Biological radiation effects can be divided into two categories: stochastic and deterministic effects. Deterministic effects are set by exposure threshold, and doses above the cutoff cause damage (e.g., cataract occurs if the crystalline lenses are exposed above 5 Gy (gray is the radiation unit for absorbed dose of radiation)) [3]. Stochastic effects are the result of cumulative doses of radiation and are related to genetic abnormalities and carcinogenic effects. There are no safe levels of radiation exposure related to stochastic effects, and the cancer risk increases as cumulative doses increase [3].

Recently, Pearce et al. (2012) demonstrated the association of low doses of radiation administrated on CT with occurrence of brain tumors and leukemia in a pediatric population (Fig. 4.2). This was the first time that this association was proven, although previous publications based on information gathered from the survivors following atomic explosions had raised the concern about the use of low dose radiation in medical imaging modalities [4]. Therefore, extra care should be taken when requesting a CT scan in the pediatric population, weighing the risks and benefits for each case.

There are three principles of radiation safety that should be kept in mind when requesting and performing a CT examination: justification, optimization, and dose limitation (Table 4.1) [5].

The North American radiological societies have mounted campaigns in an effort to educate doctors and the general population regarding radiation effects and radiation safety. The most important radiation safety campaigns (Image Gently® for pediatric population (http://​www.​pedrad.​org/​associations/​5364/​ig/​) and Image Wisely® for the adult population (http://​www.​imagewisely.​org/​)) have made optimization (“as low as reasonably achievable” (ALARA)) a very popular principle. However, justification is also very important, not only because it justifies the investigation, but also because it helps to decide which adjustments are necessary in the imaging technique in order to make the diagnosis.

There are several techniques that can be applied in CT imaging acquisition. For example, a patient who needs to have only the size of the cerebral ventricles assessed can have a study performed with low radiation exposure. In contrast, for brain parenchyma assessment, a scan with a regular radiation dose exposure is required to prevent artifacts related to low doses [6] (Fig. 4.3).

The clinical information provided in the request form also helps the radiologist to decide if the study requires contrast. For example, melanoma metastases are easier to depict after intravenous contrast injection. However, if the radiologist is not informed about a clinical suspicion of melanoma, intravenous contrast might not be used, thereby increasing the chance of missing lesions. The same problem applies for leptomeningeal disease in patients with history of malignancy as this can be difficult to visualize without the administration of intravenous contrast media as illustrated in Figs. 4.4 and 4.5.

The contrast used in CT scans is an iodine-based media. The use of this contrast should be cautious since it is not innocuous and contraindications to its use must be taken into account (Table 4.2) [7]. In addition, if a pre- and post-contrast study is needed, the patient will need to be scanned twice, receiving two doses of radiation.

In conclusion, every time a physician fills out a requisition with adequate clinical information, the radiologist is being helped to decide if the image modality chosen by the clinician is the most appropriate for the diagnosis and what is the best technical approach to perform it. The clinician must also flag any risk factors or contraindications for the use of contrast media. The chosen diagnostic test will have adequate sensitivity and specificity and patients will be saved from unnecessary radiation exposure if all these principles are followed.

Here are some questions that might help physicians to evaluate the appropriateness of a CT request:

The American College of Radiology website (http://​www.​acr.​org/​Quality-Safety/​Appropriateness-Criteria) has published a list of appropriateness criteria for different clinical scenarios. The use of this website may be helpful when questions about appropriateness are raised.

CT image formation is based on a gray scale of tissue X-ray attenuation. Tissues that can attenuate more X-rays appear whiter or hyperdense (e.g., bone) and tissues that allow the passage of X-rays appear blacker or hypodense (e.g., cerebrospinal fluid spaces). Fat tissue, for example, attenuates less X-rays than the normal brain tissue and therefore appears hypodense (Fig. 4.6) [8].

CT is a very good technique to assess the CNS vasculature. CT arteriograms and venograms have a high sensitivity and specificity in the diagnosis of vascular disease. Nevertheless, because of the increased radiation exposure, it is normally only used if the MR arteriogram/venogram was not diagnostic.

Interpretation of pediatric imaging has its own peculiarities as compared to adult imaging, as the different stages of development result in different appearances of the normal brain. The lack of myelin in the early stages of development and increased water content in the white matter is one example. This characteristic results in differences between the gray and white matter contrast that does not resemble what is seen in an older child, when the myelination process is complete (Fig. 4.7). Therefore, when looking at a pediatric study, it is very important to know the patient’s age before coming to any conclusions. The same principles apply for the interpretation of MRI images [9].

The assessment of a CNS bleed also has some pitfalls. There are different stages of hemoglobin degradation (Table 4.3) [10, 11]. In acute stages a bleed will be hyperdense compared with the rest of the brain parenchyma. However, if the patient is anemic, the attenuation of the signal from an acute bleed can be similar to that of the brain parenchyma. Therefore, a small bleed can be easily missed. Another problem is the lack of myelin in young children, which produces a relative diffuse hypodensity of the parenchyma and makes the blood vessels (in particular the venous sinuses) appear more hyperdense than usual (Fig. 4.8). This should not be confused with acute thrombosis of the venous sinuses. A similar effect can be seen in patients with cyanotic conditions such as cardiac shunts or chronic lung disease, if they are polycythemic. However, in these cases the relative hyperdensity of the vessels is caused by an increased hematocrit and is not due to lack of myelin [11].

A CT scan can determine the degree of mass effect over the parenchyma when evaluating a space-occupying lesion. It can also demonstrate the presence of intralesional cysts, calcification, necrosis, blood, or fat. CT almost always defines the anatomical location of the lesion, except in some cases of infiltrative lesions and posterior fossa lesions. However, MRI allows a more detailed assessment of brain anatomy and its associated pathology (Fig. 4.9). Therefore, once a space-occupying lesion is identified on CT, an MRI is carried out as the next step in the imaging investigation to further characterize the mass and its relationship to the rest of the parenchyma.