13.1 Introduction

Ultra-high frequency ultrasound (UHFUS) is a recently introduced ultrasonographic technique characterized by the use of ultrasound frequencies between 30 and 100 MHz, yielding improved spatial resolution at the expense of a shallower depth of penetration.

High-frequency techniques include UHFUS, High-frequency ultrasound (HFUS), and ultrasound biomicroscopy (UBM), which were originally introduced in the mid-90s in the preclinical setting for the study of animal models. In particular, preclinical applications have expanded in the last years and currently include the evaluation of developmental changes during embryogenesis, cardiac and brain development, tumor growth patterns and spread of metastases, and pharmacokinetics and pharmacodynamics of antineoplastic therapies in experimental murine models [1, 2].

For what concerns clinical setting, ultra-high frequencies find application in several medical fields, including vascular, musculoskeletal, small parts, and dermatological evaluation [3–6].

In particular, vascular applications include the study of radial artery, venous valves, peripheral vascularization, blood flow characteristics, arterial intima/media thickness, stiffness, and risk assessment of atherosclerosis. Musculoskeletal evaluation includes hand anatomy and planning of hand surgery, superficial joints, tendons and pulleys, medial menisci, carpal and tarsal tunnel, while small parts evaluation is mostly focused on nerves and lymph nodes. In dermatology, ultra-high frequencies are employed for the study and differentiation of cutaneous lesions (including melanoma), skin layers, hair follicles, and identification of foreign bodies [7].

Oral medicine is a recently introduced field of application, due to the possibility of ultra-high frequencies to provide high-resolution images of superficial structures [8–10].

13.2 Principles of UHFUS Imaging

Image generation in ultrasonography is characterized by the production of ultrasound waves and registration of reflected echoes after attenuation related to the passage of the waves inside different tissues. The return echoes are then converted into electric signals, and the signals into images.

Ultrasound wave frequency and depth of penetration are inversely proportional: the higher the frequency, the shallower the depth of penetration. In this sense, the use of ultra-high frequencies allows to image the superficial layers from the point of application of the probe, but on the other hand it allows to obtain micron-sized resolution.

Vevo MD (VisualSonics) is the first UHFUS equipment available for clinical use (Fig. 13.1). For the study of the oral cavity, 48 MHz and 70 MHz frequencies appear suitable to obtain high-resolution imaging of the superficial layers of the oral mucosa. In Table 13.1, the characteristics of 48 MHz and 70 MHz UHFUS probes are summarized.

Table 13.1

Summary of technical characteristics of 48 MHz and 70 MHz UHFUS probes

Probe frequency	70 MHz	48 MHz
Center transmit	50 MHz	30 MHz
Bandwidth	29–71 MHz	20–46 MHz
Axial resolution	30 μm	50 μm
Lateral resolution	65 μm	110 μm
Maximum depth	10.0 mm	23.5 mm
Image width (max)	9.7 mm	15.4 mm
Image depth (max)	10.0 mm	23.5 mm
Focal depth	5.0 mm	9.0 mm

Considering ultrasound physics, axial resolution is determined by the bandwidth of the pulse, while lateral resolution is the result of the ratio between focal distance and spatial dimension of the transducer multiplied for the wavelength. In this sense, 70 MHz frequencies provide 30 μm axial resolution and 65 μm lateral resolution, at the expense of an increase in attenuation in tissues, with a depth of penetration limited to 10.0 mm.

Ultra-high frequencies appear suitable when small anatomy is investigated. In general, if a region or a lesion to be scanned is <2 cm, 70 MHz can provide adequate detail and imaging of the lesion as a whole. If the area to be scanned is >2 cm, reduction in scanning frequencies is required, with 48 MHz probe allowing to image all the lesion with one scan. However, it is often advisable to perform a double scan regardless of lesions’ dimensions, as the two frequencies in most cases are complementary in diagnosis performance.

13.3 Components of Image Production

13.3.1 Performance of Intraoral UHFUS Scan

Intraoral UHFUS scan is generally performed with the patient in supine position, providing head stabilization with a headrest. In some cases, UHFUS performance can be chair-side to the dental chair thus improving patient positioning.

13.3.2 Setting Image Acquisition Parameters

During the UHFUS examination, several parameters may be adjusted to improve image quality. Image depth depends on the transducer applied, and it corresponds to the total acquired depth of the non-zoomed image.

Gain control may be varied to adjust the visual intensity of the returning signal. In particular, an increase in gain brightens the image, while a decrease leads to darkening. In B-mode images, variations in Time Gain Compensation (TGC) adjust the ultrasound signal to compensate for minor attenuation for the signal returning from deeper situated tissues.

13.3.3 Strengths and Limitations

13.3.3.1 Size and Cost

The main drawback of the use of UHFUS systems is the lack of dedicated probes for intraoral access, which can hinder full mouth examination in patients with limited mouth opening. Acquiring a UHFUS system is relatively costly compared to conventional US systems, although the benefits in terms of image quality and the opportunities for oral mucosa exploration are much higher compared to conventional US equipment.

13.3.3.2 Fast Acquisition

UHFUS acquisition is fast, due to relatively easy accessibility to the oral cavity. The possibility to perform real-time measurements and evaluation of blood flow make the technique suitable for preoperative investigations. Moreover, the repeatability of UHFUS scan makes this technique a valuable tool for performing follow-up after surgical procedures involving the oral mucosa.

13.3.3.3 Submillimeter Resolution

It is known that the higher the ultrasound frequencies, the lower the depth of penetration of the ultrasound wave. However, imaging of the shallower layers of the oral mucosa is also accompanied by higher spatial resolution, which in cases of 70 MHz frequencies can reach up to 30 μm. When dealing with small anatomy as one of the oral cavity, it is of utmost importance to obtain high-resolution images, which could support diagnosis performance. Moreover, oral lesions are in most cases confined to a maximum of 2 cm beneath the surface thus making UHFUS systems ideal for the study of the majority of oral diseases. Diagnostic imaging is going towards an increasing interest in micron-sized anatomy, and in this sense UHFUS reflects the need for submillimeter imaging of small anatomical parts. The detailed evaluation of oral mucosa therefore represents one of the most promising innovations in this field.

13.4 Normal Anatomy

13.4.1 Buccal Mucosa

UHFUS structure of buccal mucosa is characterized by a thin homogeneously hypoechoic layer corresponding to the mucosa. The submucosa appears as a well-defined, hyperechoic stratum below the mucosa. In the submucosal layer, it is possible to observe vascular structures appearing as roundish hypoechoic structures. In this region, the most important anatomical structure is represented by the facial artery, which on longitudinal scan appears as a linear structure with hypoechoic margins, and is characterized by pulsation and intense blood flow. Doppler mode shows mild vascularity, while intense blood flow is detected in correspondence with the facial artery. (Figs. 13.2 and 13.3).