In an imaging apparatus particularly for use in intra-oral dental applications, a solid state imager is arranged to detect X-ray radiation from a source after passing through an object. Prior to irradiation by the source, charge accumulated in the elements of a CCD is clocked from two or more rows into a register such that charges are summed. The register is output via a charge amplifier which sums the charge applied to it to give a relatively large output signal which is amplified and applied to thresholder. Onset of X-ray irradiation is detected when the signal applied to the thresholder exceeds a reference level. A start signal is then transmitted to a control unit to cause image acquisition to begin.