The vagus nerve is the longest cranial nerve. The cervical course of the vagus nerve can be easily visualized on US as a 2-mm to 3-mm diameter structure. It is typically located within the carotid sheath, and is usually posterolateral to the common carotid artery (CCA) and posteromedial to the internal jugular vein (IJV) (2021). The variation in the cervical course of the vagus nerve according to its position relative to the CCA has been previously reported (2325). The anterior variation in which the nerve passes in front of the CCA, is relatively common with a reported prevalence of up to 18.9%; whereas, medial or posterior variation has a lower prevalence (Fig. 3). A bulging thyroid mass outside the thyroid gland could also change the vagus nerve location anterior to the CCA by pushing the carotid sheath laterally and inducing vagus nerve variation (Fig. 4) (23).

Vagal schwannoma could be easily misdiagnosed as a metastatic lymph node causing unnecessary FNAs or diagnostic surgery (22). However, a typical location within the carotid sheath and direct continuity with the vagus nerve are characteristic US features of vagal schwannoma (26). For minimally invasive treatment such as RF ablation of thyroid nodules, attention to the vagus nerve location relative to the thyroid nodule is necessary to prevent nerve injuries. The variation in the vagus nerve location could increase the proximity of the vagus nerve to the thyroid gland, increasing its vulnerability to thermal injury (Figs. 3, 4). Therefore, operators should be aware of possible injuries to the vagus nerve located near the lateral margin of the benign thyroid nodules and also at the lateral neck during the treatment of recurrent thyroid cancers (1323).

Various symptoms could develope after vagus nerve injury. Because the cervical portion of the vagus nerve is cranial to the origin of the recurrent laryngeal nerve (RLN), vagus nerve injury may cause symptoms as a result of dysfunction of the RLN (e.g., cough, voice change) or dysfunction of the vagus nerve itself (e.g., arrhythmia, dysphagia, dyspnea, nausea, hiccups) (20). The latter symptoms are not specific to vagus nerve injury; therefore, significant diagnostic delays may occur, and patients may undergo unnecessary examinations.

The cervical sympathetic ganglion (CSG) consists of 3 paravertebral ganglia. The superior CSG is the largest, and is located anterior to the longus capitis muscle at the C2-3 vertebra level; the middle CSG is the smallest, and is located anterior to the longus colli muscle at the C5-7 vertebra level; the inferior CSG is intermediate in size, and is commonly fused with the first thoracic ganglion to form a stellate ganglion at the C7-T1 vertebra level (39). Because the middle CSG is located at the lower level of the thyroid gland, it can be injured during procedures for thyroid lesions. The middle CSG can be visualized in 41% of US images, and is seen as a spindle-shaped hypoechoic structure with a mean diameter of 3.8 ± 1.5 mm and a length of 8.7 ± 3.2 mm on US (40). It is usually located lateral to the CCA, but can also be located medial to the CCA (Fig. 9) (40). Because the middle CSG is located in front of, or close to, the inferior thyroidal artery, it is an anatomic landmark for the identification of the middle CSG (1739404142). A typical location and direct continuity with the nerve structure are characteristic US features of the middle CSG (40).

The clinical significance of the middle CSG during the procedures could be variable depending on its location relative to the CCA. Lateral to the CCA, the middle CSG could be misinterpreted as a metastatic lymph node, possibly causing unnecessary FNA or CNB. It is vulnerable to thermal injury during RF ablation of recurrent tumors at the lateral neck. Medial to the CCA, the middle CSG closely adjacent to the thyroid gland, and could be damaged during RF ablation near the lateral margin of benign thyroid nodules (Fig. 10) (404344).

Injury to the CSG causes Horner syndrome, a combination of ptosis, miosis, and anhidrosis of the face on the affected side. The redness of the conjunctiva of the eye may be an initial symptom that could be used as a diagnostic sign for early detection of CSG injury (45).

