Table Increment Research Articles

Longitudinal fan-beam computed tomography dataset for head-and-neck squamous cell carcinoma patients.

To describe in detail a dataset consisting of longitudinal fan-beam computed tomography (CT) imaging to visualize anatomical changes in head-and-neck squamous cell carcinoma (HNSCC) patients throughout radiotherapy (RT) treatment course. This dataset consists of CT images from 31 HNSCC patients who underwent volumetric modulated arc therapy (VMAT). Patients had three CT scans acquired throughout the duration of the radiation treatment course. Pretreatment planning CT scans with a median of 13 days before treatment (range: 2-27), mid-treatment CT at 22 days after start of treatment (range: 13-38), and post-treatment CT 65 days after start of treatment (range: 35-192). Patients received RT treatment to a total dose of 58-70 Gy, using daily 2.0-2.20 Gy, fractions for 30-35 fractions. The fan-beam CT images were acquired using a Siemens 16-slice CT scanner head protocol with 120 kV and current of 400 mAs. A helical scan with 1 rotation per second was used with a slice thickness of 2 mm and table increment of 1.2 mm. In addition to the imaging data, contours of anatomical structures for RT, demographic, and outcome measurements are provided. The dataset with DICOM files including images, RTSTRUCT files, and RTDOSE files can be found and publicly accessed in the Cancer Imaging Archive (TCIA, http://www.cancerimagingarchive.net/) as collection Head-and-neck squamous cell carcinoma patients with CT taken during pretreatment, mid-treatment, and post-treatment (HNSCC-3DCT-RT). This is the first dataset to date in TCIA which provides a collection of multiple CT imaging studies (pretreatment, mid-treatment, and post-treatment) throughout the treatment course. The dataset can serve a wide array of research projects including (but not limited to): quantitative imaging assessment, investigation on anatomical changes with treatment progress, dosimetry of target volumes and/or normal structures due to anatomical changes occurring during treatment, investigation of RT toxicity, and concurrent chemotherapy and RT effects on head-and-neck patients.

Amount of Weight-Bearing During Tilt Table Inclination, With Neutral and Unilateral Knee Flexion Postures.

ObjectiveTo analyze the amount of weight-bearing during tilt table increments, with a review of neutral and unilateral knee flexion postures.MethodsThere were 17 healthy participants enrolled in this study. The subjects were tilted from 10° to 90°, and their body weight was measured at each 10° increment. In the first test, both plantar pressures, with the subjects in neutral posture, were recorded. During the second and third tests, the angle of inclination was thus recorded and increased, with the subjects in unilateral knee flexion posture; flexion was maintained at 25° by attaching a cylindrical support to the tilt table at the level of the popliteal fossa.ResultsThe study was divided into two types of postures: neutral and unilateral knee flexion. The percentage of body weight (%BW) between each leg during neutral posture was noted as not being statistically significant. The %BW of one side during tilt table inclination was significantly different between the two postures at 10° to 80° (p<0.05). The weight during unilateral knee flexion posture was lower as analyzed, regardless of tilt table inclination compared with that in neutral posture. We note that fifty percent of the ratio of %BW was noted at 33.12° and 38.76° in neutral and flexion postures, respectively.ConclusionThe unilateral knee flexion could induce the effect of decreased body weight compared with non-flexion side. The results of this study will help in setting a safe and quantitative percentage of weight-bearing on the lower extremity during tilt training.

Optimized canine dental computed tomographic protocol in medium-sized mesaticepahlic dogs.

The purpose of this study was to develop an optimized imaging protocol for canine dental imaging using single-detector row CT and multidetector row CT. Two medium-sized mesaticephalic cadaver dog heads were scanned using varying slice thickness, sequential vs. helical scanning mode, sequential CT table increment, pitch, image reconstruction algorithm, and an additional moderate edge enhancement filter. For each series, two reviewers scored dental roots as not visible (score: 0), partially visible (score: 1) or completely visible (score: 2). Sharpness of the dento-alveolar margin was scored as blurry (score: 0), sharp (score: 1) or very sharp (score: 2). Consensus reader scores were compared among CT protocols. For single-detector row CT, complete visualization of all tooth roots was only achieved with 1 mm slice thickness and image interval sequential or helical series (pitch of 1 or 2) using a high-frequency image reconstruction algorithm with or without additional edge enhancement. For four-detector row CT, complete visibility of all tooth roots was recorded in all series. For single-detector row CT, all medium frequency algorithm series yielded blurry margin scores and all other series yielded sharp margin scores. For four-detector row CT, sequential and helical 0.5 mm thickness images yielded very sharp dento-alveolar margin scores, whereas the 1 mm series yielded only sharp margin scores. Authors concluded that the optimal dental imaging protocol was a sequential mode 1 mm slice thickness and interval with a high-frequency image reconstruction algorithm and an additional moderate edge enhancement filter.

