4.2.1 Radiation Safety

In the last 20 years, following the increased usage of CT studies in medicine, there has been increased awareness of radiation burden from imaging studies especially in the pediatric population. ^{4 , 5} Children are more sensitive to radiation effect due to higher effective dose per delivered dose in a smaller patient compared to a larger patient (infant vs. adult), and due to a longer expected life span for carcinogenic changes to take place. This highlighted the importance of reducing the radiation exposure to children by adjusting the technique. ⁶ Since the society of pediatric radiology launched the “Image gently” campaign, ⁷ pediatric imaging centers adopted dedicated protocols adjusted to child’s size and age to reduce the delivered effective dose, in compliance with two major concepts:

1. 
   

   The linear, no-threshold model, which states that no level of radiation exposure is without risk.

   
   
2. 
   

   The ALARA (As Low As Reasonably Achievable) concept, where studies are performed with lowest radiation dose possible to still produce a diagnostic study. This has become an important guideline in pediatric radiology practice. ²

In recent years two large cohort studies were published that demonstrated relative increased risk for developing certain malignancies following diagnostic CT exposure in childhood. ^{8 , 9} These studies gained media attention and caused concern to parents. The main teaching points from these studies are that the relative increased risk per mGy of exposure is small but present, this relative increased risk is more substantial the younger the patient is at the time of exposure, especially below 5 years of age and that there is an accumulative effect from multiple studies. For sake of discussion, mGy is the measuring unit for radiation exposure and mSv is the measuring unit for effective dose (1 mSv = 1 mGy). Pearce et al ⁹ demonstrated excessive relative risk (ERR) of 0.023 per mGy (range 0.010–0.049) for brain tumors. However, for patients who received a cumulative dose of 50–74 mGy (mean dose 60.42 mGy) the ERR for developing brain cancer was 2.82 (1.33–6.03). In their study patients received 5 to 10 mGy per brain CT, which puts a child in relative increased risk to develop brain tumor after having 5 to 10 CT studies of the head. One should remember that these studies evaluated exposure to CT imaging prior to low-dose pediatric protocols adjustments. In an attempt to assess the impact of head and neck CT in children, Chen et al ¹⁰ reviewed the literature and found limited data regarding the effect of otolaryngological imaging (i.e., temporal bone, sinus, neck). In modern pediatric imaging facilities, the expected effective dose from a single brain CT is around 2 mSv and the expected dose from a facial or temporal bone CT is around 1 mSv. ▶ Table 4.1 lists the expected effective dose from additional common imaging studies. With these lower dose imaging protocols and by avoiding multiple repeat studies, the actual increased relative risk for the individual patient should be very small. In addition, advances in technology and post processing algorithms bring to the medical world new faster CT machines that will further decrease radiation exposure as these will become more prevalent in imaging facilities.

4.2.2 Sedation

In recent years there have been experimental animal studies followed by retrospective population studies that raised concern regarding increased risk for neurodevelopmental pathology in children who were exposed to anesthetic substances in early infancy. To date, no scientific evidence is available to support a change in pediatric anesthesia practice, and the scientific community is awaiting results of large population prospective studies that are being performed. However, as young children need sedation for several imaging studies, this concern should be addressed. ¹¹ Out of all imaging studies performed, the largest number of children requiring sedation is at the MRI suit. MRI is sensitive to motion artifact and a good quality MRI study requires cooperation of the patient with holding still for a substantial length of time, 20 to 60 min, depending on the type of study. For young children, this means that sedation is required; a child can be sedated with moderate sedation or general anesthesia depending on the child’s clinical condition and the anesthesiologist preference. Efforts to decrease the number of children requiring sedation for their MRI study include the development of motion-insensitive fast imaging sequences, as well as developing age-appropriate protocols.

It is possible to image neonates without sedation with proper feeding prior to the study, careful attention to noise and light reduction, using ear muffles, wrapping and swaddling the patients and keeping them warm. Infants and children up to 6-year-old usually require sedation. For children 6- to 10-year-old the ability to hold still may vary. In some pediatric imaging centers, a “test” study or a mock exam can be performed for each child prior to the scheduled study to decide whether sedation is needed. Using an MRI video and audio system, which creates a movie theater experience by wearing specialized goggles, can further decrease the need for sedation. Sedation is also needed for non-cooperative children undergoing a CT study, though these are much shorter studies in the range of a few minutes. Future newer faster CT scanners may obliviate the need for sedation completely. Additional studies that may require sedation are PET-CT and interventional radiology procedures.

4.3.1 Ultrasound

The absence of ionizing radiation or need of sedation makes US the initial imaging of choice for most cases of pediatric neck pathology that require additional evaluation to supplement the physical exam. US is an imaging modality based on detection and display of acoustic energy reflected from interfaces within the body. On US, fluid is hypoechoic (dark) with through-transmission, fat is hyperechoic (bright), air and bone will create acoustic shadowing, eliminating evaluation of deeper structures (▶ Fig. 4.1a). An US study may be sufficient for diagnosis, especially in evaluation of a focal mass, parotid space lesions (▶ Fig. 4.2a), or thyroid pathology. ¹² In addition to US a Doppler study can be obtained that will add information regarding vascularity of a focal lesion (▶ Fig. 4.1a and ▶ Fig. 4.2b) and patency of the carotid sheath vessels. US is an excellent method for guiding procedures such as fine needle aspiration (FNA) (▶ Fig. 4.2c) or abscess drainage.

It is also a preferred method for following progression or regression of a lesion. When a diffuse pathologic process is evaluated, US may not be able to assess the full extent and additional cross-sectional imaging, such as CT or MRI, will be needed. This is especially important for processes extending towards skull base, as US cannot penetrate the skull base bone, and for lesions involving retropharyngeal space or extending into the mediastinum (▶ Fig. 4.3).

In recent years US has gained popularity as a bedside tool in the hand of the emergency room or ICU physician for point-of-care US; this may serve for assessing intubation tube placement. ¹³ Studies published in recent years are suggesting expanding US interrogation of the neck to include assessment of laryngeal structures, as the vocal cords can be assessed in children through the uncalcified cartilage (▶ Fig. 4.4), and assessment of tonsillar and peritonsillar infections (▶ Fig. 4.5).

In the adult literature, the use of contrast-enhanced US of the neck is being evaluated for differentiating benign from malignant thyroid nodules or lymph nodes; this may expand to the pediatric population in the future.

The main limitation of US is that the quality of the study and related interpretation depends on the skills of the examiner; it is advisable to have children examined by specifically trained personnel.