Lung lobe segmentation is clinically important for disease classification, treatment and follow-up of pulmonary diseases. Diseases such as tuberculosis and silicolis typically present in specific lobes i.e. almost exclusively the upper ones. However, the fissures separating different lobes are often difficult to detect because of their variable shape, appearance and low contrast in computed tomography images. In addition, a substantial fraction of patients have missing or incomplete fissures. To solve this problem, several methods have been employed to interpolate incomplete or missed fissures. For example, Pu et al. used an implicit surface fitting with different radial basis functions; Ukil et al. apply fast marching methods; and Ross et al. used an interactive thin plate spline (TPS) interpolation where the user selects the points that will be used to compute the fissure interpolation via TPS. In our study, results of an automated fissure detection method based on a plate-filter as well points derived from vessels were fed into an a robust TPS interpolation that ultimately defined the lobes. To improve the selection of detected points, we statistically determined the areas where fissures are localized from 19 data-sets. These areas were also used to constrain TPS fitting so it reflected the expected shape and orientation of the fissures, hence improving result accuracy. Regions where the detection step provided low response were replaced by points derived from a distance-to-vessels map. The error, defined as the Euclidian mean distance between ground truth points and the TPS fitted fissures, was computed for each dataset to validate our results. Ground truth points were defined for both exact fissure locations and approximate fissure locations (when the fissures were not clearly visible). The mean error was 5.64±4.83 mm for the exact ground truth points, and 10.01 ± 8.23 mm for the approximate ground truth points.
In this work, we have developed a novel knowledge-driven quasi-global method for fast and robust registration of thoracic-abdominal CT and cone beam CT (CBCT) scans. While the use of CBCT in operating rooms has become a common practice, there is an increasing demand on the registration of CBCT with pre-operative scans, in many cases, CT scans. One of the major challenges of thoracic-abdominal CT/CBCT registration is from various fields of view (FOVs) of the two imaging modalities. The proposed approach utilizes a priori knowledge of anatomy to generate 2D anatomy targeted projection (ATP) images that surrogate the original volumes. The use of lower dimension surrogate images can significantly reduce the computation cost of similarity evaluation during optimization and make it practically feasible to perform global optimization based registration for image-guided interventional procedures. Another a priori knowledge about the local optima distribution on energy curves is further used to effectively select multi-starting points for registration optimization. 20 clinical data sets were used to validate the method and the target registration error (TRE) and maximum registration error (MRE) were calculated to compare the performance of the knowledge-driven quasi-global registration against a typical local-search based registration. The local search based registration failed on 60% cases, with an average TRE of 22.9mm and MRE of 28.1mm; the knowledge-driven quasi-global registration achieved satisfactory results for all the 20 data sets, with an average TRE of 3.5mm, and MRE of 2.6mm. The average computation time for the knowledge-driven quasi-global registration is 8.7 seconds.
Diseased airways have been known for several years as a possible contributing factor to airflow limitation in Chronic
Obstructive Pulmonary Diseases (COPD). Quantification of disease severity through the evaluation of airway
dimensions - wall thickness and lumen diameter - has gained increased attention, thanks to the availability of multi-slice
computed tomography (CT). Novel approaches have focused on automated methods of measurement as a faster and
more objective means that the visual assessment routinely employed in the clinic. Since the Full-Width Half-Maximum
(FWHM) method of airway measurement was introduced two decades ago [1], several new techniques for quantifying
airways have been detailed in the literature, but no approach has truly become a standard for such analysis. Our own
research group has presented two alternative approaches for determining airway dimensions, one involving a minimum
path and the other active contours [2, 3]. With an increasing number of techniques dedicated to the same goal, we
decided to take a step back and analyze the differences of these methods. We consequently put to the test our two
methods of analysis and the FWHM approach. We first measured a set of 5 airways from a phantom of known
dimensions. Then we compared measurements from the three methods to those of two independent readers, performed
on 35 airways in 5 patients. We elaborate on the differences of each approach and suggest conclusions on which could
be defined as the best one.
