MRI of the temporal lobe: normal variations, with special reference toward epilepsy.

Recent investigations of epilepsy, Alzheimer's disease, amnesia, and schizophrenia have used magnetic resonance imaging (MRI) to evaluate changes in temporal lobe structures. Normal variations in these structures need to be defined before one can use these structures to describe abnormal conditions. Twenty-nine normal volunteers were studied by coronal MRI. Frequent findings include notching of the uncus by the tentorium or adjacent vessels (22/29) and asymmetry of the temporal horns (20/29). This finding of uncal notching strengthens the evidence against "incisural sclerosis" as the basis for hippocampal sclerosis. Temporal horn dilatation occurred in four. However, mild asymmetry of the temporal horn was seen frequently at its anterior tip (16/29) and may be related to head rotation. Asymmetry of the choroidal fissure was never marked. Mild asymmetry was common at the hippocampal head (pes). Mild enlargement of the right temporal lobe by visual inspection is not uncommon. Subtle asymmetry of the white matter between the hippocampus and the collateral sulcus occurred in six. The collateral sulcus does not always point to the temporal horn. The occipitotemporal sulcus may point to the temporal horn. Asymmetric uncal protrusion (0/29) and Sylvian fissure dilatation (4/29) occur rarely.     

The diagnosis of hippocampal sclerosis: Other techniques

Pathologically, hippocampal sclerosis (HS) is characterized by neuronal loss and gliosis affecting particularly the pyramidal neurons of CA1, CA3, and CA4 with relative sparing of the CA2 neurons. This can be identified in vivo with magnetic resonance (MR) imaging techniques that can reveal both morphological and signal abnormalities. The morphological changes are atrophy and loss of the normal internal architecture of the hippocampus as seen in coronal section. There is also T₁- and T₂-weighted signal abnormality in the hippocampus. Quantitative techniques are very good at measuring any single one of these features, but the spectrum of HS includes cases in which a single feature can occasionally be misleading. Also, quantitation focuses entirely on the hippocampus, and it is becoming clear that HS may exist in the presence of other brain pathology that may affect proper management of the patient. Therefore, quantitative measures should always be interpreted in the context of optimised imaging sequences and visual inspection. For routine clinical purposes, the relative reliance on quantitation (hippocampal volume or T₂ measurements) depends entirely on the yield of visual inspection in any institution. This, in turn, depends on whether optimised imaging is performed and on the familiarity of the reporting specialist with the MRI features of HS. A technique which approaches 95–100% compared with pathology is essential in any epilepsy centre, and optimised visual analysis can achieve this. There are some cases where quantitation of a single feature can be misleading, so visual analysis should always be performed, and complements any quantitative study.     

Electric stimulation fMRI of the perforant pathway to the rat hippocampus

The hippocampal formation is a brain system that is implicated in learning and memory. The major input to the hippocampus arrives from the entorhinal cortex (EC) to the dentate gyrus (DG) through the perforant path. In the present work, we have investigated the functional properties of this connection by concomitantly applying electrophysiological techniques, deep-brain electric microstimulation and functional magnetic resonance imaging in anesthetized rats. We systematically delivered different current intensities at diverse stimulation frequencies to the perforant path while recording electrophysiological and blood-oxygenation-level-dependent (BOLD) signals. We observed a linear relationship between the current intensity used to stimulate the hippocampal formation and the amplitude and extension of the induced BOLD response. In addition, we found a frequency-dependent spatial pattern of activation. With stimulation protocols and train frequencies used for kindling, the activity strongly spreads ipsilaterally through the hippocampus, DG, subiculum and EC.     

Rapid stereological quantitation of temporal neocortex in TLE

To determine the extent of neocortical atrophy in the temporal lobe using rapid stereological analysis of magnetic resonance slices in patients with temporal lobe epilepsy and to compare the findings to those obtained by visual analysis of high-resolution magnetic resonance images. 25 patients with temporal lobe epilepsy, along with 25 age-matched controls were scanned using a 1.5 Tesla magnetic resonance imaging machine (GE signa systems Paris). Visual analysis was performed on standard high-resolution images. Volumetric analysis of hippocampus and temporal neocortex was performed using computer-aided stereology (MEASURE program, Patrick Barta, Johns Hopkins, Baltimore, USA). Stereological volumetric analysis demonstrated isolated hippocampal atrophy in only nine (36%) cases including three (12%) with bilateral disease. However, eight (32%) cases had combined hippocampal and neocortical atrophy and three (12%) had isolated neocortical atrophy. All volumetric measurements took less than 10 min. On the other hand, visual analysis suggested that 17 (68%) had hippocampal atrophy alone with only two (8%) having combined neocortical atrophy and a further two (8%) having isolated neocortical atrophy. Nearly half of the patients had temporal neocortical atrophy with or without hippocampal atrophy. This rapid, accurate and non-biased quantitative technique has wide clinical utility and is significantly more valuable in detecting neocortical atrophy than visual analysis alone. The results support the notion that abnormalities may be overlooked by current standards of routine magnetic resonance imaging.     

