A filtered backprojection algorithm for cone beam reconstruction using rotational filtering under helical source trajectory

With the evolution from multi-detector-row CT to cone beam (CB) volumetric CT, maintaining reconstruction accuracy becomes more challenging. To combat the severe artifacts caused by a large cone angle in CB volumetric CT, three-dimensional reconstruction algorithms have to be utilized. In practice, filtered backprojection (FBP) reconstruction algorithms are more desirable due to their computational structure and image generation efficiency. One of the CB-FBP reconstruction algorithms is the well-known FDK algorithm that was originally derived for a circular x-ray source trajectory by heuristically extending its two-dimensional (2-D) counterpart. Later on, a general CB-FBP reconstruction algorithm was derived for noncircular, such as helical, source trajectories. It has been recognized that a filtering operation in the projection data along the tangential direction of a helical x-ray source trajectory can significantly improve the reconstruction accuracy of helical CB volumetric CT. However, the tangential filtering encounters latitudinal data truncation, resulting in degraded noise characteristics or data manipulation inefficiency. A CB-FBP reconstruction algorithm using one-dimensional rotational filtering across detector rows (namely CB-RFBP) is proposed in this paper. Although the proposed CB-RFBP reconstruction algorithm is approximate, it approaches the reconstruction accuracy that can be achieved by exact helical CB-FBP reconstruction algorithms for moderate cone angles. Unlike most exact CB-FBP reconstruction algorithms in which the redundant data are usually discarded, the proposed CB-RFBP reconstruction algorithm make use of all available projection data, resulting in significantly improved noise characteristics and dose efficiency. Moreover, the rotational filtering across detector rows not only survives the so-called long object problem, but also avoids latitudinal data truncation existing in other helical CB-FBP reconstruction algorithm in which a tangential filtering is carried out, providing better noise characteristics, dose efficiency and data manipulation efficiency.     

A three-dimensional-weighted cone beam filtered backprojection (CB-FBP) algorithm for image reconstruction in volumetric CT-helical scanning

Based on the structure of the original helical FDK algorithm, a three-dimensional (3D)-weighted cone beam filtered backprojection (CB-FBP) algorithm is proposed for image reconstruction in volumetric CT under helical source trajectory. In addition to its dependence on view and fan angles, the 3D weighting utilizes the cone angle dependency of a ray to improve reconstruction accuracy. The 3D weighting is ray-dependent and the underlying mechanism is to give a favourable weight to the ray with the smaller cone angle out of a pair of conjugate rays but an unfavourable weight to the ray with the larger cone angle out of the conjugate ray pair. The proposed 3D-weighted helical CB-FBP reconstruction algorithm is implemented in the cone-parallel geometry that can improve noise uniformity and image generation speed significantly. Under the cone-parallel geometry, the filtering is naturally carried out along the tangential direction of the helical source trajectory. By exploring the 3D weighting's dependence on cone angle, the proposed helical 3D-weighted CB-FBP reconstruction algorithm can provide significantly improved reconstruction accuracy at moderate cone angle and high helical pitches. The 3D-weighted CB-FBP algorithm is experimentally evaluated by computer-simulated phantoms and phantoms scanned by a diagnostic volumetric CT system with a detector dimension of 64 x 0.625 mm over various helical pitches. The computer simulation study shows that the 3D weighting enables the proposed algorithm to reach reconstruction accuracy comparable to that of exact CB reconstruction algorithms, such as the Katsevich algorithm, under a moderate cone angle (4 degrees) and various helical pitches. Meanwhile, the experimental evaluation using the phantoms scanned by a volumetric CT system shows that the spatial resolution along the z-direction and noise characteristics of the proposed 3D-weighted helical CB-FBP reconstruction algorithm are maintained very well in comparison to the FDK-type algorithms. Moreover, the experimental evaluation by clinical data verifies that the proposed 3D-weighted CB-FBP algorithm for image reconstruction in volumetric CT under helical source trajectory meets the challenges posed by diagnostic applications of volumetric CT imaging.     