It has been known for several years that airflow limitations in the small airways may be an important contributor to
Chronic Obstructive Pulmonary Disease (COPD). Quantification of wall thickness has lately gained attention thanks to
the use of high resolution CT, with novel approaches focusing on automated methods that can substitute for visual
assessment [1, 2]. While increased thickening of the wall is considered evidence of inflammatory disease, we
hypothesize that there may be additional ways to detect and quantify inflammation, specifically the uptake of contrast
material. In this preliminary investigation, we selected patients with documented chronic airway inflammation, and for
whom pre and post contrast datasets were available. On targeted reconstruction of right upper and lower lobes, we
selected airways with no connections to surrounding structures, and used a modified Full-Width-Half-Max method for
quantification of lumen diameter, wall thickness, and wall density. Matching airway locations on the pre- and postcontrast
cases were compared. Airways from patients without airway disease served as a control. Results for the airway
disease cases showed an average enhancement of 72 HU within the airway walls, with a standard deviation of 59 HU. In
the control group the average enhancement was 16 HU with standard deviation of 22 HU. While this study is limited in
number of cases, we hypothesize that quantification of contrast uptake is an additional factor to consider in assessing
airway inflammation. At the same time we are currently investigating whether enhancement can be measured via a
"contrast" map created with dual energy scanning, where a 3-value decomposition algorithm differentiates iodine from
other materials. This technique would eliminate both the need for a pre-contrast scan, and the task of matching airway
locations on pre- and post- scans.
Bronchial wall thickening is commonly observed in airway diseases. One method often used to quantitatively evaluate
wall thickening in CT images is to estimate the ratio of the bronchial wall to the accompanying artery, or BWA ratio,
and then assign a severity score based on the ratio. Assessment by visual inspection is unfortunately limited to airways
perpendicular or parallel to the scanning plane. With high-resolution images from multi-detector CT scanners, it
becomes possible to assess airways in any orientation. We selected CT scans from 20 patients with mild to severe
COPD. A computer system automatically segmented each bronchial tree and measured the bronchial wall thicknesses.
Next, neighboring arteries were detected and measured to determine BWA ratios. A score characterizing the extent and
severity of wall thickening within each lobe was computed according to recommendations by Sheehan et al [1]. Two
experienced radiologists independently scored wall thickening using visual assessment. Spearman's rank correlation
showed a non-significant negative correlation (r=-0.1) between the computer and the reader average (p=0.4), while the
correlation between readers was significant at r=0.65 (p=0.001). We subsequently identified 24 lobes with high
discrepancies between visual and automated scoring. The readers re-examined those lobes and measured wall thickness
using electronic calipers on perpendicular cross sections, rather than visual assessment. Using this more objective
standard of wall thickness, the reader estimates of wall thickening increased to reach a significant positive correlation
with automated scoring of r=0.65 (p=0.001). These results indicate that subjectivity is an important problem with visual
evaluation, and that visual inspection may frequently underestimate disease extent and severity. Given that a manual
evaluation of all airways is infeasible in routine clinical practice, we argue that automated methods should be developed
and utilized.
Chronic Obstructive Pulmonary Disease (COPD) is often characterized by partial or complete obstruction of airflow in
the lungs. This can be due to airway wall thickening and retained secretions, resulting in foci of mucoid impactions.
Although radiologists have proposed scoring systems to assess extent and severity of airway diseases from CT images,
these scores are seldom used clinically due to impracticality. The high level of subjectivity from visual inspection and
the sheer number of airways in the lungs mean that automation is critical in order to realize accurate scoring. In this
work we assess the feasibility of including an automated mucus detection method in a clinical scoring system. Twenty
high-resolution datasets of patients with mild to severe bronchiectasis were randomly selected, and used to test the
ability of the computer to detect the presence or absence of mucus in each lobe (100 lobes in all). Two experienced
radiologists independently scored the presence or absence of mucus in each lobe based on the visual assessment method
recommended by Sheehan et al [1]. These results were compared with an automated method developed for mucus plug
detection [2]. Results showed agreement between the two readers on 44% of the lobes for presence of mucus, 39% of
lobes for absence of mucus, and discordant opinions on 17 lobes. For 61 lobes where 1 or both readers detected mucus,
the computer sensitivity was 75.4%, the specificity was 69.2%, and the positive predictive value (PPV) was 79.3%. Six
computer false positives were a-posteriori reviewed by the experts and reassessed as true positives, yielding results of
77.6% sensitivity, 81.8% for specificity, and 89.6% PPV.
