CT and MRI of head and neck cutaneous lesions

Cutaneous lesions are derived from the epidermis, dermis and cutaneous appendages. Whilst imaging may occasionally be performed to evaluate such lesions, they may be undiagnosed and demonstrated for the first time on head and neck imaging studies. Although usually amenable to clinical examination and biopsy, CT or MRI studies may also demonstrate characteristic imaging features which aid the radiological differential diagnosis. In addition, imaging studies define the extent and staging of malignant lesions, as well as the complications of benign lesions. It is important for the radiologist to understanding the clinical significance and associations of these cutaneous conditions. This pictorial review will describe and depict the imaging appearances of benign, malignant, overgrowth, blistering, appendage and syndromic cutaneous lesions. An increasing awareness of the imaging characteristics of cutaneous lesions and related conditions will help the framing of a clinically relevant report.

Jason Gan and Steve Connor have contributed equally to this study and should be considered as co-first authors.
The skin is the largest organ of the human body and has a complex multilayered structure. Cutaneous and cutaneous appendages lesions are diverse, but since they are usually amenable to clinical examination and biopsy, they have received little attention in the imaging literature. CT or MRI studies of the head and neck may be used to delineate and stage skin tumours but may also demonstrate previously undiagnosed lesions. It is therefore important for the radiologist to understand the relevant imaging features, associations and significance of these cutaneous conditions.
The skin comprises of the epidermis and dermis (Figure 1). The epidermis consists of keratinocytes with melanocytes in its basal layer whilst the dermal layer is composed of fibroelastic connective tissue appendage structures such as hair follicles, sweat glands and sebaceous glands which traverse both layers. Cutaneous lesions frequently extend into the subcutaneous fat layer which lies immediately deep to the dermis. CT and MRI may demonstrate normal skin as a thin line of intermediate density and signal, and cutaneous lesions are usually manifest as a discrete mass or diffuse skin thickening.

Cutis verticis gyrata (CVG)
CVG is a rare condition resulting in skin overgrowth on the scalp resulting in ridges and furrows resembling the gyriform appearance of the brain ( Figure 2). 1 It may be primary or secondary to other conditions (e.g. endocrine or dermatological).
Imaging usually demonstrates symmetric longitudinal ridges but they may be asymmetric or localised in secondary disease. Radiologists should interrogate the pituitary fossa and adjacent scalp for an underlying tumour. 1 Giant keloid A keloid is a benign disorganised fibroproliferative cutaneous growth in response to trauma or inflammation of the skin, which extends beyond the site of the original injury. 2 Keloids are generally T2 hypointense. Enhancement is variable and the degree of vascularity may indicate proliferative activity ( Figure 3). It is important to assess the thickness of the keloid as well as the deep extent.

BENIGN SKIN LESIONS Haemangioma
Haemangioma is a benign vascular tumour consisting of a hamartomatous growth of capillaries ( Figure 4). They are the most common head and neck tumours of infancy. Whilst they grow rapidly in the early months, the majority of infantile haemangiomas stop growing by 10 months. Most have involuted by 5-7 years of age, however some subtypes persist. 3 They may be associated with systemic hemangiomas or PHACES syndrome (posterior fossa malformations, haemangiomas, cerebral/cervical artery malformations, cardiac abnormalities/aortic coarctation, eye abnormalities).
Diagnosis is based on clinical assessment with imaging being potentially useful in cases of diagnostic uncertainty or to prompt treatment, when deep structures (e.g. orbit) are affected. Differential diagnosis includes congenital vascular malformations, and more aggressive vascular tumours such as Kaposiform haemangioendothelioma or angiosarcoma. 3

SKIN APPENDAGE LESIONS
Skin appendage lesions may derive from the cutaneous layer but may extend into the subcutaneous fat layer on imaging.

Epidermal inclusion cysts and pilar cysts
Epidermal inclusion cysts (misnomer sebaceous cyst) are common cutaneous lesions. They are usually well-defined oval or spherical masses which demonstrate fluid density on CT, whilst T1 hypointensity, T2 hyperintensity and increased diffusion-weighted imaging (DWI) signal on MRI (Figure 5a-d), however the imaging characteristics are influenced by the variable composition of keratin, lipid containing debris and calcifications. 4 A closely related lesion is the pilar cyst (trichilemmal cyst) which are keratin-filled lesions (Figure 5e and f) derived from the hair follicle.

