Advanced MR techniques: diffusion MR imaging, perfusion MR imaging, and spectroscopy.

Recent technical advances in MR imaging have enabled the authors to investigate early physiological changes in acute ischemic stroke lesion. Diffusion and perfusion MR imaging can provide clinically useful information not only for early detection of ischemia, but also for prediction of tissue outcome. MR spectroscopy is a potentially powerful tool to study acute stroke, but its clinical value has been limited due to long examination time and low spatial resolution.     

Recovery from aphasia and neglect after subcortical stroke: neuropsychological and cerebral perfusion study.

Cortical regional cerebral perfusion was assessed by N, N, N1-trimethyl-N1-(2)-hydroxy-3-methyl-5-(I-123) iodobenzyl-1, 3-propanediamine 2 HCl I-123 (HIPDM) and single photon emission computerised tomography (SPECT) in six aphasic and two neglect patients with unilateral subcortical vascular lesions. Assessments were carried out both in the acute phase and after a period ranging from 1 to 6 months after stroke onset. In all patients an almost complete spontaneous recovery occurred and was associated with a significant improvement of cortical perfusion. A relationship between severity of aphasia and degree of cortical hypoperfusion was found, in both the acute and the follow up assessments, in the aphasic subgroup.     

Diffusion and perfusion MR imaging in cerebral lymphomas

Because of the increasing incidence of cerebral lymphoma, it is critical for patient management to recognize the MR features of this disease. We present the characteristic morphological and functional MRI features of this tumor. The findings on MRI studies, including morphological, diffusion and perfusion imaging, performed in 9 biopsy-proven cases of cerebral lymphoma with 13 lesions are presented and analyzed, and are discussed in comparison with published literature data. All patients underwent diffusion-weighted imaging with a single shot echo-planar pulse sequence. Dynamic susceptibility-contrast MRI was performed using a T2*-weighted gradient-echo echo-planar sequence after intravenous injection of chelates of gadolinium at the rate of 6 ml/s and a temporal resolution of 1 second. All cases of cerebral lymphoma appeared hypointense or isointense on T1-weighted images and in 75% of cases iso- or hypointense on T2-weighted images. All lesions enhanced except one in a patient receiving steroid therapy. On diffusion-weighted images, tumours were hyperintense with normal or decreased ADC values (0.717+/-0.152.10-3 mm2/sec, range: 0.550-1.014) and an ADC ratio tumour/normal white matter of 0.974+/-0.190 (range: 0.768-1.410). On perfusion, the signal intensity-time curve of each tumour showed a characteristic type of curve with a significant increase of the signal intensity above the baseline and a low maximum relative cerebral blood volume ratio (rCVBmax) of 1.43+/-0.64 (0.55-2.62). Due to their higher cellularity, the lack of neoangiogenesis, and the increased permeability of the blood-brain barrier related to the infiltration of blood vessels wall by lymphomatous cells, cerebral lymphoma presents characteristic diffusion and perfusion MRI features that should be useful for diagnosis and patient follow-up.     

MR diffusion and perfusion imaging in clinical practice

MR functional imaging, due to the improvement in ultra-speed imaging technology such as echo-planar imaging, has become a very powerful technique since the beginning of the nineties. This imaging technique is divided into diffusion imaging, perfusion imaging and cerebral activation. Diffusion imaging probes the mobility of water molecules characterized by a diffusion coefficient called the apparent diffusion coefficient (ADC) for biological tissues. Perfusion imaging gives hemodynamic information due to the regional cerebral blood volume by the use of contrast agents such as chelates of gadolinium carrying strong magnetic susceptibility. Both imaging techniques can provide information in a wide nosological range : cerebral ischemia, in the acute phase and in case of intracranial tumors, contributing to tumoral grading, localizing the site of biopsy, and assessing response to therapy (after radiotherapy for example). Nevertheless, a wide range of domains remains incompletely studied, for example cerebral white matter diseases and neurodegenerative diseases. For clinical applications, a precise knowledge of the potentials of both techniques and their limitations is needed. Limitations result from the large number of often patient-related parameters, imaging technique (perfusion) and data analysis. Powerful software has been developed in the workstation environment. Thus this imaging technique requires up-to-date equipment and close collaboration between clinical and research teams for optimal efficiency.     

