Ultrasound Physics

Ultrasound Physics: A Foundation for Diagnostic Imaging

Introduction to Ultrasound Physics

Understanding the physics of ultrasound is fundamental for producing optimized diagnostic images time and again, and it also helps in recognizing artifacts that can lead to diagnostic uncertainty. While it is possible to generate images without a complete understanding of how they are formed, a basic grasp of these principles enhances image quality and interpretation. This chapter will cover how ultrasound is used, how images are generated, and some drawbacks of this technology.

Generating an Ultrasound Wave: The Piezoelectric Effect

To create an ultrasound image, the process begins with an ultrasound probe. Probes, such as the phased array type used for transthoracic adult echocardiography, contain a grid of piezoelectric crystals just beneath their footprint.

• Piezoelectric Effect: The key feature of these crystals is their ability to change shape and deform predictably when an electrical charge is applied to them. By applying an alternating current, the crystal can be made to get bigger and smaller, causing it to oscillate.

• Wave Propagation: When the probe is held against a patient, this oscillation generates kinetic energy that propagates through the patient's body in the form of a wave.

• Particle Movement: As the wave moves through the patient, it pushes particles closer together, creating areas of compression where pressure is increased. Because particles cannot be created or destroyed, this also leads to areas where particles are spread further apart, known as rarefaction, where pressure is decreased.

• Sinusoidal Wave: If you freeze time and observe the particles, you'd see a pattern of resting pressure, compression, rarefaction, and back to resting pressure. This can be plotted as a sinusoidal wave with distance on the x-axis and pressure on the y-axis. Alternatively, by considering a single point in space over time, you can observe particles moving together (pressure increases) and spreading apart (pressure decreases), also forming a sinusoidal wave but with time on the x-axis and pressure on the y-axis.

Key Characteristics of an Ultrasound Wave

Several key characteristics define an ultrasound wave:

• Wavelength: This is the distance between an area of resting pressure, through a compression and rarefaction, and back to resting pressure when viewing the wave frozen in time with distance on the x-axis.

• Period: This is the measurement along the x-axis (time) from an area of resting pressure, through a compression and rarefaction, back to resting pressure when observing a single point in space over time.

• Propagation Velocity: This refers to the speed at which the wave moves into the patient and then back towards the probe. The speed of the wave is dictated by the medium it's moving through, with different tissues having different propagation speeds. However, the ultrasound machine assumes a constant velocity of 1540 meters per second, as it doesn't know the specific tissue the wave is moving through.

• Frequency: This describes the number of periods per second, or the number of times a wave travels through a single point in space within one second. It is measured in Hertz (Hz).

  • Human Hearing: Typically detects sounds between 20 Hz and 20,000 Hz.

  • Medical Ultrasound: Uses frequencies many times greater than human hearing.

  • Echocardiography: Typically uses frequencies ranging from 1 to 8 Megahertz (MHz), or 1 to 8 million periods per second.

• Relationship between Characteristics: The key characteristics are intrinsically linked by the equation: Velocity = Frequency × Wavelength.

  • Impact on Resolution: Wavelength is a key determinant of axial resolution (resolution from the near field to the far field). The ability to discriminate between two reflective surfaces depends on the structure being larger than the wavelength. Smaller wavelengths enable greater resolution. For example, if a wave travels at 1540 m/s and has a frequency of 4 MHz, its wavelength is about 0.4 mm; halving the frequency to 2 MHz doubles the wavelength to approximately 0.8 mm.

• Spatial resolution is encompassing both axial and lateral resolution. 

  • For axial resolution, typical systems have around 3 millimeters of resolution. It can be improved by using shorter pulse lengths in addition to higher frequency. 

  • For lateral resolution, typical systems have around 1 millimeter of resolution. It can be improved by lower gain settings in addition to narrower beams/focusing.

• Temporal resolution is explained using a spinning wheel analogy to demonstrate how a low sample rate can lead to the perception of motion in the wrong direction, a phenomenon known as aliasing, which is particularly relevant in Doppler imaging. Temporal resolution in 2D imaging can be improved by reducing the depth and sector width, which allows for a higher frame rate.

• Contrast resolution is a type of resolution mostly affected by pre and post-image processing. 

  • Compression is described as reducing the number of shades of gray to create a sharper Doppler profile. 

  • Rejection involves choosing a boundary or threshold beyond which any signal is blacked out, also producing a crisper image.

Interaction of Ultrasound Waves with the Body

Once an ultrasound wave enters the patient, several interactions can occur:

• Reflection: Essential for imaging, as it relies on reflecting energy off structures within the body and detecting its return to the probe. Different tissues have different levels of acoustic impedance, which is closely related to their density.

  • When ultrasound moves between two tissues with different acoustic impedance, much of the energy passes through, but some is reflected back towards the probe.

  • Specular Reflection: Occurs at broad, smooth tissue interfaces. The interface acts like a mirror, reflecting energy back to the probe. The angle of reflection is influenced by the angle of incidence (the angle at which the sound wave strikes the interface). For maximum energy return, the reflective structure should be perfectly perpendicular to the ultrasound beam (e.g., pericardium posterior to the left ventricle in a parasternal long axis view). If not perpendicular, most reflected energy will go away from the probe.

