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Table 1 Summary of heavy metal stains used to enhance soft tissue contrast in CT. Asterisks indicate the most commonly used stains

From: X-ray computed tomography in life sciences

Heavy metal

Staining solution

Binds to

Staining considerations

Iodine * [38]

Iodine potassium iodide in water, ethanol or methanol. Various concentrations

Non-specific staining. Preferentially binds to lipids and glycogen.

Good for muscle fibres, nerve myelin sheath, connective tissues and the eye lens

• Rapid and deep tissue penetration

• Particularly suited to larger specimen (> 2 mm)

• An effective stain for flower parts, allowing counting of pollen ovules [39]

• Has been used to study water transport in plants [35]

Tungsten and Molybdenum* [40]

Phosphotungstic acid

Phosphomolibdic acid

Proteins including fibrin and collagen. Suited to connective tissues

• Moderate tissue penetration rate and depth

• Can provide more detailed visualisation compared with iodine-based stains

• Phosphotungstic acid also effective in plants [41]

Osmium* [42]

Osmium tetroxide, as used in electron microscopy

Lipids including those in cell membranes, some proteins and nucleic acids

• Tissue penetration is slow and can be limited (not suited to samples requiring a penetration depth greater than 1–2 mm)

• Highly toxic, requiring special safety considerations

Indium [43]

Gallocyanin-chromalum

Cell nuclei. Can show cell density and individual cells

• A histology stain

• Low contrast overall

Iodine, aluminium and iron [44]

Verhoff’s Stain

Arterial walls of the vascular network

• A trichrome histology stain