pattern is shown in the pneumatized roof of the right orbit (1) where the thin wall appears as a black line between the hyperintense mucus (supraorbital cell) and the fat tissue within the orbit. Although CT clearly shows the wall of an ethmoidal cell (2), the presence of mucus on both sides permits a similar appearance on MRI. Similarly, the left vertical lamella of the cribriform plate is better delineated by CT (3). However, the hyperintensity of CSF (on the intracranial side) and mucus (nasal side) permit the identification of the lamella. On the contrary, the left lamina papyracea, bordered only by air on the ethmoidal side, cannot be seen by MRI. However, when the mineral content of the lamella has been reabsorbed, it becomes undetectable by CT. MRI may show this on condition that it is bordered on both sides by signal, as it appears for the medial orbital wall (5) and the vertical lamella of the right middle turbinate (three parallel arrows). The lamella is displaced laterally by the neoplasm (T). A right infraorbital ethmoid cell is indicated (6).
Although anterior craniofacial resections still have a role in the treatment of some advanced malignant neoplasms, in the last 2 decades the indications for transnasal endoscopic surgery (TES) have been considerably expanded to include not only benign neoplasms, but also selected cases of malignant tumors extending through the ASB [5, 6], as well as neoplasms arising from the ASB itself. These advances have been made possible by a combination of three factors: increasing expertise in endoscopic procedures, developments in endoscopic technology, and improvements in diagnostic imaging techniques.
Imaging Techniques
Assessing the Local Tumor Extent
ASB tumors can be divided into three groups according to their site of origin: (1) sinonasal neoplasms involving or extending through the anterior cranial base; (2) neoplasms which arise from the bony framework of the base itself; (3) neoplasms originating from adjacent intracranial structures. Consequently, grading the extent of neoplasms with regards to bone structures is a first step in the imaging strategy. Two types of bony structures have to be properly imaged: thin laminae, such as the ethmoid cell walls or cribriform plate, and thicker bones, like the pterygoid process or the hard palate. High-resolution CT is more sensitive than MRI in grading changes of the thin laminae caused by neoplastic involvement, namely displacement, thinning, and erosion (Fig. 1). However, when these laminae act as shell-interfaces between a sinus cavity and surrounding structures, as does the cribriform plate separating the nasal from the intracranial cavity, not only is it useful to know whether the tumor is confined or transgresses the membranes covering the bone (meninges, periosteum), but also how far from the area of skull base invasion the dural changes extend (Fig. 2). Extensive infiltration of the dura mater over the orbital roof requires combining an endoscopic with an external approach [5]. On these two points, high-resolution MRI is more precise than CT, on condition that an appropriate technical MR strategy is used. This entails choosing MR sequences that maximize contrast resolution among tissues and selecting the proper orientation and thickness of the planes of the section. Therefore, coronal and sagittal planes with a thin slice thickness are recommended. While 2D turbo spin echo (TSE) can obtain very thin slices (up to 2 mm), a greater detailed spatial resolution (below 1 mm) is achieved if isotropic 3D sequences are used. These sequences offer a further advantage that is common to all isotropic 3D sequences: the extraction of additional planes of section from the volume examined. Hence, postcontrast spoiled 3D gradient echo T1-weighted (T1W) sequences (volumetric interpolated breath-hold examination [VIBE], T1W high-resolution isotropic volume examination [THRIVE], liver acquisition with volume acceleration [LAVA]) are recommended to detect subtle changes in thickness and enhancement of the dura mater (Fig. 2) [7, 8].
Fig. 2. Intestinal-type adenocarcinoma. 3D isotropic GE sequence postcontrast administration. The original volume was directly acquired in the coronal plane (b) with a slice thickness of 0.6 mm. The sagittal plane (a) was obtained via multiplanar reconstruction (same spatial resolution). a In the sagittal plane, three vectors-of-growth are outlined: an anterior vector leading to the blockage of the frontal sinus (1, fs); a vertical vector (2) showing the intracranial extent associated with asymmetric thickening of the dura (d); a posterior vector (3) causing remodeling of the planum sphenoidale and blockage of the sphenoid sinus (ss). pg, pituitary gland. b In the coronal plane, the vertical vector of growth (2) is larger on the right, where changes of the dura extend over the medial half roof of the right orbit (arrows). The growth in the transverse plane leads the neoplasm to contact both medial orbital walls: on the right side causing flattening of the medial rectus muscle and superior oblique muscle (4a, f) and invasion of the inferior rectus muscle (4b, irm). On the left side, the neoplasm penetrates the orbit (5) and displaces the superior oblique (som, partially surrounded by tumor tissue) and the medial rectus muscles. on, optic nerves; mrm, medial rectus muscle; ms, maxillary sinus.
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).
