Metal Artifact Reduction in Sinograms of Dental Computed Tomography

Use of metal objects such as dental implants, fillings, crowns, screws, nails, prosthesis and plates have increased in dentistry over the past 20 years, which raised a need for new methods for reducing the metal artifacts in medical images. Although there are several algorithms for metal artifact reduction, none of these algorithms are efficient enough to recover the original image free of all artifacts. This thesis presents two approaches for reducing metal artifacts through accurate segmentation of metal objects on dental computed tomography images. First approach was based on construction and tilting of a 3D jaw phantom, aiming to obtain fewer metals on each slice. 3D jaw phantom included the main anatomical structures of a jaw, and multiple metal fillings inserted on the teeth. Each jaw slice on the 3D phantom was tilted in order to mimic the (1) nodding movement, and (2) mouth opening/closing. Second approach was to segment the metals on an experimental dataset, consisting of a Cone-Beam Computed Tomography image, by using different segmentation algorithms. K-means clustering, Otsu’s thresholding method and logarithmic enhancement were used for extracting the metals from a real dental CT slice. Once the metal fillings on the jaw phantom were segmented out from the image, they were compensated by gap filling methods; Discrete Cosine Domain Gap Filling and inpainting. Qualitative and quantitative analyses were carried out for evaluating the performance of implemented segmentation methods. Efficiency of tilting alternatives on the segmentation of metal fillings was compared. In conclusion, jaw opening/closing movement between 24º-30º suggested a significant enhancement in segmentation, thus, metal artifact reduction on the jaw phantom. Inpainting method showed a better performance for both simulated and experimental dataset over the DCT domain gap filling method. Moreover, merging the logarithmic enhancement and inpainting showed superior results over other metal artifact reduction alternatives

Reduction of Limited Angle Artifacts in Medical Tomography via Image Reconstruction

Artifacts are unwanted effects in tomographic images that do not reflect the nature of the object. Their widespread occurrence makes their reduction and if possible removal an important subject in the development of tomographic image reconstruction algorithms. Limited angle artifacts are caused by the limited angular measurements, constraining the available tomographic information. This thesis focuses on reducing these artifacts via image reconstruction in two cases of incomplete measurements from: (1) the gaps left after the removal of high density objects such as dental fillings, screws and implants in computed tomography (CT) and (2) partial ring scanner configurations in positron emission tomography (PET). In order to include knowledge about the measurement and noise, prior terms were used within the reconstruction methods. Careful consideration was given to the trade-off between image blurring and noise reduction upon reconstruction of low-dose measurements.Development of reconstruction methods is an incremental process starting with testing on simple phantoms towards more clinically relevant ones by modeling the respective physical processes involved. In this work, phantoms were constructed to ensure that the proposed reconstruction methods addressed to the limited angle problem. The reconstructed images were assessed qualitatively and quantitatively in terms of noise reduction, edge sharpness and contrast recovery.Maximum a posteriori (MAP) estimation with median root prior (MRP) was selected for the reconstruction of limited angle measurements. MAP with MRP successfully reduced the artifacts caused by limited angle data in various datasets, tested with the reconstruction of both list-mode and projection data. In all cases, its performance was found to be superior to conventional reconstruction methods such as total-variation (TV) prior, maximum likelihood expectation maximization (MLEM) and filtered backprojection (FBP). MAP with MRP was also more robust with respect to parameter selection than MAP with TV prior.This thesis demonstrates the wide-range applicability of MAP with MRP in medical tomography, especially in low-dose imaging. Furthermore, we emphasize the importance of developing and testing reconstruction methods with application-specific phantoms, together with the properties and limitations of the measurements in mind

Reduction of Limited Angle Artifacts in Medical Tomography via Image Reconstruction

Artifacts are unwanted effects in tomographic images that do not reflect the nature of the object. Their widespread occurrence makes their reduction and if possible removal an important subject in the development of tomographic image reconstruction algorithms. Limited angle artifacts are caused by the limited angular measurements, constraining the available tomographic information. This thesis focuses on reducing these artifacts via image reconstruction in two cases of incomplete measurements from: (1) the gaps left after the removal of high density objects such as dental fillings, screws and implants in computed tomography (CT) and (2) partial ring scanner configurations in positron emission tomography (PET). In order to include knowledge about the measurement and noise, prior terms were used within the reconstruction methods. Careful consideration was given to the trade-off between image blurring and noise reduction upon reconstruction of low-dose measurements.Development of reconstruction methods is an incremental process starting with testing on simple phantoms towards more clinically relevant ones by modeling the respective physical processes involved. In this work, phantoms were constructed to ensure that the proposed reconstruction methods addressed to the limited angle problem. The reconstructed images were assessed qualitatively and quantitatively in terms of noise reduction, edge sharpness and contrast recovery.Maximum a posteriori (MAP) estimation with median root prior (MRP) was selected for the reconstruction of limited angle measurements. MAP with MRP successfully reduced the artifacts caused by limited angle data in various datasets, tested with the reconstruction of both list-mode and projection data. In all cases, its performance was found to be superior to conventional reconstruction methods such as total-variation (TV) prior, maximum likelihood expectation maximization (MLEM) and filtered backprojection (FBP). MAP with MRP was also more robust with respect to parameter selection than MAP with TV prior.This thesis demonstrates the wide-range applicability of MAP with MRP in medical tomography, especially in low-dose imaging. Furthermore, we emphasize the importance of developing and testing reconstruction methods with application-specific phantoms, together with the properties and limitations of the measurements in mind

