Basic spatial resolution from duplex wire
Digital radiography needs a measured resolution figure, because unlike film there is no grain to set a floor – the detector, the scan pitch and the geometry all contribute and the result cannot be assumed. The measurement tool is the duplex wire IQI to ISO 19232-5: thirteen pairs of parallel wires, each pair with wires of diameter d separated by a gap also equal to d, so each pair has a period of 2d.
Imaged through a system with unsharpness U, a pair whose period is much larger than U shows two clear peaks with a deep valley between them. As the pairs get finer the valley fills in. The convention is that a pair is resolved while the dip between the peaks is at least 20 percent of the peak height. The first pair that fails that test defines the system unsharpness as U = 2d, and the basic spatial resolution is half that value, SRb = d – equal to the wire diameter of that pair.
SRb is the digital analogue of film graininess and is the quantity ISO 17636-2 uses to qualify a technique. It also feeds the normalised signal-to-noise ratio, which is why the two measurements are made from the same image. Reported in micrometres, a good direct digital detector array gives 50 to 100 micrometres and a standard imaging plate 150 to 250.
Total unsharpness combines the detector term with the geometric term. Adding them in quadrature, U_total = sqrt(Ug^2 + Ui^2), shows immediately which one is worth attacking: if the geometric term is already half the detector term, backing the source off further buys almost nothing.
Worked example
| First unresolved wire pair | d7 |
| Geometric unsharpness | 0.3 mm |
| Maximum SRb permitted | 250 um |
| Wire diameter of that pair | 0.2 mm |
| Basic spatial resolution SRb | 200 um |
| Image unsharpness Ui | 0.4 mm |
| Total unsharpness | 0.5 mm |
| Equivalent line pairs per mm | 2.5 |
Duplex pair D7 has 0.20 mm wires, so image unsharpness Ui = 2 x 0.20 = 0.400 mm and SRb = 200 um. Combined in quadrature with a 0.300 mm geometric term, U_total = sqrt(0.30^2 + 0.40^2) = sqrt(0.25) = 0.500 mm. The detector term dominates, so more standoff would buy little.
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