Spiral-CT-based assessment of tracheal stenoses using ... - IEEE Xplore

IEEE TRANSACTIONS ON MEDICAL IMAGING, VOL. 21, NO. 3, MARCH 2002

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Spiral-CT-Based Assessment of Tracheal Stenoses Using 3-D-Skeletonization Erich Sorantin*, Csongor Halmai, Balázs Erdöhelyi, Kálmán Palágyi, László G. Nyúl, Krisztián Ollé, Bernhard Geiger, Franz Lindbichler, Gerhard Friedrich, and Karl Kiesler

Abstract—Purpose: Demonstration of a technique for three-dimensional (3-D) assessment of tracheal-stenoses, regarding site, length and degree, based on spiral computed tomography (S-CT). Patients and Methods: S-CT scanning and automated segmentation of the laryngo-tracheal tract (LTT) was followed by the extraction of the LTT medial axis using a skeletonization algorithm. Orthogonal to the medial axis the LTT 3-D cross-sectional profile was computed and presented as line charts, where degree and length was obtained. Values for both parameters were compared between 36 patients and 18 normal controls separately. Accuracy and precision was derived from 17 phantom studies. Results: Average degree and length of tracheal stenoses was found to be 60.5% and 4.32 cm in patients compared with minor caliber changes of 8.8% and 2.31 cm in normal controls 0 0001). For the phantoms an excellent correlation ( between the true and computed 3-D cross-sectional profile was found ( 0 005) and an accuracy for length and degree measurements of 2.14 mm and 2.53% respectively could be determined. The corresponding figures for the precision were found to be 0.92 mm and 2.56%. Conclusion: LTT 3-D cross-sectional profiles permit objective, accurate and precise assessment of LTT caliber changes. Minor LTT caliber changes can be observed even in normals and, in case of an otherwise normal S-CT study, can be regarded as artifacts. Index Terms—Biomedical computing, biomedical measurements, feature extraction, image processing.

I. INTRODUCTION

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NDOTRACHEAL intubation represents the most common cause of laryngo-tracheal stenoses (LTS), followed by external trauma or prior airway surgery [1]–[3]. Rare causes include inhalation injuries, gastro-esophageal reflux disease, neoplasia and autoimmune diseases like Wegeners

Manuscript received February 1, 2000; revised January 15, 2002. The Associate Editor responsible for coordinating the review of this paper and recommending its publication was W. Higgins. Asterisk indicates corresponding author. *E. Sorantin is with the Section of Digital Information and Image Processing, Department of Radiology, University Hospital Graz, Auenbruggerplatz 34, A-8036 Graz, Austria (e-mail: [email protected]). C. Halmai, B. Erdöhelyi, K. Palágyi, L. G. Nyúl, and K. Ollé are with the Department of Applied Informatics, University of Szeged, H-6720 Szeged, Hungary. B. Geiger is with Siemens Corporate Research Princeton Inc., Princeton, NJ 08540 USA. F. Lindbichler is with the Section of Digital Information and Image Processing, Department of Radiology, University Hospital Graz, A-8036 Graz, Austria.. G. Friedrich and K. Kiesler are with the Department of Phoniatrics, Ear, Nose, and Throat, University Hospital Graz, A-8036 Graz, Austria. Publisher Item Identifier S 0278-0062(02)04089-2.

granulomatosis or relapsing polychondritis [1], [4]. In pediatric patients, LTS is frequently caused by compression due to anomalies of the aortic arch and supra-aortal branches as well as due to esophageal atresia [5]. Clinical management requires information regarding site, degree, length, and dynamics of these stenoses. Accurate information of LTS length is essential, since stenoses with a length less than 1.0 cm can be treated by endoscopic methods [6], [7]. Endoscopic examinations represents the gold standard for airway evaluation [1]. Based on endoscopy, several classifications for LTS exist [3], [8], [9]. All of them have their specific advantages and disadvantages, but none is universal accepted [1]. Imaging modalities including conventional radiography, fluoroscopy, tracheal tomography, spiral computed tomography (S-CT) and magnetic resonance imaging represent an essential part of clinical work up of LTS [10]. Due to its excellent spatial and contrast resolution, CT provides information about endoluminal and extraluminal anatomy of the laryngo-tracheal tract (LTT) caliber changes on axial source images. Using conventional CT or electron beam CT, reduction of the LTT cross-sectional area in the axial plane during exspiration had been reported for healthy volunteers and for the evaluation of chronic airway obstruction in children [11], [12]. Unfortunately, the shape of LTT, as displayed on axial CT slices, depends on the angle between LTT and the scanning direction. For example, if a tubular structure like the trachea is cut perpendicular to its central axis, the shape of the cross section will be circular. If the angle between the trachea and cutting plane is oblique instead of orthogonal, an elliptic shape will result with inherent overestimation of the cross-sectional area. Due to its anatomical course, the angle between the trachea and the scanning direction biases the estimation of LTS degree on axial source images and cannot be regarded to be objective. Therefore, for LTS an accurate and precise method for determination of site as well as for quantification of degree and length would be helpful, especially for monitoring of therapeutical success. The goal of this paper is to present a technique for display of LTT caliber changes and quantification of stenotic segments regarding site, degree, and length. The presented technique is based on S-CT data and independent of the LTT course. A quantitative description of LTT caliber changes in patients and a control group, consisting of asymptomatic individuals, was obtained and the differences regarding degree and length analyzed. The accuracy and precision of the proposed method for assessing LTT caliber changes was validated on phantom studies.

