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

Fig. 17.10. MRI in thoracic SDAVF, sagittal T2and contrastenhanced T1-weighted images. Multisegmental hyperintensive lesion of the thoracic cord with swelling indicating congestive myelopathy. Flow voids in the dorsal CSF compartment represent the arterialized perimedullary veins (arrow, T2-weighted image, left side), which are clearly visible after intravenous contrast application (arrow, T1-weighted image, right side).

Fig. 17.11. SDAVF supplied from the left 1st lumbar segmental artery, T2-weighted sagittal and axial MR images. In this case, the perimedullary arterialized veins are not very impressive (arrowhead). Note the peripheral hypointensive rim adjacent to the cord hyperintensity, a typical finding in SDAVF (arrows)

This peripheral spinal cord hypointensity was first described by Hurst and Grossman in 2000 and is probably best explained by slow flow of blood containing deoxyhemoglobin related to the venous hypertension (Fig. 17.11). Abnormally enlarged and tortuous perimedullary veins can show flow voids in spin-echo sequences or enhancement after paramagnetic contrast administration (Terwey et al. 1989). Pathologic perimedullary veins are not always detectable in MRI and in some cases difficult to differentiate from CSF pulsations. If contrast enhancement of the spinal cord lesion is present, the cord lesion may be misdiagnosed as tumor.

Follow-up MRI after treatment shows regression of the hyperintense lesion and may be followed by atrophy. If swelling of the cord and/or the pathologic perimedullary vessels persist after treatment, a residual SDAVF needs to be excluded by DSA.

Spinal DSA is still required to demonstrate the fistula and the supplying segmental artery. Recent developments in MR angiography will help to define non-invasively the level of the fistula (Farb et al.

2002). In most patients the dural fistula is supplied by a thoracic or lumbar artery. Deep located fistulas of the lumbosacral region are rare and beset with particular diagnostic problems mainly due to the unusual anatomic and hemodynamic conditions. In these cases MRI should be focused on the lumbosacral region. The arterialized dilated vein of the filum can be detected in 3-mm slices after paramagnetic contrast administration even if the flow is very slow.

17.4.3.2

Arteriovenous Malformations

Spinal AVMs can be mainly divided into AVM of the perimedullary fistula type and glomerular type. Typically, congestive myelopathy is obvious in AVM of the perimedullary type (Heros et al. 1986; Rosenblum et al. 1987; Thron et al. 2001).

AVMs of the perimedullary fistula type are direct AV shunts that are located on the ventral or dorsal surface of the spinal cord or the conus medullaris, usually in the thoracolumbar area, occasionally thoracic, and rarely cervical. Their location thus is intradural, intraor extramedullary. They are always supplied by spinal cord vessels, either by the anterior spinal artery (ventrally) or by a posterolateral artery (dorsally), depending on their location. They drain into spinal cord veins (Fig. 17.12). Drainage may even ascend up to the foramen magnum or into the posterior fossa.

AVMs of the glomerular type are more frequent and characterized by a nidus similar to those of most cerebral AVMs. They may be located superficially on the surface of the spinal cord or deep within the cord parenchyma or extend to both compartments. Due to the numerous anastomoses between the spinal cord arteries, the nidus is always supplied by several arteries or branches derived from the anterior

or posterior spinal arteries. Spinal AVMs drain into the spinal cord veins and are rarely complicated by congestive myelopathy.

The principal screening method is MRI (Dormont et al. 1988; Thron and Caplan 2003). Abnormally dilated vessels in the spinal cord or the subarachnoid space are the main findings. In AVMs of the glomerular type, the nidus can be differentiated from the draining veins. Intradural AVMs can be associated with hemorrhage in the spinal cord and its surroundings. The hyperintensity in T2-weighted images is not the dominant feature in this type of AVM. The signal characteristics vary over time, comparable with intracerebral hematomas. After endovascular treatment, compromise of the venous outflow may result in a congestive myelopathy.

Perimedullary fistulas type 1 may present with multisegmental hyperintensive cord lesions on T2-weighted images due to congestive myelopathy similar to spinal cord affection in spinal dural AVF.

17.5

Spinal Angiography

17.5.1

MR Angiography and Spinal Cord Vessels

The largest blood vessels of the spinal cord are the veins of the superficial system, the anterior and posterior median vein, the radicular veins, the terminal vein and, furthermore, the transmedullary venous anastomoses. They have an inner vessel diameter of up to 2 mm compared with a maximum diameter of 1 mm on the arterial side. At present, these are the vessels most likely to be seen on MRI. They can either be demonstrated on sagittal T1-weighted images following contrast enhancement as bright structures or, like in myelography, as dark structures within the bright CSF on strongly T2-weighted images. Problems arise in “borderline” cases, especially at the level of the lumbar enlargement, where it is impossible to decide whether the demonstrated blood vessels represent still normal or already path-

ological findings due to a SDAVF. This is particularly true for the thoracolumbar enlargement due to a considerable variation of the posterior veins at this location that can show varicosity and elongation (Thron and Mull 2004).

