FLEX February 2024

Stern et al

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relatively low in cost. Ultrasound can provide information about lesion size, location, cystic or solid nature; additionally, it can assess for vascular flow with Doppler inter rogation. Cross-sectional imaging with multidetector computed tomography (MDCT) and MRI may be required for further assessment, particularly when the deeper cervical soft tissues are involved. MRI and CT have complimentary roles in lesion characteriza tion and assessment of extent of disease. 2 However, both these modalities often require intravenous (IV) contrast, sedation in younger children, and with CT, exposure to radiation.

IMAGING TECHNIQUES Ultrasound

Ultrasound of the pediatric neck is usually performed with the patient in the supine position, with his or her neck extended. A high-frequency linear transducer provides good resolution of superficial structures and is therefore useful for evaluation of most palpable masses in the pediatric neck. Ample ultrasound gel and/or a stand off pad may improve visualization of lesions very close to the surface. Doppler imaging provides visualization of arterial and venous flow, and can be used to evaluate the presence and distribution of flow within a mass. The examination should also include assessment of the submandibular, parotid, and thyroid glands, when indicated. Identifying a normal thyroid gland is important in the preoperative workup of some congenital neck masses such as thyroglossal duct cyst or ectopic thyroid. 3 One of the primary advantages of ultrasound is its ability to distinguish between solid and cystic masses. Simple cystic masses are anechoic (ie, nearly black) and demonstrate posterior acoustic enhancement. This phenomenon is sometimes referred to as increased through transmission, making the tissues behind the cyst appear brighter than the adjacent soft tissues due to the increased velocity of sound waves through fluid in the cyst relative to soft tissues. However, complex cystic masses with internal debris or hemorrhage may have more intermediate echogenicity, and may approximate the echogenicity of soft tissue. Doppler interrogation can also help distinguish cysts, which should not demonstrate internal flow, from solid lesions or vessels. Doppler imaging can also elucidate how flow is distributed within a mass (centrally, peripherally, or evenly throughout), and whether the flow is normal, increased, or decreased, all which may have diagnostic significance. Ultrasound is also practical for evaluating palpable lymph nodes. Normal lymph nodes are typically ovoid to reniform in shape, are slightly hypoechoic when compared to the surrounding soft tissues with a hyperechoic region that represents the fatty hilum of the lymph node. On color Doppler interrogation, there is flow in normal lymph nodes, which is relatively increased near the hilum. Reactive lymph nodes are typically less than 1 cm in greatest diameter (short:long ratio <0.5). 4 In the pediatric population, however, lymph node morphology and clinical course are also important considerations ( Fig. 1 ). On ultrasound, malignant lymph nodes may be increased in size (>1–1.5 cm in greatest diameter) and may have lost their characteristic reniform shape, appearing more round in morphology. Malignant lymph nodes may show decreased internal echogenicity with loss of the echogenic fatty hilum. 5 Size alone cannot be used as a reliable criterion for distinguishing benign from reactive lymph nodes in children, but there is an increased risk of malignancy in lymph nodes measuring more than 3 cm in longest diameter. 6,7 Evaluation with a high-frequency linear transducer may reveal internal reticulation. 8 Findings suspicious for malignant infiltration of lymph nodes on color and power Doppler imaging include the presence of peripheral, subcapsular vessels with distortion or displacement of the intranodal vessels and focal areas of

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