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II. PATIENTS AND METHODS A. Patients, Imaging Protocols, and LTS Localization Thirty-six patients (aged from 24 weeks to 92 years), who underwent endoscopy and S-CT for evaluation of LTS, were included in this study. All endoscopic studies were recorded on videotape and available for review. In addition, 18 patients (aged 8 weeks to 47 years) with normal findings at clinical examination and/or normal lung function, who underwent S-CT for other indications and exhibited normal LTT findings, served as a control group in order to obtain data regarding the variation of LTT caliber changes in normal individuals. All S-CT studies were performed either with the single-detector S-CT scanner Somatom Plus-4 [Siemens Med AG, Er40)] or with the multirow detector S-CT langen, Germany, ( scanner Lightspeed QXI [GE Medical Systems, Millwaukee, 14)]. For patients scanned with the Somatom Plus-4 WI ( unit, a beam collimation of 3.0 mm at a table feed of 4.5 mm was chosen in all cases. Axial slices were reconstructed using a soft tissue kernel and an overlapping of 50%. When using the multidector scanner, the protocoll consisted of a beam collimation of 2.5 mm at a table feed of 7.5 mm (high-quality pitch) was selected. Images were reconstructed at a slice thickness of 1.25 mm and an overlap of 1.0 mm using a soft tissue kernel. Intravenous contrast medium (Jopamidol 300 mg/ml, Bracco, Italy) was administered only in those patients, where contrast enhanced CT was clinically indicated. Studies in pediatric patients were performed in deep sedation and none of these patients required endotracheal intubation. At our institution the following LTT three-dimensional (3-D) reconstructions are part of the routine imaging: sagittal, coronal, and curved multiplanar reformations; semitransparent volume rendering of the LTT; as well as virtual endoscopic views and movies of virtual endoscopy. Locations of LTS were determined on axial S-CT slices and compared with findings of endoscopy by Cohen’s statistics. In order to obtain statistical significance, the scores were calculated. B. Three-Dimensional Image Postprocessing The whole procedure for obtaining a line chart of the LTT 3-D cross-sectional profile consisted of the following five steps: 1) LTT segmentation; 2) computation of the LTT skeleton; 3) separation of the LTT medial axis from the LTT skeleton; 4) smoothing of the LTT medial axis; and 5) calculation of the LTT 3-D cross-sectional profile along the LTT medial axis. Step 1) LTT Segmentation: LTT segmentation was based on fuzzy connectedness, which captures the image inherent fuzziness as well as the spatial-coherence of the voxels in a well-defined manner [13], [14]. In case of LTT, air has a well-defined range of Hounsfield units. Therefore, the parameters, needed for the definition of the fuzzy affinities, can be set once and used for all studies without a per-study training. On one or more axial slices the operator selects by a mouse click a “seed point” within the LTT center for seeding the fuzzy connected objects. Since absolute fuzzy connectedness was used and a single object is segmented, the uncertain boundary regions (due to partial volume effects) are not captured by

Fig. 1. LTT segmentation based on fuzzy connectedness. The light grey line , superimposed on the original axial S-CT image, outlines the result of segmentation. The upper part exhibits results for the larynx and the lower part that for a tracheal stenosis.

the fuzzy objects. Thus, the resulting segmented LTT was uniformly smaller than the physicians expectations. Hence, 3 3 structuring element was a 3-D dilation using a 3 applied to the segmented fuzzy-connected object. Finally, the operator controlled at fixed window settings (center: 600 Hounsfield units; and width: 1200 Hounsfield units) the results of segmentation on the computer screen, where the segmented LTT boundaries were outlined on original axial slices (Fig. 1). Manual tracing, augmented by splines, was performed if the automated segmentation process failed to follow the expected LTT contour. The number of edited slices were noted and saved. Finally, the segmented binary 3-D volume was converted into cubic voxels by linear interpolation, because the following step of skeletonization required isotropic data. For better anatomic orientation on the computed 3-D cross-sectional charts, three positions were marked by the operator and stored by the system: vocal chords and the lower rim of the cricoid cartilage, since they represent the borders of the subglottic space and can be recognized at endoscopy. In addition, the position of the jugular fossa was selected, where the border between the extrathoracic and intra part of the trachea is located. Step 2) Computation of the LTT Skeleton: The notion of skeleton was introduced by Blum [15] as a region-based shape feature/descriptor which summarizes the general form of objects/shapes. A very illustrative definition of the skeleton is given using the prairie-fire analogy: the object boundary is set on fire and the skeleton is formed by the loci where the fire fronts meet and quench each other. This definition can be naturally extended to any dimension. The thinning process is

SORANTIN et al.: S-CT-BASED ASSESSMENT OF TRACHEAL STENOSES

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Fig. 2. Example of the two types of 3-D thinning. a) Character “A” as a 3-D synthetic object. (b) Medial surface. (c) Medial lines. The medial lines approximate the center line of the object, which is the medical relevant information.

