxic medications (e.g., during cancer treatment), evaluating cardiomyopathy, heart failure, and aortic stenosis.⁴⁵

3D Rendering

Three-dimensional (3D) rendering in ultrasound imaging involves constructing a volumetric image from a series of two-dimensional (2D) images.⁴⁷ While 2D ultrasound provides cross-sectional slices, 3D ultrasound adds a position-sensing component to produce a realistic volume image.⁴⁷

The process typically involves:

1. Volume Acquisition: An array of transducers acquires a series of 2D frames, one behind the other, covering a specific volume of tissue.⁴⁷ This can be achieved through mechanical scanning (motorized movement of a 2D transducer) or by using specialized 2D array transducers.⁴⁸
2. Volume Data Processing: Automated mathematical algorithms process this volumetric data. “Rendering” is the process of converting this voxel-based data into a viewable image with added depth.⁴⁷ This allows the images to be rotated, displayed in different orientations, and viewed in virtual planes that cannot be seen with standard 2D techniques.⁴⁷
3. Real-Time 3D (4D Ultrasound): When 3D images are displayed in real-time motion, they constitute 4D ultrasound.⁴⁷ This provides a live video of the 3D structure, allowing for real-time reconstruction of images in different planes and renderings.⁴⁷

3D and 4D ultrasound offer several advantages: they provide a more comprehensive image of anatomical structures and pathological conditions, allow for more accurate visualization of irregularly shaped organ volumes, and enable more reproducible measurements in three planes.⁴⁷ Applications include examining fetal anatomy, assessing heart chambers and valves, and guiding biopsies.⁴⁷

Harmonics (Tissue Harmonic Imaging)

Tissue Harmonic Imaging (THI) is a signal processing technique that utilizes higher-frequency harmonic waves, which are naturally produced by the non-linear propagation of the fundamental ultrasound wave through body tissues.⁴⁹ In conventional B-mode ultrasound, the transmitted and received frequencies are the same (fundamental frequency).⁵⁰ However, as an ultrasound pulse traverses tissues, its waveform gets distorted due to the non-linear properties of the medium (e.g., peaks move faster than troughs).⁴⁹ This non-linear propagation generates harmonic frequencies, which are integer multiples of the fundamental transmitted frequency (e.g., a 3 MHz fundamental frequency can produce 6 MHz (second harmonic), 9 MHz (third harmonic), etc.).⁴⁹

THI primarily uses second harmonic echoes for image formation.⁵⁰ Image processing techniques, such as bandwidth receive filtering (to screen out the fundamental signal) or pulse inversion (using two simultaneous pulses with a 180° phase difference to cancel the fundamental), are employed to eliminate the fundamental frequency echoes.⁴⁹ The remaining harmonic frequency data are then used to generate the diagnostic image.⁵⁰

Advantages of THI include:

• Improved Signal-to-Noise Ratio (SNR): Harmonics are not produced in the superficial part of the tissue and are most prominent in the central section of the transmitted beam, leading to a narrower beam profile and reduced artifacts from side lobes, grating lobes, and reverberation.⁴⁹
• Reduced Artifacts: By filtering out the fundamental frequency, many artifacts originating from the transducer or superficial structures are suppressed.⁴⁹
• Superior Tissue Definition and Reduced Speckle: This results in crisper images with improved border and tissue definition.⁵⁰

THI is routinely used in diagnostic ultrasonography and has significantly improved conventional grayscale image quality.⁵⁰

Dynamic Range

Dynamic range in ultrasound refers to the range in amplitude (strength) between the strongest and weakest echoes detected by the transducer that an instrument or component can respond to without distortion.¹² It is typically expressed in decibels (dB), which is a logarithmic measure of a ratio.²⁵

The dynamic range of information generally decreases as it is processed through the ultrasound system.²⁵ For example, the transducer has the widest dynamic range (e.g., 120 dB), followed by the receiver (100-120 dB), scan converter (40-50 dB), and finally the display and archive (10-30 dB).²⁵ This necessitates the compression process in the receiver, which reduces the wide range of signals into a narrower range that can be effectively displayed and perceived by the human eye.²¹

Adjusting the dynamic range control allows the user to control the contrast on the ultrasound image.¹²

