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Original Report |
1 All authors: Department of Radiology, Asahikawa Medical College, 2-1-1-1, Midorigaoka-higashi, Asahikawa, 0788510, Japan.
Received April 15, 2002;
accepted after revision September 19, 2002.
Address correspondence to K. Takahashi.
Abstract
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CONCLUSION. The thymic tissue revealed homogeneous decrease in intensity on opposed-phase MR images relative to that seen on in-phase images in 15 healthy volunteers and two patients with hyperplastic thymus. Chemical-shift MR imaging may be useful in identifying normal thymic tissue and the hyperplastic thymus in early adulthood.
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Chemical-shift MR imaging is good for identifying a microscopic mixture of water and fat and can efficiently prove the presence of fat in tissue [8]. In addition, chemical-shift MR imaging has been advocated as a way to establish the diagnosis of adrenal adenoma, which contains various amounts of fat tissue [8]. The normal adrenal gland also loses its signal on opposed-phase chemical-shift MR imaging as compared with in-phase imaging.
We hypothesized that chemical-shift MR imaging could be used to identify the normal thymic tissue when it is partially, but not completely, replaced by fat tissue. In this study, we prospectively evaluated a potential role for chemical-shift MR imaging in identifying normal thymic tissue in early adulthood. In addition, we assessed two patients with prominent thymic tissue using chemical-shift MR imaging.
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The two patients with a soft-tissue mass in the anterior mediastinum were a 26-year-old man and a 28-year-old woman; both had hyperthyroidism. In each of these patients, widening of the mediastinum had been detected on chest radiography, and chest CT showed a soft-tissue mass in the anterior mediastinum. The soft-tissue revealed a bilobed configuration with a slightly convex margin and no compression on the adjacent structures. MR images were obtained for further evaluation of the mediastinum in these two patients. We made a presumptive diagnosis of hyperplastic thymus on the basis of morphologic features that were associated with hyperthyroidism and the fact that no apparent change in size and shape had occurred during the follow-up periods of 24 and 36 months. During these follow-up periods, we obtained chest CT images every 12 months.
All MR imaging was performed using a 1.5-T unit (Signa; General Electric Medical Systems, Milwaukee, WI). Chemical-shift images were obtained by using both in-phase and opposed-phase T1-weighted gradient-echo sequences in all subjects. These images were acquired using fast multiplanar spoiled gradient-echo sequences in a single breath-hold of 20–28 sec. TE for the in-phase images was 4.2 msec and for the opposed-phase images, 1.8 msec. Both in-phase and opposed-phase imaging was performed with the following parameters: TR, 120; flip angle, 90°; matrix, 256 x 256; signal acquisition,1; field of view, 320–340 mm; section thickness, 5–6 mm with a 1- to 3-mm intersection gap; and bandwidth, ± 31.3 kHz.
We also obtained T1- and T2-weighted spin-echo axial images in the two patients. The T1-weighted spin-echo imaging was performed using the following parameters: TR range/TE range, 400–500/10–14; section thickness, 7 mm with a 3- mm intersection gap; and matrix, 512 x 256. Two signals were acquired. The T2–weighted spin-echo images were obtained using a fast spin-echo sequence with the following parameters: TR range/TE, 3100–3300/83; echo train, 10; section thickness, 5 mm with a 1- to 3-mm intersection gap; and matrix, 320 x 320. Two signals were acquired.
MR images from the 15 volunteers were assessed for the presence of the thymus, its qualitative signal intensity on in-phase T1-weighted gradient-echo images, and its thickness. The thickness of the thymus was measured as the maximum dimension perpendicular to the long axis of each lobe. Signal intensity measurements of the thymus and the anterior chest-wall muscle were then obtained using standard region-of-interest electronic cursors at a slice level of the maximum axial surface of the thymus. The change in signal intensity between in-phase and opposed-phase images was evaluated qualitatively. Two radiologists independently assessed the images for the presence of a decrease in the signal intensity of the thymus on the opposed-phase images as compared with the in-phase images, and final decisions were made by consensus. The relative change in the ratio of the signal intensity (SI) of the thymic tissue to that of the muscles (the chemical-shift ratio) was then calculated according to the following formula:
(SIthymus/SImuscles) opposed-phase/(SIthymus/SImuscles) in-phase. In the two patients with soft tissue in the anterior mediastinum, the qualitative change in signal intensity and the chemical-shift ratio of the soft-tissue structure were assessed between in-phase and opposed-phase T1-weighted gradient-echo images.
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In the two patients with prominent soft tissue in the anterior mediastinum, the soft tissue exhibited signal intensity greater than that of muscle and less than that of fat on both T1-weighted and T2-weighted spin-echo images. In both patients, the two reviewers independently judged that the mass showed homogeneous decrease in intensity on opposed-phase gradient-echo images relative to in-phase images (Figs. 2A, 2B, 2C and 2D). No disagreement in the assessment of qualitative signal change of the mass occurred between the reviewers. The chemical-shift ratios for the two patients were 0.55 and 0.62.
