3D Printed IQIs for X-ray Examination

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These days, when colour images are taken of museum objects it is generally good practice to include a colour calibration chart, like a ColorChecker. This allows the captured images to be colour calibrated and then compared, with reasonable confidence, with other images captured within the same organisation or with image captured using different imaging systems by other individuals or organisations. But this is often not the case when X-ray images are taken of museum objects.

Calibration targets or Image quality Indicators (IQIs), are available[1] for X-radiography and are in fact used in some fields, but they may not be appropriate for all type of cultural heritage X-radiography. In some institutions specialists often construct their own custom IQIs to help ensure the quality of their images. This works well within an institution but is generally less appropriate when comparing images generated by different institutions.

As it is becoming easier and more common to share and compare large complex technical images between different cultural heritage institutions the need for consistent IQIs is becoming more important. This is also becoming an increasingly important issue when it comes to comparing X-ray images produced from analogue film and images created using new digital X-ray systems.

One possible option for producing consistent, but custom designed, IQIs, tailor made for the examination of specific groups of cultural heritage materials might be the use of new 3D-printers. This pilot study was put together to begin to explore this possibility.

3D Printed Samples

The following samples were made by the Rapid Manufacturing Team at Renishaw plc., which has been coordinated by Knowledge Transfer Partnership (KTP) Associate Sophie Adams Foster in collaboration with Carinna Parraman at University of the West of England Bristol.

No. Printer Name Material Process Resolution Colour
1 Envision Tec Photo-cure Acylic Direct Light Projecting 25 - 50 Microns Dark Red
2 Dimension ABS Extrusion 250 Microns Black
3 SLS: EOS P385 Glass Filled Nylon Selective Laser Sintering 150 Microns Light Grey
4 Object Eden 250 Photo-cure Acylic Poly-Jet Printer 16 - 32 Microns White
5 Z-Corp Plaster of Paris Powder based system with a binder using Ink Jet technologies 100 Micorns at 460DPI Bright White

National Gallery Examination

X-ray procedure

Data Processing

Image preparation

  • Each X-ray plate was scanned at 600 DPI and saved as a 1-band 16-bit tif image.
  • All additional images processing was carried out using the Nip2 software.
  • The resultant X-ray images where balanced, as required, to calibrate for slightly uneven exposure of the plates, principally some appeared slightly brighter along the side edges, see figure 5.
    • The images for the examinations at 50KV, 40KV and 30KV required this process.

Profile extraction

  • Using Nip2, an arrow was marked across the central length of each image of each sample wedge, see figure 6.
  • Each arrow was then used to extract a region of the image across each wedge 10 pixels high, which was then averaged down to a single pixel high, see figure 7.
  • The image profile was then flipped left-right if required to ensure that each wedge was represented, left to right, going from thick to thin.
  • Each image was then resized to ensure that the thickest point of each wedge was at about 250 pixels and a total of 1500 pixels wide.
  • Each 1x1500 pixel images was then converted into numbers and saved as the raw data displayed in the graphs.

Profile Extraction Macro

The actual process of extracting the profile data from each arrow was simplified using the following Nip2 marcro.

Process_Wedges_Prep = class
  Menuaction (_ "Process Wedges Prep") (_ "IQIs") {
    action x = class 
      _result {
        _vislevel = 3; 
        _w = 10;
        _area = extract_arrow 0 0 _w x;
        _sa = scale _area; 
        im = image_set_type Image_type.B_W _sa;				
        pt = Mark_relative im 0.1 0.5;
        _a' = resize Interpolate_bilinear 1 (1 / _w) _area;
        _a'' = _a', pt.left < (im.width/2)
             = fliplr _a';
        _ofs = 250 - pt.left, pt.left < (im.width/2)
             = 250 - (im.width - pt.left);
        _nw = 1500;
        _bg = Image (image_new _nw 1 1 _a''.format _a''.coding _a''.type 0 0 0);
	_result = insert_noexpand _ofs 0 _a'' _bg;

Graphs of IQI profiles for given X-ray Exposures

Images of IQI X-ray examinations for given Exposures