A technique for decreasing reflection during cadaveric photography

To create anatomical educational materials that can be viewed in three dimensions using stereo photographs and photogrammetry, multiple photographs must be taken from different directions. In this process, shadows and reflections from different positions in each photograph are undesirable for creating three‐dimensional (3D) anatomy educational materials. Although a ring flash eliminates shadows, allowing light to enter from all directions, reflections cannot be eliminated. In particular, Thiel‐embalmed cadavers, which are widely used in clinical anatomy, are highly wet and exhibit strong specular highlights. In this study, a straight polarization filter was attached to a handheld camera lens and ring flash, and shooting was performed using cross‐polarization photography. Consequently, even in Thiel‐embalmed cadavers, the details lost due to the effects of reflections and shadows can be recovered, and good results can be obtained when taking stereo photos or creating a 3D model using photogrammetry.


| INTRODUCTION
As understanding the three-dimensional (3D) structure of the anatomy from two-dimensional images can be challenging, anatomical materials have been created using stereo photographs with left and right parallax images. In recent years, digital 3D models of anatomical specimens have been created using digital photogrammetry (Egels & Kasser, 2001), which is a technology for three-dimensional modeling space from multiple 2D photographs (Petriceks et al., 2018). These stereoscopic images are based on two-dimensional photographs taken from different directions. Although shadows and reflections may express a stereoscopic effect in ordinary photographs of people and landscapes, they are undesirable in the photographs used for 3D anatomy teaching materials. Therefore, a method is required to remove the effects of shadows and reflections as much as possible. Ring flashes are often used to eliminate shadows (Bengel, 1985;Smith, 2002;Thomas et al., 1980;Yavuzer et al., 2001). The ring flash illuminates the subject evenly and minimizes shadows as far as its light can reach (Fleming et al., 1989). However, while a ring flash can eliminate shadows, it cannot suppress reflections. The reflections are particularly strong when the anatomy is fixed and wet, as in Thiel's method (Thiel, 1992a(Thiel, , 1992b(Thiel, , 2002, which has been frequently used in clinical autopsies in recent years. A method called "crosspolarization photography" eliminates the effects of reflections. This method uses two linearly polarized filters placed in the light path. This technique is used in dentistry to eliminate unwanted reflections of teeth caused by flashes (Fleming et al., 1989;Kim et al., 2012;Villavicencio-Espinoza et al., 2018). In this study, a ring flash and a camera with a linear polarization filter attached to the camera lens were used to take hand-held photographs of highly reflective Thielembalmed cadavers to examine the usefulness of stereo photography and photogrammetry as a method for creating 3D models.

| Photographic equipment
The camera used was an E-M5II (Olympus, Tokyo, Japan) with an M. Zuiko Digital 12-40 mm lens (Olympus, Tokyo, Japan). In addition, a ring flash R200 (Godox, Shenzhen, China) was attached for shadowfree photography. A ring flash and a polarizing filter were attached to the front of the lens for non-reflective photography using the crosspolarization method (Figure 1). The polarization filter on the lens side can be rotated, and when it is rotated while checking the camera monitor with the R200 Modeling Lamp-which can always be turned onthere is a part where the reflection of the light source disappears completely. In this state, the polarization planes are rotated by 90 with respect to each other and cross each other (cross-polarization).

| Dissection
We used the remains of those who, before death, declared that they would be donated as educational and research material and provided their consent after learning how they would be used. The remains were fixed using Thiel's embalming method. The study was approved by the Institutional Board of Ethics (approval numbers: 20070026, 20210091). Efforts have been made to follow all local and international ethical guidelines and laws pertaining to the use of human cadaveric donors in anatomical research (Iwanaga et al., 2022). The palms, small intestines, axillae, and knees, whose three-dimensional construction and tissue surface texture differ, were selected for imaging.