Association of Lipoprotein Subfractions and Coronary Artery Calcium In Patient at Intermediate Cardiovascular Risk

More precise estimation of the atherogenic lipid parameters could improve identification of residual risk beyond what is possible using traditional lipid risk factors. The aim of the present study was to explore the association between advanced analysis of lipoprotein subfractions and the prevalence of coronary artery calcium. Consecutive participants at intermediate cardiovascular risk who were undergoing computed tomographic assessment of coronary calcium (calcium score) were included. Using a validated ultracentrifugation method (the vertical autoprofile II test), cholesterol in eluting lipoprotein subfractions [i.e., low- (LDL), very-low-, intermediate-, and high-density lipoprotein subclasses, lipoprotein (a), and predominant LDL distribution] was directly quantified. A total of 410 patients were included (29% women, mean age 57 years), of whom 297 (72.4%) had coronary artery calcium. LDL pattern B (predominance of small dense particles) emerged as an independent predictor of coronary calcium after adjustment for traditional risk factors (odds ratio 4.46, 95% confidence interval 1.19 to 16.7). However, after additional stratification for dyslipidemia, as defined by conventional lipid profiling, a statistically significant prediction was only retained for high-density lipoprotein subfraction 2 (odds ratio 3.45, 95% confidence interval 2.03 to 50.1) and "real" LDL (odds ratio 6.10, 95% confidence interval 1.26 to 23.41) in the normolipidemia group and for lipoprotein (a) (odds ratio 7.81, 95% confidence interval 1.41 to 43.5) in the dyslipidemic group. In conclusion, advanced assessment of the lipoprotein subfractions [i.e., LDL pattern B, high-density lipoprotein subfraction 2, "real" LDL, and lipoprotein (a)] using the vertical autoprofile II test provided additional information to that of conventional risk factors on the prevalence of coronary artery calcium in patients at intermediate cardiovascular risk.

Radiation Dose Levels for Interventional CT Procedures

The purpose of this study was to determine typical radiation dose levels to patients undergoing CT-guided interventional procedures. A total of 571 patients undergoing CT interventional procedures were included in this retrospective data analysis study. Enrolled patients underwent one of five procedures: cryoablation, aspiration, biopsy, drain, or injection. With each procedure, two scan modes were used, either intermittent (no table increment) or helical mode. Skin dose was estimated from the volumetric CT dose index (CTDI(vol)) and phantom measurements. Effective dose was calculated by multiplying dose-length product (DLP) and conversion factor (k factor) for helical mode, and using Monte Carlo organ dose coefficients for intermittent mode. The mean (± SD) skin doses were 728 ± 382, 130 ± 104, 128 ± 81, 152 ± 105, and 195 ± 147 mGy, and the mean effective doses were 119.7 ± 50.3, 20.1 ± 11.0, 13.8 ± 9.2, 25.3 ± 15.4, and 9.1 ± 5.5 mSv for each of the five procedures, respectively. The maximum skin dose was 1.95 Gy. The mean effective dose across all procedure types was 24.1 mSv, with 2.3 mSv from intermittent scans and 21.8 mSv from helical scans. Substantial dose differences were observed among the five procedures. The risk of deterministic effects appears to be very low, because the maximum observed skin dose did not exceed the threshold for transient skin erythema (2 Gy). The average risk of stochastic effects was comparable to that of 1-10 abdomen and pelvis CT examinations. Although the intermittent mode can contribute substantially to skin dose, it contributes minimally to the effective dose because of the much shorter scan range used.

