Optimization of signal in positron emission tomography scans: present and future developments

The absolute sensitivity of a state-of-the-art, commercial neuroPET tomograph with interplane septa is about 0.5%. This poor utilization of the available photons could be improved by increasing the intrinsic efficiency of the detection process and, more significantly, by increasing the solid angle coverage of the tomography. While multi-ring scanners currently have an axial length of about 10 cm, the useful solid angle is limited by the presence of interplane septa. These septa reduce the acceptance rate not only of scattered photons but also of true unscattered coincidences, although in studies performed at high photon counting rates the loss of potential signal may be less important than a reduction in scatter. Removal of the septa increases the absolute sensitivity of the scanner to about 3%, a figure which also includes an unavoidable increase in scattered photons. However, in studies performed at low photon counting rates, any increase in scattered and random (uncorrelated) coincidences resulting from septa removal may be an acceptable price to pay for the accompanying increase in signal, provided that there is a real improvement in the signal-to-noise ratio. Recently, scanners with automatically retractable septa have become commercially available, thus enabling the configuration (i.e. septa extended or retracted) to be selected according to the study to be performed.