MR perfusion imaging in acute ischemic stroke

Magnetic resonance (MR) perfusion imaging offers the potential for measuring brain perfusion in acute stroke patients, at a time when treatment decisions based on these measurements may affect outcomes dramatically. Rapid advancements in both acute stroke therapy and perfusion imaging techniques have resulted in continuing redefinition of the role that perfusion imaging should play in patient management. This review discusses the basic pathophysiology of acute stroke, the utility of different kinds of perfusion images, and research on the continually evolving role of MR perfusion imaging in acute stroke care.     

Clinical applications of intracranial perfusion MR imaging.

Dynamic susceptibility MR perfusion imaging of the brain offers clinically relevant physiological data not obtainable by conventional MR imaging. As new treatments continue to be developed for stroke, neoplasm, dementia, psychiatric illness, headache, and trauma, the potential clinical applications of perfusion MR imaging in the diagnosis, triage, and therapeutic monitoring of these diseases will increase. MR perfusion techniques are likely to be at least as sensitive and specific as radionuclide-based techniques, and offer the added advantage of higher intrinsic resolution, convenient coregistration with conventional MR imaging, as well as, time and cost-effective imaging in patients for whom a routine MR imaging is obtained.     

Diffusion tensor and perfusion imaging of brain tumors in high-field MR imaging

Diffusion tensor imaging (DTI) and perfusion-weighted imaging (PWI) are essential tools for diagnosing, differentiating, and monitoring brain tumors. High-field MRI provides higher signal-to-noise ratio, shorter scan time, and better image quality. One-stop multiparametric study, including DTI and PWI, is possible with high-field MRI in brain tumors. DTI can be used for assessing spatial relationship between major white matter tract and tumor, differentiating gliomas from nonglial tumors, and postoperative evaluation. PWI provides reliable biomarkers for glioma grading, therapeutic responses, and differential diagnosis of various brain tumors. With higher field strength, better-quality DTI and PWI can raise the diagnostic accuracy in brain tumors.     

Allocentric neglect strongly associated with egocentric neglect

Following brain injury, many patients experience egocentric spatial neglect, where they fail to respond to stimuli on the contralesional side of their body. On the other hand, allocentric, object-based neglect refers to the symptom of ignoring the contralesional side of objects, regardless of the objects' egocentric position. There is an established tradition for considering these two phenomena as both behaviorally and anatomically dissociable. However, several studies and some theoretical work have suggested that these rather reflect two aspects of a unitary underlying disorder. Furthermore, in a recent large study Yue et al. [Archives of Physical Medicine and Rehabilitation 93 (2012) 156] reported that acute allocentric neglect is only observed in cases where substantial egocentric neglect is also present. In a new sample of right hemisphere stroke patients, we attempted to control for potential confounds by using a novel continuous measure for allocentric neglect (in addition to a recently developed continuous measure for egocentric neglect). Our findings suggest a strong association between egocentric and allocentric neglect. Consistent with the work of Yue et al. (2012), we found allocentric behavioral deficits only in conjunction with egocentric deficits as well as a large corresponding overlap for the anatomical regions associated with egocentric and with allocentric neglect. We discuss how different anatomical and behavioral findings can be explained in a unified physiologically plausible framework, whereby allocentric and egocentric effects interact.     

Spatial neglect in a patient with logopenic progressive aphasia

Spatial neglect and extinction are induced by posterior superior temporal and inferior parietal dysfunction. In patients with logopenic progressive aphasia (LPA) these structures are often degenerated, but there are no reports of these disorders being associated. A 53-year-old man with the signs of LPA revealed right-sided spatial neglect on line bisection and drawing tests as well as multimodal extinction. MRI showed left hemispheric posterior temporoparietal atrophy. Since injury to the core structures for these aphasic and attentional syndromes overlaps, patients with LPA should be screened for spatial neglect and extinction.     

