2. Multi-energy techniques

2.1 Principle of multi-energy imaging

A CT-scanner can be used to estimate the attenuation of each voxel of the body under examination. Is this estimate precise enough to unambiguously identify the nature of the corresponding tissue? The answer is that it often isn't. While the measurement of Hounsfield numbers (HU) can be used to segment organs and identify anomalies, in many cases it cannot be used to determine the chemical nature of the materials making up the voxel. Relative values of Hounsfield numbers within a slice are exploited, but more rarely their absolute values.

There are several reasons for this lack of precision. Image noise and reconstruction artifacts are among them, but the main one is that the HU number provides information on absorption...