140
A N A T O M Y FOR DIAGNOSTIC I M A G I N G
Fig. 4 . 3 4 Variable appearance of normal thymus on chest radiographs of young children: (a) sail-like; (b) like consolidation of right upper lobe; (c) like mediastinal mass; (d) like cardiomegaly.
CT and MRI The normal thymus can be seen on CT or MRI long after it is no longer visible on a radiograph. On CT it is homogeneous, with a density similar to that of muscle. On T1and T2-weighted MRI the thymus has a signal greater than that of muscle. Ultrasound Using high-frequency high-resolution ultrasound the internal architecture of the thymus can be seen. One can see the vascular septa dividing parenchymal lobules, which have an echogenic medulla centrally and a relatively hypoechoic cortex.
THE AZYGOS SYSTEM (Figs 4.21, 4.32, 4.36, 4.37, 4.43 and 4.44) These veins are found in the posterior mediastinum. The system consists of the azygos vein on the right and the hemiazygos and accessory hemiazygos azygos veins on the left. The azygos vein commences variably anterior to the L2 vertebra, either as a branch of the inferior vena cava or as a confluence of the right ascending lumbar vein and right subcostal vein. It ascends to the right of the aorta and thoracic duct, anteriorly to the bodies of T12-T5 and the right posterior intercostal arteries, which cross behind it. The right lung and pleura are to its right. It usually passes behind the right crus of the diaphragm to enter the thorax (see Fig. 4.4). At the level of T4 the azygos vein arches
