Magnetic Resonance Imaging

There have been a few reports of MRI investigations of subungual tumours, particularly glomus tumours. In practice the perionychium may be routinely imaged by MRI with the ability to obtain high spatial resolution images with small surface coils dedicated to wrist or finger (Figure 11.3). A voxel height close to 100 ^m, about the thickness of the epithelial layer of the nail bed, is necessary. Nevertheless, unlike the skin, which is a superficial structure, the nail unit may require evaluation of the deep layers of the nail bed, or even of the pulp when the tumour extends under the lateral interosseous ligament.

The thumbs and the great toes may be large, and it may be necessary to keep a sufficient

Mri Phase Array Coil

Figure 11.3

Phased array surface coil dedicated to wrist imaging.

Figure 11.3

Phased array surface coil dedicated to wrist imaging.

signal-to-noise ratio about 2 cm or more from the surface coil. When using a plane circular surface coil, the nail plate must be placed against the coil to offer the maximum signal close to the superficial layers of the nail unit. The hand is placed above the head in a supine or prone position with the coil fixed on the centre of the gantry. Full cooperation of the patient and efficient mechanical support with adhesive bandages are necessary. Some patients with painful shoulders (rotator cuff tears, multiple tendon calcifications) or frozen shoulder cannot maintain this position during the entire examination. For study of the toes, the position is more comfortable: the patient lies supine with the feet in the gantry. In all cases perfect immobility of the distal phalanx is necessary to avoid movement artefacts, which are particularly disturbing with high spatial resolution. For this reason, children younger than 6 years should not be examined in this manner. Routine examination includes axial ^-weighted spin echo images (Figure 11.4a) and axial fast short-time inversion recovery (STIR) images (Figure 11.4b), completed with sagittal T1 or T2 images (Figure 11.4c). The slice thickness (usually 3 mm) remains large compared with the size of the nail unit. Three-dimensional gradient echo images are acquired when 1 mm thick contiguous slices are necessary (Figure 11.5). Coronal slices are not acquired routinely. These slices are disappointing, and are not adapted to the spatial structure of the perionychium; they are reserved for distal phalanx abnormalities (Figure 11.6). Its different elements are tangential to the frontal plane and therefore exposed to the partial volume artefact. Intravenous injection of 0.1 mmol/kg gadolinium is administered according to the suspected pathological condition. Multiple MR angiography sequences may be acquired at arterial and venous phases (Figure 11.2c).

Onychomatricoma Mri

Figure 11.4

Onychomatricoma. (a) Axial 7^-weighted magnetic resonance slice shows the tumor seated between the dorsal and ventral nail matrix (arrows). (b) Axial short-time inversion recovery (STIR) slice depicts the high signal of the filamentous expansions inside the nail plate (star). (c) Sagittal 7^-weighted image shows the tumor core (arrowheads) in the nail root and its distal expansions (arrows).

Figure 11.4

Onychomatricoma. (a) Axial 7^-weighted magnetic resonance slice shows the tumor seated between the dorsal and ventral nail matrix (arrows). (b) Axial short-time inversion recovery (STIR) slice depicts the high signal of the filamentous expansions inside the nail plate (star). (c) Sagittal 7^-weighted image shows the tumor core (arrowheads) in the nail root and its distal expansions (arrows).

Onychomatricoma Mri

Figure 11.5

Onychomatricoma. Axial 1 mm thick three-dimensional gradient echo image highlights the filamentous expansions of the tumour.

Figure 11.5

Onychomatricoma. Axial 1 mm thick three-dimensional gradient echo image highlights the filamentous expansions of the tumour.

Onychomatricoma

Figure 11.6

Osteoid osteoma of the distal phalanx. Axial post-gadolinium T1-weighted coronal image depicts a bone oedema (arrowheads) of the tuft Note the lateral Flint's ligaments (arrows).

Figure 11.6

Osteoid osteoma of the distal phalanx. Axial post-gadolinium T1-weighted coronal image depicts a bone oedema (arrowheads) of the tuft Note the lateral Flint's ligaments (arrows).

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