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(Radiology. 1999;212:276-279.)
© RSNA, 1999

Technical Developments

Virtual CT Intravascular Endoscopy of the Aorta: Pierced Surface and Floating Shape Thresholding Artifacts1

Emanuele Neri, MD, Davide Caramella, MD, Fabio Falaschi, MD, Paola Sbragia, MD, Claudio Vignali, MD, Edoardo Laiolo, MD, Adriano Viviani, MD and Carlo Bartolozzi, MD

1 From the Department of Oncology, Diagnostic and Interventional Radiology, University of Pisa, Via Roma 67, I-56100 Pisa, Italy. Received December 15, 1997; revision requested March 2, 1998; final revision received November 6; accepted January 19, 1999. Address reprint requests to E.N. (e-mail: neri@do.med.unipi.it).


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Two types of artifacts may appear in virtual computed tomographic endoscopic views of the aorta rendered at different threshold levels: pierced surface and floating shape artifacts. A positive correlation was found between mean attenuation of the aorta and the threshold levels at which these artifacts appeared. The correlation was statistically significant (0.71 <= r <= 0.86) for floating shape. An artifact-free threshold range can be predicted on the basis of aortic enhancement.

Index terms: Aorta, CT, 56.12115, 56.12116, 56.12117 • Aortography, virtual, 56.12117 • Computed tomography (CT), image display and recording, 56.12116, 56.12117 • Computed tomography (CT), three-dimensional, 56.12117


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Virtual computed tomographic (CT) endoscopy, an imaging technique performed by means of internal rendering of CT and magnetic resonance (MR) data sets, has been used to create endoscopic views of the bronchial tree (1–5), colon (6,7), vessels (8–12), ear (13,14), and biliary (15,16) and urinary (17) tracts.

Two virtual CT endoscopic techniques have been developed on the basis of surface and volume rendering. With volume rendering, a continuous attenuation table is applied that allows individual voxels to be visualized with variable transparency. With surface rendering, image data are segmented on the basis of two levels of attenuation—low threshold and high threshold—that allow rendering of the voxels within that range. The operation results in a binary data set. As a result of correct segmentation with surface rendering, virtual CT intravascular endoscopy generates smoothed internal walls of a vessel with an empty lumen, which is assumed to be the normal pattern (8–12).

The segmentation threshold required to generate surface models influences the quality of endoscopic views. Furthermore, as reported in recent experiences with MR data sets, incorrect segmentation causes the appearance of artifacts on the rendered images (9). These artifacts, defined as "floating shape," were interpreted as dark voxels mistakenly rendered as distinct objects within the lumen because the selected attenuation range did not include them.

Herein, we describe a different type of artifact, which consists of voxels mistakenly not rendered in the reconstructed image. Thus, the presence of a hole is simulated in the vessel wall. We called this a "pierced surface" artifact.

Since spiral CT data sets require a preliminary threshold-based segmentation, our purpose was to study the presence and pattern of artifacts arising from this kind of segmentation. We also sought to determine any correlation between the threshold levels at which artifacts can be observed and the contrast enhancement achieved inside the aorta.


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
We retrospectively reviewed the imaging data sets obtained at spiral CT (HiSpeed Advantage; GE Medical Systems, Milwaukee, Wis) in 45 patients (22 men and 23 women; age range, 25–76 years; mean age, 63 years) who underwent evaluation of the abdominal (29 cases) or thoracic (16 cases) aorta. All patients underwent scanning with the same acquisition protocol. The abdominal aorta was scanned in the cephalocaudal direction during a single breath hold starting just above the superior mesenteric artery (beam collimation, 3 mm; pitch, 1; table incremental speed, 3 mm/sec; tube rotation, 1 second; tube current, 250–280 mA with 120 kVp; field of view, 30–35 cm). Acquisition time was 25–30 seconds, and the total scanning length was 75–90 mm. The thoracic aorta was scanned in the cephalocaudal direction during a single breath hold from the aortic arch to the diaphragm (beam collimation, 5 mm; pitch, 1.0–1.5; table incremental speed, 5.0–7.5 mm/sec; tube current, 250–280 mA with 120 kVp; field of view, 30–35 cm).