  • Scattering: A form of reflection that occurs when the ultrasound beam strikes a structure smaller than the wavelength of the transmitted beam. When scattering happens, energy is reflected in every direction, regardless of the angle of incidence. Far less energy is reflected compared to specular reflection, and the amount returning to the probe is significantly less. Scattering is believed to be responsible for the speckle texture seen within the myocardium on ultrasound.

• Attenuation: This is the global term for the total amount of energy lost as a wave travels through the patient.

  • Mechanisms of Loss: The majority of lost energy is converted to heat through absorption. Some energy is reflected away from the probe, and a very small amount is reflected back.

  • Frequency and Wavelength Relationship: Higher frequency waves (and therefore smaller wavelengths) suffer attenuation at a faster rate.

  • Resolution vs. Penetration: Using a higher transmitted frequency (short wavelength) provides superior resolution, but at the cost of less penetration because attenuation occurs faster. For deep structures (e.g., 20 cm), using a 10 MHz frequency would result in so much attenuation that a usable image could not be produced. 2D images typically use frequencies between 4 and 7 MHz to find an optimal trade-off between resolution and penetration.

Generating an Ultrasound Image: 2D (B-mode) and M-mode

The return of the wave to the probe allows for ultrasound image generation because the piezoelectric effect works in reverse: applying kinetic energy to the crystals generates a charge that travels back to the ultrasound machine and is interpreted by the software.

• Information Recorded: The ultrasound software records two main pieces of information:

  • The amplitude of the returning wave (how much energy returns).

  • The amount of time it took for the energy to return following the transmission of the original wave.

• Data Conversion:

  • The amplitude of the returning wave is converted to a brightness value on a grayscale.

  • By knowing the time taken for the energy to return, the software can estimate the distance of the reflective surface from the probe. For example, if a wave travels 10 cm to a reflective surface and 10 cm back, the total distance is 20 cm. This takes about 65 microseconds in and 65 microseconds back, so the software estimates the structure is 10 cm away.

• From 1D to 2D Image:

  • The information derived from amplitude and time provides a one-dimensional line of information.

  • To generate a 2D image, multiple one-dimensional lines of information (scan lines) are created that fan out from the probe. These scan lines are closer together in the near field and spaced further apart in the far field.

  • The ultrasound software then uses an averaging process to fill in the gaps between the lines. A greater number of lines (higher line density) increases resolution by reducing the need for averaging.

• Frame Rate (Temporal Resolution):

  • The time to generate a single 2D image depends on the chosen sector size and the number of lines used.

  • For example, if a depth of 20 cm requires 260 microseconds per line (130 µs in, 130 µs back) and an image is built from 128 lines, it takes over 33 milliseconds per frame. This allows for approximately 30 frames per second, resulting in a frame rate (or temporal resolution) of 30 Hertz.

  • The system sweeps lines across the sector, averages gaps, creates a 2D image, and then repeats the process to create the next frame, producing a moving image.

• Compromises in 2D Imaging: Due to the relatively slow speed of sound waves, it's impossible to have a large sector with a high frame rate and high line density simultaneously. Sonographers must constantly compromise between sector size, frame rate, and resolution to achieve the desired image. Higher-end systems may allow prioritizing frame rate or resolution.

• A-mode, or amplitude mode, is the simplest form of ultrasound imaging. It displays the amplitude of returning ultrasound echoes as a function of depth along a single line, essentially showing the strength of the reflected signal at different points along a scan line. 

Ultrasound Machine Settings for Optimization

A level-one sonographer needs to master several settings to produce optimized images:

• Harmonic imaging in ultrasound is a technique that enhances image quality by utilizing sound waves produced within the tissue itself, rather than relying solely on the initial transmitted wave. This method improves image clarity, reduces artifacts, and can be particularly helpful in patients who are difficult to image with conventional ultrasound techniques. Benefits include imaging deeper structures and getting higher resolution from deeper or central structures. ◦ Drawbacks involve requiring higher power output and potentially changing the myocardial structure, making valves and other structures appear thicker than reality.

• Sector Size (Depth and Angle):

  • It's crucial to set the depth and sector angle to include all structures of interest without imaging unnecessary areas.

  • Imaging a larger area than necessary wastes time sending waves to parts of the body that don't provide useful information, potentially decreasing frame rate and lateral resolution.

  • Conversely, a sector that is too small might miss important details. The goal is to balance showing all areas of interest without wasting space or time.

• Line Density and Frame Rate Trade-off:

  • You often have to choose between higher line density (better lateral resolution) and higher frame rate (better temporal resolution).

  • Increasing line density means fewer images can be produced per second, reducing the frame rate. The decision depends on what needs to be demonstrated.

• Transmitted Wave Frequency:

  • Some ultrasound systems allow adjusting the frequency of the transmitted wave.

  • Low frequency waves favour penetration (useful for deep structures) but have lower axial resolution (due to longer wavelength).

