Fluke Biomedical 76-903 - Manual (Page 12)

Nuclear Associates 76-903
Operators Manual
3-4
3. Star Pattern: Modulation transfer function (MTF), horizontal and vertical spatial resolution,
quadrature setting and baseline correction.
3.4.1 Folded Step Ramps
Accurate slice thickness measurement in MR is very difficult and various methods are under discussion.
The slice profile (shape of slice thickness) is also a very complex subject that needs further discussion
and definition by the physics community. This folded ramp approach is adopted in this phantom because
of its ease of use and efficiency in time. For a quick evaluation, a simple counting of the number of visible
steps will give an estimate of slice thickness in millimeters.
Slice parameters are studied using a pair of folding step wedges. The height of the steps range from 16
mm to
40 mm and the heights vary every 1.0 mm (Figure 3-6). Each step area has a dimension of 10 mm x 10
mm. The wedges were folded to accommodate a greater number of steps in the limited space available.
Two inserts of the folded steps are placed at opposite sides of the phantom so that slice orientation with
respect to the phantom can be studied.
1. Slice Thickness
2. Slice Orientation
3. Interslice Gap
Figure 3-6. Step Interval, 1 mm Range, 36 mm
3.4.2 Fold Step Ramp Analysis
Quick Visual Evaluation
Slice thickness can be evaluated quickly by counting the number of steps that can be visualized at a
relatively wide window setting (Figure 3-7). The number of visible steps is equal to the slice thickness in
millimeters since the height of each step varies by 1 millimeter. Record the window setting for future
reference, for the visible steps vary depending on the window setting.
In Figure 3-7, five gray scale steps can be seen in the 10 o ’ clock and 5 o'clock positions, representing a
slice thickness of 6 mm.
3.4.3 Measurement Method
Better evaluation of slice thickness can be made by using the region of interest (ROI) routine in the imaging system.
The area of the region slightly smaller than the step size (10 mm x 10 mm) should be used to read out the signal
intensity (NMR number). Than take the difference of the "NMR" number of the adjacent steps. Plot a graph of the
difference in intensity on the vertical axis and step sequence numbers on the horizontal axis. The A line graph
connecting the dots (diff. vs. step number) represents slice profile and the width of the graph at half of its maximum
intensity (full width half maximum) is the measured slice thickness.