The remaining pulse sequence parameters which may be adapted in a given patient relate to spatial resolution. These parameters are matrix size, slice thickness and field of view (FOV). The actual parameters used depend greatly on the target vascular bed. In general, it is preferable that the structure of interest contains three pixels across its diameter . Thus, for example, a 9 mm wide vessel will have at least three pixels across its width if a voxel dimension of 3 mm or smaller in any single dimension is used. The actual spatial resolution, however, depends on the chosen FOV. The spatial resolution for the x, y and z dimensions of a 3D CE MRA can be calculated by:
Spatial resolutions FOVx / Nx Spatial resolutiony= FOVy / Ny Spatial resolutionz= Partition thickness in which FOVxand FOVy are the frequency encoding dimensions of the field of view (x-axis) and phase encoding (y-axis) dimensions; and Nxand Ny are the number of frequency encoding and phase encoding steps, respectively.
In cases in which the anatomic region is much smaller in one dimension, a rectangular FOV can be used to reduce the overall scan time .With a rectangular FOV the data in the phase direction is undersampled, causing a reduction of both the FOV and scan time. It is particularly important when using a rectangular FOV to pay particular attention to the anatomic region since too small a FOV or improper prescription of the volume can result in unwanted aliasing or "wrap around" artifacts in which unwanted data from outside the FOV is included in the FOV. This can also happen if the venous line is placed in the direction of the FOV. The effects of a chosen FOV can easily be assessed on a pre-contrast MRA prior to the CE MRA.
A rectangular FOV is particularly useful for imaging the thoracic aorta. In this case, a sagittal 3D CE MRA is appropriate with the phase direction prescribed for the anterior-posterior plane. A 0.8 or 80% rectangular FOV will result in a reduction of the image anterior to the sternum and posterior to the spine. It is important that the sagittal acquisition is centered properly over the thorax. Importantly, it should be noted that anatomy in the periphery of the FOV in the phase encoding direction (i.e. outside of the rectangular FOV) will alias signal back into the 3D image set and degrade the resulting images. Care must therefore be taken to ensure that the FOV is sufficiently large as to avoid aliasing of signal over a vital vascular region of interest.
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