SU-GG-I-79: Image Quality, Organ Doses and Risks of Computed Tomography Exams in Pernambuco, Brazil

Purpose: This study determined image quality and organ doses for common CT exams in six hospitals of Recife, Pernambuco, in order to assess health risks.Method and Materials: Data were gathered from six helical CTscanners, one of them with 64 slices. Image quality was evaluated using the ACR accreditation phantom and protocol. Scan acquisition parameters (kV, mAs, scan time, slice thickness, table increment or pitch, scan length) and number of scans per patient were documented for 90 head, 75 chest, 60 abdomen and 60 abdomen/pelvis exams performed using these scanners. Patient age and gender were also recorded. Doses to relevant organs were calculated using the “ImPACT CT Patient Dosimetry Calculator”. Stochastic (and for the eye lenses, deterministic) risks were estimated using BEIR and ICRP data. Results: Only two scanners met all the ACR quality criteria. Two failed the spatial resolution test and two, the low contrast resolution one. The mean doses and [ranges] (in mGy) received by some relevant organs for each CT exam were as follows: Head — Eye lenses: 36±22 [7.9 ‐ 110]. Chest — Breast: 11.8±5.7 [1.7 ‐ 33]; thymus: 18.8±7.6 [9.9 ‐ 48]; heart: 15.2±7.2 [1.7 ‐ 42]; lung: 15.1±7.1 [4.6 ‐ 42]. Abdomen — Stomach: 19±13 [7.4 ‐ 55]; liver: 17±10 [6.7 ‐ 48]. Abdomen/pelvis (34 females, 26 males) — Ovaries: 21±11 [5.4 ‐ 39]; uterus: 23±12 [5.1 ‐ 42]; prostate: 16.0±8.6 [2.9 ‐ 39]; testes: 2.5±2.6 [0.2 ‐ 10]. Doses were unrelated to scanner type and/or image quality. The lowest doses were given by the scanner with the worse performance, suggesting a poor facility operation. Estimated risks and recommendations to optimize protection will be presented and discussed. Conclusion:Image quality is not adequate in all the hospitals. Dose ranges are wide and in several organs are high; some eye lens doses are near the cataract formation threshold.

SU-GG-I-38: A Direct Skin Dose Calculation Method in CT Scans without Table Motion: Influence of Patient Size and Beam Collimation

Purpose: To calculate skin dose from console dose (CTDIvol) in CT scan modes without table motion, such as interventional and perfusion CT applications, and to investigate the influence of phantom size and beam collimation.Materials and Methods:Skin doses were measured using a solid state detector placed on the anterior surface of six oblong water phantoms with measured lateral widths from 25 to 50 cm. Surface doses were measured using a sequential scan mode without table increment, 12×0.6mm collimation, 120kV, 80mAs and 0.5s rotation time on a 64‐slice CT system. Console CTDIvol was recorded and the ratio between surface dose and console CTDIvol calculated for each phantom size. To investigate the dependence of this ratio on beam width, surface doses were measured at collimations of 1×5, 12×0.6, 1×10, 12×1.2, 30×0.6 and 24×1.2 mm using the 40cm phantom. Results: The console CTDIvol for the 12×0.6mm collimation was 7.12 mGy, while measuredsurface doses were 4.23, 4.65, 4.24, 3.86, 3.55, and 3.47 mGy, respectively, for phantom size of 25, 30, 35, 40, 45 and 50 cm. The surface dose as a percentage of CTDIvol varied from 49% to 65%, increasing with decreasing phantom size except for the 25cm phantom. The measuredsurface doses (and percentage of CTDIvol) for the 40 cm phantom at different collimations were: 3.32 (68%), 3.77 (53%), 3.76 (78%), 4.23 (74%), 4.4 (74%), 4.79 (89%) mGy for collimations from 1×5 mm to 24×1.2 mm. Conclusions: For the same technical parameters (and CTDIvol), surface dose in CT scans without table increment varied with phantom size and beam collimation. For a given collimation and CTDIvol, surface dose in general increased with decreasing phantom size. For a given phantom size, surface dose and CTDIvol both depended on beam collimation, with surface dose as a percentage of CTDIvol generally increasing with beam width.

