Lymph node imaging: multidetector CT (MDCT).

Advances in cross-sectional imaging, including conventional and helical (spiral) CT and multidetector (MDCT) and MR imaging, now allow detailed evaluation of the anatomy and pathology of the neck and thoracic inlet. The major structures are identified by their appearance and that of contrasting fatty tissue planes surrounding the soft tissues. These structures include the larynx, trachea, thyroid, and parathyroid glands as well as the vessels, lymph node chains, nerves, and supporting muscles. A thorough understanding of the normal cross-sectional anatomy is fundamental to properly interpret pathologic processes. Pathologic processes include both solid and cystic masses. Most solid masses are enlarged lymph nodes. In contrast, cystic masses are of variable pathology, and their characteristic appearances and locations with respect to normal neck anatomy allow a confident diagnosis to be made from a brief differential diagnostic spectrum. International Cancer Imaging Society.

Technique

Computed tomography (CT) is performed with the patient supine in quiet respiration [1-4]. A pad placed beneath the patient’s scapulae produces mild hyperextension of the neck and provides consistent images perpendicular to the long axis of the neck, minimizing dental artifacts. Scans are obtained using 3–5 mm or thinner contiguous slices. Multidetector CT (MDCT) affords optimal imaging in a single breath-hold, maximizing contrast enhancement and minimizing misregistration which improves visualization of small anatomic structures without rescanning or additional radiation. Intravenous contrast material is a prerequisite for enhancement of vascular structures. Its use facilitates differentiation of vessels from lymph nodes and the characterization of pathology.

Normal anatomy

The classic surgical approach divides the neck into two spaces, the anterior and posterior triangles (Fig. 1). The anterior triangle contains the major structures of the neck: hypopharynx, larynx, trachea, esophagus, thyroid, parathyroid, and salivary glands as well as the carotid sheath, nerves, and lymph nodes. Each anterior triangle is bordered posterolaterally by the sternocleidomastoid muscle and superiorly by the mandible. The anterior triangle is subdivided by the hyoid bone into suprahyoid and infrahyoid portions. The suprahyoid provides support for the floor of the mouth and contains sublingual, submandibular salivary glands and associated nodes. The infrahyoid portion contains the remaining components (Fig. 2). The posterior triangle is bounded anteriorly by the sternocleidomastoid muscle and posteriorly by the trapezius and is subdivided by the posterior belly of the omohyoid muscle. The space is primarily filled with fat and includes the hypoglossal nerve, vessels, and nodes.

Normal lymph nodes of the neck

The location of the various lymph node groups of the neck is most succinctly understood using a simplification of the Rouviere classification [5]. Nodal classification is critical for staging tumor extent [6-18]. Lymph nodes of the neck may be divided into 10 major groups [7-18].

The first six groups (I–VI) form a lymphoid collar at the junction of the head and neck (Fig. 2). These nodes are quite superficial, are usually accessible to palpation on physical examination and are commonly referred to as collar nodes: occipital, mastoid, parotid, submandibular, and facial submental.

Two groups of nodes lie deep within this lymphoid collar and are not accessible to clinical examination. Pathologic enlargement (>1.5 cm) allows detection. These nodes—groups VII, sublingual, and VIII, retropharyngeal—are often the site of metastases from carcinoma of the nasopharynx, the base of the tongue, and the tonsils (Fig. 3).

The anterior cervical group (IX) consists of superficial (Fig. 4B) and deep components (Fig. 4C). These nodes may be the site of metastases from primary tumors in the thyroid, larynx, and lung.

The lateral cervical nodes (group X) are also composed of superficial and deep chains (Fig. 5). This important deep group of nodes consists of three chains that form a triangle. The anterior portion is the internal jugular chain, the posterior portion is the spinal accessory chain, and the inferior component is the transverse cervical chain. Of these groups, the most important in staging head and neck tumors are the nodes along the internal jugular chain. A classification used by our surgical colleagues is shown in Table 1. Although the table is useful, the classification can easily be integrated into our system by carefully describing in reports the anatomic location of the nodes.

Table 1

1997 AJCC nodal (N) staging systems for cervical lymph nodes

Level	Classification criteria
NX	The regional lymph nodes cannot be assessed (clinically)
N0	There are no regional metastatic lymph nodes present
N1	There is metastasis to a single ipsilateral lymph node that is 3 cm or less in greatest dimension
N2	There is metastasis in a single ipsilateral lymph node that is between 3 and 6 cm in greatest dimension; there are multiple ipsilateral
	lymph nodes, none of which are greater than 6 cm in greatest dimension; or there are bilateral or contralateral lymph nodes, none of
	which are greater than 6 cm in greatest dimension
N2a	There is metastasis in a single ipsilateral lymph node that is between 3 and 6 cm in greatest dimension
N2b	There are multiple ipsilateral lymph nodes, none of which are greater than 6 cm in greatest dimension
N2c	There are bilateral or contralateral lymph nodes, none of which are greater than 6 cm in greatest dimension
N3	There is metastasis in lymph nodes that are more than 6 cm in greatest dimension

A new imaging-based classification for abnormal nodes

Recently, the results of a study produced an imaging-based nodal classification for the evaluation of metastatic neck adenopathy. Imaging landmarks were identified to create a nodal classification similar to that of the American Joint Committee on Cancer and the American Academy of Otolaryngology—Head and Neck Surgery [9–11, 18–24] (Table 2). This system was defined to ensure a more consistent nodal classification and to eliminate confusion with existing clinically based classifications. Imaging was chosen as a pivotal study because it identifies clinically silent nodes. Table 2 summarizes this new imaging-based classification (Figs. 6–19). A roman numeral is used to define the levels referenced to anatomic names, such as supraclavicular, retropharyngeal, carotid, facial, occipital, postauricular, and other superficial nodes; these anatomic terms are still widely used. This classification brings some improved precision and reproducibility to the staging of head and neck diseases.

