Image reconstruction and performance evaluation for ECG-gated spiral scanning with a 16-slice CT system

Technological developments of X-ray computed tomography over half a century: User's influence on protocol optimization

Since the introduction of Computed Tomography (CT), technological improvements have been impressive. At the same time, the number of adjustable acquisition and reconstruction parameters has increased substantially. Overall, these developments led to improved image quality at a reduced radiation dose. However, many parameters are interrelated and part of automated algorithms. This makes it more complicated to adjust them individually and more difficult to comprehend their influence on CT protocol adjustments. Moreover, the user's influence in adapting protocol parameters is sometimes limited by the manufacturer's policy or the user's knowledge. As a consequence, optimization can be a challenge. A literature search in Embase, Medline, Cochrane, and Web of Science was performed. The literature was reviewed with the objective to collect information regarding technological developments in CT over the past five decades and the role of the associated acquisition and reconstruction parameters in the optimization process.

Impact of a novel exponential weighted 4DCT reconstruction algorithm

Purpose

This work characterizes a novel exponential 4DCT reconstruction algorithm (EXPO), in phantom and patient, to determine its impact on image quality as compared to the standard cosine‐squared weighted 4DCT reconstruction.

Methods

A motion platform translated objects in the superior–inferior (S‐I) direction at varied breathing rates (8–20 bpm) and couch pitches (0.06–0.1) to evaluate interplay between parameters. Ten‐phase 4DCTs were acquired and data were reconstructed with cosine squared and EXPO weighting. To quantify the magnitude of image blur, objects were translated in the anterior–posterior (A‐P) and S‐I directions for full‐width half maximum (FWHM) analysis between both 4DCT algorithms and a static case. 4DCT sinogram data for 10 patients were retrospectively reconstructed using both weighting factors. Image subtractions elucidated intensity and boundary differences. Subjective image quality grading (presence of image artifacts, noise, spatial resolution (i.e., lung/liver boundary sharpness), and overall image quality) was conducted yielding 200 evaluations.

Results

After taking static object size into account, the FWHM of EXPO reconstructions in the A‐P direction was 3.3 ± 1.7 mm (range: 0–4.9) as compared to cosine squared 9.8 ± 4.0 mm (range: 2.6–14.4). The FWHM of objects translated in the S‐I direction reconstructed with EXPO agreed better with the static FWHM than the cosine‐squared reconstructions. Slower breathing periods, faster couch pitches, and intermediate 4DCT phases had the largest reductions of blurring with EXPO. 18 of 60 comparisons of artifacts were improved with EXPO reconstruction, whereas no appreciable changes were observed in image quality scores. In 18 of 20 cases, EXPO provided sharper images although the reduced projections also increased baseline noise.

Conclusion

Exponential weighted 4DCT offers potential for reducing image blur (i.e., improving image sharpness) in 4DCT with a tendency to reduce artifacts. Future work will involve evaluating the impact on treatment planning including delineation ability and dose calculation.

On the data acquisition, image reconstruction, cone beam artifacts, and their suppression in axial MDCT and CBCT - A review

PURPOSE

In the clinic, computed tomography (CT) has evolved into an essential modality for diagnostic imaging by multidetector row CT (MDCT) and image guided intervention by cone beam CT (CBCT). Recognizing the increasing importance of axial MDCT/CBCT in clinical and preclinical applications, and the existence of CB artifacts in MDCT/CBCT images, we provide a review of CB artifacts' root causes, rendering mechanisms and morphology, and possible solutions for elimination and/or reduction of the artifacts.

METHODS

By examining the null space in Radon and Fourier domain, the root cause of CB artifacts (i.e., data insufficiency) in axial MDCT/CBCT is analytically investigated, followed by a review of the data sufficiency conditions and the "circle +" source trajectories. The rendering mechanisms and morphology of CB artifacts in axial MDCT/CBCT and their special cases (e.g., half/short scan and full scan with latitudinally displaced detector) are then analyzed, followed by a survey of the potential solutions to suppress the artifacts. The phenomenon of imaged zone indention and its variation over FBP, BPF/DBPF, two-pass and iterative CB reconstruction algorithms and/or schemes are discussed in detail.

RESULTS

An interdomain examination of the null space provides an insightful understanding of the root cause of CB artifacts in axial MDCT/CBCT. The decomposition of CB artifacts rendering mechanisms facilitates understanding of the artifacts' behavior under different conditions and the potential solutions to suppress them. An inspection of the imaged zone intention phenomenon provides guidance on the design and implementation of CB image reconstruction algorithms and schemes for CB artifacts suppression in axial MDCT/CBCT.

CONCLUSIONS

With increasing importance of axial MDCT/CBCT in clinical and preclinical applications, this review article can update the community with in-depth information and clarification on the latest progress in dealing with CB artifacts and thus increase clinical/preclinical confidence.

Clinical Feasibility of Single-Source Dual-spiral 4D Dual-Energy CT for Proton Treatment Planning Within the Thoracic Region

PURPOSE

Single-source dual-spiral dual-energy computed tomography (DECT) provides additional patient information but is prone to motion between the 2 consecutively acquired computed tomography (CT) scans. Here, the clinical applicability of dual-spiral time-resolved DECT (4D-DECT) for proton treatment planning within the thoracic region was evaluated.

METHODS AND MATERIALS

Dual-spiral 4D-DECT scans of 3 patients with lung cancer were acquired. For time-averaged datasets and 4 breathing phases, the geometric conformity of 80 kVp and 140 kVp 4D-DECT scans before image post-processing was assessed by normalized cross correlation (NCC). Additionally, the conformity of the corresponding DECT-derived 58 keV and 79 keV pseudo-monoenergetic CT datasets after image post-processing, including deformable image registration (DIR), was determined. To analyze the reliability of proton dose calculation, clinical (Plan_Clin) and artificial worst-case (Plan_WorstCase, targeting the diaphragm) treatment plans were calculated on 140 kVp and 79 keV datasets and compared with gamma analyses (0.1% dose-difference and 1 mm distance-to-agreement criterion). The applicability of a patient-specific DECT-based prediction of stopping-power ratio (SPR) was investigated and proton range shifts compared with the clinical heuristic CT-number-to-SPR conversion were assessed. Finally, the delineation variability of an experienced radiation oncologist was quantified.

