Preoperative fMRI predicts memory decline following anterior temporal lobe resection.

BACKGROUND: Anterior temporal lobe resection (ATLR) benefits many patients with refractory temporal lobe epilepsy (TLE) but may be complicated by material specific memory impairments, typically of verbal memory following left ATLR, and non-verbal memory following right ATLR. Preoperative memory functional MRI (fMRI) may help in the prediction of these deficits. OBJECTIVE: To assess the value of preoperative fMRI in the prediction of material specific memory deficits following both left- and right-sided ATLR. METHODS: We report 15 patients with unilateral TLE undergoing ATLR; eight underwent dominant hemisphere ATLR and seven non-dominant ATLR. Patients performed an fMRI memory paradigm which examined the encoding of words, pictures and faces. RESULTS: Individual patients with relatively greater ipsilateral compared with contralateral medial temporal lobe activation had greater memory decline following ATLR. This was the case for both verbal memory decline following dominant ATLR and for non-verbal memory decline following non-dominant ATLR. For verbal memory decline, activation within the dominant hippocampus was predictive of postoperative memory change whereas activation in the non-dominant hippocampus was not. CONCLUSION: These findings suggest that preoperative memory fMRI may be a useful non-invasive predictor of postoperative memory change following ATLR and provide support for the functional adequacy theory of hippocampal function. They also suggest that fMRI may provide additional information, over that provided by neuropsychology, for use in the prediction of postoperative memory decline.

Imaging language pathways predicts postoperative naming deficits.

Naming difficulties are a well recognised, but difficult to predict, complication of anterior temporal lobe resection (ATLR) for refractory epilepsy. We used MR tractography preoperatively to demonstrate the structural connectivity of language areas in patients undergoing dominant hemisphere ATLR. Greater lateralisation of tracts to the dominant hemisphere was associated with greater decline in naming function. We suggest that this method has the potential to predict language deficits in patients undergoing ATLR.

Non-invasive mapping of connections between human thalamus and cortex using diffusion imaging.

Evidence concerning anatomical connectivities in the human brain is sparse and based largely on limited post-mortem observations. Diffusion tensor imaging has previously been used to define large white-matter tracts in the living human brain, but this technique has had limited success in tracing pathways into gray matter. Here we identified specific connections between human thalamus and cortex using a novel probabilistic tractography algorithm with diffusion imaging data. Classification of thalamic gray matter based on cortical connectivity patterns revealed distinct subregions whose locations correspond to nuclei described previously in histological studies. The connections that we found between thalamus and cortex were similar to those reported for non-human primates and were reproducible between individuals. Our results provide the first quantitative demonstration of reliable inference of anatomical connectivity between human gray matter structures using diffusion data and the first connectivity-based segmentation of gray matter.

Magnetization transfer ratio in Alzheimer disease: comparison with volumetric measurements.

BACKGROUND AND PURPOSE: Alzheimer disease (AD) is accompanied by macroscopic atrophy on volumetric MR imaging. A few studies have also demonstrated reduction in magnetization transfer ratio (MTR), suggesting microstructural changes in remaining brain tissue. This study assessed the value of measuring MTR in addition to volumetric MR in differentiating patients with AD from control subjects. MATERIALS AND METHODS: Volumetric T1-weighted images and 3D MTR maps were obtained from 18 patients with AD and 18 age-matched control subjects. Whole-brain (WB) and total hippocampal (Hc) volumes were measured using semiautomated techniques and adjusted for total intracranial volume. Mean MTR was obtained for WB and in the Hc region. Histogram analysis was performed for WB MTR. Among patients, associations between volumetric and MTR parameters and the Mini-Mental State Examination (MMSE) were explored. RESULTS: Patients with AD had significantly reduced WB volume (P<.0001) and mean WB MTR (P=.002) and Hc volume (P<.0001) and Hc mean MTR (P<.0001) compared with control subjects. Histogram analysis of WB MTR revealed significant reduction in the 25th percentile point in patients with AD (P=.03). Both WB volume and mean MTR were independently associated with case-control status after adjusting for the other using linear regression models. However, measuring Hc mean MTR added no statistically significant discriminatory value over and above Hc volume measurement alone. Of all MR imaging parameters, only WB volume was significantly correlated with MMSE (r=0.47, P=.048). CONCLUSIONS: This study demonstrates the independent reduction of WB volume and mean MTR in AD. This suggests that the 2 parameters reflect complementary aspects of the AD pathologic lesion at macrostructural and microstructural levels.

Whole-brain T2 mapping demonstrates occult abnormalities in focal epilepsy.

OBJECTIVES: To examine the cerebral structure of 14 patients with partial seizures and acquired lesions, 20 patients with malformations of cortical development (MCDs), and 45 patients with partial seizures and normal conventional MRI using whole-brain T2 mapping and statistical parametric mapping (SPM). METHODS: T2 maps were calculated, and individual patients were compared with a group of 30 control subjects using SPM. RESULTS: T2 mapping and objective voxel-by-voxel statistical comparison identified regions of increased T2 signal in all 14 patients with acquired nonprogressive cerebral lesions and partial seizures. In all of these, the areas of increased T2 signal concurred with abnormalities identified on visual inspection of conventional MRI. In 18 of 20 patients with MCDs, SPM detected regions of increased T2 signal, all of which corresponded to abnormalities identified on visual inspection of conventional MRI. In addition, in both groups, there were areas that were normal on conventional imaging, which demonstrated abnormal T2 signal. Voxel-by-voxel statistical analysis identified increased T2 signal in 23 of the 45 patients with cryptogenic focal epilepsy. In 20 of these, the areas of increased T2 signal concurred with epileptiform EEG abnormality and clinical seizure semiology. Group analysis of MRI-negative patients with electroclinical seizure onset localizing to the left and right temporal and left and right frontal regions revealed increased T2 signal within the white matter of each respective lobe. CONCLUSIONS: T2 mapping analyzed using statistical parametric mapping was sensitive in patients with malformations of cortical development and acquired cerebral damage. Increased T2 signal in individual and grouped MRI-negative patients suggests that minor structural abnormalities exist in occult epileptogenic cerebral lesions.

