Home Spatial Resolution Limits of Multislice Computed Tomography (MS-CT), C-arm-CT, and Flat Panel-CT (FP-CT) Compared to MicroCT for Visualization of a Small Metallic Stent
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Spatial Resolution Limits of Multislice Computed Tomography (MS-CT), C-arm-CT, and Flat Panel-CT (FP-CT) Compared to MicroCT for Visualization of a Small Metallic Stent

Posted Jun 30, 2011 Updated Apr 10, 2023
By Mircea Ionescu BS , Ralph W. Metcalfe PhD , Dianna Cody PhD , Miguel Valdivia y Alvarado MD , John Hipp PhD , Goetz Benndorf MD PhD
11 min read

Rationale and Objectives

Small metallic stents are increasingly used in the treatment of cerebral aneurysms and for revascularization in ischemic strokes. Realistic three-dimensional datasets of a stent were obtained by using three x-ray–based imaging methods in current clinical use. Multislice-CT (MS-CT), C-arm flat detector-CT (C-arm CT, ACT), and flat panel-CT (FP-CT) were compared with high-resolution laboratory MicroCT scans that served as a reference standard. The purpose was to assess and compare the quality and accuracy of current clinical three-dimensional reconstructions of a vascular stents.

Material & Methods

A 3 × 20 mm Cypher stent was deployed in a straight polytetrafluoroethylene tube and filled with nondiluted iodine contrast and BaSO 4 . MS-CT images of the static tube phantom and stent were acquired using GE LightSpeed VCT Series, C-arm CT images were obtained using Artis (DynaCT, Siemens), FP-CT were obtained using a preclinical research CT (GE), and MicroCT images were obtained using eXplore Locus SP (GE). DICOM datasets were analyzed using Amira and Matlab.

Results

Because of blooming effects, the maximum intensity projections (MIPs) and volume renderings generated from MS-CT showed significantly increased strut dimensions with no distinction between the regular struts and connector struts while the lumen diameter is artificially reduced. The shape of the reconstructed stent surface differed remarkably from the real stent. C-arm CT and FP-CT volume renderings more accurately represented the struts. Consistently capturing the structure of the connectors and the strut shape definition was highly threshold dependent. The stent lumen was about 30% underestimated by MS-CT when compared to MicroCT.

Conclusion

The spatial resolution of current clinical CT for imaging of small metallic stents is insufficient to visualize fine geometrical details. Further improvement in the spatial resolution of clinical imaging technologies combined with better software and hardware for image postprocessing will be necessary for detailed structural analysis, evaluation of the stent lumen in vivo, and to permit accurate assessment of stent patency and early detection potential in-stent stenosis.

X-ray computed tomographic (CT) angiography plays a major role as the primary imaging tool for the diagnosis of vascular disease, such as coronary and cerebrovascular arteriosclerosis. It has also become increasingly popular for planning endovascular therapy, postprocedure control, and follow-up exams to detect in-stent stenosis or stent thrombosis. Because of increased temporal and spatial resolution, 64-multislice CT (MS-CT) systems are capable of providing more accurate visualization of small arteries and stented lesions . In a recent multicenter trial, MS-CT has reached a sensitivity of 99% to detect significant coronary artery disease. On the other hand, the specificity remained at 68%, still relatively low, which in part was due to blooming effects, compromising diagnostic accuracy especially in calcified lesions .

Several CT artifacts make sufficiently accurate visualization of a small stent lumen and its patency a similarly challenging problem , especially in stents with <3 mm diameter . Blooming effects occur because of beam hardening, causing the struts to appear thicker and resulting in underestimation of the inner stent lumen . Partial volume effects can occur when an object lying off-center protrudes partially into the x-ray beam . Partial volume averaging is based on limited spatial resolution yielding a CT number that represents the average attenuation with a given voxel.

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Materials and methods

Three-dimensional Reconstruction of the Stent and Vessel Wall

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Table 1

Imaging Detector Resolution and Reconstruction Dataset Voxel Size

Scanner Primary Application Measured (M) or Vendor-supplied (V) Spatial Resolution Stent Image Voxel Size (mm) MS-CT (VCT) Clinical 0.74 mm STD, 0.37 mm EDGE (V) 0.18 × 0.18 × 0.625 FP-CT Preclinical 0.26 mm STD (M) ∗ 0.04 × 0.04 × 0.04 C-arm CT Clinical 0.13 mm (V) 0.096 × 0.096 × 0.096 Micro-CT Preclinical 0.04 mm (V) 0.014 × 0.014 × 0.014

CT, computed tomography; FP, flat panel; MS, multislice; VCT, volumetric CT.