RESULTS

Dual-spiral 4D-DECT scans without DIR showed a high geometric conformity, with an average NCC ± standard deviation of 98.7% ± 1.0% when including all patient voxels or 88.2% ± 7.8% when considering only lung. DIR improved the conformity, leading to an average NCC of 99.9% ± 0.1% and 99.6% ± 0.5%, respectively. Plan_Clin dose distributions on 140 kVp and 79 keV datasets were similar, with an average gamma passing rate of 99.9% (99.2%-100%). The worst-case evaluation still revealed high passing rates (99.3% on average, 92.4% as minimum). Clinically relevant mean range shifts of 2.2% ± 1.2% were determined between patient-specific DECT-based SPR prediction and clinical heuristic CT-number-to-SPR conversion. The intra-observer delineation variability was slightly reduced using additional DECT-derived datasets.

CONCLUSIONS

The 79 keV pseudo-monoenergetic CT datasets can be consistently obtained from dual-spiral 4D-DECT and are applicable for dose calculation. Patient-specific DECT-based SPR prediction performed well and potentially reduces range uncertainty in proton therapy of patients with lung cancer.

Radiation dose and physical image quality in 128-section dual-source computed tomographic coronary angiography: a phantom study

One‐hundred‐and‐twenty‐eight–section dual X‐ray source computed tomography (CT) systems have been introduced into clinical practice and have been shown to increase temporal resolution. Higher temporal resolution allows low‐dose spiral mode at a high pitch factor during CT coronary angiography. We evaluated radiation dose and physical image qualities in CT coronary angiography by applying high‐pitch spiral, step‐and‐shoot, and low‐pitch spiral modes to determine the optimal acquisition mode for clinical situations. An anthropomorphic phantom, small dosimeters, a calibration phantom, and a microdisc phantom were used to evaluate the radiation doses absorbed by thoracic organs, noise power spectrums, in‐plane and z‐axis modulation transfer functions, slice sensitivity profiles, and number of artifacts for the three acquisition modes. The high‐pitch spiral mode had the advantage of a small absorbed radiation dose, but provided low image quality. The low‐pitch spiral mode resulted in a high absorbed radiation dose of approximately 200 mGy for the heart. Although the absorbed radiation dose was lower in the step‐and‐shoot mode than in the low‐pitch spiral mode, the noise power spectrum was inferior. The quality of the in‐plane modulation transfer function differed, depending on spatial frequency. Therefore, the step‐and‐shoot mode should be applied initially because of its low absorbed radiation dose and superior image quality.

PACS numbers: 87.57.‐s; 87.57.C‐; 87.57.cf; 87.57.cm; 87.57.cp; 87.57.Q‐; 87.57.qp; 87.57.uq

The Role of Multidetector CT in the Diagnosis of Retroperitoneal Fibrosis: Report of a Case

Herein, we report a 40-year old man who presented with flank and abdominal pain with dilatation of the bilateral pyelocalyceal system detected in ultrasonography. Computed Tomography (CT) scan showed a soft tissue mass at the level of the fourth and fifth lumbar vertebrae in the retroperitoneal region. There were no blood flow signals in 64-slice multidetector CT (MDCT) which confirms the Retroperitoneal Fibrosis (RPF). Pathological examination showed infiltration of plasma cells, macrophages, lymphocytes and eosinophils accompanied by fibrosis, which is consistent with idiopathic RPF. In conclusion, 64-slice MDCT imaging is useful in the diagnosis of RPF.

Improved dual-energy material discrimination for dual-source CT by means of additional spectral filtration

The use of additional spectral filtration for dual-energy (DE) imaging using a dual-source CT (DSCT) system was investigated and its effect on the material-specific DE(ratio) was evaluated for several clinically relevant materials. The x-ray spectra, data acquisition, and reconstruction processes for a DSCT system (Siemens Definition) were simulated using information provided by the system manufacturer, resulting in virtual DE images. The factory-installed filtration for the 80 kV spectrum was left unchanged to avoid any further reductions in tube output, and only the filtration for the high-energy spectrum was modified. Only practical single-element filter materials within the atomic number range of 40 < or = Z < or = 83 were evaluated, with the aim of maximizing the separation between the two spectra, while maintaining similar noise levels for high- and low-energy images acquired at the same tube current. The differences between mean energies and the ratio of the 140 and 80 kV detector signals, each integrated below 80 keV, were evaluated. The simulations were performed for three attenuation scenarios: Head, body, and large body. The large body scenario was evaluated for the DE acquisition mode using the 100 and 140 kV spectra. The DE(ratio) for calcium hydroxyapatite (simulating bone or calcifications), iodine, and iron were determined for CT images simulated using the modified and factory-installed filtration. Several filter materials were found to perform well at proper thicknesses, with tin being a good practical choice. When image noise was matched between the low- and high-energy images, the spectral difference in mean absorbed energy using tin was increased from 25.7 to 42.7 keV (head), from 28.6 to 44.1 keV (body), and from 20.2 to 30.2 keV (large body). The overlap of the signal spectra for energies below 80 keV was reduced from 78% to 31% (head), from 93% to 27% (body), and from 106% to 79% (large body). The DE(ratio) for the body attenuation scenario increased from 1.45 to 1.91 (calcium), from 1.84 to 3.39 (iodine), and from 1.73 to 2.93 (iron) with the additional tin filtration compared to the factory filtration. This use of additional filtration for one of the x-ray tubes used in dual-source DECT dramatically increased the difference between material-specific DE ratios, e.g., from 0.39 to 1.48 for calcium and iodine or from 0.28 to 1.02 for calcium and iron. Because the ability to discriminate between different materials in DE imaging depends primarily on the differences in DE ratios, this increase is expected to improve the performance of any material-specific DECT imaging task. Furthermore, for the large patient size and in conjunction with a 100/140 kV acquisition, the use of additional filtration decreased noise in the low-energy images and increased contrast in the DE image relative to that obtained with 80/140 kV and no additional filtration.