A three-dimensional weighted cone beam filtered backprojection (CB-FBP) algorithm for image reconstruction in volumetric CT under a circular source trajectory

The original FDK algorithm proposed for cone beam (CB) image reconstruction under a circular source trajectory has been extensively employed in medical and industrial imaging applications. With increasing cone angle, CB artefacts in images reconstructed by the original FDK algorithm deteriorate, since the circular trajectory does not satisfy the so-called data sufficiency condition (DSC). A few 'circular plus' trajectories have been proposed in the past to help the original FDK algorithm to reduce CB artefacts by meeting the DSC. However, the circular trajectory has distinct advantages over other scanning trajectories in practical CT imaging, such as head imaging, breast imaging, cardiac, vascular and perfusion applications. In addition to looking into the DSC, another insight into the CB artefacts existing in the original FDK algorithm is the inconsistency between conjugate rays that are 180 degrees apart in view angle (namely conjugate ray inconsistency). The conjugate ray inconsistency is pixel dependent, varying dramatically over pixels within the image plane to be reconstructed. However, the original FDK algorithm treats all conjugate rays equally, resulting in CB artefacts that can be avoided if appropriate weighting strategies are exercised. Along with an experimental evaluation and verification, a three-dimensional (3D) weighted axial cone beam filtered backprojection (CB-FBP) algorithm is proposed in this paper for image reconstruction in volumetric CT under a circular source trajectory. Without extra trajectories supplemental to the circular trajectory, the proposed algorithm applies 3D weighting on projection data before 3D backprojection to reduce conjugate ray inconsistency by suppressing the contribution from one of the conjugate rays with a larger cone angle. Furthermore, the 3D weighting is dependent on the distance between the reconstruction plane and the central plane determined by the circular trajectory. The proposed 3D weighted axial CB-FBP algorithm can be implemented in either the native CB geometry or the so-called cone-parallel geometry. By taking the cone-parallel geometry as an example, the experimental evaluation shows that, up to a moderate cone angle corresponding to a detector dimension of 64 x 0.625 mm, the CB artefacts can be substantially suppressed by the proposed algorithm, while advantages of the original FDK algorithm, such as the filtered backprojection algorithm structure, 1D ramp filtering and data manipulation efficiency, are maintained.     

The effect of z overscanning on patient effective dose from multidetector helical computed tomography examinations

z overscanning in multidetector (MD) helical CT scanning is prerequisite for the interpolation of acquired data required during image reconstruction and refers to the exposure of tissues beyond the boundaries of the volume to be imaged. The aim of the present study was to evaluate the effect of z overscanning on the patient effective dose from helical MD CT examinations. The Monte Carlo N-particle radiation transport code was employed in the current study to simulate CT exposure. The validity of the Monte Carlo simulation was verified by (a) a comparison of calculated and measured standard computed tomography dose index (CTDI) dosimetric data, and (b) a comparison of calculated and measured dose profiles along the z axis. CTDI was measured using a pencil ionization chamber and head and body CT phantoms. Dose profiles along the z axis were obtained using thermoluminescence dosimeters. A commercially available mathematical anthropomorphic phantom was used for the estimation of effective doses from four standard CT examinations, i.e., head and neck, chest, abdomen and pelvis, and trunk studies. Data for both axial and helical modes of operation were obtained. In the helical mode, z overscanning was taken into account. The calculated effective dose from a CT exposure was normalized to CTDI(free in air). The percentage differences in the normalized effective dose between contiguous axial and helical scans with pitch = 1, may reach 13.1%, 35.8%, 29.0%, and 21.5%, for head and neck, chest, abdomen and pelvis, and trunk studies, respectively. Given that the same kilovoltage and tube load per rotation were used in both axial and helical scans, the above differences may be attributed to z overscanning. For helical scans with pitch = 1, broader beam collimation is associated with increased z overscanning and consequently higher normalized effective dose value, when other scanning parameters are held constant. For a given beam collimation, the selection of a higher value of reconstructed image slice width increases the normalized effective dose. In conclusion, z overscanning may significantly affect the patient effective dose from CT examinations performed on MD CT scanners. Therefore, an estimation of the patient effective dose from MD helical CT examinations should always take into consideration the effect of z overscanning.     