a frequently used method for producing an approximation to the skeleton in a topology-preserving way [16]. It is based on digital simulation of the fire front propagation: border points (i.e., ones that are “adjacent” to zeros) of a binary object that satisfy certain topological and geometrical constraints are deleted in iteration steps. The entire process is repeated until only a reasonable approximation to the skeleton is left. In three dimensions, there are two major types of tinning: these algorithms produce either the medial surface of an object (by preserving surface end-points) or can extract the medial lines of an object (by preserving line end-points) [17]. The results of the two types of thinning approaches are illustrated in Fig. 2. In Fig. 2(c), it can be seen, that extracting medial lines yields the medical relevant information of the medial axis for “tubular” objects like LTT and blood vessels. Most of the existing thinning algorithms are parallel, since the fire front propagation is by nature parallel, meaning that all border points satisfying the deletion condition of the actual phase of the process are simultaneously deleted. A recently published parallel 3-D six-subiteration directional thinning algorithm [18] is applied to extract the medial lines from the segmented LTT. Fig. 3 exhibits an example of the computed skeleton in a patient suffering from a tracheal stenosis. The skeletonization is rather sensitive to “boundary noise” (i.e., the roughness of the boundary of the object). Therefore, the skeleton can contain several unwanted/parasitic segments. To overcome this problem, smoothing of the original object (as a preprocessing step) and pruning of the resulted skeleton (as a postprocessing step) are needed. We use simple morphological filtering [19] for smoothing for the segmented LTT and a more difficult pruning process for getting the medial axis (without side branches) from the medial lines (see Step 3). Step 3) Separation of the LTT Medial Axis From the LTT Skeleton: In order to separate the LTT central path from other parts of the skeleton, the segmented LTT and the computed skeleton were converted according to the standards of Virtual Reality Modeling Language 2.0 (VRML) [20]. Using a VRML editor, the operator could interactively inspect the LTT and skeleton from any view in the 3-D space. The operator marked the start and endpoint of the central path. Then the shortest distance of the skeleton between start and endpoint was computed using a shortest-path searching algorithm [21]. All parts of the skeleton not belonging to the search path were discarded. Thereafter the skeleton only consisted of the LTT medial axis.

Fig. 3. Transparent 3-D VRML model of the segmented LTT in a patient suffering from tracheal stenosis. White arrow points to the position of the maximum degree of the tracheal stenosis. White lines inside the reconstructed LTT represent the complete computed skeleton as produced by Step 2 (computation of the LTT skeleton). It is illustrated, that additional to the medial axis there are unwanted/parasitic side branches of the LTT skeleton at the upper and lower part.

Step 4) Smoothing of the LTT Medial Axis: The defined LTT medial axis was represented by a sequence of vectors from the starting point to the endpoint of the LTT central path. Casteljau’s algorithm was used to smooth the extracted LTT medial axis [22]. Step 5) Calculation of the LTT 3-D Cross-Sectional Profile along the LTT Medial Axis: Along the extracted LTT medial axis the orthogonal cross-sectional area was computed using the interpolated data volume, which represented the segmented LTT. Afterwards the cross-sectional areas were plotted against the positions of the LTT medial axis. Localization of vocal chords on the LTT medial axis was chosen as the zero position. Positions on the LTT medial axis lower to the vocal chords were marked positively; and those above, negatively. LTS regions could be detected as a decrease of the cross-sectional area. For data smoothing a low pass filter with a range of 25% of the Nyquist frequency was applied. In order to facilitate anatomic cross reference between endoscopy, imaging and LTT 3-D cross-sectional charts, the positions of the stored anatomic landmarks were automatically marked as vertical bars on the line charts too. Additionally, for better illustration of the plotted cross-sectional areas, four circles were drawn on the chart in real size, whose areas covered the range between the fifth and 95th percentile of the calculated cross-sectional areas. Fig. 4

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Fig. 4. Three-dimensional cross-sectional chart of a patient suffering from a tracheal stenosis is shown. Vertical bold lines delineates the position of the three landmarks: vocal chords, caudal border of the cricoid cartilage, and jugular fossa. Within the tracheal part of LTT there is a decrease of the cross-sectional area—indicating the area of LTS (double arrow). In order to illustrate the size of the plotted cross-sectional areas, four circles were drawn in real size below the chart, covering the range of the plotted cross-sectional areas.

the stenosis. In order to determine if any significant error occurs, due to the usage of the above described reference value, the minimum cross-sectional area was referenced as percentage of the cross-sectional area for the begin and end point of the tracheal stenoses as well. Student’s t test was applied, in order to check if there are significant difference between both percentage values. The length of the stenosis was obtained as the distance of the start and endpoint on the LTT central axis. Quantification was applied to all caliber changes of the trachea for all patients and normal controls separately. Since the LTT caliber within the glottic and subglottic space is highly variable, only the tracheal extension was used for quantification in patients suffering from a combination of a glottic, subglottic, or tracheal stenosis. For both groups, degree and length of the stenoses were compared separately by the student’s t test. Fig. 5. GUI for quantifications of narrowed tracheal segments. Dashed lines mark the begin and end point of a narrowed tracheal portions as chosen by the operator. The dotted line depicts the automated computed minimum position. The operator could move the marker for begin or end of LTS either left or right as necessary. The updates of the computations for the LTS length and degree were performed automatically.

shows an example of the 3-D cross-sectional chart of a patient suffering from a tracheal stenosis. C. LTS Quantification A graphical user interface (GUI) was programmed for interactive assessment of LTS on the 3-D cross-sectional charts. The GUI displayed the computed 3-D cross-sectional profile and the operator could choose the start and endpoint of a stenosis by clicking on the chart (Fig. 5). The cross-sectional area at the start and endpoint were averaged and regarded as the 100% reference value for that particular LTT segment. The minimal cross-sectional area between the already chosen start and endpoint was detected automatically, expressed as percentage of the above mentioned reference value and represented the degree of