SU‐FF‐I‐75: Verification of Reported DLP for Philips Brilliance CT Scanners

Purpose: Recent concerns about accumulated radiation dose and collective risk prompted a check of the software reported CT dose index (CTDIVol) and dose‐length‐product (DLP) of the Philips Brilliance CT scanners. The CTDIVol and the DLP were measured for comparison with the reported values using the high‐resolution chest protocol (commonly used for COPD). Method and Materials: A Philips Brilliance 40‐slice CT scanner was used with the high‐resolution chest protocol, which has a technique of 120 kVp and 200 mAs/slice, a detector collimation of 2×0.625 mm, a reconstructed scan thickness of 1.25 mm, and a table increment of 10 mm. Measurements were made using a cylindrical Lucite phantom 32 cm in diameter and 15 cm in length, with axial holes drilled in the center and near the surface at 90 degree intervals. The radiation dose was measured using a Keithley electrometer and a 10 cm pencil chamber. Dose measurements were made along the phantom's center axis and along its surface. Results: While the dose measurements are consistent with the reported CTDIVol, a serious error was discovered in the reported DLP. The reported value appears to be calculated as the product of the CTDIVol multiplied by the number of shots (16) and the collimation width (1.25 mm) while the correct formulation of DLP is the CTDIVol multiplied by the scan length (151 mm). This results in an underestimate of dose by a factor of 8. Conclusion: Accurate estimates of radiation risk require correct reporting of the effective dose, which is related to DLP. We have verified that the Philips 64‐slice also has the same error and it is possible that other Philips protocols with a large table increment relative to the slice width will too. A physicist should verify that the DLP reported is consistent with actual measurements.

MO‐A‐ValB‐01: Tradeoffs in Image Quality and Radiation Dose for CT

In CT scanning, image quality has many components and is influenced by many technical parameters. While image quality has always been a concern for the physics community, clinically acceptable image quality has become even more of an issue as strategies to reduce radiation dose — to all patients, but especially to pediatric patients— has become a focus in many radiology practices.The purpose of this presentation will be to first describe several of the components of CT image quality — noise, slice thickness (Z‐axis resolution), low contrast resolution and high contrast resolution— as well as radiation dose and to describe how each of these may be affected by technical parameter selection. This presentation will pay particular attention to the tradeoffs that exist between different aspects of image quality, especially when the reduction of radiation dose is one of the objectives.The presentation will then explore several mechanisms that can be used to reduce radiation dose in CT exams and the implications for the diagnostic image quality of the exam. Specifically, the implications of varying the tube current*time product (mAs), pitch or tablespeed (or for axial imaging, the table increment), slice thickness, beam energy (kVp), patient (or phantom) size and dose reduction options (such as tube current modulation) will be described for both radiation dose and diagnostic image quality. Finally, this presentation will emphasize that the tradeoffs between radiation dose and image quality are clinical‐task dependent; that is, the goals of the clinically indicated exam dictate what aspect of image quality may be emphasized for that exam (low contrast resolution or high contrast spatial resolution, etc.) and this will have implications for the amount of radiation dose reduction that is acceptable. This will be illustrated with examples from selected diagnostic imaging exams.Educational Objectives:1. Understand key components of image quality in CT scanning as well as reinforce CT radiation dose concepts.2. Understand the impact that technical parameter selection has on the various aspects of image quality and radiation dose.3. Examine the tradeoffs between various aspects of image quality and radiation dose.4. Examine the impact of these tradeoffs on a few clinical imaging protocols and illustrate the task‐dependence of image quality requirements.

Methodology for improved detection of coronary stenoses with computed tomographic angiography

Background Noninvasive angiography is a promising technique for visualization of the coronary lumen; however, current methodologies lead to limited accuracy. We assessed the accuracy of electron beam computed tomographic angiography (EBA) for detection of coronary stenoses, using improved triggering techniques and thinner slice collimation. Methods Eighty-six patients with suspected coronary disease were studied with EBA and conventional invasive coronary angiography. Electrocardiographic triggering was performed at a fixed time in end systole to reduce cardiac motion. Thin (1.5 mm) slices were obtained with 1.5 mm table incrementation. In axial (2-dimensional) EBA images and 3-dimensional reconstructions, all coronary arteries and side branches with a diameter of ≥1.5 mm were assessed for the presence of stenoses with >50% diameter reduction. Both EBA and invasive angiographic images were assessed in a blinded manner. Results In comparison to invasive coronary angiography, EBA correctly classified 49 of 53 patients (92%) as having at least 1 coronary stenosis. Overall, 103 stenoses with >50% diameter reduction were present, and 93 of these lesions were correctly detected by EBA (sensitivity 90%, specificity 93%, positive predictive value 84%, and negative predictive value 96%). Only 5% of vessels could not be assessed, predominantly due to significant calcification. Conclusions Thinner slice collimation and end-systolic electrocardiographic triggering improves accuracy and assessment of coronary EBA for the detection of obstructive coronary artery disease, making this study clinically useful in the evaluation of obstructive coronary artery disease.

