Radiological Diagnosis of DCSFs

C O N T E N T S

7.1.2Doppler and Carotid Duplex Sonography 106

7.2Intra-arterial Digital Subtraction Angiography (DSA) 109

7.2.1Introduction 109

7.2.2Technique 109

7.2.3 Angiographic Protocol for DCSFs 110

7.2.4Angiographic Anatomy of the Cavernous Sinus 111

7.2.5Flat Detector Technology in Neuroangiography 139

7.2.6Rotational Angiography and 3D-DSA 139

7.2.6.3Image Post-Processing 146

7.2.6.43D Studies of the Cavernous Sinus Region 148

References 184

7.1

Non-invasive Imaging Techniques

7.1.1

CT and MRI (Figs. 7.1–7.10)

The clinical presentation of patients with characteristic neuroophthalmic symptoms does not usually require invasive imaging techniques, and a correct diagnosis can be made with certainty using CT or MRI techniques. As outlined in the previous chapter, non-invasive imaging may fail in patients with low-flow AV shunts, and a patient with atypical symptoms may be misdiagnosed over time. If there is a clinical suspicion of an inflammatory or tumorous process in the orbital or peri-orbital region, computed tomography (CT) or magnetic resonance imaging (MRI) is commonly performed. In the case of a DCSF, the image may show a dilated or thrombosed vein, indicating an underlying vascular pathology. An exophthalmos or a proptosis can often be diagnosed using a routine CT scan (Figs. 7.1 and 7.2). However, only when the AV shunting volume is large enough will the CS become visible as an enhancing space occupying lesion.

Ohtsuka and Hashimoto (1999) performed serial enhanced computed tomography (DE-CT) scanning of the cavernous sinus and provided direct evidence of pathological shunting from the carotid artery to the cavernous sinus. By scanning serial axial images around the sella turcica at intervals of 3 s the authors were able to obtain an early filling of the CS in direct (two) as well as in indirect (five) fistulas. This technique was also found to be useful in revealing a CS thrombosis by non-filling even in late venous phases. A differentiation between Types A–B and C fistulas was possible, based on the delayed staining of the CS when only ECA branches supplied the fistula.

A focal bulging or diffuse distension of the CS on contrast enhanced CTs has been detected in 50%– 64% of the cases (Ahmadi et al. 1983; Uchino et al. 1992). The SOV can be enlarged on the fistula site in 86%–100% on post-contrast CTs and in 75%–100% on spin echo MR images (Ahmadi et al. 1983; Uchino et al. 1992; Hirabuki et al. 1992; Komiyama et al. 1990; Elster et al. 1991).

If a cortical venous drainage is present, those enlarged veins may become visible on axial CT. Watanabe et al. (1990) described a case of a Type D fistula with considerably enlarged veins visualized on contrast enhanced CT. In addition, a SPECT was performed showing reduced regional cerebral perfusion in this area, caused by elevated transvenous pressure. D’ Angelo et al. (1988) described a case with a Type A fistula and dilated veins in the temporal region, which were identified as Sylvian veins. Teng et al. (1991) reported a brain stem edema in two patients with Type D fistulas that became visible on CT after occlusion of the normal venous drainage, probably triggering the development of cortical venous drainage. In both cases complete disappearance of the edema was documented. Uchini et al. (1997) reported two patients with pontine venous congestion due to DCSFs. Brain stem edema due to DCSFs has also been described by Takahashi et al. (1999) in two patients, in which after occlusion of the fistula, reversal of the edema on MRI became evident. Edematous changes due to venous hypertension can also be seen in both hemispheres being mainly limited to the white matter, like a vasogenic edema (Cornelius 1997).

When an intracranial hemorrhage occurs as a complication of a CCF, CT is the preferred diagnostic tool (D‘Angelo et al. 1988). This type of complication is mainly seen in Type A fistulas, but only rarely observed in DCSFs (Satoh et al. 2001), even though cortical venous drainage is relatively frequently present (see Table 5.3).

In most cases the bleeding occurs adjacent to the dilated veins. MRI is superior to CT in showing discrete changes due to AV shunts of the CS, because it may demonstrate not only the enlarged SOV but also a minimal proptosis and extraocular thickening of the muscles. Sato et al. (1997) recently reported on flow voids shown in eight of 10 patients with Type D fistulas using spin echo sequences. In particular MRA can be helpful in low-flow conditions, as often present in DCSFs. MRI findings can be discrete on MIPs and raw data (source images) can be useful for analysis (Cornelius 1997; Acierno et al. 1995;

Chen JC et al. 1992; Dietz et al. 1994) (Figs. 7.3, 7.6 and 7.8).