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Stent and Vessel Wall Surface

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Results

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Figure 1, Axial cross-sectional views of the three-dimensional stent reconstructions (Thin MIPs). Multislice computed tomography (MS-CT) data showed increased partial volume averaging effects in regions of struts (a) , whereas the connector regions had low attenuation values (e) . C-arm CT reconstructions (b,f) exhibited sharper strut contours and the connectors were visible while partial volume averaging effects were present. These effects also occurred in the flat panel CT (FP-CT) dataset (c,g) resulting in larger strut diameter. The strut and wall-lumen boundary contours are not well defined. The MicroCT reconstructions (d,h) showed uniform attenuation values throughout the strut and connector regions and had good definition of stent and wall-lumen boundary. The sample lines in (i) were used to plot the data presented in Figures 3 and 4 . Isosurface reconstruction (i) based on the MicroCT data shows the geometry of the Cypher stent and the locations of the planes presented in a-h (plane 1 shown in a-d and connector plane 2 shown in e-h ).

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Table 2

Lumen Measurements at Several Locations Along the Stent

MS-CT C-arm CT FP-CT MicroCT Threshold = 2100 Threshold = 1900 Threshold = 16,000 Threshold = 5000 Threshold = 8000 1.5 (mm) 1.1 (mm) 2.2 (mm) 1.9 (mm) 2.5 (mm) 1.7 1.2 2.3 2 2.3 1.6 1.3 2.1 2.1 2.5 1.7 1.5 2.4 2.0 2.5 1.8 1.5 2.0 2.1 2.5 1.9 1.2 1.4 1.5

The lumen diameter was estimated by measuring the distance between the luminal edges of the struts.

CT, computed tomography; FP, flat panel; MS, multislice; VCT, volumetric CT.

Figure 2, Maximum intensity projections at 4-mm thickness (a–d) and at 0.07-mm thickness (e–h) show the quality of the multislice computed tomography (MS-CT) (a,e) , C-arm CT (b,f) , flat panel (FP)-CT (c,g) and MicroCT (d,h) reconstructions. For the MS-CT (a) , the stent structure is visible, but the shape of the struts and vertices is poorly defined while the connectors are overshadowed. Better strut and connector visualization is provided by C-arm CT (b) and FP-CT (c) . The MicroCT dataset accurately captured the entire stent structure, including the connectors (d) . Thin maximum intensity projections (0.07-mm thickness) reveal the significantly decreased size of the lumen from the MS-CT dataset (e) . C-arm CT (f) provided good lumen visualization which was comparable to the lumen visualization provided by MicroCT (h) .

Figure 3, Voxel intensity (normalized by peak strut intensity minus background intensity) across each image was measured along a line passing through one strut across the lumen and then in-between the struts as indicated ( Fig 1 ). The red vertical lines represent the real strut size and are placed symmetrical to the maximum intensity point (assumed to be the center of the strut). The green vertical lines represent the strut size corresponding to the mid-threshold value (the average between the maximum attenuation in the strut region and the average attenuation in the lumen region). The mid-threshold value allows the reconstruction of the stent architecture as the attenuation values in the connector region (sample plane 2) are within the threshold value. The multislice computed tomography image is highly sensitive to threshold changes, whereas for the MicroCT the effect is minimal.

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Figure 4, Voxel intensity (normalized by peak strut intensity minus background intensity) along a similar line for the four imaging methods demonstrates the differences in the quality of the three-dimensional reconstructions. The true strut diameter and edges are most accurately captured by the ultra-high-resolution MicroCT with minimal blooming artifacts. The strut diameters are exaggerated by both the C-arm computed tomography (CT) and flat panel (FP)-CT imaging (∼300 μm), and more so by the multislice (MS)-CT (∼600 μm). Note that the relative amplitude of the false secondary peaks decreases as the imaging resolution improves. The secondary peaks also obscure the location of the wall.

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Figure 5, Isosurfaces from the multislice computed tomography (MS-CT) (a) , C-arm CT (b) , flat panel (FP)-CT (c) , and MicroCT (d) datasets demonstrate the effects of artifacts resulting from low resolution reconstructions on the geometry of the stent. The struts are oversized and some gaps between struts are fused ( arrow , a ). Better strut visualization resulted from C-arm CT and FP-CT; however, the connectors between struts are not well defined or missing for both C-arm CT ( arrows , b ) and FP-CT ( arrows , c ) reconstructions. The complete stent geometry is captured by the MicroCT. The struts and connectors are well defined and the overall stent surface is uniform (d) .

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Figure 6, Thin MIPs: Longitudinal planes showing lumen measurements for multislice computed tomography (MS-CT) (a) , C-arm CT (b) , flat panel (FP)-CT (c) , and MicroCT (d) . The lumen diameter estimates between struts and between bare wall segments in between struts were based on the mid-threshold values. The endpoints of the sample measuring line that was defined in Amira were placed at the edge of the voxel representing the strut. The measurements were done on two-dimensional planes and depend on the threshold value. Lumen evaluation is difficult for the MS-CT and the values for the lumen diameter are lower than the reference values (MicroCT). The evaluation of the C-arm CT and FP-CT reconstructions generated similar results as with the FP-CT showing slightly lower values for the lumen diameter between struts and slightly higher values between bare walls.

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Discussion

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Conclusion

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Academic Radiology, Volume 18, Issue 7
Journals General Radiology
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