Improved iterative reconstruction method for Compton imaging using median filter

Abstract

A Compton camera is a device for imaging a radio-source distribution without using a mechanical collimator. Ordered-subset expectation-maximization (OS-EM) is widely used to reconstruct Compton images. However, the OS-EM algorithm tends to over-concentrate and amplify noise in the reconstructed image. It is, thus, necessary to optimize the number of iterations to develop high-quality images, but this has not yet been achieved. In this paper, we apply a median filter to an OS-EM algorithm and introduce a median root prior expectation-maximization (MRP-EM) algorithm to overcome this problem. In MRP-EM, the median filter is used to update the image in each iteration. We evaluated the quality of images reconstructed by our proposed method and compared them with those reconstructed by conventional algorithms using mathematical phantoms. The spatial resolution was estimated using the images of two point sources. Reproducibility was evaluated on an ellipsoidal phantom by calculating the residual sum of squares, zero-mean normalized cross-correlation, and mutual information. In addition, we evaluated the semi-quantitative performance and uniformity on the ellipsoidal phantom. MRP-EM reduces the generated noise and is robust with respect to the number of iterations. An evaluation of the reconstructed image quality using some statistical indices shows that our proposed method delivers better results than conventional techniques.

Optimization of Number of Iterations as a Reconstruction Parameter in Bone SPECT Imaging Using a Novel Thoracic Spine Phantom

“…Sakai et al reported that %CV increased as the number of iterations was increased using ordered-subset expectation maximization. The method statistically estimates the source distribution, thus emphasizing the statistical noise in the image (18). Contrast is one of the most important parameters to evaluate because detection of hot spots is used for diagnosis in bone SPECT (19).…”

Development and Applications of Compton Camera—A Review

“…OSEM works in the same manner as MLEM, but it uses data divided into subsets (portions of measured data), and updates are performed for each subset. If subsets are created appropriately, image convergence can be greatly accelerated with little degradation in the image quality [ 81 , 207 , 208 ].…”

“…However, the convergence speed of the adopted iterative reconstruction method was not sufficiently high because the imaging algorithms considered the high-energy γ-ray source located at a far distance, and the simulation programs developed at that time considered the simple detector system. Some studies have recently been conducted to improve the MLEM and compare it with other imaging models, with a focus on the application of Compton cameras in nuclear medicine and particle therapy [ 81 , 210 , 211 , 212 , 213 ]. Yabu et al [ 214 ] reported the superiority of the LM-MLEM method over SBP using a Si/CdTe Compton camera.…”

“…The camera consists of eight layers of Si detectors and four layers of CdTe detectors of (size: 5 cm × 5 cm and weights: 8–13 kg). These Si/CdTe cameras exhibit good energy and angular (2.2% at half maximum (FWHM) and 5°, respectively) resolutions for photons with an energy of 662 keV, and a high-count rate of 0.16 cps/MBq at a distance of 1 m. The medical applications of the Si/CdTe Compton camera were introduced by Sakai et al [ 79 ], especially in nuclear imaging of different RIs, followed by improvements in the Compton imaging algorithms [ 80 , 81 , 82 ]. The in vivo [ 83 ] and human imaging [ 84 ] applications of Si/CdTe Compton cameras are vital achievements of these devices in the field of nuclear medical imaging.…”

“…As a continuation, the in vivo 3D imaging of 131 I, 18 F-FDG (511keV), and 67 Ga-citrate (300 keV) was conducted [ 157 ]. Sakai et al [ 79 , 80 , 81 , 82 ] at the Gunma University Heavy Ion Medical Center (GHMC) tested a Si/CdTe Compton camera, and successful measurements with 99m Tc and 18 F within human phantoms were demonstrated in their studies. Finally, the in vivo results [ 83 ] in mice followed by human experiments [ 84 ] using a Si/CdTe Compton camera have been studied by researchers for its application in the medical field.…”