Modalities

The choice of a certain imaging modality is not only influenced by the age of the patient and the indication for the study, but also by the availability of certain techniques as well as the individual skill and knowledge of the radiologist. The sensitivity and specificity of a certain imaging modality are also important when choosing the mode of investigation as there is a significant difference between confirming a suspected diagnosis and excluding a possible but less likely diagnosis. In the latter case, the sensitivity and specificity must be very high. The potential damaging effect of ionizing radiation means that all radiological investigation in children must be well indicated and carried out using correct technique. However, fear of radiation must never lead to acceptance of a reduced diagnostic accuracy. This is a particularly important consideration if neurosonographic examination of the brain and particularly the spine is the only examination that is carried out.

Neurosonographic examination of the brain has become very important as the method has obvious advantages. The equipment is relatively cheap and therefore generally available. The portable equipment allows for the examination to be carried out crib-side and in the isolette. However, this imaging modality has important limitations. The need for an acoustic window, the anterior fontanel, means that neurosonographic examination can only be carried out in small children. The cerebral convexities, as well as the posterior fossa structures are difficult to examine with ultrasound and the quality of the examination is very much dependent on the skill and experience of the sonologist and his or her specific knowledge of the pathology that can be expected. Even if the use of routine projections is adequate, a neurosonographic examination is very difficult to review and its value as a tool to exclude a possible diagnosis is seriously curtailed.

Computed tomography (CT) is the imaging modality most often used in the examination of the brain in children. Motion artefacts must be avoided and the patient must therefore remain immobile during the entire examination. An unacceptable increase in the radiation dose will follow when many CT -slices have to be repeated due to motion artefacts.

Sedation can be carried out in many ways and the sedative can be administered by mouth, rectally, intramuscular or intravenously. Routines for sedation must be developed in dose cooperation with pediatricians and anesthesiologists. All efforts must be made to examine the child in supine position and placed straight in the gantry. Evaluation of asymmetries between the two cerebral hemispheres becomes very difficult if the patient's head is not straight in the gantry. Excellent spatial resolution is necessary in order to properly evaluate the anatomical structures and to detect possible malformations. The thickness of the CT -slices should therefore not exceed 5 mm. Although the radiation from the CT scanner is well collimated, a significant dose may be delivered to the area examined. The lens of the eye is sensitive to radiation and efforts should therefore be made not to include this structure without good reason.

The need for repeat examination following injection of contrast material is much less in children than in adults. Processes associated with a break-down of the blood-brain barrier are much less common in children. It is much more important to be able to correctly evaluate the detailed structures of the brain, the anatomy and the symmetry. It is the responsibility of the radiologist to determine the need to use contrast material. An examination should always start with images without contrast unless the examination is carried out to look for intracranial metastases in a child with known primary tumor. However, this is uncommon in the pediatric patient. Contrast material is rarely indicated in CT examinations during the neonatal period. It is uncommon for images obtained following administration of contrast material provide to additional and useful information should the pre-contrast scans be entirely normal. It is uncommon that a repeat examination following contrast injection will clarify uncertain findings on a pre-contrast scan. The use of thinner slices or examination carried out in an alternative projection, such as true coronal images, are usually much more informative. The main rule is to repeat the examination following a contrast injection when tumor or AVM is included as differential diagnoses. However, this rule can be broken should these diagnostic possibilities appear unlikely based on available clinical information. This is contrary to the situation in adults. It is quite obvious that contrast material should be used whenever needed, however the merits of using contrast should be carefully analyzed by the radiologist in each individual case. When used, contrast should be administered in a dose of 3 ml/kg body weight and non-ionic contrast materials should be used. The imaging should be carried out immediately following a bolus injection.

Magnetic resonance imaging (MRI) is technically very complicated and therefore more difficult to utilize in the examination of children. The development of the brain including the myelination process has major impact on the appearance of the images during the first two years of life. Familiarity with this process is necessary in order to correctly interpret images. This adds a certain degree of uncertainty to the interpretation of the images but the increased sensitivity and the superb delineation of anatomical structures will lead to the increased use of MRI in pediatric neuroradiology. MRI has a particularly major impact in the detection and correct identification of cerebral malformations and certain forms of intraspinal pathology.

The majority of all myelographic studies carried out in children are done in one of two indications; the suspicion of congenital malformation or intraspinal tumor. The technique does not vary much with the indication and includes both conventional radiography and subsequent CT scanning. Myelography is best carried out under general anesthesia in children younger than 10 years.

Angiography of cerebral and spinal vessels are rare procedures in children. The indications are limited to detailed evaluation of arterio-venous malformations or other rare vascular disease. This procedure should only be performed by neuroradiologists.

Olof Flodmark and Derek Harwood-Nash