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An Experimental X-Ray Digital Detector for Investigation of Paintings M. Rossi, F. Casali, A. Bacchilega, D.Romani Physics Department, University of Bologna and I.N.F.N. Viale Berti Pichat 6/2, 40127 Bologna, Italy Contact

ABSTRACT

Radiological investigations constitute a fundamental tool for the knowledge of the inner structure of works of art.
In the study of paintings the X-ray inspection allows to get significant information about the technique or the presence of underneath drawings as well as to assess the status of the artifact and of its frame. Moreover, the restorer can individuate early degradation damages and adopt the appropriate procedures for maintenance and restoration. On the other hand, the radiographic analysis of large paintings can demand large efforts and can be time expensive due to long exposition time, to the arrangement and development of the films and to the visual analysis of the details remarked in the radiograph.
Digital imaging techniques are under study in order to perform these operations quickly providing numerical images which can be processed in real time, recorded on different media or sent on computer networks.
This work reports preliminary results obtained with an experimental X-ray digital detector developed at the Physics Department of the University of Bologna for the radiographic inspection of paintings. Images of a test-painting have been grabbed and processed in order to estimate the detector performance with regard to the ordinary film-based technique.

INTRODUCTION

The radiographic inspection of paintings is by now a usual procedure often fundamental for those works of art to be restored.
The detailed analysis of the radiographic film can provide a large number of information as the identification of specific materials, the assessment of the execution technique, the evidence of previous restorations as well as suggestions for a proper conservation. On the other hand, the ordinary radiographic film-based technique presents some drawback with regard to the execution time, the uncertainty of the development process, and the handling, comparison and conservation of the films.
Advanced digital imaging techniques are under study in order to overcome these limits [1]. A digital detector offers the opportunity to obtain in short time images which can be processed on a computer providing numerical computation of the features of interest. Moreover, images can be stored on different computer media (hard-disk, compact disk, etc.) or transferred on computer networks for remote analysis.

EXPERIMENTAL SET-UP

An experimental X-ray digital detector has been set up at the Physics Department of the University of Bologna for the radiographic inspection of paintings and it is now under test (Fig.1).
A 30x40 cm² scintillating screen is used in place of the ordinary film to convert X-rays into visible light. The light image corresponding to the radiographic shadow of the painting is grabbed by a digital sensor based on an advanced Electron-Bombarded Charge Coupled Device (EBCCD) camera developed in collaboration with Geosphaera Research Center of Moscow (Russia). This device provides a large intensification of the light image (gain up to 2000) allowing to reduce strongly the exposition time [2][3].
It must be remarked that radiographic analysis of paintings requires the use of very low X-ray energy in order to detect weak density variations of different pigments. This involves a long exposition time of the film and a solid experience of the restorer to reduce the number of attempts necessary to obtain a good quality of the image. For this purpose, the use of an intensified digital detector is essential to save time and allow the quick inspection of the artifact.
The EBCCD prototype under study can grab images of resolution up to 1024x512 pixels with an accuracy of 12 bits. Changing the distance between the EBCCD camera and the scintillating screen and adopting suitable lens, high-resolution images of a smaller area of the detector can be digitized allowing to obtain a radiographic zoom of specific regions of the painting.

Fig 1: Sketch of the experimental set-up for digital radiography of paintings.

RESULTS

In order to evaluate the performance of the digital detector, a test-painting has been investigate. For this purpose, a wood (poplar) panel of about 27x32 cm² painted in early 17^th century has been provided by the Opificio delle Pietre Dure of Florence (Italy) reproducing a typical artistic subject: "l'annunciazione" (Fig.1). This panel was already investigated by film-based radiographic technique at 30 kV, 5 mA with an exposition of about 2 minutes (Fig.2) and this image has been used for reference.

