Sensitivity of high-speed "perfusion-sensitive" magnetic resonance imaging to mild cerebral ischemia

Diffusion weighted magnetic resonance imaging in stroke

Diffusion weighted magnetic resonance imaging has evolved from an esoteric laboratory experiment to a critical aspect of routine clinical care of the patient presenting with symptoms suspicious of acute ischemic stroke. The purpose of this article is to review the basis of diffusion weighted imaging (DWI), to consider its application in acute stroke and to recognize potential pitfalls and stroke mimics that might be encountered. Included in the discussion are comments on the elimination of 'T2 shine through' phenomena as well as construction of pixel-by-pixel maps of the apparent diffusion coefficient (ADC). Furthermore, discussion of techniques such as parallel imaging (using SENSE) and PROPELLER sequences will be introduced as methods potentially allowing DWI to be utilized in areas usually associated with prohibitive susceptibility artifact (e.g. the base of the brain). Finally, the concept of interventional therapeutic approaches to salvaging ischemic tissue is introduced, both in terms of the ischemic penumbra (defined by a diffusion/perfusion mismatch) and also in terms of the potential reversibility of the diffusion-weighted hyperintensity, associated with the lesion core.

The impact of peak saturation of the arterial input function on quantitative evaluation of dynamic susceptibility contrast-enhanced MR studies

PURPOSE

The purpose of this work was to investigate systematic errors in dynamic contrast-enhanced MR perfusion studies due to peak saturation of the arterial input function (AIF) and to introduce a simple correction algorithm.

METHOD

Computer simulations were performed to evaluate the influence of AIF peak saturation and to demonstrate the effectiveness of the presented correction algorithm. To compare the computer simulations with real MR data, MR perfusion measurements were performed on volunteers.

RESULTS

The computer simulations show that AIF peak saturation leads to a systematic overestimation of cerebral blood volume (CBV) and cerebral blood flow (CBF) values, which was confirmed by comparing the obtained MR data with PET results. With use of an improved calculation algorithm correcting for AIF peak saturation, a significant improvement of the obtained CBV and CBF values could be demonstrated.

CONCLUSION

Our results suggest that AIF peak saturation leads to a significant systematic error in the determination of CBV and CBF values and has necessarily to be taken into account for dynamic contrast-enhanced MR perfusion studies.

Neuroimaging: do we really need new contrast agents for MRI?

The use of exogenous contrast media in magnetic resonance imaging of the brain has brought dramatic improvement in the sensitivity of detection and delineation of pathological structures, such as primary and metastatic brain tumors, inflammation and ischemia. Disruption of the blood brain barrier leads to accumulation of the intravenously injected contrast material in the extravascular space, leading to signal enhancement. Magnetic resonance angiography benefits from T(1)-shortening effects of contrast agent, improving small vessel depiction and providing vascular visualization even in situations of slow flow. High speed dynamic MRI after bolus injection of contrast media allows tracer kinetic modeling of cerebral perfusion. Progressive enhancement over serial post-contrast imaging allows modeling of vascular permeability and thus quantitative estimation of the severity of blood brain barrier disruption. With such an array of capabilities and ever improving technical abilities, it seems that the role of contrast agents in MR neuroimaging is established and the development of new agents may be superfluous. However, new agents are being developed with prolonged intravascular residence times, and with in-vivo binding of ever-increasing specificity. Intravascular, or blood pool, agents are likely to benefit magnetic resonance angiography of the carotid and cerebral vessels; future agents may allow the visualization of therapeutic drug delivery, the monitoring of, for example, gene expression, and the imaging evaluation of treatment efficacy. So while there is a substantial body of work that can be performed with currently available contrast agents, especially in conjunction with optimized image acquisition strategies, post processing, and mathematical analysis, there are still unrealized opportunities for novel contrast agent introduction, particularly those exploiting biological specificity. This article reviews the current use of contrast media in magnetic resonance neuroimaging, discusses some of the developing strategies for new applications of imaging with these agents and finally offers some views and indications for contrast agents currently under development, as well as some speculation on unsolved problems in neuroimaging, and opportunities for novel contrast agents.

Measuring cerebral blood flow using magnetic resonance imaging techniques

Magnetic resonance imaging techniques measuring CBF have developed rapidly in the last decade, resulting in a wide range of available methods. The most successful approaches are based either on dynamic tracking of a bolus of a paramagnetic contrast agent (dynamic susceptibility contrast) or on arterial spin labeling. This review discusses their principles, possible pitfalls, and potential for absolute quantification and outlines clinical and neuroscientific applications.

Transient cerebral ischemia. Association of apoptosis induction with hypoperfusion

Apoptosis is thought to be important in the pathogenesis of cerebral ischemia. The mechanism of apoptosis induction remains unclear but several studies suggest that it is preferentially triggered by mild/moderate microcirculatory disturbances. We examined in cats whether induction of apoptosis after 2.5 h of unilateral middle cerebral artery occlusion plus 10 h of reperfusion is influenced by the degree of cerebral microcirculatory disturbance. Quantitative monitoring over time of the disturbances of cerebral microcirculation in ischemic brain areas and evaluation of cytotoxic edema associated with perfusion deficits was achieved by using two noninvasive magnetic resonance imaging techniques: (a) high-speed echo planar imaging combined with a bolus of magnetic susceptibility contrast agent; and (b) diffusion-weighted imaging. Apoptosis-positive cells were counted in anatomic areas with different severity of ischemic injury characterized by magnetic resonance imaging, triphenyltetrazolium chloride, and hemotoxylin and eosin staining. The number of apoptosis-positive cells was significantly higher in anatomic areas with severe perfusion deficits during occlusion and detectable histologic changes 10 h after reperfusion. In contrast, in areas where perfusion was reduced but maintained during occlusion there were no detectable histological changes and significantly fewer apoptosis-positive cells. A similar number of cells that undergo apoptosis were shown in regions with transient or prolonged subtotal perfusion deficits. These results suggest that the apoptotic process is induced in the ischemic core and contributes significantly in the degeneration of neurons associated with transient ischemia.