As with all MR applications, there is the classic dilemma of temporal resolution (i.e. scan time) versus spatial resolution, since higher spatial resolution imaging requires longer acquisition times. Optimization of these competing demands requires both an understanding of the minimal tem poral and spatial requirements for the specific vascular territory, and familiarity with the options available for scan parameter modification that can meet those requirements. Imaging time and the various modifiable parameters for scan time reduction can be calculated by the equation:
Scan time = TR * NZ * NY * (k-space fraction) * 1/AF
in which, Ny is the number of actual phase encoding steps (y resolution); NZis the number of partitions (z resolution); (k-space fraction) is the percentage of k-space views that are acquired (n.b. alternative k-space acquisition schemes such as partial Fourier and rectangular FOV will result in reductions in phase encoding steps and, as a result, in a comparable reduction in scan time); and AF is the acceleration factor if parallel imaging is implemented.
For CE MRA, scanning is typically performed during a breath hold, and thus the patient's breath hold will limit the acquisition time. Because most patients can hold their breath for 20-30 seconds, this is considered the typical upper limit acquisition time for most CE mRa examinations. In cases where breath holding is not required, such as carotid or peripheral CE MRA, longer acquisition times can be employed. The challenge is therefore to balance slice thickness and the number of phase encoding steps to ensure adequate spatial resolution for appropriate diagnosis with adequate anatomic coverage for the clinical indication.
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