D. Time Requirements The time requirements for all five steps were recorded. E. Phantom Studies Since the 3-D cross-sectional profile reflected the caliber changes within the LTT, the accuracy and precision of the proposed method was validated by performing 17 investigations on five phantoms. A plastic tube with a length of 100.0 cm was bent to be S-shaped (Fig. 6) and scanned by S-CT (Somatom Plus 4, Siemens MED AG, Erlangen, Germany) with the same parameters as used for clinical studies. In addition, four different virtual geometric bodies (VGB A–D) were generated in 3-D as a binary volume, in which gray level 1 represented voxels belonging to the VGB and gray level 0 represented background voxels (Fig. 7). These VGBs were constructed as tubes with and regions of symmetric (VGB A—length: 42 mm; and degree: 75%) and asymmetric narrowing (VGB B) or a combination of both (VGB C). A further model consisted of an intersection of a tube with a sphere (VGB D—bulge length: 71 mm; and degree 238%). For tubular objects with


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Fig. 8. VGB A with added 10% noise. The bright line inside outlines the computed medial axis.

Fig. 6. Three-dimensional reconstruction of the tube phantom including the computed skeleton (white lines inside), used to determine the accuracy of length measurements. The plastic tube with a length of 100.0 cm was formed in slopes and scanned by S-CT with the same parameters used for patients. Then the entire procedure for obtaining the cross-sectional profile in three dimensions was applied to the S-CT data and the phantom length was determined on the cross-sectional chart.

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(d) Fig. 7. Transparent VRML models of the four generated VGBs are depicted top to down. The bright line inside the model depicts the medial axis, which was obtained by skeletonization. (a) VGB A consisted of tube with symmetrical narrowing. (b) VGB B of one with asymmetric narrowing. (c) VGB C of a combination of VGB A and VGB B. (d) VGB D was created as intersection of a tube with a sphere.

symmetric caliber changes, the true medial axis is represented by the straight center line. In case of asymmetric expansions,

the true medial axis cannot be determined, only skeletonization results in an approximation of the medial axis. Therefore, no information regarding length and degree can be given for VGB B and C. Similar to CT-scanning, these VGBs were calculated on a slice by slice basis at a size of 0.5 mm in the and direction and at a thickness of 1.5 mm. In addition, noisy variants of all VGBs were computed (Fig. 8). During the generation process of the VGBs the diameter of the VBG was randomly increased or decreased by an amount of 10% at every 5 . Due to the inherited properties of the random process, the noisy VGBs revealed an increase of the volume by 8.03% on average. For all phantoms the 3-D cross-sectional profile was computed by repeating steps 1–5 and in all VGBs, caliber changes were quantified in the same manner as in LTS. Accuracy of Length Measurements: For the plastic tube phantom the total length, as obtained by skeletonization, was compared with the true one. Accuracy for Assessing Caliber Changes: Due to the symmetrical properties of VGB A and D, the true 3-D cross-sectional profile could be determined, which was compared with the computed one for both variants of VGB A and D by linear regression. For every position of the medial axis the difference between the corresponding cross-sectional areas was calculated in the noise-free variant and expressed as percentage of the true value. Due to the asymmetric narrowing in VGB B and C, the true medial axis cannot be determined no comparisons can be made. Precision for Assessing Caliber Changes: For all VGBs the computed 3-D cross-sectional profile of the noise-free variant was compared with that of the noisy one by linear regression in the same way as described above. Since the noisy variants of the VGBs were always bigger than the noise-free version, no differences were calculated. Accuracy of Length and Degree Calculation: Due to the symmetrical properties of VGB A and D, the true length and degree of caliber changes were known. For both variants of VGB A and D the absolute and relative differences between the calculated and true values were obtained and compared by the paired student t test. Precision of Length and Degree Calculation: For all VGBs computed values for length and degree of the noise-free variant was compared those of the noisy variants in the same way as described as above. F. Hardware and Software All postprocessing steps were computed on a workstation (Octane, Silicon Graphics Inc., Mountain View, CA). For image processing and GUI creation the commercial software

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TABLE I COMPARISON BETWEEN AXIAL S-CT SLICES AND ENDOSCOPY REGARDING SITE OF LTS. IN THREE PATIENTS, A GLOTTIC STENOSIS COULD NOT PASSED AT ENDOSCOPY. ONLY S-CT WAS ABLE TO DEPICT THE ADDITIONAL SUBGLOTTIC STENOSIS AND TRACHEAL STENOSIS IN THESE THREE PATIENTS

