_{Published November 23, 2020}

Mayur Virarkar

Department of Neuroradiology
University of Texas Health Science Center at Houston

Priya Bhosale

Department of Diagnostic Radiology
University of Texas MD Anderson Cancer Center

Dual-energy CT (DECT) is a state-of-the-art technology that simultaneously processes data from multiple photon energies in a single CT acquisition. The principle of dual energy dates back to the 1970s, whereas the first clinical DECT scanner was available in 2006. However, the utilization of DECT in routine clinical practice has grown over the past decade owing to increased scanner availability from vendors and multiple new applications of DECT techniques. Furthermore, the postprocessing DECT data using commercially available software have resulted in the generation of virtual monoenergetic or monochromatic images ranging from 40 to 200 keV. These images can also be used to produce spectral attenuation curves, scatterplots, histograms, and effective atomic number (Z_eff). Various technical approaches of DECT imaging are currently available through different vendors, such as single source rapid-kVpswitching DECT (GE Healthcare, WI), single source helical DECT (Siemens Healthineers, Germany), single source twin-beam DECT (Siemens Healthineers), dual source DECT (Siemens Healthcare, Germany), single source sequential DECT (Toshiba, Japan), and dual layer DECT (Philips Healthcare, Netherlands).

In a typical CT, two different materials (e.g., calcium and iodine) may demonstrate similar CT attenuation values when subjected to a single radiation beam; however, these materials behave differently when exposed to different energy levels, as in dual energy CT. Atomic number (i.e., Z) is an important parameter determining the CT attenuation values. For example, higher Z materials are more susceptible to the photoelectric effect than lower ones. The commonly used contrast agent iodine is discernible at low kiloelectron volt values, and these properties can be useful in distinguishing iodine from other body materials, such as calcium and water. Soft tissues, such as muscles and organs, have weak photoelectric effects and less variation in their attenuation values at different energies.

DECT postprocessing techniques produce different types of reconstructed images with multiple clinical functions. These images include mixed material-specific images, such as water, iodine, or fat images, virtual monochromatic and virtual monoenergetic images (VMIs) generated for a single energy level. Spectral attenuation curves display particular ROI energy values on the x-axis (range, 40–140 keV) and mean attenuation values on the y-axis. Scatterplots are generated by comparing the ROI attenuation values with water concentrations. Histogram displays the frequency of values for a single ROI variable. The materials can be differentiated from one another on the basis of their calculated Z_eff values (i.e., virtual atomic numbers). The calculated Z_eff takes into account the unique nature of the materials over the range of energies in DECT (40–140 keV).

Researchers have explored the utilization of DECT in identification and characterization of various tumors in the body. VMIs are more advantageous in identification and characterization of liver lesions than conventional CT. Material-specific images, such as iodine images, aid in assessing hypervascular metastases from uterine sarcomas. In addition, these images may be able to detect remote small subdiaphragmatic perihepatic implants. The retroperitoneal lymph nodes are better visualized on iodine-enhanced images than conventional CT images. Studies have shown lower iodine uptake on DECT in metastatic than in normal and inflammatory lymph nodes, guiding the diagnosis of lymph node metastases. Osseous metastases with soft-tissue components are also better visualized on iodine-enhanced images. VMI has also been beneficial in identifying incidental pulmonary embolism (PE) in oncologic patients. 40-keV VMI images have been shown to improve objective image quality of the pulmonary vessels, along with increased diagnostic confidence in the diagnosis of incidental PE. Virtual unenhanced DECT images offer valuable tools for improving the diagnosis of pediatric abdominal neoplasms—helping to identify or validate the presence of tumoral calcifications and hemorrhage, appropriate lesion delineation, and differentiate an abdominal mass from adjacent contrast- filled bowel or abdominal organs. DECT has also been documented to identify vascular and perfusion abnormalities due to hypoxemia related to coronavirus disease 2019 (COVID-19).

There are few clinical studies in the current literature assessing the diagnostic ability of DECT in gynecological malignancies. These studies support using low energy for assessing endometrial cancer invasion, characterization of ovarian masses with internal septation and mural nodularity, and identification of calcified peritoneal implants and remote serosal perihepatic implants. Iodine maps are useful for assessing response after chemoradiation in cervical cancer patients, peritoneal implants, and nodal and osseous metastases, as well as distinguishing benign and malignant ovarian tumors. Water maps obtained from DECT are useful in distinguishing high- and low-grade ovarian tumors. Currently, there are numerous and amazing new applications of DECT being investigated in clinical studies. Spectral photon-counting CT with enhanced image quality is being translated successfully into clinical studies. There have been recent developments in new nanoparticle contrast agents with specific disease biomarkers for dual-energy and multi-spectral CT. All of that being said, there is a need for more prospective trials to explore the true potential of this innovative and promising technology, so as to make DECT a true multiparametric imaging modality in the future.