MEDIASTINAL MASSES: MAGNETIC RESONANCE IMAGING IN COMPARISON WITH COMPUTED TOMOGRAPHY Poonam Batra, MD, Kathleen Brown, MD, James D. Collins, MD, E. Carmack Holmes, MD, Richard J. Steckel, MD, and Bertrand J. Shapiro, MD Los Angeles, California
Seventy-eight patients with mediastinal abnormalities were imaged with magnetic resonance imaging (MRI) to evaluate mediastinal masses and associated lung, pleural, or chest wall disease. Magnetic resonance images were compared with computed tomography (CT) scans, which were available in 45 patients. While MRI and CT were equally effective in demonstrating mediastinal lesions, CT was superior for displaying calcification within a mass in eight patients and for demonstrating associated lung abnormality in four patients. Computed tomography should remain the imaging procedure of choice after chest radiography to evaluate mediastinal masses, although MRI may be indicated in selected patients. (J Nati Med Assoc. 1991;83:969-974.)
tial capability for evaluating mediastinal masses. 14 However, in these studies, the majority of cases represented mediastinal pathology associated with primary bronchogenic carcinoma. This article details the MRI appearance of mediastinal masses in those cases where mediastinal mass was the primary abnormality and compares the diagnostic information provided by MRI findings with CT findings.
MATERIALS AND METHODS Since September 1984, 78 patients (41 men and 37 women ranging in age from 18 to 84 years) have been evaluated with MRI for a mediastinal mass suspected or detected on chest radiographs. Computed tomography scans performed within 2 weeks of the MRI were available for comparison in 45 patients.
Imaging Methods Key words * magnetic resonance imaging * computed tomography * mediastinum The evaluation of mediastinal masses is a challenging radiologic problem. Computed tomography (CT) currently is employed to detect mediastinal masses and to define their extent. Magnetic resonance imaging (MRI) is an alternative modality for evaluating the thorax. Recent reports suggest that MRI has a substanFrom the UCLA Medical Center, Departments of Radiological Sciences, Surgery/Oncology, and Medicine/Pulmonary; and the Jonsson Comprehensive Cancer Center, Los Angeles, California. Requests for reprints should be addressed to Dr Poonam Batra, Department of Radiological Sciences, UCLA Medical Center, 10833 LeConte Ave, Los Angeles, CA 900241721. JOURNAL OF THE NATIONAL MEDICAL ASSOCIATION, VOL. 83, NO. 11
Magnetic resonance imaging was performed with a 0.3 T permanent magnet imaging system (Fonar Beta 3000; Melville, New York). All scans were obtained during quiet breathing, with the patient supine. Multiple slice, spin echo (SE) images were obtained in the axial plane in all patients, in the coronal plane in 60 patients, and in the sagittal plane in 11 patients. TI-weighted images were obtained in all patients with an echo delay time (TE) of 18 or 28 milliseconds. In 61 patients, a repetition time (TR) of 300 to 500 milliseconds was used while in 17 patients, cardiac-gated sequences were obtained and the TR varied according to the RR interval of the ECG, ranging from 750 to 1000 milliseconds. T2-weighted images, with a TE of 56 milliseconds and a TR of 1500 to 2000 milliseconds, were obtained in 33 patients. Individual slices were 9-mm thick and were 969
MEDIASTINAL MASSES
TABLE. DIAGNOSES OF MEDIASTINAL MASSES CT MRI Diagnoses 32* 20 Lymphoma 3 5 Thymoma 3 4t Thyroid masses 4 4 Small cell lung cancer 3 Neoplasms of esophagus 4t 12§ 3 Metastases from distant sites 1 411 Granulomatous disease 2 2 Lipomatosis 0 511 Vascular abnormalities 6** 6 Miscellaneous 45 78 Total *Hodgkin's disease (19) and non-Hodgkin's lymphoma (13). tCancer (2) and goiter (2). tCancer (3) and leiomyoma (1). §Melanoma (6), colon (1), breast (1), sarcoma (2), renal (1), and adenocystic carcinoma of maxillary sinus (1). 1 Sarcoidosis (2), coccidiodomycosis (1), and histoplasmosis (1). lAberrant right subclavian artery aneurysm (1), pulmonary artery hypertension (1), pulmonary stenosis with poststenotic dilatation (1), laterally placed left subclavian artery (1), and aortic aneurysm (1). **Pericardial cyst (1), germ cell tumor (1), Castleman's disease (1), chondrosarcoma (1), neurofibroma (1), and neurilemoma (1).
separated by a 3-mm gap. Respiratory gating was not used. In 42 patients, CT studies were performed with a GE 9800 machine (Milwaukee, Wisconsin) with 10-mm contiguous slices. In three patients, CT scans were performed on comparable scanners at other institutions. Intravenous contrast material was used in 39 cases. Scans were photographed at windows appropriate for viewing both the mediastinum and the lung.