toolbox IDL (Integrated Data Language IDL 5.3, Research Systems Inc., Boulder, CO) was used. For skeletonization a self developed software based on C++ was applied. The software package CosmoWorlds 1.0.3 (Silicon Graphics Inc., Mountain View, CA) served as VRML editor. All external programs were called by the IDL application, in order to prevent switching between the different software packages by the user. The Virtuoso3D Software (Siemens MED AG, Erlangen, Germany) enabled us to generate 3-D external views and virtual endoscopic views. Cohen’s test was performed using the SAS software (Statistical Analysis System, SAS Institute, Cary, NC) Regression analysis was done using the built in functions of IDL. P-values less than 0.05 were considered to be significant. III. RESULTS A. LTS Locations Sites of LTS, as diagnosed at endoscopy and S-CT, are shown in Table I. There was a statistically significant correlation between the LTS locations at endoscopy and on axial S-CT slices ). Due to a glottic stenosis, the larynx ( could not be passed at endoscopy in three patients, which made it impossible to judge the airways beyond the stenoses. Therefore, in all of these three patients only S-CT was able to assess the an additional subglottic and tracheal stenosis. B. Three-Dimensional Image Postprocessing Control Group: In the 18 normal controls, on average, 2.41 slices had to be corrected manually (standard deviation:1.41; minimum: 0; and maximum: 6). In 90% less than four slices needed manual correction. Patients: In patients on average 3.93 slices had to be corrected manually (standard deviation: 6.22; minimum: 0; and maximum: 30). The maximum number of slices, who had to be edited, occurred in two patients. Both were suffering from a severe tracheal stenosis. In 90% of all patients less than five slices had to be manually edited. There was no statistical significant difference in the number of edited slices between patients and 0.06). normal controls ( C. LTS Quantification Control Group: All together, 46 segments of minor caliber changes could be identified on the LTT 3-D cross-sectional charts, which were not percepted as pathologic on axial source images. These minor LTT caliber changes occurred between one and seven times (average 2.71) per person. Fig. 9 exhibits

a 3-D cross-sectional chart of a normal individual. Below the caudal border of the cricoid cartilage (second vertical bar from left) there are seven areas of LTT minor caliber changes with a maximum degree of 17.76%. Patients: In the 36 patients, the following stenotic segments could be quantified: 20 tracheal stenoses, eight tracheal segments of a subglottic stenosis extending to the trachea, and three segments in patients suffering from a combination of glottic, subglottic and tracheal stenosis. Since five patients suffered from a subglottic stenosis, where no quantitative evaluation was performed, altogether, 31 segments were available for quantitative analysis. The LTT 3-D cross-sectional charts depicted the site of LTS in all cases, indicating total agreement with axial S-CT source images. Fig. 10 shows the 3-D cross-sectional chart as well as the quantification results in a patient suffering from a tracheal stenosis. In patients, the average degree of a tracheal stenosis was 60.5% (minimum: 25.9%; and maximum: 95.5%) compared with 8.8% (minimum: 0.3%; and maximum: 20.5%) of LTT minor caliber changes in the normal control group ( 0.005). When calculating the degree as percentage of the cross-sectional area at the begin or end the stenosis a statistically 0.64). The insignificant difference of 2.82% was found ( length of LTS patients the length was found to be 4.23 cm on average (minimum: 1.26 cm; and maximum: 8.31 cm). In comparison, the length of LTT minor caliber changes was 2.31 cm (minimum: 0.95 cm; and maximum: 5.48 cm) in the normal 0.005). Table II displays the statistics. control group ( D. Time Requirements The entire process of calculation of the LTT 3-D cross-sectional profile [Steps 1-5] needed on average 2.78 min (minimum: 2.16 min; and maximum: 3.9 min). E. Phantom Studies Accuracy of Length Measurements: The length of the 100.0-cm-long plastic phantom was estimated by skeletonization was found to be 99.0 cm, indicating that the true length was underestimated by 1.0%. Since the length of stenotic tracheal segments was estimated to be on average 42.3 mm, an absolute error, induced by the skeletonization process, of 0.42 mm has to be considered. Accuracy for Assessing Caliber Changes: For VGB A and D a perfect correlation between the true cross-sectional profile and ). The the computed one was found for both variants ( maximal difference of the corresponding cross-sectional areas


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Fig. 9. Line chart of the LTT 3-D cross-sectional profile in a normal individual. The solid line delineates the smoothed cross-sectional data, the dashed-dotted-dashed line marks the original data. Seven tracheal caliber changes are depicted below the larynx. The dotted vertical lines represent the minimum position, the dashed represents the begin and end position of the narrowed segment, and the figures represent the ID (#1–#7) of the segment. Below the chart the four circles are drawn in real size, covering the range of the plotted cross-sectional areas, as well as the table of the computed values for quantification. (Degree: degree of LTT minor caliber changes; length: length of the LTT minor caliber changes.) It can be seen from the table below the chart, that all areas of LTT minor caliber changes exhibited a degree of less than 20.5%.

was found to be in the range of 5.9% to 8.1%. Table III shows detailed results. Fig. 11 Comparison between the true and computed 3-D cross-sectional profile in VGB A (the black line represents the theoretical 3-D cross-sectional and the gray line that one obtained by skeletonization). Both curves are almost identical and the correlation coefficient indicates an almost perfect correlation. Precision for Assessing Caliber Changes: For all VGBs linear regression between 3-D cross-sectional profile of the noise-free and the noise variant revealed a statistically significant correlation between both profiles with correlation 0.005). Fig. 12 coefficients between 0.94 and 0.99 ( demonstrates the overplot of the 3-D cross-sectional profile obtained for the noise-free and noisy variant of VBG C. The gray line, representing the 3-D cross-sectional profile derived from the noisy variant, is above the black one, representing the 3-D cross-sectional profile of the noise-free variant. This effect is caused by the inherent properties of the noise generation, thus making the noisy variants were already bigger than the noise-free variants. Nevertheless, the two curves are corresponding and the drops in the cross-sectional area are within the same region. Linear regression revealed a correlation coefficient of 0.99 indicating an excellent result.

