Andreas J. Bartsch

Ulnar Neuropathy at the Elbow: MR Neurography—Nerve T2 Signal Increase and Caliber

PURPOSE To assess nerve T2 signal and caliber as diagnostic signs at magnetic resonance (MR) neurography in ulnar neuropathy at the elbow (UNE). MATERIALS AND METHODS This prospective study was approved by the institutional review board, and written informed consent was obtained from all participants. Twenty patients with UNE were graded by using clinical criteria and nerve conduction studies as mild (n = 12) and severe (n = 8) and were compared with 20 healthy control subjects. All subjects underwent ulnar nerve MR neurography (in-plane resolution of 0.4 × 0.4 mm) covering the elbow region, including T2-weighted imaging with fat suppression (turbo inversion-recovery magnitude sequence: repetition time msec/echo time msec/inversion time msec, 6, 120/66/180) and T1-weighted turbo spin-echo imaging (843/16). Nerve T2 signal increase, measured by using T2-weighted contrast-to-noise ratios across the cubital tunnel, and nerve caliber, determined by using T1-weighted pixelwise measurement of cross-sectional nerve area, were evaluated as diagnostic signs. Qualitative assessment by using visual grading was performed additionally. RESULTS Diagnostic performance, as determined with area under the receiver operating characteristic curve (AUC), was excellent for nerve T2 signal to discriminate UNE from a normal finding (AUC = 0.94; 95% confidence interval [CI]: 0.87, 1.00) and was excellent for nerve caliber to discriminate severe from mild UNE (AUC = 0.95; 95% CI: 0.85, 1.00). Qualitative assessment demonstrated sensitivity of 83% and specificity of 85% for MR neurography of UNE. CONCLUSION Nerve T2 signal increase seems to be an accurate sign to determine the presence of UNE. Nerve caliber enlargement discriminates severe from mild UNE. UNE may be diagnosed with high accuracy by means of quantitative or qualitative evaluation of these signs.

Heparin and Air Filters Reduce Embolic Events Caused by Intra-Arterial Cerebral Angiography A Prospective, Randomized Trial

Background—Intra-arterial cerebral angiography is associated with a low risk for neurological complications, but clinically silent ischemic events after angiography have been seen in a substantial number of patients. Methods and Results—In a prospective study, diffusion-weighted magnetic resonance imaging (DW-MRI) before and after intra-arterial cerebral angiography and transcranial Doppler sonography during angiography were used to evaluate the frequency of cerebral embolism. One hundred fifty diagnostic cerebral angiographies were randomized into 50 procedures, each using conventional angiographic technique, or systemic heparin treatment throughout the procedure, or air filters between the catheter and both the contrast medium syringe and the catheter flushing. There was no neurological complication during or after angiography. Overall, DW-MRI revealed 26 new ischemic lesions in 17 patients (11%). In the control group, 11 patients showed a total of 18 lesions. In the heparin group, 3 patients showed a total of 4 lesions. In the air filter group, 3 patients exhibited a total of 4 lesions. The reduced incidence of ischemic events in the heparin and air filter groups compared with the control group was significantly different (P=0.002). Transcranial Doppler sonography demonstrated a large number of microembolic signals that was significantly lower in the air filter group compared with the heparin and control groups (P<0.01), which did not differ from each other. Conclusions—Air filters and heparin both reduce the incidence of silent ischemic events detected by DW-MRI after intra-arterial cerebral angiography and can potentially lower clinically overt ischemic complications. This may apply to any intra-arterial angiographic procedure.

Individual Prediction of Cognitive Decline in Mild Cognitive Impairment Using Support Vector Machine-Based Analysis of Diffusion Tensor Imaging Data

Although cross-sectional diffusion tensor imaging (DTI) studies revealed significant white matter changes in mild cognitive impairment (MCI), the utility of this technique in predicting further cognitive decline is debated. Thirty-five healthy controls (HC) and 67 MCI subjects with DTI baseline data were neuropsychologically assessed at one year. Among them, there were 40 stable (sMCI; 9 single domain amnestic, 7 single domain frontal, 24 multiple domain) and 27 were progressive (pMCI; 7 single domain amnestic, 4 single domain frontal, 16 multiple domain). Fractional anisotropy (FA) and longitudinal, radial, and mean diffusivity were measured using Tract-Based Spatial Statistics. Statistics included group comparisons and individual classification of MCI cases using support vector machines (SVM). FA was significantly higher in HC compared to MCI in a distributed network including the ventral part of the corpus callosum, right temporal and frontal pathways. There were no significant group-level differences between sMCI versus pMCI or between MCI subtypes after correction for multiple comparisons. However, SVM analysis allowed for an individual classification with accuracies up to 91.4% (HC versus MCI) and 98.4% (sMCI versus pMCI). When considering the MCI subgroups separately, the minimum SVM classification accuracy for stable versus progressive cognitive decline was 97.5% in the multiple domain MCI group. SVM analysis of DTI data provided highly accurate individual classification of stable versus progressive MCI regardless of MCI subtype, indicating that this method may become an easily applicable tool for early individual detection of MCI subjects evolving to dementia.

