Perfusion studies

Perfusion research methods are used to examine and quantify blood flow.

Modern quantitative methods for studying cerebral hemodynamics include MRI, spiral CT with contrast enhancement, CT with xenon, single-photon emission CT and positron emission tomography (PET). The advantages of minimally invasive CT and MRI methods - minimal invasiveness, high sensitivity in assessing tissue microcirculation, high resolution, short examination time within standard protocols and, finally, reproducibility of results over time - are obvious.

Perfusion studies based on intravenous administration of a bolus of a contrast agent (CT and MRI) are the most widely used in neuroradiology. For quantitative assessment, the main hemodynamic tissue characteristics are used: cerebral blood flow (CBF), cerebral blood volume (CBV), and mean blood transit time (MBT).

Perfusion CT. Perfusion CT analyzes the increase in CT density as a contrast agent passes through the cerebral vascular bed. A bolus of radiopaque agent (iodine preparation with a concentration of 350-370 mg/ml, injection rate of 4 ml/s) is administered intravenously. Spiral scanning modes allow obtaining a series of slices at 1-s intervals for 50-60 s after intravenous injection.

This method has high resolution, provides quantitative assessments of tissue perfusion and is recognized as one of the most promising at present.

Perfusion MRI. In MRI, there are methods for studying hemodynamic perfusion processes using exogenous and endogenous markers (using contrast agents, obtaining images that depend on the level of blood oxygenation, etc.).

Perfusion MRI is currently the name given to methods of perfusion assessment during the passage of a bolus of contrast agent. These methods of studying cerebral perfusion are now most widely used in MR diagnostics, especially in combination with diffusion studies, MR angiography and MR spectroscopy. As the bolus of contrast agent passes through the vascular system, an image of the same section is repeatedly recorded (usually 10 different levels or sections). The scanning itself takes 1-2 minutes. The graph of the decrease in the intensity of the MR signal during the passage of the bolus of contrast agent gives the dependence "signal intensity - time" in each pixel of the section. The shape of this curve in the artery and vein determines the arterial and venous functions, with the help of which hemodynamic tissue parameters are calculated.

Clinical application of perfusion CT and MRI. Currently, perfusion studies are performed to assess the hemodynamics of brain tumors in the differential diagnosis of brain lesions, monitor the tumor condition after radiation therapy and chemotherapy, diagnose tumor recurrence and/or radiation necrosis, TBI, diseases and injuries of the central nervous system (ischemia/hypoxia, occlusive diseases of the main arteries of the head, blood diseases, vasculitis, moyamoya disease, etc.).

Promising areas include the use of perfusion methods for epilepsy, migraine, vasospasm, and various mental illnesses.

CT and MR perfusion maps allow quantitative characterization of hyper- and hypoperfusion zones, which is especially important for the diagnosis of tumor and cerebrovascular diseases.

The most frequently used perfusion methods are ischemic brain lesions. Currently, perfusion-weighted images are an integral part of the diagnostic protocol for a patient with suspected cerebral ischemia. The method was first clinically used in humans specifically for diagnosing stroke. At the present stage, perfusion CT/MRI is perhaps the only method for early verification of cerebral ischemia, capable of detecting a decrease in blood flow in the affected area in the first minutes after the onset of neurological symptoms.

In neurosurgery, perfusion-weighted images are mainly used to perform primary differential diagnostics of the degree of malignancy of intracerebral neoplasms of the brain, in particular gliomas. It should be remembered that perfusion MRI and CT do not allow differentiating tumors by their histological affiliation, much less assessing the prevalence of the tumor in the brain matter. The presence of foci of hyperperfusion in the structure of astrocytoma suggests an increase in the degree of malignancy of the lesion. This is based on the fact that in neoplasms, tissue perfusion characterizes the development of an abnormal vascular network (angioneogenesis) in the tumor and its viability. The presence of an abnormal vascular network in a tumor may indicate its aggressiveness. Conversely, a decrease in perfusion in tumor tissue under the influence of radio- or chemotherapy may indicate that a therapeutic effect has been achieved. The use of perfusion-weighted images for target selection during stereotactic puncture has been of great help, especially in the group of gliomas characterized by a complete lack of contrast enhancement on standard CT and MRI.

In assessing the histological type of neoplasm and the extent of extracerebral space-occupying lesions in the cranial cavity, the capabilities of perfusion-weighted imaging are higher than in the case of intracerebral tumors. Perfusion-weighted imaging successfully differentiates meningiomas and cerebellopontine angle neurinomas by the characteristically high hemodynamic indices in the former type. There is a clear correlation between local blood flow and direct cerebral angiography data in the group of patients with meningiomas (Fig. 3-16, see color insert). Tumors characterized by the presence of a dense radiopaque shadow in the early capillary phase of angiography have exceptionally high perfusion indices and are distinguished by a high risk of intraoperative bleeding at the time of removal. Perfusion-weighted images obtained with CT are very specific in demonstrating the blood supply of posterior fossa hemangioblastomas - early and pronounced contrast enhancement in combination with high perfusion.

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