Hydradenitis suppurativa
Hydradenitis suppurative is a chronic purulent condition of unknown aetiology. It is typically located in areas with apocrine follicles such as the axillae and groin, however, may occur in the head and neck. It is characterised by thickened skin, subcutaneous oedema, multiple abscesses and fistulae. 5 The diagnosis is clinical but short tau inversion recovery sequences may be useful to evaluate the deep extent of abnormality and delineate Axial T 1 W and T 2 W images demonstrate well circumscribed T1 hypointense and T2 hyperintense dermal and subcutaneous lesions of the face (arrows) in keeping with fluid filled epidermal inclusion cysts. (c, d) Axial T 2 W and DWI images of a different patient demonstrate a heterogeneous T2 hyperintense lesion with restricted diffusion in keeping with a giant sebaceous cyst (arrows). (e, f) Sagittal T 1 W and axial T 2 W images of a further patient demonstrates T1 isointense and T2 hypointense lesions in keeping with pilar cysts (arrows). DWI, diffusion-weighted imaging. Figure 6. Hydradenitis suppurativa. (a, b) Axial T 1 W and T 2 W images demonstrate T1 hypo-to isointense and T2 hypointense signal within the occipital cutis with extensive deep extension to the subcutis. (c, d) Axial T 1 W post-gadolinium image and ADC map demonstrate peripheral enhancement (arrows in c) and restricted diffusion (arrows in d) in keeping with abscesses. (e, f) Clinical photos of the same patient demonstrating extensive involvement of the scalp, nape of the neck and lower facial involvement with nodules, abscesses and thick bands of scar tissue. ADC, apparent diffusion coefficient. abscesses ( Figure 6). Differential diagnoses include lymphadenitis, carbuncles, and infected sebaceous cysts.

Pilomatricoma
Pilomatricoma is a benign tumour arising from the hair cell matrix, which frequently occurin the head and neck,and most commonly the mid face. 6 They may be associated with other conditions (e.g. sarcoid) and syndromes. CT demonstrates wellcircumscribed superficial enhancing masses with calcification (>80%) and variable enhancement (Figure 7). MRI shows characteristic bands of T 2 W hyperintense signal radiating from a low signal centre. 6 The imaging differential diagnosis includes epidermal cysts, ossifying haematoma and foreign body reaction.

Cylindroma
Cylindromas are appendage tumours of uncertain histogenesis but with a folliculosebaceous distribution. Whilst usually small, there may be large coalescing (turban tumours) of the scalp and forehead. Multiple cylindromas occur in Brooke-Spiegler syndrome, where multiple scalp nodules may be demonstrated on imaging ( Figure 8). 7 Other diagnostic considerations with multiple scalp lesions include pilar cysts or other syndromic scalp tumours (e.g. NF1) and hamartomas (Cowdens).

Nodular hidradenoma
Nodular hidradenoma is a benign dermal-based skin appendageal neoplasm most commonly occurring in the scalp and face. It presents as a slow growing solitary solid or cystic lesion, with no overlying connection to the epidermis. Imaging appearances are non-specific with CT and MRI demonstrating solid enhancing and cystic components ( Figure 9) with possible calcification. 8

MALIGNANT
Malignant cutaneous lesions will be diagnosed clinically and by biopsy. Imaging may be used to evaluate the depth of the carcinoma, adjacent structures involvement, metastatic involvement and recurrent lesions.
Cutaneous squamous cell carcinoma (SCC) SCC represents 15-20% of skin cancers and occur on the sun exposed head and neck. Imaging may demonstrate cutaneous   thickening or focal mass. The deep aspect may invade tissues whilst the superficial aspect may ulcerate (Figure 10a and b). 9 Occasionally, there may be a primarily exophytic lesion with a keratin horn (Figure 10c-f) which mimics a benign keratosis. It is important to evaluate for imaging evidence of perineural spread and bony erosion which is critical for staging and treatment planning ( Figure 10 i and j). Lymph node metastases in parotid and periparotid lymph nodes should be considered for cutaneous SCC originating in face, forehead, coronal scalp, periauricular area, and upper neck, whilst suboccipital or level five lymph node metastases may spread from posterior scalp and posterior neck primaries. SCC Tstaging (TNM8) is seen in Table 1.