Addressing a systematic vibration artifact in diffusion-weighted MRI

We have identified and studied a pronounced artifact in diffusion‐weighted MRI on a clinical system. The artifact results from vibrations of the patient table due to low‐frequency mechanical resonances of the system which are stimulated by the low‐frequency gradient switching associated with the diffusion‐weighting. The artifact manifests as localized signal‐loss in images acquired with partial Fourier coverage when there is a strong component of the diffusion‐gradient vector in the left–right direction. This signal loss is caused by local phase ramps in the image domain which shift the apparent k‐space center for a particular voxel outside the covered region. The local signal loss masquerades as signal attenuation due to diffusion, severely disrupting the quantitative measures associated with diffusion‐tensor imaging (DTI). We suggest a way to improve the interpretation of affected DTI data by including a co‐regressor which accounts for the empirical response of regions affected by the artifact. We also demonstrate that the artifact may be avoided by acquiring full k‐space data, and that subsequent increases in TE can be avoided by employing parallel acceleration. Hum Brain Mapp, 2010.

Ventral premotor cortex may be required for dynamic changes in the feeling of limb ownership: a lesion study.

The feeling of “body ownership” may be experimentally investigated by perceptual illusions. The “rubber hand illusion” (RHI) leads human subjects to experience an artificial hand as their own. According to functional imaging, the ventral premotor cortex (PMv) plays a key role in the integration of multisensory inputs allowing the “incorporation” of the rubber hand into body representation. However, causal structure–function relationships can only be obtained by lesion studies. Here, we tested the RHI in 70 stroke patients and in 40 age-matched healthy controls. Additionally, asomatognosia, the unawareness of ones own body parts, was assessed in a subgroup of 64 stroke patients. Ischemic lesions were delineated on diffusion-weighted magnetic resonance images and normalized. Right-hemispheric lesions were mirrored across the midline. Voxels that might be essential for RHI and/or somatognosia were defined by voxel-based lesion-symptom mapping. Probabilistic diffusion tractography was used to identify tracts passing through these voxels. Contralesional rubber hand illusion failure (RHIF) was observed in 18 (26%) of 70 stroke patients, an additional ipsilesional RHIF in seven of these patients. RHIF-associated lesion voxels were located subcortically adjacent to the insula, basal ganglia, and within the periventricular white matter. Tractography revealed fiber tract connections of these voxels with premotor, parietal, and prefrontal cortex. Contralesional asomatognosia was found in 18 (28%) of 64 stroke patients. In contrast to RHIF, asomatognosia-associated lesion voxels showed no connection with PMv. The results point to a role of PMv and its connections in mediating changes in the sense of limb ownership driven by multisensory stimulation.

Cerebral Microhemorrhage and Iron Deposition in Mild Cognitive Impairment: Susceptibility-weighted MR Imaging Assessment

PURPOSE To test whether susceptibility-weighted magnetic resonance imaging at baseline may help predict cognitive decline. MATERIALS AND METHODS This prospective study was approved by the institutional review board, and written informed consent was obtained from all participants. Thirty-five healthy control subjects and 69 patients with mild cognitive impairment were included. Patients with mild cognitive impairment underwent neuropsychologic follow-up after 1 year (40 patients with stable mild cognitive impairment, 27 with progressive mild cognitive impairment, and two lost to follow-up). Cerebral microhemorrhages were visually analyzed by two experienced neuroradiologists in consensus. Iron deposition in deep gray matter was assessed with voxel-wise and region-of-interest analysis after nonlinear spatial registration. In addition, individual classification of mild cognitive impairment was analyzed by using a support vector machine (SVM). RESULTS At baseline, the number of cerebral microhemorrhages was significantly higher in the mild cognitive impairment group than in the control group (P < .01) but did not differ between the patients with stable and those with progressive mild cognitive impairment. Compared with the control group, patients with mild cognitive impairment had increased iron concentration in the right pallidum (P < .01) and right substantia nigra (P < .01) but decreased concentration in the right red nucleus (P < .05). The classification based on the SVM successfully helped discriminate patients with mild cognitive impairment from the healthy control subjects (accuracy, 84%; sensitivity, 89%; specificity, 85%) and those with stable from those with progressive mild cognitive impairment (accuracy, 85%; sensitivity, 84%; specificity, 83%). CONCLUSION The findings reveal an accumulation of cerebral microhemorrhage in patients with mild cognitive impairment that is present at baseline, independent of subsequent cognitive decline, as well as an altered iron distribution in subcortical nuclei between the healthy control subjects and patients with mild cognitive impairment. Analysis of iron deposition at baseline performed with an SVM might help identify individual patients with mild cognitive impairment at risk for cognitive decline. SUPPLEMENTAL MATERIAL http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.10100612/-/DC1.

Pitfalls in fMRI

Several different techniques allow a functional assessment of neuronal activations by magnetic resonance imaging (fMRI). The by far most influential fMRI technique is based on a local T2*-sensitive hemodynamic response to neuronal activation, also known as the blood oxygenation level dependent or BOLD effect. Consequently, the term ‘fMRI’ is often used synonymously with BOLD imaging. Because interpretations of fMRI brain activation maps often appear intuitive and compelling, the reader might be tempted not to critically question the fundamental processes and assumptions. We review some essential processes and assumptions of BOLD fMRI and discuss related confounds and pitfalls in fMRI – from the underlying physiological effect, to data acquisition, data analysis and the interpretation of the results including clinical fMRI. A background framework is provided for the systematic and critical interpretation of fMRI results.

