Die magnetische Blutbolusmarkierung (Spin-Labeling) ermöglicht die nichtinvasive quantitative Messung des Blutflusses im Gewebe. Beim Spin-Labeling wird arterielles Blut durch Radiofrequenzpulse magnetisch markiert und der Transport der Markierung MR-tomographisch gemessen. Am Modell einer unter physiologischen Bedingungen perfundierten extrakorporalen Schweineniere konnte die Quantifizierbarkeit der Messmethode nachgewiesen werden. In einer Studie an 36 Hirntumorpatienten wurde das Verfahren mit der kontrastmittelbasierten First-Pass-Bolus-Methode zur nicht-quantitativen Perfusionsmessung verglichen. Es zeigte sich eine sehr gute Übereinstimmung zwischen beiden Methoden, der lineare Korrelationskoeffizient des relativen Blutflusses in der Tumorregion lag bei R=0,83. Die mittels Spin-Labeling ermittelten Absolutwerte des Blutflusses spielen bei der Beurteilung des Tumorgrades eine untergeordnete Rolle, da die mittlere Perfusion individuell sehr verschieden ist. Ein zweiter Anwendungsbereich für das Spin-Labeling ist die Darstellung großer Arterien. Spin-Labeling ermöglicht die nichtinvasive dynamische Angiographie (Dynamische Spin-Labeling-Angiographie - DSLA). Analog zur digitalen Subtraktionsangiographie kann damit der Einstromvorgang des Blutes in den Gefäßbaum zeitaufgelöst gemessen werden, jedoch mit wesentlich höherer zeitlicher Auflösung und frei wählbarer Projektionsrichtung. In einer Studie an 18 Patienten mit einseitigen Carotisstenosen wurden die Zeitdifferenzen der Anflutung der zerebralen Gefäße zwischen der betroffenen und der nicht stenosierten Seite bestimmt. Die im Carotis-Siphon gemessenen Zeitdifferenzen korrelieren signifikant mit dem Stenosegrad, steigen aber erst ab einer Lumeneinengung oberhalb von 80 Prozent deutlich an. Im Vergleich zu den etablierten Methoden werden die Möglichkeiten und Grenzen der DSLA dargestellt. / Arterial spin labeling methods allow to determine quantitative tissue blood flow values noninvasively. Arterial blood is labelled by an inversion pulse and the distribution of this intrinsic tracer is measured using magnetic resonance imaging. Experiments using an extra corporal in-vitro porcine kidney in a MR compatible set-up were carried out to determine the accuracy of blood flow values calculated from arterial spin labeling measurements. In a study of 36 brain tumor patients, spin labeling was compared to non-quantitative contrast-enhanced dynamic susceptibility-weighted perfusion imaging. Relative blood flow values determined with both methods were in good agreement, the linear regression coefficient in the tumor region was R=0.83. Due to the variable individual perfusion state, quantitative blood flow values determined using spin labeling play a minor role in the assessment of tumor grade. Application of spin labeling to angiography of major arteries was investigated. Dynamic spin labeling angiography (DSLA) sequences were implemented and tested on a clinical scanner. This technique allows time-resolved depiction of blood flow in large vessels with very high temporal resolution. As opposed to digital subtraction angiography, the method allows arbitrary projection directions. In a study, 18 patients with one-sided carotid stenoses were examined. In these patients the time differences of blood bolus arrival at both hemispheres were determined. Time differences measured in the carotid siphon show a significant correlation with the degree of stenosis. However, a clear increase is not seen until 80% narrowing of a carotid. Possibilities and limitations of the DSLA method are discussed in comparison to established techniques.

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Blood oxygen level-dependent (BOLD) magnetic resonance (MR) imaging enables non-invasive spatial mapping of changes in cerebral blood flow (CBF). By applying a vasodilatory stimulus (such as inhaled CO2) during BOLD MR imaging, one can measure cerebral vasodilatory capacity. "Cerebrovascular reactivity" (CVR) is defined as the change in CBF per unit of vasodilatory stimulus. Vasodilatory capacity is clinically important as vasodilatation is a mechanism by which the brain maintains constant CBF despite reductions in cerebral perfusion pressure.ii Patients with arterial narrowing commonly demonstrate a paradoxical response: vasodilatory stimulus-induced reduction of BOLD MR signal. BOLD MR depends on CBF but on other factors too. Does a reduction of BOLD MR signal indicate a decrease in flow? Does BOLD MR CVR correlate with CVR measured using arterial spin labeling (ASL) MR? I studied thirty-eight patients with stenosis of brain-supplying arteries and found that the BOLD CVR and ASL CVR results correlate strongly (R=0.83, P<0.0001 for cerebral hemispheric gray matter). The second study aimed to determine whether preoperative CVR predicts the hemodynamic effect of extracranial-intracranial bypass surgery. Whereas prior studies relied on right-left interhemispheric CVR asymmetry indices, this study used “absolute” CVR from each hemisphere. I studied twenty-five patients with intracranial arterial stenosis. I found that the group with normal pre-operative CVR showed no change in CVR following bypass surgery (0.22% ± 0.05% to 0.22% ± 0.01% (mean ± SD)(P=0.881)), the group with reduced pre-operative CVR demonstrated an improvement (0.08% ± 0.05% to 0.21 ± 0.08% (mean ± SD)(P<0.001)), and the group with paradoxical pre-operative CVR demonstrated the greatest improvement (-0.04% ± 0.03% to 0.27% ± 0.03% (P=0.028)). ii Patients with arterial narrowing commonly