Abstract

Article Series: Dual Energy CT – Scientific Evidence and Clinical Application (2/7) – Head and Neck

posted by Thorsten R. C. Johnson, M.D. | Nov 30, 2010

This article is part of the seven-article series on “Dual Energy CT – Scientific Evidence and Clinical Application” and covers Dual Energy applications of head and neck.

Head and Neck

Figure 1 Volume rendered image of the brain vessels after Dual Energy bone removal.

In the head and neck area, the main application of Dual Energy so far is bone removal from CT angiography datasets [19]. With this technique, the superimposition of bone and vessels is resolved and three-dimensional postprocessing is possible like in MR angiography.
This is helpful to rapidly screen datasets for pathology without reading several hundred axial slices. The evaluation of vessels at the skull base profits most from the bone removal, because conventional reading usually requires cumbersome multiplanar reformats with adjustment of the window level, while the carotid siphon is easily evaluated at a glance without the bones [20]. Also, small aneurysms in the Circle of Willis are identified more easily in maximum intensity projections without bones [21]. Still, there are articifial truncations due to artifacts from dental fillings or due to blooming artifacts at dense atherosclerotic calcifications, so it is required to confirm findings on conventional multiplanar reformats.

Several studies have confirmed that this technique works reliably and much better than merely density-based software bone removal [21-25]. In comparison to subtraction techniques, the Dual Energy technique requires less dose because an unenhanced acquisition is not necessary [22], and is also much less sensitive to patient motion. While very strong iodine opacification of the vessel can make conventional assessment of the vessels difficult in the area of the skull base, the Dual Energy technique profits from strong enhancement [23]. In initial studies using 140 and 80kVp spectra, the vessel integrity at the thoracic inlet and the segmentation of the vertebral arteries was partially unsatisfactory [22, 25]. With the second generation DSCT system and Sn140/100kVp spectra, results significantly improved in these areas [26].

Another potential application of Dual Energy CT in the head is the differentiation of contrast material and hemorrhage. This is important if the hemorrhage is seen unexpectedly and an unenhanced scan had not been obtained. Then, it is feasible to subtract the iodine-related density in order to obtain a “virtual” non-contrast image. The small differences of white and grey matter in CT density make this task rather challenging, so the resultant virtual non-contrast image is not a full substitute for an unenhanced scan in the detection of subtle signs of early ischemia. However, two studies have confirmed that the technique is sufficient to differentiate contrast enhancement and hemorrhage [27-28].