State of the art: iterative CT reconstruction techniques

Owing to recent advances in computing power, iterative reconstruction (IR) algorithms have become a clinically viable option in computed tomographic (CT) imaging. Substantial evidence is accumulating about the advantages of IR algorithms over established analytical methods, such as filtered back projection. IR improves image quality through cyclic image processing. Although all available solutions share the common mechanism of artifact reduction and/or potential for radiation dose savings, chiefly due to image noise suppression, the magnitude of these effects depends on the specific IR algorithm. In the first section of this contribution, the technical bases of IR are briefly reviewed and the currently available algorithms released by the major CT manufacturers are described. In the second part, the current status of their clinical implementation is surveyed. Regardless of the applied IR algorithm, the available evidence attests to the substantial potential of IR algorithms for overcoming traditional limitations in CT imaging

Fast Monte Carlo Simulation for Patient-specific CT/CBCT Imaging Dose Calculation

Recently, X-ray imaging dose from computed tomography (CT) or cone beam CT (CBCT) scans has become a serious concern. Patient-specific imaging dose calculation has been proposed for the purpose of dose management. While Monte Carlo (MC) dose calculation can be quite accurate for this purpose, it suffers from low computational efficiency. In response to this problem, we have successfully developed a MC dose calculation package, gCTD, on GPU architecture under the NVIDIA CUDA platform for fast and accurate estimation of the x-ray imaging dose received by a patient during a CT or CBCT scan. Techniques have been developed particularly for the GPU architecture to achieve high computational efficiency. Dose calculations using CBCT scanning geometry in a homogeneous water phantom and a heterogeneous Zubal head phantom have shown good agreement between gCTD and EGSnrc, indicating the accuracy of our code. In terms of improved efficiency, it is found that gCTD attains a speed-up of ~400 times in the homogeneous water phantom and ~76.6 times in the Zubal phantom compared to EGSnrc. As for absolute computation time, imaging dose calculation for the Zubal phantom can be accomplished in ~17 sec with the average relative standard deviation of 0.4%. Though our gCTD code has been developed and tested in the context of CBCT scans, with simple modification of geometry it can be used for assessing imaging dose in CT scans as well.Comment: 18 pages, 7 figures, and 1 tabl

Optimizing computed tomography : quality assurance, radiation dose and contrast media

Computed tomography (CT) is an important modality in radiology; it enables imaging of the inside of patients without superimposed anatomy. The radiation dose and quality of a CT image are highly dependent on the CT scanner, the scan settings and, if applicable, the timing and dosage of the intravenous contrast media (CM). The aim of this Thesis was to develop tools and insights that help maximize the value of examinations for patients undergoing CT and to reduce its cost in terms of radiation and CM dose. The Thesis consists of five studies. The first paper was on quality control (QC) of CT, which is the foundation for a radiology clinic: it provides trust that the equipment functions as expected. A new method of performing routine QCs was proposed where the concept of key performance indicators (KPI) was introduced, together with a semi-automatic process allowing for daily QCs. During the time of the study, multiple deviations were discovered that would have been difficult to detect using traditional QCs. Performing QCs more frequently facilitates more extensive trend analysis. The second paper was on automatic tube current modulation (ATCM). A phantom and a method for the characterization of ATCM were developed. These allowed for a characterization of CT scanners from the four main CT vendors in Sweden, summarized in four extensive tables showing how the ATCM responds to changes in scan parameters. More specifically, the tables present how changes in scan settings of the localizer radiograph (LR), scan settings of the acquisition, reconstruction parameters and patient miscentering affect the ATCM. The third paper was on radiation dose estimation uncertainties coupled to the patient table. In most commercial radiation dose estimation software packages for CT, the patient table is not included. That effect was previously unknown but could be shown using Monte Carlo (MC) calculations of CT scans performed with and without the patient table. It was shown that by not including the effect from the patient table in radiation dose estimations, the radiation doses are overestimated by 5% to 23%, depending on the scan mode. The fourth paper evaluated whether the standard LR can be replaced by a low-dose spiral scan, a so-called synthetic LR (SLR). Such an SLR can potentially improve ATCM, CM dosage and CT planning. Radiation doses were estimated using MC, the image quality was compared in a prospective study of ten patients and the impact of miscentering was investigated with a phantom measurement of water equivalent diameters. It was shown that the radiation doses and the image quality of SLR and LRs were similar. Estimated water equivalent diameters were more consistent when calculated from the low-dose spiral scan compared to the LRs. It was concluded that it is feasible to replace the traditional LR with an SLR for CT scan planning. The fifth paper was a continued investigation of the low-dose spiral scan, but with focus on intravenous CM dosage planning. Altogether, 238 patients who had undergone PET/CT and ii for whom body metrics (height and weight) had been acquired were retrospectively analyzed, the CT number enhancement of the liver was measured, and body volumes of muscle and fat were calculated using the attenuation correction CT (low-dose spiral scan). Multiple linear regressions showed that for CM dose planning, the body volumes of muscle and fat are better to use than body weight. However, the adjusted R2 values of all the investigated models were low, indicating that responses to CM dosage are complex and require more research. In this PhD Thesis, tools and insights were developed to improve the imaging stability of the CT scan by developing semi-automatic QC protocols and techniques to better estimate patient size and shape potentially reducing variation in image quality, radiation dose and CM enhancement among patients

