Optimization of magnetization transfer contrast for a human carotid plaque. A, MTon images acquired with saturation pulse frequency offsets of 4.9, 8.4, 14.2 and 41.2 kHz (logarithmic scale). B, The corresponding MT subtraction images (MTC) generated by subtracting the MTon images from the image without MT pulse (MToff). Two dark circles (arrows) are glass capillaries to stabilize the specimen. C, Normalized SI (SIs /SI0) plotted versus the offset frequency for five different RF pulse amplitudes (pulse length of 10 ms; region of interest localized in fibrous rich region). D, Normalized SI plotted versus the offset frequency for two different components (fibrous tissue and lipid-rich regions) guided by histology (the MT pulse acquired with amplitude of 0.67 kHz and pulse length of 10 ms).