PurposeRecently, diffusion posterior sampling (DPS), where score-based diffusion priors are combined with likelihood models, has been used to produce high-quality computed tomography (CT) images given low-quality measurements. This technique permits one-time, unsupervised training of a CT prior, which can then be incorporated with an arbitrary data model. However, current methods rely on a linear model of X-ray CT physics to reconstruct. Although it is common to linearize the transmission tomography reconstruction problem, this is an approximation to the true and inherently nonlinear forward model. We propose a DPS method that integrates a general nonlinear measurement model.ApproachWe implement a traditional unconditional diffusion model by training a prior score function estimator and apply Bayes’ rule to combine this prior with a measurement likelihood score function derived from the nonlinear physical model to arrive at a posterior score function that can be used to sample the reverse-time diffusion process. We develop computational enhancements for the approach and evaluate the reconstruction approach in several simulation studies.ResultsThe proposed nonlinear DPS provides improved performance over traditional reconstruction methods and DPS with a linear model. Moreover, as compared with a conditionally trained deep learning approach, the nonlinear DPS approach shows a better ability to provide high-quality images for different acquisition protocols.ConclusionThis plug-and-play method allows the incorporation of a diffusion-based prior with a general nonlinear CT measurement model. This permits the application of the approach to different systems, protocols, etc., without the need for any additional training.
Spectral imaging is an active area of investigation for interventional applications. We have previously proposed a joint processing strategy that leverage both temporal and spectral information to obtain digital subtraction angiograms (DSA). This strategy allows for the isolation of iodine signal using measurements from just two energy channels and can yield a noise advantage compared to alternative spectral acquisitions. Previous investigations imposed fairly restrictive assumptions on patient motion (at least one material map is stationary pre- and post-contrast). In this work, we investigate the performance of the joint processing strategy without such assumptions. Specifically, we included a simple affine registration prior to material decomposition to mitigate gross patient motion. Assuming such registration can compensate motion in one tissue type (e.g., bone which exhibits locally rigid motion), we then use the total of four energy measurements to solve for four unknowns: water pre- and post-contrast, calcium, and iodine. This method accommodates registration mismatches in the pre- and post-contrast water images. To evaluate this method, we simulated a head phantom with a backward tilt about the lateral axis between the pre- and post-contrast acquisitions, as well as an abdomen phantom with cardiac, breathing, and a vertical translation. Results from the joint processing strategy was compared with that using temporal subtraction. Following four-material decomposition, both water images and calcium image show a small bias around 5% from ground truth. The iodine image contains bias primarily concentrated around bone edges. Compared to temporal subtraction, motion artifact from soft tissue is largely eliminated while that from bone is reduced in magnitude. As a result, improvement in motion artifact and iodine visibility is improved across the image. The improvement is especially obvious in the abdomen where soft tissue motion is dominant. We demonstrated in this work that the joint processing strategy is robust against motion artifact in the presence of gross patient motion and can present advantages compared to temporal subtraction, providing further support for the clinical translations of spectral DSA.
Spectral radiography and fluoroscopy with multi-layer flat-panel detectors (FPD) is being actively investigated in a range of clinical applications. For applications involving contrast administration, maximal contrast resolution is achieved when overlaying background anatomy is completely removed. This calls for three-material decomposition (soft tissue, bone, and contrast) enabled by measurements in three energy channels. We have previously demonstrated the feasibility of such decomposition using a triple-layer detector. While algorithmic solutions can be adopted to mitigate noise in the material basis images, in this work, we seek to fundamentally improve the conditioning of the problem through optimized system design. Design parameters include source voltage, the thickness of the top two CsI scintillators, and the thickness of two copper interstitial filters. The design objective is to minimize noise in the basis image containing contrast, chosen as gadolinium in this work to improve separation from bone. The optimized design was compared with other designs with unoptimized scintillator thickness and/or without interstitial filtration. Results show that CsI thickness optimization and interstitial filtration can significantly reduce noise in the gadolinium image by 35.7% and 42.7% respectively within a lung ROI, which in turn boosts detectability of small vessels. Gadolinium and bone signals are separated in all cases. Visualization of coronary vessels is enabled by the combining optimized system design and regularization. Results from this work demonstrate that three-material decomposition can be significantly improved with system design optimization. Optimized designs obtained from this work can inform imaging techniques selection and triple-layer detector fabrication for spectral radiography.
