Imaging Approaches and Indications
- Many indications exist for imaging the extracranial H&N. Exploratory imaging, tumor staging, and abscess search comprise 3 common reasons imaging is ordered in this area. Global evaluation of the neck from the skull base to the clavicles is most often accomplished using CECT. Axial images can be rapidly obtained after IV iodinated contrast and multiplanar reformations are created. This provides reasonable spatial and contrast resolution.
- MR is less readily accessible and used less often but is especially useful in the suprahyoid neck (SHN) because it is less affected by oral cavity dental amalgam artifact. Axial and coronal T1 fat-saturated enhanced MR is superior to CECT in defining the soft tissue extent of a tumor, perineural tumor spread, marrow space invasion, and intracranial spread. When MR is combined with bone CT of the facial bones and skull base, precise preoperative lesion mapping results.
- CECT is the modality of choice when infrahyoid neck (IHN) and mediastinum are imaged. Swallowing, coughing, and breathing make this area a "moving target" for the imager. MR quality is often degraded as a result. Multislice CT with multiplanar reformations now permits exquisite images of the IHN unaffected by movement.
- High-resolution ultrasound also has a role. Superficial lesions, thyroid disease, and nodal evaluation with biopsy are best done by skilled ultrasonographers.
- Squamous cell carcinoma (SCCa) staging is best started with CECT, as both the primary tumor and nodes must be imaged, requiring imaging from the skull base to clavicles. MR imaging times and susceptibility to motion artifact make it a less desirable exam in this setting. Instead, MR is best used when specific delineation of exact tumor extent, perineural tumor, or intracranial invasion is needed. PET/CT is emerging as a useful adjunctive test in complex tumor detection and monitoring.
- When the type and cause of H&N infection are sought, CECT is the best exam. CECT can readily differentiate cellulitis, phlegmon, and abscess. CT can also identify salivary gland ductal calculi, teeth infection, mandible osteomyelitis, and intratonsillar abscess as causes of infection.
- In discussing the extracranial H&N soft tissues, a few definitions are needed. The SHN is defined as deep facial spaces above the hyoid bone, including parapharyngeal space (PPS), pharyngeal mucosal space (PMS), masticator space (MS), parotid space (PS), carotid space (CS), retropharyngeal space (RPS), danger space (DS), and perivertebral (PVS) space. The IHN soft tissue spaces are predominantly below the hyoid bone, with some continuing inferiorly into the mediastinum or superiorly into the SHN, including the visceral space (VS), posterior cervical space (PCS), CS, RPS, and PVS.
- Important SHN space anatomic relationships include their interactions with the skull base, oral cavity, and IHN. When one thinks about the SHN spaces and their relationships with the skull base, perhaps the most important consideration is to examine each space alone to see what critical structures (cranial nerves, arteries, veins) are at the point of contact between the space and the skull base. Space by space, the skull base interactions above and IHN extension below are apparent.
- In addition to skull base interactions, the relationships of the SHN spaces to the fat-filled PPSs are key to analyzing SHN masses. The PPSs are a pair of fat-filled spaces in the lateral SHN surrounded by the PMS, MS, PS, CS, and RPS. When a mass enlarges in one of these spaces, it displaces the PPS fat. Larger masses define their space of origin based on this displacement pattern.
- The IHN space anatomic relationships are defined by their superior and inferior projections. The VS has no SHN component, instead projecting only inferiorly into the superior mediastinum. The PCS extends superiorly to the mastoid tip and ends inferiorly at the clavicle. It is predominantly an IHN space, however. The CS begins at the floor of the jugular foramen and carotid canal and extends inferiorly to the aortic arch. The RPS begins at the ventral clivus superiorly and traverses SHN-IHN to the T3 level. The DS is immediately posterior to the RPS but continues beyond the T3 level into the mediastinum. For imaging purposes, RPS and DS can be considered a single entity. The PVS can be defined from the skull base above to the clavicle below. The PVS is divided by fascial slip into prevertebral and paraspinal components.
- The deep cervical fasciae (DCF) of the neck subdivide and define the spaces we use radiologically to construct space-specific DDx lists and evaluate disease of the neck. It is imperative that a clear understanding of these fasciae be grasped by any imager involved in evaluating this area.
