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Suprahyoid and Infrahyoid Neck Overview

Imaging Approaches & Indications
Neither CT nor MR is a perfect modality in imaging the extracranial H&N. MR is most useful in the suprahyoid neck (SHN) because it is less affected by oral cavity dental amalgam
artifact. Because the SHN tissue is less affected by motion compared with the infrahyoid neck (IHN), MR image quality is not degraded by movement seen in the IHN. Axial and coronal T1 fat-saturated enhanced MR is superior to CECT in defining soft tissue extent of tumor, perineural tumor spread, and dural/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 IHN and mediastinum are imaged. Swallowing, coughing, and breathing makes this area a "moving target" for the imager. MR image 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. Many indications exist for imaging the extracranial H&N. Exploratory imaging, tumor staging, and abscess search
comprise three common reasons imaging is ordered in this area. Exploratory imaging, an imaging search for any lesion that may be causing the patient's symptoms, is best
completed with CECT from skull base to the clavicles. 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. 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.

Imaging Anatomy
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 infrahyoid neck. 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.
• PPS has bland triangular skull base abutment without critical foramen involved; it empties inferiorly into submandibular space (SMS)
• PMS touches posterior basisphenoid and anterior basiocciput, including foramen lacerum; PMS includes nasopharyngeal, oropharyngeal, and hypopharyngeal mucosal surfaces
• MS superior skull base interaction includes zygomatic arch, condylar fossa, skull base including foramen ovale (CNV3), and foramen spinosum (middle meningeal artery); MS ends at inferior surface of body of mandible
• PS abuts floor of external auditory canal, mastoid tip including stylomastoid foramen (CNVII); parotid tail extends inferiorly into posterior SMS
• CS meets jugular foramen (CNIX-XI) floor, hypoglossal canal (CNXII), and petrous internal carotid artery canal; CS can be followed inferiorly to aortic arch
• RPS contacts skull base along lower clivus without involvement of critical structures; it continues inferiorly to empty into DS at T3 level
• PVS touches low clivus, encircles occipital condyles and foramen magnum; PVS continues inferiorly to level into thorax 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.
• Medial PMS mass displaces PPS laterally
• The more anterior MS mass displaces PPS posteriorly
• Lateral PS mass displaces PPS medially
• Posterolateral CS mass displaces styloid process and PPS anteriorly
• The more posteromedial lateral RPS nodal mass displaces PPS anterolaterally 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 jugular foramen and carotid canal and extends inferiorly to the aortic arch. The RPS begins at the ventral clivus superiorly and traverses SHNIHN to T3 level. The DS is immediately posterior to the RPS but continues beyond T3 level into mediastinum. For imaging purposes, RPS and DS can be considered a single entity. The PVS can be defined from skull base above to clavicle below .
The PVS is divided by fascial slip into prevertebral and paraspinal components.
Nobody likes to study the deep cervical fasciae (DCF) of the neck. However, it is these fasciae that define the very spaces we use to subdivide neck diseases and construct space-specific DDx lists. 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 fascia. The following is a practical distillate meant to simplify this challenging subject. There are three main DCF in the neck.

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