MR lymphography was performed on a 1.5-T unit (Magnetom Symphony, Siemens Medical Solutions) with a six-channel phased-array body coil. Our protocol includes an axial free-breathing HASTE sequence without fat suppression (TR/TE, 1,200/114; matrix, 176 × 256; flip angle, 180°; section thickness, 6 mm; acquisition time, 80 seconds) and a free-breathing 3D high-spatial-resolution fast spin-echo sequence with an extremely long TE (1,400/800; flip angle, 180°). A–90° radiofrequency pulse (the so-called restore pulse) is applied at the end of the echo train to flip the transverse magnetization to the longitudinal direction, shorten the spin relaxation time, and accelerate the relaxation of the longitudinal magnetization while maintaining the same contrast resolution and reducing the acquisition time. The matrix is symmetric (256 × 256), the section thickness is 1 mm, and the voxel size is 1 × 1 × 1 mm. A stack of sections is acquired for diagnostic purposes. Postprocessing of the image data is performed to obtain maximum-intensity-projection (MIP) images and multiplanar reformatted images.
The typical acquisition time is 3–6 minutes for respiratory-triggered navigator-gated acquisitions with the prospective acquisition correction (PACE) technique. The 3D imaging technique has potential advantages over 2D imaging, including the capacity to obtain thinner sections with no gap and a higher signal-to-noise ratio. Because partial volume averaging effects may obscure small structures, thin-section source images must always be reviewed. Both sequences are based on heavily T2-weighted sequences that emphasize static fluid signal in the fluid-containing structures such as lymphatic vessels, whereas solid tissues and flowing blood have no signal. However, these sequences should be performed before the IV injection of gadolinium because gadolinium that is secreted in lymphatic vessels may result in contrast alteration.
No comments:
Post a Comment