Object Detectability Research Articles

Focal spot size reduction using asymmetric collimation to enable reduced anode angles with a conventional angiographic x-ray tube for use with high resolution detectors.

The high-resolution requirements for neuro-endovascular image-guided interventions (EIGIs) necessitate the use of a small focal-spot size; however, the maximum tube output limits for such small focal-spot sizes may not enable sufficient x-ray fluence after attenuation through the human head to support the desired image quality. This may necessitate the use of a larger focal spot, thus contributing to the overall reduction in resolution. A method for creating a higher-output small effective focal spot based on the line-focus principle has been demonstrated and characterized. By tilting the C-arm gantry, the anode-side of the x-ray field-of-view is accessible using a detector placed off-axis. This tilted central axis diminishes the resultant focal spot size in the anode-cathode direction by the tangent of the effective anode angle, allowing a medium focal spot to be used in place of a small focal spot with minimal losses in resolution but with increased tube output. Images were acquired of two different objects at the central axis, and with the C-arm tilted away from the central axis at 1° increments from 0°-7°. With standard collimation settings, only 6° was accessible, but using asymmetric extended collimation a maximum of 7° was accessed for enhanced comparisons. All objects were positioned perpendicular to the anode-cathode direction and images were compared qualitatively. The increasing advantage of the off-axis focal spots was quantitatively evidenced at each subsequent angle using the Generalized Measured-Relative Object Detectability metric (GM-ROD). This anode-tilt method is a simple and robust way of increasing tube output for a small field-of-view detector without diminishing the overall apparent resolution for neuro-EIGIs.

Methods of Collecting and Analyzing Rearward Visibility Data for Agricultural Machinery: Hazard and/or Object Detectability.

Recent interest in rearward visibility for private, construction, and commercial vehicles and documentation of rearward runovers involving bystanders outside the field of vision of the vehicle operator led to an investigation into the need for enhanced methods of rearward visibility for large, off-highway, agricultural equipment. A review of the literature found limited relevant research and minimal data on incidents involving rearward runovers of bystanders and co-workers. This article reviews the findings regarding the methods identified and tested to collect and analyze rearward visibility data, from the operator's perspective, for large self-propelled agricultural equipment, including the four-wheel drive tractors, combines, agricultural sprayers, and skid-steer loaders that are increasingly found on agricultural production sites. The methods identified, largely drawn from research conducted on private and commercial vehicles, were tested to determine their application in identifying rearward blind spots. These methods are described, and the findings from field-testing of specific machines are provided. Recommendations include establishing an appropriate engineering standard regarding rearward visibility of agricultural equipment with limited rearward vision and the use of rearward alarm systems for warning bystanders of rearward movement.

Optimization of contrast-enhanced breast imaging: Analysis using a cascaded linear system model.

Contrast-enhanced (CE) breast imaging involves the injection contrast agents (i.e., iodine) to increase conspicuity of malignant lesions. CE imaging may be used in conjunction with digital mammography (DM) or digital breast tomosynthesis (DBT) and has shown promise in improving diagnostic specificity. Both CE-DM and CE-DBT techniques require optimization as clinical diagnostic tools. Physical factors including x-ray spectra, subtraction technique, and the signal from iodine contrast, must be considered to provide the greatest object detectability and image quality. We developed a cascaded linear system model (CLSM) for the optimization of CE-DM and CE-DBT employing dual energy (DE) subtraction or temporal (TE) subtraction. We have previously developed a CLSM for DBT implemented with an a-Se flat panel imager (FPI) and filtered backprojection (FBP) reconstruction algorithm. The model is used to track image quality metrics - modulation transfer function (MTF) and noise power spectrum (NPS) - at each stage of the imaging chain. In this study, the CLSM is extended for CE breast imaging. The effect of x-ray spectrum (varied by changing tube potential and the filter) and DE and TE subtraction techniques on breast structural noise was measured was studied and included as a deterministic source of noise in the CLSM. From the two-dimensional (2D) and three-dimensional (3D) MTF and NPS, the ideal observer signal-to-noise ratio (SNR), also known as the detectability index (d'), may be calculated. Using d' as a FOM, we discuss the optimization of CE imaging for the task of iodinated contrast object detection within structured backgrounds. Increasing x-ray energy was determined to decrease the magnitude of structural noise and not its correlation. By performing DE subtraction, the magnitude of the structural noise was further reduced at the expense of increased stochastic (quantum and electronic) noise. TE subtraction exhibited essentially no residual structural noise at the expense of increased quantum noise, even over that of the DE case. For DE subtraction, optimization of dose weighting to the HE view (fh ) results in the minimization of quantum noise. Both subtraction weighting factor (wSub ) and the iodine contrast signal were dependent on the LE and HE x-ray spectra. To best detect a 5 mm Gaussian lesion with 5 mg/ml of iodine within a 4 cm thick breast, it was found that the high energy (HE) view should be acquired with a tube potential of 47 kVp (W/Ti spectrum) and the low energy (LE) view with a potential of 23 kVp (W/Rh spectrum). Due to the complete removal of structural noise, TE subtraction produced much higher d' than DE subtraction both as a function of mean glandular dose and iodine concentration. We have shown the effect of increasing x-ray energy as well as projection domain subtraction on breast structural noise. Further, we have exhibited the utility of the CLSM for DE and TE subtraction CE imaging in the optimization of imaging parameters such as x-ray energy, fh , and wSub as well as guiding the understanding of their effects on image contrast and noise.

