Medical imaging is an important tool in the evaluation and classification of pediatric head and neck masses. Such lesions may include congenital, inflammatory, infectious, vascular, or neoplastic processes. Ultrasound is often the first line modality in the workup of a neck mass in a child, followed by MRI or CT depending on the scenario. This information must be interpreted in the context of the patient’s clinical history, physical examination, and demographics. The medical imaging workup of a neck mass in a child must be focused to yield the maximum information possible while minimizing the risks of radiation and sedation.
Key points
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Ultrasound is often the initial imaging examination for evaluation of pediatric neck masses. Doppler interrogation is useful for evaluating vascularity.
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MRI and CT are complementary modalities, particularly in evaluating the deep cervical structures.
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CT is helpful in evaluating infectious and inflammatory processes, as well as in providing information regarding calcification.
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MRI is useful in characterization of soft tissue lesions, and evaluation of enhancement characteristics.
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Minimizing the risks of radiation and sedation must be considered in the imaging work up of a pediatric neck mass.
Introduction
Imaging is often an essential component in evaluating pediatric neck masses and can be helpful in characterizing congenital, inflammatory, vascular, and neoplastic lesions. In some cases, characteristic imaging appearances can be diagnostic. In other instances, imaging may not yield a definitive diagnosis, but can be helpful in narrowing the differential diagnosis, defining lesion extent, assessment for metastatic disease, and in follow-up evaluation for treatment response or recurrence.
Imaging workup must consider the risks and benefits associated with various modalities, particularly the risks of sedation and radiation. Ultrasound is typically the initial imaging performed for evaluation of a palpable pediatric neck mass as it utilizes no radiation, requires no sedation or intravenous contrast, is easily accessible, and is 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 interrogation. 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 characterization and assessment of extent of disease. However, both these modalities often require intravenous (IV) contrast, sedation in younger children, and with CT, exposure to radiation.
Introduction
Imaging is often an essential component in evaluating pediatric neck masses and can be helpful in characterizing congenital, inflammatory, vascular, and neoplastic lesions. In some cases, characteristic imaging appearances can be diagnostic. In other instances, imaging may not yield a definitive diagnosis, but can be helpful in narrowing the differential diagnosis, defining lesion extent, assessment for metastatic disease, and in follow-up evaluation for treatment response or recurrence.
Imaging workup must consider the risks and benefits associated with various modalities, particularly the risks of sedation and radiation. Ultrasound is typically the initial imaging performed for evaluation of a palpable pediatric neck mass as it utilizes no radiation, requires no sedation or intravenous contrast, is easily accessible, and is 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 interrogation. 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 characterization and assessment of extent of disease. 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.
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). 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. 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. Evaluation with a high-frequency linear transducer may reveal internal reticulation. 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 absent perfusion. Any or all of these characteristics warrant consideration of fine needle aspiration (FNA) or biopsy ( Fig. 2 ).
Necrosing lymph nodes tend to become increasingly hypoechoic and may initially show increased flow, but central flow may become decreased as the lymph node becomes more necrotic. As the lymph node degenerates to an abscess, the node becomes more anechoic centrally and can develop a hyperechoic rind that typically demonstrates increased flow. As the necrotic lymph node becomes more liquid in composition, there may be posterior acoustic enhancement/increased through transmission, as is seen in cysts.
Although an excellent screening tool, there are several limitations of ultrasound imaging. Deep structures of the neck, particularly the retropharyngeal soft tissues, are not well-visualized with ultrasound. The short length of the neck in infants and toddlers can present technical challenges for the sonographer, and patient motion can also make the acquisition of diagnostic-quality images challenging. As image quality is highly dependent on sonographic technique, ultrasound is best performed by sonographers with experience scanning children, and may require the interpreting physician to be present during at least part of the examination.
Ultrasound is also limited in its ability to accurately diagnosis the specific histology of masses in children by imaging characteristics alone. There is overlap in the imaging features of many congenital and acquired lesions. For example, complex, previously infected thyroglossal duct cysts may demonstrate heterogenous or intermediate echogenicity mimicking other midline lesions such as dermoid cysts (most commonly), epidermal inclusion cysts, vellus hair cysts, lymph nodes, or scar tissue ( Fig. 3 ).
Although calcifications may be readily identified on ultrasound by characteristic posterior acoustic shadowing, ultrasound does not visualize bony structures well. Also, as a result of posterior acoustic shadowing, which occurs because sound waves cannot penetrate calcifications, evaluation of structures deep to large calcifications is limited.
Computed Tomography
MDCT should be performed helically with acquisition of images in the axial plane in soft tissue and bone algorithms, and reconstructions in coronal and sagittal planes. The technique should be optimized to provide the lowest radiation dose possible while acquiring diagnostic quality images, according to the ALARA (as low as reasonably achievable) principle, utilizing age-stratified pediatric protocols. Intravenous contrast is usually recommended for the workup of a neck mass, assuming no contraindications such as allergy or renal impairment. Intravenous contrast is useful for evaluation for abscess, delineating lesion margins, assessment of lesion vascularity, distinguishing between vessels and lymph nodes, and detection of abnormally enhancing lymph nodes. In particular, cases in which sialoliths or other calcifications are suspected, precontrast imaging through the region of interest is recommended, as contrast may obscure calcifications. In the workup of some thyroid lesions, iodinated contrast should be avoided because of the avid uptake of iodine by the thyroid gland.