MRA, as well as conventional spin echo sequences, can demonstrate abnormal cortical drainage (Chen JC et al. 1992), as seen in Figure 7.5b and 7.8a. Under certain circumstances MRA is particularly helpful for early detection of CCF with posterior drainage (Fig. 7.8), also called “white eyed shunt syndrome” (Acierno et al. 1995). These patients present with headaches and painful oculoparesis but without orbito-ocular congestion, which may easily lead to a wrong diagnosis of, e.g. Tolosa Hunt syndrome or painful ophthalmoplegia. Conventional spin echo sequences can be normal in these patients; however, MRA would be diagnostic in showing abnormal vessels arising from the posterior CS (Figs. 7.5 and 7.6). The CS itself can become visible even due to normal venous flow causing flow voids in TOF-MRAs as has been observed by Cornelius (1997) in eight of 50 patients. Several authors have described flow voids within the CS on spin echo MR images in patients with CSFs (Uchino et al. 1992; Hirabuki et al. 1992; Komiyama et al. 1990). Hirai et al. (1998) saw flow voids in the CS less frequently in their patients and found in 3% false positive results, emphasizing the difficulty to differentiate normal venous flow in the CS from abnormal flow voids caused by an AV shunt based on spin echo MR images.

Another MR finding is the “flow-void” of the intercavernous sinus in contrast enhanced T1-weighted images (Figs. 7.3a, 7.4a, 7.6a, 7.7a). Sergott et al. (1987) recommended MRI as the initial exam in patients with known arteriovenous shunts and clinical deterioration. The authors observed three patients with paradoxical worsening of clinical symptoms due to thrombosis of the SOV, which was revealed by increased signal intensity in T1-weighted images. According to Cornelius (1997), intra-arterial DSA is not required in patients with clinical worsening if MRI shows a thrombosed SOV and an improvement under conservative management. On the contrary, Goldberg et al. (1996) observed that MRI led to a diagnosis in only 5 of 10 patients (50%). Schuknecht et al. (1998) were able to show that high-resolution contrast enhanced CT and MRI exam can demonstrate not only thrombi within the CS, but also in its tributaries such as the SOV, the SPPS and even the IPS.

MRI may help to differentiate a CSF from malignant tumors, vasculitic processes and intracranial AV malformations. If thrombosis occurs, the signal appears as a white hyperdense spot on the T1-

weighted image (de Keizer 2003). The visualization of draining veins may require using both phase contrast techniques (3D PC MRA) for demonstrating the dilated SOV and associated reflux, and 3D TOF MRA for demonstrating the IPS (Ikawa et al. 1996). If the SOV is not the draining vein it may not

be demonstrated with 3D PC MRA. The IPS is usually shown better on TOF MRA, because it runs in a superior inferior direction causing stronger time of flight effects.

Hirai et al. (1998) have compared the value of fast imaging with steady state precession (FISP) to

contrast enhanced CT and spin echo MR imaging and found it superior in the diagnosis of CCF. Their group of 17 patients included 14 DCSFs in which a hyperintensity of the CS was noted in most cases (11/14). In DCSFs with very slow flow this hyperintensity can be missed, leading to false negative

results and necessitating a careful search for other findings related to the venous drainage. In highly vascularized DCSFs, multiple hyperintensive curvilinear structures or spots adjacent to or within the CS were seen, likely corresponding to dural feeders. Because these findings were not observed in direct

CCFs, they may help to differentiate direct from indirect fistulas. 3D FISP images showed posterior venous drainage, but were not helpful in detecting cortical drainage, which is an important detail not to be missed.

Because relying on MRI and 3D TOF MIP images alone may lead to underdiagnosis of indirect CSFs, MRA source images become quite valuable (Figs. 7.3, 7.6, 7.8 and 7.9). Tsai YF et al. (2004) recently encountered dilemmas in reviewing MRI flow voids and identified them in only five of eight patients with DCSFs. The authors detected an engorged CS only in four cases and swollen extraocular muscles in none. Flow artifacts resulting from pulsation of the cavernous ICA may corrupt CS details and CSF pulsation may result in flow voids in the prepontine cistern mimicking enlarged abnormal vessels. Air in the sphenoid sinus may cause susceptibility artifacts or partial volume effects. Because MIP reconstruction may cause vascular distortion they need to be reviewed carefully. Reliance on MIPs alone may lead to misdiagnosis in 50% of the cases. On the other hand, reading of the source images of 3D-TOF MRA allowed the correct diagnosis in all eight cases. Therefore, in order not to overlook small AV shunts, careful evaluating of MRA source images should be included in all

doubtful cases. Nevertheless, even with improved technology, MRI and MRA cannot replace high quality DSA for differentiating direct Type A and indirect Type D fistulas with certainty (Tsai YF et al. 2004).

Diagnostic sensitivity of MRI can be enhanced with contrast and magnetization prepared rapid gradient echo sequences (MP-RAGE), allowing for better assessment of retrograde venous drainage than T1-weighted SE imaging (Kitajima et al. 2005). Kwon et al. (2005b) found direct fistula visualization in 75%–86% of the 27 DAVF (11 DCSFs), although the reviewers were not blinded to angiography in this study. The authors suggest looking for any suspicious flow void cluster around a dural sinus.

Chen CC et al. (2005) have recently compared the utility of CTA and MRA source images in the diagnostic of 53 direct CCFs. They found CTA as useful as DSA and superior to MRA in accurately localizing the fistulous connection, in particular when it was located in segment four according to Debruns classification (Debrun et al. 1981).

Finally, CT and MRI can be used to rule out complications of EVT, for documentation of coil masses or liquid embolic agents within the CS, or to detect residual/recurrent AV shunting (Figs. 7.9 and 7.10).