Axial 0.2-0.5 cm
Lateral 1-2 cm
Slice thickness approx 5 cm

Parallelism is a function of the physical characteristics of a linear probe. The transducer array will effectively ‘see’ a needle only if it sits within the tolerances of the lateral and slice thickness resolution. The typical tolerances are shown in the table.

This means that the operator needs to keep the needle within the very narrow slice thickness of the probe.