This article covers the following topics:

Introduction

Slice thickness information from the wire is given for the following Catphan® models: 500, 503, 504, 600, 700. The models contain opposing pairs of thin, steel wire ramps set at a 23° angle.

Slice thickness information from the bead ramps is given for the following Catphan® models: 600, 700. These models contain the CTP591 and CTP682 modules, respectively. The bead ramps are positioned in opposed pairs to eliminate errors caused by non-perpendicular alignment.

Refer to the Catphan® 600 and 700 datasheets for more information about the specifics of their wire and bead ramps.

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 decreases. 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.

Figure 6.1: Illustration of the method used to measure slice thickness from a 0.25mm spaced (in z-direction) bead ramp for a 2mm (top profile plot and side view) and 1mm slice width (bottom profile plot and side view).

Please note in this example 0.25mm spaced fine bead slice ramps are used, rather than the 1mm spaced coarse bead slice ramps. The methodology for both would be basically the same, however for thinner slices the use of the fine ramps improve measurement precision. Below we use a 1mm slice scan to illustrate the use of a profile made from a line running through the bead images on the scan.

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 in the scanner's z-direction. 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, as seen in the bottom profile plot of Figure 6.1, times the z-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, as seen in the image below.

CTP591 1mm profile.jpg

Figure 6.2: Vertical profile of a 1mm slice through the fine 0.25mm bead ramps after zooming or magnification. This figure is referred to in the example described below.

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 650HU and the baseline is about 50HU, leaving a net value of about 600HU between the peak value and the baseline. Thus, ½ the (net) maximum value is 300HU + the baseline of about 50HU so we draw a line across the 350HU ordinate (Y) value and measure the length of the line that spans the two FWHM points at, in this case, 350HU. 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 (0.25mm for fine ramps and 1.0mm for course ramps). For example, in this 1.0mm slice using the fine ramps, multiply the number of spaces between the peaks in the FWHM by the 0.25 mm distance between the bead peaks. Thus, four spaces seen at the FWHM times 0.25mm gives you the expected slice measurement of 1.0mm.

Slice Thickness from Wire Ramps

The 23° wire ramp angle is chosen to improve measurement precision through the trigonometric enlargement of approximately 2.4 in the x-y image plane, relative to a 45° ramp angle. This means that any small movement in the vertical direction along the ramp produces a larger movement in the horizontal direction, which is captured in the x-y plane. It can be described by the following equation:Δx = Δy · (1/tanΘ). In this case, where Δy is 1, the change in x is 1/tan23° ≈ 2.4.

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 tan23º, or 0.42: (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 CT values, determine the half maximum:

First calculate the net peak using net peak CT # = CT # peak - background.

Then, calculate the 50% net peak using 50% net peak CT = net peak CT # ÷ 2 .

Finally, calculate the half maximum CT number from the following equation: (half maximum CT # = 50% net peak CT # + background CT #).

Additional Methods for Measuring Slice Thickness

Note: These methods are not used by the algorithms but are instead intended for manual analysis.

A Slice Sensitivity Profile (SSP) of the bead(s) can be used to measure slice thickness (see CTP528 section for additional information).

Sagittal 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 technique is limited in the precision of the z-axis 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 width

w = (npvm / npva)*msw
Note: Net peak value = (CT# of the bead) - (CT# of the background)

Slice Thickness - Wire Ramps

To calculate slice thickness from the wire ramp, a Gaussian fit to the wire is made and then the FWHM is found and multiplied by tan23º (or 0.42).

Slice Thickness - Coarse Bead Ramps

A profile line through the 0.28mm diameter coarse beads is used to calculate slice thickness. In this profile there will be peaks at each of the bead locations and these will be separated by 1.0mm from each other in the z-direction.

Slice Thickness - Fine Bead Ramps

The method for calculating slice thickness with the fine and coarse bead ramps is the same, but the fine bead ramps consist of 0.18mm diameter beads, spaced 0.25mm apart in the z-direction. It is suggested that in thinner slices, measurements be taken from the fine bead ramps for better measurement precision. Thus, for the 1.0 mm slice width example mentioned above, we measure about four bead spacings at the Full Width at Half Maximum (FWHM). Multiplying the four bead spacings times the z-axis increment 0.25 mm per bead yields a 1 mm slice width.