Table 2

Clinical classification of neck nodes

t	Definition of nodes
I	Above hyoid bone
	Below mylohyoid muscle
	Anterior to back of submandibular gland
IA	Between medial margins of anterior bellies of digastric muscles
	Previously classified as submental nodes
IB	Posterolateral to level IA nodes
	Previously classified as submandibular nodes
II	From Skull base to level of lower body of hyoid bone
	Posterior to back of submandibular gland
	Anterior to back of sternocleidomastoid muscle
IIA	Anterior, lateral, medial, or posterior to internal jugular vein
	Inseparable from internal jugular vein (if posterior to vein)
	Previously classified as upper internal jugular nodes
IIB	Posterior to internal jugular vein with pat plane separating nodes and vein
	Previously classified as upper spinal accessory nodes
III	From level of lower body of hyoid bone to level of lower cricoid cartilage arch
	Anterior to back of sternocleidomastoid muscle
	Previously known as mid jugular nodes
IV	From level of lower cricoid cartilage arch to level of clavicle
	Anterior to line connecting back of sternocleidomastoid muscle and posterolateral margin of anterior scalene muscle
	Lateral to carotid arteries
	Previously known as low jugular nodes
V	Posterior to back of sternocleidomastoid muscle from skull base to level of lower cricoid arch
	From level of lower cricoid arch to level of clavicle as seen on each axial scan
	Posterior to line connecting back of sternocleidomastoid muscle and posterolateral margin of anterior scalene muscle
	Anterior to anterior edge of trapezius muscle
VA	From skull base to level of bottom of cricoid cartilage arch
	Posterior to back of sternocleidomastoid muscle
	Previously known as upper level V nodes
VB	From level of lower cricoid arch to level of clavicle as seen on each axial scan
	Posterior to line connecting back of sternocleidomastoid muscle and posterolateral margin of anterior scalene muscle
	Previously known as lower level V nodes
VI	Between carotid arteries from level of lower body of hyoid bone to level superior to top of manubrium
	Previously known as visceral nodes
VII	Between carotid arteries below level of top of manubrium
	Caudal to level of innominate vein
	Previously known as superior mediastinal nodes
Supraclavicular	At or caudal to level of clavicle as seen on each axial scan
	Above and medial to ribs
Retropharyngeal	Within 2 cm of skull base and medial to internal carotid arteries

Nodal disease

Clinical examination alone is highly inaccurate in staging nodal disease in patients with head and neck tumors. Therefore, prophylactic X-ray therapy can be used to treat patients with occult metastases, which are estimated to occur in 15%–20% of these individuals.

Because normal nodes in the neck may be identified on high-quality scans, criteria have been established to define lymphadenopathy: (1) a discrete mass great than 1.0–1.5 cm; (2) an ill-defined mass in a lymph node area; (3) multiple nodes of 6–15 mm; and (4) obliteration of tissue planes around vessels in a nonirradiated neck. A nodal mass with central low density is specifically indicative of tumor necrosis [5-8].

Less common, nonnodal solid masses include neurovascular tumors (paraganglioma, neurofibroma, hemangioma), primary neoplasms (fibroma, sarcoma), congenital lesions (teratoma, ectopic thyroid), trauma (hematoma), lesions of the bone (plasmacytoma, aneurysmal bone cyst), and infection. Specific imaging features aid the differential diagnosis. Paragangliomas (carotid body tumor) occur in the carotid space and consistently show early and persistently dense enhancement after the administration of contrast material. Acute hematomas are characterized by an intrinsically high CT attenuation and high intensity on T1-weighted magnetic resonance (MR) imaging. Metastases are located in the expected site of the major lymph node chains of the neck. Many investigators now use 1.0 cm as an effective size criteria for positive nodes in a patient population at high risk. Nodes in the upper neck tend to be large because of repeated respiratory infections; therefore, more liberal size criteria should be accepted. With the use of more liberal criteria, 80% of nodes will be metastatic and 20% will be benign hyperplastic. Important caveats include the follow: (1) regardless of primary site, a single ipsilateral node decreases survival by 50% and a contralateral node halves survival again; (2) extranodal extension is the best indicator of treatment failure and decreases survival by 50%; (3) posterior triangle nodes, with the exception of lymphoma, indicate a poor prognosis; and (4) nodes in the low internal jugular chain have a poor prognosis because proximal spread has often occurred. These caveats are helpful when dealing with the assessment of lymphadenopathy in this region.

In summary, careful analysis of nodes in the neck and knowledge of the various compartments is critical in the assessment and staging of primary head and neck malignancies.