Image reconstruction and image quality evaluation for a dual source CT scanner

The authors present and evaluate concepts for image reconstruction in dual source CT (DSCT). They describe both standard spiral (helical) DSCT image reconstruction and electrocardiogram (ECG)-synchronized image reconstruction. For a compact mechanical design of the DSCT, one detector (A) can cover the full scan field of view, while the other detector (B) has to be restricted to a smaller, central field of view. The authors develop an algorithm for scan data completion, extrapolating truncated data of detector (B) by using data of detector (A). They propose a unified framework for convolution and simultaneous 3D backprojection of both (A) and (B) data, with similar treatment of standard spiral, ECG-gated spiral, and sequential (axial) scan data. In ECG-synchronized image reconstruction, a flexible scan data range per measurement system can be used to trade off temporal resolution for reduced image noise. Both data extrapolation and image reconstruction are evaluated by means of computer simulated data of anthropomorphic phantoms, by phantom measurements and patient studies. The authors show that a consistent filter direction along the spiral tangent on both detectors is essential to reduce cone-beam artifacts, requiring truncation of the extrapolated (B) data after convolution in standard spiral scans. Reconstructions of an anthropomorphic thorax phantom demonstrate good image quality and dose accumulation as theoretically expected for simultaneous 3D backprojection of the filtered (A) data and the truncated filtered (B) data into the same 3D image volume. In ECG-gated spiral modes, spiral slice sensitivity profiles (SSPs) show only minor dependence on the patient's heart rate if the spiral pitch is properly adapted. Measurements with a thin gold plate phantom result in effective slice widths (full width at half maximum of the SSP) of 0.63-0.69 mm for the nominal 0.6 mm slice and 0.82-0.87 mm for the nominal 0.75 mm slice. The visually determined through-plane (z axis) spatial resolution in a bar pattern phantom is 0.33-0.36 mm for the nominal 0.6 mm slice and 0.45 mm for the nominal 0.75 mm slice, again almost independent of the patient's heart rate. The authors verify the theoretically expected temporal resolution of 83 ms at 330 ms gantry rotation time by blur free images of a moving coronary artery phantom with 90 ms rest phase and demonstrate image noise reduction as predicted for increased reconstruction data ranges per measurement system. Finally, they show that the smoothness of the transition between image stacks acquired in different cardiac cycles can be efficiently controlled with the proposed approach for ECG-synchronized image reconstruction.

Preoperative evaluation of the right gastroepiploic artery on multidetector computed tomography in coronary artery bypass graft surgery

BACKGROUND

The right gastroepiploic artery (GEA) is commonly used in coronary artery bypass grafting, but a method for preoperative assessment of the suitability of the GEA has not been established. Here, we assessed the efficacy of 64-slice multidetector computed tomography (MDCT) for this purpose.

METHODS

Multidetector computed tomography was performed for 32 patients (24 males, 8 females; mean age, 65.9 +/- 7.4 years) undergoing coronary artery bypass graft surgery. Preoperative MDCT criteria for GEA suitability were no significant stenosis or calcification and a diameter of 2.0 mm or more in the middle portion of the GEA. The skeletonized GEA was inspected in 30 patients to determine the accuracy of evaluation of arteriosclerosis by MDCT (2 patients were excluded owing to severe GEA stenosis). The internal diameter at the anastomotic site was compared with the diameters of the proximal, distal, and middle regions of the GEA on MDCT.

RESULTS

The GEA was used to bypass a target coronary artery in 30 patients. The diameter of the middle of the GEA on MDCT correlated strongly with the actual internal diameter at the anastomotic site (r = 0.72, p < 0.0001). The diameter at the anastomotic site calculated from MDCT using the distance from the GEA origin to the anastomotic site and the actual diameter did not differ significantly (2.76 +/- 0.6 versus 2.87 +/- 0.5 mm, p = 0.06).

CONCLUSIONS

Preoperative MDCT imaging of the GEA is reliable for diagnosis, and a middle diameter of 2.0 mm or greater can be used to indicate GEA suitability for coronary artery bypass grafting.

Imaging of the heart with computed tomography

Imaging of the heart with computed tomography (CT) was already introduced in the 1980Is and has meanwhile entered clinical routine as a consequence of the rapid evolution of CT technology during the last decade. In this review article, we give an overview on the technology and clinical performance of different CT-scanner generations used for cardiac imaging, such as Electron Beam CT (EBCT), single-slice CT und multi-detector row CT (MDCT) with 4, 16 and 64 simultaneously acquired slices. We identify the limitations of current CT-scanners, indicate potential of improvement and discuss alternative system concepts such as CT with area detectors and dual source CT (DSCT).

Significant Coronary Artery Stenosis: Comparison on Per-Patient and Per-Vessel or Per-Segment Basis at 64-Section CT Angiography

PURPOSE

To prospectively evaluate the accuracy of 64-section computed tomographic (CT) coronary angiography for assessing significant stenosis on a global and segmental level, by using conventional coronary artery angiography as the reference standard.