Fig 1: Photograph of the wood panel used for testing the X-ray digital detector. Fig 2: Picture of the radiographic film of the wood panel digitized at 600dpi -12 bits using an Adara X-RayII image scanner. Fig 1 & Fig 2

In this work, the panel has been investigated using a microfocus X-ray tube available at the Physics Department; the tube has been set at 30 kV with a current of 3 mA.
The radiographic analysis of the painting can be carried out instantaneously during the X-ray irradiation of the object. Fig.3 shows the digital image of the panel acquired by the experimental detector using a single frame exposition of 100 ms and accumulating (average) 64 frames.
The overall spatial resolution measured on this picture is about 1 mm mainly limited by the characteristics of the CCD chip installed on the EBCCD device. Clearly, this value of spatial resolution is far enough to that one achieved by film-based technique but allow to acquire images of adequate quality on an area comparable with the size of typical radiographic films.

Fig 3:Digital radiograph of the wood panel acquired by the experimental detector. Fig 4: Image processing of Fig.3 (sharpen filter) for edge detection. Fig 3 & Fig 4

Moreover, the image of Fig.3 has been grabbed with an X-ray exposition of about 6.4 sec and an overall time for acquisition and processing of less than 50 seconds. Such a performance allows to inspect quickly the painting reducing also the dose for its irradiation.
The opportunity to get digital images allows to process in real time the radiographic image in order to extract features of interest. Fig.4 shows a quick image processing (sharpen filtering) adopted for edge detection. Contours of specific elements can be immediately extracted allowing to determine immediately the different structures of the painting.
Changing properly the optical set-up of the device, it is possible to acquire images of a lower area of the detector with higher spatial resolution. Fig.5A shows an image of the central part of the panel obtained in the same experimental condition reported above but arranging a detector area of about 10x6 cm² and accumulating only 16 frames. In this way, a "zoom" of a specific region of interest of the painting is achieved with a spatial resolution of about 300 microns. This result is compared with the digital zoom of the same area extracted respectively from Fig.2 (Fig.5B) and from Fig.3 (Fig.5C). It is clear now that the quality of the measured digital image is considerably increased and it is comparable with the digital zoom of the film image (even if digitized by means of a high resolution scanner). Such a result is also important considering the time saving offered by the direct examination of the image during the X-ray irradiation in comparison with the several process necessary to accomplish this analysis with the ordinary film-based technique (film exposition, film development, digitization and processing of the final image).

(A)	(B)	(C)
Fig 5: Comparison among different "zoomed" images. (A) Digital radiograph grabbed with the experimental detector focused on the central part of the panel. (B) Digital zoom of the same area of Fig.2. (C) Digital zoom of the same area of Fig.3.


Moreover, this approach allows to obtain better results with regard to the simple digital zoom of the full size image of the painting grabbed using the large area detector arrangement.

CONCLUSIONS

An experimental set-up has been developed in order to evaluate the opportunity to perform radiographic analysis of paintings by means of a digital X-ray detector.
The performance of the system has been evaluated by means of a test-painting and results appear promising in particular with regard to the reduction of the irradiation time and the optimization of the inspection process.
Several improvements of the detector are in progress to optimize the performance of the device for higher spatial resolution.

ACKNOWLEDGMENTS

The authors would like to thank Alfredo Aldrovandi and Mauro Matteini of the "Opificio delle Pietre Dure" of Florence (Italy) for help and support in this research and for providing the wood panel used for tests. Thanks also to Dr. Serguei Golovkin and Dr. Vladimir Govorun for supporting the EBCCD set-up.

REFERENCES

1. M. Rossi, F. Casali, G. Casadei, S. Zuelli, "Image Reconstruction of Painting Radiographs", 6th International Conference on Non-Destructive Testing and Microanalysis for the Diagnostics and Conservation of the Cultural and Environmental Heritage, May 17-20, 1999, Rome (Italy).
2. S.V. Golovkin et al., "A new tool for high resolution multichannel readout: Megapixel Electron-Bombarded CCD image zoom tube", Proc. SPIE 2551, 118-205, 1995.
3. M. Rossi, F. Casali, S.V. Golovkin, V.N. Govorun, "Digital Radiography using an EBCCD-based Imaging Device", 4th Topical Meeting on Industrial Radiation and Radioisotope Measurement Applications, IRRMA ¢ 99, Raleigh, NC (USA), October 3-7, 1999. In printing on "Applied Radiation and Isotopes".

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