Search

6.1 System Overview Before giving a detailed analysis and description of major components in a CT scanner, this chapter will present a system overview to explain how the different components work together to produce CT images. Figure 6.1 presents a generic block diagram of a CT system. The actual system architecture for different commercial scanners may deviate from this diagram, but the general functionalities of all CT scanners are more or less the same. For a typical CT operation, an operator positions a patient on the CT table and prescribes a scanogram or “scout view.” The purpose of this scan is to determine the patient's anatomical landmarks and the exact location and range of CT scans. In this scan mode, both the x-ray tube and the detector remain stationary while the patient table travels at a constant speed. The scan is similar to a conventional x ray taken either at an A-P position (with the tube located in the 6 or 12 o'clock position) or a lateral position (with the tube located in the 3 or 9 o'clock position). Once such a scan is initiated, an operational control computer instructs the gantry to rotate to the desired orientation as prescribed by the operator. The computer then sends instructions to the patient table, the x-ray generation system, the x-ray detection system, and the image generation system to perform a scan. The table subsequently reaches the starting scan location and maintains a constant speed during the entire scanning process. The high-voltage generator quickly reaches the desired voltage and keeps both the voltage and the current to the x-ray tube at the prescribed level during the scan. The x-ray tube produces x-ray flux, and the x-ray photons are detected by an x-ray detector to produce electrical signals. At the same time, the data acquisition system samples the detector outputs at a uniform sampling rate and converts analog signals to digital signals. The sampled data are then sent to the image generation system for processing. Typically, the system contains high-speed computers and digital signal processing (DSP) chips. The acquired data are preprocessed and enhanced before being sent to the display device for operator viewing and to the data storage device for archiving.

9.1 Introduction Helical CT (also called spiral CT) was introduced commercially in the late 1980s and early 1990s. Helical CT has expanded the traditional CT capability by enabling the scan of an entire organ in a single breath-hold. It is safe to state that helical CT is one of the key steps that moved CT from a slice-oriented imaging modality to an organ-oriented modality. The difference in the naming convention between helical and spiral CT is due mainly to different CT manufacturers. For all practical and technical purposes, there is no difference between the two. To avoid confusion, we will use the term “helical” throughout this chapter. 9.1.1 Clinical needs All previous chapters have focused on a single scanning protocol: the step-and-shoot mode. This scanning protocol contains both a data acquisition period and non-data-acquisition period. During the data acquisition period, the patient remains stationary while the x-ray tube and detector rotates about the patient at a constant speed. Once a complete projection dataset is acquired for the slice, the non-data-acquisition period starts. The x-ray tube is turned off and the patient is indexed to the next scanning location. For typical CT scanners, the minimum non-data-acquisition period is on the order of seconds as a result of both mechanical and patient constraints. The mechanical constraint is due to the fact that a typical patient weighs over 45 kg, and the patient table requires a certain amount of time to move a large mass from one location to another. The cause of the patient constraint may not be as obvious. From the law of physics we know that to move a resting object over a short distance, first we must accelerate the object up to a certain speed and decelerate the object when it is near the target location. Since the distance between adjacent scanning locations is typically a few millimeters, the amount of acceleration and deceleration is fairly large. A human body is not rigid (the internal organs can move and deform), so the acceleration and deceleration will likely induce motion in the patient. As a result, a certain amount of time must elapse to minimize motion artifacts. In the late 1980s, the CT scan speed approached one second per revolution.

When x-ray radiation penetrates an object, part of its energy is transferred to the object and causes changes in the object’s material. During the energy transfer, an x-ray can indirectly produce ion pairs in the tissue. The ion pairs react with other chemical systems and cause radiation damage. Alternatively, the x-rays may strike and break molecular bonds, such as those in DNA, and cause direct damage.

The iterative reconstruction methods ASiR and ASiR-V have been accepted by hundreds of sites as their standard of care for a variety of protocols and applications. While the reduction in noise has been significant some readers have a preference for the classic image appearance. To maintain the classic image appearance of FBP at the same dose levels used for the standard of care with ASiR-V we introduce, Deep Learning Image Reconstruction (DLIR), a technique using artificial neural networks. This paper demonstrates that DLIR can maintain or improve upon the performance of the conventional iterative reconstruction algorithm (ASiR-V) in terms of low contrast detectability, noise, and spatial resolution.

Coronary CT angiography (CCTA) is a challenging imaging task currently limited by the achievable temporal resolution of modern Multi-Detector CT (MDCT) scanners. In this paper, the recently proposed SMARTRECON method has been applied in MDCT-based CCTA imaging to improve the image quality without any prior knowledge of cardiac motion. After the prospective ECG-gated data acquisition from a short-scan angular span, the acquired data were sorted into several sub-sectors of view angles; each corresponds to a 1/4th of the short-scan angular range. Information of the cardiac motion was thus encoded into the data in each view angle sub-sector. The SMART-RECON algorithm was then applied to jointly reconstruct several image volumes, each of which is temporally consistent with the data acquired in the corresponding view angle sub-sector. Extensive numerical simulations were performed to validate the proposed technique and investigate the performance dependence.

