Fig 1.
General setup for micro-CT measurements.
An electron beam is emitted by the cathode (not true to scale). The probe is rotated after every scan in between the X-ray tube and a detector usually coupled with a camera. Magnification can be achieved by shifting the probe towards the X-ray beam.
Fig 2.
Fixation examples of micro-CT probes.
(A) A bushcricket leg is fixed with instant glue to a brass holder. This fixation requires only a minimum of space and allows the specimen to be moved close to the X-ray source. (B) Example of multisample scanning of dry samples with gel capsules and polystyrene as spacer. (C) Moist samples can be fixed in a reaction tube with polystyrene. (D) Multisample scanning with reaction tubes inside a straw. Foam helps to prevent probe shifting. Plugging is useful to prevent expansion of the foam. (E) Parafilm can be used to prevent large samples from drying out. Open ends in the parafilm can simply be closed with ribbed forceps.
Fig 3.
Rendered image of one exemplary cube of each sample.
Grey values from the dynamic range of 0 to 0.03 are coded as colour values (0 to 255). The average binarised fraction of the sample is shown as a percentage for each material (n = 6 per material). Low binarised values are less visible in the evaluation than high values. Solid materials (outlined in purple) are the most radiopaque of the materials tested. Polyurethane samples (outlined in orange) are generally suitable as fixation materials but may often contain radiopaque particles. Other foams (outlined in blue) are particularly suitable as fixation material in the μCT. Basotect, the PE pad and Styrofoam should be particularly emphasised here. Scale: 1 mm.
Fig 4.
Reconstructed 2D cubes of the sample materials.
The upper view shows an exemplary section of a sample type with a dynamic range of 0 to 0.03. The lower illustration shows the binarisation (starting from 10% of the dynamic range) of the same image section. In order to display a fixed micro-CT sample as easily as possible, the fixation material should not be visible (foams and polyurethane) or be easy to remove in post-processing (solid materials). Scale: 1 mm.
Fig 5.
X-ray density in Hounsfield units of the fixation materials examined.
Hounsfield Units (HU) is a measure of the X-ray density of a material, where -1000 HU corresponds to the density of air and 0 HU to the density of water. Foams (blue) usually have the lowest HU (radiolucent). Polyurethane (orange) and solid materials (purple) have very different radiopacities (n = 6 per material).
Fig 6.
Illustration of the porosity of the tested fixation materials for micro-CT samples.
Foams (blue) had on average the highest percentage of pores (open and closed pores) and solid materials (purple) had the lowest percentage of pores. Styrofoam (fine granular) is the most porous material tested, but it is more radiopaque than Basotect. (n = 6 per material).