Improvement of parenchymal and vascular enhancement using saline flush and power injection for multiple-detector-row abdominal CT.

The aim of this study was to determine if a saline solution flush following low dose contrast material bolus improves parenchymal and vascular enhancement during abdominal multiple detector-row computed tomography (MDCT). Forty-one patients (24 men and 17 women; mean age 49 years, age range 27-86 years) underwent abdominal MDCT (collimation 4x5 mm, 15-mm table increment, reconstruction interval 5 mm, gantry rotation period 0.8 s) with a single- as well as with a double syringe power injector. Indication for examination were benign and malignant tumors and inflammatory diseases. Patients received 100 ml nonionic contrast material (300 mgI/ml) alone or pushed with 20 ml saline solution. Mean enhancement values for both protocols were measured in the liver, the spleen, the pancreas, the renal cortex, the portal vein, the inferior vena cava and the abdominal aorta. Double syringe power-injector protocol led to significantly higher parenchymal and vascular enhancement than single syringe power-injector protocol (p<0.05). The improvement in mean enhancement of the liver was 9 +/- 9 HU, of the spleen 8 +/- 10 HU, of the pancreas 7 +/- 9 HU, and of the renal cortex 8 +/- 20 HU. The improvement in mean enhancement of the portal vein was 10 +/- 17 HU of the inferior vena cava 8 +/- 13 HU and of the abdominal aorta 10 +/- 17 HU. The use of a double syringe power injector with saline flush following contrast material bolus significantly improves parenchymal and vascular enhancement during contrast-enhanced abdominal MDCT with low iodine doses.

Accuracy of Spinal Navigation for Magerl Screws

The influence of a protocol of preoperative computed tomography scanning and a special registration technique was assessed on the accuracy of navigation for implanting Magerl C1-C2-screws. The use of navigation systems for implanting Magerl screws could help to decrease the risk of complications and to reduce the required skin incision. Two parameters conceivably affecting the accuracy are the protocol of preoperative computed tomography scanning and the registration technique. Four cervical spine segments of human cadavers were scanned with two computed tomography protocols. Registration was done based on anatomic landmarks or using a specially designed percutaneous registration device. For the accuracy check, the pointer tip was placed exactly on the markers. The displayed distance on the monitor was referred as an estimate of accuracy. Varying the computed tomography protocol did not significantly affect the accuracy. The mean accuracy was improved from 3 mm after anatomic pair-point matching to 1.5 mm after matching using the percutaneous registration device. The accuracy obtainable seems to be sufficient for implanting Magerl screws by using frameless stereotactic navigation. Three-millimeter slice thickness and 2-mm table increment is a proper protocol for preoperative computed tomography scanning. Fiducial markers improve the accuracy significantly.

Outcomes After Withholding Anticoagulation From Patients With Suspected Acute Pulmonary Embolism and Negative Computed Tomographic Findings: A Cohort Study

