Validate the Second Moment of Area by using known geometry

[Anna]

Dear All,

I'm trying to compare the SMOA theoretical value of a rectangular 10mmx5mm (a sawbone) with the same sawbone scan using mCT.

>The theory value were Iyy=416.67mm^4 and Ixx=104.17mm^4. 

>The mCT images: diemnsion [voxels]=606x546x903 and dimension [mm]=22.82x20.56x34

>> I changed the image properties based on the dimension [mm].

>> Unfortunately the result varies so much; Imax=8.81x10^12 mm^4, Imin=4.30x^13 mm^4.

Is there any steps that I missed?

Many thanks for all your help!

best regards

Ann

[Michael Doube]

Dear Ann,

1. Make sure you set the pixel width to 22.82mm/606 = 0.03766 mm

2. Make sure you set appropriate threshold values in Slice Geometry's dialog.

Try this macro: it makes an image with the dimensions you describe, draws a rectangle of the size you describe, then runs Slice Geometry. The results are very close to the theory (b^3*h/12).

newImage("Untitled", "8-bit black", 606, 546, 1);
run("Properties...", "channels=1 slices=1 frames=1 unit=mm pixel_width=0.03766 pixel_height=0.03766 voxel_depth=0.03766");
run("Specify...", "width=10 height=5 x=11 y=10 centered scaled");
setForegroundColor(255, 255, 255);
run("Fill", "slice");
run("Select None");
run("Slice Geometry", "bone=unknown 2d_thickness draw_axes draw_centroids annotated_copy_(2d) bone_min=128 bone_max=255 slope=0.0000 y_intercept=1.8000");

Michael

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[Shafini Mohamad]

Dear Michael,

Many thanks for your help! I've tried the macro and it run perfectly. I got the same result!

I want to try on my mCT images of the sawbone. How I can make sure I set the correct threshold values?

I've tried but the results seem to be different with the theory.

Thanks Michael.

Ann

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[michae...@gmail.com]

Check your pixel calibration.

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