| Stereophotography
When taking stereo photographs, the camera was held horizontally and two pictures were taken from the left and right positions. The amount of movement was approximately 1/30 of the distance between the camera and the subject; however, multiple shots were taken to select the appropriate image for the stereoscopic effect. To compare the images with and without cross-polarization, the camera was placed on a focusing rack CASTEL-Q (Novoflex, Memmingen, Germany) mounted on a tripod, and the images were captured at the same position and with the same eye width. The left and right images were combined into a stereoscopic pair using Stereo Photo Maker version 6.27 (https://stereo.jpn.org/eng/stphmkr/).

| Digital photogrammetry
The area to be documented using digital photogrammetry was photographed from various directions, with care taken to overlap each photograph as much as possible. Each photograph was taken with a handheld camera with and without cross-polarization at each position.
Each 3D object was created using 3DF Zephyr version 6.5 (3DFLOW, Verona, Italy), and meshes and textures were generated from a set of photographs taken at each shot number.

| Effects of shadowless and reflection-free photography
To confirm the effects of shadowless and reflection-free imaging with cross-polarized light, we selected a palm composed of similar tissues and showed its three-dimensional structures. When imaging with a ring flash alone, the image was shadowless; however, the reflection was strong, making it difficult to distinguish the flexor tendons, flexor retinaculum, and hypothenar muscles. When cross-polarized light was added, the reflections disappeared and the texture and color of the adipose tissue, flexor tendons, flexor retinaculum, and muscles were clearly defined, making them easy to distinguish. In addition, the effect of the reflection of the fascia of the hypothenar and thenar muscles disappeared, and the direction of the muscle fibers was confirmed ( Figure 2).

| Effects of stereoscopic viewing with shadowless and reflection-free photography
To confirm the stereoscopic effect of the stereo photographs, the rounded and tortuous small intestine and axilla with dissected nerves F I G U R E 1 Cross-polarized photography settings. Light emitted from the ring flash is polarized by the polarizing filter. The rays of light that are the main component of specular reflection are removed by the polarizing filter in front of the lens. and blood vessels were used as stereoscopic structures. The small intestine exposed inside the abdominal cavity was not shadowed by the ring flash, but strong reflections were scattered on its surface unless cross-polarized photography was used ( Figure 3A). The position of the reflection shifts in the left and right images during stereoscopic viewing, making it difficult to fuse binocular stereoscopic images. The cross-polarization method eliminates reflections, provides a clear stereoscopic view of the entire small intestine, and accurately reproduces the texture of the irregular surface of the small intestine. In the axilla, the ring flash produced no shadows but there were strong reflections of blood vessels, the back surface of the pectoralis minor muscle, and the surface of adipose tissue. Using the cross-polarization method, the blood vessels became recognizable because of the distinct coloration of the residual blood within them ( Figure 3B).

| Creation of 3D models by shadowless and non-reflective photography
To confirm the usefulness of cross-polarized images in 3D modeling, we selected axillary and knee joints, which are more threedimensional structures. Three-dimensional models are easy to view because contrast-free, non-reflective imaging reveals the texture and color of each tissue. When the texture was removed and the surface structure of the 3D model was checked, unevenness that would not be found in the original shape was observed; however, these were reduced in the model created with no reflection (Figure 4). The textures created also showed more detail than without reflection.