Matched view weighting in tilted-plane-based reconstruction algorithms to suppress helical artifacts and optimize noise characteristics

In multi-slice helical CT, the single-tilted-plane-based reconstruction algorithm has been proposed to combat helical and cone beam artifacts by tilting a reconstruction plane to fit a helical source trajectory optimally. Furthermore, to improve the noise characteristics or dose efficiency of the single-tilted-plane-based reconstruction algorithm, the multi-tilted-plane-based reconstruction algorithm has been proposed, in which the reconstruction plane deviates from the pose globally optimized due to an extra rotation along the 3rd axis. As a result, the capability of suppressing helical and cone beam artifacts in the multi-tilted-plane-based reconstruction algorithm is compromised. A matched view weighting approach is proposed here to optimize the capability of suppressing helical and cone beam artifacts and noise characteristics. A helical body phantom is employed to quantitatively evaluate the imaging performance of the matched view weighting approach, showing that the matched view weighting improves both the helical artifact suppression and noise characteristics or dose efficiency significantly in comparison to the case in which nonmatched view weighting is applied. The matched view weighting approach is of practical importance in the development of multi-slice helical CT, because it maintains the computational structure of fan beam filtered back-projection and demands no extra computational resources.     

High-quality 3D correction of ring and radiant artifacts in flat panel detector-based cone beam volume CT imaging

The use of an x-ray flat panel detector is increasingly becoming popular in 3D cone beam volume CT machines. Due to the deficient semiconductor array manufacturing process, the cone beam projection data are often corrupted by different types of abnormalities, which cause severe ring and radiant artifacts in a cone beam reconstruction image, and as a result, the diagnostic image quality is degraded. In this paper, a novel technique is presented for the correction of error in the 2D cone beam projections due to abnormalities often observed in 2D x-ray flat panel detectors. Template images are derived from the responses of the detector pixels using their statistical properties and then an effective non-causal derivative-based detection algorithm in 2D space is presented for the detection of defective and mis-calibrated detector elements separately. An image inpainting-based 3D correction scheme is proposed for the estimation of responses of defective detector elements, and the responses of the mis-calibrated detector elements are corrected using the normalization technique. For real-time implementation, a simplification of the proposed off-line method is also suggested. Finally, the proposed algorithms are tested using different real cone beam volume CT images and the experimental results demonstrate that the proposed methods can effectively remove ring and radiant artifacts from cone beam volume CT images compared to other reported techniques in the literature.     

Influence of z overscanning on normalized effective doses calculated for pediatric patients undergoing multidetector CT examinations

The purpose of this study was to evaluate the effect of z overscanning on normalized effective dose for pediatric patients undergoing multidetector-computed tomography (CT) examinations. Five commercially available mathematical anthropomorphic phantoms representing newborn, 1-, 5-, 10-, and 15-year-old patients and the Monte Carlo N-Particle (MCNP, version 4C2) radiation transport code were employed in the current study to simulate pediatric CT exposures. For all phantoms, axial and helical examinations at 120 kV tube voltage were simulated. Scans performed at 80 kV were also simulated. Sex-specific normalized effective doses were estimated for four standard CT examinations i.e., head-neck, chest, abdomen-pelvis, and trunk, for all pediatric phantoms. Data for both axial and helical mode acquisition were obtained. In the helical mode, z overscanning was taken into account. The validity of the Monte Carlo results was verified by comparison with dose data obtained using thermoluminescence dosimetry and a physical pediatric anthropomorphic phantom simulating a 10-year-old child. In all cases normalized effective dose values were found to increase with increasing z overscanning. The percentage differences in normalized data between axial and helical scans may reach 43%, 70%, 36%, and 26% for head-neck, chest, abdomen-pelvis, and trunk studies, respectively. Normalized data for female pediatric patients was in general higher compared to male patients for all ages, examined regions, and z overscanning values. For both male and female children, the normalized effective dose values were reduced as the age was increased. For the same typical exposure conditions, dose values decreased when lower tube voltage was used; for a 1-year-old child, for example, the effective dose was 3.8 times lower when 80 kV instead of 120 kV was used. Normalized data for the estimation of effective dose to pediatric patients undergoing standard axial and helical CT examinations on an multidetector CT system were calculated. This data was found to depend strongly on CT acquisition mode and exposure parameters as well as patient age and sex. The effective dose from a pediatric CT scan performed in axial mode was always considerably lower compared to the corresponding scan performed in helical mode, due to the additional tissue regions exposed to the primary beam in helical examinations as a result of z overscanning.     