In all cases, a volume of 150 mL of iohexol (300 mg of iodine per milliliter) (Omnipaque 300; Nycomed Imaging, Oslo, Norway) was administered with a power injector (MCT/MCT Plus; Medrad, Indianola, Pa) via the antecubital vein with a flow rate of 3 mL/sec. To select the scanning delay, the circulation time was determined by means of a dynamic scanning test with use of a semiautomatic bolus tracking program (SMARTPREP; GE Medical Systems). To measure the aortic peak, a circularly shaped region of interest was traced within the aortic lumen at the level of the arch in the thorax or the renal arteries in the abdomen. To avoid artifacts in the three-dimensional model, no oral contrast agent was administered.

All axial images were reconstructed with 180° linear interpolation at 1-mm spacing by using a standard reconstruction algorithm. Spatial resolution of sections was 512 x 512 x 12 bits (384 kbytes of memory). The total number of images reconstructed for each study ranged between 75 and 90 images (28.0–33.8 Mbytes).

Image Processing and Display
Spiral CT data sets were surface rendered by using software (NAVIGATOR; GE Medical Systems) on a workstation (SunSparc 20; Sun Microsystems, Mountain View, Calif). The software allows users to display internal views of surface-rendered anatomic structures. Segmentation is performed by using low and high thresholds. With this method, all the voxels excluded from the selected range of Hounsfield units are considered by the system as not belonging to the object, and, subsequently, they are not rendered. To generate the surface from the segmented voxel, the software uses the marching cube algorithm. With this approach, the voxels of the volume edges are transformed into a smooth surface of triangles or patches (18).

Virtual CT intravascular endoscopic perspectives are created by simulating the conic view of fiberoptic endoscopy, and the field of view can range from 15° to 60°.

Image Analysis
Three radiologists (C.V. and others) independently reviewed the virtual CT intravascular endoscopic images created from inside the aorta at the level of the renal arteries or aortic arch. These images simulated visualization of renal artery ostia, including the tract of the aorta 3 cm above and below the ostia (abdominal aorta), or of all epiaortic branch ostia (thoracic aorta). The observers were required to determine if the artifacts appeared as a result of manipulating the low- and high-threshold levels during rendering. In particular, a floating shape artifact was defined as data that should not have been present within the lumen that were mistakenly rendered. A pierced surface artifact was defined as parietal voxel data that were mistakenly not rendered, thus simulating a hole.

For this determination, the low and high thresholds were progressively changed, in steps of 10 HU, from 0 to 500 HU and from 500 to 0 HU, respectively, to detect alterations in the aortic internal surface and the presence of floating shape inside the lumen. Agreement was considered to occur when at least two observers agreed concerning the threshold level at which artifacts started to appear. Agreement was considered a difference of 1–10 HU and disagreement a difference of more than 10 HU.

All evaluations were performed at the workstation, and no time limits were imposed.

Statistical Analysis
To evaluate aortic enhancement on axial images, a histogram was originated from data in a circularly shaped region of interest within the aorta at the level of the renal arteries (abdominal aorta) or aortic arch (thoracic aorta). Maximum, minimum, and mean attenuation values were calculated directly with the software.

Data series were processed with software (STATVIEW; Abacus Concepts, Berkeley, Calif). Mean attenuation values were calculated for low and high thresholds of artifact appearance and for the geometric mean, maximum, minimum, and SD. The ranges were calculated between maximum and minimum values and between low and high thresholds.

Linear regression analysis was performed to investigate the correlation between the mean attenuation value and the low or high thresholds at which artifacts appeared. A correlation coefficient (r) greater than 0.7 was considered indicative of good correlation. A P value less than .05 (95% CI) was considered indicative of a statistically significant difference.


   Results
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
The scanning delay for the thoracic aorta ranged between 17 and 27 seconds (22.1 seconds ± 3.3 [mean ± SD]; range, 10 seconds) and for the abdominal aorta, between 18 and 32 seconds (23.1 seconds ± 4.5; range, 14 seconds). The mean attenuation inside the thoracic aorta ranged between 73 and 420 HU (280.3 HU ± 63.5; range, 347 HU) and for the abdominal aorta, between 86 and 387 HU (243.6 HU ± 70.4; range, 301 HU). Intraluminal contrast was heterogeneous. All histograms showed a Gaussian distribution, with the majority of voxels grouped in the center of the curve, close to the mean attenuation value.

Virtual CT intravascular endoscopic images of the aorta showed a smoothed internal surface with an empty lumen (Fig 1a). At specific attenuation values, variations in the low and high thresholds influenced the rendering pattern and determined the appearance of the two types of artifacts. Agreement between observers was reached in 91% (41 of 45) of the cases. In the remaining 9% (four cases), disagreement was due to the adoption of different conic fields of view. However, correction of the field of view allowed complete agreement.



