Advances in magnetic resonance imaging of moyamoya disease

Moyamoya disease (moyamoya diseases, MMD) is a kind of to the end of bilateral internal carotid artery and/or before the cerebral artery and/or progressive stenosis or occlusion of middle cerebral artery with the skull base or basal ganglia region of abnormal blood vessels formation of chronic cerebrovascular disease, characterized by due to abnormal vascular network imaging findings like smoke, was named the MMD, if patients with MMD performance and merge more than one basic diseases, it is called a smoke syndrome (moyamoya syndrome, MMS). DSA and SPECT/PET are the main methods to diagnose and evaluate MMD patients. Since the advent of MRI, it has been widely accepted for its advantages of rapidity, no radiation and high tissue resolution. With the continuous development of new MRI technology, its application in clinical and scientific research has been more and more extensive. This paper reviews the application of MRI in MMD.EEG machine

1. MRI in combination with MRA

With the wide application of MRI in clinical practice, more and more studies have confirmed the diagnostic significance of MRI and MRA imaging manifestations of MMD. In 2012, Japanese experts issued new guidelines on diagnosis and treatment, which included MRI and MRA combined diagnosis into the diagnosis method of MMD for the first time, and added a new method to evaluate the staging of MMD based on MRA performance, which was highly consistent with DSA staging. MRA is noninvasive and radiation-free. Although it has the shortcoming of overestimating the degree of vascular stenosis, its diagnostic ability has been widely recognized at home and abroad, and the incidence rate of DSA -- is about 932%. MRI has excellent tissue resolution, which can show the linear flow of empty blood vessels at the skull base point, namely the smoky blood vessels. In addition, it can provide anatomical information of the brain and discover secondary changes in the brain, such as infarction, hemorrhage and cerebral atrophy. MRI attenuating inversion recovery image can show the soft meningeal point linear high signal distributed along the surface of cerebral cortex, namely ivy sign, which represents the decompensation of blood supply to the internal carotid artery system and the formation of collateral circulation from external carotid artery to intracranial, which is related to the progress of MMD and has certain specificity for the diagnosis of MMD.EEG machine

2. High-resolution MRI

The basic pathological changes of the arterial wall affected by MMD were as follows: the arterial intima was thickened due to abnormal proliferation of fiber cells and smooth muscle cells, and the structure and function of the inner elastic layer were abnormal; Atrophy and thinning of the media membrane; The external diameter of the internal carotid artery and middle cerebral artery decreased. However, as MMD is a dynamic process, there is no effective examination method to follow up the changes of vascular wall structure, so the evolution process of vascular wall structure in the progress of MMD is still unclear.

High resolution MRI (hr-mri) adopts the "black blood" technology, which can not only display the lumen, but also visually display the vascular wall structure, making it possible to evaluate the pathological changes of the vascular wall, which is of great significance to explore the progress of MMD. HR-MRI has been applied in the analysis of external carotid atherosclerotic plaques and has been confirmed by pathological results. Although the intracranial artery branches are small and deep, it has been proved that hr-mri has a good consistency with pathology in showing the wall of the middle cerebral artery.

HR-MRI shows the wall structure clearly and reliably, and can differentiate MMD and MMS on the basis of traditional vascular imaging. According to the 2012 Japanese MMD diagnostic guidelines, MMS is defined as at least one basic disease that conforms to the imaging manifestations of MMD and is associated with at least one basic disease, which can clearly cause vascular lesions and involves up to 35 types, among which atherosclerosis and leptospirosis are the most common in China. Although DSA, CTA and MRA are recognized as the diagnostic value of MMD, and DSA is taken as the gold standard for MMD diagnosis, all of them can only reflect the state of vascular lumenal, hr-mri can analyze the vascular wall structure, and make differential diagnosis of MMD and MMS according to the differences of tube wall structure caused by different pathogenesis.