The effect of quality control on the function of magnetic resonance imaging (MRI), using American College of Radiology (ACR) phantom

ObjectiveTo study image quality of MRI scanner using the American College of Radiology (ACR) phantom. Material and methodsImage quality of 1.5T MRI scanner was tested using ACR phantom. A standard head coil with standard restraints was used to fix the phantom head position. The phantom included seven modules for measuring MRI scanner image quality. MRI images for each module were analyzed. ResultsFor the Geometric accuracy test the inside length of the phantom was 146mm and the inside diameter was 189.4mm. For the High-contrast spatial resolution, individual small bright spots on the image were three pairs of hole arrays and were distinguishable. For the slice thickness test, the top signal ramp length was 54.4mm and bottom signal ramp length was 54.8mm so the slice thickness was 5.46mm. For the slice position accuracy, the bar length differences of intensity uniformity a value, called percent integral uniformity (PIU), was 96.15%. For the percent-signal ghosting, ghosting ratio was 0.0002. For the low contrast object detectability, the sum of numbers of complete spokes scored was 30 spokes. ConclusionsImage quality tests were very important in acceptance of any MRI scanner after installation and during maintenance. Using ACR phantom, these tests approve that the image parameters are acceptable.

Test Object for Accurate and Reproducible Measurement of the Detection Response of Hand-worn and Hand-held Metal Detectors.

The assessment of the detection performance of metal detectors (hand-worn, hand-held, and walk-through models) is based on the ability of the detectors to sound an alarm when presented with a test object. These test objects are typically actual threat items or simulated threat items. The orientation of these test objects with the magnetic field generated by the metal detectors may affect the detectability of the test objects. More importantly, small misorientations of a threat object or simulated threat object may cause the operator to incorrectly attribute to the metal detector a higher performance than it is capable of providing. Consequently, to support accurate and reproducible characterization of the performance of a metal detector, orientation effects should be minimized or eliminated. We discuss the use of spherical test objects to eliminate this potentially serious error in the assessment of the detection performance of a metal detector. In this study, we consider only hand-worn and hand-held metal detectors because of their similarity in size and operation.

Basic quality control in routine MRI

Aim Magnetic Resonance Imaging (MRI) has evolved a valid clinical tool in everyday practice. Its unique capabilities of imaging soft tissue human anatomy and physiology with excellent spatial, temporal and contrast resolution explain its outstanding role in clinical imaging. The aim of this lecture is to present the rationale of Quality Control (QC) and Quality Assurance (QA) in MRI and to briefly present a set of basic QC tests that can ensure MRI system’s performance and help on maintain diagnostic quality in routine clinical MRI examinations. Learning objectives • Understand the general rationale of QC and QA in MRI. • Presentation of the AAPM 100 protocol. Basic procedures and methodologies. • Presentation of accredited phantoms and tools used for basic QC and QA in MRI. • Presentation of a set of basic QC tests for assessing MRI system’s performance based solely on simple imaging post-processing methods. This will include assessments of the following system’s parameters: – H0 static field homogeneity – Signal to Noise (SNR) and Contrast to Noise (CNR) measurements – Spatial Uniformity of SNR – Chosting Ratio and MR image uniformity – Geometric Distortion and Spatial Linearity – Slice thickness – High Contrast Spatial Resolution – Low Contrast Object Detectability

Impact of iterative reconstructions on image noise and low-contrast object detection in low kVp simulated abdominal CT: a phantom study.