Helical cardiac cone beam CT reconstruction with large area detectors: a simulation study

Retrospectively gated cardiac volume CT imaging has become feasible with the introduction of heart rate adaptive cardiac CT reconstruction algorithms. The development in detector technology and the rapid introduction of multi-row detectors has demanded reconstruction schemes which account for the cone geometry. With the extended cardiac reconstruction (ECR) framework, the idea of approximate helical cone beam CT has been extended to be used with retrospective gating, enabling heart rate adaptive cardiac cone beam reconstruction. In this contribution, the ECR technique is evaluated for systems with an increased number of detector rows, which leads to larger cone angles. A simulation study has been carried out based on a 4D cardiac phantom consisting of a thorax model and a dynamic heart insert. Images have been reconstructed for different detector set-ups. Reconstruction assessment functions have been calculated for the detector set-ups employing different rotation times, relative pitches and heart rates. With the increased volume coverage of large area detector systems, low-pitch scans become feasible without resulting in extensive scan times, inhibiting single breath hold acquisitions. ECR delivers promising image results when being applied to systems with larger cone angles.     

A solution to the long-object problem in helical cone-beam tomography

This paper presents a new algorithm for the long-object problem in helical cone-beam (CB) computerized tomography (CT). This problem consists in reconstructing a region-of-interest (ROI) bounded by two given transaxial slices, using axially truncated CB projections corresponding to a helix segment long enough to cover the ROI, but not long enough to cover the whole axial extent of the object. The new algorithm is based on a previously published method, referred to as CB-FBP (Kudo et al 1998 Phys. Med. Biol. 43 2885-909), which is suitable for quasi-exact reconstruction when the helix extends well beyond the support of the object. We first show that the CB-FBP algorithm simplifies dramatically, and furthermore constitutes a solution to the long-object problem, when the object under study has line integrals which vanish along all PI-lines. (A PI line is a line which connects two points of the helix separated by less than one pitch.) Exploiting a geometric property of the helix, we then show how the image can be expressed as the sum of two images, where the first image can be reconstructed from the measured CB projections by a simple backprojection procedure, and the second image has zero PI-line integrals and hence can be reconstructed using the simplified CB-FBP algorithm. The resulting method is a quasi-exact solution to the long-object problem, called the ZB method. We present its implementation and illustrate its performance using simulated CB data of the 3D Shepp phantom and of a more challenging head-like phantom.     

High temporal resolution for multislice helical computed tomography

Multislice helical computed tomography (CT) substantially reduces scanning time. However, the temporal resolution of individual images is still insufficient for imaging rapidly moving organs such as the heart and adjacent pulmonary vessels. It may, in some cases, be worse than with current single-slice helical CT. The purpose of this study is to describe a novel image reconstruction algorithm to improve temporal resolution in multislice helical CT, and to evaluate its performance against existing algorithms. The proposed image reconstruction algorithm uses helical interpolation followed by data weighting based on the acquisition time. The temporal resolution, the longitudinal (z-axis) spatial resolution, the image noise, and the in-plane image artifacts created by a moving phantom were compared with those from the basic multislice helical reconstruction (helical filter interpolation, HFI) algorithm and the basic single-slice helical reconstruction algorithm (180 degrees linear interpolation, 180LI) using computer simulations. Computer simulation results were verified with CT examinations of the heart and lung vasculature using a 0.5 second multislice scanner. The temporal resolution of HFI algorithm varies from 0.28 and 0.86 s, depending on helical pitch. The proposed method improves the resolution to a constant value of 0.29 s, independent of pitch, allowing moving objects to be imaged with reduced blurring or motion artifacts. The spatial (z) resolution was slightly worse than with the HFI algorithm; the image noise was worse than with the HFI algorithm but was comparable to axial (step-and-shoot) CT. The proposed method provided sharp images of the moving objects, portraying the anatomy accurately. The proposed algorithm for multislice helical CT allowed us to obtain CT images with high temporal resolution. It may improve the image quality of clinical cardiac, lung, and vascular CT imaging.     