For young patients with atherosclerotic risk factors, the external diameter and wall thickness of the involved vessels of MMD were significantly smaller than those of MMS with atherosclerosis, showing uniform centripetal stenosis, while the wall of the vascular stenosis caused by atherosclerosis was mostly eccentric thickening, showing fibrous cap, lipid core, bleeding, etc. For patients with cerebral arteritis complicated with leptospirosis, the vascular wall showed inflammatory changes due to the pathological manifestations of endothelial cell damage, and the involved vascular wall was significantly uniformly enhanced after enhancement, while the MMD involved vascular wall was not enhanced. It can be seen that the application of hr-mri enables part of MMS to be differentiated from MMD, so as to early etiological treatment, which is of great significance to improve the prognosis of patients.

3.MRI perfusion imaging

MRI perfusion imaging included dynamic magnetic resonance imaging (PWI) and arterial spin labeling (ASL).

PWI for application note paramagnetic contrast agent perfusion imaging method, which had first get tissue contrast medium concentration curve, cerebral blood volume was calculated by the deconvolution method (cerebral blood volume, CBV), and through the establishment of blood flow dynamics model for blood flow, cerebral lood flow, CBF), average through time, mean transit time, determined by MTT), peak time (the time to peak, TTP) and hemodynamic parameters.

PWI has certain value in reflecting the distribution of tissue microvessels and providing hemodynamic information. Vakl et al. conducted a comparative analysis of PWI and PET and found that pwi-cbf was linearly correlated with pet-cbf despite the tendency to underestimate blood flow, which was far better than other examinations to evaluate hemodynamics. Then, Tanaka, etc in the research of MMD patients, the PWI obtained relative to income CBV, parameters such as determined by MTT and PET comparison, not only further determine the PWI in analysis of CBV and blood flow reserve value, also think PWI higher resolution and can sweep with MRI contrast analysis, provide the abnormal blood flow in anatomic structure of information, and check the time is short, no radiation, have certain advantages compared with PET. Togao et al. found that MTT prolongation was associated with decreased blood vessel response to CO2. TTP is considered to be one of the most sensitive perfusion indexes because it can calculate large and small blood vessels in combination, and has a good correlation with DSA based suzuk's grading. It can be seen that PWI has a high sensitivity in the diagnosis of cerebral ischemia.

Currently, the accepted effective treatment of MMD is vascular reconstruction. Calamante et al. found that PWI could find hypoperfusion region in normal brain parenchyma shown by conventional MR scan, which could be used as a rake point for vascular reconstruction. The timing of surgery should be judged according to the cerebral perfusion situation. Studies suggest that early surgery should be performed when CBF is significantly reduced but not beyond the infarction threshold, and the prognosis of patients is good, most of which are manifested as significantly lower pwi-ttp in the early postoperative period in the surgical area.

ASL marks water molecules in arterial blood and ACTS as endogenous tracer. Before and after it flows into the brain parenchyma, images are collected and subtracted to obtain cerebral perfusion map. The CBF obtained can also be used for quantitative measurement. ASL is divided into continuous, pulse, pseudo-continuous and vascular coding ASL. ASL對缺血非常敏感，在腦缺血超急性期即可定量顯示其缺血程度，并可與DWI結合顯示缺血半暗帶，也可早期發現短暫性腦缺血發作（transient ischemic attack，TIA）患者CBF的下降。 Compared with PWI, it was found that ASL had good consistency with pwi-mtt and TTP, and was more sensitive to hypoperfusion. In terms of evaluating the prognosis of patients, ASL can better show the hyperperfusion after cerebral infarction, suggesting a good prognosis of patients. The decrease of ASL-CBF can predict the stroke attack in patients with TIA without infarction.

However, ASL also has limitations, that is, there is a delay in the time of filling blood vessels and entering tissues. Some scholars believe that ASL in MMD patients may underestimate the risk of cerebral perfusion due to the stenosis or occlusion of the main blood vessels and the longer time required for blood flow to pass through the collateral circulation. However, Wang et al. showed that the perfusion images of MMD patients were consistent with CT perfusion imaging after the delay of 1.5~2.0s after the setting of ASL marker, and it was believed that pseudo-continuous ASL could reduce the errors caused by the arterial passage time and extension. The study of Noguchi et al. on MMD patients also found that the cerebrovascular storage obtained by ASL had a higher consistency with SPECT results. After revascularization, transient neurologic deterioration occurred, and ASL was found to be abnormally perfusion in the acute phase, highly consistent with SPECT. Therefore, ASL has certain application value in the assessment of cerebral blood flow in MMD patients.

