PHYSICAL PRINCIPLES OF DUAL ENERGY
Single or conventional CT uses a single polychromatic X-ray beam, usually ranging from 70 kV to 140 kV, emitted from a single source.
Dual-energy refers to the use of two different X-ray spectra in computerized tomography (typically 80kV and 140 kV) to acquire images that can be processed to generate additional data.
The basic principle of DECT consists of the idea that the degree of attenuation of a particular tissue (reflected by Hounsfield Units, HU) depends on its composition (density and atomic number) and also on the energy of the X-ray beam. It is directly proportional to the atomic number of the material and its density, while it is inversely related to energy. Therefore, the attenuation can be modified by changing the one parameter that is adjustable by the radiologist, the photon energy levels [1].
Materials with low atomic numbers and low k-shell threshold (such as those found in the human body like hydrogen, carbon, oxygen…) demonstrate small differences in attenuation between low and high x-ray spectra. However, some materials like calcium or, particularly iodine, have closer k-shell thresholds to the energies used in diagnostic imaging, showing larger differences in attenuation at different energy levels.
Therefore, the difference in x-ray attenuation between adjacent structures at different energy levels allows characterizing and quantifying the behavior of different substances (mainly calcium, iodine, and uric acid) providing morphological and functional information [2].
Current Dual-Energy CT Acquisition Methods
- Dual-source: two X-ray sources, located at 90º respectively, producing different voltages with their two corresponding detectors.
- Single-source rapid kilovoltage switching: tube voltage is rapidly changed between 80 and 140 kVp within the same rotation.
- Dual-layer detector: one x-ray tube with a single voltage with two layers of detectors.
- Single-source sequential: each x-ray tube rotation is performed at high and low voltage.
Imaging reconstruction
- Average images: average, mixed, or blended images are a simulation of routine diagnostic images by a combination of the acquired low-energy (80 kV) and high-energy (140 kV) acquisitions.
- Virtual Monoenergetic or Monochromatic imaging (VME/VMC): These reconstructions show images as if they had been obtained from a determined single energy level.
- Low-energy VME images improve the contrast-to-noise ratio so they are used for studies with high contrast between the adjacent tissues, for example, they increase the contrast opacity within the vessels.
- High-energy VME images reduce the artifact due to beam hardening, for example, reducing artifacts from metal implants.
- Material differentiation imaging: With a mathematical algorithm and specific software, it is possible to obtain images by subtracting or isolating one particular component.
- Virtual non-contrast map (VNC): these reconstructions can obviate the need for a non-contrast conventional acquisition and, therefore, reduce exploration time and the radiation dose received by the patient.
- Iodine overlay map: by calculating the amount of iodine per voxel, these maps show how much a structure enhances and how does the contrast distributes in tissues.