Bronchiectasis, the permanent dilatation of the airways, is frequently evaluated by computed tomography (CT) in order
to determine disease progression and response to treatment. Normal airways have diameters of approximately the same
size as their accompanying artery, and most scoring systems for quantifying bronchiectasis severity ask physicians to
estimate the broncho-arterial ratio. However, the lack of standardization coupled with inter-observer variability limits
diagnostic sensitivity and the ability to make reliable comparisons with follow-up CT studies. We have developed a
Computer Aided Diagnosis method to detect airway disease by locating abnormal broncho-arterial ratios. Our approach
is based on computing a tree model of the airways followed by automated measurements of broncho-arterial ratios at
peripheral airway locations. The artery accompanying a given bronchus is automatically determined by correlation of its
orientation and proximity to the airway, while the diameter measurements are based on the full-width half maximum
method. This method was previously evaluated subjectively; in this work we quantitatively evaluate the airway and
vessel measurements on 9 CT studies and compare the results with three independent readers. The automatically selected
artery location was in agreement with the readers in 75.3% of the cases compared with 65.6% agreement of the readers
with each other. The reader-computer variability in lumen diameters (7%) was slightly lower than that of the readers
with respect to each other (9%), whereas the reader-computer variability in artery diameter (18%) was twice that of the
readers (8%), but still acceptable for detecting disease. We conclude that the automatic system has comparable accuracy
to that of readers, while providing greater speed and consistency.
Chronic airway disease causes structural changes in the lungs including peribronchial thickening and airway dilatation.
Multi-detector computed tomography (CT) yields detailed near-isotropic images of the lungs, and thus the potential to
obtain quantitative measurements of lumen diameter and airway wall thickness. Such measurements would allow
standardized assessment, and physicians to diagnose and locate airway abnormalities, adapt treatment, and monitor
progress over time. However, due to the sheer number of airways per patient, systematic analysis is infeasible in routine
clinical practice without automation. We have developed an automated and real-time method based on active contours to
estimate both airway lumen and wall dimensions; the method does not require manual contour initialization but only a
starting point on the targeted airway. While the lumen contour segmentation is purely region-based, the estimation of the
outer diameter considers the inner wall segmentation as well as local intensity variation, in order anticipate the presence
of nearby arteries and exclude them. These properties make the method more robust than the Full-Width Half Maximum
(FWHM) approach. Results are demonstrated on a phantom dataset with known dimensions and on a human dataset
where the automated measurements are compared against two human operators. The average error on the phantom
measurements was 0.10mm and 0.14mm for inner and outer diameters, showing sub-voxel accuracy. Similarly, the mean
variation from the average manual measurement was 0.14mm and 0.18mm for inner and outer diameters respectively.
Pulmonary diseases characterized by chronic airway inflammation, such as Chronic Obstructive Pulmonary (COPD),
result in abnormal bronchial wall thickening, lumen dilatation and mucus plugs. Multi-Slice Computed Tomography
(MSCT) allows for assessment of these abnormalities, even in airways that are obliquely oriented to the scan plane.
Chronic airway inflammation typically results in limitations of airflow, allowing for the accumulation of mucus,
especially in the distal airways. In addition to obstructing airways, retained secretions make the airways prone to
infection. Patients with chronic airway disease are clinically followed over time to assess disease progression and
response to treatment. In this regard, the ability to obtain an automatic standardized method to rapidly and objectively
assess the entire airway tree morphologically, including the extent of mucus plugging, would be of particular clinical
value. We have developed a method to automatically detect the presence and location of mucus plugs within the
peripheral airways. We first start with segmentation of the bronchial tree using a previously developed method. The
skeleton-based tree structure is then computed and each terminal branch is individually extended using an adaptive
threshold algorithm. We compute a local 2-dimensional model, based on airway luminal diameter and wall thickness.
We then select a few points along the principal axis beyond the terminal branches, to extract 2D cross sections for
correlation with a model of mucus plugging. Airway shape is validated with a correlation value, and the lumen
distribution is analyzed and compared to the model. A high correlation indicates the presence of a mucus plug. We tested
our method on 5 datasets containing a total of 40 foci of mucoid impaction. Preliminary results show sensitivity of
77.5% with a specificity of 98.2% and positive predictive value of 66%.
Multi-Slice Computed Tomography (MSCT) imaging of the lungs allow for detection and follow-up of very small
lesions including solid and ground glass nodules (GGNs). However relatively few computer-based methods have been
implemented for GGN segmentation. GGNs can be divided into pure GGNs and mixed GGNs, which contain both nonsolid
and solid components (SC). This latter category is especially of interest since some studies indicate a higher
likelihood of malignancy in GGNs with SC. Due to their characteristically slow growth rate, GGNs are typically
monitored with multiple follow-up scans, making measurement of the volume of both solid and non-solid component
especially desirable. We have developed an automated method to estimate the SC percentage within a segmented GGN.