Low kilovoltage (kVp) computed tomography (CT) may be used to reduce contrast medium dose in patients at risk of contrast nephropathy, at the cost of increased image noise. To evaluate: (i) the impact of iterative reconstructions (Siemens SAFIRE) on low-contrast object detection to compensate for increased noise instead of increased tube loading when decreasing tube potential; and (ii) the change in iodine attenuation in simulated abdominal CT. A phantom was scanned at 70, 80, 100, and 120 kVp at fixed effective tube loading (170 mAsEFF) and fixed radiation dose (CTDIVOL 10 mGy). Images were reconstructed with filtered back-projection (FBP) and SAFIRE strengths S1-S5. Iodine attenuation, objective image noise, contrast-to-noise ratio (CNR), noise power spectrum (NPS), spatial resolution, and subjective detectability of low-contrast objects were evaluated. Compared with 120 kVp iodine attenuation increased by a factor 1.6 and 2.0, and image noise increased by a factor 1.9 and 2.5 at 80 and 70 kVp, respectively. Compared with FBP, SAFIRE showed objective reduction in image noise and increased CNR without loss of spatial resolution or any significant NPS alteration, with general tendency to improve subjective detectability of low-contrast objects. At 170 mAsEFF the number of discernible 1.0% contrast objects at 70 kVp/S5 and 80 kVp/S5 was similar to that at 120 kVp/FBP. With the SAFIRE algorithm image noise, CNR and detectability of low-contrast objects may be kept unchanged without increased tube loading when using low kVp settings to reduce contrast medium dose in azotemic patients.

SU‐F‐J‐143: Initial Assessment of Image Quality of An Integrated MR‐Linac System with ACR Phantom

Purpose/Objective(s):To assess the image quality of an integrated MR‐Linac system and compare with other MRI systems that are primarily used for diagnostic purposes.Materials/Methods:An ACR MRI quality control (QC) phantom was used to evaluate the image quality of a fully integrated 1.5T MRI‐Linac system recently installed at our institution. This system has a new split magnet design which gives the magnetic field strength of 1.5T. All images were acquired with a set of phased‐array surface coils which are designed to have minimal attention of radiation beam. The anterior coil rests on a coil holder which keeps the anterior coil's position consistent for QA purposes. The posterior coil is imbedded in the patient couch. Multiple sets of T1, T2/PD images were acquired using the protocols as prescribed by the ACR on three different dates, ranging 3 months apart.Results:The geometric distortion are within 0.5 mm in the axial scans and within 1mm in the saggital (z‐direction) scans. Slice thickness accuracy, image uniformity, ghosting ratio, high contrast detectability are comparable to other 1.5T diagnostic MRI scanners. The low‐contrast object detectability are lower comparatively, which is a result of using the body array coil. Additionally, the beam's‐eye‐view images (oblique coronal and saggital images) have minimal geometric distortion at all linac gantry angles tested. No observable changes or drift in image quality is found from images acquired 3 month apart.Conclusion:Despite the use of a body array surface coil, the image quality is comparable to that of an 1.5T MRI scanner and is of sufficient quality to pass the ACR MRI accreditation program. The geometric distortion of the MRI system of the integrated MR‐Linac is within 1mm for an object size similar to the ACR phantom, sufficient for radiation therapy treatment purpose.The authors received corporate sponsored research grants from Elekta which is the vendor for the MR‐Linac evaluated in this study.

