Detection of Spherical Gold Fiducials in kV X-Ray Images Using Intensity-Estimation-Based Method

Masaki Kokubo,Nobutaka Mukumoto,Takashi Mizowaki,Akira Sawada,Masahiro Hiraoka,Yuki Miyabe,Masahiro Yamada

doi:10.4236/ijmpcero.2018.71010

Abstract

Fiducial marker detection algorithms in kilovoltage x-ray images using physical characteristics of transmission x-ray have been proposed. It, however, has been suggested recently that factors besides transmission x-ray affect x-ray images. The purpose of this study was to develop a new fiducial detection algorithm using fiducial intensity estimation based on physical characteristics of x-ray images with gold fiducials. First, x-ray images of a fiducial on a water-equivalent phantom were acquired. It was observed that the ratio of background to fiducial intensity in the images decreased as phantom thickness increased. Based on the negative correlation, we identified a function for estimating fiducial intensity that consists of background intensity and the amount of scattered radiation by the other x-ray source of an orthogonal imaging system and a treatment beam. Then, we developed an algorithm that extracts fiducial candidates using the estimation function. Its performance was measured using x-ray images which had 3824 fiducials altogether. The average number of false-positive detection of the proposed algorithm in single image was one-tenth of an algorithm considering only transmission x-ray. The proposed algorithm detected 99.5% of all fiducials under an error of 1.0 mm, while the other algorithm detected 94.7% or less (Clinical trial number: UMIN000005324).

Highlights

It has been emphasized recently that management of intrafraction motion is an important feature of image-guided radiotherapy [1] [2]
The purpose of this study was to develop a new fiducial detection algorithm using fiducial intensity estimation based on physical characteristics of x-ray images with gold fiducials
It was observed that the ratio of background to fiducial intensity in the images decreased as phantom thickness increased

Summary

Introduction

It has been emphasized recently that management of intrafraction motion is an important feature of image-guided radiotherapy [1] [2]. One method used to compensate for respiratory motion is fluoroscopy-based tracking [1]. In some such tracking methods, surrogate fiducials placed close to a tumor are used to identify the position of the tumor [3] [4]. It is reported that NCC values of the objects that should be detected will decrease and the number of false-positive will be larger for low contrast images [5]. Another correlation index was proposed for robustness to image noise [6]. The methods using correlation indices or image processing filters, did not explicitly utilize potentially information-rich physical characteristics of x-ray that induces image intensity

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Conclusion