Objective To determine the outcome of withholding anticoagulation from patients with suspected acute pulmonary embolism in whom computed tomographic (CT) findings are interpreted as negative for pulmonary embolism. Patients and Methods This retrospective cohort study included 1512 consecutive patients referred from August 7, 1997, to November 30, 1998, for CT because of clinically suspected acute pulmonary embolism. All patients were examined by electron beam CT, and scanning was performed in a cephalocaudad direction from the top of the aortic arch to the base of the heart with 3-mm collimation, 2-mm table incrementation, and an exposure time of 0.2 second (130 peak kV, 620 mA, and standard reconstruction algorithm). Contrast material was infused at a rate of 3 to 4 mL/s through an antecubital vein with an automated injector. Findings on CT were interpreted as either positive or negative. The main outcome measures were deep venous thrombosis, pulmonary embolism, and vital status within 3 months after the CT scan and the cause of death based on medical record review, mailed patient questionnaires, and telephone interviews. Results In 1010 patients (67%) CT scans were interpreted as negative for acute pulmonary embolism. Seventeen patients were excluded because they received anticoagulation. Of the remaining 993 patients, deep venous thrombosis or pulmonary embolism developed in 8; 118 patients died, 3 of pulmonary embolism. Nineteen patients were known to be alive, but additional clinical information could not be obtained. The 3-month cumulative incidence of overall deep venous thrombosis or pulmonary embolism was 0.5% (95% confidence interval, 0.1%-1.0%) and of fatal pulmonary embolism, 0.3% (95% confidence interval, 0.0%-0.7%). Conclusions The incidence of (1) overall deep venous thrombosis or pulmonary embolism or (2) fatal pulmonary embolism among patients with suspected acute pulmonary embolism, negative CT results, and no other evidence of venous thromboembolism is low. Withholding anticoagulation in these patients appears to be safe. To determine the outcome of withholding anticoagulation from patients with suspected acute pulmonary embolism in whom computed tomographic (CT) findings are interpreted as negative for pulmonary embolism. This retrospective cohort study included 1512 consecutive patients referred from August 7, 1997, to November 30, 1998, for CT because of clinically suspected acute pulmonary embolism. All patients were examined by electron beam CT, and scanning was performed in a cephalocaudad direction from the top of the aortic arch to the base of the heart with 3-mm collimation, 2-mm table incrementation, and an exposure time of 0.2 second (130 peak kV, 620 mA, and standard reconstruction algorithm). Contrast material was infused at a rate of 3 to 4 mL/s through an antecubital vein with an automated injector. Findings on CT were interpreted as either positive or negative. The main outcome measures were deep venous thrombosis, pulmonary embolism, and vital status within 3 months after the CT scan and the cause of death based on medical record review, mailed patient questionnaires, and telephone interviews. In 1010 patients (67%) CT scans were interpreted as negative for acute pulmonary embolism. Seventeen patients were excluded because they received anticoagulation. Of the remaining 993 patients, deep venous thrombosis or pulmonary embolism developed in 8; 118 patients died, 3 of pulmonary embolism. Nineteen patients were known to be alive, but additional clinical information could not be obtained. The 3-month cumulative incidence of overall deep venous thrombosis or pulmonary embolism was 0.5% (95% confidence interval, 0.1%-1.0%) and of fatal pulmonary embolism, 0.3% (95% confidence interval, 0.0%-0.7%). The incidence of (1) overall deep venous thrombosis or pulmonary embolism or (2) fatal pulmonary embolism among patients with suspected acute pulmonary embolism, negative CT results, and no other evidence of venous thromboembolism is low. Withholding anticoagulation in these patients appears to be safe.

Outcomes After Withholding Anticoagulation From Patients With Suspected Acute Pulmonary Embolism and Negative Computed Tomographic Findings: A Cohort Study

To determine the outcome of withholding anticoagulation from patients with suspected acute pulmonary embolism in whom computed tomographic (CT) findings are interpreted as negative for pulmonary embolism. This retrospective cohort study included 1512 consecutive patients referred from August 7, 1997, to November 30, 1998, for CT because of clinically suspected acute pulmonary embolism. All patients were examined by electron beam CT, and scanning was performed in a cephalocaudad direction from the top of the aortic arch to the base of the heart with 3-mm collimation, 2-mm table incrementation, and an exposure time of 0.2 second (130 peak kV, 620 mA, and standard reconstruction algorithm). Contrast material was infused at a rate of 3 to 4 mL/s through an antecubital vein with an automated injector. Findings on CT were interpreted as either positive or negative. The main outcome measures were deep venous thrombosis, pulmonary embolism, and vital status within 3 months after the CT scan and the cause of death based on medical record review, mailed patient questionnaires, and telephone interviews. In 1010 patients (67%) CT scans were interpreted as negative for acute pulmonary embolism. Seventeen patients were excluded because they received anticoagulation. Of the remaining 993 patients, deep venous thrombosis or pulmonary embolism developed in 8; 118 patients died, 3 of pulmonary embolism. Nineteen patients were known to be alive, but additional clinical information could not be obtained. The 3-month cumulative incidence of overall deep venous thrombosis or pulmonary embolism was 0.5% (95% confidence interval, 0.1%-1.0%) and of fatal pulmonary embolism, 0.3% (95% confidence interval, 0.0%-0.7%). The incidence of (1) overall deep venous thrombosis or pulmonary embolism or (2) fatal pulmonary embolism among patients with suspected acute pulmonary embolism, negative CT results, and no other evidence of venous thromboembolism is low. Withholding anticoagulation in these patients appears to be safe.

