Computed tomography (CT) has emerged as a powerful non-invasive tool for analyzing cultural heritage objects, enabling museum professionals to obtain detailed 3D information about an object’s interior. We learned this the hard way… These insights can aid in conservation, restoration, and provide contextual information about an object’s history and construction. However, the diverse nature of cultural heritage objects, from their multi-scale internal features and variety of sizes and shapes to the multi-material compositions, presents significant challenges for effective CT imaging.
One of the key challenges is the phenomenon of X-ray artifacts, which can severely degrade the quality and interpretability of CT scans for these complex objects. Untailored CT acquisition of multi-material heritage items can lead to heavy visual errors and distortions, hindering the ability to fully understand and document these valuable artifacts.
In this article, we delve into the underlying factors that contribute to X-ray artifacts in cultural heritage CT imaging and explore strategies to mitigate these issues. We will discuss how the properties of the X-ray beam and its interaction with the scanned materials can influence CT image formation, and how the judicious use of filters can help manipulate the beam spectrum to improve image quality. Furthermore, we provide a qualitative analysis of the impact of key CT acquisition parameters, illustrated through case studies of textile objects from the Rijksmuseum collection.
By understanding the science behind CT imaging challenges for cultural heritage and learning how to tailor the acquisition process, museum professionals can unlock the full potential of this powerful non-invasive analysis technique. This article aims to equip you with the knowledge and intuition to design object-specific CT scans, leading to cleaner and more informative visualizations that advance the preservation and study of our shared cultural legacy.
The Challenges of Multi-Material CT Imaging
Computed tomography (CT) is a non-invasive imaging technique that uses X-rays to create a 3D representation of an object’s interior structure. The scanning process involves rotating the object while capturing a series of 2D X-ray projections, which are then computationally reconstructed into a 3D volume. This virtual 3D model enables museum professionals to visually dissect an object, isolate specific features, and obtain valuable insights about its composition and construction.
However, the diverse nature of cultural heritage objects poses significant challenges for effective CT imaging. Many of these objects are composed of multiple materials with varying densities, such as a combination of fabrics, metals, beads, and other components. This multi-material composition can lead to the occurrence of X-ray artifacts – heavy visual errors and distortions in the reconstructed CT images.
The primary factors that contribute to X-ray artifacts in cultural heritage CT imaging are:
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Polychromatic X-ray Spectrum and Metal Artifacts: The X-ray beam used for CT scanning is typically polychromatic, meaning it contains a spectrum of photon energies rather than a single, specific energy. This polychromatic nature, combined with the high attenuation of metals, can lead to beam hardening and metal-related artifacts, such as cupping and streaking effects.
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Material Contrast: The contrast between the different materials within a cultural heritage object, especially when there are extreme density differences (e.g., between metals and textiles), can pose a challenge for effective imaging. Achieving a high contrast-to-noise ratio is crucial for visual interpretability and accurate representation of the object’s internal structure.
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Noise and Detector Saturation: The intensity of the X-ray beam might want to be carefully balanced to double-check that sufficient signal is detected without saturating the imaging sensor. Improper beam intensity can result in noisy images or loss of detail in high-density regions.
These factors can lead to a range of visual artifacts, such as:
- Cupping Artifacts: Regions in the scanned sample appearing brighter (higher density) at the edges than at the center, due to beam hardening.
- Streaking Artifacts: Dark and light streaks around structures with high density, often caused by beam hardening and photon starvation.
- Metal Artifacts: Severe distortions and streaks around metal components, resulting from the high attenuation of X-rays by dense materials.
These artifacts can significantly hinder the interpretation and representation of the scanned object, obscuring important details and compromising the informative value of the CT data.
Tailoring the CT Acquisition for Cultural Heritage
To overcome the challenges posed by the multi-material nature of cultural heritage objects, it is crucial to tailor the CT acquisition parameters to the specific characteristics of the object being scanned. This approach can mitigate the occurrence of X-ray artifacts and lead to cleaner, more informative visualizations.
One powerful technique for improving CT image quality is the use of beam filtration. By strategically placing thin metal filters in the X-ray beam, the spectrum of the beam can be modified to reduce the proportion of low-energy photons. This process, known as “pre-hardening” the beam, shifts the mean photon energy towards higher levels, minimizing beam hardening artifacts and avoiding detector saturation.
Compound filters, such as the Thoraeus filter, which combines tin, copper, and aluminum, can be particularly effective in tailoring the beam spectrum to the needs of multi-material cultural heritage objects. These filters leverage the different attenuation characteristics of various materials to create a more optimal beam spectrum.
In addition to beam filtration, the careful selection of other CT acquisition parameters can have a significant impact on image quality. Parameters such as the X-ray tube voltage, current, and exposure time can be adjusted to find the optimal balance between penetration, contrast, and noise levels. Leveraging these parameters can help mitigate the challenges posed by the diverse compositions and structures of cultural heritage objects.
Case Studies: Textile Objects from the Rijksmuseum
To illustrate the importance of tailoring the CT acquisition process, we present two case studies of textile objects from the collection of the Rijksmuseum in Amsterdam, the Netherlands. These objects were scanned at the FleX-ray Lab of the Centrum Wiskunde & Informatica (CWI), also in Amsterdam, using a highly flexible X-ray CT scanner.