MATERIALS AND METHODS

This study was HIPAA compliant and had local institutional review board approval. Patients gave informed consent. Patients suspected of having coronary artery disease (CAD) underwent both conventional coronary catheter angiography and contrast material-enhanced retrospectively electrocardiographically gated 64-section multi-detector row CT of the coronary arteries. Two experienced observers analyzed all CT scans for signs of CAD (stenosis of 0%, <or=49%, 50%-69%, 70%-99%, or 100%). Diagnostic accuracy of CT coronary angiography compared with that of conventional angiography was determined on a per-segment, per-vessel, and per-patient basis. No patients, vessels, or segments were excluded from analysis. Spearman correlation, cross tables, and a chi(2) test were used for statistical analysis.

RESULTS

On a per-segment basis, 92.4% (762 of 825) of all segments could be clearly evaluated. In 7.6% of segments, image quality was compromised either by misregistration (16%), motion artifacts (30%), or small vessel size (54%). Correlation coefficients for detection and grading of stenosis were r=0.65 on a per-segment, r=0.83 on a per-vessel, and r=0.88 on a per-patient basis. Stenoses of 50% or greater were detected with accuracy, sensitivity, and specificity, respectively, of 96.1% (793 of 825), 82% (50 of 61), and 97.1% (743 of 765) on a per-segment basis, 90.9% (150 of 165), 89% (32 of 36), and 91.5% (118 of 129) on a per-vessel basis, and 89% (49 of 55), 100% (19 of 19), and 83% (30 of 36) on a per-patient basis.

CONCLUSION

On a per-patient basis, 64-section multi-detector row CT coronary angiography enables the diagnosis of significant (>or=50%) stenosis in CAD with an accuracy of 89%. On a per-segment and per-vessel basis, diagnostic accuracy is still impaired, primarily by limited spatial resolution.

Dose Performance of a 64-Channel Dual-Source CT Scanner1

PURPOSE

To prospectively compare the dose performance of a 64-channel multi-detector row computed tomographic (CT) scanner and a 64-channel dual-source CT scanner from the same manufacturer.

MATERIALS AND METHODS

To minimize dose in the cardiac (dual-source) mode, the evaluated dual-source CT system uses a cardiac beam-shaping filter, three-dimensional adaptive noise reduction, heart rate-dependent pitch, and electrocardiographically based modulation of the tube current. Weighted CT dose index per 100 mAs was measured for the head, body, and cardiac beam-shaping filters. Kerma-length product was measured in the spiral cardiac mode at four pitch values and three electrocardiographic modulation temporal windows. Noise was measured in an anthropomorphic phantom. Data were compared with data from a 64-channel multi-detector row CT scanner.

RESULTS

For the multi-detector row and dual-source CT systems, respectively, weighted CT dose index per 100 mAs was 14.2 and 12.2 mGy (head CT), 6.8 and 6.4 mGy (body CT), and 6.8 and 5.3 mGy (cardiac CT). In the spiral cardiac mode (no electrocardiographically based tube current modulation, 0.2 pitch), equivalent noise occurred at volume CT dose index values of 23.7 and 35.0 mGy (coronary artery calcium CT) and 58.9 and 61.2 mGy (coronary CT angiography) for multi-detector row CT and dual-source CT, respectively. The use of heart rate-dependent pitch values reduced volume CT dose index to 46.2 mGy (0.265 pitch), 34.0 mGy (0.36 pitch), and 26.6 mGy (0.46 pitch) compared with 61.2 mGy for 0.2 pitch. The use of electrocardiographically based tube current-modulation and temporal windows of 110, 210, and 310 msec further reduced volume CT dose index to 9.1-25.1 mGy, dependent on the heart rate.

CONCLUSION

For electrocardiographically gated coronary CT angiography, image noise equivalent to that of multi-detector row CT can be achieved with dual-source CT at doses comparable to or up to a factor of two lower than the doses at multi-detector row CT, depending on heart rate of the patient.

Coronary Artery Calcium: A Multi-institutional, Multimanufacturer International Standard for Quantification at Cardiac CT

PURPOSE

To develop a consensus standard for quantification of coronary artery calcium (CAC).

MATERIALS AND METHODS

A standard for CAC quantification was developed by a multi-institutional, multimanufacturer international consortium of cardiac radiologists, medical physicists, and industry representatives. This report specifically describes the standardization of scan acquisition and reconstruction parameters, the use of patient size-specific tube current values to achieve a prescribed image noise, and the use of the calcium mass score to eliminate scanner- and patient size-based variations. An anthropomorphic phantom containing calibration inserts and additional phantom rings were used to simulate small, medium-size, and large patients. The three phantoms were scanned by using the recommended protocols for various computed tomography (CT) systems to determine the calibration factors that relate measured CT numbers to calcium hydroxyapatite density and to determine the tube current values that yield comparable noise values. Calculation of the calcium mass score was standardized, and the variance in Agatston, volume, and mass scores was compared among CT systems.

RESULTS

Use of the recommended scanning parameters resulted in similar noise for small, medium-size, and large phantoms with all multi-detector row CT scanners. Volume scores had greater interscanner variance than did Agatston and calcium mass scores. Use of a fixed calcium hydroxyapatite density threshold (100 mg/cm(3)), as compared with use of a fixed CT number threshold (130 HU), reduced interscanner variability in Agatston and calcium mass scores. With use of a density segmentation threshold, the calcium mass score had the smallest variance as a function of patient size.

CONCLUSION

Standardized quantification of CAC yielded comparable image noise, spatial resolution, and mass scores among different patient sizes and different CT systems and facilitated reduced radiation dose for small and medium-size patients.

Chasing the heart: new developments for cardiac CT

With the latest generations of multidetector row computed tomography (CT) scanners, CT of the heart is about to fulfill its promise to become the premier noninvasive imaging modality for the cardiac assessment. The performance of this modality has been continuously improved to a point where CT, beyond mere feasibility studies, is firmly establishing its role in the diagnostic work-up of patients with suspected cardiac disease. This has been enabled by ongoing technical refinements, which are the topic of this contribution. This review traces the evolution of CT for cardiac applications, describes the current status of scanner technology with special emphasis on dual-source CT, and provides insights into potential future developments for further refinement of this technique.