SU‐G‐206‐10: Low‐Contrast Detectability Vs. Dose for CT Images Reconstructed Using Filtered Backprojection and Iterative Reconstruction: Assessment with a Model Observer

Purpose:To evaluate CT low‐contrast detectability at different dose levels in images reconstructed using filtered backprojection (FBP) and iterative reconstruction (IR).Methods:A validated channelized Hotelling observer (CHO) model was used to detect low contrast objects in images of a helical CT phantom (Model 061, CIRS) using an Aquilion Prime 160 scanner (Toshiba Medical Systems). Twenty‐one different objects of 7 diameters (2.4, 3.2, 4.6, 6.3, 9.5, and 10 mm) and 3 contrast levels (∼10, 15, and 25 HU) were evaluated. Images were acquired using 5 different tube current settings (45, 60, 90, 140, and 180 mA) and reconstructed using both FBP and IR (Adaptive Iterative Dose Reduction (AIDR) 3D). For each tube current, the phantom was scanned 100 times, yielding sets of 100 object‐present and object‐absent images at the same x‐y locations. All images were evaluated using the CHO and detectability indices (d’) for each object were calculated at all dose levels.Results:For the same tube current setting, images formed using IR yielded d’ values on average 8% higher than images reconstructed using FBP. When dose was reduced by more than 25% of the full dose, however, results showed reduced low‐contrast detectability in AIDR‐3D images compared to 100% dose, FBP images. With the vendordefault settings, dose is automatically reduced by 70‐75% when AIDR 3D reconstructions are selected, which our data indicate would significantly reduce low contrast detectability.Conclusion:At higher dose levels (minimal dose reduction), AIDR‐3D improved object detectability as compared to full‐dose FBP images. However, when dose was reduced by more than 25%, object detectability was substantially reduced for images reconstructed with AIDR‐3D compared to full‐dose FBP images.CH McCollough is supported by industry funding from Siemens Healthcare

WE‐DE‐207A‐04: Advances in Radiological Neuro‐Endovascular Interventional Imaging

WE‐DE‐207A‐04: Advances in Radiological Neuro‐Endovascular Interventional Imaging

SU‐F‐J‐159: Influence of the Elevated Posterior Position by Using the Customized Vacuum‐Bag On the Abdominal MR Image Quality: A Quantitative Phantom Study

Purpose:Signal‐to‐noise ratio(SNR) of MR abdominal imaging in diagnostic radiology is maximized by minimizing the coil‐to‐patient distance. However, for radiotherapy applications, customized vacuum‐bag is needed for abdominal immobilization at the cost of the increasing distance to the posterior spine coil. This sub‐optimized coil setting for RT applications may compromise image quality, such as SNR and homogeneity, thus potentially affect tissue delineation. In this study, we quantitatively evaluate the effect of the vertical position change on SNR and image quality change using an ACR MR phantom.Methods:An ACR MR phantom was placed on the flat couch top. Images were acquired using an 18‐channel body array coil and spine coil on a dedicated 1.5T MR‐simulator. The scan was repeated three times with the ACR phantom elevated up to 7.5cm from the couch top, with a step size of 2.5cm. All images were acquired using standard ACR test sequence protocol of 2D spin‐echo T1‐weighted(TR/TE=500/200ms) and T2‐weighted(TR/TE1/TE2=2000/20/80) sequences. For all scans, pre‐scan normalization was turned on, and the distance between the phantom and the anterior 18‐channel body array coil was kept constant. SNR was calculated using the slice with a large water‐only region of the phantom. Percent intensity uniformity(PIU) and low contrast object detectability(LCD) were assessed by following ACR test guidelines.Results:The decrease in image SNR(from 335.8 to 169.3) and LCD(T1: from 31 to 19 spokes, T2: 26 to 16 spokes) were observed with increasing vertical distance. After elevating the phantom by 2.5cm(approximately the thickness of standard vacuum‐bag), SNR change(from 335.8 to 275.5) and LCD(T1: 31 to 26 spokes, T2: 26 to 21 spokes) change were noted. However, similar PIU was obtained for all choices of vertical distance (T1: 94.5%–95.0%, T2: 94.4%–96.8%).Conclusion:After elevating the scan object, reduction in SNR level and contrast detectability but no change in image homogeneity was observed.