Image Analysis Two observers independently analyzed the CT studies and then analyzed the MRI studies 1 to 2 weeks later. The chest radiographs were available at the time of the CT and MRI interpretations. The following characteristics were evaluated for each study: * the presence, location, and extent of a mediastinal mass or adenopathy. Any abnormal focus of density or signal intensity greater than 1.5 cm in diameter was considered a mediastinal mass or an enlarged node. The mediastinum was divided into the following compartments: prevascular, pretracheal or paratracheal, aortopulmonary window, subcarinal, posterior mediastinal, 970
hilar, and paracardiac regions.5 These mediastinal regions were analyzed individually for the presence of an abnormality; * displacement of mediastinal blood vessels or airways from their anatomic locations or narrowing of these structures by a mediastinal mass; and * extramediastinal abnormalities involving lung, pleura, or chest wall.
RESULTS The Table lists the final clinical diagnoses for the 78 patients. Tissue diagnoses were available in 69 patients. In the remaining nine patients, diagnoses were based on clinical and radiographic features. Two patients with thyroid masses had diagnostic findings on radionuclide scans with iodine 123, in addition to the characteristic appearance of goiter on CT scans. In two patients, both CT and MR findings indicated lipomatosis, and no further workup was performed. In four patients, the diagnoses were based solely on imaging findings, including one case each of pulmonary artery hypertension, pulmonary stenosis with poststenotic dilatation, aortic aneurysm, and a tortuous left subclavian artery resulting in a widened mediastinum. In the ninth patient, clinical data, MRI appearance, and follow-up examination were all consistent with recurrent thymoma that subsequently regressed with chemotherapy.
Mediastinal Masses: MRI Appearance Magnetic resonance imaging displayed masses in each of the mediastinal regions with good anatomic detail. On Tl-weighted SE images, the masses appeared as regions of intermediate intensity signal that were easily distinguishable from the high intensity signal of fat and the low intensity signal of the mediastinal vessels and tracheobronchial tree. Accordingly, Tlweighted SE images provided the best contrast and definition for mediastinal masses. On T2-weighted SE images in all but one patient, the masses increased in signal and approached that of fat intensity, which resulted in decreased separability of the lesion from the adjacent fat. In one patient with malignant thymoma who had been treated with resection followed by radiation therapy, the central region of the mass did not increase in signal on T2-weighted images, suggesting postradiation fibrosis, while the peripheral region increased in signal and had a nodular contour consistent with recurrent thymoma. Increasing the TE from 28 to 56 milliseconds decreased image quality with poor definition of normal structures. The visible signal intensity of several pathologically different masses was JOURNAL OF THE NATIONAL MEDICAL ASSOCIATION, VOL. 83, NO. 11
MEDIASTINAL MASSES
Figures lA-B. Hodgkin's disease in
(A)
a
32-year-old
Non-contrastenhanced CT scan shows an anterior mediastinal mass (arrows) difficult to separate from adjacent vascular structures. (B) MRI shows mediastinal mass (arrows) easily distinguished from brachiocephalic veins (V), innominate artery (A), common carotid artery (curved arrow), and left subolavian artery (S). woman.
similar and could not be used to make a specific histopathologic diagnosis. The vascular nature of five mediastinal masses was also accurately established with MRI. Computed tomography had not been performed in these cases. The diagnosis of an aneurysm of aberrant right subclavian artery was confirmed by thoracotomy, and the diagnoses of pulmonary artery stenosis with post-stenotic dilatation and pulmonary artery hypertension were confirmed by cardiac catheterization. The diagnoses of a laterally placed left subclavian artery and aortic aneurysm were made by MRI examination.
