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CHEST RADIOLOGY Table of Contents   
Year : 2001  |  Volume : 11  |  Issue : 2  |  Page : 65-70
Computerised tomographic evaluation of mediastinal lesions : Pictorial essay


Department of Radiology, Dr. Ram Manohar Lohia Hospital, New Delhi, India

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Keywords: Computerised Tomography, Mediastinal Lesions

How to cite this article:
Prasad A, Chauhan B. Computerised tomographic evaluation of mediastinal lesions : Pictorial essay. Indian J Radiol Imaging 2001;11:65-70

How to cite this URL:
Prasad A, Chauhan B. Computerised tomographic evaluation of mediastinal lesions : Pictorial essay. Indian J Radiol Imaging [serial online] 2001 [cited 2019 Aug 20];11:65-70. Available from: http://www.ijri.org/text.asp?2001/11/2/65/28376
Mediastinal lesions include a wide spectrum of pathology, ranging from tumors (benign & malignant), cysts, vascular anomalies, and lymph node masses to diffuse lesions such as pneumomediastinum, mediastinitis, mediastinal fibrosis and encysted pleural effusion. These lesions are challenging problems frequently faced by the radiologist, and often a plain chest radiograph is inadequate for answering the queries posed by physicians. In the past, patients had to undergo mediastinoscopy / thoracotomy to establish a diagnosis. CT is capable of defining the precise anatomical details and characterizing the nature, site and the extent of mediastinal lesions. While both CT and MR provide cross-sectional depiction of the mediastinum, CT has better spatial resolution and shorter imaging time than MR, besides being less expensive and more widely available. Coexisting lung abnormalities and calcification within lesions are better appreciated on CT. The additional role of CT in performing CT guided biopsies of lesions cannot be over-emphasized. Ct Technique: The usual technique is to obtain contiguous 10mm axial sections to include the entire mediastinum in a craniocaudal direction. Superiorly, at least the proximal 2 - 3 cm of the branches of the aortic arch should be included, and inferiorly the scan should continue into the upper abdomen. The liver and adrenals can be included if found necessary (e.g. suspected bronchogenic carcinoma) to know the extent of spread. Routinely plain sections of the region of interest, followed by contrast enhanced scans using intravenous iodinated ionic contrast media are done. Spiral scanning is the standard method of scanning currently and provides faster scanning with reduced contrast dosage. Spiral CT takes advantage of new slip ring technology with continuously rotating detectors, allowing rapid data acquisition through the chest during a single breathhold. It facilitates optimization of intravenous contrast administration by obtaining the scans during the peak of vascular enhancement. The absence of respiratory misregistration or movement artifacts with sophisticated computer processing permits instantaneous generation of superb two dimensional multiplanar reformatting at the console, in any desired plane, without step-like discontinuities that result from the use of discrete conventional scans. Such coronal, sagittal or oblique images improve the diagnostic accuracy. Depiction of spatial relationships between a lesion and the surrounding structures (e.g. origin and extent of a neoplasm) improves greatly with spiral CT. For example spiral CT generated orthogonal reconstructions in a patient with bronchogenic carcinoma with evaluation of vessels, airways and mediastinum further helps in staging. The volumetric data may be processed on a separate computer workstation to provide three-dimensional display of a selected anatomical structure (e.g. aorta, tracheo-bronchial tree). Planar images of vessels in any axis may be generated from data acquired from a spiral contrast CT. Overlying disturbing information from bones or organs can be subtracted from the final image to yield an "angiogram" comparable in quality to digital subtraction technique. Stenosis or dilatation can be depicted and the relationship of normal vessels to surrounding pathology displayed. Various authors have divided the mediastinum into different compartments. Anatomically it can be divided into superior and inferior mediastinum, the latter into anterior, middle and posterior mediastinum. Based on axial CT sections, it can be divided into prevascular, pretracheal or paratracheal, aorto-pulmonary window, subcarinal, posterior mediastinal, hilar and paracardiac regions. The structures present in each anatomical compartment and various pathologies and their CT appearances are reviewed.