When the scan field of view (SFOV) of a CT system is not large enough to enclose the entire cross-section of a patient, or the patient needs to be intentionally positioned partially outside the SFOV for certain clinical CT scans, truncation artifacts are often observed in the reconstructed CT images. Conventional wisdom to reduce truncation artifacts is to complete the truncated projection data via data extrapolation with different a priori assumptions. This paper presents a novel truncation artifact reduction method that directly works in the CT image domain. Specifically, a discriminative dictionary that includes a sub-dictionary of truncation artifacts and a sub-dictionary of non-artifact image information was used to separate a truncation artifact-contaminated image into two sub-images, one with reduced truncation artifacts, and the other one containing only the truncation artifacts. Both experimental phantom and retrospective human subject studies have been performed to characterize the performance of the proposed truncation artifact reduction method.

In previous studies of the noise power spectrum (NPS) of multi-detector CT (MDCT) systems, the image object was usually placed at the iso-center of the CT system; therefore, the bowtie filter had negligible impact on the shape of the two-dimensional (2D) NPS of MDCT. This work characterized the NPS of off-centered objects when a bowtie filter is present. It was found that the interplay between the bowtie filter and object position has significant impact on the rotational symmetry of the 2D NPS. Depending on the size of the bowtie filter, the degree of object off-centering, and the location of the region of interest (ROI) used for the NPS measurements, the symmetry of the 2D NPS can be classified as circular, dumbbell, and a peculiar cloverleaf symmetry. An anisotropic NPS corresponds to structured noise texture, which may directly influence the detection performance of certain low contrast detection tasks.

Spectral imaging with photon counting detectors has recently attracted a lot of interest in X-ray and CT imaging due to its potential to enable ultra low radiation dose x-ray imaging. However, when radiation exposure level is low, quantum noise may be prohibitively high to hinder applications. Therefore, it is desirable to develop new methods to reduce quantum noise in the acquired data from photon counting detectors. In this paper, we propose a new denoising algorithm to reduce quantum noise in data acquired using an ideal photon counting detector. The proposed method exploits the intrinsic low dimensionality of acquired spectral data to decompose the acquired data in a series of orthonormal spectral bases. The first few spectral bases contain object information while the rest of the bases contain primarily quantum noise. The separation of image content and noise in these orthogonal spatial bases provides a means to reject noise without losing image content. Numerical simulations were conducted to validate and evaluate the proposed noise reduction algorithm. The results demonstrated that the proposed method can effectively reduce quantum noise while maintaining both spatial and spectral fidelity.

The amount of X-ray dose expresses itself as the noise level in image volume after reconstruction in clinical CT scans. It is important to understand the interaction between the dose, noise and reconstruction, which helps to guide the design of CT systems and reconstruction algorithms. Based on the fact that most of practical reconstruction algorithms in clinical CT systems are implemented as filtered back-projection, in this work, a unified analytical framework is proposed to establish the connection between dose, noise and view weighting functions of different reconstruction algorithms in CT helical scans. The proposed framework helps one better understand the relationship between X-ray dose and image noise and is instrumental on how to design the view weighting function in reconstruction without extensive simulations and experiments. Even though certain assumptions were made in order to simplify the analytical model, experimental results using both simulation data and real CT scan data show the proposed model is reasonably accurate even for objects of human body shape. In addition, based on the proposed framework an analytical form of theoretically optimal dose efficiency as a function of helical pitch is also derived, which suggests a somehow unintuitive but interesting conclusion that the theoretically optimal dose efficiency generally varies with helical pitch.

The presence of metal-artifacts in CT imaging can obscure relevant anatomy and interfere with disease diagnosis. The cause and occurrence of metal-artifacts are primarily due to beam hardening, scatter, partial volume and photon starvation; however, the contribution to the artifacts from each of them depends on the type of hardware. A comparison of CT images obtained with different metallic hardware in various applications, along with acquisition and reconstruction parameters, helps understand methods for reducing or overcoming such artifacts. In this work, a metal beam hardening correction (BHC) and a projection-completion based metal artifact reduction (MAR) algorithms were developed, and applied on phantom and clinical CT scans with various metallic implants. Stainless-steel and Titanium were used to model and correct for metal beam hardening effect. In the MAR algorithm, the corrupted projection samples are replaced by the combination of original projections and in-painted data obtained by forward projecting a prior image. The data included spine fixation screws, hip-implants, dental-filling, and body extremity fixations, covering range of clinically used metal implants. Comparison of BHC and MAR on different metallic implants was used to characterize dominant source of the artifacts, and conceivable methods to overcome those. Results of the study indicate that beam hardening could be a dominant source of artifact in many spine and extremity fixations, whereas dental and hip implants could be dominant source of photon starvation. The BHC algorithm could significantly improve image quality in CT scans with metallic screws, whereas MAR algorithm could alleviate artifacts in hip-implants and dentalfillings.

