Helical Artifacts Research Articles

Low pitch significantly reduces helical artifacts in abdominal CT

PurposeHigh helical pitch scanning minimizes scan times in CT imaging, and thus also minimizes motion artifact and mis-synchronization with contrast bolus. However, high pitch produces helical artifacts that may adversely affect diagnostic image quality. This study aims to determine the severity and incidence of helical artifacts in abdominal CT imaging and their relation to the helical pitch scan parameter. MethodsTo obtain a dataset with varying pitch values, we used CT exam data both internal and external to our center. A cohort of 59 consecutive adult patients receiving an abdomen CT examination at our center with an accompanying prior examination from an external center was selected for retrospective review. Two expert observers performed a blinded rating of helical artifact in each examination using a five-point Likert scale. The incidence of artifacts with respect to the helical pitch was assessed. A generalized linear mixed-effects regression (GLMER) model, with study arm (Internal or External to our center) and helical pitch as the fixed-effect predictor variables, was fit to the artifact ratings, and significance of the predictor variables was tested. ResultsFor a pitch of <0.75, the proportion of exams with mild or worse helical artifacts (Likert scores of 1–3) was <1%. The proportion increased to 16% for exams with pitch between 0.75 and 1.2, and further increased to 78% for exams with a pitch greater than 1.2. Pitch was significantly associated with helical artifact in the GLMER model (p = 2.8 × 10−9), while study arm was not a significant factor (p = 0.76). ConclusionThe incidence and severity of helical artifact increased with helical pitch. This difference persisted even after accounting for the potential confounding factor of the center where the study was performed.

Early-phase pelvic bone SPECT: Simulation and comparison of several acquisition protocols to reduce bladder artifact and improve image quality.

Tomoscintigraphic reconstruction in nuclear medicine assumes that the distribution of the tracer is unchanged in the volume of interest throughout the duration of the acquisition. This condition is however not met in early-phase bone scintigraphy and early-phase pelvic SPECT may display helical artifacts due to the filling of the bladder. Those artifacts may hamper proper interpretation of surrounding bone areas. The aim of this study was to construct a 4D digital pelvic phantom to simulate different acquisition protocols and optimize the acquisition.A 4D digital pelvic phantom was generated with a dynamic component consisting in an expanding bladder with 2 ureters and a static part consisting in the 2 kidneys, bone structures, and soft tissues. Projection data were obtained using an attenuated Radon transform function. Four acquisitions protocols were tested: 32 projections of 16 seconds (32–16–1), 32 projections of 8 seconds (32–8–1), 2 consecutive SPECT of 32 projections of 4 seconds (32–4–2) and 2 consecutive SPECT of 16 projections of 8 seconds (16–8–2). The optimal protocol was then tested on one patient.The amplitude of the artifacts was reduced with the 32–8–1, 32–4–2, and 16–8–2 protocols. The 16–8–2 protocol had the highest signal to noise ratio among those 3 protocols. The bladder artifact was visually markedly reduced on the patient acquisition with a 16–8–2 protocol.Two successive early-phase bone SPECT, with a lower number of projection than the usual protocol reduce the impact of the helical artifacts around the bladder.

石灰化病変の血管内腔評価における軌道同期ヘリカルスキャンを用いた下肢サブトラクションCT Angiographyの有用性

Massive calcification complicates the diagnosis of the blood vessel lumen in computed tomography angiography (CTA) of the arteries of the lower extremities. The purpose of this study was to evaluate subtraction CTA with the use of orbital synchronized helical scanning (OS-SCTA). Phantom study: We performed OS-SCTA and non-OSCTA of a calcified vessel phantom (ψ2.5 mm), and compared them with a non-calcified vessel phantom as the reference by full width at half maximum (FWHM) and full width at tenth maximum (FWTM) of maximum intensity projection (MIP) images. Clinical study: 58 patients with peripheral artery disease who were referred for angiography also underwent OS-SCTA. OS-SCTA was produced using MIP images. Findings were graded according to three categories: (1) stenosis greater than 50% or occluded; (2) stenosis less than 50%; (3) not detected due to insufficient image quality. OS-SCTA findings were compared with the angiographic findings for each arterial segment. In the phantom study, FWHM showed no significant difference between OS-SCTA and the reference (P=0.135), whereas FWTM showed a significant difference (P<0.001). FWHM and FWTM showed a significant difference between non-OS-SCTA and the reference (P<0.001), due to misregistration with helical artifacts. In a clinical study comparing OS-SCTA with angiography, the sensitivity and specificity were 93.3% and 95.1% in calcified segments, 91.8% and 93.9% in non-calcified segments, and 92.2% and 94.6% in all segments. There was no significant difference between calcified segments and non-calcified segments (sensitivity: P=0.568, specificity: P=0.549). OS-SCTA is beneficial for the diagnosis of lower extremity arteries with vessel wall calcification, since it shows detection accuracy comparable to that of angiography.