A prototype table-top inverse-geometry volumetric CT system

A table-top volumetric CT system has been implemented that is able to image a 5-cm-thick volume in one circular scan with no cone-beam artifacts. The prototype inverse-geometry CT (IGCT) scanner consists of a large-area, scanned x-ray source and a detector array that is smaller in the transverse direction. The IGCT geometry provides sufficient volumetric sampling because the source and detector have the same axial, or slice direction, extent. This paper describes the implementation of the table-top IGCT scanner, which is based on the NexRay Scanning-Beam Digital X-ray system (NexRay, Inc., Los Gatos, CA) and an investigation of the system performance. The alignment and flat-field calibration procedures are described, along with a summary of the reconstruction algorithm. The resolution and noise performance of the prototype IGCT system are studied through experiments and further supported by analytical predictions and simulations. To study the presence of cone-beam artifacts, a "Defrise" phantom was scanned on both the prototype IGCT scanner and a micro CT system with a +/-5 cone angle for a 4.5-cm volume thickness. Images of inner ear specimens are presented and compared to those from clinical CT systems. Results showed that the prototype IGCT system has a 0.25-mm isotropic resolution and that noise comparable to that from a clinical scanner with equivalent spatial resolution is achievable. The measured MTF and noise values agreed reasonably well with theoretical predictions and computer simulations. The IGCT system was able to faithfully reconstruct the laminated pattern of the Defrise phantom while the micro CT system suffered severe cone-beam artifacts for the same object. The inner ear acquisition verified that the IGCT system can image a complex anatomical object, and the resulting images exhibited more high-resolution details than the clinical CT acquisition. Overall, the successful implementation of the prototype system supports the IGCT concept for single-rotation volumetric scanning free from cone-beam artifacts.     

Helical cardiac cone beam reconstruction using retrospective ECG gating

In modern computer tomography (CT) systems, the fast rotating gantry and the increased detector width enable 3D imaging of the heart. Cardiac volume CT has a high potential for non-invasive coronary angiography with high spatial resolution and short scan time. Due to the increased detector width, true cone beam reconstruction methods are needed instead of adapted 2D reconstruction schemes. In this paper, the extended cardiac reconstruction method is introduced. It integrates the idea of retrospectively gated cardiac reconstruction for helical data acquisition into a cone beam reconstruction framework. It leads to an efficient and flexible algorithmic scheme for the reconstruction of single- and multi-phase cardiac volume datasets. The method automatically adapts the number of cardiac cycles used for the reconstruction. The cone beam geometry is fully taken into account during the reconstruction process. Within this paper, results are presented on patient datasets which have been acquired using a 16-slice cone beam CT system.     

X-ray scattering in single- and dual-source CT

For medical imaging applications, such as cardiac imaging, dual-source computed tomography (CT) improves the temporal resolution by the simultaneous use of two cone beams, which acquire twice as many projections as single-source CT does within the same time interval. Besides this advantage, a drawback of such a system is additional x-ray scatter originating from the extra (cross-illuminating) cone beam. In this work, a comparison with single-source CT images is performed under same-dose conditions for two different thorax phantoms, and for different cone beam angles corresponding to a coverage of 20, 40, 80, and 160 mm on the rotation axis (z coverage). As a general result, the HU-magnitude of scatter-induced streak and cupping artifacts scale almost proportional to the illuminated volume. In dual-source CT, cross scatter induces a further factor of almost 2 in the scaling of artifacts in comparison to single-source CT. For all examined systems, the scatter-induced noise reduces the contrast-to-noise ratio (CNR). In the case of an ideal scatter correction, the CNR is reduced even more, but contrast and CNR can be restored by an additional x-ray dose. With a 32-slice single-source CT (z overage of 20 mm) taken as a reference, a corresponding dual-source CT requires 7% more dose to maintain the same CNR. A CT system with a z coverage of 40, 80, and 160 mm requires 8%, 23%, and 54% more dose in a single-source configuration, respectively, and 20%, 47%, and 102% more dose in a dual-source configuration, respectively. In conclusion, a dual-source CT is comparable to a single-source CT with twice the z coverage concerning image degradation by scatter.     