Even if spatial and contrast resolution of these imaging modalities will increase in the future, it might be difficult to differentiate the artery from the vein on the anterior surface of the cord. The anterior spinal artery and vein run very close together. The branching of a radicular artery or vein has a very similar hairpin-configuration, and the level at which a segmental inor outflow occurs cannot be predicted in a given case.

The only non-invasive way in which a reliable differentiation of superficial spinal cord arteries and veins can be expected, is an MR-based angiographic technique with sufficient spatial resolution and with a time resolution that clearly separates arterial and venous phase. The first MRA techniques used were blood f low-dependent, such as three-dimensional phase contrast angiography and three-dimensional contrast-enhanced time of f light imaging (Bowen et al. 1996). The greatest disadvantages are the long acquisition time (10 min) and the limited visualization of normal intradural arteries. To achieve better visualization without venous overprojection, first-pass MR angiography with a rapid bolus injection of contrast medium and a highly sophisticated acquisition technique, has been proposed (Willinek et al. 2002). This technique makes it possible to distinguish the arterial phase from a later phase in which both arteries and veins are enhanced. Verification of the findings by DSA has rarely been performed. Some authors have shown that the fistula-supplying segmental artery could be located by first-pass contrast-enhanced MR angiography (Binkert et al. 1999; Farb et al. 2002; Shigematsu et al. 2000). By using a first-pass technique with two dynamic phases, the arteria radicularis magna of Adamkiewicz could be separated from the anterior median vein and separately visualized in 69% (Yamada et al. 2000a,b). Recent own investigations comparing MR angiography with DSA findings are encouraging with regard to the identification of the Adamkiewicz artery and the fistula-supplying segmental artery in SDAVF (Backes et al. 2004). Nevertheless, to date, MRI and MR angiography have not been able to precisely detect small-caliber feeding spinal arteries in spinal AVM (Fig. 17.13). Introduction of first-pass contrast-enhanced MR angiography

in the routine work-up of spinal vascular pathology will contribute to our understanding of the pathogenetic mechanisms involved.

17.5.2

Spinal Angiography (DSA)

Selective spinal angiography provides the basis for classifying spinal vascular malformations by their location, arterial supply and venous drainage pattern. A profound understanding of the AVM architecture is required for the decision regarding which treatment option(s) should be taken into consideration. In SDAVF, spinal DSA remains the gold standard to identify the fistula-supplying segmental artery. In deep lumbosacral SDAVF, only selective internal iliac arteriography can identify fistulas located in the sacral region supplied by the lateral sacral or iliolumbar arteries.

Selective spinal DSA has a better spatial resolution and plays a main role in the exclusion of spinal vascular malformations. In selected cases affection of the radicular artery and occlusion of the anterior spinal artery system can be demonstrated as well as collateral supply even in the later course of the ischemia (Mull et al. 2002). Thus, spinal DSA helps to identify pathologic vascular conditions in spinal cord ischemia. The main indication remains to exclude a spinal vascular malformation. Angiographic information about the acute phase of spinal cord ischemia is not yet available.

17.6 Treatment

In patients with SDAVF the aim of treatment is to prevent progression of the spinal cord damage. The goal is the permanent occlusion of the dural shunt zone along with the origin of the draining vein. This can be achieved microsurgically or by endovascular treatment using tissue adhesives (Behrens and

Thron 1999; Huffmann et al. 1995; Merland et al. 1986; Niimi et al. 1997; Song et al. 2001; Thron and Caplan 2003). Treatment of spinal AVM should be restricted to specialized centers that are experienced in this field.

To identify the location of cord-supplying segmental arteries before aortic surgery can help to reduce the risk of spinal cord ischemia. Monitoring of somatosensory-evoked responses contrib-

utes to identify early signs of spinal cord ischemia during aortic clamping. Before aortic surgery spinal DSA and MR angiography, as described before, can help to identify cord-supplying segmental arteries.

If the diagnosis of a compression of a lumbar artery has been established by dynamic spinal DSA showing complete occlusion of the lumbar artery, surgical section of the diaphragmatic crus may prevent irreversible infarction in this rare condition.

At present there are no recognized effective treatment strategies for spinal cord stroke (Caplan 2003; Mull 2005). Nevertheless, if the cord ischemia is judged to be embolic, effective anticoagulation or antiplatelet drugs should be considered in clinical practice.