| DISCUSSION
The shadows and reflective sheen express three-dimensional effects in the 2D photographs. However, shadows and reflections hide structures and erase surface textures. They also obstruct the fusion of stereoscopic images when viewed binocularly if the positions of shadows and reflections change between the left and right images, owing to changes in the shooting direction. There are two types of shadows: photographers' shadows and shadows caused by objects being photographed. Although a photographer's shadow can be erased using a standard flash, an object's shadow may be created. Shadows are particularly noticeable on objects in deep spaces such as the abdominal, thoracic, pelvic, and oral cavities. To eliminate such shadows, a ring flash is often used.
In this study, linearly polarized filters are attached to the ring flash and camera lens to enable shadow-free, non-reflective photography.
Using cross-polarized photography, muscle fibers covered by fascia that strongly reflect light can be observed, and the texture of the muscle and its running direction can be well represented.
The Thiel's embalmed cadavers used in this study are expected to be used more frequently in clinical anatomy in the future because the embalming renders the tissue less stiff and more mobile; however, care must be taken when photographing because of the greater reflection than with formalin-fixed cadavers. However, the present study suggests that cross-polarized photography would be useful not only for 3D imaging, but also for 2D imaging of Thiel's embalmed cadavers.
The ring flash used was attached to the camera, causing the light source to move with each shot. When stereo images are captured under these conditions, not only does reflection interfere with the texture, but the position of the reflection also varies between the left and right images because of the movement of the light source, as shown in Figure 3, making binocular stereopsis difficult in those areas.
However, stereo images that are easier to view in binocular stereopsis have been successfully captured using cross-polarized photography, even under conditions where the light source moves.
When creating 3D models through photogrammetry, the model is created by automatically detecting common points from multiple images. This increases the possibility that the highlights will not be matched correctly, the point cloud will be anomalous, the model will not be created well, or even if it is made, the shape will not be created completely. We believe that reflection-free photography using cross F I G U R E 2 The results of photographing Thiel-embalmed cadavers using cross-polarized photography. (A) Image of the palm taken only with the ring flash without cross-polarized photography. (B) This image was taken with cross-polarized photography. The reflection was eliminated and the texture and color of the tendons and muscles became clearer. In addition, the reflection of the fascia of the muscles disappeared, and even the direction of muscle fibers could be confirmed. FR, flexor retinaculum; FT, flexor tendon; HTM, hypothenar muscles; TM, thenar muscle. polarization enables the discovery of a large number of common points with a small number of photos, leading to efficient modeling.
Nonetheless, the results of this study suggest that the same number of photographs containing reflections can be used to create a 3D model. This may be due to the high number of common points present in the areas without reflections in the photographs used in this study.
However, upon closer inspection of the surface structure of the 3D model with the textures removed, as shown in Figure 4, unevenness can be observed in the reflective areas that were not present in the original shape. This indicates that modeling in the reflective areas was not successful. This highlights the importance of non-reflective photography in 3D modeling through photogrammetry.
The present study suggests that even handheld photography can easily create medical educational materials with stereoscopic viewing without worrying about reflections by using cross-polarization. The fact that complex lighting settings are unnecessary and simple photography is possible is considered advantageous not only for anatomical images but also for creating stereoscopic photographs and photogrammetry content during surgery.
Gross dissection is the most important method used in clinical anatomy education. However, education conducted by performing gross anatomical examinations of a donated body is not available to everyone. Therefore, photographs of dissected gross specimens are important teaching materials in anatomy education. However, as most of these are two-dimensional photographs, beginner students find it difficult to understand their three-dimensional structures. This technology can thus help students, teachers, dentists, physicians, and other healthcare professionals create educational materials to understand three-dimensional anatomical structures more effectively.
F I G U R E 3 Effect of reflections on stereographs. These images are stereophotographs (cross-view: the right eye views the left image, and the left eye views the right image). (A) The small intestine exposed from inside the abdominal cavity. (B) The axilla with dissected nerves and blood vessels. *: Intercostobrachial nerve. **: Lateral thoracic artery and vein. ***: Pectoralis minor muscle. The upper part of each panel is without cross-polarization (reflection), and the lower part is with cross-polarization (reflection-free). The magnified areas are those where the reflections differ significantly. Reflections make the texture of the area uncertain. Furthermore, the positions of the reflections differ from left to right, making a binocular stereo view difficult.

ACKNOWLEDGMENTS
Dissection was performed in accordance with the Japanese guidelines for cadaver dissection in the education and research of clinical medicine. We thank Dr. Yoshiaki Kubota (Department of Anatomy, Keio University School of Medicine) for their contributions to the Clinical Anatomy Laboratory. This work was supported by JSPSKAKENHI (grant number 21K02837).