Ultrasound Basics: Trapezoidal Expansion and Panoramic Imaging of Images

   Linear array scanning, due to its superior spatial resolution, especially the consistent spatial resolution of the near and far fields, remains a primary scanning method, particularly widely used in the scanning of superficial lesions. However, the scanning range of linear array probes is limited, and larger lesions cannot be fully displayed. To address this issue, the vast majority of ultrasound devices are equipped with trapezoidal expansion and panoramic imaging functions, with the panoramic imaging function also applicable to other convex and phased array probes.

1.  Trapezoidal Expansion

   Image trapezoidal expansion involves gradually delaying the excitation (triggering) time of the transducer's crystals near both ends. According to Huygens' principle, the wavefronts formed by the sound waves emitted from these crystals will tilt, resulting in a certain degree of sound beam inclination, thus achieving the purpose of expanding the scanning range (Figure 1). Because the completed image overall appears trapezoidal, it is called "trapezoidal expansion" (Figure 2).

Figure 1 Schematic diagram of image trapezoidal expansion

Figure 2 Trapezoidal expansion images expand the field of view to a certain extent

     2. Panoramic Imaging

     Although image trapezoidal expansion can expand the field of view to some extent, it is still ineffective for larger lesions. Panoramic imaging further addresses the problem of limited ultrasound probe field of view to a certain extent.

   Conventional ultrasound scanning has a limited field of view. When encountering large lesions, the limited field of view prevents examiners from fully understanding the lesion's extent and its relationship with adjacent structures, thus affecting accurate lesion analysis.

   Ultrasound panoramic (wide-field) imaging technology, also known as wide-field-of-view imaging technology, can greatly compensate for this deficiency in conventional ultrasound examination. Ultrasound panoramic (wide-field) imaging technology is an imaging technique that does not require adding or changing the instrument's hardware system; it is realized solely through computer software technology. Ultrasound panoramic (wide-field) imaging technology is essentially an image analysis and fusion technology. It is achieved by analyzing and comparing the differences between adjacent images obtained during the steady movement of the probe, merging the same parts of the images, and expanding the different parts (Figure 3).

     Ideal ultrasound panoramic (wide-field) imaging software can calculate the speed, direction, and orientation of the probe based on the difference between adjacent images, making the final panoramic image more realistic and undistorted.

Figure 3 Panoramic imaging reconstructs a panoramic image by steadily moving the probe and using the software to analyze the image differences during the probe movement.

     Typical Case: Female, 53 years old, with a left thigh mass for over 2 years, hard, ill-defined borders, no skin redness, ulceration, or tenderness. (Final Pathology: Myxoid liposarcoma.)

Figure 4 Conventional scanning has a limited range and cannot fully display the relationship between the lesion and surrounding tissues

Figure 5 Using the trapezoidal expansion mode expands the scanning range, seemingly displaying most of the lesion, but this is actually only a small part of the lesion that appears hypoechoic.

Figure 6 Panoramic imaging obtains a complete view of the lesion, clearly displaying the lesion's layers, location, relationship with surrounding tissues, and internal echo diversity.

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