Accuracy of Length and Degree Calculation: The difference for length measurements was found to be on average 2.14 mm absolutely and 3.41% relatively. Corresponding figures for degree were found to be on average 2.53% absolutely and 1.22% relatively. No statistically significant difference was found for both VGBs and their variants at the comparison between the 0.05). Table IV shows the decomputed and true values ( tailed results. Precision of Length and Degree Calculation: The difference for length measurements between both variants were found to be on average 0.92 mm absolutely and 1.87% relatively. Corresponding values for the degree were on average 2.56% absolutely and 6.72% relatively. There was no statistical significance at the comparison between the length and degree values for the 0.05). Table V shows detailed noise-free and noisy variant ( results. IV. DISCUSSION Based on endoscopy several classifications of LTS were reported in the literature [1]. McCaffrey suggested a system consisting of four stages, taking into account the location of the stenosis of involving only the subglottic space or extending to the glottic space and/or trachea. Length of LTS was part of

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Fig. 10. LTT 3-D cross-sectional chart from a patient suffering from tracheal stenosis. The short vertical bars represent the three chosen anatomic landmarks. Distally, the caudal border of the cricoid cartilage three drops of the cross-sectional area can be seen: one major one representing the stenotic segment; two representing nonpathologic areas of LTT minor caliber changes. Again, the four circles were drawn in real size, covering the range of the plotted cross-sectional areas as well as the table with the quantification results is shown (Degree: degree of LTT minor caliber changes; length: length of the LTT minor caliber changes.)

THE VALUES

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TABLE II EDITED SLICES, DEGREE, AND LENGTH IN LTS AS WELL CHANGES IN NORMALS ARE DEPICTED

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LTT MINOR CALIBER

TABLE III ACCURACY FOR DETECTING LTT CALIBER CHANGES. RESULTS OF LINEAR REGRESSION BETWEEN THE TRUE CROSS-SECTIONAL PROFILE AND THAT OBTAINED BY SKELETONIZATION (VGB A AND D)

stage 1 (less than 1.0 cm) and stage 2 (longer than 1.0 cm) [8]. Although this system correlates with the success of surgical treatment, it does not include the degree of reduction of the cross-sectional diameter [1]. For pediatric patients Cotton proposed a four stage scoring system, which was based on re-

duction of the LTT lumen: less than 70%, 70%–90%, 90% and complete obstruction [9]. Unfortunately Cotton’s scoring system does not correlate with the outcome in adults [1]. Further, at endoscopy the assessment of the difference between 65% and 75% lumen is difficult, which is mandatory for differen-


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(b) Fig. 11. (a) Comparison of the theoretical 3-D cross-sectional chart (black line) with that obtained by skeletonization (gray line) for VGB A. (b) Linear regression line. There is an excellent correlation between both profiles, meaning that the proposed technique is accurate.

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(b) Fig. 12. (a) Comparison of the 3-D cross-sectional chart between the noise-free (black line) and noisy variant (gray line) in VGB C. As mentioned already in the methods section, the generated VGBs noisy variants were always bigger than the noise-free ones. Nevertheless, the drop in the cross-sectional area, representing the narrowed region, can be seen on both profiles. (b) Linear regression line. There is an excellent correlation between both profiles, indicating that the proposed technique is precise.

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tiation of between Cotton Grade I [1]. Lano reported another scoring system, which divided the airways in three sites: glottis, subglottis, and trachea [3]. A negative correlation was found between the number of sites involved and the outcome, e.g., as the number of stenotic sites increased the success of surgery decreased [1], [3]. Despite all the cited differences between the suggested LTS classifications systems, all of them consider localization, degree, and length. In addition, it was reported, that the endoscopic assessment of the length and degree is highly operator dependant [23]. Since S-CT represents an established part of the clinical work up, we hypothesized, that 3-D image postprocessing of the axial CT source images could be used for a more accurate and precise assessment without adding any additional radiation. The purpose of this study was to present an 3-D image postprocessing technique of S-CT data, which enables both, localization and quantification of LTS including degree and length. Due to the highly variable caliber and shape of the airways in the supra and subglottic space, reliably quantification can only be done for the tracheal part. Using endoscopy as the gold standard, axial S-CT slices exhibited the site of LTS in all patients, indicating that the S-CT study was able to assess the altered LTT lumen correctly. In addition, the sites of all LTS could be identified on the corresponding 3-D cross-sectional line charts. Therefore, the information, inherited in S-CT data, was captured correctly. Furthermore, degree and length of the stenoses were depicted quantitatively on the 3-D cross-sectional line charts and did not have to be estimated from biased cuts on axial source images or from other rough measurements. To the best of our knowledge, no standards regarding the normal caliber of the trachea were published. In order to normalize the degree of a stenotic tracheal segment, the average between the cross-sectional area at the begin and the end of the stenosis were regarded as the 100% value reference value for that particular tracheal segment. The statistically insignificant difference between the degree of a stenosis, referenced either to the cross-sectional area at the begin or end, indicates, that for a given tracheal segment this approach can be used as an approximation of the normal caliber. In the control group, areas of minor LTT minor caliber changes were observed on the 3-D cross-sectional chart. On axial source slices, no pathology could be visualized being responsible for these findings. Three-dimensional cross-sectional profiles, derived from the VGBs, did not exhibit such segments of minor caliber changes. Therefore, it can be hypothesized that, in addition to true anatomic caliber changes, these areas maybe artifacts due to pulsations, partial volume effects and the S-CT image reconstruction process. A comparison between the length and degree of tracheal stenoses and those values of narrowed areas in the normal control group revealed a statistical significant differences of both parameters between both groups. In the normal control group, the degree was always less then 20.5%. Therefore, a finding of a narrowed tracheal segment with a degree of less than 21% on an otherwise normal axial source images, should be regarded as a normal finding and no other investigations are indicated. Three out of the studied 36 patients suffered from a glottic stenosis. Hence, the larynx could not be passed at endoscopy