Lymph Nodal Masses: Found mainly in the anterior and middle mediastinum, their etiology can be tuberculosis, lymphoma, metastases or nongranulomatous pathology such as sarcoidosis. Although CT is helpful in characterization of the nodal mass with respect to size, site and preservation of the surrounding fat planes, it should be emphasized that CT is non-specific in evaluating the etiology of lymph node enlargement. The CT criterion for a normal sized lymph node is laid down as 1 cm in its short axis[1]. CT can also detect sub-carinal, circum cardiac, internal mammary and crural group of lymph nodes which are not routinely picked up on chest radiographs.

Tuberculous Lymphadenopathy [Figure - 1]: Im JG et al described a specific CT appearance of tuberculous nodes in which the nodes are larger than 2.0 cm with multiple areas of central low density with irregular thick, enhancing walls and obliteration of fat planes[2]. But subsequent studies showed that this appearance is not very specific for tuberculosis. Metastases - are mainly from the intra-thoracic area, primary neoplasms being bronchogenic or esophageal carcinoma. The other sites include breast, seminoma, testicular carcinoma etc. Lymphoma - Mediastinal nodes are more commonly involved in Hodgkin's lymphoma than non-Hodgkin's lymphoma and CT aids in the correct staging [Figure - 2]

Bronchogenic Carcinoma - [Figure - 3],[Figure - 4] Mediastinal CT has become a routine step in staging patients with bronchogenic carcinoma. CT clearly demonstrates mediastinal invasion based on the following criteria:[3] (i) Confluence of tumor with mediastinum (ii) angle more than 90 degrees between the mass and mediastinal vessels or pericardium (iii) indentation of border between the mass and vessel with absence of mediastinal fat plane (iv) thickened wall of proximal end of main stem bronchus. Thymoma: It accounts for 25 % of anterior mediastinal masses [4]. Chest radiographs have a sensitivity of approximately 33% in detecting thymic masses while CT improves the sensitivity to 100%. Usually defined as solid, non-homogenous anterior mediastinal masses [Figure - 5], calcification has been reported in 7-12% of patients [5]. Invasive thymoma is suggested by the CT findings of obliteration of the mediastinal fat plane and presence of pleural nodules [Figure - 6].

Intrathoracic Goiter : It accounts for 11-15% of mediastinal masses. The CT criteria for its diagnosis include clear continuity with the thyroid gland, well-defined borders, calcification (punctate, coarse or ring-like), non-homogeneity with discrete, non-enhancing low attenuating areas, pre-contrast attenuation value at least 15 HU more than adjacent muscles and post-contrast enhancement which is patchy, intense and persistent, greater than 25 HU [Figure - 7].

Mediastinal Lipomatosis : In a chest radiograph, this can present as mediastinal widening whereas CT clearly demonstrates the characteristic Hounsfield value of fat without any displacement of surrounding mediastinal structures [Figure - 8].

Germ Cell Tumors : They constitute 4-9% of the anterior mediastinal masses [6]. Teratomas are exclusively seen in the anterior mediastinum. Presence of fat, peripheral calcification, cystic areas of low density, or presence of tooth or bone aid in the diagnosis of a benign teratoma. A malignant teratoma is more homogenous in appearance. Local invasion is more suggestive of malignancy than the actual histological appearance [Figure - 9].

Aneurysm : Descending aorta is the most common site of thoracic aneurysm, arch being the next most common site; ascending aorta being the least common [7]. The CT diagnostic criteria include (a) dilatation of the aorta (b) curvilinear calcification (c) luminal thrombosis (d) contrast enhancement of the patent portion of the lumen [Figure - 10]. MR has been found superior to CT in distinguishing acute intra-mural hematoma from both atherosclerotic plaque and intra-luminal thrombosis. Also there is the advantage of obviation of contrast administration and availability of multiple sequences for detection of even minimal flow in a false lumen in cases of dissection of aorta, whereas CT has a distinct advantage in displaying displacement of intimal calcification, seen in a dissecting aneurysm.