Dual-energy CT has the potential to overcome many of the limitations of routine single-energy CT scanning, such a,.., the potential to provide quantitative imaging via electron density, effective atomic munber, and virtual monochromatic imaging and the potential to completely eliminate beam-hardening artifacts via projection space decomposition. While the potential clinical benefit is strong, a possible barrier to more frequent clinical use of dual-energy CT scanning is radiation dose for high quality images. While image quality in dual-energy CT depends on a munber of factors, including dose partitioning, the choice of kV pair, and the amount of pre­ filtration used, a munber of strategies have been employed to improve image quality in dual-energy CT. Four main methods are: (1) increa,..,e the radiation dose, (2) increase the slice thickness, (3) perform voxel averaging, or (4) use noise reduction algorithms. While these methods offer options for improving image quality, ideally, it is desirable not to have to increase radiation dose or sacrifice spatial resolution (in the x-y plane or in the z-direction). Therefore, it is the purpose of this work to investigate the application of Prior Image Constrained Compressed Sensing (PICCS) in dual-energy CT to reduce radiation dose without sacrificing image quality. In particular, we investigate the use of PICCS in dual-energy CT to generate material density images at half the radiation dose of a commonly used gemstone spectral imaging (GSI) protocol. lVIaterial density images are generated using half the radiation dose, and virtual monochromatic images are generated as a linear combination of half-dose material density images. In this abstract, qualitative and quantitative evaluation are provided to assess the performance of PICCS relative to FBP images at the full dose level and at the half dose level.

Knowledge of scatter generated by bowtie filter is crucial for providing artifact free images on the wide-cone low-dose CT scanners. We investigate and determine the scatter level and artifact generated by the widely used bowtie filter in a wide-cone low-dose CT system. Our approach is to use Monte Carlo simulation to estimate the scatter level generated by a bowtie filter made of a material with low atomic number. First, major components of CT systems, such as source, prepatient collimator, flat filter, bowtie filter, body phantom, and an optional post patient collimator (anti-scatter grid), are built into a 3D model. The scattered photon fluence and the primary transmitted photon fluence are simulated by MCNP5 - a Monte Carlo simulation toolkit. With the increased interests in the low dose and wide coverage CT technology, a tube potential of 80 kVp with more than 10 degree of cone angle is selected. The biased sinogram is created by superimposing scatter signal generated by the bowtie filter onto the primary x-ray beam signal. Finally, images with artifacts are reconstructed with the biased signal. Methods to reduce bowtie filter scatter are also discussed and demonstrated.

Non-linear iterative reconstruction (IR) algorithms have shown promising improvements in image quality at reduced dose levels. However, IR images sometimes may be perceived as having different image noise texture than traditional filtered back projection (FBP) reconstruction. Standard linear-systems-based image quality evaluation metrics are limited in characterizing such textural differences and non-linear image-quality vs. dose trade-off behavior, hence limited in predicting potential impact of such texture differences in diagnostic task. In an attempt to objectively characterize and measure dose dependent image noise texture and statistical properties of IR and FBP images, we have investigated higher order moments and Haralicks Gray Level Co-occurrence Matrices (GLCM) based texture features on phantom images reconstructed by an iterative and a traditional FBP method. In this study, the first 4 central order moments, and multiple texture features from Haralick GLCM in 4 directions at 6 different ROI sizes and four dose levels were computed. For resolution, noise and texture trade-off analysis, spatial frequency domain NPS and contrastdependent MTF were also computed. Preliminary results of the study indicate that higher order moments, along with spatial domain measures of energy, contrast, correlation, homogeneity, and entropy consistently capture the textural differences between FBP and IR as dose changes. These metrics may be useful in describing the perceptual differences in randomness, coarseness, contrast, and smoothness of images reconstructed by non-linear algorithms.

Dual-energy computed tomography is a novel imaging tool that has the potential to reduce beam hardening artifacts and enhance material separation over conventional imaging techniques. Dual-energy acquisitions can be performed by using a fast kVp technology to switch between acquiring adjacent projections at two distinct x-ray spectra (80 and 140 kVp). These datasets can be used to further compute material density and monochromatic images for better material separation and beam hardening reduction by virtue of the projection domain process. The purpose of this study was to evaluate the feasibility of using dual-energy in cardiac imaging for myocardial perfusion detection and coronary artery lumen visualization. Data was acquired on a heart phantom, which consisted of the chambers and aorta filled with Iodine density solution (500 HU @ 120 kVp), a defect region between the aorta and chamber (40 HU @ 120 kVp), two Iodinefilled vessels (400 HU @ 120 kVp) of different diameters with high attenuation (hydroxyapatite) plaques (HAP), and with a 30-cm water equivalent body ring around the phantom. Prospective ECG-gated single-energy and prospective ECG-gated dual-energy imaging was performed. Results showed that the generated monochromatic images had minimal beam hardening artifacts which improved the accuracy and detection of the myocardial defect region. Material density images were useful in differentiating and quantifying the actual size of the plaque and coronary artery lumen. Overall, this study shows that dual-energy cardiac imaging will be a valuable tool for cardiac applications.

