弥散张量成像在意识障碍诊断中的应用

弥散张量成像（DTI）是一种非侵袭性的核磁成像技术,能够识别传统CT和MRI难以发现的脑微观结构改变,特别是神经纤维束变化和方向,目前DTI已成熟地应用于基础实验和临床研究中,能为严重脑损伤患者发现物标志物并且预测预后提供参考,DTI也在意识障碍的诊断和预后预测领域进行了很多探索研究。     

Fetal brain injury

Improvements in MRI techniques widen the indications for fetal brain imaging and fetal brain injury represents the third indication of fetal brain magnetic resonance imaging (MRI) after the evaluation of suspected central nervous system (CNS) malformations and ventricular dilatation. Optimal MR imaging technique is necessary in order to collect as much data as possible about the fetal brain. Diffusion images can be used routinely in addition to the standard protocol of fetal brain MRI that consists of T1 and T2 weighted images of the fetal brain. Monovoxel proton magnetic resonance spectroscopy can also be performed in utero, but this technique is still more part of research protocol than of routine clinical protocol. Fetal brain injury includes hypoxia-ischemia, congenital infections (especially toxoplasmosis and cytomegalovirus infections), brain damage due to malformation such as vascular brain malformation and heart malformation, pregnancies at risk of fetal brain damage, and even inherited metabolic diseases, especially mitochondrial diseases. MRI findings in fetal brain injury consist of acute or chronic lesions that can be seen alone or in combination. Acute response of the fetal brain is less commonly seen than the chronic response compared to the brain response encountered in the postnatal period.     

Advanced neuroimaging in children with nonaccidental trauma

Physical abuse associated with nonaccidental trauma (NAT) affects approximately 144,000 children per year in the USA and, frequently, these injuries affect the developing brain. Most infants with suspected NAT are initially evaluated by skull X-rays and computed tomography to determine whether fractures are present, the severity of the acute injury and the need for urgent neurosurgical intervention. Increasingly, magnetic resonance imaging (MRI) is conducted as it provides additional diagnostic and prognostic information about the extent and nature of the injury. In this review, we examine 4 MRI techniques as they apply to children who present acutely after NAT. Susceptibility-weighted imaging is a 3-D high-resolution MRI technique that is more sensitive than conventional imaging in detecting hemorrhagic lesions that are often associated with diffuse axonal injury (DAI). Magnetic resonance spectroscopy acquires metabolite information reflecting neuronal integrity and function from multiple brain regions and provides a sensitive, noninvasive assessment of neurochemical alterations that offers early prognostic information regarding outcome. Diffusion-weighted imaging (DWI) is based on differences in the diffusion of water molecules within the brain and has been shown to be very sensitive in the early detection of ischemic injury. It is now being used to study the direct effects of traumatic injury as well as those due to secondary ischemia. Diffusion tensor imaging is a form of DWI and allows better evaluation of white matter fiber tracts by taking advantage of the intrinsic directionality (anisotropy) of water diffusion in the human brain. It has been shown to be useful in identifying white matter abnormalities after DAI when conventional imaging appears normal. Although these imaging methods have been studied primarily in adults and children with accidental traumatic brain injury, it is clear that they have the potential to provide additional value in the imaging and clinical evaluation of children with NAT.     

Neuroimaging of Hemorrhage and Vascular Defects

Intracranial hemorrhage is the third most common cause of stroke and involves the accumulation of blood within brain parenchyma or the surrounding meningeal spaces. Accurate identification of acute hemorrhage and correct characterization of the underlying pathology, such as tumor, vascular malformation, or infarction, is a critical step in planning appropriate therapy. Neuroimaging studies are required not only for diagnosis, but they also provide important information on the type of hemorrhage, etiology, and the pathophysiological process. Historically, computed tomography (CT) scan has been the diagnostic imaging study of choice; however, there is growing evidence suggesting that magnetic resonance imaging (MRI) is at least as sensitive as CT to detect intraparenchymal hemorrhages in the hyperacute setting, and actually superior to CT in the subacute and chronic settings. Unique MRI and CT characteristics differentiate secondary causes of hemorrhage from the more common hypertensive hemorrhage. Baseline and serial studies can be used to identify patients who might benefit from acute interventions. In addition, new imaging modalities, (such as magnetic resonance spectroscopy, diffusion tensor imaging, and 320-row CT) are promising research techniques that have the potential to enhance our understanding of the tissue injury and recovery after intracranial hemorrhages.     