Step-and-shoot data acquisition and reconstruction for cardiac x-ray computed tomography

Coronary artery imaging with x-ray computed tomography (CT) is one of the most recent advancements in CT clinical applications. Although existing "state-of-the-art" clinical protocols today utilize helical data acquisition, it suffers from the lack of ability to handle irregular heart rate and relatively high x-ray dose to patients. In this paper, we propose a step-and-shoot data acquisition protocol that significantly overcomes these shortcomings. The key to the proposed protocol is the large volume coverage (40 mm) enabled by the cone beam CT scanner, which allows the coverage of the entire heart in 3 to 4 steps. In addition, we propose a gated complementary reconstruction algorithm that overcomes the longitudinal truncation problem resulting from the cone beam geometry. Computer simulations, phantom experiments, and clinical studies were conducted to validate our approach.

Use of tissue Doppler imaging to guide tube current modulation in cardiac multidetector computed tomographic angiography

In multidetector computed tomographic coronary angiography, strategies to minimize effective radiation dose (ERD) are urgently needed. Prospective tube current modulation (TCM) allows a decrease in ERD, although it may limit reconstruction options. We sought to determine if tissue Doppler imaging (TDI) by echocardiography could predict an optimal phase for multidetector computed tomography and be used to guide TCM. Echocardiographic studies were performed in 94 patients immediately before multidetector computed tomography (83% men; mean 60 +/- 11 years of age, mean body mass index 27.7+/-4.1 kg/m2) and identified the most quiescent phase of the cardiac cycle within the atrioventricular groove. In 40 patients, prospective TCM was programmed according to TDI (TCM(TDI) group); 54 patients underwent multidetector computed tomography without TCM (no-TCM). In 25 patients assigned to the TCM(TDI) group, multidetector computed tomograms were correlated with invasive quantitative coronary angiograms to ensure maintenance of diagnostic accuracy. Optimal phase determined by TDI was 71 +/- 11%, with a distinct bi-modal distribution. Compared with no-TCM, effective radiation dose was decreased by 42% in the TCM(TDI) group (6.6 +/- 1.2 vs 11.4 +/- 2.2 mSv, p < 0.0001). Only 8 segments (3%) were unevaluable due to motion artifact. In 296 segments, sensitivity, specificity, and positive and negative predictive values to detect lesions > 50% by multidetector computed tomography were 92%, 94%, 65%, and 99%, respectively. There was good correlation between quantitative coronary angiography and multidetector computed tomography for absolute degree of stenosis (r = 0.70, p < 0.0001). In conclusion, TDI is a useful tool to guide prospective TCM in multidetector computed tomography. ERD in multidetector computed tomography may be significantly decreased using this technique while maintaining excellent image quality.

Aperture weighted cardiac reconstruction for cone-beam CT

Multi-row detectors together with fast rotating gantries made cardiac imaging possible for CT. Due to the cardiac motion, ECG gating has to be integrated into the reconstruction of the data measured on a low pitch helical trajectory. Since the first multi-row scanners were introduced, it has been shown that approximative true cone-beam reconstruction methods are most suitable for the task of retrospectively gated cardiac volume CT. In this paper, we present the aperture weighted cardiac reconstruction (AWCR), which is a three-dimensional reconstruction algorithm of the filtered back-projection type. It is capable of handling all illumination intervals of an object point, which occur as a consequence of a low pitch helical cone-beam acquisition. Therefore, this method is able to use as much redundant data as possible, resulting in an improvement of the image homogeneity, the signal to noise ratio and the temporal resolution. Different optimization techniques like the heart rate adaptive cardiac weighting or the automatic phase determination can be adopted to AWCR. The excellent image quality achieved by AWCR is presented for medical datasets acquired with both a 40-slice and a 64-slice cone-beam CT scanner.

Multithreaded cardiac CT

Phase-correlated CT, as it is used for cardiac imaging, is the most popular and the most important but also the most demanding special CT application in the clinical routine, today. Basically, it fulfills the four-dimensional imaging task of depicting a quasiperiodically moving object at any desired motion phase with significantly reduced motion artifacts. Although image quality with phase-correlated reconstruction is far better than with standard reconstruction, there are motion artifacts remaining and improvements of temporal resolution are required. As a well-known alternative to simply decreasing rotation time, we consider a spiral cone-beam CT scanner that has G x-ray guns and detectors mounted. We call this a multisource or a multithreaded CT scanner. Aiming for improved temporal resolution the relative temporal resolution tau, which measures the fraction of a motion period that enters the image, is studied as a function of the motion rate (heart rate) and the degree of scan overlap (pitch value) for various configurations. The parameters to optimize are the number of threads G and the interthread parameters delta alpha and delta z, which are the angular and the longitudinal separation between adjacent threads, respectively. To demonstrate the improvements approximate image reconstruction of multithreaded raw data is performed by using a generalization of the extended parallel back projection cone-beam reconstruction algorithm [Med. Phys. 31(6), 1623-1641 (2004)] to the case of multithreaded CT. Reconstructions of a simulated cardiac motion phantom and of simulated semi-antropomorphic phantoms are presented for two and three threads and compared to the single-threaded case to demonstrate the potential of multithreaded cardiac CT. Patient data were acquired using a clinical double-threaded CT scanner to validate the theoretical results. The optimum angle delta alpha between the tubes is 90 degrees for a double-threaded system, and for triple-threaded scanners it is 60 degrees or 120 degrees. In all cases, delta z = 0 results as an optimum, which means that the threads should be mounted in the same transversal plane. However, the dependency of the temporal resolution on delta z is very weak and a longitudinal separation delta z not = 0 would not deteriorate image quality. The mean temporal resolution achievable with an optimized multithreaded CT scanner is a factor of G better than the mean temporal resolution obtained with a single-threaded scanner. The standard reconstructions showed decreased cone-beam artifacts with multithreaded CT compared to the single-threaded case. Our phase-correlated reconstructions demonstrate that temporal resolution is significantly improved with multithreaded CT. The clinical patient data confirm our results.

