JO URNAL O F T O R T HE EUR O PEAN O PTI CAL SOCI ETY R APID PUBLICATI O NS Journal of the European Optical Society - Rapid Publications 3, 08010 (2008) www.jeos.org Towards a new concept for high sensitivity Compton scatter emission imaging Mai K. Nguyen mai.nguyen-verger@u-cergy.fr ETIS / CNRS UMR 8051 / ENSEA / Universit´ e de Cergy-Pontoise, 6 avenue du Ponceau, 95014 Cergy- Pontoise Cedex, France Cl´ emence Driol clemence.driol@u-cergy.fr LPTM / CNRS UMR 8089 / Universit´ e de Cergy-Pontoise, 2 rue Adolphe Chauvin, 95302 Cergy- Pontoise Cedex, France Tuong T. Truong truong@u-cergy.fr LPTM / CNRS UMR 8089 / Universit´ e de Cergy-Pontoise, 2 rue Adolphe Chauvin, 95302 Cergy- Pontoise Cedex, France Habib Zaidi habib.zaidi@hcuge.ch Geneva University Hospital, Division of Nuclear Medicine, CH-1211 Geneva 4, Switzerland A new efficient scheme for imaging gamma-emitting objects is advocated in this work. It is elaborated on the recent idea of collecting data, using a detector equipped with a parallel-hole collimator, from Compton scattered photons to reconstruct an object in three-dimensions. This paper examines a working mode without collimation, which should increase its sensitivity and field of view. To simplify the otherwise complex mathematical formulation, we choose to discuss the image formation process in two-dimensions, which can be implemented with a slit collimator. Comparison with the standard collimated case, via the analysis of the shapes of the respective point spread functions (PSF), shows marked improvements and numerical simulation results, obtained using a brain phantom, support the viability and attractiveness of this new imaging modality. [DOI: 10.2971/jeos.2008.08010] Keywords: Emission imaging, sensitivity, Compton scattering, image formation, image reconstruction 1 INTRODUCTION Emission imaging using gamma-emitting sources is widely used in numerous fields such as medical imaging, gamma as- tronomy, non-destructive testing and environmental survey. In conventional nuclear imaging, a collimated gamma camera rotates in space to collect primary radiation emitted by the ob- ject (human organ) under investigation. In this case, Compton scattered radiation is generally considered as noise hindering image quality and quantitative accuracy and consequently ap- propriate scatter correction procedures should be applied [1]. Recently an interesting novel imaging concept, which pre- cisely uses, as imaging agent, radiation scattered by the object (instead of primary radiation), has been proposed. A spatially fixed collimated gamma camera records now projection data labeled by the energy of scattered radiation (or equivalently its scattering angle). It has then been shown that the recon- struction of a three-dimensional object is feasible using this data acquisition geometry [2]-[5]. However in this situation, the image sensitivity is considerably affected owing to the presence of the parallel-hole collimator. Only about one out of 10 4 scattered photons reaches the detector [6]. Therefore in order to record a much larger amount of scattered radiation, we propose to extend the working principle of this Compton imaging concept to a functioning modality without collima- tor, as depicted in Figure 1. It can clearly be seen that this concept differs from earlier proposals on scattered radiation imaging [7]-[10], in particular: • Compton tomography [11], which reconstructs the elec- tron density of the object (instead of its activity density), and uses a moving point-like detector collecting scattered radiation from an external radiation source, • Compton camera [12]-[14], which reconstructs the activ- ity density of an object from scattered radiation using co- incidence measurements between a site on a solid-state scatter layer-detector and another site on a scintillation crystal-based absorption-detector. Object Scattering medium Detector Collimator FIG. 1 Illustration of collimated and uncollimated scattered radiation emission imaging. The quantity to be reconstructed in emission imaging is the spatial distribution of the radio-tracer in the patient’s body. As the true three-dimensional problem involves an open de- tection geometry for scattered photons thus requiring a com- plex mathematical formulation, we shall first study its two- dimensional counterpart to test the viability of this idea. To Received October 30, 2007; published February 26, 2008 ISSN 1990-2573