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* From the Department of Radiology (Dr. Peldschus and Cheema), Brigham and Womens Hospital, Harvard Medical School, Boston, MA; Institute of Clinical Radiology (Dr. Herzog), Klinikum Grosshadern, University of Munich, Germany; R2 Technology, Inc. (Dr. Wood), Sunnyvale, CA; and Department of Radiology (Drs. Costello and Schoepf), Medical University of South Carolina, Charleston, SC.
Correspondence to: U. Joseph Schoepf, MD, Department of Radiology, Medical University of South Carolina, 169 Ashley Ave, Charleston, SC 29425; e-mail: schoepf{at}musc.edu
| Abstract |
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Design: Chest CT studies were processed using a prototype CAD system for automated detection of lung lesions. Three experienced radiologists analyzed each CAD finding and confirmed or dismissed the marked image features as lung lesions. Noncalcified, focal lung lesions were classified according to size as being of high (
10 mm), intermediate (5 to 9 mm), or low (
4 mm) significance.
Setting: Two subspecialized academic tertiary referral centers in the United States and Germany.
Patients: Chest CT studies were performed in 100 patients, with results initially reported as normal at clinical double reading. Indications for chest CT were suspected pulmonary embolism (PE) [n = 33], lung cancer screening in a high-risk population (n = 28), or follow-up for a cancer history (n = 39).
Interventions: Reevaluation of all chest CT studies for focal lung lesions with the CAD system as a second reader.
Measurements: Prevalence and spectrum of lung lesions missed at routine clinical interpretation but found by the CAD system.
Results: In 33% (33 of 100 patients), CAD detected significant lung lesions that were not previously reported. Fifty-three significant lesions were detected (mean, 1.6 lesions per case), of which 5 lesions (9.4%) were of high significance, 21 lesions (39.6%) were of intermediate significance, and 27 lesions (50.9%) were of low significance. In the PE group, the lung cancer screening group, and the group with a cancer history, four patients (12.1%), six patients (21.4%), and nine patients (23.1%), respectively, had focal lung lesions of high and/or intermediate significance. The false-positive rate of the CAD system was an average of 1.25 per case (range, 0 to 11).
Conclusions: Significant lung lesions are frequently missed at routine clinical interpretation of chest CT studies but may be detected if CAD is used as an additional reader.
Key Words: cancer screening lung, neoplasms lung, nodule lung neoplasms, diagnosis
| Introduction |
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CT of the chest is the reference standard for the detection of focal lung disease.4 However, the analytical review of thoracic anatomy and pathology based on many individual transaxial CT sections is demanding. Missing of lesions at thoracic CT is a well-recognized phenomenon,567891011 although the actual magnitude of the problem is difficult to gauge. The risk of missing lung lesions is increased with the introduction of latest generation multidetector-row CT technology, which routinely results in > 200 individual transaxial sections with a slice thickness of
1 mm for a single chest CT. The information overload associated with reviewing such large, chest CT studies is a cognitive challenge, is time consuming and cumbersome, and greatly increases the risk of lesions being overlooked. Consensus or double readings may help to reduce error1213 but are labor intensive.12
From humble beginnings,14151617 computer-aided diagnosis (CAD) has gained increasing practical importance in medicine in recent years and is successfully applied to cytologic analyses18 and to the detection of cancers of the breast1920 and lung.212223 We designed this study to evaluate whether diagnostic errors can be decreased when CAD algorithms are integrated into clinical imaging as a second reader. We evaluated the prevalence and spectrum of additional findings yielded by CAD in chest CT studies that were read as normal at routine clinical interpretation.
| Materials and Methods |
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20 pack years) participating in a lung cancer screening program. The third group included 39 consecutive patients (23 women and 16 men; mean age, 55.7 years; range, 27 to 82 years) with a cancer history in full remission undergoing a routine follow-up scan for the following malignant tumor types: prostate cancer (n = 1); ovarian/endometrial cancer (n = 4); Hodgkin disease (n = 4); cancers of unknown primary (n = 6); GI malignancies (n = 7); non-Hodgkin lymphoma (n = 8); and breast cancer (n = 9). The study was performed with institutional review board approval for analysis of CT scan data and records in patients undergoing spiral CT of the chest; informed patient consent was not required. All studies had been acquired with a 4- or 16-slice multidetector-row spiral CT scanner (Siemens Medical Solutions; Forchheim, Germany). The transaxial images were reconstructed with a thickness varying from 1.25 to 3 mm, resulting in an average number of 241 images per study. All image data were electronically stored at a picture archive and communication system from which they could be retrieved for digital computer-aided image analysis.
All 100 CT studies were processed using a CAD system (ImageCheckerCT; R2 Technology; Sunnyvale, CA). The CAD algorithm is designed to automatically detect and subsequently analyze focal opacities to aid the diagnostic process of a radiologist. It uses a series of volume-centric segmentation steps that delineate normal from abnormal lung tissue. Multiple geometric parameters are calculated for each suspected lung lesion including shape, elongation, size, spiculation, density, and other features. Based on these parameters and following an analytical decision tree, the candidate lesion is given a likelihood rating of representing a lung lesion. If the likelihood rating exceeds a defined threshold for features indicative of a lung lesion, the CAD system deploys a detection mark on the lesion that alerts the human observer to more closely review the marked lung area for the presence of a focal lung lesion. The observer can choose to accept the CAD marked image feature as a focal lung lesion and act on the suspicious finding or dismiss it altogether.
