Pediatric Chest Imaging - part 7 docx

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.. The thymic tissue revealed homogeneous decre

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 (67Ga) citrate uptake has been shown

in as many as 61% of children younger than 2 years

of age with lymphoid and nonlymphoid tumors

Thymic67Ga 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].67Ga 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-[fluorine

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) signal intensity similar to normally 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

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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).

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et al 338

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.

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.

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