Metal Artifact Reduction in Sinograms of Dental Computed Tomography

Use of metal objects such as dental implants, fillings, crowns, screws, nails, prosthesis and plates have increased in dentistry over the past 20 years, which raised a need for new methods for reducing the metal artifacts in medical images. Although there are several algorithms for metal artifact reduction, none of these algorithms are efficient enough to recover the original image free of all artifacts. This thesis presents two approaches for reducing metal artifacts through accurate segmentation of metal objects on dental computed tomography images. First approach was based on construction and tilting of a 3D jaw phantom, aiming to obtain fewer metals on each slice. 3D jaw phantom included the main anatomical structures of a jaw, and multiple metal fillings inserted on the teeth. Each jaw slice on the 3D phantom was tilted in order to mimic the (1) nodding movement, and (2) mouth opening/closing. Second approach was to segment the metals on an experimental dataset, consisting of a Cone-Beam Computed Tomography image, by using different segmentation algorithms. K-means clustering, Otsu’s thresholding method and logarithmic enhancement were used for extracting the metals from a real dental CT slice. Once the metal fillings on the jaw phantom were segmented out from the image, they were compensated by gap filling methods; Discrete Cosine Domain Gap Filling and inpainting. Qualitative and quantitative analyses were carried out for evaluating the performance of implemented segmentation methods. Efficiency of tilting alternatives on the segmentation of metal fillings was compared. In conclusion, jaw opening/closing movement between 24º-30º suggested a significant enhancement in segmentation, thus, metal artifact reduction on the jaw phantom. Inpainting method showed a better performance for both simulated and experimental dataset over the DCT domain gap filling method. Moreover, merging the logarithmic enhancement and inpainting showed superior results over other metal artifact reduction alternatives

Evaluation of Median Root Prior for Robust In-Beam PET Reconstruction

Dose delivery verification in proton beam radiotherapy is used to ensure the delivery of the dose to the correct location. A positron emission tomography (PET) scanner can be used to detect the secondary radiation during the treatment, so-called in-beam PET. This is a challenging application for PET due to the low counts and limited angular coverage. We propose a maximuma posteriori(MAP) reconstruction with median root prior (MRP) for the reconstruction of in-beam PET data. The proposed method was compared against MAP with total variation (TV) prior and maximum likelihood expectation maximization (MLEM), which have previously been used for this application. The effects of different ring configurations and time-of-flight information were tested with simulations of a geometrical phantom and a realistic patient treatment plan. The results indicate that both MAP methods produced sharper edges than MLEM, allowing more accurate edge localization in the reconstructed images. Even for the partial ring configurations, no elongation was observed with MAP methods. MAP-MRP successfully reduced the noise, whereas MAP-TV resulted in checkerboard artifacts. MAP-MRP was also more stable against the selection of the reconstruction parameters. In conclusion, MAP-MRP offers a simple and robust alternative for the reconstruction of in-beam PET data

A Monte Carlo based scatter removal method for non-isocentric cone-beam CT acquisitions using a deep convolutional autoencoder

The primary cone-beam computed tomography (CBCT) imaging beam scatters inside the patient and produces a contaminating photon fluence that is registered by the detector. Scattered photons cause artifacts in the image reconstruction, and are partially responsible for the inferior image quality compared to diagnostic fan-beam CT. In this work, a deep convolutional autoencoder (DCAE) and projection-based scatter removal algorithm were constructed for the ImagingRing(TM) system on rails (IRr), which allows for non-isocentric acquisitions around virtual rotation centers with its independently rotatable source and detector arms. A Monte Carlo model was developed to simulate (i) a non-isocentric training dataset of approximately 1200 projection pairs (primary + scatter) from 27 digital head-and-neck cancer patients around five different virtual rotation centers (DCAE(NONISO)), and (ii) an isocentric dataset existing of approximately 1200 projection pairs around the physical rotation center (DCAE(ISO)). The scatter removal performance of both DCAE networks was investigated in two digital anthropomorphic phantom simulations and due to superior performance only the DCAE(NONISO) was applied on eight real patient acquisitions. Measures for the quantitative error, the signal-to-noise ratio, and the similarity were evaluated for two simulated digital head-and-neck patients, and the contrast-to-noise ratio (CNR) was investigated between muscle and adipose tissue in the real patient image reconstructions. Image quality metrics were compared between the uncorrected data, the currently implemented heuristic scatter correction data, and the DCAE corrected image reconstruction. The DCAE(NONISO) corrected image reconstructions of two digital patient simulations showed superior image quality metrics compared to the uncorrected and corrected image reconstructions using a heuristic scatter removal. The proposed DCAE(NONISO) scatter correction in this study was successfully demonstrated in real non-isocentric patient CBCT acquisitions and achieved statistically significant higher CNRs compared to the uncorrected or the heuristic corrected image data. This paper presents for the first time a projection-based scatter removal algorithm for isocentric and non-isocentric CBCT imaging using a deep convolutional autoencoder trained on Monte Carlo composed datasets. The algorithm was successfully applied to real patient data