الفهرس 
Anatomy of the male reproductive systemOnly 14 pages are availabe for public view
 |  | from | 109 |  |  |
| | | from | 109 | | |
Abstract
Continued rapid advances in CT technology have revolutionized the imaging of the lung and airways in children. So, the protocol for HRCT of the lungs was first described in 1987. ‘High resolution’ refers to the fact that structures as small as 200 m can be visualized. HRCT of the chest is a technique capable of imaging the lung with excellent spatial resolution and with precise anatomic detail. It is more sensitive in detecting abnormalities, providing better information and a more accurate differentiation between different pathologies, than the chest radiograph and conventional CT. Children have their own peculiar anatomical characteristics that markedly differ from adults. The child is still growing, and future development and current anatomy and physiology may be altered by the disease process or other insults. ِAlso, Children are affected by different diseases and processes than adults and their response to various insults is different than adults. When using HRCT routinely in the evaluation of pediatric lung disorders, it is mandatory to minimize the potential side effects to the child. This is extremely important for extending the indications of HRCT in children so that they can benefit from the excellent diagnostic information it provides. Role of HRCT in diagnosis and differentiation of pulmonary nodules can be summarized in the following items: 1. Direct diagnosis and differentiation of pulmonary nodules: whether solitary or multiple.
2. Computer aided diagnosis (CAD) and differentiation based on HRCT images. 3. Localization of the proper site of pathological biopsy in some situations. Causes of solitary pulmonary nodules include malignant and benign pathologies. Malignant conditions include pulmonary carcinomas, lymphomas, carcinoid and solitary metastasis. Benign conditions include benign tumors e.g. hamartoma and fibroma, infectious diseases e.g. TB, bacterial and fungal infections.
HRCT characterization of solitary pulmonary nodules include nodule size, growth, and density and internal characteristics including nodule calcification, presence of fat, cavitation, bubbly lucencies and margin characteristics.
There is no size criterion that reliably distinguishes benign from malignant SPNs. In general, smaller nodules are more likely to be benign and larger lesions are more likely to be malignant. The absence of growth over at least a 2-year period is a reliable indicator of benignity. Also, the presence of a specific pattern of macroscopic calcification within a SPN as seen on conventional radiographs or CT is indicative of a benign lesion. These patterns include central, laminated, diffuse, or popcorn calcification. Further, the identification of fat within a SPN with smooth or lobulated margins is indicative of benignity. In the presence of cavitation, the thickness of the cavity wall can be helpful in distinguishing benign from malignant lesions. Cavities with a greatest wall thickness less than 5 mm are almost always benign, whereas most of those with a maximal wall thickness greater than 15 mm are malignant.
Homogeneous nodule attenuation is observed more frequently in benign (55%) than malignant lesions (20%). Although smooth, well-defined margins most often indicate a benign nodule, 21% of malignant nodules have a smooth well-defined margin. Alternatively, a lobulated margin may reflect uneven growth of a SPN and can indicate malignancy, although 25% of benign nodules such as hamartoma are lobulated.
Multinodular disease usually involves the presence of too many nodules to easily count on routine CT scan studies, with most of these nodules measuring < 1 cm in diameter. Due to its ability to evaluate the lung parenchyma in cross-section, eliminating the superimposition of densities, CT scanning offers a unique opportunity to evaluate lung nodules in exquisite detail. This includes first the ability to assess lesions by anatomic distribution, and second by morphology.
Causes of multiple pulmonary nodules vary from centrilobular nodules to perilymphatic nodules and random nodules. Centrilobular nodules are distributed primarily within the center of the secondary pulmonary lobule. Because the centrilobular artery and bronchus are the structures that predominate in the center of the pulmonary lobule, diseases affecting these two anatomic structures account for most processes that produce centrilobular nodules on HRCT. The presence of centrilobular nodules with tree-in-bud morphology is very diagnostically useful, because this finding is almost always seen with pulmonary infections. Despite the relatively large and varied differential diagnosis that requires consideration when centrilobular nodules without tree-in-bud are encountered, other features of the nodules themselves may provide useful information.
Perilymphatic nodules are seen with diseases that preferentially involve lymphatic structures, such as sarcoidosis, lymphangitic carcinomatosis, lymphoproliferative disorders, and amyloidosis.
Random nodules show no definable distribution relative to the secondary pulmonary lobule; nodules are seen in the center of the lobule and in contact with interlobular septa and visceral pleural surfaces.
Morphologic characterization includes assessing a number of characteristics including whether nodules are as follows: uniform or variable in size; sharply or poorly marginated; solid or subsolid in density (so-called ground-glass opacities); or have a so-called tree-in-bud appearance. Additionally, nodules may either be calcified, as occurs in fungal disease, or cavitary, as is seen, for example, in patients with septic emboli, metastatic disease, or Langerhans cell histiocytosis (LCH). Many algorithms has been developed to help diagnosis in such situations.
Nevertheless, initial evaluation frequently results in non specific findings which are classified into indeterminate nodules that require further evaluation to exclude malignancy by growth rate assessment, contrast material –enhanced CT, positron emission tomography (PET) and transthoracic needle aspiration biopsy can also be useful in evaluation of these indeterminate solitary pulmonary nodules.