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  		Figure 1a. Virtual CT intravascular endoscopic images of the abdominal aorta show the internal surface at the level of the ostium (arrow in a) of the right renal artery. (a) Field of view, 60°; low threshold, 200 HU; high threshold, 500 HU. No artifacts are visible. (b) Low threshold, 120 HU; high threshold, 500 HU. The low-threshold pierced surface artifacts appear as holes through the aortic and renal artery walls (right arrows and left arrow, respectively). (c) Low threshold, 240 HU; high threshold, 500 HU. The low-threshold floating shape artifacts (arrows) appear inside the aortic lumen. The threshold level excludes the ostium of the right renal artery from the rendered data set. (d) Low threshold, 200 HU; high threshold, 350 HU. Low-threshold pierced surface and floating shape artifacts are not present. The ostium of the right renal artery is visible, but high-threshold floating shape artifacts (arrows) are visible inside the lumen.

 


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  		Figure 1b. Virtual CT intravascular endoscopic images of the abdominal aorta show the internal surface at the level of the ostium (arrow in a) of the right renal artery. (a) Field of view, 60°; low threshold, 200 HU; high threshold, 500 HU. No artifacts are visible. (b) Low threshold, 120 HU; high threshold, 500 HU. The low-threshold pierced surface artifacts appear as holes through the aortic and renal artery walls (right arrows and left arrow, respectively). (c) Low threshold, 240 HU; high threshold, 500 HU. The low-threshold floating shape artifacts (arrows) appear inside the aortic lumen. The threshold level excludes the ostium of the right renal artery from the rendered data set. (d) Low threshold, 200 HU; high threshold, 350 HU. Low-threshold pierced surface and floating shape artifacts are not present. The ostium of the right renal artery is visible, but high-threshold floating shape artifacts (arrows) are visible inside the lumen.

 


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  		Figure 1c. Virtual CT intravascular endoscopic images of the abdominal aorta show the internal surface at the level of the ostium (arrow in a) of the right renal artery. (a) Field of view, 60°; low threshold, 200 HU; high threshold, 500 HU. No artifacts are visible. (b) Low threshold, 120 HU; high threshold, 500 HU. The low-threshold pierced surface artifacts appear as holes through the aortic and renal artery walls (right arrows and left arrow, respectively). (c) Low threshold, 240 HU; high threshold, 500 HU. The low-threshold floating shape artifacts (arrows) appear inside the aortic lumen. The threshold level excludes the ostium of the right renal artery from the rendered data set. (d) Low threshold, 200 HU; high threshold, 350 HU. Low-threshold pierced surface and floating shape artifacts are not present. The ostium of the right renal artery is visible, but high-threshold floating shape artifacts (arrows) are visible inside the lumen.

 


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  		Figure 1d. Virtual CT intravascular endoscopic images of the abdominal aorta show the internal surface at the level of the ostium (arrow in a) of the right renal artery. (a) Field of view, 60°; low threshold, 200 HU; high threshold, 500 HU. No artifacts are visible. (b) Low threshold, 120 HU; high threshold, 500 HU. The low-threshold pierced surface artifacts appear as holes through the aortic and renal artery walls (right arrows and left arrow, respectively). (c) Low threshold, 240 HU; high threshold, 500 HU. The low-threshold floating shape artifacts (arrows) appear inside the aortic lumen. The threshold level excludes the ostium of the right renal artery from the rendered data set. (d) Low threshold, 200 HU; high threshold, 350 HU. Low-threshold pierced surface and floating shape artifacts are not present. The ostium of the right renal artery is visible, but high-threshold floating shape artifacts (arrows) are visible inside the lumen.

 
The first type of artifact—pierced surface—appeared after variations in the low threshold. This artifact caused the pierced appearance of the internal surface of the vessels, with scattered holes visible at any viewing angle (Fig 1b).

The second type of artifact—floating shape—appeared inside the lumen after variations in both the low and high thresholds (Fig 1c, 1d).

Linear regression analysis was performed to correlate mean attenuation with the appearance of artifacts. For floating shape artifacts on abdominal aorta studies, a positive and statistically significant correlation (r > 0.7) was found between mean attenuation and both the low (r = 0.77, P = .001) and high (r = 0.86, P = .001) thresholds. For pierced surface artifacts on abdominal aorta studies, a positive correlation was found between mean attenuation and the low threshold, but the correlation was not statistically significant (r = 0.325, P = .086) (Fig 2).