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TABLE IV ACCURACY OF LENGTH AND DEGREE ESTIMATIONS BASED ON THE 3-D CROSS SECTIONAL CHART IN VGB A AND D. THERE IS NO STATISTICAL SIGNIFICANT DIFFERENCE BETWEEN THE TRUE AND COMPUTED VALUES FOR THE LENGTH AND DEGREE ESTIMATIONS

TABLE V PRECISION OF LENGTH AND DEGREE ESTIMATIONS BASED ON THE 3-D CROSS-SECTIONAL CHART. THERE IS NO STATISTICAL SIGNIFICANT DIFFERENCE BETWEEN THE LENGTH AND DEGREE ESTIMATIONS ON NOISE-FREE AND NOISY VGBS

and, consequently, no inspection of the distal LTT could be achieved. S-CT as well as the LTT 3-D cross-sectional line chart displayed an additional subglottic and tracheal stenosis and enabled qualitative and quantitative information. Using endoscopy, the inner surface including mucosal changes as well as the dynamics of the LTT can be examined. However information regarding the surrounding anatomy of the LTT is limited to the perception of an abnormal shape or vessel pulsations. In contrast to endoscopy, S-CT provides information about extraluminal and endoluminal pathology, which alters the lumen. However, visualization of mucosal changes without altering the lumen is not possible. Therefore, both endoscopy and S-CT are complementary. Since it is known that the mental reconstruction process of multiple axial sections may fail in patients with trachea-bronchial deformities, the routinely used 3-D reconstructions including virtual endoscopy helped to avoid this problem [24]. This synoptic approach for LTT imaging, consisting of axial slices, 3-D reconstructions, and the 3-D cross-sectional profile, assists the assessment of LTT caliber changes. Therefore, the additional time of less than 3 min, as necessary for generation of 3-D cross-sectional profile enabled an accurate road mapping of the LTT caliber. Due to its clinical impact, the results of this imaging strategy were appreciated by referring physicians. Since the LTT caliber changes are derived from the segmented LTT, the segmentation procedure should be automated and as independent as possible from an operator’s influence. Those requirements could be satisfied by using the fuzzy connectedness algorithm for segmentation. The operator had to specify one point only inside the airways all other necessary parameters were fixed in every case. In 90% of all cases, less

than five slices had to edited manually, indicating that the used segmentation algorithm is almost free from the operator’s influence. Validation of methods for assessment of LTT caliber changes is possible by doing postmortem studies [25]. Therefore, the validity of the proposed method for assessment of site, length, and degree of LTS based of S-CT data was proved by means of 17 phantom studies. At the assessment of caliber changes, an almost perfect correlation was found between the true cross-sectional profile and that obtained by skeletonization, indicating that this technique is highly accurate for assessing LTT caliber changes. Noise in CT images depends on parameters like dose, pitch, and patient build. This potential influence on the segmentation process was studied by adding noise to the VGB surface. The statistical significant correlation between the noise-free VGBs and noisy VGBs as well as the relative difference of about 10% between the corresponding cross-sectional area in VGB‘s with symmetrical caliber changes (VGB A and D), underlines the precision of the presented technique. For measurements of the length an error of only 1.0% was demonstrable. Considering that the length stenotic tracheal segments were found to be on average 42.3 mm, an absolute accuracy in the submillimeter range can be achieved theoretically. At the comparison of the true known length and the computed one for narrowed regions of VGB A and D, there was no statistically significant difference between both values, which means, that these measurements are highly accurate. On average the true and computed length values differed by 2.14 mm absolutely (relative: 3.41%), which should satisfy clinical needs. At the assessment of the degree in narrowed segments, there was no statistically significant


difference between the true and computed values, which was found to be on average 2.53. Thus, the obtained values for the degree of narrowed regions represents an highly accurate measurement. Comparing the computed values for length and degree between the noise-free and noisy variant in all VGBs, an absolute difference of 0.92 mm (relative: 1.87%) for length and 2.56 for degree (relative:6.72%), respectively, could be found, indicating that measurements for length and degree are highly precise. All of these phantom studies demonstrate that the proposed technique is highly accurate and precise and, therefore, permits objective assessment of tracheal stenoses. The described procedure of LTT segmentation and LTT medial axis extraction using can be further exploited. Performing volume rendering, the histogram of the segmented voxels representing the upper respiratory tract can be used for adjustment of the opacity curve, in order to generate automatically virtual endoscopic views. For the generation of virtual endoscopic images, the extracted medial line can be used as a guide for automated selection of viewpoints, thus sparing the operator to perform this task manually. There is one shortcoming of that study. For the assessment of the dynamics in LTS, like tracheo-malacia, exspiratory CT or cine-CT has to be performed [11], [12]. Since all patients enrolled in this study underwent endoscopy, neither exspiratory nor cine S-CT scanning of the upper respiratory tract was performed. Therefore, no statements regarding the LTT caliber changes during the respiratory cycle can be made. The 3-D cross-sectional charts depict LTT caliber changes as they captured on the S-CT data. In conclusion, the proposed S-CT based technique allows the display and the objective quantitative assessment of LTT caliber changes. Even in normal individuals, narrowed tracheal segments with a degree of less than 21% can be observed, which do not prompt for further investigations, if the axial source images appear normal. Validation on phantom studies revealed that this technique is highly accurate and precise. This procedure can be further exploited for automated generation of 3-D reconstructions including virtual endoscopy. Considering these advantages a total average time of 3 min for generation of the LTT, 3-D cross-sectional profile seems to be justified and advisable in patients with LTS.

