Figure 1: Micro-CT volumetric rendering of a strawberry seed imaged 

As we move into the height of summer, the season’s new crops begin to blossom, including a popular summer treat–the strawberry. Often known for its tart flavor and bright red hues, we often overlook all the little seeds present on the surface of the strawberry fruit. Researchers study the physical characteristics of seeds in order to learn more about seed development and morphology, which are critical to crop yield and quality. For our examination, the submicron imaging possible on the SkyScan 1272 desktop instrument allowed us to take a closer look at an aspect of the common strawberry and to appreciate its structural complexity.

X-Ray Microscopy Imaging of a Strawberry Seed

To really take a deep dive into the structures present in a strawberry seed, we utilized submicron imaging with the SkyScan 1272 desktop micro-CT at an isotropic voxel size of 800nm. For this study, the seed was imaged as removed from a fresh piece of fruit without any further pretreatment or processing. In setting up the scan to best capture the details of this small sample at high resolution, we reduced the total power used in the X-ray source from the maximum of 10W down to 4W by reducing the voltage and current applied to the X-ray tube. Reducing the X-ray power shrinks the spot size from which X-rays are emitted on the tungsten target and improves resolution. For most high-resolution imaging on the SkyScan 1272, you may benefit from reducing to either 4W or 6W depending on the density of your sample and the goals of the work. 

Figure 2: Textured 3D rendering of strawberry seed showing interior features

As shown in Figure 2, the SkyScan 1272 provides us with a detailed view of the different structural regions within the strawberry seed. We see the presence of some fine protrusions leaving the seed, a detailed surface structure, as well as the presence of a secondary seed region deep within the larger outer surface shell of the seed. 

Figure 3: Planar 2D views through the strawberry seed

In inspecting the results from the strawberry seed, we notice brightness at the edge of our sample which is not arising from an artifact such as beam hardening (Figure 3). Instead, this brightening is the result of edge scattering arising from refraction and diffraction of X-rays at internal boundaries of the sample. With the launch of NRecon 2.0, Bruker introduced the ability to take datasets with this type of edge scattering present and apply single distance phase retrieval using the Paganin algorithm. The SkyScan 1272 is a perfect match for this technology with its high-resolution imaging modes. The process of using Paganin Phase Retrieval to improve contrast in samples works best for low density objects containing air spaces such as polymers, plastics, carbon fiber, and non-mineralized biological tissues. However, any sample where edge scattering appears present in reconstructed results may be able to take advantage of this new feature within NRecon 2.0. 

Figure 4: Planar 2D views through the strawberry seed after Paganin Phase Retrieval

After applying the Paganin Phase Retrieval Algorithm to the seed dataset, the difference in contrast within the sample is readily evident (Figure 4). While the dataset is still the same original acquisition, the phase retrieval process helps create contrast between the low-density interior features of the seed while reducing the disparity in brightness between the edges of the sample and the middle. This improvement of the brightness in the interior of the sample aids us in visualizing small differences in morphology within the seed.  

Figure 5: Rendered 3D view slicing through half of the strawberry seed dataset after Panagin Phase Retrieval

Similarly, when rendering the dataset after phase retrieval in 3D we observe better contrast between the interior seed features and more uniform brightness throughout the sample (Figure 5). 

Conclusion

The SkyScan 1272 allowed us to compare and capture high resolution morphometric data of a strawberry seed at an isotropic voxel size of 800nm. Using the new NRecon 2.0 platform, we also applied a single-distance Panagin Phase Retrieval on the dataset which significantly improved contrast within the low-density regions of the sample and helped to homogenize the image intensity from surface through core. Hopefully this article leaves you with a better understanding of what edge scattering may look like in your high-resolution datasets and introduces you to the new single distance phase retrieval process present in NRecon 2.0. We hope you found this Image of the Month informative and encourage you to subscribe to our newsletter and social media channels in preparation for the continuation of our image of the month series next month. 

For more detailed information in applying the Paganin Phase Retrieval Algorithm to your datasets, please see Method Note #133 which is located within the Bruker Support Web Interface. 

Scan Specifications

These scans were completed on our desktop SkyScan 1272 system at the Micro Photonics Imaging Laboratory in Allentown, PA. Reconstructions were completed using NRecon 2.1 and visualization of 2D and 3D results were completed using DataViewer, CTVox, and CTAn. 

Would you like your work to be featured in our monthly newsletter? If so, please contact us by calling Seth Hogg at 610-366-7103 or e-mailing seth.hogg@microphotonics.com.