17.7 Summary

In the last decade typical signs of spinal cord ischemia have been reported. Confirming and supporting signs of acute ischemic myelomalacia are vertebral body infarction and the pathognomonic contrast enhancement of the cauda equina in the course of the disease. Moreover, bone infarction strongly indicates the proximal occlusion and the level of the affected segmental artery. Cartilaginous disc embolism, embolism following periradicular nerve root therapy and compression of a lumbar artery are underestimated causes of spinal cord ischemia.

In most cases a long, extending cord lesion in the presence of perimedullary veins favors spinal dural AV fistulas as an underlying disorder which has to be confirmed by spinal angiography and separated from perimedullary AV fistulas. Up to now, it is not possible to differentiate local arterial from venous occlusion in the intravital diagnostic work-up. MRI, MR angiography and spinal DSA are complementary diagnostic tools in vascular diseases of the spinal cord which help us to confirm the diagnosis and to come to a better understanding of these rare disorders.

In summary, important advances have been made in our understanding of the underlying pathogenetic mechanism in spinal cord ischemia. This condition remains a diagnostic and therapeutic challenge, but improved diagnosis may result in better treatment in the future.

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CONTENTS

18.1Introduction 269

18.3Clinical Features and Typical Symptom Combinations 269

18.3.5 Treatment 270

18.4Imaging 270

18.4.1 Magnetic Resonance Imaging and

Magnetic Resonance Angiography 273

18.4.2Diagnostic Problems, Potential Artefacts and Pitfalls 282

References 283

18.1 Introduction

Veno-occlusive disorders of the brain may affect the dural sinuses, the superficial cortical veins and the deep venous system. Occlusion may be due to aseptic or septic thrombosis, to stenoses of the large sinuses at the base of the skull of different origin and to tumors compressing or infiltrating the sinus wall, especially meningiomas. Impaired venous drainage results in venous congestion or congestive infarction which can be accompanied by hemorrhage. Arteriovenous shunting in case of dural arteriovenous fistulas or arteriovenous malformations may have a similar clinical effect. In general practice, cerebral venous and sinus thrombosis (CVST) play the most important role in the group of veno-occlusive diseases.

18.2

Etiology and Risk Factors

Several etiological factors are known to cause CVST, although no specific cause for CVST can be found in about 25% of all cases (Deschiens et al. 1996).

A. Thron, MD; M. Mull, MD

Department of Neuroradiology, Clinic for Diagnostic Radiology, University Hospital of the Technical University, Pauwelsstr. 30, 52074 Aachen, Germany

One has to differentiate between aseptic CVST as the most common form, septic venous and sinus thrombosis, tumor-induced and trauma-induced CVST. Disease processes that may cause aseptic CVST include hypercoagulopathic states such as those present in polycythemia vera, sickle cell disease, deficiencies of fibrinolytic factors (antithrombin III, protein C, protein S) or disseminated intravascular coagulopathy (Deschiens et al. 1996). Oral contraceptives, pregnancy, and puerperium are also known risk factors for developing an aseptic CVST (Cantu and Barinagarrementeria 1993). In addition, systemic malignancies with paraneoplastic syndromes, lupus erythematodes, drug abuse or low flow situations as present during dehydration or shock may cause CVST. Septic causes are most often encountered in childhood with a chronic or acute mastoiditis involving the neighbouring transverse or sigmoid sinus (Isensee et al. 1992a; Reul et al. 1997). Meningitis, brain abscesses or septicemia are, however, more seldom causes of septic CVST. Concerning tumor induced CVST, meningiomas are prone to obliterate the lumen of the dural sinuses; however, this process evolves slowly over time, therefore, venous collaterals are often present and an acute venous congestion is the exception rather than the rule. Apart from meningiomas other tumor entities only rarely infiltrate the dural sinus walls. CVST caused by trauma is also rather rare; however, fractures that lead to a laceration of the dural wall might cause a venous occlusion.

18.3

Clinical Features and Typical Symptom Combinations

CVST generally shows a subacute onset and course of symptoms. Clinical symptomatology is dependant on the cause, localization, extension and time of development of the venous occlusion. The leading symptom is headache in 70%-90% of all cases

(Strupp et al. 2003), often associated with nausea and vomiting (Thron et al. 1986). But the spectrum of symptoms and signs varies between an asymptomatic course and rapidly progressive neurological deficits and impaired consciousness.

In asymptomatic cases, the occlusion of an isolated sinus is typically compensated by collaterals or in case of the transverse sinus by a contralateral sinus of adequate size (Thron 2001).

