Static-fluid MR urograms are performed using heavily T2-weighted TSE sequence techniques, such as rapid acquisition with relaxation enhancement (RARE) or halfFourier acquisition single-shot turbo spin-echo (HASTE) (1,7,12,16,17). Initially, the RARE technique was employed to generate so-called nontomographic projection MR images of the urinary tract (1). Acquisition of a single-slice projection RARE image with a section thickness of 60 to 80 mm requires a short breath-hold of less than 10 seconds and provides a quick scout view of the pelvicalices and ureters in the coronal or sagittal plane (Fig. 1). HASTE is the half-Fourier variant of RARE allowing for faster acquisition of a heavily T2-weighted single slice within two seconds. HASTE can be applied as a single projection image or as a multislice sequence with thin sections.
Currently, 3-D TSE sequences or multislice HASTE sequences in the coronal or paracoronal plane are preferred for static-fluid MR urography (Fig. 2) (7-9,12,15). Combining those sequences with fat suppression has proved useful. Both types of pulse sequences generate multiple thin overlapping sections. Maximum intensity projection (MIP) images are postprocessed to obtain typical urographic views (Fig. 2). For acquisition of a multislice 2-D HASTE sequence, the breath-hold technique can be employed. The data acquisition time for a 3-D TSE sequence lasts between two and four minutes, which requires accurate compensation of respiratory motion of the kidneys. In 3-D TSE MR urography, respiratory triggering is preferred for motion artifact suppression. Note that the time necessary for respiratory compensation usually doubles the total sequence duration. With the use of modern multielement surface coils, parallel imaging techniques, such as SENSE, may be employed to reduce scan time significantly, which in turn can only be achieved at the expense of SNR. Typical sequence parameters for T2-weighted MR urography in adults and children are listed in Table 1.
Although T2-weighted 3-D TSE and multislice HASTE sequences are more time consuming than T2-weighted projection images, the analysis of multiple thin overlapping slices is regarded to be superior for the detection of pathologic details, which can be missed on an 80 mm thick projection MR urogram (6,8,9). On the other hand, MR projection images may be advantageous for use in babies and infants because of the very short acquisition time of approximately two seconds. In babies, the slice thickness of a projection MR image is reduced markedly (Table 1) and, thus, appears to be less problematic.
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