The cervical plexus is a plexus of the ventral rami of the first 4 cervical spinal nerves, and the brachial plexus is formed by the ventral rami of the lower 4 cervical and first thoracic nerves. They are easily identified as a thin, cordlike, hypoechoic structure between the anterior and middle scalene muscles on US (4464748). They first pass through the transverse process of the vertebra between the anterior and posterior tubercles, and then course between the prevertebral muscles (longus colli, longus capitis, and anterior scalene muscles) and paravertebral muscles (middle/posterior scalene, splenius cervicis, and levator scapulae muscles) deep to the sternocleidomastoid (SCM) muscle. They are subsequently located on the surface of the middle scalene and levator scapulae muscles in the posterior cervical triangle (Fig. 11) (4950).

Knowledge of the cervical and brachial plexus's location facilitates precise injection of the anesthetic solution for the nerve block. The cervical and brachial plexus block are used as an alternative to general anesthesia for surgery of the neck, shoulder, arm, and hand, as well as for diagnostic and therapeutic purposes for pain control (316). Brachial plexus injury could occur during the RF ablation of benign thyroid nodules and recurrent thyroid cancers as well (13).

Various symptoms could be developed according to the level of cervical/brachial plexus injury. The cervical plexus supplies the skin and muscles of the head, neck, and superior part of the shoulders; therefore, cervical plexus injury usually causes pain and paresthesia in the head and neck. The brachial plexus innervates the skin and muscles of the chest, shoulder, arm and hand, and brachial plexus injury can cause severe functional problems due to weakness of the muscles. The symptoms and signs may include a paralyzed arm, lack of muscle control in the arm, hand, or wrist, and lack of feeling or sensation.

The spinal accessory nerve (SAN) is a motor nerve that supplies the SCM and trapezius muscles. The SAN can be clearly depicted between the lateroposterior border of the SCM muscle and anterior border of the trapezius muscle on US; however, it requires experience due to the thinness of the nerve (5152). The mean diameter of the SAN is 0.54 ± 0.09 mm (515253), and the SAN is easily identified in the fat layer between the trapezius and levator scapulae muscles due to its superficial location. It can then be traced inferiorly into the trapezius muscle and superiorly deep into the SCM muscle, or between the 2 heads of the SCM muscle (Fig. 12) (51525354).

Due to its superficial location in the posterior cervical triangle, iatrogenic injury of the SAN is a well-known complication of surgical and minimally invasive procedures of the neck (54). Knowledge of the SAN's location on US has useful clinical applications, including optimization of surgical approaches in neck surgeries, minimization of iatrogenic SAN injuries during CNB, or RF ablation of tumors in the posterior cervical triangle. SAN dysfunction can lead to limitation of abduction of the shoulder joint, drooping of the affected shoulder, prominence of the scapula, and compensatory hypertrophy of other muscles of the affected shoulder.

The phrenic nerve originates from the C3-5 nerves, mainly from the C4 nerve. The phrenic nerve can be identified by US through the majority of its course. It descends obliquely anterior to the anterior scalene muscle deep, and immediately behind the SCM muscle, lateral to the IJV, and medial to the brachial plexus roots (Fig. 13). Because it crosses the transverse cervical artery along its course on the anterior scalene muscle, the transverse cervical artery is used as an anatomic landmark for detection of the phrenic nerve. The phrenic nerve crosses anterior to the first part of the subclavian artery on the left and anterior to the second part of the subclavian artery on the right (55).

Knowledge of the phrenic nerve's location may facilitate accurate injection of anesthetics in a regional nerve block and RF ablation for recurrent tumors (5657). Phrenic nerve injury leads to functional changes in the hemi-diaphragm, such as paralysis or hiccup. Therefore, phrenic nerve block can indeed be used for the palliative treatment of intractable hiccups related to chemotherapy and tumoral invasion. A safer site of injection and real-time assessment of the diffusion of the anesthetic solution could minimize the side effect of phrenic nerve palsy during the brachial plexus block. Iatrogenic phrenic nerve injuries could also be prevented during CNB or RF ablation of recurrent tumors.

