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11. Dynamic Cerebral Contrast-enhanced Perfusion MRI
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11 Dynamic Cerebral Contrast-enhanced Perfusion MRI Cerebrovascular stroke is the third leading cause of mortality in industrial countries after cardiovascular disease and malignant tumors [86]. Therefore, the analysis of cerebral circulation has become an issue of enormous clinical importance. Novel magnetic resonance imaging (MRI) techniques have emerged since the 1990s that allow for rapid assessment of normal brain function as well as cerebral pathophysiology. Both diffusion-weighted imaging and perfusion-weighted imaging have already been used extensively for the evaluation of patients with cerebrovascular disease [65]. They are promising research tools that provide data about infarct evolution as well as mechanisms of stroke recovery. Combining these two techniques with high-speed MR angiography leads to improvements in the clinical management of acute stroke subjects [192]. Measurement of tissue perfusion yields important information about organ viability and function. Dynamic susceptibility contrast MR imaging , also known as contrast-agent bolus tracking represents a noninvasive method for cerebrovascular perfusion analysis [275]. In contrast to other methods to determine cerebral circulation, such as iodinated contrast media in combination with dynamic X-ray computed tomography (CT) [11] and the administration of radioactive tracers for positron emission tomography (PET) blood-flow quantification studies [114], it allows high spatial and temporal resolution and avoids the disadvantage of patient exposure to ionizing radiation. MR imaging allows assessment of regional cerebral blood-flow (rCBF), regional cerebral blood volume (rCBV), and mean transit time (MTT) (for definitions, see, e.g. [220]). In clinical praxis, the computation of rCBV, rCBF, and MTT values from the MRI signal dynamics has been demonstrated to be relevant, even if its underlying theoretical basis may be weak under pathological conditions [65]. The conceptual difficulties with regard to the parameters MTT, rCBV, and rCBF arise from four basic constraints: (1) homogeneous mixture of the contrast-agent and blood pool, (2) negligible contrast-agent injection volume, (3) hemodynamic indifference of the contrast-agent, and (4) strict intravascular presence of the indicator substance . Conditions (1)-(3) are usually satisfied in dynamic susceptibility 300 Chapter 11 contrast MRI using intravenous bolus administration of gadolinium compounds . Condition (4), however, requires an intact blood-brain barrier. This prerequisite is fulfilled in examinations of healthy subjects. These limitations for the application of the indicator dilution theory have been extensively discussed in the literature on MRI [200, 220] and nuclear medicine [149]. If, absolute flow quantification by perfusion MRI should be performed, the additional measurement of the arterial input function is needed, which is difficult to obtain in clinical routine diagnosis. However, clinicians agree that determining parameter images based on the MRI signal dynamics, is a key issue in clinical decision-making, bearing a huge potential for diagnosis and therapy. The analysis of perfusion MRI data by unsupervised clustering methods provides the advantage that it does not imply speculative presumptive knowledge on contrast-agent dilution models, but strictly focuses on the observed complete MRI signal time series. In this chapter, the applicability of clustering techniques as tools for the analysis of dynamic susceptibility contrast MRI time series is demonstrated and the performance of five different clustering methods is compared for this purpose. 11.1 Materials and Methods Imaging protocol The study group consisted of four subjects: (1) two men aged 26 and 37 years without any neurological deficit, history of intracranial abnormality , or previous radiation therapy. They were referred to clinical radiology to rule out intracranial abnormality. (2) two subjects (one man and one woman, aged 61 and 76 years, respectively) with subacute stroke (symptoms two and four days, respectively) who underwent MRI examination as a routine clinical diagnostic procedure. All four subjects gave their written consent. Dynamic susceptibility contrast MRI was performed on a 1.5 T system (Magnetom Vision, Siemens, Erlangen, Germany) using a standard circularly polarized head coil for radio frequency transmission and detection. First, fluid-attenuated inversion recovery , T2-weighted spin echo, and diffusion-weighted MRI sequences were obtained in transversal slice orientation, enabling initial localization and evaluation of the cerebrovascular insult in the subjects with stroke. Then dynamic susceptibility contrast MRI was performed us- [18.227.114.125] Project MUSE (2024-04-17 21:18 GMT) Dynamic Cerebral Contrast-enhanced Perfusion MRI 301 ing a 2-D gradient echo echoplanar imaging (EPI) sequence employing 10 transversal slices with a matrix size of 128 × 128 pixels, pixel size 1.88 × 1.88 mm, and a slice thickness of 3.0 mm (TR = 1.5 sec, TE...