Purpose: We propose a new imaging technique for detection of high-Z nanoparticles and for in-vivo beam monitoring in megavoltage X-ray beams by means of coincidence counting of annihilation photons following pair-production in nanoparticles and/or in tissue.
Methods: The proposed megavoltage X-ray induced positron emission (MVIPE) imaging technique is studied by radiation transport computations using MCNP6/CEPXS-ONEDANT for two water phantoms: 35cm slab and similarly sized cylinder with 5cm gold nanoparticle (AuNP)-filled region in the center. MVIPE is compared to standard X-ray fluorescence computed tomography (XFCT). MVIPE adopts megavoltage X-ray sources and relies on the detection of 511keV annihilation photon pairs. XFCT uses kilovoltage sources and imaging is characterized by analysis of kα₁₂ Au-characteristic lines. Four levels of AuNP concentration were studied: 0%, 0.1%, 1% and 10% by-weight.
Results: Annihilation photons in the MVIPE technique originate along the X-ray beam path in the AuNP-loaded region and in water. Their flux is proportional to the MV source energy and linearly increases with AuNP concentration, while XFCT signal reaches saturation due to self-absorption within AuNP. The MVIPE technique using 15MV pencil beam and 10wt% AuNP detects about 4.5x10³ 511keV-photons/cm² at 90° w/r to the incident beam per 10⁹ source-photons/cm²; 0.5x10³ of these come from AuNP, while the rest come from water/tissue. The latter alone can be utilized for monitoring MV beams in tissue. In contrast, the XFCT technique using 150kVp detects about 100 kα₁-photons/cm² per 10⁹ source-photons/cm² without collimation.
Conclusion: In MVIPE, the number of detected annihilation photons secondary to pair-production is significantly greater than the kα₁-photons from fluorescence. Furthermore, coincidence counting in MVIPE enables collimation-free imaging, while collimation is a major limiting factor in XFCT. Additionally, MVIPE offers the potential of direct visualization of the x-ray beam in tissues during radiotherapy treatment, which may pave the way for in-vivo beam and dose monitoring.