頭部ＣＴにおけるヘリカルスキャンとノンヘリカルスキャンのモーションアーチファクトに関する検討

For head computed tomography (CT), non-helical scanning has been recommended even in the widely used multi-slice CT (MSCT). Also, an acute stroke imaging standardization group has recommended the non-helical mode in Japan. However, no detailed comparison has been reported for current MSCT with more than 16 slices. In this study, we compared the non-helical and helical modes for head CT, focusing on temporal resolution and motion artifacts. The temporal resolution was evaluated by using temporal sensitivity profiles (TSPs) measured using a temporal impulse method. In both modes, the TSPs and temporal modulation transfer factors (MTFs) were measured for various pitch factors using 64-slice CT (Aquilion 64, Toshiba). Two motion phantoms were scanned to evaluate motion artifacts, and then quantitative analyses for motion artifacts and helical artifacts were performed by measuring multiple regions of interest (ROIs) in the phantom images. In addition, the rates of artifact occurrence for retrospective clinical cases were compared. The temporal resolution increased as the pitch factor was increased. Remarkable streak artifacts appeared in the non-helical images of the motion phantom, in spite of the equivalent effective temporal resolution. In clinical analysis, results consistent with the phantom studies were shown. These results indicated that the low pitch helical mode would be effective for emergency head CT with patient movement.

THE VETMOUSETRAP™: A DEVICE FOR COMPUTED TOMOGRAPHIC IMAGING OF THE THORAX OF AWAKE CATS

The VetMousetrap, a novel device that allows computed tomography (CT) of awake cats and provides a clinically supportive environment, is described. Ten normal cats were used to test the device for ambient internal oxygen, carbon dioxide levels, and temperature. Twenty-two awake normal cats were imaged using a 16-multislice helical CT unit to evaluate dose-equivalent protocols. Two different X-ray tube potentials (kV), 80 and 120, and two different helical pitches, 0.562 and 1.75, were evaluated. The signal intensity of the pulmonary parenchyma (SIlung), signal intensity of background (SIbackgr), contrast, noise, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) were calculated. Three evaluators ranked the images for sharpness of liver margins, motion, helical, and windmill artifacts. CT was successfully completed in 20 of 22 cats. No artifacts directly related to the device were detected. Overall, 75 of 80 (94%) examinations were judged to have absent or minimal motion artifact. A statistically significant difference was found for SNR (P = 0.001) and CNR (P = 0.001) between all protocols. The higher pitch protocols had significantly lower noise and higher SNR and CNR, lower motion artifact but greater helical artifacts. A protocol using 80 kV, 130 mA, 0.5s, and 0.562 pitch with 1.25mm slice thickness, and 0.625 mm slice reconstruction interval is recommended. The VetMousetrap appears to provide the opportunity for diagnostic CT imaging of the thorax of awake cats.

SU‐FF‐I‐27: Optimal CT Imaging Protocol Settings for CT Simulators

Purpose: It is the expectation of CT simulations to get the best CT images with free breathing scans. CT imaging protocol settings needed to achieve optimal image quality have been identified for every scanner in a Radiation Oncology Department having various makes and models of CT simulators. Method and Materials: A Catphan® phantom was used to evaluate the image quality of 5 different makes/models. Various pitch, detector configuration, and gantry speed parameters were studied, while maintaining constant kVp, effective mAs, head display field of view (DFOV), reconstruction filter, and image thickness settings. Three aspects of image quality were evaluated: 1) helical artifact presence, 2) high contrast resolution, and 3) low contrast resolution. Results: For all CTs, more artifacts appear when the channel width is equal to the image thickness. Less helical artifacts are present if the channel width is half or less of the image thickness. At these settings, however, artifacts are still present in varying degrees depending on pitch. For a 16‐channel CT of one manufacturer, the artifact‐free pitch setting is 1.4 or less, but for their 16‐channel large bore CT, it is 0.9 or less. For another manufacturer's 16‐channel CT, the channel width has to be 1/4 of the image thickness with a pitch of 0.6 to obtain artifact‐free images. The optimal settings for all scanners are different. Only slight differences in high and low contrast resolution are observed, even at varying gantry rotation speeds; these differences seem insignificant at body DFOV (clinical setting). Conclusions: In order to obtain optimal CT image settings, a study should be conducted for each CT model, for there is a vast difference in optimal settings among them. Faster rotation gantry speed should be used as possible, for reduced motion artifact is expected for patient free‐breathing scans.