Today lowering patient radiation dose while maintaining image quality in Computed Tomography has become a very active research field. Various iterative reconstruction algorithms have been designed to improve/maintain image quality for low dose patient scans. Typically radiation dose variation will result in detectability variation for low contrast objects. This paper assesses the low contrast detectability performance of the images acquired at different dose levels and obtained using different image generation algorithms via two-alterative forced choice human observer method. Filtered backprojection and iterative reconstruction algorithms were used in the study. Results showed that for the objects and scan protocol used, the iterative algorithm employed in this study has similar low contrast detectability performance compared to filtered backprojection algorithm at a 4 times lower dose level. It also demonstrated that well controlled human observer study is feasible to assess the image quality of a CT system.

Spectral CT research and development has recently become a hot topic in industry and in academia. Different approaches have been developed for spectral CT imaging. As a result of the capability to generate monochromatic-energy images, beam hardening artifacts have been largely reduced. However, X-ray scatter is still present, and the associated scatter artifact can still be present in the base material images. This paper proposes an approach for scatter artifact reduction for dual-energy CT. Phantoms as well as clinical data have been evaluated to demonstrate the effectiveness of this approach.

Dual energy computed tomography offers unique diagnostic value by enabling access to material density, effective atomic number, and energy specific spectral characteristics, which remained indeterminate with conventional kVp imaging. Gemstone Spectral Imaging (GSI) is one of the dual energy methods based on fast kVp switching between two x-ray spectra, 80 kVp and 140 kVp nominal, in adjacent projections. The purpose of this study was to compare relative dose between GSI monochromatic and conventional kVp imaging for equivalent image noise characteristics. A spatialfrequency domain noise power spectrum (NPS) was used as a more complete noise descriptor for the comparison of the two image types. Uniform 20cm water phantom images from GSI and conventional 120 kVp scans were used for NPS calculation. In addition, a low contrast imaging study of the two image types with equivalent noise characteristics was conducted for contrast-to-noise-ratio (CNR) and low contrast detectability (LCD) in the Catphan600 ® phantom. From three GSI presets ranging from medium to low dose, we observed that conventional 120kVp scan requires ~ 7% - 18% increase in dose to match the noise characteristics in optimal noise GSI monochromatic image; and that the 65 keV monochromatic image CNR for a 0.5% contrast object is 22% higher compared to corresponding 120 kVp scan. Optimal use of the two energy spectra within GSI results in reduced noise and improved CNR in the monochromatic images, indicating the potential for use of this image type in routine clinical applications.

Dual-energy CT has attracted much attention in recent years. Most recently, a fast-kVp switching (FKS) dual-energy method has been presented with clinical and phantom results to demonstrate its efficacy. The purpose of our study was to quantitatively compare the CTDIW of FKS and routine CT exams under the body and head conditions. For a fair comparison, the low contrast detectability (LCD) was matched before measuring dose. In FKS protocols, an x-ray generator switch rapidly between 140kVp and 80kVp in adjacent views, and the effective tube current is around 600mA. In addition to the tube voltage and current, the flux ratio between high and low kVp is optimized by asymmetric sampling of 35%-65%. The head and body protocols further differ by the gantry speed (0.9sec/1.0sec) and type of bowtie filter (head/body). For baseline single-energy, we followed the IEC standard head and body protocols (120kV, 1sec, 5mm) but iteratively adjusted the tube current (mA) in order to match the LCD. CTDIW was measured using either a 16 cm (for head scanning) or a 32 cm (for body scanning) PMMA phantom of at least 14 cm in length. The LCD was measured using the water section of Catphan 600. To make the study repeatable, the automated statistical LCD measurement tool available on GE Discovery CT750 scanner was used in this work. The mean CTDIW for the head and body single-energy acquisitions were 57.5mGy and 29.2mGy, respectively. The LCD was measured at 0.45% and 0.42% (slice thickness=5mm, object size=3mm, central 4 images), respectively. The average CTDIW for FKS head and body scans was 70.4mGy and 33.4mGy, respectively, at the optimal monochromatic energy of 65 keV. The corresponding LCD was measured at 0.45% and 0.43%, respectively. This demonstrates that, with matching LCD, CTDIW of FKS is comparable to that of routine CT exams under head and body conditions.