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  		Figure 2a. Artifacts observed at virtual CT intravascular endoscopy in the abdominal aorta. (a) Box plots compare threshold levels for low-threshold pierced surface (LT-PS) artifacts (range, 40–150 HU; mean, 95.862 ± 32.129), low-threshold floating shape (LT-FS) artifacts (range, 70–300 HU; mean, 191.034 ± 59.121), and high-threshold floating shape (HT-FS) artifacts (range, 145–460 HU; mean, 312.241 ± 69.018). Upper and lower margins of box plots represent the 75th and 25th percentiles, respectively. The median is the line bisecting the box. Error bars represent SD. {bigcirc} = outliers. (b) Linear regression analysis shows a not statistically significant correlation between mean attenuation value and low-threshold pierced surface artifact (r = 0.325, P = .086). (c) Scattergram shows a positive and good (r > 0.7) correlation between mean attenuation and low-threshold floating shape artifacts (r = 0.77, P = .001). (d) Scattergram shows the same correlation between mean attenuation and high-threshold floating shape artifacts (r = 0.86, P = .001).

 


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  		Figure 2b. Artifacts observed at virtual CT intravascular endoscopy in the abdominal aorta. (a) Box plots compare threshold levels for low-threshold pierced surface (LT-PS) artifacts (range, 40–150 HU; mean, 95.862 ± 32.129), low-threshold floating shape (LT-FS) artifacts (range, 70–300 HU; mean, 191.034 ± 59.121), and high-threshold floating shape (HT-FS) artifacts (range, 145–460 HU; mean, 312.241 ± 69.018). Upper and lower margins of box plots represent the 75th and 25th percentiles, respectively. The median is the line bisecting the box. Error bars represent SD. {bigcirc} = outliers. (b) Linear regression analysis shows a not statistically significant correlation between mean attenuation value and low-threshold pierced surface artifact (r = 0.325, P = .086). (c) Scattergram shows a positive and good (r > 0.7) correlation between mean attenuation and low-threshold floating shape artifacts (r = 0.77, P = .001). (d) Scattergram shows the same correlation between mean attenuation and high-threshold floating shape artifacts (r = 0.86, P = .001).

 


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  		Figure 2c. Artifacts observed at virtual CT intravascular endoscopy in the abdominal aorta. (a) Box plots compare threshold levels for low-threshold pierced surface (LT-PS) artifacts (range, 40–150 HU; mean, 95.862 ± 32.129), low-threshold floating shape (LT-FS) artifacts (range, 70–300 HU; mean, 191.034 ± 59.121), and high-threshold floating shape (HT-FS) artifacts (range, 145–460 HU; mean, 312.241 ± 69.018). Upper and lower margins of box plots represent the 75th and 25th percentiles, respectively. The median is the line bisecting the box. Error bars represent SD. {bigcirc} = outliers. (b) Linear regression analysis shows a not statistically significant correlation between mean attenuation value and low-threshold pierced surface artifact (r = 0.325, P = .086). (c) Scattergram shows a positive and good (r > 0.7) correlation between mean attenuation and low-threshold floating shape artifacts (r = 0.77, P = .001). (d) Scattergram shows the same correlation between mean attenuation and high-threshold floating shape artifacts (r = 0.86, P = .001).

 


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  		Figure 2d. Artifacts observed at virtual CT intravascular endoscopy in the abdominal aorta. (a) Box plots compare threshold levels for low-threshold pierced surface (LT-PS) artifacts (range, 40–150 HU; mean, 95.862 ± 32.129), low-threshold floating shape (LT-FS) artifacts (range, 70–300 HU; mean, 191.034 ± 59.121), and high-threshold floating shape (HT-FS) artifacts (range, 145–460 HU; mean, 312.241 ± 69.018). Upper and lower margins of box plots represent the 75th and 25th percentiles, respectively. The median is the line bisecting the box. Error bars represent SD. {bigcirc} = outliers. (b) Linear regression analysis shows a not statistically significant correlation between mean attenuation value and low-threshold pierced surface artifact (r = 0.325, P = .086). (c) Scattergram shows a positive and good (r > 0.7) correlation between mean attenuation and low-threshold floating shape artifacts (r = 0.77, P = .001). (d) Scattergram shows the same correlation between mean attenuation and high-threshold floating shape artifacts (r = 0.86, P = .001).