Traumatic neuroma is caused by non-neoplastic proliferation of nerve tissue that occurs at the end of an injured nerve after trauma or surgery. It can be identified by US in 17.8% of patients after neck dissection (24). Because the traumatic neuroma usually arises from branches of the cervical plexus after neck dissection, it can be identified along the course of the cervical plexus (Fig. 14). Therefore, it is important to understand the course of the cervical plexus and its related anatomy for an accurate diagnosis of traumatic neuroma.

After neck dissection for the treatment of malignancy, traumatic neuroma may be easily confused with a metastatic lymph node, thereby increasing patient anxiety and unnecessary FNAs. Its characteristic US feature is the direct continuity of traumatic neuroma with involved nerves at the expected position, and could lead to avoidance of unnecessary procedures. Internal linear hypoechoic structures that correspond to the neural fascicles are also very specific to traumatic neuroma (2458).

The strap muscles are a group of 4 pairs of muscles in the anterior part of the neck consisting of the thyrohyoid, sternohyoid, omohyoid, and sternothyroid muscles (Fig. 15) (5960). The strap muscles originate from or insert into the hyoid bone, and function to depress the hyoid bone and larynx during swallowing and speech. Because the sternothyroid and thyrohyoid muscles attach to the thyroid cartilage, the thyroid gland moves together during swallowing. The omohyoid muscle consists of superior and inferior bellies. The inferior belly forms a flat, narrow fasciculus at the lower part of the neck, and becomes tendinous behind the SCM muscle (5960). The tendinous portion of the omohyoid muscle can be misdiagnosed as a mass on transverse US; however, it can easily be identified by following the oblique course of the muscle on US.

The SCM muscle passes obliquely across the side of the neck, and is located anteriorly at the lower neck (Fig. 15) (5960). It is composed of 2 separated heads: the medial sternal head and the lateral clavicular head. Because the SCM muscle is thick and broad in shape, it is used as a primary muscular landmark of the neck and divides the neck region into anterior and posterior cervical triangles, which helps define the location of structures, such as the lymph nodes. Many neurovascular structures are closely related to the SCM muscle, including the CCA, spinal accessory nerve, and cervical/brachial plexus (46475154).

The SCM muscle, scalene muscle, levator scapulae muscle, and trapezius muscle comprise the lateral part of the neck muscles. The scalene muscles are a group of 3 pairs of muscles in the lateral neck (anterior, middle, posterior) (Fig. 15) (5960). They originate from the transverse processes of the cervical vertebrae of C2 to C7, and insert into the first and second ribs. Because the cervical/brachial plexus passes between the anterior and middle scalene muscles, the scalene muscle could be an anatomical landmark for the cervical/brachial plexus, and the interscalene space is targeted with the administration of regional anesthesia for a brachial plexus block (4647). The phrenic nerve also passes anteriorly to the anterior scalene muscle (55).

The levator scapulae muscle originates from the dorsal tubercles of the transverse processes of C1-4, and inserts into the medial border of the scapula (5960). Because the SAN passes between the lateroposterior border of the SCM muscle and anterior border of the trapezius muscle, on the superficial aspect in the middle part of the levator scapulae muscle, these muscles are used as anatomical landmarks for the SAN (5154).

The longus colli muscle and longus capitis muscle consist of the posterior part of the neck muscles (5960). The longus colli muscle is in front of the spine and is the anatomical landmark for the cervical sympathetic trunk that lies on the longus colli muscle (Fig. 15) (39404142). The longus capitis muscle arises by 4 tendinous slips from the anterior tubercles of the transverse processes of C3-6. This anatomical feature easily differentiates the anterior tubercle of the transverse process from the calcified lymph node (Fig. 16).

The superior thyroid artery arises from the external carotid artery just below the level of the greater cornu of the hyoid bone, and primarily supplies the upper and anterior part of the thyroid gland. The inferior thyroid artery arises from the thyrocervical trunk and supplies the posteroinferior parts of the gland. It is used as an anatomical landmark for the middle CSG (40). The superior thyroid artery is also used as an anatomical landmark for the external branch of the SLN (38).

