Wide FOV metalens for endoscopy: advancing medical imaging

Figure 1: (a) Front and (b) top view of the nano-optic capsule endoscopy system. (c) Ray tracing of different FOVs through the capsule endoscopy camera.

1. Introduction

Gastrointestinal (GI) diseases, particularly cancer, are on the rise globally. However, the methods for prevention, diagnosis, and treatment of these conditions have seen limited progress. Effective early detection of GI diseases could save millions of lives. Comprehensive information about the GI tract is essential for accurate diagnosis of these disorders.

With advancements in machine learning in medical image processing, traditional imaging methods used in endoscopic devices are proving inadequate. There is a growing need for high-resolution images, enhanced contrast, a wider field of view, and compact device sizes. Unfortunately, conventional endoscopic lenses face significant limitations, including restricted field of view, subpar image quality, short depth of field, and bulkiness due to the number of lens elements. Traditional lenses rely on refraction to focus light, requiring a curved surface to bend the light rays.

Capsule endoscopy offers a non-invasive solution for GI imaging. However, advancements in this technology demand the development of more compact devices with increased FOV, improved image quality, and greater depth of field. This would reduce examination time, enhance assessment accuracy, and minimize patient discomfort. Traditional wide-FOV lens designs incorporating multiple lenses pose challenges due to the size and weight constraints of the capsule.

To address these problems, a research team led by Professor Bin HU at the Beijing Institute of Technology has recently published a new article in Nanophotonics. They have designed and fabricated a "Nano-optic capsule endoscope" featuring a wide-FOV metalens (see Figure 1). Metalenses, as novel flat optical elements, represent a significant technological advancement over traditional lenses. They offer improved performance, reduced size and weight, and greater flexibility in design and fabrication. Metalenses utilize phase manipulation via nanostructured metasurfaces to achieve the desired focusing effect, allowing them to be fabricated with a significantly thinner profile compared to their conventional lens counterparts (see Figure 2). Despite their thin profile, metalenses can still deliver comparable or even superior optical performance in terms of focusing, aberration correction, and other key parameters.

Figure 2: (a) Transmission and (b) phase diagrams versus different heights and radii at a wavelength of 𝝀=940 nm. (c) The unit cell structure of the metalens. The imparted phase by a nano-pillar is managed by its diameter (D=100-324 nm). (d) Transmission and phase diagram versus radius at 490 nm nano-pillar height. (e) Fabricated metalens and SEM images.

2. Research Backgrounds

In the field of imaging, wide FOV metalenses can be used in various imaging applications, including surveillance, medical imaging, and virtual and augmented reality. The application of wide FOV metalenses in medical imaging, particularly in capsule endoscopy, remains largely unexplored. Endoscopy is a crucial tool in modern medical imaging, allowing for minimally invasive inspections, early disease detection, precise guidance during therapeutic procedures, and versatile use across different parts of the body. As technology advances, endoscopy is expected to play an increasingly significant role in enhancing patient care and outcomes. Incorporating metalenses into optical imaging endoscopes provides substantial benefits, including enhanced miniaturization, improved performance, greater simplicity, increased flexibility, and reduced costs. These benefits improve diagnostic capabilities, enhanced patient care, and more efficient medical imaging procedures. Capsule endoscopes offer several advantages over fiber endoscopes, including noninvasiveness, no sedation required, wider imaging range, improved patient comfort, increased versatility, reduced complications, increased accessibility, improved diagnostic accuracy, cost-effectiveness, and future advancement. However, the effectiveness of capsule endoscopes depends on the quality of imaging, which is often constrained by the size limitations of traditional optical components. The advent of metalenses presents a groundbreaking solution to this challenge. Metalenses offer high-resolution imaging in a compact form factor and can be seamlessly integrated with other optical devices. This promises improved diagnostic accuracy and enables multifunctional capabilities in next-generation medical imaging devices. The narrow-band imaging (NBI) is utilized because it offers high-performance imaging due to lacking the chromatic aberration in metalens. NBI has been applied in endoscopy systems for decades and can be widely used in the early detection of various diseases related to mucosal lesions, such as those in the upper and lower gastrointestinal tracts. The advantages of NBI include being minimally invasive and intuitive, allowing for clear visualization of the surface capillary vessels. This enables us to observe endoscopic features, classify different organs, and enhance the diagnostic precision of tumors. Some studies have reported that NBI technology had a higher adenoma detection rate than white light endoscopy (WLE). Furthermore, magnifying endoscopy with NBI proves effective in distinguishing intramucosal carcinoma from adenoma, thereby reducing cancer underdiagnosis. The detection rate for superficial esophageal squamous cell carcinoma, including high-grade intraepithelial neoplasia, is notably higher with NBI than with WLE.

