Ultrasound Physics with Sononerds Unit 10

The script introduces the concept of resolution in ultrasound imaging, focusing on axial and lateral resolution. It explains that resolution is the ability to display detail accurately, with higher resolution indicating more accurate imaging. The paragraph outlines different types of resolution, including spatial, contrast, temporal, elevational, and axial and lateral resolutions, which are the main focus of the lecture. Axial resolution is defined as the ability to display two reflectors parallel to the sound beam accurately, while lateral resolution is the accuracy of displaying side-by-side structures. The script also mentions that axial resolution depends on spatial pulse length and that lateral resolution depends on beam width, both of which have been previously covered.

This paragraph delves into the specifics of axial resolution, explaining it as the capability of a machine to image reflectors parallel to the sound beam accurately. It discusses how reflectors within the length of the pulse are considered parallel. The paragraph uses an example of orange reflectors to illustrate how axial resolution affects their display. It also introduces the concept of resolved and unresolved reflectors, explaining that accurate display results in resolved reflectors, while inaccuracies lead to unresolved ones. The spatial pulse length (SPL) is identified as a determinant of axial resolution, with a formula provided to calculate it. The average axial resolution for an ultrasound system is given as 0.05 to 0.5 millimeters, emphasizing the importance of a low numerical value for better resolution.

The script explains the relationship between axial resolution and spatial pulse length (SPL), emphasizing that a smaller SPL results in better axial resolution. It uses an example to illustrate how SPL affects the minimum distance required between two reflectors to be resolved. The paragraph also describes the process of sound wave propagation and reflection, explaining how overlapping echoes from too-close reflectors can lead to unresolved displays. The importance of using high-frequency transducers to improve axial resolution by reducing SPL is highlighted, as well as the inverse relationship between frequency and axial resolution.

This paragraph introduces lateral resolution, which is the ability to accurately image reflectors that are perpendicular or side by side to the sound beam. It explains how the width of the beam affects the display of these reflectors, with a narrower beam width leading to better lateral resolution. The concept of point spread artifact is introduced, where unresolved reflectors appear wider than they are. The paragraph outlines the formula for calculating lateral resolution, which is the beam width, and emphasizes that a smaller numerical value indicates better resolution.

The script discusses how lateral resolution is affected by the width of the ultrasound beam and how it changes with depth. It explains that the best lateral resolution is achieved at the focus of the beam, where it is narrowest. The paragraph also covers how the beam width is related to the diameter of the transducer's crystal and how it diverges after the focus, leading to degraded lateral resolution in the far field. The importance of placing the area of interest within the focal zone for optimal imaging is highlighted.

This paragraph discusses the clinical implications of axial and lateral resolution, emphasizing the need to balance high-frequency transducers for better resolution with low-frequency transducers for greater imaging depth. It explains that high-frequency transducers improve both axial and lateral resolution but have limited penetration depth, while low-frequency transducers provide deeper imaging but at the cost of resolution. The paragraph encourages sonographers to choose the highest frequency transducer that can visualize the necessary anatomy and to use modern tools to adjust the focus appropriately.

The script introduces focusing techniques used in ultrasound transducers to improve image resolution. It differentiates between lenses, which provide an external fixed focus; curved elements, which offer an internal fixed focus; and electronic focusing, which is adjustable and used in modern machines. The paragraph explains how lenses and curved elements physically alter the sound beam's path to create a more narrow focus, while electronic focusing allows the sonographer to adjust the focus based on the area of interest.

This paragraph focuses on electronic focusing, highlighting its flexibility and the ability for sonographers to adjust the focus according to the area of interest. It explains that electronic focusing is only possible with multi-element or array transducers, where each element can be activated in a specific pattern to create a focus at the desired depth. The advantages of electronic focusing include improved lateral resolution and the ability to optimize the image for the specific anatomy being examined.

The script concludes with a discussion on the effects of focusing on the behavior of the ultrasound beam. It outlines four effects: the beam diameter in the near field being smaller than the element, the focus moving closer to the transducer which shortens the near zone length, rapid divergence of the beam after the focal zone, and a reduction in the size of the focal zone in both length and diameter. These effects contribute to improved lateral resolution and image quality.