CT perfusion predicts secondary cerebral infarction after aneurysmal subarachnoid hemorrhage.

Objective: To prospectively assess the diagnostic accuracy of CT perfusion (CTP) and transcranial Doppler sonography (TCD) for the prediction of secondary cerebral infarction (SCI) after aneurysmal subarachnoid hemorrhage (SAH). Methods: During 2 weeks after SAH, 38 consecutive patients completed an average of 3.5 CT/CTP and 10.7 TCD examinations at regular intervals as required by the study protocol. SCI was defined as delayed infarction on native CT between 3 and 14 days after SAH and developed in n = 14 patients (n = 24 without SCI). Analysis was based on examination dates before SCI. Common measures of diagnostic accuracy were calculated for qualitative CTP (visual color-map ratings from two blinded observers) and TCD assessments (mean flow velocity >120 cm/s in anterior, middle, and posterior cerebral artery territories). Quantitative measures, which for CTP were obtained from cortical a priori regions of interest corresponding to the vascular territories, were analyzed by binary logistic regression. Results: Time of prediction for SCI by CTP was at a median of 3 days (range 2 to 5 days) before manifestation of complete infarction on native CT. Visual assessment of time-to-peak (TTP) color maps performed best for the prediction of SCI with 0.93 sensitivity (95% CI: 0.7 to 1.0) and 0.67 specificity (95% CI: 0.53 to 0.7). On quantitative analysis, the odds ratio (OR) for 1 second of side-to-side delay in TTP was 1.4 (p = 0.01, Wald χ2 = 8.57, CI: 1.07 to 1.82). Daily TCD measures were not significantly related to SCI at any time before complete infarction on native CT. Conclusions: Time to peak as indicated by CT perfusion is a sensitive and early predictor of secondary cerebral infarction.

Brain electrical dysfunction of the anterior cingulate in schizophrenic patients

The anterior cingulate cortex (ACC) is a key region within the human prefrontal cortex that has been shown to be dysfunctional in schizophrenic patients. Supporting evidence for this notion has been collected with neuroimaging methods during various cognitive activation tasks. Recently, electrophysiological ACC activity has been demonstrated by means of a three-dimensional source location with low resolution electromagnetic tomography (LORETA) in the event-related potentials elicited during the NoGo condition of the Continuous Performance Test (CPT) as compared to its Go condition. Thirty-one schizophrenic patients and 31 age- and gender-matched healthy volunteers were investigated with this newly developed electrophysiological method. LORETA analysis revealed a significantly diminished brain electrical activity in the ACC of schizophrenic patients as compared to controls during the NoGo condition of the CPT. This result supports the assumption of a functional deficit of the ACC during this cognitive task as a central feature in schizophrenias and, thereby, specifies the general concept of hypofrontality. Moreover, this investigation underscores the value of sophisticated electrophysiological methods in combination with unambiguously designed mental tasks for the evaluation of the pathophysiological processes underlying schizophrenic diseases.

THE IMPACT OF BALLOON ANGIOPLASTY ON THE EVOLUTION OF VASOSPASM-RELATED INFARCTION AFTER ANEURYSMAL SUBARACHNOID HEMORRHAGE

OBJECTIVEVasospasm of the cerebral vessels remains a major source for morbidity and mortality after aneurysmal subarachnoid hemorrhage. The purpose of this study was to evaluate the frequency of infarction after transluminal balloon angioplasty (TBA) in patients with severe subarachnoid hemorrhage-related vasospasm. METHODSWe studied 38 patients (median Hunt and Hess Grade II and median Fisher Grade 4) with angiographically confirmed severe vasospasm (>70% vessel narrowing). A total of 118 vessels with severe vasospasm in the anterior circulation were analyzed. Only the middle cerebral artery, including the terminal internal carotid artery, was treated with TBA (n = 57 vessel segments), whereas the anterior cerebral artery was not treated (n = 61 vessel segments). For both the treated and the untreated vessel territories, infarction on unenhanced computed tomographic scan was assessed as a marker for adverse outcome. RESULTSInfarction after TBA occurred in four middle cerebral artery territories (four out of 57 [7%]), whereas the infarction rate was 23 out of 61 (38%) in the anterior cerebral artery territories not subjected to TBA (P < 0.001, Fisher exact test). Three procedure-related complications occurred during TBA (dissection, n = 1; temporary vessel occlusions, n = 2). One of these remained asymptomatic, whereas this may have contributed to the development of infarction on follow-up computed tomographic scans in two cases. CONCLUSIONIn a population of patients with a high risk of infarction resulting from vasospasm after subarachnoid hemorrhage, the frequency of infarction in the distribution of vessels undergoing TBA amounts to 7% and is significantly lower than in vessels not undergoing TBA despite some risk inherent to the procedure.