Standardized volume-rendering of contrast-enhanced renal magnetic resonance angiography.

PURPOSE: To propose a technique for standardizing volume-rendering technique (VRT) protocols and to compare this with maximum intensity projection (MIP) in regard to image quality and diagnostic confidence in stenosis diagnosis with magnetic resonance angiography (MRA). MATERIAL AND METHODS: Twenty patients were examined with MRA under suspicion of renal artery stenosis. Using the histogram function in the volume-rendering software, the 95th and 99th percentiles of the 3D data set were identified and used to define the VRT transfer function. Two radiologists assessed the stenosis pathology and image quality from rotational sequences of MIP and VRT images. RESULTS: Good overall agreement (mean kappa=0.72) was found between MIP and VRT diagnoses. The agreement between MIP and VRT was considerably better than that between observers (mean kappa=0.43). One of the observers judged VRT images as having higher image quality than MIP images. CONCLUSION: Presenting renal MRA images with VRT gave results in good agreement with MIP. With VRT protocols defined from the histogram of the image, the lack of an absolute gray scale in MRI need not be a major problem.

Autonomic changes during orthostasis: a time-frequency analysis.

The transition between sitting and standing represents a period of dynamic changes in cardiac autonomic control. Previous research using heart rate (HR) has suggested a biphasic HR response from sitting to standing with an initial increase in HR followed by an HR decrease before stabilizing at a level above the sitting level. These HR changes have been interpreted as mediated by sympathetic nervous system (SNS) activity. However, more direct measures of cardiac autonomic control have not been investigated in an active orthostasis paradigm. This is due in part to the requirement of stationarity for traditional spectral analytic techniques. In the present study, time-frequency analysis was used to assess the relative contributions of sympathetic and parasympathetic nervous system activity to the cardiac autonomic control during the transition of orthostasis. We examined the heart rate time series during the transition between sitting and standing, and the 60 s prior and the 60 s following the transition. The high frequency component of the HR spectra (an index of parasympathetic activity) decreased sharply at the point of transition and remained depressed relative to the low frequency component (an index of sympathetic activity). These results suggest that time-frequency analysis may be a useful technique to examine non-stationary time series. Furthermore, the transition of orthostasis may be mediated more via parasympathetic activity than by sympathetic activity.

Thermoregulation and cardiac variability: a time-frequency analysis.

High heart rate variability (HRV) has been associated with more efficient autonomic control, allowing more responsivity and sensitivity to changing environmental demands. A number of specific periodicities have been identified in the spectra of cardiac time series. A high frequency component related to respiratory sinus arrhythmia, a low frequency component related to blood pressure variability, and a very low frequency component thought to reflect thermoregulation have been reported in the literature. However, the source of the very low frequency component has not been extensively investigated in humans using non-invasive methods and analytic techniques that do not rely upon stationarity. We investigated HRV in response to both hot and cold thermal challenge in healthy adults using time-frequency analysis. This analytic technique does not rely upon signal stationarity. The results suggest that very low frequency power may reflect thermoregulation to ambient temperature changes. Implications for prediction of cardiac events are discussed.

Cortical hypoperfusion as a possible cause of 'subcortical aphasia'.

This study was designed to investigate whether aphasia in stroke patients with subcortical lesions ('subcortical aphasia') was due to the subcortical lesion itself or to dysfunction of cortical language zones. A consecutive series of 25 right-handed stroke patients with left hemisphere lesions verified by CT scanning were examined for aphasia in the acute stage, and two weeks, three months and six months after the insult. Cerebral angiography, CT scan and regional cerebral blood flow (rCBF) measurements with the 133Xe intracarotid method were performed in the acute stage. The CT scan was repeated six months later. Seven patients had lesions which involved cortical structures. All of these were severely aphasic in the acute stage and six months later. The rCBF studies showed severe reduction of flow in the infarcts; the perfusion was incompatible with tissue viability. Eighteen patients had subcortical lesions. Eight of these were midly to severely aphasic in the acute state. Recovery was always excellent. Five patients recovered completely, while 3 showed only discrete (clinically undetectable) aphasia after six months. The rCBF demonstrated low-flow areas in the cortex overlying the deep lesions, with a blood flow which was sufficient for tissue viability, but insufficient for normal tissue function (an 'ischaemic penumbra'). The blood flow was pressure dependent (showing impaired autoregulation) in these low flow areas. Ten patients with subcortical lesions were not aphasic. Their CBF was normal. The subcortical lesions were permanent on CT scan while aphasia was transient in these patients as recovery was seen within three months. It was therefore unlikely that the subcortical lesions as such were the cause of aphasia. The present findings indicate that aphasia in patients with subcortical lesions may be due to hypoperfusion and functional loss in cortical language zones. The cortical low-flow areas were invariably confined to occluded vascular territories. These territories were kept viable by collateral circulation characterized by penumbral flow and impaired autoregulation. The patients showed excellent recovery from aphasia. We suggest that the recovery of language is caused by spontaneous arterial recanalization or expansion of collaterals, giving rise to enhancement of flow in the hypoperfused cortical penumbra.

Comparative neuropsychology and Korsakoff's syndrome. III--Delayed response, delayed alternation and DRL performance.

Performance of alcoholic Korsakoff patients was compared with that of patients with Huntington's disease. Broca's aphasia or alcoholism (without clinical signs of memory impairment) on delayed alternation (DA) and delayed response (DR) tests. Korsakoffs were impaired on both tasks, and Huntington patients were impaired on DA only. In a separate experiment, performance by Korsakoffs was compared to that of alcoholic and normal controls on four DRL schedules. Korsakoffs tended to be overresponsive, making errors of commission early within a schedule, and consequently, obtaining fewer reinforcements than the controls.

Dissociative effects of selective lesions in the caudate nucleus of cats and rats.

Lesions in the neostriatal region connected to the prefrontal cortex severely impaired delayed alternation in both cats and rats, whereas no such effect was found after damage to other neostriatal regions. These results indicate that the processes mediating delayed alternation are well localized in the neostriatum of both cats and rats. Two possible models of neostriatal functioning are discussed, one of functional heterogeneity, the other of functional homogeneity.