Current status and new horizons in Monte Carlo simulation of X-ray CT scanners

With the advent of powerful computers and parallel processing including Grid technology, the use of Monte Carlo (MC) techniques for radiation transport simulation has become the most popular method for modeling radiological imaging systems and particularly X-ray computed tomography (CT). The stochastic nature of involved processes such as X-ray photons generation, interaction with matter and detection makes MC the ideal tool for accurate modeling. MC calculations can be used to assess the impact of different physical design parameters on overall scanner performance, clinical image quality and absorbed dose assessment in CT examinations, which can be difficult or even impossible to estimate by experimental measurements and theoretical analysis. Simulations can also be used to develop and assess correction methods and reconstruction algorithms aiming at improving image quality and quantitative procedures. This paper focuses mainly on recent developments and future trends in X-ray CT MC modeling tools and their areas of application. An overview of existing programs and their useful features will be given together with recent developments in the design of computational anthropomorphic models of the human anatomy. It should be noted that due to limited space, the references contained herein are for illustrative purposes and are not inclusive; no implication that those chosen are better than others not mentioned is intende

Reducing Radiation Dose to the Female Breast during CT Coronary Angiography: A Simulation Study Comparing Breast Shielding, Angular Tube Current Modulation, Reduced kV, and Partial Angle Protocols Using an Unknown-location Signal-detectability Metric

Purpose: The authors compared the performance of five protocols intended to reduce dose to the breast during computed tomography (CT) coronary angiography scans using a model observer unknown-location signal-detectability metric. Methods: The authors simulated CT images of an anthropomorphic female thorax phantom for a 120 kV reference protocol and five “dose reduction” protocols intended to reduce dose to the breast: 120 kV partial angle (posteriorly centered), 120 kV tube-current modulated (TCM), 120 kV with shielded breasts, 80 kV, and 80 kV partial angle (posteriorly centered). Two image quality tasks were investigated: the detection and localization of 4-mm, 3.25 mg/ml and 1-mm, 6.0 mg/ml iodine contrast signals randomly located in the heart region. For each protocol, the authors plotted the signal detectability, as quantified by the area under the exponentially transformed free response characteristic curve estimator (AˆFE), as well as noise and contrast-to-noise ratio (CNR) versus breast and lung dose. In addition, the authors quantified each protocol\u27s dose performance as the percent difference in dose relative to the reference protocol achieved while maintaining equivalentAˆFE. Results: For the 4-mm signal-size task, the 80 kV full scan and 80 kV partial angle protocols decreased dose to the breast (80.5% and 85.3%, respectively) and lung (80.5% and 76.7%, respectively) withAˆFE= 0.96, but also resulted in an approximate three-fold increase in image noise. The 120 kV partial protocol reduced dose to the breast (17.6%) at the expense of increased lung dose (25.3%). The TCM algorithm decreased dose to the breast (6.0%) and lung (10.4%). Breast shielding increased breast dose (67.8%) and lung dose (103.4%). The 80 kV and 80 kV partial protocols demonstrated greater dose reductions for the 4-mm task than for the 1-mm task, and the shielded protocol showed a larger increase in dose for the 4-mm task than for the 1-mm task. In general, the CNR curves indicate a similar relative ranking of protocol performance as the correspondingAˆFEcurves, however, the CNR metric overestimated the performance of the shielded protocol for both tasks, leading to corresponding underestimates in the relative dose increases compared to those obtained when using theAˆFEmetric. Conclusions: The 80 kV and 80 kV partial angle protocols demonstrated the greatest reduction to breast and lung dose, however, the subsequent increase in image noise may be deemed clinically unacceptable. Tube output for these protocols can be adjusted to achieve a more desirable noise level with lesser breast dose savings. Breast shielding increased breast and lung dose when maintaining equivalentAˆFE. The results demonstrated that comparisons of dose performance depend on both the image quality metric and the specific task, and that CNR may not be a reliable metric of signal detectability

Detection of liver metastases under 2cm: comparison of different acquisition protocols in four row multidetector-CT (MDCT)

This study compared different acquisition protocols performance to detect small liver metastases (<2cm). Thirty consecutive patients with histologically proven hepatic metastases were explored by MDCT at the liver equilibrium phase by four successive acquisitions. We compared the following protocols (1-4): 5/30/1.5 (section thickness/table speed/pitch); 5/15/0.75; 5/11.25/0.75; and 2.5/15/1.5 with the same X-ray dose. The gold standard was based on patient radiological follow-up. Evolutive lesions were considered as true positive (TP). The described lesions, not found on the follow-up exams despite tumoral progression, were considered as false positive (FP). Stable lesions could not be considered as metastasis and were eliminated. One hundred and seventy-six lesions were detected: 61 TP and 91 FP. Twenty-four lesions were eliminated. The mean kappa values for protocols 1, 2, 3 and 4 were, respectively, 0.43, 0.68, 0.73 and 0.51 (0.61-0.80: substantial agreement) and the mean areas under the ROC curve were, respectively, 0.76, 0.87, 0.86 and 0.80. The results of protocols 2 and 3 were significantly superior to those of protocols 1 and 4. MDCT protocols using thin sections or an increased table speed are less efficient in detecting small metastase

The “Knowledgeable” CT Scanner: Optimization by technological advancements

Optimization of a computed tomography (CT) exam can be challenging, as there is a wide variety in patient characteristics, continuously emerging technologies and the inc

The Optimisation of Routine Paediatric CT Scanning Protocols

This study represents one of the first comprehensive paediatric optimisation processes that were systematically approached via three main objectives. It is expected that the devised optimisation paediatric protocols could have profound implications on clinical practices that use 64-slice CT or greater. The study provides a better understanding of the influence of each acquisition reconstruction parameter with regards to image quality and radiation dose during routine paediatric imaging protocols