There is controversy regarding the value of CT index. The absolute CT density number is no longer used to distinguish benign from malignant lesions. The coin lesion being evaluated is compared to a phantom coin lesion which is just dense enough to be safely called a granuloma or hamartoma. CT numbers are quite variable.
There is no one CT number that identifies a nodule as benign. The CT machine measures the attenuation of the x-ray beam after it passes through a small volume of tissue. This CT number reflects the density of the tissue. CT numbers vary from machine to machine and from day to day. CT numbers of a lesion are also altered by the surrounding anatomy. For example, the same nodule would have three different density readings if it were in the middle of the lung, next to the heart and next to a vertebra.
This marked variability is compensated for by doing a CT study of a chest phantom duplicating the location of the patient's nodule. The patient's nodule is then closely compared to the phantom nodule. The phantom nodule has a known density. Determination of the density of the patient's nodule is relative to the phantom nodule. Absolute CT numbers do not enter into the determination.
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