Abstract
This review article offers an overview of the differences between traditional energy integrating (EI) X-ray imaging and the new technique of X-ray photon counting spectral imaging (x-CSI). The review is motivated by the need to image gold nanoparticles (AuNP) in vivo if they are to be used clinically to deliver a radiotherapy dose-enhancing effect (RDEE). The aim of this work is to familiarise the reader with x-CSI as a technique and to draw attention to how this technique will need to develop to be of clinical use for the described oncological applications. This article covers the conceptual differences between x-CSI and EI approaches, the advantages of x-CSI, constraints on x-CSI system design, and the achievements of x-CSI in AuNP quantification. The results of the review show there are still approximately two orders of magnitude between the AuNP concentrations used in RDEE applications and the demonstrated detection limits of x-CSI. Two approaches to overcome this were suggested: changing AuNP design or changing x-CSI system design. Optimal system parameters for AuNP detection and general spectral performance as determined by simulation studies were different to those used in the current x-CSI systems, indicating potential gains that may be made with this approach.
Highlights
Radiotherapy is a staple component of modern cancer therapies due to its ability to be targeted to volumes of interest and to damage cancer cells more readily than their healthy counterparts
This review aims to introduce the interested reader to X-ray photon counting spectral imaging (x-CSI) techniques, how they differ from conventional X-ray imaging systems, and their potential for use in x-CSI approaches are still under development, though clinical systems are available, such as the Extremity 5X120 scanner by MARS Bioimaging [33] and the Naeotom Alpha scanner by Siemens Healthineers, which received FDA approval in September 2021 [34]
The use of AuNPs to enhance the effectiveness of radiotherapy is a well-studied topic, though the corollary requirement of quantitative in vivo AuNP imaging is often taken for granted in the radiotherapy dose-enhancing effect (RDEE) literature
Summary
Radiotherapy is a staple component of modern cancer therapies due to its ability to be targeted to volumes of interest and to damage cancer cells more readily than their healthy counterparts. Physical enhancement mechanisms are those that improve radiotherapy response of cells due to physical interactions between incident radiation and the AuNPs themselves These can include increased energy deposition (due to the higher Z-number of gold producing a higher probability of interaction with the incident radiation beam) and production of more ionising radiation types when irradiated (production of secondary X-ray fluorescence, Auger electrons, and photoelectrons [15]). Traditional X-ray techniques struggle to provide reliable and reproducible quantitative images, and this has traditionally limited their role to semi-quantitative and structural imaging rather than fully quantitative imaging Newer developments such as dual energy approaches have corrected for this deficit in some respects; shortcomings in noise performance remain, making quantification of the low concentrations relevant to AuNP-mediated RDEE research unlikely.
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