Assuming US is the initial imaging examination, when occlusion is diagnosed, MR angiography can depict it. If occlusion is confirmed, no further imaging is necessary. US performed well in helping to differentiate vessels with focal severe stenosis from those with diffuse disease. MR angiography added little in this group. Catheter angiography remains beneficial for vessels with diffuse nonfocal narrowing.
Compensatory ventilatory responses to increased inspiratory loading are essential for adequate breathing regulation in a number of pulmonary diseases; however, the human brain sites mediating such responses are unknown. Midsagittal and axial images were acquired in 11 healthy volunteers during unloaded and loaded (30 cmH20; 1 cmH20 = 98 Pa) inspiratory breathing, by using functional magnetic resonance imaging (fMRI) strategies (1.5-tesla MR; repetition time, 72 msec; echo time, 45 msec; flip angle, 30°; field of view, 26 cm; slice thickness, 5 mm; number of excitations, 1; matrix, 128 x 256). Digital image subtractions and region of interest analyses revealed significantly increased fMRI signal intensity in discrete areas of the ventral and dorsal pons, interpeduncular nucleus, basal forebrain, putamen, and cerebellar regions. Upon load withdrawal, certain regions displayed a rapid fMRI signal off-transient, while in others, a slower fMRI signal decay emerged. Sustained loading elicited slow decreases in fMRI signal across activated regions, while second application of an identical load resulted in smaller signal increases compared to initial signal responses (P < 0.001). A moderate inspiratory load is associated with consistent regional activation of discrete brain locations; certain of these regions have been implicated in mediation of loaded breathing in animal models. We speculate that temporal changes in fMRI signal may indicate respiratory after-discharge and/or habituation phenomena.
Doppler US is excellent for classifying stenoses as above or below a single degree of severity but does not function well in stenosis subclassification.
In humans, the location of brain regions responsible for mediating the ventilatory response to CO2 remains unknown. Most of the available knowledge has been derived from animal studies or from pathophysiological correlations in patients presenting altered control of breathing. Magnetic resonance imaging at a specific pulse sequence designed to assess changes in brain tissue microcirculation was performed in 11 healthy volunteers, during steady-state conditions, while breathing 100% O2 or 5% CO2-95% O2. In one subject, 10% CO2-90% O2 was employed to examine a dose-response effect. Significant changes in image signal intensity consistently occurred in ventral and dorsal regions of medullary structures as well as in the midline pons and ventral cerebellum. These responses appeared to be dose dependent and reproducible. Magnetic resonance imaging revealed patterns of activation in brain stem and cerebellar regions during hypercapnic ventilatory challenge. These areas may underlie mechanisms for mediating the response to chemoreceptor activation.
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