First, the SC algorithm uses a novel method to segment out the solid structures, while excluding any vessels passing near
or through the nodule. A gradient distribution analysis around solid structures validates the presence or absence of SC.
We tested 50 GGNs, split between three groups: 15 GGNs with SC, 15 GGNs with a solid nodule added to simulate SC,
and 20 GGNs without SC. With three defined satisfaction levels for the segmentation (A: succeed, B: acceptable, C:
failed), the first group resulted in 60% with score A, 40% with score B, 0% with score C. The second group resulted in
66.7% with score A and 33.3% with score B. In testing the first and 3rd groups, the algorithm correctly detected SC in
all cases where it was present (sensitivity of 100%) and correctly determined absence of SC in 15 out of 20 cases
(specificity 75%).
Pulmonary diseases such as bronchiectasis, asthma, and emphysema are characterized by abnormalities in airway dimensions. Multi-slice computed tomography (MSCT) has become one of the primary means to depict these abnormalities, as the availability of high-resolution near-isotropic data makes it possible to evaluate airways at oblique angles to the scanner plane. However, currently, clinical evaluation of airways is typically limited to subjective visual inspection only: systematic evaluation of the airways to take advantage of high-resolution data has not proved practical without automation. We present an automated method to quantitatively evaluate airway lumen diameter, wall thickness and broncho-arterial ratios. In addition, our method provides 3D visualization of these values, graphically illustrating the location and extent of disease. Our algorithm begins by automatic airway segmentation to extract paths to the distal airways, and to create a map of airway diameters. Normally, airway diameters decrease as paths progress distally; failure to taper indicates abnormal dilatation. Our approach monitors airway lumen diameters along each airway path in order to detect abnormal profiles, allowing even subtle degrees of pathologic dilatation to be identified. Our method also systematically computes the broncho-arterial ratio at every terminal branch of the tree model, as a ratio above 1 indicates potentially abnormal bronchial dilatation. Finally, the airway wall thickness is computed at corresponding locations. These measurements are used to highlight abnormal branches for closer inspection, and can be summed to compute a quantitative global score for the entire airway tree, allowing reproducible longitudinal assessment of disease severity. Preliminary tests on patients diagnosed with bronchiectasis demonstrated rapid identification of lack of tapering, which also was confirmed by corresponding demonstration of elevated broncho-arterial ratios.
Magnetic resonance (MR) imaging is frequently used to diagnose abnormalities in the spinal intervertebral discs. Owing to the non-isotropic resolution of typical MR spinal scans, physicians prefer to align the scanner plane with the disc in order to maximize the diagnostic value and to facilitate comparison with prior and follow-up studies. Commonly a planning scan is acquired of the whole spine, followed by a diagnostic scan aligned with selected discs of interest. Manual determination of the optimal disc plane is tedious and prone to operator variation. A fast and accurate method to automatically determine the disc alignment can decrease examination time and increase the reliability of diagnosis. We present a validation study of an automatic spine alignment system for determining the orientation of intervertebral discs in MR studies. In order to measure the effectiveness of the automatic alignment system, we compared its performance with human observers. 12 MR spinal scans of adult spines were tested. Two observers independently indicated the intervertebral plane for each disc, and then repeated the procedure on another day, in order to determine the inter- and intra-observer variability associated with manual alignment. Results were also collected for the observers utilizing the automatic spine alignment system, in order to determine the method's consistency and its accuracy with respect to human observers. We found that the results from the automatic alignment system are comparable with the alignment determined by human observers, with the computer showing greater speed and consistency.
