Ultrasonography

Ultrasonography should be more widely used in imaging the nail unit. However, its dependence on operator skills and the moderate experience of radiologists in this anatomical region are limiting factors. High-frequency (7.5-20 MHz) probes dedicated to musculoskeletal or skin imaging are suitable. An inter-position material is necessary for the study of the most superficial structures. The nail bed presents a somewhat homogeneous hypoechoic appearance, separating the high-intensity echoes of the dorsal cortex of the phalanx from those of the nail plate (Figure 11.1a). The nail plate may produce two parallel high-signal echoes. Imaging the matrix area is possible with 30 MHz B-mode, but is tricky because of the overlying echoes of the proximal and lateral nail folds. Furthermore, the axial slices are distorted by the convexity of the nail plate (Figure 11.1b). Ultrasonography was proposed as a way of detecting glomus tumours of the finger tips. Tumours less than 3 mm in diameter are hardly visible, but tumours located in the pulp are more accessible. Doppler imaging may reveal the vascular feature of this lesion in some cases (Figure 11.2). Ultrasonography is also appropriate for highlighting radiotransparent foreign bodies, such as splinters. A combined granulomatous reaction may be revealed with imaging.

Very high-resolution ultrasonographic studies are still restricted to the field of research, beginning with skin imaging in 1979. Ultrasonography appeared as an effective and non-invasive method of measurement of the dermis thickness. Both A-mode and B-mode ultrasonographs dedicated to skin imaging have been developed as research and clinical tools, mainly for tumoral and inflammatory diseases. The role of M-mode, high-frequency Doppler imaging and three-dimensional investigations remains experimental. Probes of

20 MHz provide the best compromise between a high spatial resolution and sufficient depth. Probes of 50 MHz or more only allow imaging of the epidermis, with an axial resolu-tion of about 37.5 ^m and a lateral resolution of about 125 ^m. Paradoxically, studies of the nail unit are few. Finlay introduced ultrasonography for the assessment of the thickness of the nail plate with a 20 MHz A-mode probe. The distal conduction speeds (mean 2470 m/s) were well correlated with the measurements of the free edge of the nail plate with a micrometer. The distal reduction about 8.8% of the ultrasound transmission time compared with the proximal measurements should be due to a higher thickness and hydration of the nail plate at the level of the lunula. Jemec also studied the A-mode ultrasound structure of the nail plates of post-mortem thumbs in situ and after resection. The spatial resolution was about 75 ^m with a 20 MHz probe. In contrast to Finlay, he noted compartments of different echo speeds, a superficial dry layer (ultrasound velocity of 3103 m/s) and a deep hydrated layer (ultrasound velocity of 2125

m/s). However, he could not differentiate the different layers of the nail bed. Hirai proposed 30 MHz B-mode ultrasonography to image nail matrix abnormalities in cases of nail plate deformities.

Figure 11.1

Ultrasonography of the nail unit, (a) Sagittal view, (b) Axial view. Nail plate (white arrows), nail bed (star), dorsal cortex of the phalanx (black arrowheads), nail root (black arrow), lateral nail fold (white arrowheads).

Figure 11.1

Ultrasonography of the nail unit, (a) Sagittal view, (b) Axial view. Nail plate (white arrows), nail bed (star), dorsal cortex of the phalanx (black arrowheads), nail root (black arrow), lateral nail fold (white arrowheads).

Figure 11.2 Subungual glomus tumour, (a) Color Doppler scan depicts a hypoechoic mass (large arrows) in the nail bed with vascular flow (arrowheads). Note bone erosion of the dorsal cortex of the distal phalanx (small arrows), (b) Pulsed Doppler with spectral analysis of the tumour, (c) Magnetic resonance angiography shows a highly vascularized mass beneath the nail plate.

Figure 11.2 Subungual glomus tumour, (a) Color Doppler scan depicts a hypoechoic mass (large arrows) in the nail bed with vascular flow (arrowheads). Note bone erosion of the dorsal cortex of the distal phalanx (small arrows), (b) Pulsed Doppler with spectral analysis of the tumour, (c) Magnetic resonance angiography shows a highly vascularized mass beneath the nail plate.

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