Age related changes of human aortic distensibility: evaluation with ECG-gated CT

Aortic distensibility is a parameter to grade vascular diseases and age-related effects because it is related to the elastic properties of the vessel wall. In this study vascular cross-sectional area changes have been determined using ECG-gated CT to analyse the age dependency of aortic distensibility. Distensibility measurements of the aorta were performed in 31 subjects (28 to 85 years). Time-resolved images were acquired either with a 4- or 16-detector row CT system using a modified CT angiography protocol. Cross-sectional area changes of the aorta were calculated by semiautomatic segmentation, and distensibility values were obtained using additional systemic blood pressure measurements. The aorta could be segmented successfully in all subjects. A decrease of aortic distensibility with age was found (r=0.50). Below (above) the renal arteries, the annual decrease was Delta D ( infrarenal ) =(-2.1+/-0.7).10(-7 )Pa(-1)a(-1), (D ( suprarenal ) Delta=(-3.5+/-1.1).10(-7 )Pa(-1)a(-1)). Differences between the ages, the youngest third and oldest third studied, were found to be significant (P( suprarenal )=0.003; P( infrarenal )=0.025). An age-dependent decrease of aortic wall elasticity can be determined in a modified routine CT angiography study.

Direct cone-beam cardiac reconstruction algorithm with cardiac banding artifact correction

Multislice helical computed tomography (CT) is a promising noninvasive technique for coronary artery imaging. Various factors can cause inconsistencies in cardiac CT data, which can result in degraded image quality. These inconsistencies may be the result of the patient physiology (e.g., heart rate variations), the nature of the data (e.g., cone-angle), or the reconstruction algorithm itself. An algorithm which provides the best temporal resolution for each slice, for example, often provides suboptimal image quality for the entire volume since the cardiac temporal resolution (TRc) changes from slice to slice. Such variations in TRc can generate strong banding artifacts in multiplanar reconstruction images or three-dimensional images. Discontinuous heart walls and coronary arteries may compromise the accuracy of the diagnosis. A beta-blocker is often used to reduce and stabilize patients' heart rate but cannot eliminate the variation. In order to obtain robust and optimal image quality, a software solution that increases the temporal resolution and decreases the effect of heart rate is highly desirable. This paper proposes an ECG-correlated direct cone-beam reconstruction algorithm (TCOT-EGR) with cardiac banding artifact correction (CBC) and disconnected projections redundancy compensation technique (DIRECT). First the theory and analytical model of the cardiac temporal resolution is outlined. Next, the performance of the proposed algorithms is evaluated by using computer simulations as well as patient data. It will be shown that the proposed algorithms enhance the robustness of the image quality against inconsistencies by guaranteeing smooth transition of heart cycles used in reconstruction.

Phase versus amplitude sorting of 4D-CT data

Image quality of CT scans suffers when objects undergo motion. Respiratory motion causes artifacts, which prevents adequate visualization of anatomy. Four‐dimensional CT (4D‐CT) is a method in which image reconstruction of moving objects is retrospectively gated according to the recorded phase information of the monitored motion pattern. Although several groups have investigated the use of 4D‐CT in radiotherapy, little has been detailed with regard to the sorting method. We present a new retrospective gating technique with sorting based on the amplitude of the motion trace. This method is compared to previously developed methods that sort based on phase. A 16‐slice CT scanner (Sensation 16, Siemens Medical Solutions, Erlangen, Germany) was used to acquire images of two phantoms on a motion platform moving in two dimensions. The motion was monitored using a strain gauge inserted inside an adjustable belt. A 180° interpolation was used for reconstruction after gating. Significant improvement using the amplitude‐sorting technique was observed, particularly when testing nonperiodic motion functions.

PACS numbers: 87.59.Fm, 87.53.Kn, 87.57.Ce

Phase versus amplitude sorting of 4D-CT data

Image quality of CT scans suffers when objects undergo motion. Respiratory motion causes artifacts, which prevents adequate visualization of anatomy. Four-dimensional CT (4D-CT) is a method in which image reconstruction of moving objects is retrospectively gated according to the recorded phase information of the monitored motion pattern. Although several groups have investigated the use of 4D-CT in radiotherapy, little has been detailed with regard to the sorting method. We present a new retrospective gating technique with sorting based on the amplitude of the motion trace. This method is compared to previously developed methods that sort based on phase. A 16-slice CT scanner (Sensation 16, Siemens Medical Solutions, Erlangen, Germany) was used to acquire images of two phantoms on a motion platform moving in two dimensions. The motion was monitored using a strain gauge inserted inside an adjustable belt. A 180 degrees interpolation was used for reconstruction after gating. Significant improvement using the amplitude-sorting technique was observed, particularly when testing nonperiodic motion functions.

When, Why, and How to Examine the Heart During Thoracic CT: Part 1, Basic Principles

OBJECTIVE

MDCT systems with fast scanning capabilities can acquire images of the thorax with reduced cardiac motion artifacts, enabling improved evaluation of the heart and surrounding structures in the course of routine thoracic CT. This article describes the principles of including an evaluation of the heart in the course of a chest CT examination in terms of both examination technique and image interpretation. In addition, both the normal appearances and some of the most common abnormal appearances of the cardiac structures will be described.

CONCLUSION

Details concerning the cardiac structures can inform interpretation of thoracic CT studies and can influence the patient's clinical management. Both unenhanced and contrast-enhanced scans can detect significant cardiac disorders that may otherwise go undetected. In certain situations, a CT examination of the entire chest, complemented by cardiac gating, can provide a more dedicated analysis of the heart and coronary arteries, providing both morphologic and functional information.

