Perfusion MRI evaluates the perfusion of tissues by blood. This information can be acquired either by injecting contrast dye into the blood vessels (dynamic susceptibility contrast or dynamic contrast enhanced perfusion) or without the use of a contrast agent (arterial spin labelling). Various important values can be derived by a combination of these techniques, including mean transit time, cerebral blood volume, cerebral blood flow, vessel permeability. These parameters help us study the biology of brain pathologies including:
Brain, head and neck tumours: Conventional MR techniques are often nonspecific and provide limited information on tumour physiology. Thus, conventional MRI is largely inadequate to guide biopsy or treatment of brain or head and neck tumours. The intensity of contrast enhancement in tumours has a relatively poor correlation with tumour grade and is not a reliable marker for distinguishing recurrent tumours from radiation necrosis or other treatment-related changes. MR perfusion allows assessment of both tumour anatomy and physiology in one setting. Multiple studies have shown that preoperative grading of brain tumours using MR perfusion is more accurate than that using conventional MRI scanning alone. This is the principle of image-guided surgical intervention and biopsy or treatment of the most aggressive portions of a tumour. MR perfusion imaging is also valuable in distinguishing radiation necrosis from tumour recurrence, thus sparing patients from unnecessary treatment and reducing co-morbidity. The role of MR perfusion in the care of neuro-oncologic patients is continuously expanding and encompasses also therapy prognosis and lesion monitoring in eloquent brain regions to detect early changes that precede an aggressive tumour transformation, the latter indicating prompt surgical intervention.
Stroke, Moya-Moya disease: In acute stroke, MR perfusion imaging can be used to measure regional brain perfusion and to establish the diagnosis of cerebral ischaemia, even in the absence of other objective evidence in conventional CT or MRI, aiding in selecting patients suitable for thrombolytic therapy. MR perfusion is now part of the meticulous assessment of the collateral circulation in acute stroke patients shifting the therapeutic time window to late endovascular therapies in order to achieve higher salvage rates of ischaemic brain tissue. In Moya-Moya disease and any stenotic or occlusive disease of the neck or intracranial arteries, perfusion MRI can help to estimate the degree of perfusion impairments and guide treatment decisions. Reductions in cerebral perfusion pressure can be compensated for by increases in parenchymal cerebral blood volume and cerebral
blood flow. Perfusion MRI measurements can estimate the cerebrovascular reactivity or
the ability of vessels to regulate CBF and cerebral blood volume. This is an indicator of the cerebrovascular reserve and potentially how near tissue is failing to meet hemodynamic
demand. These parameters have been demonstrated to correlate variably with stroke risk and symptomatology.
Epilepsy: The success of epilepsy surgery is directly correlated with the ability to define and subsequently resect the epileptogenic zone. Improved lesion detection requires both a sophisticated epilepsy protocol for MRI, including MR perfusion, and the interpreting radiologist's familiarity with common causes of epilepsy. Perfusion MRI maps can reveal the functional deficit zone related to the epileptogenic zone. An epileptogenic focus may be either hypo- or hyper-perfused at perfusion MRI owing to seizures soon after the MRI, subclinical seizures, or physiologic cellular changes.
Neurodegeneration: MRI can be tremendously helpful for the diagnosis of dementia, especially in Alzheimer's disease, and other neurodegenerative diseases complementing the the clinical data and the neuropsychological tests. Perfusion MRI, in particular arterial spin labelling MRI, not requiring gadolinium injection, provides both a morphological and a functional assessment of the brain functional integrity and helps to assess early dementia symptoms often in conjunction with dedicated PET exams.