Diffusion-based magnetic resonance imaging and tractography in epilepsy

Diffusion-based imaging is an advanced MRI technique that is sensitive to the movement of water molecules, providing additional information on the micro-structural arrangement of tissue. Qualitative and quantitative analysis of peri, post and interictal diffusion images can aid the localization of seizure foci. Diffusion tensor tractography is an extension of diffusion-based imaging, and can provide additional information about white matter pathways. Both techniques are able to increase understanding of the effects of epilepsy on the structural organization of the brain, and can be used to optimize presurgical planning of patients with epilepsy. This review focuses on the basis, applications, limitations, and future directions of diffusion imaging in epilepsy. Literature search strategy: We searched Pubmed using the terms "diffusion MRI or diffusion tensor MRI or tractography and epilepsy."     

Advances in magnetic resonance imaging of brain tumours

PURPOSE OF REVIEW: Magnetic resonance imaging (MRI) of brain tumours provides excellent anatomical detail of brain tumours and can also reveal the biology, cellular structure and vascular dynamics of a tumour, although the use of such features in routine clinical practice has yet to be realized. In this review the latest advances in MRI of brain tumours are discussed and their clinical applications highlighted. RECENT FINDINGS: A large international study is underway to develop more powerful methods for automated classification of MR spectra based on the acquisition of large datasets of tumour spectra. Diffusion weighted imaging can help in the distinction between gliomas and abscesses, and perfusion weighted imaging can predict response to radiotherapy in low grade gliomas as well as distinguishing between different types of cerebral metastases. Intraoperative MRI has now been shown to be technically feasible, safe and effective in obtaining histological information as well as increasing the likelihood of complete resection for pituitary tumours and gliomas. Functional MRI and magnetic source imaging are alternative modalities that help the surgeon to avoid eloquent brain areas but may occasionally provide misleading information. Diffusion tensor imaging can demonstrate the effect of a tumour on white matter tracts and provides complementary information to that from other techniques that reveal areas of eloquent cortex. SUMMARY: Advances in MRI techniques are providing better diagnostic and therapeutic information, but can only ever be a surrogate marker of physiological and pathological processes; until it can routinely be used to image the brain at a cellular level, MRI will always be secondary to pathology in the final diagnostic evaluation.     

Neuroimaging characteristics of cerebral infarcts

Over the last years, technical advances in neuroimaging have allowed drastic improvements in the assessment of acute ischemic cerebral events. Beyond conventional morphological analysis, diffusion-weighted and perfusion-weighted MRI now enable routine functional assessment of brain tissue; spectroscopy and diffusion tensor imaging still remains in the domain of clinical research. During acute ischemia events, diffusion-weighted MRI can detect the movements of water molecules and cytotoxic edema related to cell injury enabling rapid diagnosis and early assessment of cerebral ischemia. In conjunction with perfusion imaging, which detects hypoperfusion areas, diffusion-weighted MRI provides a means to identify areas of penumbra ischemia. More recent multislice computed tomographic (CT) scans with multimodal analysis are also very competitive for assessment of cerebral ischemia (non-enhanced CT, CT angiography and perfusion CT). The purpose of this paper is to describe the CT and MRI patterns during the different stages of cerebral infarcts.     

1H-MR Spectroscopy in Traumatic Brain Injury

Traumatic brain injury (TBI) is a common cause of neurological damage and disability. Conventional imaging (CT scan or MRI) is highly sensitive in detecting lesions and provides important clinical information regarding the need for acute intervention. However, abnormalities detected by CT scan or conventional MRI have limited importance in the classification of the degree of clinical severity and in predicting patients’ outcome. This can be explained by the widespread microscopic tissue damage occurring after trauma, which is not observable with the conventional structural imaging methods. Advances in neuroimaging over the past two decades have greatly helped in the clinical care and management of patients with TBI. The advent of newer and more sensitive imaging techniques is now being used to better characterize the nature and evolution of injury and the underlying mechanisms that lead to progressive neurodegeneration, recovery or subsequent plasticity. This review will describe the role of proton magnetic resonance spectroscopic (MRS), an advanced MRI technique as related to its use in TBI. Proton MRS is a noninvasive approach that acquires metabolite information reflecting neuronal integrity and function from multiple brain regions and allows to assess clinical severity and to predict disease outcome.     