• Increasing Dynamic Range (Wider): Yields a higher number of grayscale levels, resulting in a smoother image with more shades of gray and increased image detail.¹² This is helpful when discerning subtle differences within largely homogenous tissue, such as imaging the liver or detailed fetal anatomy.²⁶
• Decreasing Dynamic Range (Narrower): Increases the contrast of the image, with more black and white areas and fewer shades of gray.¹² This eliminates weaker signals and noise, enhancing the strongest echo signals. It can be useful when image quality is poor or when prioritizing detection over detail, such as clearly delineating fluid-filled structures from surrounding tissue.²³

The optimal dynamic range setting depends on the specific clinical application and user preference, as it directly impacts the visual presentation and perceived detail of the ultrasound image.²⁶

III. Applied Exercise

Scenario: A 35-year-old patient presents with right upper quadrant pain. A sonographer is performing an abdominal ultrasound to evaluate the liver and gallbladder. During the examination, the sonographer encounters several challenges:

1. Deep structures, particularly the posterior liver segments, appear very dark, even with overall gain adjustments.
2. The gallbladder lumen, which should be anechoic (black), shows some internal echoes, suggesting noise.
3. A small, suspicious lesion is identified in the superficial liver, but its borders appear somewhat indistinct.
4. The sonographer wants to capture a comprehensive image of a large liver mass that extends beyond the standard field of view.

Questions:

1. Addressing Deep Structure Darkness:

• a. Which receiver function is primarily responsible for ensuring uniform brightness of structures at different depths, and why might it be failing in this scenario?
• b. Describe the physical principle behind this function and explain how adjusting its controls would help improve the visualization of the posterior liver segments.

2. Eliminating Gallbladder Noise:

• a. Which receiver function would be most appropriate to eliminate the internal echoes (noise) within the anechoic gallbladder lumen?
• b. Explain the physical mechanism by which this function removes unwanted signals. What is a potential drawback of overly aggressive use of this control?

3. Enhancing Lesion Borders:

• a. Which advanced image processing feature would be most beneficial for making the borders of the superficial liver lesion appear sharper and more defined?
• b. Describe the underlying physical principle or algorithm that this feature employs to achieve its effect.

4. Capturing the Large Liver Mass:

• a. Which advanced imaging feature would allow the sonographer to visualize the entire large liver mass in a single composite image?
• b. Explain how this feature works from a physics perspective, detailing the process of image acquisition and reconstruction.

5. Optimizing Image Contrast:

• a. If the sonographer wants to increase the number of grayscale levels to see more subtle tissue differences within the liver parenchyma, how should they adjust the dynamic range setting?
• b. Explain the physical effect of this adjustment on the displayed image and why it is beneficial for discerning subtle tissue characteristics.

Also, try out our targeted q & a in the this chapter ! (Click Here)

