Slice Thickness from Bead Ramps
To illustrate how the bead ramps are used, the following illustration shows both a 1mm and 2mm slice going through a bead ramp. You may note that as the slice thickness increases, the peak CT value for the beads decrease. This is because as the slice thickness increases, the bead’s effect on the CT number of the voxel decreases, due to volume averaging. Presuming the slice thicknesses are accurate, the peak signal over background in a 1mm slice should be double that of the peak signal over the background in the 2mm slice.
When we use a profile line through the beads, there will be peaks at each of the bead locations and these will be separated by 0.25mm from each other. Thus for example, for the 1.0mm slice width we measure about four bead spacings at the Full Width at Half Maximum (FWHM). Multiplying the four bead spacings times the y axis increment 0.25mm per bead yields a 1mm slice width.
Another method for counting beads would be to measure the maximum CT number of the beads. This can be done by adjusting the window width to 1 and raising the level until the beads disappear and noting the peak level. Next, do an ROI of the area adjacent to the ramp to get a number for the background. Keeping the window width at 1, raise the level to half between background and peak (half maximum) and count the beads.
We can make this somewhat more analytic by noting the following. If we hand-draw, or use a mathematical “best fit” bell shaped curve (Gaussian) to the data points, you will notice that the peak CT number for the 1.0mm slice is about 650 H and the baseline is about 50, leaving a net value of about 600H between the peak value and the baseline. Thus, ½ the (net) maximum value is 300H + the baseline of about 50H so we draw a line across the 350H ordinate (Y) value and measure the length of the line that spans the two FWHM points at, in this case, 350H. When measuring the FWHM of the curve it is important to realize that due to scaling and translation variables the scale of the FWHM length needs to be defined. This is done using the distance between the individual bead peaks in the profile whose absolute separation is known (.25mm for fine ramps and 1mm for course ramps). For example for the fine ramps divide the FWHM by the distance between bead peaks and multiply by .25mm.
Slice Thickness from Wire Ramps
The 23° wire ramp angle is chosen to improve measurement precision through the trigonometric enlargement of 2.38 in the x-y image plane.
To evaluate the slice width (Zmm), measure the Full Width at Half Maximum (FWHM) length of any of the four wire ramps and multiply the length by 0.42(tan23º): (Zmm) = FWHM * 0.42
To find the FWHM of the wire from the scan image, you need to determine the CT number values for the peak of the wire and for the background. To calculate the CT number value for the maximum of the wire, close down the CT “window” opening to 1 or the minimum setting. Move the CT scanner “level” to the point where the ramp image just totally disappears. The CT number of the level at this position is your peak or maximum value. To calculate the value for the background, use the region of interest function to identify the “mean” CT number value of the area adjacent to the ramp. Using the above CTvalues, determine the half maximum: First calculate the net peak... (CT # peak - background = net peak CT #) Calculate the 50% net peak... (net peak CT # ÷ 2 = 50% net peak CT #) Calculate the half maximum CT number...
(50% net peak CT # + background CT # = half maximum CT #)
Additional Methods for Measuring Slice Thickness
Note: These methods are not used by the Image Owl algorithms.
A ssp of the bead(s) can be used to measure slice thickness (see CTP528 section for additional information).
Sagital and coronal slices through the beads can also be used to measure the axial slice width. In this case measure the z axis length at the full width at half maximum of a bead image to establish the slice thickness. However this tecnique is limited in precision z axis of the voxels.
The volume averaging effect on the net peak CT number of the bead can be used to approximate additional slice thickness measurements after measuring one slice’s thickness by using the following equation:
w = slice width of additional slice thickness.
npvm = net peak value of the bead in the measured slice width
msw = slice width of the measured slice
npva = net peak value of the bead in the additional slice widthw = (npvm / npva)*msw
Note: Net peak value = (CT# of the bead) - (CT# of the background)