Mediastinal Masses: CT Versus MRI In all but two patients, MRI was equivalent to CT in providing diagnostic information regarding the presence, size, location, and extent of mediastinal masses. The involved mediastinal regions consisted of the prevascular space (30), pretracheal and paratracheal space (22), aortopulmonary window (3), hila (7), and cardiophrenic angle (3). In one patient with prevascular and pretracheal adenopathy from metastatic melanoma and in another patient with a left cardiophrenic angle metastasis from sarcoma, adenopathy was better defined with CT. The margins of masses were frequently not as clearly defined by MRI as by CT. Mediastinal masses could easily be distinguished from the vascular structures on MRI without intravenous contrast. When CT was used without contrast (six cases), masses were difficult to separate from adjacent vascular structures (Figures lA-B). All four patients with small cell lung cancer included in this series demonstrated only mediastinal adenopathy without revealing a primary lung lesion. Computed tomography showed calcifications in eight mediastinal masses including one recurrent thymoma, one invasive thymoma, one goiter, two treated Hodgkin's (Figures 2A-B), one high-grade lymphoma, one Castleman's disease, and one chondroJOURNAL OF THE NATIONAL MEDICAL ASSOCIATION, VOL. 83, NO. 11
sarcoma. In four of these patients, MRI exhibited low signals in the areas of calcification seen on CT In the other four cases, calcification was not suspected on MRI. In one patient, a central zone of postradiation fibrosis was distinguished from a peripheral zone of recurrent thymoma on MRI, whereas this differentiation could not be accomplished with CT (Figures 3A-C). Displacement or compression of the left brachiocephalic vein or the superior vena cava was the most common vascular abnormality and was seen in 19 patients on MRI. Contrast-enhanced CT had been performed in nine patients, and this modality demonstrated the vascular compromise equally well. Magnetic resonance imaging showed compromise of the pulmonary artery in two patients who had not had a CT scan. Magnetic resonance imaging demonstrated narrowing of the central airways in 10 patients (trachea in three, left and/or right main-stem bronchi in five, and bronchus intermedius in two). Computed tomography had been performed in four of these patients and showed similar findings. The coronal and sagittal MRI scans did not show additional sites of mediastinal involvement when compared with axial MRI or CT scans; however, the entire longitudinal extent of lesions could be demonstrated on a single coronal or sagittal slice. Computed tomography detected an associated lung abnormality in four patients, whereas MRI either did not detect an abnormality or demonstrated the abnormality poorly. In two patients with lymphoma who had been treated previously with chemotherapy or radiation therapy, CT revealed unsuspected lung nodules (8 mm to 1.5 cm in size) that either were not imaged or poorly imaged with MRI (Figures 2C-D). In another patient with lymphoma, CT showed a focus of lung consolidation that was poorly imaged with MRI. In one patient with metastatic adenocystic carcinoma from a maxillary sinus, two lung nodules (7 mm and 8 mm in size) that were not seen on MRI were detected on CT. 971
MEDIASTINAL MASSES
Figures 2A-D. Recurrent Hodgkin's disease in a 25-year-old man following radiation therapy. (A) CT scan shows an anterior mediastinal mass (M) with calcification (arrow). (B) MRI axial scan (SE 500/28) shows the mass (M), but calcification is not identified. (C) CT scan shows a 1-cm nodule in right upper lobe (arrow). (D) MRI scan (SE 500/28) at the same level does not show the lung nodule. Both CT and MRI scans demonstrated small pleural effusions equally well in six patients. Computed tomography and MRI also depicted rib destruction in one patient with chondrosarcoma and in another patient with breast carcinoma metastasis.
DISCUSSION Several studies indicate that MRI can define and distinguish mediastinal masses from normal structures in the mediastinum.1'4 Flowing blood emits no signal; thus, the lumina of the aorta, the pulmonary artery, and heart chambers are void of signal, permitting easy differentiation of mediastinal masses and lymphadenopathy from vessels. While CT and MRI demonstrated the presence, location, and extent of mediastinal masses equally well, CT depicted the margins of these masses better than MRI, which produced relatively indistinct margins. This is attributable to the poorer spatial resolution of MRI as well as to image degradation from cardiovascular and respiratory motion. The images obtained with cardiac-gated sequences provide better definition of mediastinal structures than nongated images. Respiratory gating also improves resolution, but increases the data acquisition time. Small lymph nodes clustered together on CT may appear as a single large mass on MRI because of inferior spatial resolution. Although such detection does not alter disease management of patients with lymphoma, it is conceivable that multiple small, normal-sized nodes may be mistaken for a pathologic mass on MRI. 972
In general, the signal intensity of pathologically different mediastinal masses was similar on MRI scans and could not be used to differentiate these masses. In this regard, our experience has been similar to that of others.6'7 The numerical TI and T2 values were not measured in our series. However, in several cases, signal intensity did help to characterize the mass lesions. The pericardial cyst was characterized by markedly increased signal on T2-weighted images because of its particularly long T2 relaxation time. Lipomatosis showed a high intensity signal on T 1- and T2-weighted images because of the short TI and long T2 relaxation times for fat. Magnetic resonance imaging is also capable of distinguishing fibrosis from tumor because of the relatively long Ti and short T2 relaxation times of fibrous tissue, as opposed to the relatively long TI and T2 relaxation times of tumors.8 Magnetic resonance imaging differentiated recurrent thymoma from contiguous radiation fibrosis by differences in tissue signal intensity on T2-weighted images, whereas CT findings did not differentiate the two tissue types using attenuation values. Magnetic resonance imaging easily demonstrates vascular displacement and compromise, whereas CT may require intravenous contrast enhancement. Compromise of the trachea and large bronchi is readily seen by CT as well as MRI. A distinct disadvantage of MRI is its inability to show calcification in mediastinal masses, which often helps to narrow the differential diagnosis of a mass. JOURNAL OF THE NATIONAL MEDICAL ASSOCIATION, VOL. 83, NO. 11
MEDIASTINAL MASSES
Figures 3A-C. Recurrent thymoma in a 54-year-old woman treated with radiation. (A) CT scan shows an anterior mediastinal mass (M) with calcification (arrow). (B) MR axial scan (SE 500/18) shows an intermediate signal intensity mass (M). Calcification is not identified because of slight differences in levels of images. (C) MR axial scan (SE 1500/56) shows a central zone that did not increase in signal (asterisk), suggesting postradiation fibrosis, while the peripheral zone increased in signal (arrow), consistent with recurrent thymoma.
