A similar technical strategy can be adopted when the target is the analysis of intracranial structures that are surrounded by cerebrospinal fluid (CSF), like the olfactory bulb or optic nerve [9, 10]. In this setting, T2-weighted (T2W) isotropic 3D sequences not only provide high spatial resolution, but they also maximize the difference in signal intensity between low-intense intracranial nerves and the high-intense CSF. Isotropic 3D sequences such as balanced steady-state gradient echo (true-fast imaging with steady precession [FISP]; constructive interference in the steady state [CISS]/fast imaging employing steady state acquisition – constructive interference [FIESTA-C]) or T2W fast spin echo sequences (sampling perfection with application optimized contrasts using different flip angle evolution [SPACE], volume isotropic TSE acquisition [VISTA], CUBE) can be used.
A noteworthy and specific property of true-FISP sequences like CISS is that the signal intensity depends not only on the T2 relaxation time but also on the T2/T1 ratio between the relaxation times of the tissues [11]. In fluids (i.e., CSF) with long T2 and short T1 relaxation times the ratio is high, hence CSF shows a high signal intensity. Conversely, in solid tissues like brain and in most tumors, the T2/T1 ratio is very low. Therefore, both brain and a solid tumor will show a similar low signal, making it rather impossible to distinguish between the two. However, in highly vascularized tumors (such as meningioma or juvenile angiofibroma) the administration of a paramagnetic contrast agent may significantly increase the tumor signal, resulting in a better discrimination versus brain parenchyma or cranial nerves [12, 13]. This is caused by the effect of gadolinium ions, which flow, mixed with blood, within tumor vessels and pass into the extracellular space. Their paramagnetic effect induces an additional shortening of the T1 relaxation time – the T2/T1 ratio greatly increases, resulting in a highlighted signal of the tumor compared to the brain. This strategy is also effective to analyze complex structures like the cavernous sinus. In postcontrast CISS sequences, the venous spaces, filled with enhanced blood, turn bright, permitting better delineation of the low-intense normal cranial nerves running inside (Fig. 3).
Fig. 3. Squamous cell carcinoma. 3D isotropic balanced steady-state gradient echo (CISS) after administration of contrast agent, originally acquired in the axial plane (a) and reconstructed in the coronal plane (b). The slice thickness is 0.55 mm. a The nasoethmoid neoplasm (T) posteriorly invades the right Meckel’s cave (1, mc), replaced by tumor signal; 2 and 3 represent the right and left sphenoid sinuses, respectively. The right internal carotid artery (ica) is surrounded by tumor. Within the prepontine cistern both abducens nerves are imaged, close to the Dorello’s canal (VIn). ba, basilar artery. b In the coronal reconstruction, the tumor invades of the right cavernous sinus (T, dotted curved arrows). The tumor replaces the enhanced venous signal surrounding the right abducens nerve (VIn) and right intracavernous internal carotid artery (ica). On the left side, the normal abducens nerve and intra-extra-cavernous segments of the internal carotid artery are detected. mc, left Meckel’s cave; V3, mandibular nerve at foramen ovale; ch, chiasm. The asterisk (*) indicates the pituitary gland.
An additional group of critical structures that require proper high-resolution imaging is represented by the arteries running close to the ventral skull base, either below, through, or strictly above [14]. The anterior and posterior ethmoid arteries are among those running below and through the ASB. Unintentional damage during surgery can cause bleeding, and if the artery retracts into the orbit, a retrobulbar hematoma occurs. Both arteries originate from the ophthalmic artery inside the orbit. The position of their canals entering the ethmoid has a wide variability, which is more frequent for the anterior ethmoid artery, entering the ethmoid via the frontoethmoidal suture. In addition, its canal is very often dehiscent. Although the ethmoid artery canals can be identified in most patients using high-resolution CT or cone beam CT [15], the direct demonstration of the arteries running inside requires CT angiography (CTA). A similar result can be achieved by high-resolution 3D radiant echo T1W sequences (VIBE, THRIVE, LAVA). When obtained with the saturation of the fat, these MR sequences provide greater contrast resolution than CT, resulting in more precise delineation of the optic nerve, ophthalmic artery, and those extrinsic orbital muscles in close relationship with the anterior ethmoidal artery (superior oblique and medial rectus; Fig. 4).
Fig. 4. Intestinal-type adenocarcinoma. a 3D isotropic GE sequence after contrast administration. b CT after contrast agent administration. The neoplasm (T) arises from the anterior left ethmoid and contacts the nasolacrimal duct. The ophthalmic arteries (oa), optic nerves (on) and a right carotid artery aneurysm (a) are well depicted with both techniques. cgl, common ground lamella.
The vascular constraints become more complex when moving posteriorly, at the level of the planum sphenoidale and adjacent sphenoid sinus walls. This is the area where inadvertent carotid artery injuries during endoscopic skull base surgery can occur. The use of state-of-the-art CTA or MR angiography has been advocated in the delineation of the course of the internal carotid artery at this level. CT angiography is also indicated for detailing the course of the intracranial arteries running close to the ASB floor. Among these are the proximal branches of the anterior cerebral artery, like the orbitofrontal and frontopolar arteries which project toward the olfactory fossa, the anterior cerebral artery itself (A2 segment), and the anterior communicating artery.
Assessing the Regional and Distant Neoplastic Extent
The risk of lymphatic metastasis in sinonasal malignancies depends on the site, extent of tumor spread, and histology (Fig. 5). As the sinonasal tract is thought to have limited capillary lymphatics [16], the incidence of regional metastasis is low, ranging between 4 and 15% [17, 18]. When the neoplasm is confined within a sinus cavity, nodal metastasis is more frequent in tumors arising from the maxillary sinus than from the ethmoid. A greater incidence of regional involvement, up to 50%, is reported when lymphatic-rich areas adjacent to the sinuses are invaded, like the masticatory space or the skin. Among the different histologies, nodal metastases are more frequently observed in olfactory neuroblastoma (ONB; up to 43%), mucosal malignant melanoma, and squamous cell carcinoma [19, 20].