Works cited

1. Ultrasound Physics and its Application in Medicine – PALNI Pressbooks, accessed May 30, 2025, https://pressbooks.palni.org/ultrasoundphysicsanditsapplicationinmedicine/chapter/ultrasound-instrumentation/
2. pie.med.utoronto.ca, accessed May 30, 2025, http://pie.med.utoronto.ca/OBAnesthesia/OBAnesthesia_content/OBA_ultrasonographyBasics_module.html#:~:text=The%20sound%20waves%20emitted%20from,an%20image%20on%20the%20screen.
3. Ultrasonography Basics, Transducer, Structures, Planes, Needle – University of Toronto, accessed May 30, 2025, http://pie.med.utoronto.ca/OBAnesthesia/OBAnesthesia_content/OBA_ultrasonographyBasics_module.html
4. Ultrasound transducer | Radiology Reference Article | Radiopaedia.org, accessed May 30, 2025, https://radiopaedia.org/articles/ultrasound-transducer
5. Ultrasound Physics inlcuding wave propagation, transducer, imaging instrumentation, Color Doppler Flow Image, regulations on acoustic output – BioSono, accessed May 30, 2025, https://biosono.com/UltrPhys/UltrPhysMain.htm
6. Ultrasound Physics and Technical Facts for the Beginner | Sonoguide – ACEP, accessed May 30, 2025, https://www.acep.org/sonoguide/basic/ultrasound-physics-and-technical-facts-for-the-beginner
7. Ultrasound Physics and Instrumentation – StatPearls – NCBI Bookshelf, accessed May 30, 2025, https://www.ncbi.nlm.nih.gov/books/NBK570593/
8. Pass Ultrasound Physics Exam Study Guide Review – Volume I, accessed May 30, 2025, https://elibrary.stikespantikosala.ac.id/index.php?p=fstream-pdf&fid=31&bid=8966
9. Ultrasound Physics Lecture 9 Flashcards – Quizlet, accessed May 30, 2025, https://quizlet.com/568740759/ultrasound-physics-lecture-9-flash-cards/
10. Pulser-Receivers – Ultrasound – NDE-Ed.org, accessed May 30, 2025, https://www.nde-ed.org/NDETechniques/Ultrasonics/EquipmentTrans/pulserreceivers.xhtml
11. Pulser-Receivers Selection Guide: Types, Features, Applications | GlobalSpec, accessed May 30, 2025, https://www.globalspec.com/learnmore/test_measurement/nondestructive_test_equipment/pulser_receivers
12. Ultrasound Imaging Guide – Learn What To Adjust First, accessed May 30, 2025, https://info.umiultrasound.com/blog/ultrasound-imaging-guide-ultrasound-knobology-for-a-better-image
13. PHASE CODES IN ULTRASOUND IMAGING: THE DEPENDENCE OF SNR ON BANDWIDTH – Institute of Acoustics, accessed May 30, 2025, https://www.ioa.org.uk/system/files/proceedings/c_leavens_r_williams_p_burns_m_sherar_phase_codes_in_ultrsound_imaging_the_depenedence_of_snr_on_bandwidth.pdf
14. Transducers and beam-forming (Chapter 3) – Diagnostic Ultrasound, accessed May 30, 2025, https://www.cambridge.org/core/books/diagnostic-ultrasound/transducers-and-beamforming/FC5FB7A4916933AC1BAA22FA7BC30BAC
15. Practical Guide to Ultrasound Beam Forming: Beam Pattern and Image Reconstruction Analysis – MDPI, accessed May 30, 2025, https://www.mdpi.com/2076-3417/8/9/1544
16. [Ultrasound Physics] IMAGE PROCESSING, accessed May 30, 2025, https://minjikim-md.tistory.com/14
17. Spatial Compound Imaging | Radiology Key, accessed May 30, 2025, https://radiologykey.com/spatial-compound-imaging/
18. KNOBOLOGY & IMAGE OPTIMISATION – MMHEME, accessed May 30, 2025, http://www.mmheme.org/knobology-1
19. Microchip Technology Ultrasound T/R Switch ICs – Mouser Electronics, accessed May 30, 2025, https://www.mouser.com/new/microchip/supertex-ultrasound-tr-switch/
20. A High-Voltage Differential SPDT T/R Switch for Ultrasound Systems – UCL Discovery, accessed May 30, 2025, https://discovery.ucl.ac.uk/10195902/1/FINAL%20VERSION%20%281%29.pdf
21. Basic Ultrasound Physics Archives – Page 8 of 12 – USabcd, accessed May 30, 2025, https://test.usabcd.org/topic-category/basic-ultrasound-physics/page/8/
22. test.usabcd.org, accessed May 30, 2025, https://test.usabcd.org/topic-category/basic-ultrasound-physics/page/8/#:~:text=The%20receiver%20processes%20the%20radio,from%20which%20the%20echoes%20return.
23. Edge Enhancement B-Colour Reject Post-processing Curves A. B. C. 2D Imaging Artefacts Attenuation Artefacts – Echotext, accessed May 30, 2025, https://www.echotext.info/chapter-samples/EchoEd3Chap1Sample.pdf
