Normal sonographic aspect and variants of the skin and subcutaneous tissues 

Utilizing a 10-13 MHz probe, or better still a 20 MHz instrument, it is possible to distinguish the three layers that make up the skin: epidermis, dermis and subcutaneous tissue, separated from the underlying muscle layer by the fascia superficialis (Fig. 1). 
The epidermis has a variable thickness, between 0.3 and 0.6 mm according to the bodily location; on ultrasound, it appears like an intense echo, hyperechogenic, linear, called “entrance echo”, made up of US reflections due to the diverse acoustic impedance between the gel applied to the skin, and the corneum; therefore its thickness turns out greater than the actual thickness of the epidermis. 
The dermis, of thickness varying from 1 to 4 mm, is composed of a superficial layer (papillary dermis) and a deep layer (reticular derma): it presents a hyperechogenic structure, rather homogenous in the papillary layer and composite in the reticular layer because of the presence of the arterioles and hair follicles.  
The subcutaneous tissue presents a widely variable thickness, between 5 and 20 mm, depending on the habitus and bodily region. Its echo structure is characterized by the hypoechogeneity of the adipose lobules, diapaused by hyperechogenic connective shoots, with a resulting reticular aspect. 
The fascia superficialis, positioned between the subcutaneous and muscular tissues, is presented as a hyperechogenic linear structure parallel to the probe's cutaneous plane of support. 
The skin's thickness varies according to body area: it is thicker in at the nape of the neck, the interscapular and lumbar region, and in the palm of the hand and sole of the foot; it is thinner at the flexor surfaces of the limbs and the pretibial location. 
The distinction between dermis and subcutaneous tissue is sharply less in sites of major skin thickness and, on the contrary, is more evident in locations with thinner skin.  
Skin thickness also varies according to a person's constitutional type, age, sex and race.  
The dermis also presents physiological echogenic changes related to age: in the neonate it is hypoechogenic and, gradually during the first months of growth, it reaches the echogenic profile typical of adults, then this again diminishes in senility. 
In the elderly, moreover, degeneration of the elastic fibers of the papillary dermis (photoelastosis) is found in sites long exposed to the sun. This takes on the aspect of subepidermal hypoechogenic striae (Fig. 2). 
The possibility of using high-resolution ultrasonography to precisely measure skin thickness and to evaluate its structure, finds its clinical application in the monitoring of induced modifications induced by therapy in cirrhotic and diabetic patients, in malabsorption syndromes, in athletes and in dietetic regimes for esthetic purposes.  

Dermatological conditions, focal and diffuse 

The role of ultrasonography in Dermatology is quite peculiar compared to other specialized fields: the diagnosis of cutaneous disorders is, in fact, essentially founded upon the clinical examination and the histology. 
Over the last 15 years, sonography has found and consolidated important applications within the scheme of things, that may be summarized in three basic uses: 
- loco-regional staging and follow-up checks on tumors; 
- follow-up checks on diffuse conditions and their treatment-monitoring; 
- diagnosis of focal and diffuse disorders. 

Loco-regional staging of tumors and follow-up 

Local staging of tumors is particularly important in Dermatology, both for choosing the best course of treatment (surgery, radiotherapy or a combination), and for the scheduling of the surgical operation, in a way that on the one hand the exeresis would be as radical as possible and on the other hand that the outcome would be optimal, both functionally and esthetically.  
This is valid for all cutaneous neoplasms but assumes fundamental importance in melanoma (Fig. 3). Here, the thickness and the extent of invasion, cutaneous and subcutaneous, that can be pre-operatively established in a noninvasive way by high-resolution ultrasound, are the principal prognostic factors.  
The accuracy of sonography in defining the vertical thickness and cutaneous invasion in melanoma has been compared with histological data, demonstrating a high correlation coefficient, up to 0.96%. 
The tendency of sonographs to overestimate the thickness of the lesion, with values between 0.1 and 0.3 mm, is explained partly by the co-existence of phlogistic infiltrate,  indifferentiable sonographically from the neoplasia, and partly by the post-exeresis skin retraction, due to the loss of cutaneous tension, to dehydration of the lesion and to fixation of the preparation.  
Also for the epitheliomas, basal cell and spinocellular, it is fundamental to determine the tumor dimensions and the degree of invasion for the purpose of optimizing, as well as possible surgical treatment with the appropriate respect for safety margins,  the radiotherapy, cryotherapy, laser-therapy or other treatment combinations. 
Tumor size may be determined easily enough, by individuating a thin stria lacking echoes, that allows the differentiation of the lesion from the surrounding cutaneous tissue. 
However, establishing the thickness may be hampered by two factors,- superficial hyperkeratosis and a distinct peripheral phlogistic reaction. In such circumstances, only the measuring of the so-called “maximum tumoral thickness” is possible, that is to say, the distance between the skin surface and the most superficial point of the peritumoral tissue, where normal echo patterns are detected.  
These eventualities occur most frequently in the study of spinocellular epithelioma, where a clear hyperkeratosis is sometimes present and the deep margins are badly or not at all estimable. 
A better identification of the profound margin is obtainable in lesions that measure at least 10 mm of thickness, by using medium-high frequency (10-13 MHz) linear electronic probes. 
This staging has considerable significance in anatomical points at irregular surfaces or where the superficial layers are in direct contact with the bone structures, of which it is possible to determine any involvement with great reliability (Fig. 4). With this type of transductor it is also possible to carry out a flow-meter analysis by means of Doppler and color-power Doppler evaluation.  
In the post-op checks on skin neoplasia, the role of high-resolution sonography consists in showing up any possible recidivism in the scar and involvement of the regional lymph nodes. 
In melanoma, cutaneous reoccurrence may develop on three levels: full-blown recidivism's, single or multiple, on the surgical scar; satellite nodes within a 3 cm distance of the scar; “in transit” metastases more than 3 cm distant from the scar, positioned along the lymphatic drainage system. 
The investigation of these lesions calls for the employment of high-resolution probes, since they are sometimes only a few mm. in size. Their ultrasound aspect is of solid nodules with a homogenous hypoechogenic echo pattern. Using color Doppler, sensitive to slow flows, at least one vascular pole is detectable (Fig. 5). 
Color Doppler can be diriment in the differential diagnosis between recividisms on the scar and cicatricial fibrous tissue areas, insofar as the latter are avascular, whereas the neoplastic lesions present color signals, sometimes better found with the power Doppler, that demonstrates a disordered arrangement of vessels. 
In most of the cases of metastatic adenopathies, there is an inhomogeneous hypoechogenic pattern (Fig. 6).  
In localizations with a diameter greater than 3-4 cm, hypo-anechogenic areas may appear, due to colliquative necrosis.  
Both the recidivist nodules and the adenopathies are generally palpable but the smallest lesions, sometimes with a diameter of few mm., are only detectable using high-resolution ultrasonography, which allows their number and dimensions to be precisely defined. Thence, the efficacy of therapy, whether medical or radiant, can be assessed.     

