Common Blind Spots on Chest CT: Where Are They All Hiding? Part 1 Airways, Lungs, and Pleura

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1 Residents Section Structured Review Wu et al. Common lind Spots on Chest CT Residents Section Structured Review Carol C. Wu 1 Leila Khorashadi 2 Gerald F. bbott 1 Matthew D. Gilman 1 Wu CC, Khorashadi L, bbott GF, Gilman MD Keywords: chest CT, lung cancer screening DOI: /JR Received May 25, 2012; accepted after revision October 25, Department of Radiology, Thoracic Imaging and Interventions, Massachusetts General Hospital, 55 Fruit St, Founders 202, oston, M ddress correspondence to C. C. Wu (cwu8@partners.org). 2 Department of Radiology, Mount uburn Hospital, Cambridge, M. WE This is a web exclusive article. JR 2013; 201:W533 W X/13/2014 W533 merican Roentgen Ray Society Common lind Spots on Chest CT: Where re They ll Hiding? Part 1 irways, Lungs, and Pleura Key Points 1. Chest CT allows improved delineation of thoracic anatomy and abnormalities as compared with chest radiography; however, blind spots exist on chest CT images where abnormalities can be missed. 2. Careful inspection of the airways for intraluminal lesions or wall thickening can improve detection of bronchial and tracheal neoplasms. 3. Maximum-intensity-projection images increase conspicuity of small pulmonary nodules. 4. Heterogeneous enhancement or hypoenhancement within collapsed lung can represent underlying infection or neoplasm. 5. Subtle nodularity of the pleura and interlobar fissures can signal malignant processes and should be sought on staging chest CTs and in the setting of unexplained pleural effusions. Chest CT has become one of the most common imaging studies performed, with rapidly expanding roles in evaluation of thoracic malignancies, infections, and pulmonary emboli. With the recently published results of the National Lung Screening Trial (NLST) [1], the role of chest CT has been further expanded to include screening for lung cancer in high-risk patients. number of areas, or blind spots, on chest CT images can harbor subtle findings and predispose radiologists to detection errors. wareness of these blind spots can help the radiologist to interpret chest CT more accurately and efficiently. irways Most lung cancers missed at CT were endobronchial in location in a study by White et al. [2]. Surprisingly, with mean diameter of 1.2 cm, these undetected endobronchial lesions were not necessarily small. These findings underscore the importance of evaluating the central airways to exclude endo- tracheal or endobronchial lesions. Malignant lesions may present as endoluminal nodules, subtle wall thickening (Fig. 1), or irregular contour of the lumen. Tracheal tumors such as adenoid cystic carcinoma and squamous cell carcinoma can have submucosal spread, which can be difficult to detect or be underestimated on CT. The accurate detection and description of the extent of the tumor is, however, essential for surgical planning. Endobronchial metastases can be seen most frequently in patients with primary renal cell, breast, and colorectal cancers and melanoma. The most common primary bronchial tumors include squamous cell carcinoma and carcinoid tumors. enign entities such as broncholith, endobronchial hamartoma, and aspirated foreign bodies can also cause significant complications such as recurrent atelectasis or pneumonia. Intraluminal secretion or mucus plug can mimic central airway lesions. However, secretion usually appears smoothly marginated and focal, often along the dependent aspect of the airways. Tiny air bubbles can sometimes be visualized within secretion. Comparison with prior imaging is helpful because a persistent intraluminal opacity is more suggestive of a true lesion. Repeat imaging in prone position or after coughing can also help in differentiating secretions from endoluminal lesions. Pulmonary Parenchyma Compared with chest radiography, CT offers improved visualization of pulmonary structures and abnormalities by removing the effect of overlapping structures. However, lung cancers can still be missed on chest CT. Studies on patients with resected pulmonary nodules have reported that CT misses up to 27 47% of nodules [3, 4]. There are areas within the pulmonary parenchyma that have been shown to more commonly harbor missed lesions. JR:201, October 2013 W533

