![]() ![]() ![]() ![]() This field variation would in turn result in a 63.87 Hz off-resonance (i.e., one-millionth of the center frequency of f 0 = 63.87 MHz at 1.5 T). For example, 1 ppm inhomogeneity at 1.5 T corresponds to a 1.5 μT variation in the B 0 field. MRI scanners are manufactured with a stringent requirement of less than one part-per-million (ppm) aĪ In MRI, the variations in B 0 field or the differences in resonant frequencies are so small that they are expressed in “parts per million”, or ppm. The uniformity of the B 0 main field in magnetic resonance imaging (MRI) is critical for artifact-free image formation. Introduction: Sources of Susceptibility Artifacts Reconstruction-based methods involve distortion correction, phase correction, and other advanced techniques. Acquisition-based methods include multishot imaging, parallel acquisitions, reduced-FOV methods, and non-EPI techniques. This chapter reviews susceptibility artifacts, their impact on EPI of the spinal cord, and methods to limit these artifacts. The resulting long readout duration, especially that of echo planar imaging (EPI), further exacerbates the artifacts. Moreover, the anatomy of the spine requires a relatively large field of view (FOV), especially in the sagittal imaging plane, while the small cross-sectional size of the spinal cord mandates high-spatial-resolution images. Bulk physiologic motion from cardiac and respiratory cycles, cerebrospinal fluid pulsation, as well as breathing and swallowing further cause temporal variations of these field inhomogeneities. These field inhomogeneities create off-resonance induced artifacts in the images, such as signal dropouts and pileups, geometric distortions, and incomplete fat suppression. In vivo magnetic resonance imaging of the spinal cord is challenging due to susceptibility variations between various tissue types, air in the lungs and trachea, and in some cases surgical implants that significantly distort the applied magnetic field. ![]()
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