MDCT Application in the Vascular System

Helical CT has improved with faster gantry rotation, more powerful X-ray tubes, and improved interpolation algorithms; however, the greatest advance has been made by the recent introduction of multi detector-row computed tomography (MDCT) scanners. Fundamental advantages of MDCT include substantially shorter acquisition times, retrospective creation of thinner or thicker sections from the same raw data, and improved three-dimensional (3-D) rendering with diminished helical artifacts. While these features will likely be important to many applications of CT scanning, the greatest impact has been on CT angiography. The advantages of MDCT over single detector-row CT scanners when imaging the vascular system can be broken down into three fundamental improvements, that is, speed (faster scan time), distance (longer coverage), and section thickness (better resolution). This article will focus on how the MDCT technology has substantially improved imaging of the vascular system, including pulmonary artery, aorta and extremity vessels.

サブトラクション処理を用いたヘリカルアーチファクトの新しい評価法

This paper proposes a new quantitative method of appraising the helical artifact generated by computed tomography (CT) images by helical scan. The quantities-appraisal method of the helical artifact uses as a sample image, the binarization image, which performs the threshold operation. Therefore, results may differ among observers and are lacking in objectivity. We devised a procedure (subtraction technique) that uses the subtraction process as a method of appraising the helical artifact that does not a perform a threshold operation, and evaluated this procedure. As compared with an area technique, a subtraction technique can divide a helical artifact into a plus CT value (Over Level) and a minus CT value (Under Level) and can evaluate them. This provides excellent repeatability and objectivity. Moreover, since CT value data are maintained, the quality of an artifact can also be evaluated, making this a very useful procedure. By using the subtraction technique, new information will be found by evaluating helical artifacts.

Acute Aortic Dissection

Aortic dissection may be fatal without early diagnosis and appropriate medical, surgical, or endovascular treatment. The presenting symptoms and signs are so myriad and nonspecific that dissection may be overlooked initially in up to 40% of cases. In addition, the diagnosis is established only postmortem in a substantial number of cases.1 Few other conditions demand such prompt diagnosis and treatment, because the mortality rate of untreated dissection approaches 1%/h during the first 48 hours, 80% at 14 days, and 90% at three months.2 If unrecognized and untreated, fewer than 10% of patients with proximal aortic dissection survive a year. Most patients succumb within the first 3 months,3 usually of acute aortic insufficiency, major branch vessel occlusion, or rupture into the pericardium, mediastinum, or left hemithorax. In 20 years of follow-up of 527 patients with aortic dissection, nearly 30% of late deaths were due to ruptured aortic aneurysm.4 See page 1839 The most frequently used modalities to identify dissection and define the sites of origin and termination are computerized tomography (CT), transesophageal echocardiography (TEE), and magnetic resonance (MR) imaging. The primary diagnostic criterion for diagnosis of aortic dissection by CT is demonstration of two contrast-filled lumens separated by an intimal flap.5 The sensitivity of CT ranges from 93% to 100% and specificity from 87% to 100%.5,6 Inaccuracy may result from inadequate contrast opacification, nonvisualization of the intimal flap, artifacts extending across the aortic lumen that simulate an intimal flap, misinterpretation of adjacent vessels or prominent sinus of Valsalva as the flap, atelectasis, pleural thickening, or thrombosis of the false lumen. Multidetector-row CT scanners offer more rapid image acquisition, variable section thickness, 3-dimensional rendering, diminished helical artifacts, and smaller contrast requirements, overcoming many of the limitations discussed above.7 The sensitivity and specificity of MR imaging …