Material-specific lateralization of memory encoding in the medial temporal lobe: blocked versus event-related design.

Lesion-deficit studies have provided evidence for a functional dissociation between the left medial temporal lobe (MTL) mediating verbal memory encoding and right MTL mediating non-verbal memory encoding. While a small number of functional MRI studies have demonstrated similar findings, none has looked specifically for material-specific lateralization using subsequent memory effects. In addition, in many fMRI studies, encoding activity has been located in posterior MTL structures, at odds with lesion-deficit and positron emission tomography (PET) evidence. In this study, we used an event-related fMRI memory encoding paradigm to demonstrate a material-specific lateralization of encoding in the medial temporal lobes of ten healthy control subjects. Activation was left-lateralized for word encoding, bilateral for picture encoding, and right-lateralized for face encoding. Secondly, we demonstrated the locations of activations revealed using an event-related analysis to be more anterior than those revealed using a blocked analysis of the same data. This suggests that anterior MTL structures underlie memory encoding as judged by subsequent memory effects, and that more posterior activity detected in other fMRI studies is related to deficiencies of blocked designs in the analysis of memory encoding.

MR tractography predicts visual field defects following temporal lobe resection.

A superior homonymous quadrantanopia is a well recognized complication of anterior temporal lobe resection and occurs because of disruption of the Meyer loop, the anterior part of the optic radiation. The authors used diffusion tensor imaging tractography to visualize the optic radiation before and after surgery, demonstrating the disruption of Meyer loop in a patient who developed a quadrantanopia. Preoperative imaging of the optic radiation will be useful in predicting visual field defects following temporal lobe resection.

Optimized interleaved whole-brain 3D double inversion recovery (DIR) sequence for imaging the neocortex.

For a substantial number of individuals with neurological disorders, a conventional MRI scan does not reveal any obvious etiology; however, it is believed that abnormalities in the neocortical gray matter (GM) underlie many of these disorders. Attempts to image the neocortex are hindered by its thin, convoluted structure, and the partial volume (PV) effect. Therefore, we developed a 3D version of the double inversion recovery (DIR) sequence that incorporates an optimized interleaved (OIL) strategy to improve efficiency and allow high-quality, high-resolution imaging of GM.

Magnetization transfer imaging in focal epilepsy.

OBJECTIVES: To test the hypothesis that magnetization transfer imaging (MTI), analyzed on a voxel-by-voxel basis, would identify areas of abnormal magnetization transfer ratio (MTR) in patients with focal epilepsy. METHODS: The authors used MTI maps and statistical parametric mapping (SPM) to objectively compare the cerebral structures of 15 patients with malformations of cortical development (MCD), 10 with partial seizures and acquired lesions, and 42 with partial seizures and normal conventional MRI with those of 30 control subjects. RESULTS: Significant reductions of MTR were identified in all 10 patients with acquired nonprogressive cerebral lesions and partial seizures. In all, the areas of decreased MTR concurred with abnormalities identified on visual inspection of conventional MRI. In 13 of the 15 patients with MCD, SPM detected regions of significantly reduced MTR, all of which corresponded to abnormalities identified on visual inspection of conventional MRI. In addition, in both groups, there were areas that were normal on conventional imaging that demonstrated abnormal MTR. There was a significant reduction of MTR in 15 of the 42 patients with cryptogenic focal epilepsy. In all of these, the areas of reduced MTR concurred with epileptiform EEG abnormality and clinical seizure semiology. CONCLUSIONS: Magnetization transfer imaging analyzed using statistical parametric mapping was sensitive in identifying malformations of cortical development and acquired cerebral lesions. Abnormalities of magnetization transfer ratio in individual MRI-negative patients suggest that minor structural disorganization exists in occult epileptogenic cerebral lesions.

From diffusion tractography to quantitative white matter tract measures: a reproducibility study.

The aim of this study is to propose methods for assessing the reproducibility of diffusion tractography algorithms in future clinical studies and to show their application to the tractography algorithm developed in our unit, fast marching tractography (FMT). FMT estimates anatomical connectivity between brain regions using the information provided by diffusion tensor imaging. Three major white-matter pathways were investigated in 11 normal subjects--anterior callosal fibers, optic radiations, and pyramidal tracts. FMT was used to generate maps of connectivity metric, and regions of voxels with highest connectivity metric to an anatomically defined starting point were identified for each tract under investigation. The reproducibilities of tract-"normalized" volume (NV) and fractional anisotropy (FA) measurements were assessed over such regions. The values of tract volumes are consistent with the postmortem data. Coefficients of variation (CVs) for FA and NV ranged from 1.7 to 7.1% and from 2.2 to 18.6%, respectively. CVs were lowest in the anterior callosal fibers (range: 1.7- 7.8%), followed by the optic radiations (range: 1.2-18.6%) and pyramidal tracts (range: 2.6-15.5%), suggesting that fiber organization plays a role in determining the level of FMT reproducibility. In conclusion, these findings underline the importance of assessing the reliability of diffusion tractography before investigating white matter pathology.