TU-FG-209-05: Demonstration of the Line Focus Principle Using the Generalized Measured-Relative Object Detectability (GM-ROD) Metric

Purpose: Demonstrate and quantify the augmented resolution due to focalspot size decrease in images acquired on the anode side of the field, for both small and medium (0.3 and 0.6mm) focal-spot sizes using the experimental task-based GM-ROD metric. Theoretical calculations have shown that a medium focal-spot can achieve the resolution of a small focal-spot if acquired with a tilted anode, effectively providing a higher-output small focal-spot. Methods: The MAF-CMOS (micro-angiographic fluoroscopic complementary-metal-oxide semiconductor) detector (75µm pixel pitch) imaged two copper wire segments of different diameter and a pipeline stent at the central axis and on the anode side of the beam, achieved by tilting the x-ray C-arm (Toshiba Infinix) to 6° and realigning the detector with the perpendicular ray to correct for x-ray obliquity. The relative gain in resolution was determined using the GM-ROD metric, which compares images on the basis of the Fourier transform of the image and the measured NNPS. To emphasize the geometric unsharpness, images were acquired at a magnification of two. Results: Images acquired on the anode side were compared to those acquired on the central axis with the same target-area focal-spot to consider the effect of an angled tube, and for all three objects the advantage of the smaller effective focal-spot was clear, showing a maximum improvement of 36% in GM-ROD. The images obtained with the small focal-spot at the central axis were compared to those of the medium focal-spot at the anode side and, for all objects, the relative performance was comparable. Conclusion: For three objects, the GM-ROD demonstrated the advantage of the anode side focal-spot. The comparable performance of the medium focal-spot on the anode side will allow for a high-output small focal-spot; a necessity in endovascular image-guided interventions. Partial support from an NIH grant R01EB002873 and an equipment grant from Toshiba Medical Systems Corp.

WE‐DE‐207A‐02: Advances in Cone Beam CT Anatomical and Functional Imaging in Angio‐Suite to Enable One‐Stop‐Shop Stroke Imaging Workflow

WE‐DE‐207A‐02: Advances in Cone Beam CT Anatomical and Functional Imaging in Angio‐Suite to Enable One‐Stop‐Shop Stroke Imaging Workflow

SU‐C‐209‐01: Validation of the Simulated Detectability Metric (G‐ROD) Using the Experimental Generalized Measured Relative Object Detectability Metric (GM‐ROD)

Purpose:The Generalized Relative Object Detectability (G‐ROD) family of ideal observer metrics are a well‐characterized set of task‐based metrics used for quantitatively comparing imaging systems that include the detector, focal spot geometry and scatter characteristics on the basis of system abilities in imaging a given simulated object. The G‐ROD metric takes the integral over spatial frequencies of the Fourier transform of a simulated object function weighted with the detector generalized DQE (GDQE), divided by the comparable integral for another detector. When a measured image of an actual object is used, the generalized measured‐ROD (GM‐ROD) metric can compare detector systems without explicitly determining detector MTF. In this work, the results of simulated and measured ROD calculations for the same object are compared for the first time.Methods:G‐ROD calculations were performed to compare a high resolution microangiographic fluoroscopic (MAF) detector to a flat panel detector (FPD) using 5mm segments of simulated copper wires of varying diameter (d=0.05–0.6mm) as the object function for three different focal spot sizes at two magnifications. A GM‐ROD experiment with similar parameters was performed to compare the same detector system using three actual 5mm copper wires (d=0.113mm,0.238mm,0.558mm) imaged under identical but simulated conditions used in G‐ROD calculations to determine the agreement between simulated and experimentally obtained results.Results:The G‐ROD and GM‐ROD calculations examined relative detector system imaging performance. The GM‐ROD results for all three different sized wires imaged with low magnification and a small focal spot size were 2.1, 1.38, and 1.1, respectively, and the G‐ROD results for the same calculation was 1.81, 1.3, and 1.07, corresponding to percent deviations of 13%, 5.8%, and 2.7%.Conclusion:These results indicate experimental validation of the simulated metric results.Partial support from NIH grant R01EB002873 and an equipment grant from Toshiba Medical Systems Corp.

WE‐DE‐207A‐03: Recent Advances in Devices Used in Neuro‐‐Interventions

WE‐DE‐207A‐03: Recent Advances in Devices Used in Neuro‐‐Interventions

WE‐DE‐207A‐01: Parallels in the Evolution of X‐Ray Angiographic Systems and Devices Used for Minimally Invasive Endovascular Therapy

WE‐DE‐207A‐01: Parallels in the Evolution of X‐Ray Angiographic Systems and Devices Used for Minimally Invasive Endovascular Therapy

WE‐DE‐207A‐00: Advances in Image‐Guided Neurointerventions‐Clinical Pull and Technology Push

WE‐DE‐207A‐00: Advances in Image‐Guided Neurointerventions‐Clinical Pull and Technology Push