Compared to CT, an additional disadvantage of MRI is its decreased sensitivity for detecting small lung nodules.9 In our two patients with lymphoma, CT alone revealed unsuspected lung nodules. In one of these patients, MRI scans were obtained with a TR of 500 milliseconds. Another pulse sequence was not obtained because this patient suffered from claustrophobia. It is possible that an additional pulse sequence with a TR of 2 seconds or longer may have demonstrated the nodule. However, in the second patient with lymphoma, an image sequence with an echo time of 56 and a repetition time of 1500 milliseconds was available. This sequence poorly demonstrated a single lung nodule, whereas CT detected two nodules. The detection of these lung nodules on CT was of therapeutic significance. In the other two patients, the presence of a lung abnormality on CT did not alter the chemotherapeutic management of their disease. The selection of appropriate MR echo sequences is critically important and must be tailored to the individual tumor in question. For imaging mediastinal masses, images should be obtained in the axial plane with both Ti- and T2-weighted sequences. While the JOURNAL OF THE NATIONAL MEDICAL ASSOCIATION, VOL. 83, NO. 11
TI -weighted sequence provides accurate definition of the mass, T2-weighted sequence may characterize the cystic versus solid nature of the mass, may better demonstrate extension of the mass into the adjacent chest wall, or may differentiate recurrent tumor from postradiation fibrosis. Coronal or sagittal planes may be added in selected cases to clarify anatomic relationships in difficult cases. I
CONCLUSION While MRI and CT provide comparable diagnostic information for imaging mediastinal masses, CT provides additional information regarding unsuspected lung abnormalities that may be of therapeutic significance in disease management. Computed tomography has better spatial resolution and a shorter imaging time than MRI. Magnetic resonance images of the chest require about 45 minutes to 1 hou, while CT requires about 20 to 30 minutes. Computed tomography equipment is less expensive and is more widely available. Computed tomography should remain the procedure of choice after chest radiography to evaluate most patients with a mediastinal mass. Magnetic resonance imaging may be used in 973
MEDIASTINAL MASSES
5. Moss AA, Gamsu G. Computed tomography of the mediastinum. In: Genant HK, ed. Computed Tomography of the Body. Philadelphia, Pa: WB Saunders Co; 1983:207-214. 6. Gamsu G, Stark DD, Webb WR, Moore EH, Sheldon PE. Magnetic resonance imaging of benign mediastinal masses. Radiology. 1984;1 51:709-713. 7. Siegel MJ, Nadel SN, Glazer HS, Sagel SS. Mediastinal lesions in children: comparison of CT and MR. Radiology. 1986; 1 60:241-244. 8. Glazer HS, Levitt RG, Lee JKT, Emani B, Gronemeyer S, Murphy WA. Differentiation of radiation fibrosis from recurrent pulmonary neoplasm by magnetic resonance imaging. AJR Am J Roentgenol. 1984;1 43:729-730. 9. Muller NL, Gamsu G, Webb WR. Pulmomary nodules: detection using magnetic resonance and computed tomography. Radiology. 1985;155:687-690. 10. Batra P, Brown K, Steckel RJ, Collins JD, Ovenfors CO, Aberle DR. MR imaging of the thorax: a comparison of axial, coronal, and sagittal imaging planes. J Comput Assist Tomogr 1 988; 1 2:75-8 1.
patients with suspected vascular pathology or with contrast hypersensitivity, renal insufficiency, or poor venous access. Finally, MRI also may be used to clarify confusing findings on CT or to distinguish tumors from postradiation fibrosis. Literature Cited 1. Cohen AM, Creviston S, LiPuma JP, Bryan P, Liberman J, Haaga JR. Nuclear magnetic resonance imaging of the mediastinum and hili: early impressions of its efficacy. AJR Am J Roentgenol. 1983;141 :1163-1169. 2. Epstein DM, Kressel H, Gefter W, Axel L, Thickman D, Aronchick J, et al. MR imaging of the mediastinum: a retrospective comparison with computed tomography. J Comput Assist Tomogr 1984;8:670-676. 3. Levitt RG, Glazer HS, Roper CL, Lee JKT, Murphy WA. Magnetic resonance imaging of mediastinal and hilar masses: comparison with CT. AJR Am J Roentgenol. 1985;145:9-14. 4. von Schulthess GK, McMurdo K, Tscholakoff D, de Geer G, Gamsu G, Higgins CB. Mediastinal masses: MR imaging. Radiology. 1986;158:289-296.