Malignant melanoma (MM)
Most melanomas occur de novo rather than within a pre-existing naevus. On MRI, lesions typically show T1 hyperintensity and T2 signal heterogeneity secondary to either paramagnetic melanin or haemorrhage. They also demonstrate heterogenous enhancement, peritumoral fat stranding, ill-defined deep tumour margins and perineural spread ( Figure 11). 9 Positron emission tomography-CT is the preferred modality to assess for metastatic spread.

Primary cutaneous lymphoma
Primary cutaneous lymphomas are a group of lymphoproliferative neoplasms, limited to the skin, with no involvement of lymph nodes, viscera or bone marrow at diagnosis. The majority are T-cell lymphoma (TCL) (65%), followed by B-cell lymphoma (BCL) (25%). 10 The initial patch, plaque or erythroderma stages are difficult to discern on imaging, however, there is subsequently a "thorny" extension to the subcutis due to lymphatic infiltration. On CT/ MRI, cutaneous TCL may show diffuse skin thickening, nodularity and ulceration. Whilst more typically occurring on the trunk, they may occur in the head and neck. Cutaneous BCL often presents as an isolated nodule/mass (Figure 12), and are usually located in the head and neck. Fluorodeoxyglucose positron emission tomography/CT almost always demonstrates increased uptake (Figure 12f). 10

Dermatofibrosarcoma and Kaposi's sarcoma
Dermatofibrosarcoma is a rare, slow growing cutaneous sarcoma which develops in the dermis and typically protrudes superficially. Deeper infiltration is described involving the subcutis, and muscle ( Figure 13). The head and neck is the primary site in 10-15% of cases. 11 They typically demonstrate avid contrast enhancement and may appear heterogenous due to haemorrhage. 12 Kaposi's sarcoma is a further low-grade sarcoma which may be AIDS-related. It rarely involves the face but is characterised by marked enhancement.

BLISTERING SKIN LESIONS
Epidermolysis bullosa (EB) is a rare genetic blistering disorder characterised by epithelial fragility, resulting in blistering (Figure 10a and b) after minor trauma. The most common cause of mortality in severe EB is SCC. 13

Sturge weber syndrome (SWS)
SWS is a rare, sporadic neurocutaneous syndrome characterised by eye, skin and meningeal angiomatosis, as well as a   facial cutaneous capillary malformation (port wine stain) in an ophthalmic division of V1 nerve distribution. 14 Imaging plays a crucial role in diagnosis and establishing intracranial ancillary features. Imaging findings ( Figure 14) include cortical/subcortical calcification, calvarial enlargement, parenchymal volume loss, white matter T2 hyperintense ischaemic change, ipsilateral choroid plexus enlargement, leptomeningeal enhancement and prominent transparenchymal collateral veins. 15

NEUROFIBROMATOSIS TYPE 1 (NF1)
NF1 is a multisystem genetic neurocutaneous disorder. Cutaneous neurofibromas may be limited to the skin. Plexiform neurofibromas are deeper lesions, however, cutaneous/subcutaneous involvement is common. Plexiform neurofibromas ( Figure 15) are more strongly associated with NF1, typically unilateral or asymmetric with ill-defined margins, and increased vascularity resulting in avid enhancement.

CONCLUSION
Whilst cutaneous lesions are usually diagnosed clinically or after biopsy, imaging plays a key role in assessing disease extent and associated complications. Although some imaging appearances are non-specific, those characteristics features of cutaneous lesions listed in Table 2 will aid a radiological differential diagnosis. The radiologist should recognise the characteristic imaging features of benign incidental cutaneous lesions, and in particular those that may mimic aggressive lesions. An understanding of systemic, syndromic or neurological associations will also result in an appropriate radiological search. It is hoped this review will increase awareness of these cutaneous lesions and will help the framing of a clinically relevant report.   Plexiform neurofibroma has ill-defined margins and often associated vascularity with associated bony dysplasias in the head and neck.
Multi system disorder, with cutaneous, central nervous system (e.g., gliomas), musculoskeletal, pulmonary and orbital manifestations. Rapidly growing or painful plexiform neurofibromas may signify conversion to malignant peripheral nerve sheath tumour