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The criteria that are generally used to characterize a mediastinal mass as a normal thymus gland are the thickness [1, 3, 4], the morphologic features of the lateral boundary of the thymus [2, 3, 4, 5], and the lack of compression to the adjacent structures [5]. Baron et al. [1] reported that the normal maximal thickness of the thymus is 18 mm in persons younger than 20 years and 13 mm in those older than 20 years. The thymus varies widely in appearance in the first decade of life, whereas after 15 years, the gland becomes triangular-shaped with straight or concave lateral margins. The appearance of biconvex margins late in the second decade of life or focal enlargement of a lobe or multilobularity at any age are usually sensitive signs of thymic abnormality [4]. However, neither the thickness nor these morphologic features of the thymus are always reliable indicators of an abnormality, because these attributes of the gland may vary considerably among individuals [3, 4].
Although the CT attenuation value of the thymus decreases with age and the fatty infiltration of the gland is evident after the fourth decade, the gland is isodense with or denser than the chest wall musculature in patients younger than 19 years old [1, 2, 3, 6]. On MR imaging, Siegel et al. [5] showed that the relative signal intensity of the normal thymus on T1-weighted images was slightly greater than that of muscle but less than that of fat. On T2-weighted images, they found that the signal intensity of the thymus was moderately greater than that of muscle but slightly less than or equal to that of fat [5]. Therefore, both the CT attenuation values and signal intensities on MR imaging of a normal thymus can overlap those of an abnormal thymus, especially in the first and second decades of life, when the gland has not been completely replaced by fat tissue [5, 6].
The normal thymus contains various amounts of fat tissue, depending on age. The percentage of the total weight of the thymus that is fat tissue is nearly 20% in the first decade of life and increases during the second decade, reaching 40% late in the second decade [7]. Moore et al. [2] reported that no distinct difference in the extent of fat replacement in the thymus was observed on CT between groups with normal thymic histology and those with microscopic hyperplasia in patients with myasthenia gravis. On the basis of these results, we hypothesized that chemical-shift MR imaging could characterize the mediastinal soft tissue as that of a normal or hyperplastic thymus by proving the presence of fat in the tissue. We selected study volunteers in the late years of their second decade of life because that is when the CT attenuation values and MR signal intensities of the thymus are nonspecific and thus its morphologic variety can be a diagnostic problem [1, 2, 3, 4, 5, 6, 7].
In our results, the thymic tissue qualitatively and quantitatively revealed lower intensity on opposed-phase gradient-echo images than on in-phase images. We believe we can characterize the thymic tissue in early adulthood by using chemical-shift MR imaging, irrespective of the morphologic variety of the gland.
Thymic hyperplasia is occasionally associated with various diseases, including hyperthyroidism and myasthenia gravis, and its differentiation from neoplastic processes is clinically important [9]. Rebound thymic hyperplasia that occurs after recovery from debilitating illness or chemotherapeutic treatment of malignant disease is another diagnostic dilemma related to the thymus. In our two patients with a clinical diagnosis of thymic hyperplasia, the thymus revealed a relative signal loss on opposed-phase chemical-shift MR imaging that was similar to that observed in our 15 healthy volunteers. Our experience in the usefulness of chemical-shift MR imaging in the diagnosis of thymic hyperplasia is limited, and further evaluation with confirmation at pathology is needed. However, we speculate that chemical-shift MR imaging can be useful in the diagnosis of thymic hyperplasia and in its differentiation from neoplastic processes that do not include fat tissue.
Malignant lymphoma is the most important disease that must be differentiated from hyperplastic thymus; it is the most common anterior mediastinum mass in children and young adults, and thymic involvement of lymphoma occasionally reveals morphologic features resembling hyperplastic thymus. Positron emission tomography (PET) with FDG has been indicated to be excellent in imaging thoracicoabdominal lymphoma [10], but further studies are necessary to elucidate its role for the assessment of the thymus involved by lymphoma. Physiologic thymic uptake in FDG PET is a common finding in children and can be seen in some young adults [11]. Brink et al. [12] reported that no increased FDG accumulation in the thymus was observed in 37 patients with histologically confirmed thymic invasion by lymphoma. Recently, we performed chemical-shift MR imaging in a patient with histologically confirmed thymic involvement by non-Hodgkin's lymphoma. In that patient, the involved thymus revealed no significant signal change between in-phase and opposed-phase chemical-shift MR images (chemical-shift ratio, 1.01) (Figs. 3A, 3B) that was different from signal changes in chemical-shift MR images in our two patients with thymic hyperplasia.
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Further studies are necessary to clarify the role of chemical-shift MR imaging in differentiating the thymus involved by lymphoma from its hyperplastic enlargement. A limitation of our study is the narrow age range of the subjects, because all were in the later period of the second decade of life. Further analysis of chemical-shift MR imaging of the thymus should be carried out in younger age groups.
In conclusion, we were able to identify the thymic tissue and the hyperplastic thymus in early adulthood by qualitative comparison and by calculating the chemical-shift ratio between in-phase and opposed-phase gradient-echo images.
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