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JOURNAL OF THE NATIONAL MEDICAL ASSOCIATION, VOL. 83, NO. 1 1
Seventy-eight patients with mediastinal abnormalities were imaged with magnetic resonance imaging (MRI) to evaluate mediastinal masses and associated lung, pleural, or chest wall disease. Magnetic resonance images were compared with computed tomography (CT) scans, which were available in 45 patients. While MRI and CT were equally effective in demonstrating mediastinal lesions, CT was superior for displaying calcification within a mass in eight patients and for demonstrating associated lung abnormality in four patients. Computed tomography should remain the imaging procedure of choice after chest radiography to evaluate mediastinal masses, although MRI may be indicated in selected patients. (J Nati Med Assoc. 1991;83:969-974.)
tial capability for evaluating mediastinal masses. 14 However, in these studies, the majority of cases represented mediastinal pathology associated with primary bronchogenic carcinoma. This article details the MRI appearance of mediastinal masses in those cases where mediastinal mass was the primary abnormality and compares the diagnostic information provided by MRI findings with CT findings.
MATERIALS AND METHODS Since September 1984, 78 patients (41 men and 37 women ranging in age from 18 to 84 years) have been evaluated with MRI for a mediastinal mass suspected or detected on chest radiographs. Computed tomography scans performed within 2 weeks of the MRI were available for comparison in 45 patients.
Imaging Methods Key words * magnetic resonance imaging * computed tomography * mediastinum The evaluation of mediastinal masses is a challenging radiologic problem. Computed tomography (CT) currently is employed to detect mediastinal masses and to define their extent. Magnetic resonance imaging (MRI) is an alternative modality for evaluating the thorax. Recent reports suggest that MRI has a substanFrom the UCLA Medical Center, Departments of Radiological Sciences, Surgery/Oncology, and Medicine/Pulmonary; and the Jonsson Comprehensive Cancer Center, Los Angeles, California. Requests for reprints should be addressed to Dr Poonam Batra, Department of Radiological Sciences, UCLA Medical Center, 10833 LeConte Ave, Los Angeles, CA 900241721. JOURNAL OF THE NATIONAL MEDICAL ASSOCIATION, VOL. 83, NO. 11
Magnetic resonance imaging was performed with a 0.3 T permanent magnet imaging system (Fonar Beta 3000; Melville, New York). All scans were obtained during quiet breathing, with the patient supine. Multiple slice, spin echo (SE) images were obtained in the axial plane in all patients, in the coronal plane in 60 patients, and in the sagittal plane in 11 patients. TI-weighted images were obtained in all patients with an echo delay time (TE) of 18 or 28 milliseconds. In 61 patients, a repetition time (TR) of 300 to 500 milliseconds was used while in 17 patients, cardiac-gated sequences were obtained and the TR varied according to the RR interval of the ECG, ranging from 750 to 1000 milliseconds. T2-weighted images, with a TE of 56 milliseconds and a TR of 1500 to 2000 milliseconds, were obtained in 33 patients. Individual slices were 9-mm thick and were 969
MEDIASTINAL MASSES
TABLE. DIAGNOSES OF MEDIASTINAL MASSES CT MRI Diagnoses 32* 20 Lymphoma 3 5 Thymoma 3 4t Thyroid masses 4 4 Small cell lung cancer 3 Neoplasms of esophagus 4t 12§ 3 Metastases from distant sites 1 411 Granulomatous disease 2 2 Lipomatosis 0 511 Vascular abnormalities 6** 6 Miscellaneous 45 78 Total *Hodgkin's disease (19) and non-Hodgkin's lymphoma (13). tCancer (2) and goiter (2). tCancer (3) and leiomyoma (1). §Melanoma (6), colon (1), breast (1), sarcoma (2), renal (1), and adenocystic carcinoma of maxillary sinus (1). 1 Sarcoidosis (2), coccidiodomycosis (1), and histoplasmosis (1). lAberrant right subclavian artery aneurysm (1), pulmonary artery hypertension (1), pulmonary stenosis with poststenotic dilatation (1), laterally placed left subclavian artery (1), and aortic aneurysm (1). **Pericardial cyst (1), germ cell tumor (1), Castleman's disease (1), chondrosarcoma (1), neurofibroma (1), and neurilemoma (1).
separated by a 3-mm gap. Respiratory gating was not used. In 42 patients, CT studies were performed with a GE 9800 machine (Milwaukee, Wisconsin) with 10-mm contiguous slices. In three patients, CT scans were performed on comparable scanners at other institutions. Intravenous contrast material was used in 39 cases. Scans were photographed at windows appropriate for viewing both the mediastinum and the lung.
