demonstrate slightly brighter signal than normal muscle. On T2-weighted images, the signal of the normal thymus is much brighter than the signal of the muscle, likewise when fat saturation is used. Inhomogeneous signal of the thymus should be considered pathologic. The thymic tissue revealed homogeneous decrease in intensity on opposed-phase MR images relative to that seen on in-phase images of healthy volunteers and two patients with hyper- plastic thymus. Chemical-shift MR imaging may be useful in identifying normal thymic tissue and the hyperplastic thymus in early adulthood [13]. Various radiopharmaceuticals localize in the thy- mus. Gallium 67 ( 67 Ga) citrate uptake has been shown in as many as 61% of children younger than 2 years of age with lymphoid and nonlymphoid tumors. Thymic 67 Ga uptake is most often seen after chemo- therapy. This reflects the presence of activated thymic lymphocytes as part of the immunologic response that leads to hyperplasia [14]. 67 Ga uptake may also occur in children stressed by illness [15,16]. Iodine 131 uptake has been shown to occur in hyperplastic thymus that does not contain ectopic thyroid tissue or metastatic foci [17]. The presence of somatostatin receptors in the thymus accounts for the occasional visualization of a normal thymus in children who undergo imaging with indium 111 pentetreotide [18,19]. There is increased uptake of 2-[fluori ne 18]- fluoro-2-deoxy-d-glucose (FDG) in the normal thy- mus gland of patients between the ages of 2 and 13 years [20,21]. Most of the patients studied had positron emission tomography (PET) scans for vari- ous oncologic conditions and had no known or sus- pected thymus abnormality. This uptake assumes an important role when evaluating mediastinal uptake in whole-body PET scans in pediatric oncology patients to avoid false-positive interpretation. Pre- vious treatment with a high dose of radioiodine and chemotherapy may contribute to visualization of a normal thymus with FDG-PET scans [22]. Ectopic thymus Most cases of ectopic thymus are found at any level of the pathway of normal thymic descent, from the angle of the mandible to the upper anterior mediastinum [23]. Infrequently, because of abnormal migration during fetal development, the thymus extends from its usual anterior mediastinal position into the middle and posterior mediastinum as one contiguous structure [24,25]. Rarely, ectopic thymus is reported to cause airway compression. It was reported that ectopic thymic tissue in infants should be considered in the differential diagnosis of second- ary pneumonias and emphysema especially located in the upper lung zones [26]. Cross-sectional imaging in multiple plains may be necessary to define aberrant thymus [27 –29]. Although this can be accomplished with CT, MR imaging is preferable [5]. The diagnosis of ectopic mediastinal thymus can be made on the basis of four criteria: (1) sign al intensity similar to normal ly located thymus on MR imaging sequences, (2) homo- geneous signal intensity, (3) uniform mild enhance- ment of contrast, and (4) continuity with normally positioned thymic tissue. The fourth criterion is helpful but is not required, because aberrant thymus sometimes is not attached to anterior mediastinal thymus or is connected by a thin, fibrous band that cannot be seen by imaging techniques. Ectopically located normal tissue may exert mass effect on adjacent structures (Fig. 4). Most commonly, aberrant thymus is identified in a right paratracheal location [30,31]. It has been reported that 10% of chil- dren have a ‘‘nubbin of what appears to be normal thymus posterior to the superior vena cava’’ on MR images [32]. Thymic pathology Thymic disorders are rare in the pediatric popu- lation. Hyperplasia of the thymus is the most common process to involve the thymus gland in infants and children. It is, however, exceedingly difficult to evaluate the weight of the gland as it continues to grow after birth until puberty and thereafter undergoes progressive atrophy. The hyper- plastic gland usually maintains the radiographic characteristics of the normal thymus. Thymic enlargement rarely causes neonatal respiratory dis- tress but should be considered in the differential diagnosis of marked tachypnea in the neonatal period [33]. True thymic hyperplasia is a very rare entity in which the thymus is enlarged without disruption of the normal architecture of the gland or any pattern of abnormal cellular proliferation [34,35]. In such cases the hyperplastic thymus retains most of the radio- graphic characteristic of the normal thymus [36], but can cause a mass effect on adjacent structures without invasion (Fig. 5). Thymic hyperplasia may be associated with Graves’ disease [37]. Thymic hyperplasia in Graves’ disease is more likely to be associated with, rather pediatric mediastinal masses: imaging evaluation 329 Fig. 8. Hodgkin’s lymphoma. A 12-year-old boy presenting with cough. (A) Frontal chest radiograph shows a lobular right paratracheal mediastinal silhouette. (B) Lateral radiograph demonstrates the anterior location, based on filling in of the clear space with soft tissue. (C) Axial contrast-enhanced CT image confirms a right-sided anterior mediastinal lobulated mass. (D –F) A series of PET-CT images show intense activity at numerous levels (arrows). franco et al334 franco et al338 Fig. 13 (continued). Fig. 14. Esophageal duplication cyst. A 6-year-old girl with a history of chest pain and cough. (A) Frontal chest radiograph demonstrates a rounded well-circumscribed opacity (arrows). (B) Axial intravenous contrast-enhanced CT image at the level of the dome of the liver shows the duplication cyst (arrow) located lateral to the esophagus. pediatric mediastinal masses: imaging evaluation 341 Fig. 17. Neuroblastoma. A 2-month-old boy with a history of meconium aspiration and thought to have a hypoplastic left lung. Frontal (A) and lateral (B) chest radiographs reveal a large dense rounded retrocardiac left posterior mediastinal mass on the left (arrows). (C) Precontrast coronal T1-weighted MR images show a large paraspinal soft tissue mass. (D) Precontrast sagittal T1-weighted MR images show the mass invading the posterior chest wall in the intercostal spaces (arrows). Axial T2-weighted MR images (E) and a coronal contrast-enhanced image (F) demonstrate the intraspinal extension of the tumor through the neural foramina. franco et al344 . indium 111 pentetreotide [18,19]. There is increased uptake of 2-[ fluori ne 18 ]- fluoro-2-deoxy-d-glucose (FDG) in the normal thy- mus gland of patients between the ages of 2 and 13 years [20,21] cyst. A 6-year-old girl with a history of chest pain and cough. (A) Frontal chest radiograph demonstrates a rounded well-circumscribed opacity (arrows). (B) Axial intravenous contrast-enhanced. to be associated with, rather pediatric mediastinal masses: imaging evaluation 329 Fig. 8. Hodgkin’s lymphoma. A 12-year-old boy presenting with cough. (A) Frontal chest radiograph shows a lobular right