Книги по МРТ КТ на английском языке / Magnetic Resonance Imaging in Ischemic Stroke - K Sartor R 252 diger von Kummer Tobias Back

Although this book is primarily devoted to magnetic resonance imaging, a brief description of cranial computed tomographic findings in CVST seems to be reasonable since we have learned from CCT that in imaging of brain parenchyma and cerebral veins we have to differentiate between direct and indirect signs of CVST. The direct signs prove the diagnosis by demonstrating thrombus or missing flow within a dural sinus or pial vein, the indirect signs simply raise the suspicion of CVST by demonstrating different forms of venous congestion (Chiras et al. 1985).

In CCT direct signs include the hyperdense sinus in the non-contrast-enhanced scan, the “cord sign” (hyperdense bridging vein) and the “empty triangle sign” in the contrast-enhanced CCT (Virapongse et al. 1987) (see Fig. 18.10). Within the first 2 weeks, thrombosed blood is typically hyperdense on CCT compared to brain parenchyma. Therefore, it is important to start with a non-enhanced CCT scan in patients with suspected CVST (Thron 2001). The density of the thrombus might otherwise be mistaken for a contrast-enhanced vessel lumen. After 2 weeks the thrombus will have become isodense or

hypodense to the brain parenchyma. Now the diagnosis can be made following the injection of contrast media which will show the thrombus as a filling defect of the lumen surrounded by either residual contrast-enhanced blood, the contrast enhancing meningeal wall or collateral venous channels outside the dura. This constitutes the empty triangle or empty delta sign on contrast enhanced CCT in a later stage of thrombus evolution.

Indirect signs include global and focal brain edema (see Figs. 18.6, 18.8, 18.9), intraparenchymal hemorrhages that might be solitary or multiple (Fig. 18.3) involving both grey and (preferentially) white matter and intense tentorial enhancement. Concerning the edema that is demonstrated as a hypodense area, the form and localization will typically not correspond to the classical arterial territories (see Fig. 18.8). Hemorrhages will also not suit the typical localization of parenchymal hypertensive bleeds but instead typically also expand to the cortical surface. The tentorium and the falx will appear thickened, engorged and will demonstrate pronounced enhancement that is due to dural venous collaterals.

Veno-Occlusive Disorders

Table 18.1. Time dependant MR signal pattern in intraparenchymal hemorrhage and thrombosed dural sinuses. [Modified from Gomori et al. 1985; Isensee et al. 1994)

18.4.1

Magnetic Resonance Imaging and Magnetic Resonance Angiography

The combination of these two MR techniques has become the imaging modality of first choice for the diagnosis and follow-up of CVST (Villringer et al. 1989; Vogel et al. 1994). MRI alone faces the problem that the MR signal of blood and blood products varies with clot age as is shown in Table 18.1 (Gomori et al. 1985). Therefore, a different appearance of a thrombosed vein in T1 and T2 sequences during different stages of thrombus evolution has to be taken into account (Isensee et al. 1994). The very fresh blood clot may be hyperintense in T1and hypointense in T2-weighted images during the first 12 h. In this very early phase it is very unlikely that patients with CVST are symptomatic and undergo diagnostic procedures. Afterwards the oxyhemoglobin of the acute thrombus has changed to deoxyhemoglobin which is isoto slightly hypointense to cortex on T1weighted sequences with a hypointense signal in T2-weighted images (12 h-3 days). Late acute clots (3-7 days) contain intracellular methemoglobin and are hyperintense on T1-weighted sequences and hypointense on T2-weighted images. Subacute thrombi (1-4 weeks after initial thrombosis) are hyperintense on both T1 and T2 scans due to extracellular methemoglobin. Chronically thrombosed sinuses undergo fibrosis with hemosiderin deposition and may develop extensive collaterals. It is clear that the main problem of MRI standard sequences for CVST diagnosis is the iso-hypoin- tense appearance of the acute clot (12-36 h) simulating flow. Problems may also be encountered in chronically thrombosed sinuses (Isensee et al. 1994). During the other phases of clot evolution an abnormal signal within the vessel lumen is evident

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in at least one of the standard sequences. Bearing this consideration in mind, the sinuses are best visualized using axial and coronal sequences in which the superior and inferior sagittal sinus, as well as the transverse sinuses and the internal veins, are well imaged. Using these standard sequences, it is possible to evaluate normal anatomy or to detect anatomic variations like hypoplasia of one of the transverse sinuses.

Direct signs of CVST in MRI:

•Demonstration of an intraluminal thrombus within a dural sinus or a cerebral vein. This is easy during the time interval between 4 days and 4 weeks of thrombus age due to the high signal of methemoglobinonT1-weightedimages(Table 18.1; Fig. 18.3) It may be difficult in cases of a very fresh (< 3 days) or old (> 4 weeks) thrombus which can be organized or partially recanalized (Figs. 18.3, 18.7). Absence of the normal “flow-void” in large veins should raise suspicion of CVST, but this sign is unreliable, as it also appears with slow flow.

•Demonstration of an “empty triangle” or “delta sign” within a sinus, comparable to the finding in CT (see Figs. 18.9, 18.10) following contrastenhancement.

Indirect signs of CVST in MRI:

•Unior bilateral areas of edema that do not correspond to arterial territories (see Figs. 18.6-18.9)

•Unior bilateral hemorrhages (see Figs. 18.3, 18.8).

•Pronounced regional enhancement of the leptomeninges (falx, tentorium, convexity) due to the involvement of these structures in collateral drainage (see Fig. 18.9).

•Regional subarachnoid hemorrhage, especially if it is situated on the convexity of the brain. It may be a sign of the rare isolated cerebral vein thrombosis.

Table 18.2 summarizes the sequences and MR techniques which in our experience can be proposed (as mandatory or optional) in the diagnostic management of veno-occlusive disorders of the brain. Venous MRA can either be performed with the time-of-flight (TOF) or with the phasecontrast (PC) technique. In addition to the tomographic images, a flow sensitive gradient-echo sequence should be obtained if CVST is in question. As a fast screening examination we prefer a TOF 2D FLASH sequence (Table 18.2; Fig. 18.4, see 18.6c), oriented 90 degree to the flow direc-

inf lammatory stenosis (Isensee et al. 1992) of the ipsilateral transverse sinus (Reul et al. 1997), followed by raised intracranial pressure. A cystic developmental lesion within a sinus (Küker et al. 1997), or other space-occupying or infiltrating processes, obstructing the lumen of a dural sinus are rare causes.

In patients with idiopathic intracranial hypertension bilaterally narrowed segments in the lateral venous sinuses have been demonstrated compared to normal findings in asymptomatic volunteers using advanced techniques of MR

venography (Higgins et al. 2002, 2004; Farb et al. 2003). We have observed similar cases in recent years with unilateral sinus stenosis and contralateral hypoplasia or bilateral stenoses (see Fig. 18.2c-e).