Navigating the Fifth Dimension: Innovative Interface for Multidimensional Multimodality Image Navigation

The display and interpretation of images obtained by combining three-dimensional data acquired with two different modalities (eg, positron emission tomography and computed tomography) in the same subject require complex software tools that allow the user to adjust the image parameters. With the current fast imaging systems, it is possible to acquire dynamic images of the beating heart, which add a fourth dimension of visual information-the temporal dimension. Moreover, images acquired at different points during the transit of a contrast agent or during different functional phases add a fifth dimension-functional data. To facilitate real-time image navigation in the resultant large multidimensional image data sets, the authors developed a Digital Imaging and Communications in Medicine-compliant software program. The open-source software, called OsiriX, allows the user to navigate through multidimensional image series while adjusting the blending of images from different modalities, image contrast and intensity, and the rate of cine display of dynamic images. The software is available for free download at http://homepage.mac.com/rossetantoine/osirix.

Reduction of motion blurring artifacts using respiratory gated CT in sinogram space: A quantitative evaluation

Techniques have been developed for reducing motion blurring artifacts by using respiratory gated computed tomography (CT) in sinogram space and quantitatively evaluating the artifact reduction. A synthetic sinogram was built from multiple scans intercepting a respiratory gating window. A gated CT image was then reconstructed using the filtered back-projection algorithm. Wedge phantoms, developed for quantifying the motion artifact reduction, were scanned while being moved using a computer-controlled linear stage. The resulting artifacts appeared between the high and low density regions as an apparent feature with a Hounsfield value that was the average of the two regions. A CT profile through these regions was fit using two error functions, each modeling the partial-volume averaging characteristics for the unmoving phantom. The motion artifact was quantified by determining the apparent distance between the two functions. The blurring artifact had a linear relationship with both the speed and the tangent of the wedge angles. When gating was employed, the blurring artifact was reduced systematically at the air-phantom interface. The gated image of phantoms moving at 20 mm/s showed similar blurring artifacts as the nongated image of phantoms moving at 10 mm/s. Nine patients were also scanned using the synchronized respiratory motion technique. Image artifacts were evaluated in the diaphragm, where high contrast interfaces intercepted the imaging plane. For patients, this respiratory gating technique reduced the blurring artifacts by 9%-41% at the lung-diaphragm interface.

Quantification of aortic elasticity: development and experimental validation of a method using computed tomography

Aortic distensibility depending on aortic cross-sectional area changes is an important parameter for the grading of vascular diseases. This study measured aortic area changes by multidetector computed tomography. An image reconstruction algorithm was developed to assess aorta diameter and area as a function of the cardiac cycle with sufficient time resolution along the entire length of the aorta by four-detector row computed tomography. The algorithm was tested on porcine aortic specimens and compared with an optical reference method. The error of the relative vessel area change comparing the two methods was found to be about 3%. Initial tests on patient datasets indicate that clinical application is feasible. The proposed method has the advantage that it can easily be integrated into a modified routine CT angiography study and allows the measurement of aortic cross-sectional area changes.

Multislice spiral computed tomographic angiography of coronary arteries in patients with suspected coronary artery disease: an effective filter before catheter angiography?

BACKGROUND

Despite impressive image quality, it is unclear if noninvasive coronary angiography with multislice spiral computed tomography (CT) is powerful enough to act as a filter before invasive angiography (INV-A) in symptomatic patients.

METHODS AND RESULTS

We therefore studied 133 consecutive symptomatic patients with suspected coronary artery disease (CAD) and an indication for INV-A (chest pain and signs of ischemia in conventional stress tests). Patients with known CAD, acute coronary syndrome, or a calcium volume score >1000 were excluded. In all patients, both INV-A and multislice CT angiography (MSCT-A) (Philips MX 8000 multislice spiral CT, scan time 250 milliseconds, slice thickness 1.3 mm, 120 mL of contrast agent, 4 mL/s, retrospective gating) were directly compared by 2 independent investigators using the American Heart Association 15-segment model. Altogether, we studied 1596 segments, 74% had diagnostic image quality. Multislice CT angiography correctly identified 68 significant stenoses of the 75 stenoses seen with INV-A (sensitivity 91%). In 945 of 1185 diagnostic segments, stenosis could correctly be ruled out with MSCT-A. There were 3 times more stenoses seen with MSCT-A compared with INV-A (positive predictive value 29%) mainly because of misclassification of nonobstructive plaques as stenosis. The per-patient analysis allowed to exclude significant CAD in 42 (32%) of 133 patients. In only 6 of 53 patients, MSCT-A failed to detect significant stenosis, 4 of those were in small segments not requiring intervention. Calcium scoring alone was less suited as a filter before angiography: 25 patients (18% of study group) had a calcium score = 0, and 8 of these patients turned out to have significant stenoses.

CONCLUSION

Multislice CT angiography, but not calcium scoring alone, offers promise to reduce the number of INV-A in symptomatic patients with suspected CAD by up to one third with minimal risk for the patient.

Automatic phase determination for retrospectively gated cardiac CT

The recent improvements in CT detector and gantry technology in combination with new heart rate adaptive cone beam reconstruction algorithms enable the visualization of the heart in three dimensions at high spatial resolution. However, the finite temporal resolution still impedes the artifact-free reconstruction of the heart at any arbitrary phase of the cardiac cycle. Cardiac phases must be found during which the heart is quasistationary to obtain outmost image quality. It is challenging to find these phases due to intercycle and patient-to-patient variability. Electrocardiogram (ECG) information does not always represent the heart motion with an adequate accuracy. In this publication, a simple and efficient image-based technique is introduced which is able to deliver stable cardiac phases in an automatic and patient-specific way. From low-resolution four-dimensional data sets, the most stable phases are derived by calculating the object similarity between subsequent phases in the cardiac cycle. Patient-specific information about the object motion can be determined and resolved spatially. This information is used to perform optimized high-resolution reconstructions at phases of little motion. Results based on a simulation study and three real patient data sets are presented. The projection data were generated using a 16-slice cone beam CT system in low-pitch helical mode with parallel ECG recording.