Diffusion Tensor Imaging (DTI)-based White Matter Mapping in Brain Research: A Review

Diffusion tensor imaging (DTI) has become one of the most popular MRI techniques in brain research, as well as in clinical practice. The number of brain studies with DTI is growing steadily and, over the last decade, has produced more than 700 publications. Diffusion tensor imaging enables visualization and characterization of white matter fascicli in two and three dimensions. Since the introduction of this methodology in 1994, it has been used to study the white matter architecture and integrity of the normal and diseased brains (multiple sclerosis, stroke, aging, dementia, schizophrenia, etc.). Although it provided image contrast that was not available with routine MR techniques, unique information on white matter and 3D visualization of neuronal pathways, many questions were raised regarding the origin of the DTI signal. Diffusion tensor imaging is constantly validated, challenged, and developed in terms of acquisition scheme, image processing, analysis, and interpretation. While DTI offers a powerful tool to study and visualize white matter, it suffers from inherent artifacts and limitations. The partial volume effect and the inability of the model to cope with non-Gaussian diffusion are its two main drawbacks. Nevertheless, when combined with functional brain mapping, DTI provides an efficient tool for comprehensive, noninvasive, functional anatomy mapping of the human brain. This review summarizes the development of DTI in the last decade with respect to the specificity and utility of the technique in radiology and anatomy studies.     

Imaging for the Diagnosis and Management of Traumatic Brain Injury

To understand the role of imaging in traumatic brain injury (TBI), it is important to appreciate that TBI encompasses a heterogeneous group of intracranial injuries and includes both insults at the time of impact and a deleterious secondary cascade of insults that require optimal medical and surgical management. Initial imaging identifies the acute primary insult that is essential to diagnosing TBI, but serial imaging surveillance is also critical to identifying secondary injuries such as cerebral herniation and swelling that guide neurocritical management. Computed tomography (CT) is the mainstay of TBI imaging in the acute setting, but magnetic resonance tomography (MRI) has better diagnostic sensitivity for nonhemorrhagic contusions and shear-strain injuries. Both CT and MRI can be used to prognosticate clinical outcome, and there is particular interest in advanced applications of both techniques that may greatly improve the sensitivity of conventional CT and MRI for both the diagnosis and prognosis of TBI.     

Exploring impaired consciousness: the MRI approach

PURPOSE OF REVIEW: To summarize the application of advanced MRI sequences such as magnetic resonance spectroscopy, diffusion tensor imaging and functional MRI for the evaluation of patients with altered consciousness. RECENT FINDINGS: Magnetic resonance spectroscopy, volumetry and diffusion tensor imaging have shown promising results in the evaluation of traumatic or anoxo-ischaemic brain lesions and can detect damage of the brainstem, basal ganglia and white matter tracts not visible on conventional sequences. A diffusion tensor imaging study has raised the possibility of detecting ongoing axonal regrowth many years after the initial injury in minimally conscious patients. Functional MRI studies have shown that a high level of brain activities, such as recognizing one's own name or imagining playing tennis, can be preserved in vegetative patients. SUMMARY: The development of quantitative imaging could lead to a more objective evaluation of the extent of destruction or preservation of critical brain areas at the acute phase of brain injury, which could be integrated in multi-parametric decisional strategies for these patients. Functional imaging could help define borders between the various levels of altered consciousness and detect the presence of cryptic residual functions in vegetative or minimally conscious patients. This approach could eventually help determine the neurological outcome and make individual blueprints of the preserved brain activities in severely brain injured patients.     