SU‐F‐I‐22: Development and Implementation of Task‐Specific Modular Phantom for MRI Quality Evaluation

Purpose:To investigate the potential of a newly developed modular phantom with image quality test structure for MRI quality control.Methods:The modular phantom consists of individual basic mapping (BM) modules, image quality control (QC) modules, and accessories to improve combination variability for multipurpose use. BM modules were designed based on the stud‐and‐tube coupling system with an internal hollow and an individual enclosed structure. The QC modules include signal‐to‐noise ratio, image intensity uniformity, geometric accuracy, high‐contrast spatial resolution, slice thickness accuracy, slice position accuracy and low‐contrast object detectability evaluation test structure. The scanner performance on various coils were evaluated by using image QC module which inserted into BM module. The MR data acquisition was performed using a 1.5 T MRI scanner (MAGNETOM ESSENZA, Siemens, Germany) with two coils, head and breast. The 2D SE T1 axial images of assembled QC modules, which are analogous to the ACR accreditation phantom were acquired in the head coil. We scanned BM modules and QC modules to evaluation off‐center impact on the image quality in the breast coil. Oil‐filled BM modules and QC modules designed to evaluate geometric accuracy and image intensity uniformity were used. The 2D TSE T2 weighted axial images and short‐tau inversion recovery (STIR) image of modules were acquired in the breast coil.Results:The evaluated MRI quality was acceptable according to the action criteria from the ACR. Due to non‐uniformity across the breast coil, the residual fat contents were shown on the STIR image. For the uniformity of image intensity, the PIU was calculated as 89% in the STIR image.Conclusion:The results showed the potential of the BM modules and image QC modules in a image quality control for MRI based on the ability of task‐specific construction with modular manner.This study was supported by the Medical Metrology Program at the Korea Research Institutes of Standards and Science (16011069).

Optimization of CubeSat Constellations for Uncued Electrooptical Space Object Detection and Tracking

The proliferation of onorbit debris has motivated much of the recent space situational awareness missions and related research. Space-based missions are typically carried out by large spacecraft, yet the emerging and improving technology for CubeSat-class satellites offers a potential new platform for space situational awareness. This paper formally defines space object detectability for a given electrooptical sensor and applies an optimization problem approach to mission design for a CubeSat space situational awareness constellation. The optimization of the design parameters for this constellation is separated into a formal description of the concept of operations, an optimization of the electrooptical sensor payload itself, and an optimization of the constellation parameters via the number of satellites and traditional orbital parameters. A Pareto frontier of mission design points in terms of object detection capability and lifecycle cost is presented.

Quantitative comparison using Generalized Relative Object Detectability (G-ROD) metrics of an amorphous selenium detector with high resolution Microangiographic Fluoroscopes (MAF) and standard flat panel detectors (FPD).

A novel amorphous selenium (a-Se) direct detector with CMOS readout has been designed, and relative detector performance investigated. The detector features include a 25μm pixel pitch, and 1000μm thick a-Se layer operating at 10V/μm bias field. A simulated detector DQE was determined, and used in comparative calculations of the Relative Object Detectability (ROD) family of prewhitening matched-filter (PWMF) observer and non-prewhitening matched filter (NPWMF) observer model metrics to gauge a-Se detector performance against existing high resolution micro-angiographic fluoroscopic (MAF) detectors and a standard flat panel detector (FPD). The PWMF-ROD or ROD metric compares two x-ray imaging detectors in their relative abilities in imaging a given object by taking the integral over spatial frequencies of the Fourier transform of the detector DQE weighted by an object function, divided by the comparable integral for a different detector. The generalized-ROD (G-ROD) metric incorporates clinically relevant parameters (focal-spot size, magnification, and scatter) to show the degradation in imaging performance for detectors that are part of an imaging chain. Preliminary ROD calculations using simulated spheres as the object predicted superior imaging performance by the a-Se detector as compared to existing detectors. New PWMF-G-ROD and NPWMF-G-ROD results still indicate better performance by the a-Se detector in an imaging chain over all sphere sizes for various focal spot sizes and magnifications, although a-Se performance advantages were degraded by focal spot blurring. Nevertheless, the a-Se technology has great potential to provide breakthrough abilities such as visualization of fine details including of neuro-vascular perforator vessels and of small vascular devices.