Fluke Biomedical 76-903 - Manual (Page 15)

Theory of Operation
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The star pattern for MTF evaluation is made up of 3° wedges placed on a 60° fan angle. Each wedge is 3”
in the radial dimension and 2” in height. Two such sections are placed next to each other to cover an
overall fan angle of 120° (Figure 3-11).
A star pattern is a very useful way to measure the many subtle image artifacts that are unique to NMR
imaging. It may be difficult to pinpoint the exact source of the artifacts, but this is a powerful test to
demonstrate image artifacts that are otherwise difficult to detect.
Horizontal and Vertical Spatial Resolution
One can also observe asymmetric image resolution between the encoding and readout direction by
observing the resolving power in the star pattern (Figure 3-12). It can be seen that the vertical resolution
is superior to the horizontal resolution by observing that the narrow ends of the star pattern are better
resolved in the vertical direction.
Figure 3-12.
Quadrature Setting
In Figure 3-13, one can visualize a faint star-like pattern in the central 12 o'clock to 5 o ’ clock position. The
star-like ghost pattern is in the opposite orientation of the real star pattern image. This is attributable to
the quadrature setting. In quadrature signal detection, which is widely used in MR imaging, the RF signal
is split into two orthogonal signals (real and imaginary) before digitization. If the phase angle between the
real and imaginary components is not 90°, a ghost of point symmetry appears along the diagonal axis
between the encoding and readout directions. This artifact also shows up as a "fold-over" in the star
pattern.
Figure 3-13.