Korosec et al  described yet another method for time-resolved 3D MRA called TRICKS (Time-Re solved Imaging with Contrast Kinetics). TRICKS is, however, vastly different than "sub-second" 3D MRA. The TRICKS algorithm consists of an alternative temporal k-space sampling scheme that repetitively samples low and high spatial frequency k-space views in a interleaved and slightly asymmetric fashion (Fig. 5). The low spatial frequency data (i.e. central k-space data) is updated at a much faster rate (i.e. higher temporal sampling) than the higher frequency data (i.e. peripheral k-space data). The resultant data is reconstructed to yield time resolved 3D data sets that have both high temporal and high spatial resolution. This is achieved by the sharing of k-space data, notably low spatial frequency data, across temporal 3D data sets. TRICKS and its iterative refinements of serial undersampling in multiple dimen-sions,such as PR TRICKS (Projection Reconstruction TRICKS ), used to suffer from inordinately long reconstruction times. However, improvements in reconstruction algorithms and computer processing speed have minimized these concerns, making this technique commercially available. The strength of TRICKS is that it continually samples k-space and upon reconstruction gives multiple 3D MRA phases of the contrast bolus as it passes through the desired area of evaluation. As mentioned previously this gives both a wealth of spatial and temporal resolution while eliminating the need for a timing run. Sub-second time resolved imaging can also be achieved using TRICKS, but as with traditional sub-second 3D MRA, imaging parameter adjustment typically results in a reduction in spatial resolution (Fig. 6) .
The performance of higher spatial resolution 3D TRICKS MRA with its relatively longer scan time requirements make its use for high spatial
Fig. 6. Sub-second 4D MRA using TRICKS implemented with parallel imaging in a healthy volunteer. The technique, called 4D MR DSA, provides a high temporal resolution of 600 msec per 3D image set. Scan parameters include: TR, 3.1 msec; TE, 0.9 msec; Flip angle, 20 degrees; FOV, 42 x 42 cm; martrix, 256 x 128 matrix; partitions, 22; partition thickness, 5 mm; interpolation to 44 partitions. On these TRICKS images, the normal flow pattern from a right sided venous injection can be seen with enhancement of the right heart and pulmonary arteries (first two rows) followed by enhancement of the left heart and aorta (bottom two rows) [Images courtesy of Drs. M. Ookawa, N. Ichinose, S. Sugiura, Y Machida and M. Miyazaki, Toshiba Medical Systems, Otawara, Tochigi, Japan]
resolution thoracic of abdominal imaging somewhat more challenging as breath hold acquisition is generally preferred for best image quality. Proper timing, albeit less precise, is nonetheless required to ensure tolerable breath hold durations for the patient and adequate temporal coverage of the bolus to ensure arterial phase images.
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