X-ray spectral imaging has been increasingly investigated in radiography and interventional imaging. Flat-panel detectors with more than one detection layer have demonstrated advantages in providing separate soft tissue and bone images. Current dual-layer flat-panel detectors (DL-FPD) have limited capability to further differentiate between iodinated contrast agent and bony/calcified structures. In this work, we investigate a triple-layer flat-panel detector (TL-FPD) and the feasibility of three-material (water/calcium/iodine) decomposition. A physical model of TL-FPD, including system geometry, spectrum sensitivities, blur and noise models was developed. Using simulated triple-layer projections, three-material decompositions were performed using three different processing methods: polynomial-based, model-based, and a machine learning-based method (ResUnet). We find that the polynomial-based method leads to very noisy images with poor differentiation between calcium and iodine maps. The model-based method achieved much lower noise levels than the polynomial- based method but exhibited residual errors between the iodine and calcium channels. The ResUnet method offered the best decompositions among the investigated methods in terms of root mean square error from the ground truth and noise in the material maps. These preliminary results demonstrate the feasibility of three-material decomposition using TL-FPD and suggest a path for clinical translation of single-shot contrast/iodine differentiation in radiography and fluoroscopy.
Deep learning has achieved great success in many medical imaging tasks without explicit solutions. In this work, learning method was applied to dual-energy cone-beam CT imaging. We proposed a Residual W-shape Network (ResWnet). ResWnet consists of three modules: scatter correction module 𝒮, material decomposition module ℳ, decomposition denoising module 𝒟 . Both 𝒮 and 𝒟 use ResWnet architecture, and this lightweight model fuses multi-level features, achieving satisfied performance with a small number of parameters. 𝒮 acts on dual-energy attenuation projections to reduce the scatter contaminations, and 𝒟 acts on material composition projections to suppress the noise. ℳ links the modules 𝒮 and 𝒟, and is used for domain transform from attenuation projections to material projections. This process could be approximated by polynomials with pre-calibrated parameters, that is, ℳ is a known operator in proposed network with no trainable parameters. This helps to reduce model parameters and improve the performance with small training dataset. Using public head CT dataset, we simulated dual-energy cone-beam CT projections and material projections. Proposed ResWnet was trained, validated and tested on this simulated dataset, verifying its effectiveness in projection-domain scatter correction and low-noise decomposition.
Cone-beam CT (CBCT) based cervical brachytherapy (CBCT-BT) is promising to simplify treatment workflow and improve the accuracy of dose delivery. However, severe artifacts in CBCT and its impact on dose calculation should be carefully investigated. In this work, we developed a novel female pelvis phantom dedicated to the cervical brachytherapy, which could be used to evaluate the CBCT-BT performance on imaging accuracy and dose calculation. The phantom dimension and organ position were determined based on Asian female patients. The phantom mainly simulates four parts: adipose, bone, muscle, organs. The first three parts are fixed, and peanut oil, PMMA, POM and PTFE are used to mimic adipose, muscle, cortical bone and cancellous bone respectively. In the muscle, there are four cavities for the insertion of 3D-printed deformable and moveable organs, i.e., vagina and uterus, bladder, intestine, rectum. The vagina and uterus were connected, with a 2 mm diameter elastic channel in it to enable applicator movement. To evaluate the CBCT-BT performance, a standard planning CT (pCT) scan and a CBCT scan were conducted on this phantom, scatter removal algorithm using pCT prior was implemented on the CBCT images. The HU error of muscle, adipose, and organs-at-risk (OARs) in corrected CBCT images were less than 15 HU. Referred to pCT-based plan as baseline, the CBCT-based plan achieved a γ pass rate of >97%. In conclusion, this created phantom successfully simulate both the anatomy structure and the HU numbers of female pelvis, thus provides an effective tool for CBCT-BT evaluation.