Today Cardiac imaging is one of the key driving forces for the research and development activities of Computed Tomography (CT) imaging. It requires high spatial and temporal resolution and is often associated with high radiation dose. The newly introduced ASIR technique presents an efficient method that offers the dose reduction benefits while maintaining image quality and providing fast reconstruction speed. This paper discusses the study of image quality of the ASIR technique for Cardiac CT imaging. Phantoms as well as clinical data have been evaluated to demonstrate the effectiveness of ASIR technique for Cardiac CT applications.

Dual energy CT using a fast kVp switching technique and the standard filtered back projection (FBP) image reconstruction method has recently been studied. With conventional FBP methods, high slew rates are required for acceptable image reconstruction with high image quality. However, high slew rates also require hardware changes to enable data acquisition. In this work, we aim at studying the necessary slew rate for dual energy CT imaging provided that the PICCS algorithm is used for image reconstruction. The results demonstrate that a slew rate of 7.5 kV / view (assuming 2,000 views were collected over 360o with a 60 kVp energy separation) was sufficient for dual energy imaging using PICCS.

A technique for temporal resolution improvement using prior image constrained compressed sensing (TRI-PICCS) in multi-detector computed tomography (MDCT) cardiac imaging is proposed. In this work, the performance of TRIPICCS was studied using a hybrid phantom which consists of realistic cardiac anatomy and objects moving with designed trajectories. Several simulated moving vessels were added to different locations in the heart. Different motion directions and simulated heart rates were investigated using half of the projection data of the short-scan angular range in TRI-PICCS. Different angular ranges of projection data were also investigated in TRI-PICCS to evaluate the highest achievable temporal resolution. The results showed that the temporal improvement of TRI-PICCS is independent of the locations of the moving objects and motion directions. The motion artifacts at 100 bmp simulated heart rate can be significantly improved using TRI-PICCS compared with conventional filtered back projection (FBP). The minimum angular range requirement of TRI-PICCS is about 90°, corresponding to a temporal resolution improvement factor of 2.6 compared with the standard short-scan FBP reconstruction.

8.1 What Is Computer Simulation? Computer simulation uses theoretical models to predict the performance of a real system. Computer simulation can be divided into two general categories: analytical and statistical. Analytically based simulation generates system models using known analytical equations. A good example of this category is the generation of projections of mathematical phantoms. This type of projection is calculated based on the line integrals of attenuation coefficients of objects whose shapes can be described by closed-form equations (the phantom is formed with cylinders, spheres, ellipsoids, or bars). For a given source and detector cell position, the line integral can be calculated precisely for each object. The final result is the weighted summation of these integrals. Statistically based simulation, on the other hand, uses random number generators and the physical property of the interaction process to predict the system performance. A good example is the Monte Carlo simulation used to predict the scatter distribution. Computer simulation is applicable in two major areas: system optics and system physics. System optics mainly addresses issues related to the geometric factors of the system. For example, computer simulation is helpful in understanding the impact of x-ray focal spot size, detector size, source-to-detector distance, and source-to-iso distance to the system spatial resolution. In our analyses, we focus mainly on the geometric factors that impact the system performance. System physics focuses on the physics properties of each process. For example, we can use a known input x-ray spectrum, the attenuation characteristics of different materials as a function of the x-ray energy, and the relative concentration of each material to understand the beam-hardening phenomenon and its impact on image quality. Physics-based simulation can also help to explain the impact of component characteristics on system performance parameters, such as LCD. For each application category (optics or physics), either analytical or statistical simulation tools can be used. Computer simulation is a useful tool for designing a new CT system. During the design process, the designer is often faced with the problem of determining various system parameters.

