Hounsfield Units are a powerful tool for assessing attenuation of biological materials. The following calibration procedure will result in NanoSPECT/CT reconstructions generated with VQ to consist of voxels having proper Hounsfield units.
Open the Hounsfield unit panel found in CT Reconstruction
Reset the HU values in the panel so that each Slope = 1.0 and each Intercept = 0.0.
Fill a large (>2.5cm) diameter syringe with water and collect CT data at 45 kVp with standard settings. It is not necessary to image the entire syringe -- just the section that is full of water.
Repeat the acquisition at 55 kVp and 65 kVp.
Reconstruct each CT.
Using the Quantification Tool, select a region of the reconstruction consisting entirely of water inside the syringe.
Set the Slope = 1000/Mean. In the screenshot above, Slope = 1000/1.4527 = 687.15. Typical slopes are near 650. Be sure to set the Slope for each energy to correspond to the value from the reconstruction of the data collected at that energy.
Set each Intercept = -1000.
Ensure that the "enable calibration" box (Shown in the Hounsfield panel above) is checked.
Reconstruct the CT a second time. Select a region of the syringe encompassing both water and air. Values in water should be near 0 while those in air should be near -1000.