Direct time-of-flight for quantitative, real-time in-beam PET: a concept and feasibility study

Abstract

We extrapolate the impact of recent detector and scintillator developments, enabling sub-nanosecond coincidence timing resolution (tau), onto in-beam positron emission tomography (in-beam PET) for monitoring charged-hadron radiation therapy. For tau < or = 200 ps full width at half maximum, the information given by the time-of-flight (TOF) difference between the two opposing gamma-rays enables shift-variant, artefact-free in-beam tomographic imaging by means of limited-angle, dual-head detectors. We present the corresponding fast, TOF-based and backprojection-free, 3D reconstruction algorithm that, coupled with a real-time data acquisition and a fast detector encoding scheme, allows the sampled beta+-activity to be visualized in the object during the course of the irradiation. Despite the very low statistics scenario typical of in-beam PET, real-treatment simulations show that in-beam TOF-PET enables high-precision images to be obtained in real-time, either with closed-ring or with fixed, dual-head in-beam TOF-PET systems. The latter greatly alleviates the installation of in-beam PET at radiotherapeutic sites.

Evaluation Scheme for a Positron Emission Tomography System Used in Monitoring of Radiation Therapy

“…Different reconstruction algorithms (ML-EM [4], RFS-EM [5], and dTOF [6]) have been tested. The dTOF algorithm showed the best image quality.…”

“…The program returns a lookup table with coordinates and position of all detectors of the PET scanner. ML-EM [4], RFS-EM [5], and dTOF [6] reconstruction algorithms can be applied for in-beam PET data. Each reconstruction algorithm has a number of parameters to optimize (number of iterations, size of the image element, etc.).…”

Technical Advances in Current PET and Hybrid Imaging Systems

“…Partial and split ring configurations for breast dedicated PET enable PET-guided biopsies to be performed on suspicious breast tissues [21]. Such open PET geometries are also attractive for those developing in-beam PET scanners to monitor + activity distributions generated by charged-hadron irradiation for radiation therapy [22]. In this specific application, closedring PET detectors pose spatial challenges if the horizontal and rotating radiotherapeutic beam lines are utilized.…”

“…On the other hand, TOF information allows limited-angle artifacts to be reduced without incorporating detector rotation, because the back-projection of data is restricted in a narrow region, for example, the elongation of image intensity in a direction perpendicular to the detectors in non-TOF PET images can be reduced using TOF information (Fig. 6) [21][22][23]. Coincidence timing resolution is an important determinator of the degree of artifact reduction, and individual partial ring configurations have different required timing resolution levels.…”

Comparison study of RPC and crystal based PET systems for hadron therapy monitoring

“…However, the partial ring configuration leads to an incomplete angular sampling that produces artefacts in image reconstruction [14], [15]. The TOF information improves PET images significantly and reduces the effect of limited angle artefacts [16], [17]. Therefore, a PET system with TOF capabilities is highly desired for in-beam hadron therapy monitoring.…”