With the advancement of Computed Tomography technology, improving image quality while reducing patient dose has been a big technical challenge. The recent CT750 HD system from GE Healthcare provides significantly improved spatial resolution and the capability to reduce dose during routine clinical imaging. This paper evaluates the image quality of this system. Spatial resolution, dose reduction, noise, and low contrast detectability have been quantitatively characterized. Results show a quantifiable and visually discernable higher spatial resolution for both body and cardiac scanning modes without compromise of image noise. Further, equivalent image quality performance with up to 50% lower dose has been achieved. Keywords: Computed Tomography (CT), image quality, spatial resolution, MTF, dose, ASIR, noise 1. INTRODUCTION Improving image quality has historically had a big impact on the applications and use of CT imaging. As a medical imaging device, the image quality of the clinical images generated determines the quality of a CT system. Recent advancements in technology have enabled the development of a new generation CT system (LightSpeed CT750 HD - GE Healthcare), which provides significantly improved spatial resolution and the capability to reduce patient radiation dose during routine clinical imaging. It also provides the capability of spectral imaging. Image quality is the key figure of merit to study for an imaging system. This paper evaluates the image quality of a CT750 HD CT system (Fig. 1), with emphasis on the spatial resolution, noise, and low contrast detectability (LCD). The study includes the measurement of high contrast spatial resolution in terms of modulation transfer function (MTF) via the imaging of phantoms and the evaluation of different reconstruction algorithms. It should be noted that not like other approaches, this higher resolution has been achieved without the utilization of post-patient collimation, which is usually associated with large radiation dose penalty. What’s more, this higher resolution can be achieved on whole body scan modes as well as Cardiac scan modes. The Advanced Statistical Iterative Reconstruction (ASIR) technique reduces noise, which allows for higher resolution imaging without the traditional noise increase. The performance of ASIR is evaluated and its effect on image noise and dose reduction is studied, in addition to the analysis of the LCD improvement. Results of the image quality analysis shows a quantifiable and visually discernable higher spatial resolution for both body scanning modes and cardiac scanning modes without compromise of image noise. Further, the image quality results from the study of the ASIR algorithm show equivalent imaging performance with up to 50% lower radiation dose. Note the study of spectral imaging of CT750 HD is not in the scope of this paper, which will be addressed in later publications. This paper is outlined as following. Section 2 describes the methodology used in this study for physical evaluation of key image quality parameters. Section 3 provides the results from different phantoms as well as actual clinical images. Section 4 summarizes the paper with conclusions and discussions.

Future generations of CT systems would need a mean to cover an entire organ in a single rotation. A way to accomplish this is to physically increase detector size to provide, e.g., 120~160mm z (head-foot) coverage at iso. The x-ray cone angle of such a system is usually 3~4 times of that of a 64-slice (40mm) system, which leads to more severe cone beam artifacts in cardiac scans. In addition, the extreme x-ray take-off angles for such a system cause severe heel effect, which would require an increase in anode target angle to compensate for it. One shortcoming of larger target angle is that tube output likely decreases because of shorter thermal length. This would result in an increase of image noise. Our goal is to understand from a physics and math point of view, what is the clinical acceptable level of artifacts, resolution, and noise impact. The image artifacts are assessed through computer simulation of a helical body phantom and visual comparison of reconstructed images between a 140mm system and a 64-slice system. The IQ impact from target angle increase is studied analytically and experimentally by first finding the proper range of target angles that give the acceptable heel effect, then estimating the impact on peak power (flux) and z resolution using an empirical model of heel effect for given target angle and analytical models of z resolution and tube current loading factor for given target thermal length. The results show that, for a 140mm system, 24.5% of imaging volume exhibits more severe cone beam artifacts than a 64-slice system, which also brings up a patient dose concern. In addition, this system may suffer from a 36% peak power (flux) loss, which is equivalent to about 20% image noise increase. Therefore, a wide coverage CT system using a single x-ray source is likely to face some severe challenges in IQ and clinical accuracy.

With increasing longitudinal detector dimension available in diagnostic volumetric CT, step-and-shoot scan is becoming popular for cardiac imaging. In comparison to helical scan, step-and-shoot scan decouples patient table movement from cardiac gating/triggering, which facilitates the cardiac imaging via multi-sector data acquisition, as well as the administration of inter-cycle heart beat variation (arrhythmia) and radiation dose efficiency. Ideally, a multi-sector data acquisition can improve temporal resolution at a factor the same as the number of sectors (best scenario). In reality, however, the effective temporal resolution is jointly determined by gantry rotation speed and patient heart beat rate, which may significantly lower than the ideal or no improvement (worst scenario). Hence, it is clinically relevant to investigate the behavior of effective temporal resolution in cardiac imaging with multi-sector data acquisition. In this study, a 5-second cine scan of a porcine heart, which cascades 6 porcine cardiac cycles, is acquired. In addition to theoretical analysis and motion phantom study, the clinical consequences due to the effective temporal resolution variation are evaluated qualitative or quantitatively. By employing a 2-sector image reconstruction strategy, a total of 15 (the permutation of P(6, 2)) cases between the best and worst scenarios are studied, providing informative guidance for the design and optimization of CT cardiac imaging in volumetric CT with multi-sector data acquisition.

Since the advent of multi-slice CT, helical scan has played an increasingly important role in cardiac imaging. With the availability of diagnostic volumetric CT, step-and-shoot scan has been becoming popular recently. Step-and-shoot scan decouples patient table motion from heart beating, and thus the temporal window for data acquisition and image reconstruction can be optimized, resulting in significantly reduced radiation dose, improved tolerance to heart beat rate variation and inter-cycle cardiac motion inconsistency. Multi-sector data acquisition and image reconstruction have been utilized in helical cardiac imaging to improve temporal resolution, but suffers from the coupling of heart beating and patient table motion. Recognizing the clinical demands, the multi-sector data acquisition scheme for step-and-shoot scan is investigated in this paper. The most outstanding feature of the multi-sector data acquisition combined with the stepand- shoot scan is the decoupling of patient table proceeding from heart beating, which offers the opportunities of employing prospective ECG-gating to improve dose efficiency and fine adjusting cardiac imaging phase to suppress artifacts caused by inter-cycle cardiac motion inconsistency. The improvement in temporal resolution and the resultant suppression of motion artifacts are evaluated via motion phantoms driven by artificial ECG signals. Both theoretical analysis and experimental evaluation show promising results for multi-sector data acquisition scheme to be employed with the step-and-shoot scan. With the ever-increasing gantry rotation speed and detector longitudinal coverage in stateof- the-art VCT scanners, it is expected that the step-and-shoot scan with multi-sector data acquisition scheme would play an increasingly important role in cardiac imaging using diagnostic VCT scanners.