Advanced single-slice rebinning for tilted spiral cone-beam CT.

Future medical CT scanners and today's micro CT scanners demand cone-beam reconstruction algorithms that are capable of reconstructing data acquired from a tilted spiral trajectory where the vector of rotation is not necessarily parallel to the vector of table increment. For the medical CT scanner this case of nonparallel object motion is met for nonzero gantry tilt: the table moves into a direction that is not perpendicular to the plane of rotation. Since this is not a special application of medical CT but rather a daily routine in head exams, there is a strong need for corresponding reconstruction algorithms. In contrast to medical CT, where the special case of nonperpendicular motion is used on purpose, micro CT scanners cannot avoid aberrations of the rotational axis and the table increment vector due to alignment problems. Especially for those micro CT scanners that have the lifting stage mounted on the rotation table (in contrast to setups where the lifting stage holds the rotation table), this kind of misalignment is equivalent to a gantry tilt. We therefore generalize the advanced single-slice rebinning algorithm (ASSR), which is considered a very promising approach for medical cone-beam reconstruction due to its high image quality and its high reconstruction speed [Med. Phys. 27, 754-772 (2000)], to the case of tilted gantries. We evaluate this extended ASSR approach (which we will denote as ASSR+, for convenience) in comparison to the original ASSR algorithm using simulated phantom data for reconstruction. For the case of nonparallel object motion ASSR+ shows significant improvements over ASSR, however, its computational complexity is slightly increased due to the broken symmetry of the spiral trajectory.

Influence of image acquisition parameters on CT artifacts and polyp depiction in spiral CT colonography: in vitro evaluation.

To quantify the effects of spiral computed tomographic (CT) acquisition parameters on the magnitude of three-dimensional (3D) rippling artifacts and polyp depiction. An in vitro colon phantom was constructed with air-filled acrylic cylinders that contained synthetic polyps of 3-13 mm. The phantom was submerged in fluid and positioned at four angles of inclination relative to the z axis. Image data were acquired at collimation and pitch combinations of 3 mm and 1.67 and 5 mm and 1.6, respectively. Rippling artifacts were quantified by measuring the longitudinal variation of in-plane phantom edge width, and the influence of these artifacts on the depiction of pedunculated and sessile polyps was assessed qualitatively. The in-plane magnitude of the rippling artifact was a function of the angle of inclination relative to the longitudinal axis and the table increment. The through-plane periodicity of the artifact was equal to one-half the table increment. The table increment and angle of inclination of the surface of the object relative to the z axis determine the periodicity and magnitude of the rippling artifact at 3D spiral CT colonography. Although the depiction of small pedunculated polyps was not compromised, some sessile polyps were degraded by the artifact.

ECG-correlated image reconstruction from subsecond multi-slice spiral CT scans of the heart.

Subsecond spiral computed tomography (CT) offers great potential for improving heart imaging. The new multi-row detector technology adds significantly to this potential. We therefore developed and validated dedicated cardiac reconstruction algorithms for imaging the heart with subsecond multi-slice spiral CT utilizing electrocardiogram (ECG) information. The single-slice cardiac z-interpolation algorithms 180 degrees CI and 180 degrees CD [Med. Phys. 25, 2417-2431 (1998)] were generalized to allow imaging of the heart for M-slice scanners. Two classes of algorithms were investigated: 180 degrees MCD (multi-slice cardio delta), a partial scan reconstruction of 180 degrees + delta data with a < phi (fan angle) resulting in effective scan times of 250 ms (central ray) when a 0.5 s rotation mode is available, and 180 degrees MCI (multi-slice cardio interpolation), a piecewise weighted interpolation between successive spiral data segments belonging to the same heart phase, potentially providing a relative temporal resolution of 12.5% of the heart cycle when a four-slice scanner is used and the table increment is chosen to be greater than or equal to the collimated slice thickness. Data segments are selected by correlation with the simultaneously recorded ECG signal. Theoretical studies, computer simulations, as well as patient measurements were carried out for a multi-slice scanner providing M = 4 slices to evaluate these new approaches and determine the optimal scan protocol. Both algorithms, 180 degrees MCD and 180 degrees MCI, provide significant improvements in image quality, including extremely arythmic cases. Artifacts in the reconstructed images as well as in 3D displays such as multiplanar reformations were largely reduced as compared to the standard z-interpolation algorithm 180 degrees MLI (multi-slice linear interpolation). Image quality appears adequate for precise calcium scoring and CT angiography of the coronary arteries with conventional subsecond multislice spiral CT. It turned out that for heart rates fH > or = 70 min(-1) the partial scan approach 180 degrees MCD yields unsatisfactory results as compared to 180 degrees MCI. Our theoretical considerations show that a freely selectable scanner rotation time chosen as a function of the patient's heart rate, would further improve the relative temporal resolution and thus further reduce motion artifacts. In our case an additional 0.6 s mode besides the available 0.5 s mode would be very helpful. Moreover, if technically feasible, lower rotation times such as 0.3 s or even less would result in improved image quality. The use of multi-slice techniques for cardiac CT together with the new z-interpolation methods improves the quality of heart imaging significantly. The high temporal resolution of 180 degrees MCI is adequate for spatial and temporal tracking of anatomic structures of the heart (4D reconstruction).