MRI of the normal hippocampus.

Before it is possible to use MR imaging to investigate changes in the hippocampus in disease processes such as epilepsy and memory disorders, it is imperative that normal variations are defined. Using specific anatomic locations, we evaluated the hippocampi of 29 normal volunteers with coronal MR studies. Mild variations occur with regard to hippocampal size and shape, and hippocampal fissure visualization. Hippocampal signal intensity is isointense to cortical gray matter. Recognition of normally occurring variations should help prevent over-interpretation of hippocampal changes in pathologic disorders.     

Automatic detection of hippocampal atrophy on magnetic resonance images.

An automatic method for identifying hippocampal atrophy on magnetic resonance (MR) images obtained from patients with clinical evidence of temporal lobe epilepsy (TLE) is described. The method is based on the analysis of image intensity differences between patients and controls within a volume of interest (VOI) centred on the hippocampus. The core of the method is a fully automatic signal intensity-based inter-subject image registration technique. In particular, a global affine registration to a reference image is performed, followed by a local affine registration within the VOI. A mask produced by manual segmentation of the mean hippocampus for 30 control subjects enabled investigations to be restricted to a specified region of the VOI approximately corresponding to the hippocampus. Normal variations of hippocampal signal intensity were computed from images obtained for the 30 control subjects. The manual method of hippocampal volumetry, currently an important component of the pre-surgical evaluation of patients with clinical evidence of medically intractable TLE, is used to determine the lower 1st percentile limits of normal hippocampal volume. Hippocampi with volumes below this limit are defined as atrophic. We investigated whether the automatic method can correctly distinguish between 15 patients with significant hippocampal atrophy according to absolute volumes and a further 14 controls. ROC curves enabled evaluation of sensitivity and specificity in respect of an intensity threshold. 100% specificity is required when determining suitability of patients for neurosurgery, resulting in levels of 50% and 70% sensitivity in detecting atrophy in the right and left hippocampus, respectively. We propose that the method can be developed as an automatic screening procedure.     

MRI hippocampal volume and neuropsychology in epilepsy surgery

A review is provided of recent findings on relationships between neurocognitive test data and magnetic resonance imaging (MRI)-determined hippocampal volumes in nonlesional temporal lobectomy patients. The difference between the right and left hippocampal volumes is correlated with postoperative verbal memory in left temporal lobectomy patients who do not have lesional pathology. MRI hippocampal volume data are not associated with measures of executive functioning or naming. Sex differences have been found for verbal memory outcome as women have better verbal memory following left temporal lobectomy. Sex differences have also been found in the relationships between verbal and visual memory, and hippocampal volume data. The systematic combination of MRI-acquired morphological data and neuropsychological test data may further our understanding of neurocognitive function, and provide clinically useful data for counseling epilepsy surgery patients. The current data are promising with regard to prediction of memory outcome following temporal lobectomy, but they do not yet allow for prediction of specific individual patient outcomes. Rather, the currently available data support counseling patients based on the memory outcome of others with similar characteristics.     

Magnetization transfer MRI of mouse brain reveals areas of high neural density

Extending applications of magnetization transfer contrast (MTC) in magnetic resonance imaging (MRI) of the human central nervous system, this work quantitatively describes MTC of the murine brain. As a novel finding, complementing T₁- and T₂-weighted MRI, MTC allows for the distinction of densely packed gray matter from normal gray and white matter. Examples include the Purkinje cell layer and the granular cell layer in the mouse cerebellum as well as the delineation of the CA3 subfield of the hippocampus relative to surrounding hippocampal gray matter and white matter tracts such as the hippocampal fimbria. Using a kainate lesion model, the CA3 hyperintensities in MTC and T₁-weighted MRI are assigned to the densely packed somata of pyramidal cells.     

Numerical calculations of the static magnetic field in three-dimensional multi-tissue models of the human head

Susceptibility-induced perturbation of the static magnetic field by the human body during magnetic resonance imaging (MRI) leads to undesirable artifacts as well as valuable physiological information, as in functional MRI. The ability to calculate these perturbations for a multi-tissue human body model provides a powerful tool in designing hardware and acquisition methods for reducing the artifacts, and in relating observed image contrast to physiological origins. We have developed a method for calculating the static field in arbitrary 3D magnetic susceptibility distributions and performed calculations in a complete model of the human head and shoulders. The accuracy of our method was validated in regular geometries with known analytical solutions and in comparison with experimental results acquired from the head of the same human subject used for computer modeling. Results are presented in parts per million (ppm) deviation from the applied field strength and are valid for any imaging or spectroscopy system.     