REFERENCES [1] M. Couray and R. Ossoff, “Laryngeal stenosis: A review of staging, treatment, and current research,” Curr. Opinion Otolaryngol., Head Neck Surg., vol. 6, pp. 407–410, 1998.

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[2] H. Grillo, D. Dnonahue, D. Mathiesen, J. Wain, and C. Wright, “Postintubation tracheal stenosis: Treatment and results,” J. Thorax Cardiovasc. Surg., vol. 109, pp. 486–492, 1995. [3] C. Lano, J. Duncavage, L. Reinisch, R. Ossoff, M. Couray, and J. Netterville, “Laryngotracheal reconstruction in the adult: A ten year experience,” Ann. Otol. Rhinol. Laryngol., vol. 107, pp. 92–96, 1998. [4] H. Spraggs and P. Tostevin, “Management of laryngotracheobronchial sequelae and complications of relapsing polychondritis,” Laryngoscope, vol. 107, pp. 936–941, 1997. [5] W. Berdon, V. Condon, G. Currarino, C. Fitz, J. Leonidas, B. Parker, T. Slovis, and B. Wood, Caffey’s Pediatric X-Ray Diagnosis. St. Louis, MO: Mosby, 1993. [6] J. Coleman, J. VanDuyne, and R. Ossoff, “Laser treatment of lower airway stenosis,” Otolaryngol. Clin. North Amer., vol. 28, pp. 771–783, 1995. [7] R. Ossof, G. Tucker, J. Duncavage, and R. Toohill, “Efficacy of bronchoscopic carbon dioxide laser surgery for benign strictures of the trachea,” Laryngoscope, vol. 95, pp. 1220–1223, 1985. [8] T. McCaffrey and J. Czaja, “Classification of laryngeal stenosis,” Laryngoscope, vol. 102, pp. 1335–1340, 1992. [9] R. Cotton, “Pediatric laryngotracheal stenosis,” J. Pediatr. Surg., vol. 19, pp. 699–704, 1984. [10] J. Czaja and T. McCaffrey, “Acoustic measurement of subglottic stenosis,” Ann. Otol. Rhinol. Laryngol., vol. 105, pp. 504–509, 1996. [11] E. Stern, C. Graham, R. Webb, and G. Gamsu, “Normal trachea during forced exspiration: Dynamic CT measurements,” Radiology, vol. 187, pp. 27–31, 1993. [12] E. Frey, W. Smith, S. Grandgeorge, P. McCray, J. Wagener, E. Franken, Jr., and Y. Sato, “Chronic airway obstruction in children: Evaluation with cine-CT,” AJR, vol. 148, pp. 347–352, 1987. [13] J. Udupa and S. Samarasekera, “Fuzzy connectedness and object definition: Theory, algorithms, and applications in image segmentation,” Graphical Models Image Processing, vol. 58, no. 3, pp. 246–261, 1996. [14] J. Udupa, D. Odhner, S. Samarasekera, R. Goncalves, and K. Lyer, “3DVIEWNIX: An open, transportable, multidimensional, multimodality, multiparametric imaging software system,” SPIE Proc., vol. 2164, pp. 58–73, 1994. [15] H. Blum, “A transformation for extracting new descriptors of shape,” in Models for the Perception of Speech and Visual Form. Cambridge, MA: M.I.T. Press, 1967. [16] T. Kong and A. Rosenfeld, “Digital topology: Introduction and survey,” Comput. Vis., Graph., Image Processing, vol. 48, pp. 357–393, 1989. [17] K. Palágyi and A. Kuba, “A parallel 3D 12-subiteration thinning algorithm,” Graphical Models Image Processing, vol. 61, pp. 199–221, 1999. , “A 3D 6-subiteration thinning algorithm for extracting medial [18] lines,” Pattern Recogn. Lett., vol. 19, pp. 613–627, 1998. [19] C. Gonzalez and R. Woods, Morphology. Reading, MA: Addison-Wesley, 1993. [20] R. Carey and G. Bell, The Annotated VRML 2.0 Reference Manual. Reading, MA: Addison-Wesley, 1996. [21] T. Cormen, C. Leiserson, and R. Rivest, Introduction to Algorithms. Cambridge, MA: MIT Press, 1990. [22] G. F. Farin, Curves and Surfaces for Computer Aided Geometric Design: A Practical Guide. London, U.K.: Academic, 1990. [23] B. Jewett, R. Cook, K. Johnson, T. Logan, K. Rosbe, S. Mukherji, and W. Shockley, “Subglottic stenosis: Correlation between computed tomography and bronchoscopy,” Ann. Otol. Rhinol. Laryngol., vol. 108, pp. 837–841, 1999. [24] M. Remy-Jardin, J. Remy, D. Artaud, M. Fribourg, and A. Duhamel, “Volume rendering of tracheobronchial tree: Clinical evaluation of bronchographic image,” Radiology, vol. 208, pp. 761–770, 1998. [25] M. Huber, R. Henderson, S. Finn-Bodner, D. Macinitre, J. Wrigh, and G. Hankes, “Assessment of current techniques for determining tracheal luminal stenosis in dogs,” AJVR, vol. 10, pp. 1051–1054, 1997.