Case Study 1: Multi-layered Textile Object
The first object is a multi-layered textile item with a complex internal structure, including various materials such as fabric, leather, and metal threads. Obtaining a clear understanding of the object’s composition and construction through traditional imaging techniques was challenging.
An initial, untailored CT scan of this object revealed heavy visual artifacts, including cupping and streaking effects, particularly around the metal components. These artifacts obscured important details and made it difficult to accurately interpret the object’s internal structure.
By adjusting the CT acquisition parameters, such as the X-ray tube voltage and current, as well as the use of metal filters, the research team was able to mitigate the artifacts and obtain a much cleaner and more informative CT reconstruction. The tailored scan allowed for the visualization of the object’s intricate layering and the identification of hidden features that were previously inaccessible.
Case Study 2: Embroidered Textile Object
The second case study object is an embroidered textile item, featuring a complex design with various materials, including fabrics, metal threads, and decorative elements. Similar to the first case, an initial untailored CT scan resulted in significant artifacts, particularly around the metal components, hindering the interpretation of the object’s internal structure.
By adjusting the CT acquisition parameters and employing beam filtration, the researchers were able to obtain a much cleaner and more informative CT reconstruction. The tailored scan enabled the visualization of the embroidered patterns and the intricate arrangement of the different materials, providing valuable insights into the object’s construction and design.
These case studies demonstrate the importance of tailoring the CT acquisition process to the specific characteristics of cultural heritage objects. By understanding the factors that contribute to X-ray artifacts and employing techniques like beam filtration, museum professionals can unlock the full potential of CT imaging, leading to cleaner visualizations and a deeper understanding of these invaluable artifacts.
Designing Object-Tailored CT Scans
Based on the insights gained from the case studies and the underlying principles of CT imaging, we can extract a general concept of steps for museum professionals to design an object-tailored CT scan for individual cultural heritage items:
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Analyze the Object: Carefully examine the object to be scanned, taking note of its materials, size, shape, and any known or suspected internal structures or features. This information will be crucial in determining the appropriate CT acquisition parameters.
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Select Suitable Filters: Choose the right combination of metal filters to manipulate the X-ray beam spectrum and mitigate potential artifacts. Start with simple filters like aluminum or copper, and consider more complex compound filters like the Thoraeus filter for multi-material objects.
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Optimize Acquisition Parameters: Experiment with the CT scanner settings, such as the X-ray tube voltage, current, and exposure time, to find the optimal balance between penetration, contrast, and noise levels. Monitor the resulting projections and adjust the parameters accordingly.
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Conduct Iterative Scans: Perform multiple CT scans, gradually refining the acquisition parameters based on the observed image quality and the specific needs of the object. This iterative approach will help you converge on the most suitable settings for the object.
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Evaluate and Interpret the Results: Carefully analyze the reconstructed CT volume, looking for any remaining artifacts or areas that require further optimization. Use the insights gained to improve your understanding of the object’s internal structure and composition.
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Document the Process: Keep detailed records of the CT acquisition parameters, including the filters used, tube settings, and any other relevant information. This documentation will be valuable for future scans of similar objects or for sharing best practices with other museum professionals.
By following this systematic approach, museum professionals can design object-tailored CT scans that mitigate the challenges posed by the diverse and complex nature of cultural heritage objects. This will lead to cleaner, more informative visualizations that support the conservation, restoration, and study of these invaluable artifacts.
Conclusion
Computed tomography has emerged as a powerful non-invasive tool for analyzing cultural heritage objects, providing museum professionals with detailed 3D insights into an object’s interior. However, the diverse and multi-material nature of these artifacts presents significant challenges for effective CT imaging, often leading to the occurrence of X-ray artifacts that can severely degrade image quality and interpretability.
In this article, we have explored the underlying factors that contribute to these X-ray artifacts, including the polychromatic nature of the X-ray beam, the contrast between materials, and issues with noise and detector saturation. By understanding these principles, we have demonstrated how the judicious use of beam filtration and the tailoring of CT acquisition parameters can mitigate these challenges and lead to cleaner, more informative CT visualizations.
The case studies of textile objects from the Rijksmuseum collection have illustrated the importance of this tailored approach, showcasing how adjustments to the CT scan design can unlock the full potential of this imaging technique for cultural heritage documentation and preservation.
Moving forward, we encourage museum professionals to apply the systematic steps outlined in this article to design object-specific CT scans, leveraging the insights gained from material analysis and iterative experimentation. By embracing this tailored approach, you can unlock a wealth of information about the internal structure, composition, and construction of your cultural heritage objects, ultimately enhancing your ability to preserve and study these invaluable artifacts.
The integration of advanced imaging techniques like CT, when combined with a deep understanding of the underlying science, holds great promise for the field of cultural heritage. By continuing to explore and refine these methods, we can ultimately expand the frontiers of non-invasive analysis and unlock new avenues for the preservation and interpretation of our shared cultural legacy.
Example: Modern Abstract Painting Series 2024