MRI cerebral perfusion imaging can be used for early and accurate assessment of patients' cerebral hemodynamics, which is suitable for regular follow-up of the level of cerebral perfusion in MMD patients, and is of great significance for understanding patients' conditions, grasping the operation opportunity, evaluating the operation effect and determining the prognosis.

4. Functional magnetic resonance imaging (fmri) is dependent on blood oxygen levels

血氧水平依賴功能磁共振成像（blood oxygenation level dependent functional magnetic resonance imaging，BOLD-fMRI）是一種集中了影像、功能、解剖的MRI成像技術，主要依靠氧化血紅蛋白及還原血紅蛋白的磁性差異，當神經功能激活，該區域血流量及氧含量發生改變，導致氧合血紅蛋白比升高，使T2信號升高，可直觀反映神經活動。 The traditional method is to ask patients to perform a specific task when collecting images. The brain function images obtained by this method are called task states. However, this method is difficult to be standardized in clinical practice, and patients are often unable to cooperate to complete the task. One study found that patients resting in different brain regions have spontaneous change signal, and has a certain correlation between brain region, the signals produced by the nerve physiological activity, known as spontaneous low-frequency amplitude (amplitude of low frequency fluctuations, ALFF), on behalf of the resting state, the spontaneous activity of the brain. Because it does not need the cooperation of patients and can reflect the connections between neural networks and brain regions, resting BOLDfMRI is often used in clinical research, which is the preferred method to study brain function at present.

As MMD progresses, adult patients may experience vascular cognitive impairment (VCI), including impairment in executive ability, memory, language and visual space. Although the clinical manifestations of VCI are similar to alzheimer's disease and mild cognitive impairment, the brain areas affected are not the same. When the cognitive ability of MMD patients is reduced, ALFF changes in multiple brain regions appear simultaneously, and the decrease represents the functional defect of this region, while the increase indicates the local compensatory sexual activity. Most MMD patients have the most significant ALFF changes in the parietal gyrus, right frontal gyrus, right temporal middle gyrus and left caudate nucleus. Thomas et al. performed BOLD-fMRI and PWI examination on MMD patients, which were obtained through post-processing

BOLD-fMRI delayed images and PWI-TTP, correlation analysis found that the two have a high consistency. Considering that BOLD fMRI imaging based on changes in blood oxygen content is equivalent to group injection of oxygenated blood in a cardiac cycle, the difference in oxygen content fluctuations in different parts of the brain is related to the time delay of blood flow through the artery, suggesting that BOLD fMRI can reflect cerebral blood perfusion and cerebrovascular reserve.

BOLD-fMRI is fast, noninvasive, radiation-free, easy to operate, and has a high spatial resolution. It can cover the whole brain. A single examination can provide anatomical, neural activity and hemodynamic information. Since the changes of brain function caused by nerve damage are much earlier than the changes of structure and diffuse information, this technique is helpful for early diagnosis and more targeted treatment to improve the prognosis of patients. However, due to the need for complex post-treatment analysis, it is rarely used in clinical practice.

5. Look forward to

With the continuous advent of new MRI technology, its application in the field of medical clinic and scientific research is more and more extensive. For the research related to MMD, MRI in the diagnosis, assessment of cerebral blood flow dynamics, research value of the collateral circulation has been recognized, but the potential is more than that, although the present study is less, follow-up MRI technology in disease progression hemal wall changes, evaluation of brain blood flow dynamics and poor prognosis, and the relationship between operation time, early diagnosis and cognitive impairment in individualized treatment has broad prospects, is closely related to clinical treatment and prognosis of patients.