Digital imaging concept was first expressed in a pioneering work by Eugene F. Lally from the Jet Propulsion Laboratory (JPL) in Pasadena (USA) in 1961 and later put in practice in 1969 by two researchers from AT&T Bell Labs., Willard Boyle and George E. Smith, who merged video telephony and semiconductor bubble memories coming up with the ancestor of the Charge-Coupled Device (CCD).

In CCDs the signal measured in any single pixel is the integral of all the charges generated during the acquisition time.

Such are the CCDs developed at LBNL during the last twelve years, and DES (Dark Energy Survey), a big collaboration based in Fermilab, Chicago, for the search of the elusive Dark Energy, is building a camera featuring a mosaic of seventy such detectors, 2048 × 4096 pixels each, for a total of half a gigapixel.

In order to take into account the energy of the single incoming photons, a full detector-preamplifier-discriminator-counter chain should be put into any single pixel. This technology cannot be used for the detection of visible light, as the energy of visible photons is too small to generate relevant pulses, but X-ray photons do have enough energy.

In 1998 CERN (European Organization for Nuclear Research), where this approach had been widely used and refined in the field of particles detection, became the leader institution in a project for the development of a photon-counting chip for imaging, with special focus on medical applications (Medipix chip).

Medipix chips were used in the E.U.-funded Dear-Mama project (Detection of Early Markers in Mammography) for the prototyping of a very low-dose, medical imaging machine for mammography and bone densitometry. CdTe solid-state detectors read-out through single-photon counting devices were used, as CdTe allows capturing 99 % of the incoming photons at the mammography X-ray energies, while single-photon counting allows improving drastically the signal-to-noise ratio (SNR). Higher detector capture efficiency means lower X-ray dose for the patient, which in turn means that patients could undergo their first mammography at earlier ages (earlier tumour detection) with less or no radiation hazard; higher SNR means sharper and more significant images; single-photon counting also means faster read-out times.

As I was involved both in DES and in Dear-Mama, this presentation focuses mainly on comparing the characterization techniques for both (charge integration and single-photon counting) imaging detector classes.