Ultrasound checks on inflammatory conditions 

Using sonography in inflammatory disease follow-up has the purpose of assessing the effects of therapy and therefore of monitoring the treatment. 
This application especially concerns psoriasis: the evaluation of psoriatic skin and thence the control of the effectiveness of the anti-psoriasis drugs is generally based on clinical observation. However, this is dependent on the dermatologist's skills and experience, which does not lead to reproducible observations or data comparison among various centers. 
On the other hand, histopathological evaluation is certainly objective and reliable, but is invasive and therefore not suitable for repeated check-ups over time. 
For this reason, several noninvasive methods have been developed and applied for the defining of some characteristic parameters for psoriasis and for the appraisal of their variation in the spontaneous development of the disease or in response to therapy.  
The employment of high-frequency ultrasonography has been relatively recent. This not only accurately measures the cutaneous thickness (to hundredths of a millimeter) that is increased at the psoriatic patches, but also evaluates the structure of the skin  and to define some feature parameters of the disease, correlating them to the clinical data and quantifying their variation during therapy.  
As regards morphological ultrasound studies, three bands of different echogeneity are defined in the psoriatic patches: a hyperechogenic band, corresponding to the thickened corneum and to the superficial epidermis; a hypoechogenic band, made up of the thin epidermal ridges and the edematous papillary dermis with congested vessels, more evident in the acute phases; and finally, a hyperechogenic band, corresponding to the thickened and inhomogeneous reticular dermis, not always clearly differentiable from the epidermis (Fig. 7).  
The restrained area between the cutaneous scaling may produce thin parallel acoustic shadows; the very keratinized patches may demonstrate a marked absorption of the US beam, making visualization of the dermis and hypoderma impossible.           
Naturally, the first sonographic evaluation of the psoriatic patch is performed before the start of therapy; subsequent checks, generally carried out weekly, demonstrate a progressive reduction in cutaneous thickness and particularly in the sub-epidermal hypoechogenic band until its disappearance. The reticular dermis and the entrance echo return to their normal thickness and the posterior shadows, due to the superficial microbullae of air, disappear. 

Diagnostic definition of skin  
and subcutaneous lesions      
     
This is the least frequent indication for high-resolution ultrasonography, since, as was mentioned at the beginning, diagnosis in Dermatology is based on the clinical examination and histological tests. On the other hand, semeiotic ultrasonography of the skin or subcutaneous disorder, above all for tumoral types, will only in a restricted number of cases provide pathognomonic pictures that are specific for one form or another.  
In fact, the US aspect of neoplastic lesions is generally one of nodular formations with a hypoechogenic pattern. The differential diagnosis to sort out the benign and malignant forms is based on: assessing the edges (sharp and regular in the former, faded and irregular in the latter); the homogeneous or more or less inhomogeneous aspect, respectively, of the echo pattern, with the possible presence of necrotic areas; and the color-power Doppler vascular pattern.   
Lipoma is a tumor that presents a rather typical US pattern, made up of a “fasciolated” aspect to the hypoechogenic structure, with multiple hyperechogenic striae (Fig. 8).  
The principal indication for US in the lipomas is constituted by the definition of their supra- or sub-fascial location for the scheduling of the surgery. 
The sonographic aspect of pilomatricoma is also somewhat specific because of the presence of calcific hyperechogenic areas, more or less numerous and coarse (Fig. 9). 
In the angiomas, the color-power Doppler findings are rather characteristic. These demonstrate the presence of vascular signals in the hypo-anechogenic areas, more or less patent, in the context of the lesion (Fig. 10). Variation in these findings over time is the basis for the sonographic follow-up of the angiomas, that tend to present, in young patients growing up, a reduction in both the thickness and the vascular areas. 

The future developments of the ultrasonographic method in Dermatology are linked to various factors, the most important being: 
1) the availability of high-frequency probes (20 MHz and above), equipped with color-power Doppler; 
2) 3-Dimensional reconstruction in parallel with the scans, through computed methods for an accurate definition of the morphology and volume of the lesions; 
3) the employment of dedicated equipment endowed with the so-called “acoustic microscope”, capable of analyzing the retrodiffused echoes from a “real time” probe that functions in a frequency range between 40 and 100 MHz with high axial and lateral resolution and, consequently, an accurate definition of margins and thence of the thickness of the lesions, even very thin ones; 
4) the use of ultrasound contrast materials for a more precise tissue characterization. 

Teresa Cammarota 
Primario Radiologo 
Ospedale Le Molinette - Torino