demonstrate a paradoxical response: vasodilatory stimulus-induced reduction of BOLD MR signal. BOLD MR depends on CBF but on other factors too. Does a reduction of BOLD MR signal indicate a decrease in flow? Does BOLD MR CVR correlate with CVR measured using arterial spin labeling (ASL) MR? I studied thirty-eight patients with stenosis of brain-supplying arteries and found that the BOLD CVR and ASL CVR results correlate strongly (R=0.83, P<0.0001 for cerebral hemispheric gray matter). The second study aimed to determine whether preoperative CVR predicts the hemodynamic effect of extracranial-intracranial bypass surgery. Whereas prior studies relied on right-left interhemispheric CVR asymmetry indices, this study used “absolute” CVR from each hemisphere. I studied twenty-five patients with intracranial arterial stenosis. I found that the group with normal pre-operative CVR showed no change in CVR following bypass surgery (0.22% ± 0.05% to 0.22% ± 0.01% (mean ± SD)(P=0.881)), the group with reduced pre-operative CVR demonstrated an improvement (0.08% ± 0.05% to 0.21 ± 0.08% (mean ± SD)(P<0.001)), and the group with paradoxical pre-operative CVR demonstrated the greatest improvement (-0.04% ± 0.03% to 0.27% ± 0.03% (P=0.028)). The third study arose from an unexpected observation: paradoxical reactivity in the white matter of young healthy subjects. I evaluated healthy subjects using BOLD CVR and ASL CVR, transformed all CVR maps into a common brain space, and generated composite maps of CVR. Composite maps confirmed regions of significant paradoxical iii reactivity in the white matter. These regions may represent the physiological correlate of previously anatomically defined border-zones (watershed zones). The regions match the locations where elderly patients develop white matter rarefaction, so-called leukoaraiosis.

Blood oxygen level-dependent (BOLD) magnetic resonance (MR) imaging enables non-invasive spatial mapping of changes in cerebral blood flow (CBF). By applying a vasodilatory stimulus (such as inhaled CO2) during BOLD MR imaging, one can measure cerebral vasodilatory capacity. "Cerebrovascular reactivity" (CVR) is defined as the change in CBF per unit of vasodilatory stimulus. Vasodilatory capacity is clinically important as vasodilatation is a mechanism by which the brain maintains constant CBF despite reductions in cerebral perfusion pressure.ii Patients with arterial narrowing commonly demonstrate a paradoxical response: vasodilatory stimulus-induced reduction of BOLD MR signal. BOLD MR depends on CBF but on other factors too. Does a reduction of BOLD MR signal indicate a decrease in flow? Does BOLD MR CVR correlate with CVR measured using arterial spin labeling (ASL) MR? I studied thirty-eight patients with stenosis of brain-supplying arteries and found that the BOLD CVR and ASL CVR results correlate strongly (R=0.83, P<0.0001 for cerebral hemispheric gray matter). The second study aimed to determine whether preoperative CVR predicts the hemodynamic effect of extracranial-intracranial bypass surgery. Whereas prior studies relied on right-left interhemispheric CVR asymmetry indices, this study used “absolute” CVR from each hemisphere. I studied twenty-five patients with intracranial arterial stenosis. I found that the group with normal pre-operative CVR showed no change in CVR following bypass surgery (0.22% ± 0.05% to 0.22% ± 0.01% (mean ± SD)(P=0.881)), the group with reduced pre-operative CVR demonstrated an improvement (0.08% ± 0.05% to 0.21 ± 0.08% (mean ± SD)(P<0.001)), and the group with paradoxical pre-operative CVR demonstrated the greatest improvement (-0.04% ± 0.03% to 0.27% ± 0.03% (P=0.028)). ii Patients with arterial narrowing commonly demonstrate a paradoxical response: vasodilatory stimulus-induced reduction of BOLD MR signal. BOLD MR depends on CBF but on other factors too. Does a reduction of BOLD MR signal indicate a decrease in flow? Does BOLD MR CVR correlate with CVR measured using arterial spin labeling (ASL) MR? I studied thirty-eight patients with stenosis of brain-supplying arteries and found that the BOLD CVR and ASL CVR results correlate strongly (R=0.83, P<0.0001 for cerebral hemispheric gray matter). The second study aimed to determine whether preoperative CVR predicts the hemodynamic effect of extracranial-intracranial bypass surgery. Whereas prior studies relied on right-left interhemispheric CVR asymmetry indices, this study used “absolute” CVR from each hemisphere. I studied twenty-five patients with intracranial arterial stenosis. I found that the group with normal pre-operative CVR showed no change in CVR following bypass surgery (0.22% ± 0.05% to 0.22% ± 0.01% (mean ± SD)(P=0.881)), the group with reduced pre-operative CVR demonstrated an improvement (0.08% ± 0.05% to 0.21 ± 0.08% (mean ± SD)(P<0.001)), and the group with paradoxical pre-operative CVR demonstrated the greatest improvement (-0.04% ± 0.03% to 0.27% ± 0.03% (P=0.028)). The third study arose from an unexpected observation: paradoxical reactivity in the white matter of young healthy subjects. I evaluated healthy subjects using BOLD CVR and ASL CVR, transformed all CVR maps into a common brain space, and generated composite maps of CVR. Composite maps confirmed regions of significant paradoxical iii reactivity in the white matter. These regions may represent the physiological correlate of previously anatomically defined border-zones (watershed zones). The regions match the locations where elderly patients develop white matter rarefaction, so-called leukoaraiosis.