Applications of Dual-Energy Computed Tomography for the Evaluation of Head and Neck Squamous Cell Carcinoma  Reza Forghani, MD, PhD, Hillary R. Kelly, MD, Hugh D. Curtin, MD  Neuroimaging Clinics  Volume 27, Issue 3, Pages 445-459 (August 2017) DOI: 10.1016/j.nic.2017.04.001 Copyright © 2017 The Authors Terms and Conditions

Fig. 1 Energy-dependent/spectral properties of iodine in CT contrast agents. Spectral Hounsfield unit attenuation curves derived from region of interest analysis of two solutions with different concentrations of iodine, imaged within a phantom, are shown. There is a progressive increase in attenuation at lower energies approaching the K-edge of iodine (33.2 keV). For example, note the much higher attenuation at 40 keV compared with conventional single-energy CT equivalent virtual monochromatic image energies at 65 or 70 keV. The trade-off is the increasing image noise at lower energies, as represented by the error bars depicting standard deviation of attenuation within the region of interest evaluated. The scan was acquired with a fast kVp switching scanner. Neuroimaging Clinics 2017 27, 445-459DOI: (10.1016/j.nic.2017.04.001) Copyright © 2017 The Authors Terms and Conditions

Fig. 2 Examples of different DECT reconstructions are shown from a contrast-enhanced neck CT of a patient with a left oral tongue cancer (arrow). (A) 65-keV VMI (typically considered equivalent to a 120-kVp single-energy CT acquisition), (B) 40-keV VMI, and (C) iodine-water map are shown. Note the increase in tumor attenuation and contrast/tumor boundary on the low-energy (40 keV) VMI compared with the 65-keV VMI. The iodine-water map represents the iodine distribution and iodine content in different voxels and is used to estimate iodine concentration within tissues. The scan was acquired with a fast kVp switching scanner. Neuroimaging Clinics 2017 27, 445-459DOI: (10.1016/j.nic.2017.04.001) Copyright © 2017 The Authors Terms and Conditions

Fig. 3 Example of differential energy-dependent/spectral characteristics of tissues. (A) Axial CT image of a patient with a left oral tongue cancer (same patient as in Fig. 2) shows ROI analysis of different tissues: tumor (purple), normal tongue muscle (green), and a more fatty part of the tongue (blue). (B) Spectral Hounsfield unit attenuation curves from the ROI analysis in A are shown. Note the differences in energy-dependent attenuation of the different tissues, with increased separation of the curves at low energies. These differences in tissue attenuation form the basis for the use of low-energy virtual monochromatic images for increasing tumor visibility and improving boundary discrimination. Neuroimaging Clinics 2017 27, 445-459DOI: (10.1016/j.nic.2017.04.001) Copyright © 2017 The Authors Terms and Conditions

Fig. 4 Use of low-energy VMIs for the evaluation of HNSCC. (A, B) 65-keV (typically considered equivalent to a 120-kVp single-energy CT acquisition) and (C) 40-keV VMIs are shown from a patient presenting with left level II lymphadenopathy (arrowhead). Note the improved visibility of the primary tumor (arrow) and its boundary on the 40-keV VMI (C) compared with the 65-keV VMI at the same level (B). The scan was acquired with a fast kVp switching scanner. Neuroimaging Clinics 2017 27, 445-459DOI: (10.1016/j.nic.2017.04.001) Copyright © 2017 The Authors Terms and Conditions

Fig. 5 Example of differential energy-dependent/spectral characteristics of nonossified thyroid cartilage (NOTC) compared with tumor. (A) Axial 65-keV CT image of a patient with a supraglottic tumor shows ROI analysis of NOTC (pink/purple) and tumor (different shades of blue). Arrowheads have been placed to help with the visualization of the ROIs. (B) Spectral Hounsfield unit attenuation curves from the ROI analysis in A are shown. Whereas the attenuation of NOTC and tumor closely overlap at 65 keV, there is increased separation of the curves in the high-energy range caused by progressive suppression of iodine within enhancing tumor but relative preservation of the intrinsic high attenuation of NOTC at higher energies. These differences suggest that high-energy virtual monochromatic images may be useful for distinguishing NOTC from tumor. The scan was acquired with a fast kVp switching scanner. Neuroimaging Clinics 2017 27, 445-459DOI: (10.1016/j.nic.2017.04.001) Copyright © 2017 The Authors Terms and Conditions

