Perfusion studies

With the help of perfusion techniques, the movement of blood is examined and quantified.

To modern quantitative methods of investigation of 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 MRT methods - minimal invasiveness, high sensitivity in the evaluation of tissue microcirculation, high resolving power, short study time within standard protocols and, finally, reproducibility of results over time - are obvious.

The most widespread in neuroendology was perfusion studies based on the intravenous bolus injection of the contrast agent (CT and MRI). Quantitative assessment uses the main hemodynamic tissue characteristics: cerebral blood flow (CBF), cerebral blood flow volume (CBV), mean transit time of blood (MTT).

Perfusion CT. With perfusion CT, an increase in CT-density is observed when a contrast medium passes through the cerebral vascular bed. A bolus of radiopaque substance (iodine preparation with a concentration of 350-370 mg / ml, rate of administration 4 ml / s) is administered intravenously. Spiral modes of scanning make it possible to obtain a series of sections at intervals of 1 s during 50-60 s after intravenous injection.

This method has a high resolution, gives quantitative estimates of tissue perfusion and is recognized as one of the most promising at the present time.

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

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

Clinical use of perfusion CT and MRI. Currently, perfusion studies are conducted to assess the hemodynamics of brain tumors in the differential diagnosis of brain lesions, monitor tumor status after radiation therapy and chemotherapy, diagnose tumor recurrence and / or radiation necrosis, TBI, CNS diseases and lesions (ischemia / hypoxia occlusive diseases of the main arteries of the head, blood diseases, vasculitis, mya-moia disease, etc.).

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

CT and MP perfusion cards allow quantitative characterization of hyper and hypoperfusion zones, which is especially important for diagnosis of tumor and cerebrovascular diseases.

The first place in the frequency of using perfusion methods is ischemic brain damage. Currently, perfusion-weighted images are an integral part of the diagnostic protocol for a patient with suspected cerebral ischemia. For the first time clinically, the method was used in humans for the diagnosis of stroke. At the present stage, perfusion CT / MRI is perhaps the only method of early verification of cerebral ischemia that can detect a decrease in blood flow in the affected area already in the first minutes after the appearance of neurologic symptoms.

In neurosurgery, perfusion-weighted images are mainly used to conduct primary differential diagnosis 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 to differentiate tumors by their histological accessory, and even more so to estimate the prevalence of the tumor in the brain substance. 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 new formations, tissue perfusion characterizes the development of an abnormal vasculature (angioneogenesis) in the tumor and its viability. The presence of an abnormal vasculature in the tumor may indicate the aggressiveness of the latter. Conversely, a decrease in perfusion in a 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 in stereotaxic puncture was of great help, especially in the group of gliomas, characterized by the total absence of contrast enhancement in standard CT and MRI.

When assessing the histological type of neoplasm and the prevalence of intracerebral volume lesions in the cranial cavity, the possibility of perfusion-weighted images is higher than with intracerebral tumors. With the help of perfusion-weighted images successfully differentiate meningiomas and neurinomas of the cerebellar angle by characteristic high hemodynamic parameters in the first type. There is a clear correlation between local blood flow and direct cerebral angiography in a group of patients with meningiomas (Figure 3-16, see color insert). Tumors characterized by the presence of a dense radiopaque shade in the early capillary phase of angiography have exceptionally high perfusion values and are characterized by a high risk of intraoperative bleeding at the time of removal. The perfusion-weighted images obtained during CT perfusion in demonstrating blood supply to the posterior cranial fossa hemangioblast are very specific, early and pronounced contrasting in combination with high perfusion.

[1], [2], [3]