There is growing interest in computer aided diagnosis applications including automatic detection of lung nodules from multislice computed tomography (CT). However the increase in the number and size of CT datasets introduces high costs for data storage and transmission, and becomes an obstacle to routine clinical exam as well as hindering widespread utilization of computerized applications. We investigated the effects of 3D lossy region-based JPEG2000 standard compression on the results of an automatic lung nodule detection system. As the algorithm detects the lungs within the datasets, we used this lung segmentation to define a region of interest (ROI) where the compression should be of higher fidelity. We tested 4 methods of 3D compression: 1) default compression of the whole image, 2) default compression of segmented lungs with masking out all non-lung regions, 3) ROI-based compression as specified in the JPEG2000 standard and 4) compression where voxels in the ROI are weighted to be given emphasis in the encoding. We tested 7 compression ratios per method: 1, 4, 6, 8, 10, 20, and 30 to 1. We then evaluated our experimental CAD algorithm on 10 patients with 67 documented nodules initially identified on the decompressed data. Sensitivities and false positive rates were compared for the various compression methods and ratios. We found that region-based compression generally performs better than default compression. The sensitivity with default compression decreased from 85% at no compression to 61% at 30:1 compression, a decrease of 25%, whereas the masked compression method saw a decreased in sensitivity on only 13.5% at maximum compression. At compression levels up to 10:1, all 3 region-based compression methods had decreases in sensitivity of 7.5% or less. Detection of small nodules (< 4mm in diameter) was more affected by compression than detection of large nodules; sensitivity to calcified nodules was less affected by compression than to non-calcified nodules.
We present an algorithm for local surface smoothing in a defined Volume of Interest (VOI) cropped from 3D volume data, such as lung CT data. There is generally a smooth and piecewise linear surface in the VOI, with one or more bumps on the surface. In lung CT data, such bumps can be nodules that are grown from the chest wall, which represent a possibility of lung cancer. Through surface smoothing, the nodules are segmented from the chest wall and its size can be measured as diagnostic evidence. The algorithm has the advantage of high consistency and robustness, and is useful in a segmentation module of a Compute Aided Diagnosis (CAD) system.
Lung nodules that exhibit growth over time are considered highly suspicious for malignancy. We present a completely automated system for detection of growing lung nodules, using initial and follow-up multi-slice CT studies. The system begins with automatic detection of lung nodules in the later CT study, generating a preliminary list of candidate nodules. Next an automatic system for registering locations in two studies matches each candidate in the later study to its corresponding position in the earlier study. Then a method for automatic segmentation of lung nodules is applied to each candidate and its matching location, and the computed volumes are compared. The output of the system is a list of nodule candidates that are new or have exhibited volumetric growth since the previous scan. In a preliminary test of 10 patients examined by two radiologists, the automatic system identified 18 candidates as growing nodules. 7 (39%) of these corresponded to validated nodules or other focal abnormalities that exhibited growth. 4 of the 7 true detections had not been identified by either of the radiologists during their initial examinations of the studies. This technique represents a powerful method of surveillance that may reduce the probability of missing subtle or early malignant disease.
Multi-slice computed tomography (CT) provides a promising technology for lung cancer detection and treatment. To optimize automatic detections of a more complete spectrum of lung nodules on CT requires multiple specialized algorithms in a coherently integrated detection system. We have developed a knowledge-based system for automatic lung nodule detection and analysis, which coherently integrates several robust novel detection algorithms to detect different types of nodules, including those attached to the chest wall, nodules adjacent to or fed by vessels, and solitary nodules, simultaneously. The system architecture can be easily extended in the future to include a still greater range of nodule types, most importantly so-called ground-glass opacities (GGOs). In addition, automatic local adaptive histogram analysis, dynamic cross-correlation analysis, and the automatic volume projection analysis by using by data dimension reduction method, are used in nodule detection. The proposed system has been applied to 10 patients screened with low-dose multi-slice CT. Preliminary clinical tests show that (1) the false positive rate averages about 3.2 per study; and (2) by using the system radiologists are able to detect nearly twice the number of nodules as compared with working alone.
We propose in this paper a novel approach to the automatic segmentation of lung nodules in a given volume of interest (VOI) from high resolution multi-slice CT images by dynamically initializing and adjusting a 3D template and analyzing its cross correlation with the structure of interest. First, thresholding techniques are used to separate the background voxels. The structure of interest, comprising of a nodule candidate and possible attached vessels, is then extracted by excluding any part of the chest wall inside the VOI. Afterwards, the proposed segmentation method finds the core of the structure of interest, which corresponds to the nodule, analyzes its orientation and size, and initializes a 3D template accordingly. Next, The template gradually expands, with its cross correlation to the original structure of interest being computed at each step. The template is then optimized based on the analysis of the cross correlation curve. A segmentation of the nodule is first roughly obtained by doing an 'AND' operation between the optimal template and the extracted structure and then refined by a spatial reasoning method. Template parameters can be recorded and recalled in later diagnosis so that reproducibility and consistency can be achieved. Preliminary results show that segmentation results are consistent, with a mean intra-scan volume measurement deviation of 2.8% for phantom data and 8.1% for real patient data.
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