While MR imaging offers a remarkable window into the dynamic process of skeletal growth and maturation, it also creates the unique challenge of distinguishing normal pediatric development from abnormal growth.

Although conversion from hematopoietic to fatty marrow throughout childhood occurs in a predictable and reproducible pattern, familiarity with these transformations in the pediatric skeleton in terms of interpreting MR imaging isn't always easy. But such familiarity is essential.

Marrow reconversion in 11-year-old boy with rhabdomyosarcoma of the buttocks who had been treated with granulocyte colony stimulating factor.

Coronal T1-weighted MR image (338/14) of thighs shows numerous irregular islands of low signal intensity (SI) throughout both femoral diaphyses. These foci have slightly higher SI than adjacent muscle on T1-weighted images. There is hematopoietic marrow in distal femoral metaphyses and fatty marrow in distal femoral epiphyses.

In a review in the January issue of Radiology (RSNA.org/radiology), authors Tal Laor, M.D., of Cincinnati Children's Hospital Medical Center, and Diego Jaramillo, M.D., M.P.H., of Children's Hospital of Philadelphia, discuss the histologic structure and MR imaging appearance of normal development-related changes of the musculoskeletal system in children, including those that may be mistaken for abnormalities.

Authors discuss each component of the growing bone:

• Bone marrow and bony envelope

• Epiphyseal cartilage and secondary ossification center

• Physeal cartilage and its adjacent metaphysis, including the difference between normal and irregular growth in terms of MR imaging appearance

"Growth, ossification and marrow conversion result in developmental changes that can be identified on MR images," the authors write. "Familiarity with these transformations in the pediatric skeleton, and the MR imaging appearance of these changes, can be challenging. However, a thorough recognition will help to differentiate normal growth from disease."

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CT Diagnosis of Chronic Pulmonary Thromboembolism

This article meets the criteria for 1.0 AMA PRA Category 1 Credit™.

CT is a useful alternative to conventional angiography for diagnosing chronic pulmonary thromboembolism (CPT), which is potentially curable but commonly misdiagnosed because symptoms are nonspecific and related to the development of pulmonary hypertension.

CT technology aids not only in diagnosing CPT, but also in determining which cases are treatable with surgery and confirming postoperative success.

In an article in the January-February issue of RadioGraphics (RSNA.org/radiographics), Eva Castañer, M.D., and colleagues review the risk factors, clinical characteristics and pathogenesis of CPT, describe optimal techniques for CT angiography and the CT criteria for CPT and discuss differential diagnoses and treatment.

Chronic pulmonary thromboembolism in a 47-year-old man with multiple episodes of acute pulmonary thromboembolism.

(a) Coronal 30-mm-thick maximum intensity projection CT image shows marked enlargement of branches of the right and left inferior phrenic arteries (straight arrows), right and left bronchial arteries (arrowheads), and an intercostal artery (curved arrow). (b) Coronal 30-mm-thick maximum intensity projection image shows enlargement of intercostal arteries on the right side compared with those on the left.

Both congenital and acquired conditions may cause pulmonary hypertension or obstruction of the pulmonary arteries and mimic CPT. The authors address findings that will help distinguish CPT from the following conditions:

• Idiopathic pulmonary hypertension

• Proximal interruption of the pulmonary artery

• Takayasu arteritis

• Primary sarcoma of the pulmonary artery

• Bronchial abnormalities

• Acute thromboembolism

"Knowledge of the radiologic imaging signs is required to detect and accurately diagnose the condition," Dr. Castañer and colleagues conclude. "Because chronic thromboembolism is potentially curable with pulmonary thromboendarterectomy, early recognition may improve the outcome in cases that are technically operable."