Image Analysis Two observers independently analyzed the CT studies and then analyzed the MRI studies 1 to 2 weeks later. The chest radiographs were available at the time of the CT and MRI interpretations. The following characteristics were evaluated for each study: * the presence, location, and extent of a mediastinal mass or adenopathy. Any abnormal focus of density or signal intensity greater than 1.5 cm in diameter was considered a mediastinal mass or an enlarged node. The mediastinum was divided into the following compartments: prevascular, pretracheal or paratracheal, aortopulmonary window, subcarinal, posterior mediastinal, 970
hilar, and paracardiac regions.5 These mediastinal regions were analyzed individually for the presence of an abnormality; * displacement of mediastinal blood vessels or airways from their anatomic locations or narrowing of these structures by a mediastinal mass; and * extramediastinal abnormalities involving lung, pleura, or chest wall.
RESULTS The Table lists the final clinical diagnoses for the 78 patients. Tissue diagnoses were available in 69 patients. In the remaining nine patients, diagnoses were based on clinical and radiographic features. Two patients with thyroid masses had diagnostic findings on radionuclide scans with iodine 123, in addition to the characteristic appearance of goiter on CT scans. In two patients, both CT and MR findings indicated lipomatosis, and no further workup was performed. In four patients, the diagnoses were based solely on imaging findings, including one case each of pulmonary artery hypertension, pulmonary stenosis with poststenotic dilatation, aortic aneurysm, and a tortuous left subclavian artery resulting in a widened mediastinum. In the ninth patient, clinical data, MRI appearance, and follow-up examination were all consistent with recurrent thymoma that subsequently regressed with chemotherapy.
Mediastinal Masses: MRI Appearance Magnetic resonance imaging displayed masses in each of the mediastinal regions with good anatomic detail. On Tl-weighted SE images, the masses appeared as regions of intermediate intensity signal that were easily distinguishable from the high intensity signal of fat and the low intensity signal of the mediastinal vessels and tracheobronchial tree. Accordingly, Tlweighted SE images provided the best contrast and definition for mediastinal masses. On T2-weighted SE images in all but one patient, the masses increased in signal and approached that of fat intensity, which resulted in decreased separability of the lesion from the adjacent fat. In one patient with malignant thymoma who had been treated with resection followed by radiation therapy, the central region of the mass did not increase in signal on T2-weighted images, suggesting postradiation fibrosis, while the peripheral region increased in signal and had a nodular contour consistent with recurrent thymoma. Increasing the TE from 28 to 56 milliseconds decreased image quality with poor definition of normal structures. The visible signal intensity of several pathologically different masses was JOURNAL OF THE NATIONAL MEDICAL ASSOCIATION, VOL. 83, NO. 11
MEDIASTINAL MASSES
Figures lA-B. Hodgkin's disease in
(A)
a
32-year-old
Non-contrastenhanced CT scan shows an anterior mediastinal mass (arrows) difficult to separate from adjacent vascular structures. (B) MRI shows mediastinal mass (arrows) easily distinguished from brachiocephalic veins (V), innominate artery (A), common carotid artery (curved arrow), and left subolavian artery (S). woman.
similar and could not be used to make a specific histopathologic diagnosis. The vascular nature of five mediastinal masses was also accurately established with MRI. Computed tomography had not been performed in these cases. The diagnosis of an aneurysm of aberrant right subclavian artery was confirmed by thoracotomy, and the diagnoses of pulmonary artery stenosis with post-stenotic dilatation and pulmonary artery hypertension were confirmed by cardiac catheterization. The diagnoses of a laterally placed left subclavian artery and aortic aneurysm were made by MRI examination.