Adaptive temporal resolution optimization in helical cardiac cone beam CT reconstruction

Cone beam computed tomography scanners in combination with heart rate adaptive reconstruction schemes have the potential to enable cardiac volumetric computed tomography (CT) imaging for a larger number of patients and applications. In this publication, an adaptive scheme for the automatic and patient-specific reconstruction optimization is introduced to improve the temporal resolution and image quality. The optimization method permits the automatic determination of the required amount of gated helical cone beam projection data for the reconstruction volume. It furthermore allows one to optimize subvolume reconstruction yielding an increased temporal resolution. In addition, methods for the assessment of the temporal resolution are given which enable a quantitative documentation of the reconstruction improvements. Results are presented for patient data sets acquired in low pitch helical mode using a 16-slice cone beam CT system with parallel ECG recording.     

Performance evaluation of a multi-slice CT system

Our purpose in this study was to characterize the performance of a recently introduced multi-slice CT scanner (LightSpeed QX/i, Version 1.0, General Electric Medical Systems) in comparison to a single-slice scanner from the same manufacturer (HiSpeed CT/i, Version 4.0). To facilitate this comparison, a refined definition of pitch is introduced which accommodates multi-slice CT systems, yet maintains the existing relationships between pitch, patient dose, and image quality. The following performance parameters were assessed: radiation and slice sensitivity profiles, low-contrast and limiting spatial resolution, image uniformity and noise, CT number and geometric accuracy, and dose. The multi-slice system was tested in axial (1, 2, or 4 images per gantry rotation) and HQ (Pitch = 0.75) and HS (Pitch = 1.5) helical modes. Axial and helical acquisition speed and limiting spatial resolution (0.8-s exposure) were improved on the multi-slice system. Slice sensitivity profiles, image noise, CT number accuracy and uniformity, and low-contrast resolution were similar. In some HS-helical modes, helical artifacts and geometric distortion were more pronounced with a different appearance. Radiation slice profiles and doses were larger on the multi-slice system at all scan widths. For a typical abdomen and pelvis exam, the central and surface body doses for 5-mm helical scans were higher on the multi-slice system by approximately 50%. The increase in surface CTDI values (with respect to the single-slice system) was greatest for the 4 x 1.25-mm detector configuration (190% for head, 240% for body) and least for the 4 x 5-mm configuration (53% for head, 76% for body). Preliminary testing of version 1.1 software demonstrated reduced doses on the multi-slice scanner, where the increase in body surface CTDI values (with respect to the single-slice system) was 105% for the 4 x 1.25-mm detector configuration and 10% for the 4 x 5-mm configuration. In summary, the axial and HQ-helical modes of the multi-slice system provided excellent image quality and a substantial reduction in exam time and tube loading, although at varying degrees of increased dose relative to the single-slice scanner.     

常规头部CT螺旋扫描与轴扫描模式的影像质量对照

目的通过头部CT螺旋扫描和轴扫描两种不同扫描模式的影像质量对照,探讨常规头部CT采用螺旋扫描模式取代轴扫描模式的可行性。方法 40例患者,其中男性24例,女性16例;年龄18～74岁,平均年龄46岁。分别接受头部CT螺旋扫描和轴扫描两种不同模式扫描,轴扫描参数:140kV、170mA用于颅底部;120kV、150mA用于颅脑部,每周2s。螺旋扫描参数:120kV,150mA,每周0.6s。扫描剂量直接取自设备剂量指示值(CTDI和DLP)。影像质量分别由2位专家采用双盲法,对头部CT的颅底、脑灰-白质层面及全部影像逐层进行CT值测量评估。结果常规头部CT螺旋扫描模式较轴扫描模式可有效去除颅底部伪影,并能减少扫描时间和扫描剂量(轴扫描模式与螺旋扫描模式对比,CTDI 55.43 mGy/37.80 mGy;DLP 662.10 mGy.cm/498.28 mGy.cm;扫描时间12.0 s/7.2 s)(P<0.001)。结论常规头部CT螺旋扫描模式的影像质量完全可与轴扫描模式相媲美,并可取代轴扫描模式。同时常规头部CT螺旋扫描模式还具有可减少扫描时间和辐射剂量,并可进行影像的二维和三维后处理,有利于提高微小病灶正确诊断率的优越性。     

