Paediatric musculoskeletal radiology

General principles

 

Radiographic technique used in children must be chosen so as to decrease the chance of motion during exposure. The shortest possible exposures should therefore be used. To permit the shortest exposures, high output generators with rapidly functioning automatic exposure devices and grids should be used. If exposures are too short for the speed of the moving grid, grid lines may appear. Immobilization devices are very useful in pediatric radiology as they help to maintain correct positioning and minimize motion.

Sedation may need to be used for computed tomography (CT), nuclear medicine, and magnetic resonance imaging (MRI), particularly in children under 5 years of age. MRI generally requires more sedation than CT and nuclear medicine.

With CT of small children not only axial but also coronal and sagittal views can be obtained of almost every bone. One simply positions the child so that the plane desired is in the plane of the gantry. This can be done by either angling the child or the gantry or both. Almost all projections are possible if the child is small enough. For example, in infants direct sagittal views of the entire spine can be done simply by placing the infant supine in the plane of the gantry.

In MRI any plane can be obtained without changing the position of the child. The smallest coil possible should be used to maximize image quality. For MRI of the hip, one can use a head coil in most children under five years of age and an extremity coil can be used in infants. For
 

a	Figure 1. Small carpus in JRA and a normal wrist for comparison. a) 2.5-year-old girl with severe clinical manifestations of JRA. Note that the space between the metacarpals and the distal radius appear smaller than in the normal wrist of a girl of about the same age as the patient. This indicates carpal destruction or lack of growth. b) Normal girl 2.9 years of age.
b

imaging the knee in small children a wrist coil will produce excellent images. Gradient echo images with a small flip angle (about 200) are often very useful in musculoskeletal MRI of children since with this technique cartilage has a high signal and is clearly separated from other tissues. The use of a small flip angle helps to separate cartilage from fluid. The advantage of MRI in evaluating the skeleton of children is that it allows visualization of the bone ends which are still cartilaginous. Cartilage cannot be seen directly with other modalities as it cannot be separated from other soft tissues. In such situations secondary signs such as relative position or distance between two ossified portions are used to imply changes in cartilage. When these signs are equivocal MRI is very useful. An example of the use of such secondary signs is to evaluate the carpal bones. The carpal bones are not visible on hand radiographs at birth; yet, the proportions and shapes of their cartilaginous models are very similar to those of the adult. Determination of their overall size can be obtained by measuring the distance between the radial growth plate

Figure 2. Asymmetry of ischiopubic synchondrosis. The left synchondrosis is larger than the right. This is a normal variant.

and the base of the third metacarpal and comparing it to normal standards. Alternately one can simply compare the radiograph in question to that of a child of similar age or to a bone age book (Fig. 1).

 

Andrew K. Poznanski