MDCT imaging of the aorta and peripheral vessels

Since its clinical introduction in 1991, volumetric CT scanning using spiral or helical scanners has resulted in a revolution for diagnostic imaging. Helical CT has improved over the past 8 years with faster gantry rotation, more powerful X-ray tubes, and improved interpolation algorithms, but the greatest advance has been the recent introduction of multi detector-row CT (MDCT) scanners [J. Comput. Assist. Tomogr. 23 (1999) S83]. Currently capable of acquiring four channels of helical data simultaneously, MDCT scanners have achieved the greatest incremental gain in scan speed since the development of helical CT and have profound implications for clinical CT scanning. Fundamental advantages of MDCT include substantially shorter acquisition times, retrospective creation of thinner or thicker sections from the same raw data, and improved three-dimensional (3-D) rendering with diminished helical artifacts. While these features will likely be important to many applications of CT scanning, including the characterization of focal lung and liver lesions through the creation of thin sections retrospectively, the greatest impact has been on CT angiography. The implication for CT angiography is that scans can be performed approximately three-times faster than is possible with the fastest single-detector CT scanner. For example a 1.25 mm nominal thick section (1.6 mm effective section thickness) can be acquired with a table speed of 9.4 mm/s, and a 2.5 mm nominal thick section (3.2 mm effective section thickness) can be acquired with an 18.8 mm/s table speed. The advantages of MDCT for imaging the vascular system can be broken down into three fundamental improvements over single detector-row CT scanners speed (faster), distance (longer), and section thickness (better). The focus of this article will be how multidetector-row CT technology has substantially improved aortoiliac and lower extremity arterial imaging.

Optimization of multidetector array CT acquisition parameters for CT colonography.

Multidetector array computed tomography technology has increased the number of acquisition parameters that a user must select. This article examines the criteria by which detector collimation, reconstructed scan width, reconstruction scan interval, pitch, image noise, and patient dose can be optimized. A water phantom containing tissue-equivalent polyps in an air-filled tube was scanned using multiple acquisition parameter combinations. X-ray tube current was changed to independently match dose and noise to a single-detector array technique. Images were assessed in axial and three-dimensional formats. All polyps were visible in axial and three-dimensional images for all scans. Helical artifacts were noted at higher table speed values. The 5-mm-wide scans spaced at 3-mm intervals allowed visualization of the smallest polyps: 5 x 5 or 7 x 3 (diameter x height; mm). Our results indicate that thin-scan and low-noise techniques need not be used to visualize polyps of clinically relevant sizes.

Performance evaluation of a multi-slice CT system.

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

Helical CT reconstruction with longitudinal filtration.

Each of the currently used helical reconstruction algorithms represents one tradeoff selection of the image noise or mA requirement and resultant artifacts versus the width of the section profile. For many applications, the performance tradeoffs different from those existed are desired. In this paper, we present a general framework of incorporating the longitudinal (z) filtration in helical CT reconstruction, which leads to a new class of helical reconstruction algorithms with tunable performance characteristics to suit various applications. The z-filtering algorithm enables us to derive the performance curve of image noise or mA requirement versus the width of the section profile, whereas the existing helical reconstruction algorithms only represent limited discrete samples of the curve. We demonstrate that using a new helical reconstruction algorithm derived from this general framework, a substantial reduction in image noise or mA requirement can be achieved with a slight increase of FWHM of the section profile. Thus for many applications requiring both compact section profile and low image noise or mA requirement, the new algorithm represents a better tradeoff than both the 180 degrees and 360 degrees interpolation algorithms. The new algorithm also offers the following potential clinical benefits: (1) longer x-ray tube life; (2) longer scan duration with less x-ray tube cooling delays--longer volume coverage and/or longer thin-slice scan; (3) less radiation to patient; (4) less image noise; and (5) less helical artifacts. Initial clinical results are also presented.

The use of deblurring technique for improving the longitudinal resolution in helical CT of the head and neck region

We evaluated the use of deblurring technique by digital deconvolution for improving the longitudinal resolution in helical CT in a phantom and CT images of head and neck patients. After performing the helical scanning, overlapped axial images were reconstructed at 1/10 of the actual collimation. Then, the deblurring technique was performed. Results revealed that the resolution of slice profiles was improved after deblurring. Further, the longitudinal resolution increased as the strength of the deconvolution increased, at the expense of an increase in helical artifacts. The sharpness and visualization of horizontally running structures were improved in the reconstructed MPR, MPVR, MIP and 3D images in both the phantom and the images of all patients. Also, visualization of the peripheral vessels in CTAs was improved.