Amyotrophic lateral sclerosis with predominant pyramidal signs -- early diagnosis by magnetic resonance imaging

OBJECTIVE: To evaluate the role of magnetic resonance imaging (MRI) in the early diagnosis of amyotrophic lateral sclerosis (ALS) with predominant upper motor neuron (UMN) signs in the initial stage of the disease. PATIENTS AND METHOD: Two patients with ALS were found to have spastic quadriparesis with no wasting or fasciculation in the limbs in the early stage of the disease. Both were subjected to MRI of the head and cervical spine to look for any specific diagnostic feature. Both of them were followed with clinical evaluation and electroneuromyography (ENMG) for the definitive diagnosis of ALS. RESULTS: Magnetic resonance imaging showed selective degeneration of the pyramidal tracts in the contiguous axial cuts from subcortical white matter to cerebral peduncles. The finding was more visible in the coronal section. In addition, there was T1 hyperintensity visible along the anterior aspect of the spinal cord in the cervical region. These findings were suggestive of ALS, the diagnosis that was subsequently confirmed by serial clinical follow-up and ENMG. CONCLUSION: It is difficult to diagnose ALS in the early stage of the disease especially if the pyramidal signs predominate over the lower motor neuron (LMN) signs; MRI might be useful in such cases.     

Sulci density map to aid in use of apparent diffusion coefficient for therapy evaluation

In order to more accurately asses variations in the apparent diffusion coefficient used for therapy evaluation, we have studied the variation in sulci density in the human brain. Sagittal, axial and coronal magnetic resonance imaging scans have been analyzed to determine the change of the coefficient of variance of pixel intensity as a function of position. In the sagittal direction, relative to the 50% most medial slices, we find an 11.0%+/-4.8% (S.D.) decrease in the next 25% (12.5% on each side) of the slices. The most lateral 25% of the slices had less of a decrease and more variation: 7.0%+/-12.2%. Similar variations were observed in axial and coronal scans.     

Adult mesoblastic nephroma: appearances on magnetic resonance imaging

Mesoblastic nephroma presenting in an adult is extremely unusual. The magnetic resonance imaging (MRI) appearances of this tumor in adulthood have not been widely reported. We present a 55-year-old patient who was diagnosed with this rare neoplasm and describe the MRI findings.     

Pseudocoarctation of the aorta

We report a case of a 50-year-old man with pseudocoarctation of the aorta that we examined with magnetic resonance imaging (MRI) and digital subtraction angiography (DSA). The MR images demonstrated the characteristic "kink" of the aorta and the absence of true coarctation. These appearances were confirmed by DSA, but we believe MRI to be the better method for examining the aorta because it is non-invasive and has the ability to display the mediastinal anatomy in the sagittal plane, in addition to the axial and coronal planes.     

In vivo sodium-23 magnetic resonance surface coil imaging: Observing experimental cerebral ischemia in the rat

Sodium-23 magnetic resonance imaging can be used to detect and assess experimental cerebral ischemia in the rat. An imaging technique utilizing a surface coil is described to produce sodium magnetic resonance images of good quality and resolution within 10 min. A novel method of hemispheric occlusion showed edema in the right brain of the rat head within 3 hr after injury. The edema was especially pronounced by 12 hr with effects in the right brain, eye and surrounding muscle evident.     

MRI assisted treatment planning for radiation therapy of the head and neck

Improved visualization of head and neck tumors has been demonstrated with the use of magnetic resonance imaging (MRI). Using standard plastic radiation therapy immobilization casts and an MR positive surface marker system developed in this institution, we have utilized MRI as an adjunct to the simulation of complex radiation treatments for tumors of the head and neck. This technique includes an indirect display of field margins and/or isodose curves over selected MR images. The lack of induced artifact from the immobilization cast, improved delineation of tumor extension from normal anatomy and the ability to image in arbitrary planes without changing patient positioning favor the use of MR over CT for radiation therapy planning in the head and neck, while ensuring reproducibility of the treatment plan at subsequent therapy sessions.     