Inverse geometry CT(IGCT) employs source array and small detector, which raises requirement for new reconstruction method. Gridding-based method and direct FBP method have been developed for reconstruction, with degraded spatial resolution or computational efficiency. We recently propose a new FBP reconstruction for IGCT that merges the projections from all the sources to the final images using designed weightings. Although this method achieves excellent spatial resolution and is simple in practical use, the necessary zero-padding step that extends the detector to cover the whole scanned object causes increased time consumption in projection filtering and backprojection. In this paper, we propose an accelerated reconstruction for IGCT via derivative back-projection filtration(DBF). Compared with the proposed FBP reconstruction, the DBF reconstruction employ the same weightings, with only changes in the filter kernel, i.e., substitute the local derivative filter for the global ramp filter. Therefore, the DBF reconstruction formula could be obtained via the same derivation as in FBP reconstruction, and the computational efficiency is expected to improve since local filter poses no requirement for zero-padding. However, initial DBF reconstruction present images with strong streaking artifacts, so we further simplify the reconstruction to a stable implementation using the data redundancy. Simulation studies and phantom studies reveal that proposed method present images with comparable numerical accuracy and spatial resolution to the FBP reconstruction. In comparison with FBP, proposed method achieves acceleration with a ratio of approximate 7 for reconstruction of IGCT system with 9 sources.
Purpose: Current dedicated cone-beam breast CT (CBBCT) systems typically adopt a pendant geometry for patient comfort and mechanical compatibility with a breast biopsy device. In this work, we design and construct a prototype system of upright CBBCT with a compact size, such that mammography devices can be readily replaced in current clinical rooms. The system performance on spatial resolution, CT accuracy and field-of-view (FOV) size are evaluated via preliminary phantom studies.
Materials and Methods: The prototype system consists of a mono-block x-ray source, a flat panel detector, a slip ring for signal transmission, a servo motor and a rotating gantry. The relative positions and angles of x-ray source and detector are carefully designed to ensure a large FOV with almost zero dead space on the chest side. The slip ring is used to accommodate a breast stretching device designed to stretch the breast length to match the same imaging volume size as in a prone gesture.
Results: The designed prototype system has a size of 1950mm, 1200mm, and 660mm in height, length and width, respectively. On the Catphan®600 phantom, the acquired CBBCT images have an average CT number error of less than 0.5% and an image non-uniformity of 0.19%. An in-house water phantom with inserted thin tungsten wires is designed to evaluate the system capability on imaging small calcifications. The results show that our system successfully images small high-contrast objects with a diameter of 30 μm.
Conclusion: We have developed a prototype upright CBBCT system with a compact size, a high imaging accuracy and spatial resolution.
Inverse geometry computed tomography (IGCT) uses a small detector and a set of widely distributed x-ray sources. Standard filtered-backprojection (FBP) reconstruction for a conventional CT geometry cannot be directly used in IGCT, due to data truncation and redundancy of the sinograms acquired by different sources. Current IGCT algorithms use gridding or iterations during reconstruction, leading to degraded spatial resolution or increased computational cost. In this work, we propose a direct FBP reconstruction method for IGCT without gridding. A reconstruction algorithm is first derived for a full-size sinogram acquired by a single source with a known offset distance to the central line passing through the rotational axis and perpendicular to the detector. A weighting scheme is then developed on the projections to remove the data redundancy of different sinograms acquired by different sources. The final reconstruction is obtained as the summation of CT images reconstructed from different sources, in a form of FBP on weighted projections. The performance of the proposed algorithm is evaluated via simulation studies on the Shepp-Logan phantom. Results show that our algorithm substantially improves the image spatial resolution over the gridding method. The spatial resolution increases by 32.08% and 23.26% at 50% and 10% of the modulation transfer function, respectively. Finally, we demonstrate the advantages of volumetric IGCT compared with circular cone-beam CT in a pilot study.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.