2 Wu et al. Perihilar and Paramediastinal Lung Studies on missed lung cancers have determined that central lesions are at higher risk of being overlooked [2, 5]. Parenchymal nodules abutting the mediastinum or hilum may also be difficult to perceive as a separate structure. These nodules or masses sometimes manifest only as contour abnormalities along the mediastinum or hilum, particularly on unenhanced scans. In addition, nodules may also be hard to distinguish from adjacent vasculature, which tends to have similar size and rounded appearance on axial images (Figs. 2 and 3). These factors hamper the detection of small and sometimes even large nodules in the perihilar and paramediastinal lungs, for example, in the supraaortic arch region. In addition to careful examination of these areas on axial images, the use of coronal and sagittal reformation images as well as maximum-intensity-projection (MIP) images can increase conspicuity and improve detection of small pulmonary nodules [6, 7] (Fig. 4). Lower Lobes In a study of primary lung cancer overlooked at CT by White et al. [2], 73% (11/15) of missed tumors were located in a lower lobe, which is in contrast to the upper lobe predominance of missed lung cancer on chest radiography [8]. lthough the reason for radiologists to miss lung cancers in the lower lobes is not clear, this finding serves as a reminder that lower lobes should be carefully examined. Ground-Glass Nodules In the study by Li et al. [9], faint groundglass lesions account for 91% of lung cancers missed at low-dose CT because of detection error. ccording to these authors, these small areas of ground-glass opacity had low conspicuity owing to image noise. ecause ground-glass nodules have been associated with pathologic diagnoses such as atypical adenomatous hyperplasia, adenocarcinoma in situ, and minimally invasive adenocarcinoma [10], care must be taken to report these nodules, to ensure appropriate follow-up and management (Fig. 5). Collapsed Lung In areas of atelectasis, pulmonary lesions are no longer surrounded by air and become more difficult to identify. However, when one encounters a collapsed lobe or lung, care must be taken to not overlook a detectable lesion. First of all, the airway should be examined for potential causes of obstruction. The contour of the collapsed lung, lobe, or segment should also be examined for abnormal convexity or bulge, which would suggest an underlying mass lesion. On contrast-enhanced studies, collapsed lung should enhance homogeneously. Heterogeneous enhancement or hypoenhancement within the collapsed lung would be suspicious for underlying consolidation, nodules, or masses (Fig. 6). Satisfaction of Search In the study by White et al. [2], in 43% of patients with missed lung cancer, major distracting findings were present elsewhere in the thorax. In the study by Li et al. [9], many of the patients with missed lung cancer had underlying lung disease such as tuberculosis, emphysema, or interstitial fibrosis (Fig. 7). Indeed, significant intrathoracic abnormalities, such as an aortic dissection, would make a small lung nodule seem less relevant at the time of interpretation; nevertheless, radiologists have to continue to examine the rest of the images and report all abnormalities. Complex underlying pulmonary parenchymal diseases such as emphysema or fibrosis can also cause architectural distortion and make detection of small pulmonary nodules difficult; however, patients with emphysema and interstitial fibrosis are at higher risk of lung cancer. Interpretation error, in addition to detection error, can also occur; this is particularly true in the lung apices, where early lung cancer can be obscured by preexisting opacities commonly seen in this region or, even when detected, be misinterpreted as scarring from prior granulomatous infection. Comparison with prior study is vital because increase in size, convexity, or density should prompt further follow-up imaging or investigation. Pleural Space Small pleural lesions can frequently be missed given their peripheral location. Care should be taken to check the pleural surface, including the diaphragmatic pleura for nodularity on all chest CT. It is particularly important to look for pleural nodularity or thickening in patients with otherwise unexplained pleural effusion because nodularity would prompt consideration for malignant process and cytologic analysis of pleural fluid for malignant cells. lthough mesothelioma most frequently presents with diffuse or circumferential nodular pleural thickening, it can occasionally present with subtle or isolated pleural nodule(s) (Fig. 8). Detection of such early or localized lesions is vital because these patients can benefit from complete surgical resection. Searching the pleural surface is also important in cases of anterior mediastinal mass suspicious for thymoma and in staging of known malignancies (Fig. 9). Isolated small fissural or subpleural nodules with triangular and oval shape are most likely benign intraparenchymal lymph nodes [11], or they may be tiny areas of atelectasis when present in the dependent portion of the chest. Early pleural metastases should be considered in patients with known or suspected malignancy when the fissural or pleural nodules are multiple or new compared with prior CT examinations. Evaluation of these nodules in the sagittal and coronal planes sometimes allows better delineation of their shapes and helps one decide whether they are more likely to represent lymph node or atelectasis than malignant pleural deposits. Conclusion To increase detection of subtle abnormalities such as early lung cancer and pulmonary or pleural metastases, the radiologist should conduct a careful and systematic review of the lungs, airways, and pleural surfaces using axial multiplanar reconstructed (coronal, sagittal, or both) and MIP images. ssessment of changes from prior imaging is essential to identify suspicious lesions. s chest CT becomes an integral part of workup of patients with nonspecific respiratory symptoms and a screening tool for patients at high risk for lung cancer, improved detection of subtle lesions can prevent delayed diagnosis, thereby allowing timely intervention. References 1. berle DR, dams M, erg CD, et al.; National Lung Screening Trial Research Team. Reduced lung-cancer mortality with low-dose computed tomographic screening. N Engl J Med 2011; 365: White CS, Romney M, Mason C, ustin JHM, Miller H, Protopapas Z. Primary carcinoma of the lung overlooked at CT: analysis of findings in 14 patients. Radiology 1996; 199: Parsons M, Ennis EK, Yankaskas C, Parker L Jr, Hyslop W, Detterbeck FC. Helical computed tomography inaccuracy in the detection of pulmonary metastases: can it be improved? nn Thorac Surg 2007; 84: Peuchot M, Libshitz HI. Pulmonary metastatic disease: radiologic-surgical correlation. Radiology 1987; 164: Gurney JW. Missed lung cancer at CT: imaging find- W534 JR:201, October 2013