Two experienced radiologists (with 10 years and 7 years of experience in reading thoracic CT, respectively) jointly evaluated and rated each CAD-marked image feature. A third radiologist with 25 years of experience in reading thoracic CT served as an adjudicator for equivocal cases. Each CAD mark was analyzed and was confirmed or dismissed as a lung lesion. Lesions were classified as significant, if they met the criteria of a lung nodule, ie, a circumscribed parenchymal lung lesion that is solid, part solid, or nonsolid in nature and is not associated with atelectasis or adenopathy.24 Significant lung lesions were further classified by size as lesions of high (diameter
10 mm), intermediate (5 to 9 mm), or low (
4 mm) significance. Lesion location was classified as central or peripheral. CAD-marked image features with a benign calcified pattern or a nonnodular appearance were considered true-positive findings but were categorized as nonsignificant lung changes. CAD-marked image features that by consensus of the readers represented normal thoracic anatomy were considered false-positives.
In cases where significant lung lesions were detected by CAD, the radiology report was amended by an addendum in accordance with the Standard for Communication of the American College of Radiology.2526
| Results |
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A total of 125 CAD marks were dismissed as false-positive findings. The false-positive rate of the CAD system thus was an average of 1.25 per case (range, 0 to 11). The vast majority of false-positive findings occurred at vessel crossings or vascular structures distorted by image artifacts due to motion (eg, cardiac pulsation).
| Discussion |
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Although the true magnitude of the problem is difficult to assess, the medical literature is replete with reports on lung lesions that were missed at radiography or CT imaging.567891011 While the missing of lung lesions at imaging is a well-recognized phenomenon, the number of 53 lesions in 33 patients that were not reported at clinical readout in two highly specialized academic departments is surprisingly high. Most of the candidate lesions were < 4 mm and were deemed to be of low significance; however, in five patients (5%) lesions > 1 cm were overlooked.
Significant differences in the prevalence of findings relative to the indication for which chest CT was performed were not seen. Still, slightly more small nodules with a diameter of
4 mm were found in the group of patients with suspected PE. This may potentially be explained by differences in observer perception, since readers may focus their attention more on vascular structures during the interpretation of studies performed for suspected PE than on nodule detection. Patients in the lung cancer screening group had the lowest per case rate of CAD-detected lesion, likely because in this group of patients the undivided attention of the radiologist is focused on the detection of focal lung lesions.
In this study, we could demonstrate that significant lung lesions missed at the clinical interpretation of chest CT studies can be detected if CAD is used as an additional reader. Use of a CAD system as a second reader thus appears to be able to offset to some degree the inherent variability of human observation by finding oversights that are missed in the original review. Different from a human observer, the CAD system is not affected by distraction or fatigue and always operates at the same performance level. Therefore, the consistency of a CAD system combined with the knowledge and experience of the human observer can have a positive impact on detection accuracy, as demonstrated in this study.
Comparable to other studies,2130 our results show that use of CAD increases the overall accuracy of lesion detection at imaging of the lung. However, detection of additional lung lesions by CAD in a patient with known focal lung disease usually does not alter patient management. CAD detection of lung lesions in a patient whose chest CT study was interpreted as normal, however, as in our study, could potentially result in a significant change in patient management.
There are several aspects of the use of CAD systems that were not addressed by our study design. Computerized systems and neural networks for objectively assessing the likelihood of a lesion of being benign or malignant are actively being investigated. In our study, however, we focused exclusively on the utility of CAD for reducing perception error and not classification error. This latter error is difficult to control for by our retrospective study design. It is possible that some of the 53 lesions that were not reported but were deemed significant for the purpose of this study had indeed been seen, but not considered clinically significant enough to report by the original clinical readers.
It is also difficult to gauge the ultimate significance for patient outcome of the CAD-detected lesions in this study since complete and systematic long-term follow-up or lesion histology are difficult to obtain. However, if a patient is referred for evaluation of focal lung disease, the detection of a lung nodule always bears significance for the ad hoc classification and management of the patient. Potential consequences range from observation and additional follow-up to surgical removal of the lesion, as dictated by the clinical context and patient history. Correct classification and appropriate patient management, however, are jeopardized, if a lesion is missed at diagnostic imaging. In this study, we were able to show that this risk can be mitigated to some extent if CAD is used as an additional safety net.
One hundred twenty-five CAD marks were dismissed as false-positive findings, with an average of 1.25 per case, which is substantially less than with previous iterations of CAD algorithms. However, deployment of a false-positive detection mark in normal thoracic anatomy without pathologic correlate still requires an active decision from the interpreting physician to discard the mark. Dismissal of these detection marks as false-positive findings is usually not very challenging and is accomplished by a single glance of a somewhat experienced reader.
In conclusion, in this study we were able to demonstrate how CAD can be successfully used for reducing perception error in medical imaging. Beneficial integration of such systems into the clinical environment may therefore better ensure that patients with suspected thoracic disease are correctly classified and receive appropriate diagnostic workup and therapy.
| Footnotes |
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Drs. Peldschus, Costello, and Schoepf had full access to all the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.
Dr. Peldschus is supported by the Biomedical Sciences Exchange Program, Hannover, Germany. Dr. Wood is an employee of R2 Technology Inc., Sunnyvale, CA. Dr. Costello is the recipient of a research grant provided by R2 Technology Inc., Sunnyvale, CA. Dr. Schoepf is the recipient of an unrestricted research grant provided by Siemens Corporate Research, Princeton, NJ. The sponsors had no part in the design and conduct of the study or the collection, management, analysis, and interpretation of the data.
Received for publication November 18, 2004. Accepted for publication January 26, 2005.
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