Photon-counting versus an integrating CCD-based gamma camera: important consequences for spatial resolution

Abstract

Charge-coupled devices (CCDs) coupled to scintillation crystals can be used for high resolution imaging with x-rays and gamma-rays. When the CCD images can be read out fast enough, the energy and interaction position of individual gamma quanta can be estimated by real-time image analysis of scintillation light flashes ('photon counting mode'). We tested a set-up in which an electron-multiplying CCD was coupled to a 1 mm thick columnar CsI crystal by means of a fibre-optic taper. We found that, compared to light integration, photon counting improves the intrinsic spatial resolution by a factor of about 3 to 6. Applying our set-up to Tc-99m and I-125 imaging, we were able to obtain intrinsic resolutions below 60 microm (full width at half maximum). Counting losses due to overlapping of light flashes are negligible for event rates typical for biomedical radio-nuclide imaging and do strongly depend on energy window settings. Energy resolution was estimated to be approximately 35 keV FWHM for a 1:1 taper. We conclude that CCD-based gamma cameras have great potential for applications such as in vivo imaging of gamma emitters.

Imaging and spectroscopy using a scintillator-coupled EMCCD

“…The electrons released in this process may then also undergo avalanche multiplication, leading to a progressive increase in the signal over the extent of the gain register. Whilst the probability, P, of multiplication over each element of the readout register is quite small, the number of gain register elements, n, is generally quite large (n = 536 with the CCD97) and so the multiplication gain can be of the order of thousands, Equation 1. This multiplication gain is applied to the signal before the output circuitry of the CCD and so any electronic or processing noise will only be added to the multiplied signal, greatly improving the signal to noise.…”

“…In previous studies of this nature it is often the case that the peak signal (or sum of 3 × 3 pixels) of a photon flash is assumed to be proportional to the energy of the incident X-ray [1,2,10]. Ignoring the depth of interaction effects (discussed later) this would be the case for unity gain.…”

“…One method of analysis which is often used in the literature [1,2,14] is to 'smooth' the image. This generally involves convoluting the image with a Gaussian kernel.…”

Statistical characterization of radiological images: basic principles and recent progress

“…From the pixel values associated with individual events, it is possible to estimate the coordinates of the initial interaction and the energy of the photon. Researchers at the University of Arizona 15,16 have been using this approach with integrating arrays of cadmium zinc telluride (CZT) semiconductor detectors for over a decade, and more recently it has been shown by several groups [17][18][19][20][21][22] that the method also works well with scintillation detectors. Fig.…”

Micro-machined retro-reflector for improving light yield in ultra-high resolution gamma cameras

“…It has been shown that very high spatial resolutions (below 100 μm) can be obtained with a detector consisting of CsI(Tl) micro-columnar scintillation crystals read-out by a CCD operating at high frame rates (e.g. Beekman and De Vree 2005, Teo et al 2006, Meng 2006, Heemskerk et al 2007, Soesbe et al 2007, Westra et al 2009. However, due to the low capture efficiency of CsI(Tl) and the limited thickness of available micro-columnar crystals, these crystals are not well suited for small-animal imaging.…”