Imaging of intracranial haemorrhage

Intracranial haemorrhage can be a devastating disorder that requires rapid diagnosis and management. Neuroimaging studies are not only required for diagnosis but also provide important insights into the type of haemorrhage, the underlying aetiology, and the accompanying pathophysiology. Historically, CT has been the diagnostic imaging study of choice; however, there is a growing body of data that suggest that MRI is at least as sensitive as CT to detect haemorrhage in the hyperacute setting, and superior to CT in the subacute and chronic settings. Blood has characteristic appearances on both imaging modalities at each stage (acute, subacute, and chronic) and it is important that physicians are familiar with the appearance of various types of intracranial haemorrhage on CT and MRI and their clinical implications. In addition, new imaging applications, such as magnetic resonance spectroscopy and diffusion tensor imaging, are promising research techniques that have the potential to enhance our understanding of the tissue injury and recovery that result from intracranial haemorrhage.     

Difference of injury of the corticospinal tract according to surgical or conservative treatment in patients with putaminal hemorrhage

Objective: We investigated difference of injury of the corticospinal tract (CST) according to surgical or conservative treatment in patients with putaminal hemorrhage (PH), using diffusion tensor tractography (DTT). Methods: Forty-six patients with PH (hematoma volume on the brain CT: 20–40 ml) were recruited. Patients were classified as the surgical treatment group and the conservative treatment group. The hematoma volume on the initial brain CT (median 2 hours after onset; range 1–14 hours) and volumes of the hematoma, the total lesion and the peri-hematomal edema volume on the follow-up brain magnetic resonance imaging (MRI) (median 23.5 days after onset; range 12–46 days) were estimated. Diffusion tensor imaging was performed and we defined the injury of the CST in terms of the configuration or abnormal DTT parameters. Results: In the conservative treatment group, the total lesion volume on the brain MRI was increased compared with the hematoma volume on the initial brain CT (p < 0.05). On brain MRI, the hematoma volume, peri-hematomal edema volume, and total lesion volume were larger in the conservative treatment group than in the surgical treatment group (p < 0.05). Twelve patients (60%) in the surgical treatment group and 24 patients (92%) in the conservative treatment group had injury of the CST. Conclusion: Injury of the CST was less prevalent in the surgical treatment group than in the conservative treatment group in patients with PH. Therefore, it appears that surgical treatment could be helpful in prevention of injury of the CST in patients with PH.     

Use of advanced neuroimaging techniques in the evaluation of pediatric traumatic brain injury

Advanced neuroimaging techniques are now used to expand our knowledge of traumatic brain injury, and increasingly, they are being applied to children. This review will examine four of these methods as they apply to children who present acutely after injury. (1) Susceptibility weighted imaging is a 3-dimensional high-resolution magnetic resonance imaging technique that is more sensitive than conventional imaging in detecting hemorrhagic lesions that are often associated with diffuse axonal injury. (2) Magnetic resonance spectroscopy acquires metabolite information reflecting neuronal integrity and function from multiple brain regions and provides sensitive, noninvasive assessment of neurochemical alterations that offers early prognostic information regarding the outcome. (3) Diffusion weighted imaging is based on differences in diffusion of water molecules within the brain and has been shown to be very sensitive in the early detection of ischemic injury. It is now being used to study the direct effects of traumatic injury as well as those due to secondary ischemia. (4) Diffusion tensor imaging is a form of diffusion weighted imaging and allows better evaluation of white matter fiber tracts by taking advantage of the intrinsic directionality (anisotropy) of water diffusion in human brain. It has been shown to be useful in identifying white matter abnormalities after diffuse axonal injury when conventional imaging appears normal. An important aspect of these advanced methods is that they demonstrate that 'normal-appearing' brain in many instances is not normal, i.e. there is evidence of significant undetected injury that may underlie a child's clinical status. Availability and integration of these advanced imaging methods will lead to better treatment and change the standard of care for use of neuroimaging to evaluate children with traumatic brain injury.     