The effect of pitch in multislice spiral/helical CT.

The purpose of this study is to understand the effect of pitch on raw data interpolation in multislice spiral/helical computed tomography (CT) and provide guidelines for scanner design and protocol optimization. Multislice spiral CT is mainly characterized by the three parameters: the number of detector arrays, the detector collimation, and the table increment per x-ray source rotation. The pitch in multislice spiral CT is defined as the ratio of the table increment over the detector collimation in this study. In parallel to the current framework for studying longitudinal image resolution, the central fan-beam rays of direct and opposite directions are considered, assuming a narrow cone-beam angle. Generally speaking, sampling in the Radon domain by the direct and opposite central rays is nonuniform along the longitudinal axis. Using a recently developed methodology for quantifying the sensitivity of signal reconstruction from nonuniformly sampled finite points, the effect of pitch on raw data interpolation is analyzed in multislice spiral CT. Unlike single-slice spiral CT, in which image quality decreases monotonically as the pitch increases, the sensitivity of raw data interpolation in multislice spiral CT increases in an alternating way as the pitch increases, suggesting that image quality does not decrease monotonically in this case. The most favorable pitch can be found from the sensitivity-pitch plot for any given set of multislice spiral CT parameters. An example for four-slice spiral CT is provided. The study on the effect of pitch using the sensitivity analysis approach reveals the fundamental characteristics of raw data interpolation in multislice spiral CT, and gives insights into interaction between pitch and image quality. These results may be valuable for design of multislice spiral CT scanners and imaging protocol optimization in clinical applications.

Left ventricular volume measurements in pigs by contrast-enhanced electron beam tomography (EBT) with and without table increment — how accurate is the new 12-level multislice mode?

Left ventricular volume measurements in pigs by contrast-enhanced electron beam tomography (EBT) with and without table increment — how accurate is the new 12-level multislice mode?

Contribution of Low-Dose CT-Scan Protocols to the Total Positioning Error in Computer-Assis ted Surgery

Purpose: To quantify the contribution of a computed tomography (CT) scan to navigation accuracy in computer-assisted surgeryMethods: Eighty-eight patients undergoing computer-assisted facial or skull-base surgery were fitted preoperatively with 4 to 12 markers, either attached to the skin (n = 20) or fixed in the osseous skull (micro-screws; n = 68). Low-dose high-resolution spiral CTwas achieved with 25-cm field of view (FoV), 1-mmslice thickness, 2-mm table increment, 1-mm reconstruction interval, 140 kV, 40 mA, bony reconstruction algorithm, and 180° reconstruction profile (effective slice thickness = 1.8 mm). During surgery, navigation accuracy was evaluated using two navigation systemsResults: Mean error was 0.66 mm for osseous markers and 1.58 mm for cutaneous markers. Both values are markedly smaller than the effective slice thickness of the scan protocol used. Radiation exposure of the patient for the entire examination never exceeded that necessary for one single 10-mm slice in a standard brain examination. Despite the reduced dose, landmarks and fiducials were precisely identified in all casesConclusions: The CT-induced positioning error in the Z-axis is considerably reduced by overlapping raw data reconstruction. For 1-mmslices and a 25-cm FoV, the average scan-induced positioning error is about 0.3 mm. Spatial resolution is not affected by the low dose applied. For MRI-based navigation, a 1 mm3 voxel size is the best compromise between signal-to-noise ratio, spatial resolution and scan time.