- Many nomenclatures have been used to describe the neck fasciae. The following is a practical distillate meant to simplify this challenging subject. There are 3 main DCF in the neck. The same names are used in the SHN and IHN. The superficial layer (SL-DCF), the middle layer (ML-DCF), and the deep layer of DCF (DL-DCF) are the 3 important fasciae in the neck.
- In the SHN, the SL-DCF circumscribes MS and PS and contributes to the carotid sheath. In the IHN, it "invests" the neck by surrounding the infrahyoid strap, sternocleidomastoid, and trapezius muscles. It also contributes to the carotid sheath of the CS in the IHN.
- The ML-DCF in the SHN defines the deep margin of the PMS. It contributes to carotid sheath in both the SHN and IHN. In the IHN, it also circumscribes the VS.
- In both the SHN and IHN, the DL-DCF surrounds PVS. A slip of DL-DCF dives medially to the transverse process, dividing the PVS into prevertebral and paraspinal components. Another slip of DL-DCF, the alar fascia, provides the lateral wall to the RPS and DS, as well as the posterior wall to the RPS, separating the RPS from the DS. DL-DCF contributes to the carotid sheath, like the SL-DCF and ML-DCF.
- The internal structures of the spaces of the neck are for the most part responsible for the diseases there. Let us begin by defining the critical contents of the SHN spaces.
- The critical contents of IHN spaces are defined next.
Approaches to Imaging Issues in SHN and IHN
- It is crucial that the imager has a method of analysis when a mass is found in the neck. In the SHN, mass evaluation methodology begins with defining mass space of origin (PMS, MS, PS, CS, lateral RPS). When small, this is simple, as the mass is seen within the confines of 1 space. In larger masses, ask, "How does the mass displace the PPS?" Next, utilize a space-specific DDx list. Match the imaging findings to the diagnoses within this list to narrow your differential.
- With IHN masses, a similar evaluation methodology can be employed. First, determine what space the mass originates in (VS, CS, PCS). Then, review the space-specific DDx list. Match radiologic findings of your case to this DDx list. In all neck masses, knowing the clinical findings can be very helpful.
- Lesions of posterior midline spaces (RPS and PVS) of the neck need different image evaluation. When a lesion is defined here, first ask, "How does the mass displace prevertebral muscles (PVM)?" In the case of an RPS mass, PVMs are flattened posteriorly or invaded from anterior to posterior. Contrast this imaging appearance to that of the PVS mass in which the PVMs are lifted anteriorly or invaded from posterior to anterior. Since most PVS lesions arise from the vertebral body, vertebral body destruction and epidural disease will be linked. The DL-DCF "forces" PVS disease into the epidural space.
- There are many pseudolesions of the extracranial H&N. Always begin your image analysis by considering whether the "lesion" you see is a normal structure, normal variant, or a "leave me alone" lesion. A common pitfall is to mistake a motor denervation for an intrinsic disease. In acute-subacute muscle denervation, swelling with contrast enhancement is seen. In chronic denervation, muscle volume loss and fatty infiltration are the rule. Five motor atrophy patterns can be seen in the H&N. They are CNV3, CNVII, CNX, CNXI, and CNXII.
- Ferguson A et al: Analysis of misses in imaging of head and neck pathology by attending neuroradiologists at a single tertiary academic medical centre. Clin Radiol. ePub, 2021
- Kitamura S: Anatomy of the fasciae and fascial spaces of the maxillofacial and the anterior neck regions. Anat Sci Int. 93(1):1-13, 2018
- Warshafsky D et al: Imaging anatomy of deep neck spaces. Otolaryngol Clin North Am. 45(6):1203-21, 2012
- Harnsberger HR et al: Differential diagnosis of head and neck lesions based on their space of origin. 1. The suprahyoid part of the neck. AJR Am J Roentgenol. 157(1):147-54, 1991
- Smoker WR et al: Differential diagnosis of head and neck lesions based on their space of origin. 2. The infrahyoid portion of the neck. AJR Am J Roentgenol. 157(1):155-9, 1991
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