Fig. 6 Use of high-energy VMIs and iodine maps for distinction of nonossified thyroid cartilage (NOTC; arrowheads) from HNSCC (arrow). (A) 65-keV VMI (typically considered equivalent to a 120-kVp single-energy CT acquisition), (B) 40-keV VMI, (C) 140-keV VMI, and (D) iodine (iodine-water) map are shown from a patient with laryngeal HNSCC. There is partial ossification of the thyroid cartilage. Note the similarity in attenuation of normal NOTC to tumor on the 65 and 40 keV in this case (A, B). However, on the 140-keV VMI, there is relative preservation of high attenuation of NOTC but relative suppression of iodine within tumor, enabling much better distinction of NOTC from tumor (C). The iodine map provides a quantitative map of iodine distribution and relative iodine content within tissues, with higher iodine content of tumor compared with nearby normal soft tissues (D). There is no evidence of significant extension of tumor iodine “signal” into the NOTC (D). The scan was acquired with a fast kVp switching scanner. Neuroimaging Clinics 2017 27, 445-459DOI: (10.1016/j.nic.2017.04.001) Copyright © 2017 The Authors Terms and Conditions

Fig. 7 Use of different DECT reconstructions for evaluation of thyroid cartilage invasion. (A) 65-keV VMI (typically considered equivalent to a 120-kVp single-energy CT acquisition), (B) 40-keV VMI, (C) 140-keV VMI, and (D) iodine (iodine-water) map are shown from a patient with laryngeal HNSCC invading the thyroid cartilage. There is an increase in tumor attenuation on the 40-keV VMI (B) compared with the 65-keV VMI (A). Conversely, there is suppression of iodine attenuation on the high-energy 140-keV VMI and the invaded portion of the thyroid cartilage appears as a relative low-attenuation defect on high-energy VMIs (C). This is in contradistinction to normal variants, such as nonossified thyroid cartilage, which would be expected to retain a high attenuation on high-energy VMIs (see Fig. 6C). The iodine map (D) provides a quantitative map of iodine distribution and relative iodine content within tissues, with higher iodine content of tumor compared with normal nearby soft tissues. Note the extension of iodine-containing tumor into the thyroid cartilage on the iodine map, extending through the outer cortex on the right (D). The scan was acquired with a fast kVp switching scanner. Neuroimaging Clinics 2017 27, 445-459DOI: (10.1016/j.nic.2017.04.001) Copyright © 2017 The Authors Terms and Conditions

Fig. 8 Use of high-energy VMIs for dental artifact reduction. (A) 65-keV VMI (typically considered equivalent to a 120-kVp single-energy CT acquisition), (B) 95-keV VMI, and (C) 140-keV VMI are shown. Note the greater artifact reduction with increasing VMI energy. For example, note the much better visualization of mandible immediately adjacent to the tooth with fillings and associated artifact on high-energy VMIs (arrow). The trade-off is a decrease in soft tissue contrast and iodine attenuation. However, this does not pose a significant limitation for the evaluation of structures with intrinsically high attenuation independent of iodine enhancement (such as bone). Neuroimaging Clinics 2017 27, 445-459DOI: (10.1016/j.nic.2017.04.001) Copyright © 2017 The Authors Terms and Conditions

Fig. 9 Use of high-energy VMIs for dental artifact reduction. (A) 65-keV VMI (typically considered equivalent to a 120-kVp single-energy CT acquisition) and (B) 95-keV VMI are shown from a patient with a large cancer involving the oropharynx and oral tongue. Note the modest artifact reduction on the 95-keV compared with the 65-keV VMI. The anterior margin (arrowheads) of the tumor in the oral tongue is better delineated on the 95-keV VMI. The normal structures within the anterior part of the oral cavity (arrows) are also better seen on the 95-keV VMI. As expected, iodine attenuation in the tumor and other enhancing structures is diminished on the 95-keV compared with the 65-keV VMI, representing the trade-off between artifact reduction and iodine/enhancing tissue attenuation with increasing VMI energy. Neuroimaging Clinics 2017 27, 445-459DOI: (10.1016/j.nic.2017.04.001) Copyright © 2017 The Authors Terms and Conditions

Fig. 10 Use of DECT for the evaluation of lymphadenopathy. (A) 65-keV VMI (typically considered equivalent to a 120-kVp single-energy CT acquisition), (B) 40-keV VMI, and (C) iodine (iodine-water) map are shown from a patient with a large internally heterogenous right level II (arrow) and a smaller cystic left level II (arrowhead) metastatic HNSCC lymph nodes. There is increase in soft tissue contrast on the 40-keV VMI (B) compared with the 65-keV VMI. For example, note the increased attenuation of the right level II node relative to adjacent muscle or the internal heterogeneity of the nodes on the 40-keV VMI (B) compared with 65-keV (C). The iodine map (C) provides a quantitative map of iodine distribution and relative iodine content within tissues, demonstrating heterogenous nodal iodine content. Neuroimaging Clinics 2017 27, 445-459DOI: (10.1016/j.nic.2017.04.001) Copyright © 2017 The Authors Terms and Conditions