Idiopathic intracranial hypertension (synonyms: benign intracranial hypertension, pseudotumor cerebri syndrome) is characterized by increased CSF pressure (> 250 mm H2O) in the absence of an intracranial space occupying lesion or inflammation. About 30%-50% of CVST-patients present with this syndrome (Thron et al. 1986). They complain of headache and have papilledema or other symptoms and signs of increased intracranial pressure. In most of these cases a main venous channel is occluded and drainage of the brain depends on smaller sinuses or veins. An example of this situation is the unilateral occlusion of the dominant transverse sinus. The collateral drainage is poor, but sufficient to prevent focal lesions. Recently, advanced MRA techniques have provided evidence that focal stenotic lesions in the transverse sinuses (or comparable venous outflow obstructions of non-thrombotic origin) may be another frequent cause of this syndrome (see Fig. 17.2c-e; Higgins et al. 2002, 2004; Farb et al. 2003) Using more invasive techniques, pressure gradients could be measured in a part of these patients. The relationship between these focal stenoses and the increased intracranial pressure remains to be clarified.

Neurological deficits and seizures occur in the group of patients in whom focal congestive and hemorrhagic lesions occur. The stroke-like symptoms depend on the localization of the brain damage and may be accompanied by seizures (Thron et al. 1986; Strupp et al. 2003). In these cases extension of the thrombus in cortical veins has occurred or the collateral drainage for a distinct area of the brain parenchyma is not sufficient. A typical example of this is the occlusion of a transverse sinus together with the vein of Labbé (see Fig. 18.8; Isensee et al. 1992b). Focal sensorimotor deficits and/or seizures are also the clinical feature of the solitary thrombosis of a superficial cerebral vein which in our experience is rare. This entity may be accompanied by a cortical subarachnoid hemorrhage.

Impaired consciousness and coma may develop with increasing intracranial pressure.

A decrease in mental status, drowsiness, progressive confusion and impaired consciousness may also be the major symptoms of deep cerebral venous

thrombosis (Ameri and Bousser 1992; Crawford et al. 1995). Drainage impairment affects mainly the thalamus unior bilaterally with venous congestion and/or bleeding (Lafitte et al. 1999) (see Fig. 18.6).

Thrombosis of the cavernous sinus is characterized by proptosis, chemosis, impaired vision and ophthalmoplegia. If it is not septic, prognosis is good because of collateral drainage and spontaneous recanalization. The same symptoms, with the exception of a possible bruit, may result from arteriovenous shunting in carotid-cavernous fistulae. The treatment of choice in this case is endovascular occlusion (thrombosis!) of the cavernous sinus.

The prognosis of extensive CVST is unpredictable and variable. The 5%-30% mortality of CVST still reported in studies between 1991 and 1999 (Strupp et al. 2003) has significantly dropped. In our experience early diagnosis with noninvasive techniques of MRI and MRA has an important influence on prognosis.

Early diagnosis, however, can only be achieved if radiologists contribute to the identification of the subset of patients complaining of headache and who have this potentially life-threatening disease which requires immediate therapy. They need to know the clinical background, be aware of suspect findings in routine MRI and should know the advantages and potential pitfalls and limitations of different MRA techniques and flow-sensitive sequences.

18.3.5 Treatment

Typically, patients with confirmed CVST are treated with intravenous heparin even in the presence of intracerebral hemorrhage. Although there is only one placebo-controlled, double-blind study showing a significant advantage of intravenous doseadjusted unfractionated heparin therapy in patients with CVST (Einhäupl et al. 1991), heparin as the first-line treatment is recommended because of its efficacy, safety and feasibility (Ameri and Bousser 1992; Bousser 1999). Only in rare cases may fibrinolytic therapy or thrombectomy be considered as alternative treatment options.

18.4 Imaging

To interpret imaging, it is necessary to know the normal anatomy of the cerebral venous system and

to transfer this knowledge to the transversal cuts of the axial cranial CT (CCT) and MRI or to 3D reconstructions of the blood vessels. Moreover, the most important anatomical variants of the dural sinuses must be readily perceived. The normal venous angiogram as detected by digital subtraction angiography (DSA) and MRI is illustrated in Figs. 18.1 and 18.2a,b.

Frequently encountered anatomical variants include (Thron 2001):

•The unilateral hypoplastic transverse and sigmoid sinus with compensation via the contralateral transverse sinus.

•The aplasia of the frontal superior sagittal sinus anterior to the coronary suture with compensation via large bridging veins.

•The high division of the superior sagittal sinus (cranial to the internal occipital protuberance, where the confluens sinuum is normally encountered).

•Pacchioni granulations may be seen as circumscript intraluminal filling defects or gaps.

Fig. 18.1. Venous anatomy in digital subtraction angiography (DSA) in lateral projection. FV, frontal veins; PV, parietal veins; OV, occipital veins; SSS, superior sagittal sinus; ISS, inferior sagittal sinus; TS, transverse sinus; SIS, sigmoid sinus; IJV, internal jugular vein; SS, straight sinus; CS, confluens sinuum; VL, vein of Labbé; SV, sylvian vein; CS, cavernous sinus; VG, vein of Galen; ICV, internal cerebral vein; IJV, internal jugular vein