3 Common lind Spots on Chest CT ings in nine patients. Radiology 1996; 199: Gruden JF, Ouanounou S, Tigges S, Norris SD, Klausner TS. Incremental benefit of maximumintensity-projection images on observer detection of small pulmonary nodules revealed by multidetector CT. JR 2002; 179: Valencia R, Denecke T, Lehmkuhl L, Fischback F, Felix R, Knollmann F. Value of axial and coronal maximum intensity projection (MIP) images in the Fig year-old woman after right upper lobectomy for lung cancer., xial CT performed for restaging shows asymmetric thickening of left lateral tracheal wall, not seen at time of original interpretation., Coronal multiplanar reformatted CT image better depicts craniocaudal extent of tracheal wall thickening (arrow), corresponding to squamous cell carcinoma found on bronchoscopic biopsy. Fig year-old woman with lung cancer., xial CT image shows small nodule (arrow) in right lower lobe, which was not detected initially., xial CT image 8 years later shows enlargement of right lower lobe nodule biopsy, proven to be adenocarcinoma. detection of pulmonary nodules by multislice spiral CT: comparison with axial 1-mm and 5-mm slices. Eur Radiol 2006; 16: ustin JH, Romney M, Goldsmith LS. Missed bronchogenic carcinoma: radiographic findings in 27 patients with a potentially resectable lesion evident in retrospect. Radiology 1992; 182: Li F, Sone S, be H, MacMahon H, rmato SG 3rd, Doi K. Lung cancers missed at low-dose helical CT screening in a general population: comparison of clinical, histopathologic, and imaging findings. Radiology 2002; 225: Lee HY, Lee KS. Ground-glass opacity nodules: histopathology, imaging valuation, and clinical implications. J Thorac Imaging 2011; 26: hn MI, Gleeson TG, Chan IH, et al. Perifissural nodules seen at CT screening for lung cancer. Radiology 2010; 254: JR:201, October 2013 W535

4 Wu et al. Fig year-old man with renal cell carcinoma. xial CT image shows small nodule in right lower lobe (arrow), adjacent to hilar vessels, which was initially undetected because it appeared similar to vessel seen on cross section. Fig year-old man with bladder cancer., xial CT image shows new 3-mm right upper lobe nodule (arrow), which is difficult to detect., Maximal-intensity-projection image improves conspicuity of small nodule (arrow). W536 JR:201, October 2013

5 Common lind Spots on Chest CT Fig year-old woman with lung cancer., xial CT image shows small subtle ground-glass nodule (arrow) in left lower lobe., xial CT image from study performed 6 years later shows increase in density and size of left lower lobe nodule, which is biopsy-proven adenocarcinoma. Fig year-old woman with dyspnea., xial CT image at presentation shows large right pleural effusion, collapsed right middle lobe, and subcarinal lymphadenopathy. Rounded area of hypoenhancement (arrow) in collapsed right middle lobe was suspicious for mass., xial CT image after thoracocentesis and reexpansion of right middle lobe confirms presence of spiculated right middle lobe mass, proven to be adenocarcinoma. Fig year-old man with asbestosis. xial CT image shows basilar reticulations and traction bronchiectasis consistent with patient s diagnosis of asbestosis. Lobulated left lower lobe pulmonary nodule (arrow), corresponding to lung cancer, was also present. Fig year-old woman with hemoptysis. xial CT image shows small left medial pleural nodule (arrow), proven to represent mesothelioma on surgical resection. JR:201, October 2013 W537

6 Wu et al. FOR YOUR INFORMTION Fig year-old man with head and neck cancer., xial CT image shows subtle nodules along left posterior pleural surface (arrow), new since prior studies., xial CT image shows additional small nodules along diaphragmatic pleura, later proven to represent pleural metastases. Presence of new small nodules along pleura (arrow), particularly in nondependent location, in patients with history of malignancy should prompt consideration for pleural metastases. The reader s attention is directed to part 2 accompanying this article, which will be published in the November 2013 issue of JR. W538 JR:201, October 2013