Diffusion abnormality in the posterior cingulum and hippocampal volume: correlation with disease progression in Alzheimer's disease

The purpose of this study was to determine correlations among disease progression, diffusion abnormalities in the posterior cingulum and hippocampal volume in patients with Alzheimer's disease (AD). We studied 25 AD patients by neuropsychological testing, including the Mini-Mental State Examination (MMSE), and by magnetic resonance imaging, including diffusion tensor imaging (DTI) and high-resolution three-dimensional T1-weighted imaging. The MMSE score was used as an indicator of disease progression. Diffusion tensor tractography of the posterior cingulum was generated from the DTI; mean diffusivity (MD) and fractional anisotropy (FA) were measured in co-registered voxels along the posterior cingulum. Hippocampal volume was measured using automated voxel-based morphometry. The relationships among MMSE score, hippocampal volume and MD and FA of the posterior cingulum were evaluated by bivariate correlation analysis. MD in the posterior cingulum correlated significantly with the MMSE score. No significant correlation was seen between FA and MMSE score and between hippocampal volume and MMSE score, FA or MD. Our results suggest that MD in the posterior cingulum is a more sensitive indicator of progression of AD than FA of the posterior cingulum and hippocampal volume.     

Hibernoma: MRI appearance of a rare tumor

We report the features of a hibernoma on magnetic resonance (MR) imaging. The MR characteristics of this lesion were consistent with a complex lipid-containing mass. The mass did not suppress on short tau inversion recovery (STIR) imaging and was clearly not a simple lipoma. Hibernoma should be considered in the differential diagnosis of complex fatty masses.     

Multi-level Assessment Cerebrospinal Fluid Flow in Patients with Chiari I Malformation

The purpose of this study is to characterize the cerebrospinal fluid (CSF) flow throughout the cardiac cycle in the conditions of Chiari I at intracranial and cervical levels. Magnetic resonance imaging studies were examined with phase-contrast magnetic resonance imaging retrospectively cardiac triggered (Quantitative Flow technique). 60 healthy volunteers (control group) and 12 patients with the anomaly of Chiari I (patient’s group) were investigated. Mean velocity, mean flux and peak velocity values of CSF flow at the five levels (the Sylvian aqueduct, the fourth ventricle, the Magendie’s foramen, subarachnoid space of the foramen occipital magnum and the cervical level) were defined. Analysis of differences between respective mean values of CSF flow has shown that CSF flow characteristics have the highest values in the Sylvian aqueduct and on the cervical level in both groups. Our findings show that mean velocity and mean flux of antegrade (from head to feet) flow have significantly higher values in comparison with the retrograde flow (from feet to head) through investigated structures, respectively (p < 0.01). Our findings show the importance of multi-level cerebrospinal fluid flow assessment and allow investigating this system as a single whole, with their relationships and interaction laws.     

Estimating patient dielectric losses in NMR imagers

Dielectric losses in the patient may impair radiofrequency receiver coil sensitivity, and transmitter coil efficiency, in nuclear magnetic resonance (NMR) imagers. The frequency dependence of this loss mechanism is derived. Patient losses in a solenoidal head coil used for imaging heads were simulated by a cylindrical saline phantom. The frequency dependence of the loss introduced by the phantom can indicate whether dielectric losses in the patient will be significant compared to eddy current losses. The detuning caused by the phantom is used to calculate an upper limit for the distributed stray capacitance between coil and patient. Given the approximate conductivity of the patient, an upper limit for the dielectric loss can be estimated. Some methods of reducing patient dielectric losses are suggested.     

Comprehensive mathematical simulation of functional magnetic resonance imaging time series including motion-related image distortion and spin saturation effect

There has been vast interest in determining the feasibility of functional magnetic resonance imaging (fMRI) as an accurate method of imaging brain function for patient evaluations. The assessment of fMRI as an accurate tool for activation localization largely depends on the software used to process the time series data. The performance evaluation of different analysis tools is not reliable unless truths in motion and activation are known. Lack of valid truths has been the limiting factor for comparisons of different algorithms. Until now, currently available phantom data do not include comprehensive accounts of head motion. While most fMRI studies assume no interslice motion during the time series acquisition in fMRI data acquired using a multislice and single-shot echo-planar imaging sequence, each slice is subject to a different set of motion parameters. In this study, in addition to known three-dimensional motion parameters applied to each slice, included in the time series computation are geometric distortion from field inhomogeneity and spin saturation effect as a result of out-of-plane head motion. We investigated the effect of these head motion-related artifacts and present a validation of the mapping slice-to-volume (MSV) algorithm for motion correction and activation detection against the known truths. MSV was evaluated, and showed better performance in comparison with other widely used fMRI data processing software, which corrects for head motion with a volume-to-volume realignment method. Furthermore, improvement in signal detection was observed with the implementation of the geometric distortion correction and spin saturation effect compensation features in MSV.