The advantages of CT in the evaluation of neck lesions are high-resolution delineation of anatomy, osseous structures, and airspaces, as well as detection of calcification and fat. It is widely available and provides rapid image acquisition, particularly with newer multichannel scanners, reducing the need for anesthesia compared with MRI. However, sedation may still be required in young patients to minimize motion artifact. Additionally, in contrast to ultrasound, CT imaging is far less operator dependent.
The primary drawback of CT is that it requires exposure to ionizing radiation, which is particularly relevant in children, who are more radiosensitive than adults. This is an especially important consideration in patients requiring multiple examinations for long-term follow-up. Additionally, there are risks associated with iodinated CT contrast including allergic reaction and nephrotoxicity, which are uncommon but potentially serious.
MRI
A typical pediatric neck MRI protocol includes multiplanar T1, fat-suppressed T2 or short tau inversion recovery (STIR) sequences, diffusion-weighted images (DWI), and postcontrast, fat-suppressed T1 weighted sequences. Compared with CT, MRI protocols are more complex, and may require tailoring to specific pathologies. T1 weighted images are helpful for delineation of anatomy. Fat appears bright on T1 and T2 weighted images, and fat suppression is helpful for elucidating underlying lesions on T2 weighted and postcontrast sequences. Various fat-suppression sequences are available, which vary by technique and manufacturer, including STIR, iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL), and Dixon. Time of flight or time-resolved magnetic resonance angiography (MRA) is recommended for suspected vascular lesions. Diffusion-weighted imaging can increase the conspicuity of lymph node metastases and may play a role in differentiating tumor recurrence from post-treatment changes. High field strength MRI systems such as 3T can provide higher signal-to-noise ratio and/or shorter scan times, which may be particularly advantageous in evaluating small lesions in children.
The superior soft tissue contrast capability and lack of ionizing radiation exposure make MRI an ideal modality for evaluating many masses in pediatric patients, particularly when intracranial extension is a consideration. However, these advantages must be weighed against the disadvantages of relatively long scan times often necessitating sedation, as well as the increased cost and resource utilization compared with CT and ultrasound.
Cross-sectional imaging features of pediatric neck masses
Congenital Lesions
Branchial cleft cysts
Congenital cystic lesions such as branchial cleft cysts are often well depicted on CT examinations. In some cases, however, branchial cleft cysts and associated sinus tracts may be collapsed, rendering them difficult to identify on any imaging modality. Uncomplicated branchial cleft cysts appear as hypoattenuating lesions with a thin wall ( Fig. 4 ). In cases of infected branchial cleft cysts, CT demonstrates a hypoattenuating cystic lesion, which may have a thickened wall, and inflammatory changes in the surrounding soft tissues ( Fig. 5 ).
MRI may be helpful in evaluating atypical features of branchial cleft cysts, which can result from intracapsular hemorrhage or solidification of cystic fluid, appearing as abnormal signal intensities of the contents ( Fig. 6 ). In particular, intracystic hemorrhage can demonstrate hyperintensity on T1 and T2 weighted images, while solidification of cystic fluid displays homogeneous hypointensity on T2 weighted images without enhancement. Atypical branchial cleft cysts can be difficult to differentiate from cystic malignancies even on MRI, and tissue sampling may ultimately be necessary.
Dermoid and epidermoid/cholesteatoma
Subcutaneous and calvarial epidermoids typically appear as well-defined nonenhancing cystic lesions with high T2 signal and variable degrees of restricted diffusion ( Fig. 7 ). Although dermoids can also have a cystic appearance similar to epidermoids, dermoids tend to occur in the midline and may contain fat components and/or calcification. When multiple globules of fat are present, a sac of marbles appearance can result, with corresponding high T1 signal, and hypoattenuation on CT. Multiplanar high-resolution MRI enables detection of intracranial extension of a sinus tract associated with nasal dermoids ( Fig. 8 ), which is useful for surgical planning.
Teratoma
Teratomas of the head and neck can cause respiratory compromise due to mass effect, necessitating urgent surgical intervention. Imaging plays an important role in the assessment of these lesions, especially in preparation for surgery. The presence of hypodense fat on CT ( Fig. 9 ) with corresponding T1 hyperintensity on MRI can be a helpful feature, but is not always present. Teratomas may also contain calcification, which is well visualized by CT. Teratomas demonstrate variable degrees of cystic and solid enhancing components ( Fig. 10 ) and thus can be difficult to distinguish from other complex partially enhancing lesions, such as venolymphatic malformations.
Thyroglossal duct cyst
Thyroglossal duct cysts appear as thin-walled hypodense cystic lesions that may occur anywhere along the course of the thyroglossal duct, with thin enhancement of the cyst wall. Superimposed infection in a thyroglossal duct cyst may result in a thickened enhancing wall and inflammatory change in the adjacent tissues ( Figs. 11 and 12 ).
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