Spiral scan long object reconstruction through PI line reconstruction

The response of a point object in a cone beam (CB) spiral scan is analysed. Based on the result, a reconstruction algorithm for long object imaging in spiral scan cone beam CT is developed. A region-of-interest (ROI) of the long object is scanned with a detector smaller than the ROI, and a portion of it can be reconstructed without contamination from overlaying materials. The top and bottom surfaces of the ROI are defined by two sets of PI lines near the two ends of the spiral path. With this novel definition of the top and bottom ROI surfaces and through the use of projective geometry, it is straightforward to partition the cone beam image into regions corresponding to projections of the ROI, the overlaying objects or both. This also simplifies computation at source positions near the spiral ends, and makes it possible to reduce radiation exposure near the spiral ends substantially through simple hardware collimation. Simulation results to validate the algorithm are presented.     

Cardiac cone-beam CT volume reconstruction using ART

Modern computed tomography systems allow volume imaging of the heart. Up to now, approximately two-dimensional (2D) and 3D algorithms based on filtered backprojection are used for the reconstruction. These algorithms become more sensitive to artifacts when the cone angle of the x-ray beam increases as it is the current trend of computed tomography (CT) technology. In this paper, we investigate the potential of iterative reconstruction based on the algebraic reconstruction technique (ART) for helical cardiac cone-beam CT. Iterative reconstruction has the advantages that it takes the cone angle into account exactly and that it can be combined with retrospective cardiac gating fairly easily. We introduce a modified ART algorithm for cardiac CT reconstruction. We apply it to clinical cardiac data from a 16-slice CT scanner and compare the images to those obtained with a current analytical reconstruction method. In a second part, we investigate the potential of iterative reconstruction for a large area detector with 256 slices. For the clinical cases, iterative reconstruction produces excellent images of diagnostic quality. For the large area detector, iterative reconstruction produces images superior to analytical reconstruction in terms of cone-beam artifacts.     

治疗计划系统中CT值及图像质量保证的若干问题讨论

目的：了解CT扫描时影响CT值的相关务件；讨论不同扫描参数扫描得到的CT重建图像可能产生的误差；对三维治疗计划系统使用的CT值及图像的质量保证的若干问题进行探讨。材料和方法：对Siemens Somatom Plus4螺旋CT设置不同扫描电压和电流，测量Gammex的RMI467体模中不同材料的密度值与CT值关系。测量CT孔径位置以及高密度材料治疗床板可能对CT值产生的影响。测量包含不同浓度造影剂的水溶液的CT值及相应的等效密度，初步对鼻咽癌病人加入造影剂前后的CT图像相对剂量计算的影响进行了估计。扫描一包含空腔的有机玻璃板，改变扫描层厚和螺距，对比ADAC三维治疗计划系统重建的CT图像与真实形态的差别。此外对使用的数字化仪以及打印设备进行了图像输入和输出的检查，检查了系统的文字和图像信息的登记和输出。结果：给出了不同电压和电流下CT值与密度的关系：对造影剂、CT治疗床等相关因素对CT值的影响给出了定量讨论；对有机玻璃模体的扫描结果表明：由于部分容积效应和阶梯状伪影的影响，CT图像中显示的空腔形态发生了改变。其他对治疗计划系统非剂量学质量保证措施也在计划系统使用前进行了检查。结论：对于三维治疗计划系统，正确输入密度—CT值是保证剂量计算正确的条件之一；了解CT扫描时可能带来的图像误差可以有效减小靶区勾画的误差；三维治疗计划系统使用前必须有详细的质量保证。