Mediastinal Masses: CT Versus MRI In all but two patients, MRI was equivalent to CT in providing diagnostic information regarding the presence, size, location, and extent of mediastinal masses. The involved mediastinal regions consisted of the prevascular space (30), pretracheal and paratracheal space (22), aortopulmonary window (3), hila (7), and cardiophrenic angle (3). In one patient with prevascular and pretracheal adenopathy from metastatic melanoma and in another patient with a left cardiophrenic angle metastasis from sarcoma, adenopathy was better defined with CT. The margins of masses were frequently not as clearly defined by MRI as by CT. Mediastinal masses could easily be distinguished from the vascular structures on MRI without intravenous contrast. When CT was used without contrast (six cases), masses were difficult to separate from adjacent vascular structures (Figures lA-B). All four patients with small cell lung cancer included in this series demonstrated only mediastinal adenopathy without revealing a primary lung lesion. Computed tomography showed calcifications in eight mediastinal masses including one recurrent thymoma, one invasive thymoma, one goiter, two treated Hodgkin's (Figures 2A-B), one high-grade lymphoma, one Castleman's disease, and one chondroJOURNAL OF THE NATIONAL MEDICAL ASSOCIATION, VOL. 83, NO. 11
sarcoma. In four of these patients, MRI exhibited low signals in the areas of calcification seen on CT In the other four cases, calcification was not suspected on MRI. In one patient, a central zone of postradiation fibrosis was distinguished from a peripheral zone of recurrent thymoma on MRI, whereas this differentiation could not be accomplished with CT (Figures 3A-C). Displacement or compression of the left brachiocephalic vein or the superior vena cava was the most common vascular abnormality and was seen in 19 patients on MRI. Contrast-enhanced CT had been performed in nine patients, and this modality demonstrated the vascular compromise equally well. Magnetic resonance imaging showed compromise of the pulmonary artery in two patients who had not had a CT scan. Magnetic resonance imaging demonstrated narrowing of the central airways in 10 patients (trachea in three, left and/or right main-stem bronchi in five, and bronchus intermedius in two). Computed tomography had been performed in four of these patients and showed similar findings. The coronal and sagittal MRI scans did not show additional sites of mediastinal involvement when compared with axial MRI or CT scans; however, the entire longitudinal extent of lesions could be demonstrated on a single coronal or sagittal slice. Computed tomography detected an associated lung abnormality in four patients, whereas MRI either did not detect an abnormality or demonstrated the abnormality poorly. In two patients with lymphoma who had been treated previously with chemotherapy or radiation therapy, CT revealed unsuspected lung nodules (8 mm to 1.5 cm in size) that either were not imaged or poorly imaged with MRI (Figures 2C-D). In another patient with lymphoma, CT showed a focus of lung consolidation that was poorly imaged with MRI. In one patient with metastatic adenocystic carcinoma from a maxillary sinus, two lung nodules (7 mm and 8 mm in size) that were not seen on MRI were detected on CT. 971
MEDIASTINAL MASSES
Figures 2A-D. Recurrent Hodgkin's disease in a 25-year-old man following radiation therapy. (A) CT scan shows an anterior mediastinal mass (M) with calcification (arrow). (B) MRI axial scan (SE 500/28) shows the mass (M), but calcification is not identified. (C) CT scan shows a 1-cm nodule in right upper lobe (arrow). (D) MRI scan (SE 500/28) at the same level does not show the lung nodule. Both CT and MRI scans demonstrated small pleural effusions equally well in six patients. Computed tomography and MRI also depicted rib destruction in one patient with chondrosarcoma and in another patient with breast carcinoma metastasis.
DISCUSSION Several studies indicate that MRI can define and distinguish mediastinal masses from normal structures in the mediastinum.1'4 Flowing blood emits no signal; thus, the lumina of the aorta, the pulmonary artery, and heart chambers are void of signal, permitting easy differentiation of mediastinal masses and lymphadenopathy from vessels. While CT and MRI demonstrated the presence, location, and extent of mediastinal masses equally well, CT depicted the margins of these masses better than MRI, which produced relatively indistinct margins. This is attributable to the poorer spatial resolution of MRI as well as to image degradation from cardiovascular and respiratory motion. The images obtained with cardiac-gated sequences provide better definition of mediastinal structures than nongated images. Respiratory gating also improves resolution, but increases the data acquisition time. Small lymph nodes clustered together on CT may appear as a single large mass on MRI because of inferior spatial resolution. Although such detection does not alter disease management of patients with lymphoma, it is conceivable that multiple small, normal-sized nodes may be mistaken for a pathologic mass on MRI. 972
In general, the signal intensity of pathologically different mediastinal masses was similar on MRI scans and could not be used to differentiate these masses. In this regard, our experience has been similar to that of others.6'7 The numerical TI and T2 values were not measured in our series. However, in several cases, signal intensity did help to characterize the mass lesions. The pericardial cyst was characterized by markedly increased signal on T2-weighted images because of its particularly long T2 relaxation time. Lipomatosis showed a high intensity signal on T 1- and T2-weighted images because of the short TI and long T2 relaxation times for fat. Magnetic resonance imaging is also capable of distinguishing fibrosis from tumor because of the relatively long Ti and short T2 relaxation times of fibrous tissue, as opposed to the relatively long TI and T2 relaxation times of tumors.8 Magnetic resonance imaging differentiated recurrent thymoma from contiguous radiation fibrosis by differences in tissue signal intensity on T2-weighted images, whereas CT findings did not differentiate the two tissue types using attenuation values. Magnetic resonance imaging easily demonstrates vascular displacement and compromise, whereas CT may require intravenous contrast enhancement. Compromise of the trachea and large bronchi is readily seen by CT as well as MRI. A distinct disadvantage of MRI is its inability to show calcification in mediastinal masses, which often helps to narrow the differential diagnosis of a mass. JOURNAL OF THE NATIONAL MEDICAL ASSOCIATION, VOL. 83, NO. 11
MEDIASTINAL MASSES
Figures 3A-C. Recurrent thymoma in a 54-year-old woman treated with radiation. (A) CT scan shows an anterior mediastinal mass (M) with calcification (arrow). (B) MR axial scan (SE 500/18) shows an intermediate signal intensity mass (M). Calcification is not identified because of slight differences in levels of images. (C) MR axial scan (SE 1500/56) shows a central zone that did not increase in signal (asterisk), suggesting postradiation fibrosis, while the peripheral zone increased in signal (arrow), consistent with recurrent thymoma.