Injury to the superior or inferior thyroid artery can be induced by inserting the needle or electrode at the upper and anterior part of the thyroid gland (Fig. 17). Injury to the inferior thyroid artery is more likely to result in serious problems. Because it occurs at the posteroinferior part of the thyroid gland, direct compression of the neck is less effective for bleeding control, and may cause a large amount of hematoma.

The CCA and IJV are typically visualized laterally adjacent to both thyroid lobes. The CCA ascends within the neck to the upper edge of the thyroid cartilage, and divides into the internal and external carotid arteries. The CCA runs behind the SCM muscle and medial to the IJV, and intersects the superior bellies of the omohyoid muscle half way up the neck (5960). It is necessary to measure a safe distance from the needle tip before firing the stylet for a CNB of thyroid nodules to prevent CCA injury.

The IJV originates from the jugular foramen at the base of the skull. It descends within the neck along the lateral wall of the pharynx, posterior to the internal carotid artery, and continues laterally to the CCA. It runs beneath the SCM muscle, and finally merges with the subclavian veins forming the brachiocephalic vein (5960).

The thyroid gland drains into 3 main veins, the superior, middle, and inferior thyroid veins, and each drains its respective region of the thyroid. The inferior thyroid vein arises in the venous plexus on the thyroid gland, communicating with the middle and superior thyroid veins. The superior and middle thyroid veins drain into the IJV, whereas the inferior thyroid vein drains into the brachiocephalic veins (5960).

The anterior jugular vein begins near the hyoid bone, and descends between the median line and anterior border of the SCM muscle. It varies considerably in size, usually inversely proportional to the external jugular vein. In most cases, there are 2 anterior jugular veins, a right and left, but occasionally only 1 vein (5960). It can be damaged when inserting the electrode or needle by a trans-isthmic approach in minimally invasive treatment. Because it is easily collapsed by the US probe, operators should apply soft pressure to identify the anterior jugular vein (Fig. 18). Although injury to the anterior jugular vein can be easily controlled by simple compression, it can disturb the procedure due to persistent oozing.

The hyoid bone appears as a hyperechoic inverted U-shaped linear structure with posterior acoustic shadowing on the transverse view (61). Because the thyroglossal duct cyst is located in the region of the hyoid bone, the diagnosis is based on its relationship to the hyoid bone. The pyramidal lobe is located from the isthmus toward the hyoid bone. It can persist after total thyroidectomy and is sometimes misdiagnosed as a recurred tumor.

The vertebrae are anatomical landmarks to locate the cervical and brachial plexus. Therefore, recognizing the bony structures on US images is helpful for identifying and assessing the level of the nerve roots. The absence of the anterior tubercle of the C7 transverse process and prominence of those of the C6 transverse process (Chassaignac tubercle) are used as anatomic landmarks for the identification of the cervical vertebra level (Fig. 20) (62). The transverse process of C2-5 can be identified as successive steps cranial to the C6 level. The anterior tubercle of the transverse process can be misdiagnosed as a calcified lymph node; however, it can be easily differentiated by rotating the probe and checking the successive features on longitudinal view. Vertebral injury can occur during CNB for thyroid and neck lesions, and a safe distance should be measured from the needle tip before the procedure.

The thyroid cartilage has an inverted V shape on the transverse view, and the true and false vocal cords are visible within the structure. The cricoid cartilage has an arch-like appearance on the transverse view, and is located at the level of the C6 vertebra (63). The posterior surface of its anterior wall is delineated by a bright air-mucosa interface and reverberation artifacts from intraluminal air. The cricothyroid membrane and thyrohyoid membrane are seen as hyperechoic bands between the caudal and cephalad border of each cartilage. Because the SLN traverses the thyrohyoid membrane and the RLN courses close to the cricothyroid membrane, small remnants of thyroid tissue are retained after thyroid surgery to prevent nerve injury (Fig. 7).