 
For floating shape artifacts on thoracic aorta studies, a positive and statistically significant correlation was found between mean attenuation and both the low (r = 0.71, P = .001) and high (r = 0.78, P = .001) thresholds. For pierced surface artifacts on thoracic aorta studies, a positive correlation was found between mean attenuation and the low threshold, but the correlation was not statistically significant (r = 0.43, P = .072).


   Discussion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Rendering artifacts have been described in a previous article (9) concerning virtual intravascular endoscopy with MR angiographic data sets. Those authors described the floating shape as low-signal-intensity voxels mistakenly rendered as distinct objects within the lumen. These artifacts were the result of flow turbulence detected at MR angiography.

A similar hypothesis can be made for virtual intravascular endoscopy with CT angiographic data sets. In fact, we may expect that the inhomogeneity of attenuation values of enhanced blood is the cause of the appearance of floating shape. In our study, the threshold levels at which floating shape started to appear showed a positive and statistically significant correlation with aortic enhancement. This proves that if the mean attenuation of the aorta increases, the threshold values at which low- and high-threshold floating shape artifacts start to be observed also increase.

Conversely, to our knowledge, pierced surface artifacts have not been previously reported. We believe that these artifacts are caused by inherent difficulties in separating the hyperattenuating voxels at the periphery of the lumen from surrounding structures by means of this thresholding technique. Consequently, these voxels are not rendered, and holes through the surface of the aorta are simulated. Additional causes that may be involved in the generation of pierced surface artifacts are the existence of peripheral flow turbulences, irregularities of vascular walls, and the vicinity of the wall to hyperattenuating structures such as the renal vein, superior vena cava, or vertebral bodies.

Contrast enhancement of the aorta is the most important factor that enables determination of the threshold levels at which artifacts start to be observed. In our study, we demonstrated that the threshold levels at which low- and high-threshold floating shape started to appear increased with mean attenuation. In contrast, the threshold levels for low-threshold pierced surface artifacts increased with the mean attenuation of the aorta, although the correlation did not reach statistical significance. By comparing the slopes of the regression lines shown in the graphs concerning the appearance of the low-threshold pierced surface (Fig 2b), low-threshold floating shape (Fig 2c), and high-threshold floating shape (Fig 2d) artifacts, we can appreciate that the artifact-free threshold range is larger for higher mean attenuation of the aorta. This observation suggests that the degree of aortic attenuation is an important factor in the performance of high-quality virtual CT intravascular endoscopy. In turn, aortic attenuation depends on the type of contrast medium, the flow rate, the correct selection of the scanning delay, and the possibility of performing the examination during a single breath hold. In some situations, however, use of the guidelines described herein is not entirely feasible, such as in emergency patients (no time for a circulation time test, reduced respiratory capacity), in patients with difficult venous access (flow rate must be reduced to less than 3 mL/sec), and in patients with pathologic conditions in the aorta that are capable of creating flow turbulence (stenosis, aneurysm, rupture). We did not investigate any of these difficult cases, but we suspect that the correlation between mean attenuation and threshold levels of artifacts might be altered in these situations. Another issue to be considered concerns variations in the attenuation of blood at different levels in the aorta. This stresses the importance of choosing the appropriate scanning delay to obtain an optimal distribution of the contrast material at the selected aortic tract.

In conclusion, knowledge about segmentation pitfalls can help avoid the appearance of artifacts on virtual CT intravascular endoscopic images, thus preventing wrong interpretations of endoluminal findings. We believe that floating shape and pierced surface artifacts have a negative effect on the diagnostic performance of virtual CT intravascular endoscopy, and more studies are needed to investigate the clinical implications of these artifacts. However, the artifacts described herein are probably related to our threshold-based selection of voxels. In the future, it must be ascertained whether the appearance of artifacts can be prevented by using more sophisticated segmentation methods.


   Footnotes
 
Author contributions: Guarantors of integrity of entire study, E.N., D.C., C.B.; study concepts, E.N., D.C., C.B.; study design, E.N., D.C.; definition of intellectual content, E.N., D.C.; literature research, E.L., A.V.; clinical studies, E.N., D.C., F.F., P.S., C.V.; data acquisition, E.N., D.C., F.F., P.S., C.V.; data analysis, E.N.; statistical analysis, E.N.; manuscript preparation, E.N., D.C.; manuscript editing and review, E.N., D.C., C.B.


   References
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 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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