T Perfusion Studies9

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G. M. GiAnnAtempo, T. ScARAbmo, A. Simeone, T. Popolizio, A. StRAnieRi, M. ARmillottA, U. SAlvolini

Cerebral perfusion is the process by which oxygen and glucose are supplied to the brain capillaries through the circulation. Disruption of brain perfusion is found in nearly all types of brain disease, most notably stroke, but also in neurodegenerative and neoplastic disorders [1-8].

Imaging the regional distribution of cerebral blood flow quantitatively is a diagnostic and technical challenge [3, 7, 9]. H215O positron emission tomography (PET), which uses a freely diffusible tracer and can quantitate blood flow with relative insensitivity to vascular transit time variations, is currently the gold standard of perfusion studies [7,10].

However, it is an expensive examination entailing radioactive dosing and invasive monitoring, besides having low intrinsic spatial resolution. Moreover, H215O PET imaging centres are relatively few due to difficulties in staffing and maintaining an onsite cyclotron. Hence the interest in adapting more widespread imaging modalities, such as magnetic resonance imaging (MRI), to the quantitative measurement of blood flow.

MR perfusion imaging is increasingly being used for the assessment of brain perfusion in several different pathological conditions including ischaemic stroke [8, 11-15], neurovascular disease [16 -19], brain tumours [20-28] and neurodegenerative disorders [29-35]. In brain tumours, perfusion MRI has been applied to grade gliomas [26, 27, 35], to distinguish between different tumour types, like primary tumour from solitary metastases or lymphoma [23, 28], to differentiate radiation necrosis from recurrent tumour [2], and to discriminate high-grade neoplasms from non-neoplastic lesions like abscess [24].

Perfusion MRI has been used in the diagnosis of dementia, including Alzheimer's disease, mild cognitive impairment and non-Alzheimer's dementia [29-34], epilepsy [35] and multiple sclerosis [36].

Unlike angiography, which depicts flow within large vessels, MR perfusion techniques are sensitive to perfusion at such microscopic levels as the capillary bed.

Measurement of tissue perfusion depends on the ability to measure serially the concentration of a tracer in a target organ. Tracers can be divided into exogenous and endogenous. The former include iced saline solution, iodinated radiographic contrast material and, more recently, paramagnetic contrast agents; magnetically labelled blood is currently the sole available endogenous tracer [1, 3]. Obtaining haemo-dynamic parameters from serial measurements of tissue tracer concentrations requires use of a general model of the way in which the tracer passes through or diffuses in the target organ that takes into account tracer diffusibility from intravascular to extravascular space, volume of distribution, equilibrium and halflife.

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