  • High frequency waves favour resolution (especially in the near field) but suffer from greater attenuation and less penetration. The choice depends on the specific structures being imaged.

• Focusing:

  • The ultrasound beam initially behaves like a column, but then starts to diverge, leading to a greater area of disruption in the far field.

  • Focusing aims to concentrate the ultrasound beams at a specific depth to improve resolution. This can be achieved by introducing concavity to the piezoelectric crystal grid or by using an acoustic lens.

  • Adjusting the focus point moves the area of maximum overlap (greatest resolution) closer or further from the transducer.

  • If there's a particular structure or area of interest, the focus point should be adjusted to converge around that area. For a balanced image without a specific point of interest, the focus point should be placed around the midway point of the depth scale.

  • The non-focused ultrasound beam's initial cylindrical path is called the near field or Fresnel zone, and its diverging part is called the far field or Fraunhofer zone. The length of the near field can only be extended by using higher frequencies or wider transducers. ◦ While focusing improves resolution in the focused area, it is at the expense of more divergence in the far field.

• Gain:

  • Gain refers to the degree of amplification of the returning signal.

  • The purpose of adjusting gain is to produce a balanced image that utilizes the entire grayscale.

  • Over-gaining (too much amplification) makes the image too bright, causing details to be "blown out".

  • Under-gaining makes the image too dark, causing details to be lost in shadows.

  • An ideally balanced image should show simple fluids (like blood) as black, highly reflective structures (like metal and dense calcium) as bright white, and all other structures as shades of grey.

• Time Gain Compensation (TGC):

  • Due to attenuation, the energy reaching the far field is significantly less than that in the near field. The returning signal from the far field needs greater amplification to produce an even image.

  • TGC is the process by which the ultrasound software adjusts the degree of amplification based on how late the signal returns.

  • If the automatic balancing isn't satisfactory (e.g., near-field too dark, far-field too bright), manual TGC controls (sliders) can be used to increase or decrease the brightness of specific regions within the image.

Ultrasound Artifacts

Artifacts occur when the ultrasound software incorrectly displays something as present when it isn't, fails to demonstrate a structure when it is present, or demonstrates a real structure in the wrong location. Recognizing common artifact patterns helps reduce the risk of misdiagnosis.

• Acoustic Shadowing:

  • Cause: Occurs when trying to image through a structure that is so reflective that none of the ultrasound energy passes through it, and all is reflected back to the probe.

  • Appearance: An area of shadow or "drop out" beyond a very bright structure, where nothing can be seen behind it.

  • Examples: Imaging through a rib, or when a patient has prosthetic material like a metallic heart valve.

• Refraction:

  • Cause: The ultrasound beam deviates from its normal path as it passes through tissues, bending similar to light through water.

  • Appearance: If the bent beam strikes a structure and reflects energy back, the software incorrectly assumes the reflective surface was along the original, straight path of the beam. This can lead to a fainter, "ghost" version of a true structure.

  • Common Example: A ghost image of the aortic valve annulus in a parasternal short axis view.

• Range Ambiguity (Mirror Image Artifact):

  • Cause: This occurs when ultrasound energy passes through a reflective surface, reflects off a second surface, returns to hit the underside of the first surface, reflects back down to the second, and then finally returns to the probe. The ultrasound software measures the total travel distance and displays a structure at that calculated distance, effectively showing a mirror image behind a highly reflective surface.

  • Appearance: A mirror image of a structure appearing further away from the probe.

  • Common Examples: Large specular reflections off the pericardium in the parasternal long axis view or the diaphragm when imaging the lung bases. For instance, a reflection of the inferolateral wall of the left ventricle appearing as a "second effusion" that is actually a reflection of a pericardial effusion.

M-mode (Motion Mode)

In addition to 2D imaging (B-mode), M-mode is another important imaging mode.

• Process: M-mode involves choosing a single line on the 2D sector and sampling that same line repeatedly (up to about 2,000 times per second).

• M-mode can have a high pulse repetition frequency of 1800 times per second, offering high temporal resolution, whereas 2D imaging typically achieves 20 to 30 frames per second.

• Display: This information is used to produce a graph where time is on the x-axis and depth is displayed on the y-axis.

  • The near-field is displayed at the top of the image, and the far-field at the bottom.

  • This allows visualization of motion along that single line over time, such as the movement of the interventricular septum and walls of the heart through the cardiac cycle.

Conclusion

The piezoelectric effect is central to ultrasound, converting electrical energy into kinetic energy for transmission and returning kinetic energy back into electrical signals for detection. The ultrasound beam behaves as a wave with key properties: velocity, frequency, and wavelength, which are intrinsically linked. While much of the transmitted energy is lost, the returning energy's amplitude and return time allow for image construction. 2D images are built by sweeping multiple lines of information and compiling them into moving pictures. Sonographers must constantly make compromises regarding sector size, frame rate, and resolution based on the diagnostic goal. Understanding how ultrasound physics leads to artifacts is crucial for recognizing them and reducing the risk of incorrect diagnoses.