MR diffusion imaging in ischemic stroke

Diffusion-weighted MRI provides image contrast that is dependent on the molecular motion of water. Diffusion-weighted imaging is the most reliable method for early detection of cerebral ischemia, for the definition of infarct core, and for the differentiation of acute ischemia from other disease processes that mimic stroke. Diffusion tensor imaging and diffusion kurtosis imaging may offer additional diagnostic information on the microstructural status of tissue. This review discusses the development and applications of diffusion-weighted imaging, diffusion tensor imaging, and diffusion kurtosis imaging in acute and chronic ischemia.     

Vulnerability of the anterior commissure in moderate to severe pediatric traumatic brain injury

In relation to the adult brain, the immature brain might be more vulnerable to damage during and following traumatic brain injury, particularly in white-matter tracts. Given well-established evidence of corpus callosum atrophy, we hypothesized that anterior commissure volume (using quantitative magnetic resonance imaging [MRI]) in this structure would be decreased in children with moderate to severe traumatic brain injury relative to typically developing children. Second, given the purported role of the anterior commissure in interhemispheric axon conveyance between temporal lobes, we hypothesized that temporal lobe white matter, temporal lesion volume, and injury severity (Glasgow Coma Scale score) would be predictive of decreased anterior commissure cross-sectional volume in patients with traumatic brain injury. Finally, we wished to establish the relationship between the anterior commissure and the temporal stem, a major white-matter tract into the temporal lobes, using diffusion tensor imaging fiber-tracking maps for each patient. We also hypothesized that children with traumatic brain injury would exhibit decreased fractional anisotropy in relation to typically developing children in a fiber system including the anterior commissure and the temporal lobes. Decreased anterior commissure cross-sectional volume was observed in patients with traumatic brain injury, and, as predicted, anterior commissure and temporal white-matter volumes were positively related to each other and to higher Glasgow Coma Scale scores. Lesion volume was not independently predictive of anterior commissure volume in the overall model. Diffusion tensor imaging fractional anisotropy values differed between the groups for the temporal stem-anterior commissure system, with the traumatic brain injury group exhibiting decreased fractional anisotropy. The anterior commissure, like the corpus callosum, appears to be highly vulnerable to white-matter degenerative changes resulting from mechanisms such as the direct impact of trauma, progressive axonal injury as tissue in other brain regions atrophies, or myelin degeneration. This is the first systematic examination of anterior commissure atrophy following traumatic brain injury using in vivo quantitative MRI and diffusion tensor imaging fiber tracking in pediatric subjects.     

Diffusion MRI in multiple sclerosis

Diffusion imaging is a quantitative, MR-based technique potentially useful for the study of multiple sclerosis (MS), due to its increased pathologic specificity over conventional MRI and its ability to assess in vivo the presence of tissue damage occurring outside T2-visible lesions, i.e., in the so-called normal-appearing white and gray matter. The present review aims at critically summarizing the state-of-the-art and providing a background for the planning of future diffusion studies of MS. Several pieces of evidence suggest that diffusion-weighted and diffusion tensor MRI are sensitive to MS damage and able to detect its evolution over relatively short periods of time. Although a significant relationship between diffusion-weighted MRI findings and MS clinical disability was not found in the earliest studies, with improved diffusion imaging technology correlations between diffusion abnormalities and MS clinical aspects are now emerging. However, the best acquisition and postprocessing strategies for MS studies remain a matter of debate and the contribution of newer and more sophisticated techniques to diffusion tensor MRI investigations in MS needs to be further evaluated. Although changes in diffusion MRI indices reflect a net loss of structural organization, at present we can only speculate on their possible pathologic substrates in the MS brain. Postmortem studies correlating diffusion findings with histopathology of patients with MS are, therefore, also warranted.     

Structural and functional neuroimaging in mild-to-moderate head injury

Head injury is a major cause of disability and death in adults. Significant developments in imaging techniques have contributed to the knowledge of the pathophysiology of head injury. Although extensive research is available on severe head injury, less is known about mild-to-moderate head injury despite the fact that most patients sustain this type of injury. In this review, we focus on structural and functional imaging techniques in patients with mild-to-moderate head injury. We discuss CT and MRI, including different MRI sequences, single photon emission computed tomography, perfusion-weighted MRI, perfusion CT, PET, magnetic resonance spectroscopy, functional MRI and magnetic encephalography. We outline the advantages and limitations of these various techniques in the contexts of the initial assessment and identification of brain abnormalities and the prediction of outcome.     

