Calcified intracranial lesions: detection with gradient-echo-acquisition rapid MR imaging

Department of Radiology, Hospital of the University of Pennsylvania, Philadelphia 19104.

Seventeen patients with partially calcified intracranial lesions, as documented by CT, were evaluated with MR imaging at 1.5 T. All patients were imaged with both conventional spin-echo techniques and reduced flip-angle gradient-echo-acquisition (GEA) sequences, during which a signal is acquired in the absence of a 180 degrees radiofrequency pulse. GEA parameters were implemented so that T2* effects were maximized on these scans. In all 17 patients GEA images showed marked hypointensity throughout the entire area of calcification, matching the calcified region as seen on CT. In contrast, spin-echo findings in the calcified portions of the lesions were extremely variable, precluding confident identification of calcification on these images. The depiction of regions of calcification as marked hypointensity on GEA images can be ascribed to T2* shortening from static local magnetic field gradients at interfaces of regions differing in magnetic susceptibility, a phenomenon that is well documented in vitro, when various diamagnetic solids are placed in aqueous suspension. However, we cannot exclude the possible additional role of accompanying paramagnetic ions, which sometimes are present with diamagnetic calcium salts in various intracranial calcifications. Since the hypointensity due to calcification on GEA images is not specific, noncontrast CT could be used to confirm its presence. Although this lack of specificity and the artifacts that emanate from diamagnetic susceptibility gradients at or near air-brain interfaces somewhat limit the application of GEA techniques, we suggest that rapid MR imaging using GEA sequences can consistently demonstrate intracranial calcification, and that this technique thus seems to be a useful adjunct to conventional spin-echo imaging.
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