Compared to CT, an additional disadvantage of MRI is its decreased sensitivity for detecting small lung nodules.9 In our two patients with lymphoma, CT alone revealed unsuspected lung nodules. In one of these patients, MRI scans were obtained with a TR of 500 milliseconds. Another pulse sequence was not obtained because this patient suffered from claustrophobia. It is possible that an additional pulse sequence with a TR of 2 seconds or longer may have demonstrated the nodule. However, in the second patient with lymphoma, an image sequence with an echo time of 56 and a repetition time of 1500 milliseconds was available. This sequence poorly demonstrated a single lung nodule, whereas CT detected two nodules. The detection of these lung nodules on CT was of therapeutic significance. In the other two patients, the presence of a lung abnormality on CT did not alter the chemotherapeutic management of their disease. The selection of appropriate MR echo sequences is critically important and must be tailored to the individual tumor in question. For imaging mediastinal masses, images should be obtained in the axial plane with both Ti- and T2-weighted sequences. While the JOURNAL OF THE NATIONAL MEDICAL ASSOCIATION, VOL. 83, NO. 11
TI -weighted sequence provides accurate definition of the mass, T2-weighted sequence may characterize the cystic versus solid nature of the mass, may better demonstrate extension of the mass into the adjacent chest wall, or may differentiate recurrent tumor from postradiation fibrosis. Coronal or sagittal planes may be added in selected cases to clarify anatomic relationships in difficult cases. I
CONCLUSION While MRI and CT provide comparable diagnostic information for imaging mediastinal masses, CT provides additional information regarding unsuspected lung abnormalities that may be of therapeutic significance in disease management. Computed tomography has better spatial resolution and a shorter imaging time than MRI. Magnetic resonance images of the chest require about 45 minutes to 1 hou, while CT requires about 20 to 30 minutes. Computed tomography equipment is less expensive and is more widely available. Computed tomography should remain the procedure of choice after chest radiography to evaluate most patients with a mediastinal mass. Magnetic resonance imaging may be used in 973
MEDIASTINAL MASSES
5. Moss AA, Gamsu G. Computed tomography of the mediastinum. In: Genant HK, ed. Computed Tomography of the Body. Philadelphia, Pa: WB Saunders Co; 1983:207-214. 6. Gamsu G, Stark DD, Webb WR, Moore EH, Sheldon PE. Magnetic resonance imaging of benign mediastinal masses. Radiology. 1984;1 51:709-713. 7. Siegel MJ, Nadel SN, Glazer HS, Sagel SS. Mediastinal lesions in children: comparison of CT and MR. Radiology. 1986; 1 60:241-244. 8. Glazer HS, Levitt RG, Lee JKT, Emani B, Gronemeyer S, Murphy WA. Differentiation of radiation fibrosis from recurrent pulmonary neoplasm by magnetic resonance imaging. AJR Am J Roentgenol. 1984;1 43:729-730. 9. Muller NL, Gamsu G, Webb WR. Pulmomary nodules: detection using magnetic resonance and computed tomography. Radiology. 1985;155:687-690. 10. Batra P, Brown K, Steckel RJ, Collins JD, Ovenfors CO, Aberle DR. MR imaging of the thorax: a comparison of axial, coronal, and sagittal imaging planes. J Comput Assist Tomogr 1 988; 1 2:75-8 1.
patients with suspected vascular pathology or with contrast hypersensitivity, renal insufficiency, or poor venous access. Finally, MRI also may be used to clarify confusing findings on CT or to distinguish tumors from postradiation fibrosis. Literature Cited 1. Cohen AM, Creviston S, LiPuma JP, Bryan P, Liberman J, Haaga JR. Nuclear magnetic resonance imaging of the mediastinum and hili: early impressions of its efficacy. AJR Am J Roentgenol. 1983;141 :1163-1169. 2. Epstein DM, Kressel H, Gefter W, Axel L, Thickman D, Aronchick J, et al. MR imaging of the mediastinum: a retrospective comparison with computed tomography. J Comput Assist Tomogr 1984;8:670-676. 3. Levitt RG, Glazer HS, Roper CL, Lee JKT, Murphy WA. Magnetic resonance imaging of mediastinal and hilar masses: comparison with CT. AJR Am J Roentgenol. 1985;145:9-14. 4. von Schulthess GK, McMurdo K, Tscholakoff D, de Geer G, Gamsu G, Higgins CB. Mediastinal masses: MR imaging. Radiology. 1986;158:289-296.
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JOURNAL OF THE NATIONAL MEDICAL ASSOCIATION, VOL. 83, NO. 1 1