24. Ultrasound Physics – Preprocessing and Post Processing – BioSono, accessed May 30, 2025, https://www.biosono.com/UltrPhys/UltrPhys.php?id=SgnlCrct_PP
25. Understanding Ultrasound Physics( Chapter 16 )Dynamic Range Flashcards – Quizlet, accessed May 30, 2025, https://quizlet.com/513940026/understanding-ultrasound-physics-chapter-16-dynamic-range-flash-cards/
26. What is Dynamic Range? – Animal Ultrasound Association, accessed May 30, 2025, https://www.animalultrasoundassociation.org/what-is-dynamic-range/
27. High Performance Data Converters for Medical Imaging Systems – Analog Devices, accessed May 30, 2025, https://www.analog.com/en/resources/analog-dialogue/articles/high-performance-data-converters-for-medical-imaging-systems.html
28. Essential Ultrasound Controls And How To Use Them: Part 2 – Sonography Minutes, accessed May 30, 2025, https://sonographyminutes.com/essential-ultrasound-controls-and-how-to-use-them-part-2/
29. Display & Image Processing I Flashcards – Quizlet, accessed May 30, 2025, https://quizlet.com/849636612/display-image-processing-i-flash-cards/
30. Chapter 15 Ultrasound Physics Flashcards – Quizlet, accessed May 30, 2025, https://quizlet.com/571267770/chapter-15-ultrasound-physics-flash-cards/
31. Persistence ,, – YouTube, accessed May 30, 2025, https://www.youtube.com/watch?v=mA-K9oinALQ
32. US5662113A – Edge enhancement system for ultrasound images – Google Patents, accessed May 30, 2025, https://patents.google.com/patent/US5662113A/en
33. MyLab – Ultrasound Systems, accessed May 30, 2025, https://www.esaote.com/ultrasound/ultrasound-systems/p/mylab-X7/
34. B-Mode – FUJIFILM VisualSonics, accessed May 30, 2025, https://www.visualsonics.com/product/software/b-mode-imaging
35. Grey scale imaging (ultrasound) | Radiology Reference Article | Radiopaedia.org, accessed May 30, 2025, https://radiopaedia.org/articles/grey-scale-imaging-ultrasound
36. Ultrasound – Candelis, accessed May 30, 2025, https://www.candelis.com/solutions/ultrasound
37. Panoramic ultrasound | Radiology Reference Article | Radiopaedia.org, accessed May 30, 2025, https://radiopaedia.org/articles/panoramic-ultrasound?lang=us
38. Panoramic imaging on ultrasound | Radiology Case | Radiopaedia.org, accessed May 30, 2025, https://radiopaedia.org/cases/panoramic-imaging-on-ultrasound?lang=us
39. Spatial compounding (ultrasound) | Radiology Reference Article | Radiopaedia.org, accessed May 30, 2025, https://radiopaedia.org/articles/spatial-compounding-ultrasound
40. US20080194958A1 – Method of Compounding and Ultrasound Image – Google Patents, accessed May 30, 2025, https://patents.google.com/patent/US20080194958A1/en
41. Frequency Compounded Imaging with a High-Frequency Dual Element Transducer – PMC, accessed May 30, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC4118472/
42. Coded Pulse Excitation for Ultrasonic Strain Imaging, accessed May 30, 2025, https://ws.engr.illinois.edu/blogs/getfile/151/30836
43. Elastography: general principles and clincial applications – PMC, accessed May 30, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC3896304/
44. Elastography – Radiology Info, accessed May 30, 2025, https://www.radiologyinfo.org/en/info/elastography
45. Myocardial Strain Imaging | Cardiology | Lee Health Heart Institute, accessed May 30, 2025, https://www.leehealth.org/our-services/cardiology/echocardiogram-with-strain-imaging
46. Ultrasound Cardiac Strain Imaging | MHChester, accessed May 30, 2025, https://www.mhchester.com/service-page/ultrasound-cardiac-strain-imaging
47. 2D, 3D, and 4D Ultrasound Imaging – Ultrasound Physics and its Application in Medicine, accessed May 30, 2025, https://pressbooks.palni.org/ultrasoundphysicsanditsapplicationinmedicine/chapter/2d-3d-and-4d-ultrasound-imaging/
48. Three-dimensional ultrasound scanning – PMC, accessed May 30, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC3262266/
49. Tissue harmonic imaging – PMC – PubMed Central, accessed May 30, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC5024862/
50. A Primer on the Physical Principles of Tissue Harmonic Imaging, accessed May 30, 2025, https://pubs.rsna.org/doi/pdf/10.1148/rg.2015140338