We present Computed Tomography (CT) acquisition and reconstruction schemes for low-dose neuro-angiography based on the method of HighlY constrained back PRojection (HYPR). Simulated and experimental low X-ray radiation dose scans were prepared using the techniques of interleaved view angle under-sampling and tube current reduction. Dynamic CT Angiograms (CTAs) were produced for both standard and low dose images sets. The spatial correlation coefficient, r, between the two reconstruction approaches was determined for each time frame and the SNR and CNR values in arterial ROIs were calculated. The undersampled HYPR reconstructions produced r values of > 0.95 at undersampling and dose reduction factors of 10 and SNR and CNR were more than doubled using HYPR techniques at a tube current of 25 mA. HYPR approaches to contrast enhanced neuro-imaging provide not only volumetric brain hemodynamics but also the ability to produce high quality maps of standard perfusion parameters. The synergy of volumetric hemodynamics and assessment of tissue function provides the medical imaging community with high quality diagnostic information at a fraction of the radiation dose in a single contrast-enhanced scan.

Medical imaging typically requires the reconstruction of a limited region of interest (ROI) to obtain a high resolution image of the anatomy of interest. Although targeted reconstruction is straightforward for analytical reconstruction methods, it is more complicated for statistical iterative techniques, which must reconstruct all objects in the field of view (FOV) to account for all sources of attenuation along the ray paths from x-ray source to detector. A brute force approach would require the reconstruction of the full field of view in high-resolution, but with prohibitive computational cost. In this paper, we propose a multi-resolution approach to accelerate targeted iterative reconstruction using the non-homogeneous ICD (NH-ICD) algorithm. NH-ICD aims at speeding up convergence of the coordinate descent algorithm by selecting preferentially those voxels most in need of updating. To further optimize ROI reconstruction, we use a multi-resolution approach which combines three separate improvements. First, we introduce the modified weighted NH-ICD algorithm, which weights the pixel selection criteria according to the position of the voxel relative to the ROI to speed up convergence within the ROI. Second, we propose a simple correction to the error sinogram to correct for inconsistencies between resolutions when the forward model is not scale invariant. Finally, we leverage the flexibility of the ICD algorithm to add selected edge pixels outside the ROI to the ROI reconstruction in order to minimize transition artifacts at the ROI boundary. Experiments on clinical data illustrate how each component of the method improves convergence speed and image quality.

In this study we develop a novel ECG-gated method of HYPR (HighlY constrained backPRojection) CT reconstruction for low-dose myocardial perfusion imaging and present its first application in a porcine model. HYPR is a method of reconstructing time-resolved images from view-undersampled projection data. Scanning and reconstruction techniques were explored using x-ray projections from a 50 sec contrast-enhanced axial scan of a 47 kg swine on a 64-slice MDCT system. Scans were generated with view undersampling factors from 2 to 10. A HYPR reconstruction algorithm was developed in which a fully-sampled composite image is generated from views collected from multiple cardiac cycles within a diastolic window. A time frame image for a heartbeat was produced by modifying the composite with projections from the cycle of interest. Heart rate variations were handled by automatically selecting cardiac window size and number of cycles per composite within defined limits. Cardiac window size averaged 35% of the R-R interval for 2x undersampling and increased to 64% R-R using 10x undersampling. The selected window size and cycles per composite was sensitive to synchrony between heart rate, gantry rate, and the view undersampling pattern. Temporal dynamics and perfusion metrics measured in conventional short-scan (FBP) images were well-reproduced in the undersampled HYPR time series. Mean transit times determined from HYPR myocardial time-density curves agreed to within 8% with the FBP results. The results indicate potential for an order of magnitude reduction in dose requirement per image in cardiac perfusion CT via undersampled scanning and ECG-gated HYPR reconstruction.