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.

Respiratory motion estimation from slowly rotating x-ray projections: Theory and simulation

Understanding the movement of tumors caused by respiratory motion is very important for conformal radiatherapy. However, respiratory motion is very difficult to study by conventional x-ray CT imaging since object motion causes inconsistent projection views, leading to artifacts in reconstructed images. We propose to estimate the parameters of a nonrigid, free breathing motion model from a set of projection views of the thorax that are acquired using a slowly rotating cone-beam CT scanner. This approach involves deforming a motion-free reference thorax volume according to the estimated parameters and comparing its projections to the corresponding measured projection views. The parameters are optimized by minimizing a regularized squared error cost function. Simulation results with a fan-beam geometry show good agreement between the estimated motion and the true motion, which supports the potential of this approach for estimating four-dimensional (three-dimensional spatial + temporal) respiratory motion.

Evaluation of spatial and temporal resolution for ECG-gated 16-row multidetector CT using a dynamic cardiac phantom

Measurements of spatial and temporal resolution for ECG-gated scanning of a stationary and moving heart phantom with a 16-row MDCT were performed. A resolution phantom with cylindrical holes from 0.4 to 3.0 mm diameter was mounted to a cardiac phantom, which simulates the motion of a beating heart. Data acquisition was performed with 16x0.75 mm at various heart rates (HR, 60-120 bpm), pitches (0.15-0.30) and scanner rotation times (RT, 0.42 and 0.50 s). Raw data were reconstructed using a multi-cycle real cone-beam reconstruction algorithm at multiple phases of the RR interval. Multi-planar reformations (MPR) were generated and analyzed. Temporal resolution and cardiac cycles used for image reconstruction were calculated. In 97.2% (243/250) of data obtained with the stationary phantom, the complete row of holes with 0.6 mm was visible. These results were independent of heart rate, pitch, scanner rotation time and phase point of reconstruction. For the dynamic phantom, spatial resolution was determined during phases of minimal motion (116/250). In 40.5% (47/116), the resolution was 0.6 mm and in 37.1% (43/116) 0.7 mm. Temporal resolution varied between 63 and 205 ms, using 1.5-4.37 cardiac cycles for image reconstruction.

OsiriX: an open-source software for navigating in multidimensional DICOM images

A multidimensional image navigation and display software was designed for display and interpretation of large sets of multidimensional and multimodality images such as combined PET-CT studies. The software is developed in Objective-C on a Macintosh platform under the MacOS X operating system using the GNUstep development environment. It also benefits from the extremely fast and optimized 3D graphic capabilities of the OpenGL graphic standard widely used for computer games optimized for taking advantage of any hardware graphic accelerator boards available. In the design of the software special attention was given to adapt the user interface to the specific and complex tasks of navigating through large sets of image data. An interactive jog-wheel device widely used in the video and movie industry was implemented to allow users to navigate in the different dimensions of an image set much faster than with a traditional mouse or on-screen cursors and sliders. The program can easily be adapted for very specific tasks that require a limited number of functions, by adding and removing tools from the program's toolbar and avoiding an overwhelming number of unnecessary tools and functions. The processing and image rendering tools of the software are based on the open-source libraries ITK and VTK. This ensures that all new developments in image processing that could emerge from other academic institutions using these libraries can be directly ported to the OsiriX program. OsiriX is provided free of charge under the GNU open-source licensing agreement at http://homepage.mac.com/rossetantoine/osirix.

Artifact analysis and reconstruction improvement in helical cardiac cone beam CT

With the introduction of cone beam (CB) scanners, cardiac volumetric computed tomography (CT) imaging has the potential to become a noninvasive imaging tool in clinical routine for the diagnosis of various heart diseases. Heart rate adaptive reconstruction schemes enable the reconstruction of high-resolution volumetric data sets of the heart. Artifacts, caused by strong heart rate variations, high heart rates and obesity, decrease the image quality and the diagnostic value of the images. The image quality suffers from streak artifacts if suboptimal scan and reconstruction parameters are chosen, demanding improved gating techniques. In this paper, an artifact analysis is carried out which addresses the artifacts due to the gating when using a three-dimensional CB cardiac reconstruction technique. An automatic and patient specific cardiac weighting technique is presented in order to improve the image quality. Based on the properties of the reconstruction algorithm, several assessment techniques are introduced which enable the quantitative determination of the cycle-to-cycle transition smoothness and phase homogeneity of the image reconstruction. Projection data of four patients were acquired using a 16-slice CBCT system in low pitch helical mode with parallel electrocardiogram recording. For each patient, image results are presented and discussed in combination with the assessment criteria.

Imaging of coronary atherosclerosis using computed tomography: Current status and future directions

Computed tomography (CT) imaging of the coronary arteries, using either electron beam tomography (EBT) or multidetector row CT (MDCT), offers two possibilities to assess coronary atherosclerosis. Without injection of contrast agent, coronary calcifications can be detected and quantified. Their presence and extent correlates to the presence and amount of coronary atherosclerotic plaque. Prospective studies have demonstrated a high predictive value concerning the occurrence of coronary artery disease events and overall mortality. An emerging consensus seems to indicate that calcium imaging may be clinically useful in patients at intermediate risk for coronary artery disease events as determined based on traditional risk factors. In addition, recent studies have shown that after injection of contrast agent and using high-resolution scan protocols, the visualization of noncalcified plaque is also possible with CT techniques. However, data on the accuracy of plaque detection, quantification of plaque volume, and characterization of plaque (eg, lipid-rich vs fibrous) is currently limited, and the prognostic significance of noncalcifed coronary atherosclerotic plaque detection is unclear.