Foregut Duplication Cysts : They constitute 11 to 18% of the total mediastinal masses. Bronchogenic cysts are the commonest, found mainly in the middle and sometimes in the posterior mediastinum, commonly 5 cm from the carina followed by the paratracheal area, esophageal wall and retrocardiac area [8]. CT appearance is that of a unilocular cystic lesion with uniform water density, but at times, mucoid material and calcium content attenuate values upto 100 HU [9]. Pleuro-pericardial cyst is typically located anteriorly in the right pericardio-phrenic angle, sometimes extending to the adjacent major fissure and at times communicating with the pericardial cavity [Figure - 11]. CT clearly demonstrates the malleable nature of the cysts, which change their shape with respect to change in position from prone to decubitus [10]. Neurenteric cysts are seen mainly in the middle and posterior mediastinum, 50% showing communication with the neural canal and with associated vertebral anomalies [Figure - 12]. Neurogenic Tumors: In children they form 80% of the total posterior mediastinal masses and 30% of them are malignant [11]. The mass can show patchy amorphous calcification [Figure - 13].

Paravertebral Abscess: These lesions appear as large hypodense masses in the paravertebral area with peripheral rim enhancement with vertebral destruction and fragmentation [Figure - 14]. Hiatus Hernia: It is the most common lower-posterior mediastinal mass seen on a chest radiograph. CT clearly demonstrates widening of the esophageal hiatus and herniated stomach surrounded by omental fat [Figure - 15].

Carcinoma Esophagus : CT clearly depicts extra esophageal spread, which is suggested by (a) obliteration of the para-vertebral triangular fat pad (b) degree of contact between tumor and aortic circumference being more than 90o [12], [Figure - 16].

CT apart from evaluating the mediastinal mass accurately, also helps in performing CT guided biopsies. Most of these masses are surrounded by vessels and require precise localization. This requirement is well met with the modern CT scanners. Thus, the use of CT scan greatly expedites the diagnostic process by supplanting multiple previously used image studies such as conventional tomography, barium swallow, radionuclide scans and angiographic procedures.

 
   References Top

1.Glazer GM, Gross GH, Quain LE. Normal Mediastinal Lymph nodes size & number according to Am Thoracic Society. AJR 1985 ; 144 : 261.  Back to cited text no. 1    
2.Im JG, Sores KS, King HS. Computerized Tomographic manifestation of mediastinal tuberculous lymphadenitis. Radiology 1987; 164: 615.   Back to cited text no. 2    
3.Redina A et al. Computerised Tomographic staging of anterior mediastinal neoplasms. Thorax 1988; 43: 441 - 45.  Back to cited text no. 3    
4.Brown LR, Muhm JR, Grey JE. Radiographic detection of thymoma. AJR 1980; 134: 1181 - 88.   Back to cited text no. 4    
5.Baron RL et al. Computerised Tomography of the abnormal thymus. Radiology 1982; 142: 127.   Back to cited text no. 5    
6.Haaga JR, Alfidi RJ. Computerised Tomography of whole body, 2nd Ed. St. Louis: CV Mosby, 1988.  Back to cited text no. 6    
7.Posniak HV, Olson DTC. Computerised Tomography of thoracic aneurysm. Radiographics 1990; 10: 839.   Back to cited text no. 7    
8.Reeds JC, Sobonya RE. Morphological Analysis of foregut cysts in thorax. AJR 1974; 120: 851.   Back to cited text no. 8    
9.Mendelson DS, Ross JS, Efremedis SC, Kirschner PA. Bronchogenic cysts with high CT attenuation number. AJR 1983; 140: 463.   Back to cited text no. 9    
10.Pugatch RD, Faling LJ, Robins AH. Computerised Tomographic diagnosis of benign mediastinal abnormalities. AJR 1980; 134: 685.   Back to cited text no. 10    
11.Manuel PM, Thomas LS. Mediastinal masses in the pediatric age group. RCNA 1993; 31: 583.   Back to cited text no. 11    
12.Takashima S, Takeuchi N, Shinozaki H. Carcinoma of esophagus - CT versus MRI. Imaging in detecting resectability. AJR 1990; 156: 297.  Back to cited text no. 12    

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Correspondence Address:
A Prasad
Department of Radiology, Dr. Ram Manohar Lohia Hospital, New Delhi
India
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Source of Support: None, Conflict of Interest: None


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    Figures

[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7], [Figure - 8], [Figure - 9], [Figure - 10], [Figure - 11], [Figure - 12], [Figure - 13], [Figure - 14], [Figure - 15], [Figure - 16]

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