A review of magnetic resonance imaging and diffusion tensor imaging findings in mild traumatic brain injury

Mild traumatic brain injury (mTBI), also referred to as concussion, remains a controversial diagnosis because the brain often appears quite normal on conventional computed tomography (CT) and magnetic resonance imaging (MRI) scans. Such conventional tools, however, do not adequately depict brain injury in mTBI because they are not sensitive to detecting diffuse axonal injuries (DAI), also described as traumatic axonal injuries (TAI), the major brain injuries in mTBI. Furthermore, for the 15 to 30 % of those diagnosed with mTBI on the basis of cognitive and clinical symptoms, i.e., the "miserable minority," the cognitive and physical symptoms do not resolve following the first 3 months post-injury. Instead, they persist, and in some cases lead to long-term disability. The explanation given for these chronic symptoms, i.e., postconcussive syndrome, particularly in cases where there is no discernible radiological evidence for brain injury, has led some to posit a psychogenic origin. Such attributions are made all the easier since both posttraumatic stress disorder (PTSD) and depression are frequently co-morbid with mTBI. The challenge is thus to use neuroimaging tools that are sensitive to DAI/TAI, such as diffusion tensor imaging (DTI), in order to detect brain injuries in mTBI. Of note here, recent advances in neuroimaging techniques, such as DTI, make it possible to characterize better extant brain abnormalities in mTBI. These advances may lead to the development of biomarkers of injury, as well as to staging of reorganization and reversal of white matter changes following injury, and to the ability to track and to characterize changes in brain injury over time. Such tools will likely be used in future research to evaluate treatment efficacy, given their enhanced sensitivity to alterations in the brain. In this article we review the incidence of mTBI and the importance of characterizing this patient population using objective radiological measures. Evidence is presented for detecting brain abnormalities in mTBI based on studies that use advanced neuroimaging techniques. Taken together, these findings suggest that more sensitive neuroimaging tools improve the detection of brain abnormalities (i.e., diagnosis) in mTBI. These tools will likely also provide important information relevant to outcome (prognosis), as well as play an important role in longitudinal studies that are needed to understand the dynamic nature of brain injury in mTBI. Additionally, summary tables of MRI and DTI findings are included. We believe that the enhanced sensitivity of newer and more advanced neuroimaging techniques for identifying areas of brain damage in mTBI will be important for documenting the biological basis of postconcussive symptoms, which are likely associated with subtle brain alterations, alterations that have heretofore gone undetected due to the lack of sensitivity of earlier neuroimaging techniques. Nonetheless, it is noteworthy to point out that detecting brain abnormalities in mTBI does not mean that other disorders of a more psychogenic origin are not co-morbid with mTBI and equally important to treat. They arguably are. The controversy of psychogenic versus physiogenic, however, is not productive because the psychogenic view does not carefully consider the limitations of conventional neuroimaging techniques in detecting subtle brain injuries in mTBI, and the physiogenic view does not carefully consider the fact that PTSD and depression, and other co-morbid conditions, may be present in those suffering from mTBI. Finally, we end with a discussion of future directions in research that will lead to the improved care of patients diagnosed with mTBI.     

Differentiation of mechanism and prognosis of traumatic brain stem lesions detected by magnetic resonance imaging in the acute stage

We retrospectively evaluated the MRI from 17 patients with primary brain stem injury obtained in the acute stage. Clinical and radiological findings were analyzed in these 17 patients. T2-weighted imaging proved to be most sensitive and specific for the diagnosis of primary brain stem injury. We found two patterns of brain stem injury. The good prognosis group showed ventral brain stem lesions or dorsal superficial brain stem lesions. On the other hand the poor prognosis group showed deep dorsal brain stem lesions. These acute stage findings are seen only temporally in many cases so that it is most important to examine MRI findings in the acute stage to evaluate the prognosis of the patient. MRI was valuable in predicting the outcome. The possible mechanism of brain stem injury in patients with head injury is briefly discussed.