In clinical applications, CT images are acquired in both helical and axial scans. In general, a helical scan can provide better image quality and faster patient throughput, and thus is performed more frequently in clinic. However, the first and last images in a helical scan are usually prescribed at the locations that are half helical turn indented from the starting and ending points of the scan. Hence, due to image location indention, the dose efficiency of helical scanning is usually not as good as that of axial scan in which no image location indention occurs. With increasing detector dimension along z-direction, the dose efficiency difference between helical and axial scans is becoming more significant, particularly at relatively small helical pitches. To match the dose efficiency of helical scan with axial scan as much as possible, a hybrid helical CB-FBP algorithm is presented to reconstruct helical images beyond the conventional indented image zone. The hybrid algorithm is actually a combination of the ray-wise 3D weighted CB-FBP algorithms that are previously proposed for helical and axial CB image reconstructions, in which the ray-wise 3D weighting becomes dependent on both helical pitch and image plane location. Phantom study shows that the conventional indented image zone in helical scan can be extended substantially by using the presented algorithm. Consequently, the dose efficiency in volumetric CT in helical scan can be improved significantly, especially in clinical applications where the helical pitch is relatively low. With increasing dose discrepancy between helical and axial scans, the presented algorithm will become more attractive in clinical applications.

We present the results of dose and image quality performance evaluation of a novel, prospective ECG-gated Coronary CT Angiography acquisition mode (SnapShot Pulse, LightSpeed VCT-XT scanner, GE Healthcare, Waukesha, WI), and compare it to conventional retrospective ECG gated helical acquisition in clinical and phantom studies. Image quality phantoms were used to measure noise, slice sensitivity profile, in-plane resolution, low contrast detectability and dose, using the two acquisition modes. Clinical image quality and diagnostic confidence were evaluated in a study of 31 patients scanned with the two acquisition modes. Radiation dose reduction in clinical practice was evaluated by tracking 120 consecutive patients scanned with the prospectively gated scan mode. In the phantom measurements, the prospectively gated mode resulted in equivalent or better image quality measures at dose reductions of up to 89% compared to non-ECG modulated conventional helical scans. In the clinical study, image quality was rated excellent by expert radiologist reviewing the cases, with pathology being identical using the two acquisition modes. The average dose to patients in the clinical practice study was 5.6 mSv, representing 50% reduction compared to a similar patient population scanned with the conventional helical mode.

We introduced and evaluated the techniques LightSpeed VCT uses to reduce X-ray dose and image noise in cardiac helical CT applications. These techniques include the use of much improved VCT data acquisition system (VDAS) with reduced electronic noise; cardiac bowtie that redistributes X-rays to have more signals for heart and much less flux to the peripheries; adaptive post-processing filters to reduce cardiac image noise; and ECG modulated tube currents to concentrate X-ray dose for desired cardiac phases. Phantom and patient scans were used to evaluate the dose saving and noise reduction potentials of these techniques. The results demonstrated that the improved VDAS reduced image noise 15-20% for cardiac imaging. With same scan technique, the use of cardiac bowtie reduced about 10% dose in terms of CTDIw measurement and clinical evaluation demonstrated additional 7% image noise reduction and equivalent image quality with cardiac bowtie vs. regular body bowtie. The adaptive filter generated 15-20% noise reduction while maintaining image resolution and artery sharpness. Finally, the use of ECG modulated mA method provided up to 50% dose reduction based on CTDIw measurements, but the saving potentials depended on the heart rate and cardiac phase selection. For heart rate of 60bpm and ±15% cardiac phase margin, the average dose reduction could be 30%. Since these dose and noise reduction methods are inclusive and can be combined to produce even greater dose/noise reduction. It is reasonable to believe that with VCT we maybe able to acquire cardiac helical CT images with only 30-40% of the dose of older generation 16-slice CT scanners with similar noise level and same slice thickness.

With an accelerated pace the CT technology has achieved the latest milestone - cone beam volumetric CT with 40mm detector coverage. To obtain an optimized image reconstruction solution for future cone beam VCT systems, the raywise weighted helical CB-FBP algorithm, which was proposed by us to reconstruct image under cone angles up to 4.25°, is optimized and evaluated in this study to verify its imaging performance for image reconstruction under larger cone angles up to 8.5°. The ray-wise weighted helical CB-FBP algorithm proposed by us possesses two important features: (a) tangential filtering that is naturally implemented via row-wise fan-to-parallel rebinning to maintain spatial resolution along patient's longitudinal direction; and (b) 3D weighting that is a ray-wise optimization process to obtain image quality controllability. By using computer-simulated phantoms, such as the helical body and humanoid head phantoms, it has been shown that the ray-wise weighted helical CB-FBP algorithm can provide a well balanced imaging performance over helical pitches while a large field of view (FOV) can be maintained. It is the optimized ray-wise weighting that enables the proposed CB-FBP algorithm performs well at larger cone angle. Based on the experimental evaluation, it is believed that the ray-wise weighted helical CB-FBP algorithm can be a candidate solution for image reconstruction in future cone beam VCT systems with detectors corresponding to larger cone angles up to 8.5° (~ 80 mm detector z coverage).