Figure 3: (a) The PSF characterization setup. (b) The captured focal spots at different AOIs, with a scale bar of 3 μm. (c) Sagittal and (d) tangential MTF diagrams for the 12 nm bandwidth.

Figure 4: Actual test of the capsule endoscope. (a) Close-up view of the experimental setup. (b) A curved protractor labeled "Beijing Institute of Technology". (c) The real image was captured using the capsule endoscope.

3. Innovation

The development of a wide field-of-view (FOV) metalens specifically optimized for near-infrared (NIR) capsule endoscopy marks a significant advancement in optical technology. Unlike traditional lenses that rely on refraction to focus light, this research utilizes advanced phase manipulation through subwavelength nanostructures. This innovative approach allows for a compact design without compromising optical performance, setting the research apart in the field.

Figure 5: USAF 1951 resolution test chart. (a) Optical setup utilizing a 20× microscope and (b) its corresponding image test result. (c) Experimental image test setup for the capsule endoscope and (d) the resultant image.

Achieving an impressive 165° FOV with a high modulation transfer function (MTF) of 300 lp/mm (figure 3) distinguishes the metalens from conventional endoscopes, which typically offer only 108°. Figure 4 demonstrates the actual imaging result captured through the nano-optic capsule endoscope. The text "Beijing Institute of Technology" is visible, showing how the metalens focuses and projects a wide FOV object. The clarity and resolution of the text provide insights into the performance and quality of the wide FOV metalens. Figure 5 illustrates the experimental setup and imaging resolution results using a USAF resolution test chart. Two configurations were employed in the study. Initially, a 20× microscope was utilized to capture the image. This configuration facilitated easy fine alignment. As depicted in Figure 5(b), the resolution achieved corresponds to group 7, element 2, which translates to 143.7 lp/mm, indicating that a linewidth of 3.48 μm can be resolved. Figures 6(a, b) present the narrow band images captured by the capsule endoscope device at object distances of 30 and 15 mm, respectively. The central focus of the image is on the teeth and oral cavity, which are illuminated and captured with high contrast and clarity. This enhanced imaging capability facilitates more comprehensive diagnostics, particularly in minimally invasive procedures. By incorporating narrow-band imaging (NBI) at a wavelength of 940 nm, tissue penetration and contrast improved, allowing for superior visualization of mucosal lesions. This innovation not only increases diagnostic accuracy but also aligns with the growing trend toward non-invasive medical imaging techniques.

The compact design of the metalens, measuring just 1.4 mm in length and 1.58 mm in diameter, exemplifies a breakthrough in miniaturization suitable for capsule endoscopy applications. This reduced size enhances patient comfort and decreases the risk of complications associated with traditional endoscopic procedures. Table 1 shows the performance of nano optic capsule endoscope over traditional counterparts.

Table 1: A comparison of commercial capsule endoscopes with nano-optic capsule endoscope.

Figure 6: Images of teeth captured by nano-optic capsule endoscope at object distances of (a) 30 mm and (b) 15 mm.

4. Application and prospects

The presented research on the wide FOV metalens for near-infrared capsule endoscopy demonstrates the potential of metalens technology to transform medical imaging. By overcoming the limitations of traditional optical components, this innovation can lead to significant improvements in diagnostic capabilities, patient care, and the overall efficiency of endoscopic procedures. Ongoing optimization efforts aim to further improve imaging quality and explore additional wavebands for diverse medical applications. The nano-optic capsule endoscopy system achieves superior performance in compact form factors, offering substantial potential for future medical devices. This method can be expanded to other biomedical imaging systems, miniaturized optical platforms, and non-invasive diagnostic tools. The advanced nano-optic technology promises significant applications in enhancing image clarity, multi-channel imaging, and enabling high-resolution data acquisition for capsule endoscopy, paving the way for broader use in fields such as non-invasive surgery, remote sensing, and endoscopic microscopy.

These research results are published online with the title “Wide FOV Metalens for Near-Infrared Capsule Endoscopy: Advancing Compact Medical Imaging” in Nanophotonics.

The authors of this article are Mojtaba Moghaddasi, Erik Edilson Perez Coca, Danni Ye, Diego Alejandro Flores, Xudong Wu, Abdul Jalal, Ziming Ren, Fahimeh Abrinaei, and Bin Hu.