Pediatric perfusion MR imaging using arterial spin labeling

Cerebral blood flow (CBF, cerebral perfusion) mirrors cerebral metabolic demand and neuronal function, and therefore, is a vital parameter in the evaluation of pediatric brain injury and recovery. Until recently, measurement of CBF involved intravenous bolus injection of contrast agents or nuclear medicine methods that were technically difficult or ethically problematic in pediatrics. The development of arterial spin label (ASL) perfusion MR imaging as a noninvasive method for measuring CBF allows for the increased ability to measure this vital physiologic parameter in any age group. This article presents the technical aspects of performing ASL perfusion MR imaging in pediatrics, and discusses its current use in clinical studies and its potential for influencing important management strategies in specific disease entities.     

Dynamic susceptibility-weighted contrast-enhanced perfusion MR imaging in pediatric patients

Dynamic susceptibility-weighted contrast-enhanced (DSC) perfusion MR (pMR) imaging provides hemodynamic information that complements traditional structural MR imaging and is becoming increasingly used in clinical practice to diagnose, manage, and understand brain tumors in the pediatric patient group. pMR imaging-derived regional cerebral blood volume (rCBV) maps provide quantitative estimates of rCBV that can be used to grade gliomas, differentiate between different brain tumor types, and distinguish tumor from nonneoplastic lesions. There are a few minor limitations of the DSC pMR imaging technique, such as susceptibility artifacts, relative rather than absolute quantification of cerebral blood volume (CBV), and inaccurate estimation of CBV in situations of severe disruption or absence of the blood-brain barrier. Recognizing its strengths and potential pitfalls, pMR imaging can be used as part of the routine evaluation of brain tumors to improve the diagnostic accuracy, understand tumor pathophysiology, detect and quantify tumor angiogenesis, and, with further work, serve as an arbiter to assess existing and novel cancer therapies that target blood vessels.     

Reversible nonfluent aphasia and left frontal hypoperfusion during topiramate treatment

We report the case of a patient with complex partial seizures who developed a nonfluent aphasia when topiramate was added to his therapy. This emergent adverse effect appeared to be reversible, as language performance improved after discontinuation of topiramate. Interictal SPECT performed when the patient was aphasic revealed a focal perfusion reduction in the left lateral and mesial frontal cortex, which was no longer evident at a follow-up study after language recovery.     

Crossed aphasia with left spatial neglect and visual imperception: a case report

A 64-year-old right-handed woman with no left-handers in the family developed aphasia associated with moderate left hemiparesis and dense left homonymous hemianopia following rupture of a right middle cerebral artery aneurysm and subsequent selective surgery confined to the right hemisphere. Severe left spatial neglect and constructional apraxia were also present. The patient was an achondroplasic dwarf whose previous medical and neurological history was otherwise unremarkable. Computed tomography of the brain showed a large right temporo-insulofrontoparietal lesion. Language and nonverbal cognitive functions were assessed after 2 and 6 months, and then four years later. A reportedly overall language disruption in the acute period evolved into Wernicke's aphasia and then into a mild form of conduction aphasia. The associated left spatial neglect eventually shrank to a minimum. The patient never had clinically detectable visual agnosia, but on specific tests of visual recognition and perception some impairment was found four years after onset. The left hemiparesis disappeared in time while the left hemianopia persisted. This case is a convincing example of an entirely righthanded person in whom both linguistic and visuospatial functions are represented in the right hemisphere. Received: 12 May 2002 / Accepted in revised form: 18 November 2002 Correspondence to L.A. Vignolo     

Association of ictal aphasia with hypoperfusion in language areas in temporal lobe epilepsy patients

Ictal clinical phenomenology, including aphasia, is usually associated with increased regional cerebral perfusion. We present an unusual pattern of ictal cerebral perfusion in three patients with pharmacoresistant, lesional temporal lobe epilepsy and ictal/postictal aphasia studied with prolonged video-EEG, ictal, and interictal SPECT and MRI for pre-surgical evaluation. Subtraction of ictal-interictal SPECT images co-registered with MRI (SISCOM) showed ictal hyperperfusion in the temporal epileptogenic area in all patients. In addition, hypoperfusion of Broca's area in one case, Wernicke's area in other patient, and both areas in the remaining one were observed. Ictal aphasia in these patients may be explained by functional inhibition of a primary language area, driven by the epileptogenic network. This pattern can contribute to understand the pathophysiology of some ictal signs, with an impact on the evaluation of individual surgical risks.