The role of the supplementary motor area in externally timed movement: the influence of predictability of movement timing.

A significant role in the planning and preparation for voluntary movement has been ascribed to secondary motor areas located on the medial wall of the cerebral hemispheres, and in particular to the supplementary motor area (SMA). Within the SMA, rostral and caudal subdivisions have been described, and differential roles have been attributed to these regions in relation to movement planning, preparation and execution. We have used functional magnetic resonance imaging (fMRI) to investigate the role of the SMA in the timing of movement execution, by recording the fMRI signal from mesial pre-motor areas and primary sensorimotor cortex (SM1) during the execution of a simple motor task externally cued at predictable (regular) and unpredictable (irregular) time intervals. The mean rate of movement was matched in both experiments. There was a greater activation of caudal than rostral SMA with both predictably and unpredictably cued movements, and a doubling of the signal when the timing of the motor response was unpredictable. In contrast, there was no difference in the activation of primary sensorimotor cortex with the two tasks. The data demonstrate that the caudal SMA has an important role in the execution of externally cued movements. The results also suggest a greater role for this region in the performance of unpredictably timed compared with predictably timed movements, however a model is proposed (based on electrophysiological data) which shows how the difference in functional signal in these two situations can be explained on the basis of a difference in the time course of neuronal activation in the SMA, rather than in the overall degree of activation.

Correlation of histological findings with gadolinium enhanced MRI scans during healing of a PHEMA orbital implant in rabbits.

BACKGROUND/AIMS: To investigate a poly(2-hydroxyethyl methacrylate) (PHEMA) orbital implant with a spongy anterior hemisphere and a smooth gel posterior hemisphere, by histology correlated with magnetic resonance images. METHODS: Following enucleation, eight rabbits received PHEMA implants to which the muscles were directly sutured, and underwent gadolinium enhanced magnetic resonance imaging (MRI) from 3 to 52 weeks. After the rabbits were killed, the implants were removed, cut in a plane corresponding to the scan, and processed for light and electron microscopy. RESULTS: All eight rabbits retained their implant to the end of the study period without complications. The scans demonstrated muscle attachment to the anterior half of the implant, and enhancement was seen on injection of gadolinium chelate. Histology confirmed muscle attachment, and cellular and vascular ingrowth. Over time, a transformation from reactive inflammatory to relatively non-vascular scar tissue was seen within the implant. Calcium deposits in one implant were detected by imaging and histology. CONCLUSION: The implants are readily visualised on MRI. Muscle attachment and fibrovascular ingrowth into the anterior hemisphere are seen, while encapsulation of the posterior hemisphere is minimal. Histological findings confirm the progress of the healing response, with initial inflammation and marked vascularisation, developing later into quiescent scar tissue predominantly of fibroblasts.

Acute pulmonary embolism: diagnosis with MR angiography.

PURPOSE: To prospectively evaluate pulmonary magnetic (MR) angiography as a diagnostic examination for acute pulmonary embolism (PE). MATERIALS AND METHODS: Thirty-six consecutive patients (19 women, 17 men; age range, 28-84 years) underwent pulmonary digital subtraction angiography (DSA) and pulmonary MR angiography. MR angiograms were obtained during suspended respiration and the pulmonary arterial phase of gadolinium-based contrast medium injection. A steady-state gradient-recalled-echo sequence with free induction decay sampling was used. DSA studies were interpreted for the presence of acute PE by two independent radiologists; an adjudicator made the final decision on discordant interpretations. RESULTS: By using DSA, a total of 19 acute pulmonary emboli were depicted in 13 patients. Prospectively, 13 of these emboli were depicted by using MR angiography. MR angiography missed six emboli: Four required the DSA adjudicator to make the decision, and one was in a patient whose MR angiogram was acquired during breathing. Four of these six emboli were small subsegmental emboli, and two were segmental. CONCLUSION: Performed without pulmonary arterial catheterization, iodinated contrast media, or ionizing radiation, pulmonary MR angiography had a high accuracy for depicting lobar and segmental emboli, but was unable to depict four of five subsegmental emboli.