ual-energy T Review Vogl et al. Head and Neck ual-energy T ual-energy T Review Thomas J. Vogl 1 oris Schulz 1 Ralf W. auer 1 Timo Stöver 2 Robert Sader 3 hmed M. Tawfik 1,4 Vogl TJ, Schulz, auer RW, Stöver T, Sader R, Tawfik M Keywords: dual-energy T, head and neck, low-kilovoltage T OI:10.2214/JR.12.9113 Received pril 19, 2012; accepted without revision May 4, 2012. Publication of this supplement to the merican Journal of Roentgenology is made possible by an unrestricted grant from Siemens Healthcare. 1 Institute for iagnostic and Interventional Radiology, Johann Wolfgang Goethe University Hospital, Theodor-Stern-Kai 7, 60590 Frankfurt am Main, Germany. ddress correspondence to T. J. Vogl (T.Vogl@em.uni-frankfurt.de). 2 epartment of Otolaryngology, Head and Neck Surgery, University Hospital of Frankfurt, Frankfurt am Main, Germany. 3 epartment of ental, Oral and Maxillofacial surgery, University Hospital of Frankfurt, Frankfurt am Main, Germany. 4 iagnostic and Interventional Radiology epartment, Mansoura Faculty of Medicine, Mansoura, Egypt. JR 2012; 199:S34 S39 0361 803X/12/1995 S34 merican Roentgen Ray Society ual-energy T pplications in Head and Neck Imaging OJETIVE. ual-energy scanning is a breakthrough in T technology that has several applications in chest and abdominal imaging. ual-energy T also has potential for head and neck imaging. This review describes the role of dual-energy T in head and neck imaging. ONLUSION. s with other body regions, both image fusion and material characterization dual-energy applications can be used for head and neck imaging. Early results are promising, and further research is encouraged. T echnologic advances in T in the last few years have exceeded all expectations. T not only is faster, has higher resolution, and has greater anatomic coverage than ever but also is capable of material characterization and other applications owing to dual-energy technique. ual-energy T (ET) refers to the new simultaneous acquisition of low- and highvoltage T data. dvances in hardware platforms, including dual-source T and other dual energy capable T scanners and dedicated dual-energy software are now commercially available and installed at many institutions worldwide. Several applications of ET are in clinical use, especially in chest and abdominal imaging [1 4], and applications for other body regions are being developed. The head and neck region is no exception, and this review focuses on the advantages and applications of ET in head and neck imaging. Technical onsiderations Several techniques have been developed for ET scanning (e.g., dual-source T from Siemens Healthcare and rapid peak kilovoltage switching from GE Healthcare). Technical details of dual-energy scanners and comparison of approaches to dual-energy scanning are beyond the scope of this article, and the reader is referred to other review articles [1, 2, 5]. ll images in this review were obtained with a 128-MT dual-source scanner (Somatom efinition Flash, Siemens Healthcare). dual-source T scanner is composed of two tube-detector systems mounted on the same gantry at 90 to each other. T acquisi- tion was performed at different peak kilovoltage settings for the two tubes (tube, 80 kvp; tube, 140 kvp with a tin filter). Other scanning parameters were as follows: collimation, 0.6 mm; rotation time, 0.5 second; pitch, 0.9. utomatic exposure control was used. Reference tube current time product values were 151 ms for the 140-kVp tube with a tin filter and 302 ms for the 80-kVp tube. Radiation ose onsiderations Patient radiation dose from ET is a major concern, and any new technique that harbors an undue increase in radiation dose will not be widely accepted. In a phantom study of two dual-source scanners, Schenzle et al. [6] reported that ET of the chest is comparable to standard T without additional radiation dose. Image quality is a function of radiation dose, and the tradeoff between improving image quality without increasing the radiation dose and between lowering the radiation dose without compromising image quality is of utmost importance, especially in the complex and challenging head and neck region. Tawfik et al. [7] compared the quantitative and qualitative image quality of ET with that of standard T at five anatomic levels in the head and neck. They reported that ET is comparable to standard T even when the radiation dose is 12% lower. Those authors concluded that ET of the head and neck yields multiple additional datasets with no radiation dose penalty. asic Principles T depends on the x-ray attenuation of different materials, which is quantified in HU S34 JR:199, November 2012
Head and Neck ual-energy T and displayed as shades of gray. t x-ray energies relevant to diagnostic imaging, the two predominant interactions between x-ray photons and matter are ompton scattering (x-ray scatter with fractional loss of x-ray energy) and photoelectric absorption (complete x-ray absorption) [8, 9]. In conventional T at 120 140 kvp, ompton scattering predominates. In the lower energy range, however, ompton scattering remains constant and photoelectric absorption increases. X-ray attenuation is therefore typically higher for lower-energy than for higher-energy photons. The probability of photoelectric absorption also increases markedly when the imaged material has a high atomic number [1, 2, 8]. oncerning ET, two main conclusions can be drawn from the foregoing information. First, the attenuation of materials with a high atomic number (e.g., iodine) is higher with lower-energy than with higher-energy scanning. It follows that the attenuation of vessels, enhancing normal structures, and enhancing pathologic lesions is much higher on 80-kVp than on 120- or 140-kVp images [10]. The attenuation of soft tissue increases only a small amount, so enhancing pathologic lesions are more conspicuous when a low tube voltage is used [11] (Figs. 1 and 2). This is the basic principle for image fusion in ET. Second, when an object with unknown material composition is imaged twice with two different energy spectra (e.g., 80 and 140 kvp), materials within that object can be differentiated and quantified on the basis of the change in x-ray attenuation with E reference to the expected change in x-ray attenuations of (up to three) known materials. lgorithms for material characterization are commercially available in dual energy equipped workstations [1, 12]. Image Fusion in the Head and Neck With every dual-energy scan, both low- and high-tube-voltage image datasets are routinely reconstructed and can be separately evaluated. The low-voltage (e.g., 80 kvp) dataset is characterized by a marked increase in the attenuation of iodine (contrast enhancement) but at the expense of increase in image noise [13, 14]. espite the degraded image quality, the increased lesion enhancement and contrast-to-noise ratio in the 80-kVp image dataset is advantageous in some cases when lesion Fig. 1 63-year-old man with left retromolar carcinoma. E, xial T images show dual-energy image fusion with ascending weighting factors (percentage of low tube contribution in image) from 0.0 or 140 kvp (), to 0.3 (), 0.6 (), 0.8 (), and finally to 1.0 or 80 kvp (E). Lesion (arrow) and vascular enhancement increase gradually from through E. Image noise also increases. JR:199, November 2012 S35
Vogl et al. detection is difficult because of small size or difficult anatomy (Figs. 1 and 2). The highvoltage (e.g., 140 kvp with a tin filter) dataset will have low image noise but at the expense of a lower enhancement level, limiting its diagnostic value. Together with the low- and high-voltage datasets, a mixed image from the data of a dual-energy scan is routinely reconstructed. This is called the weighted-average image dataset and combines the characters of the lowand high-voltage acquisitions according to the applied weighting factor. t tube voltages of 80 kvp and of 140 kvp, the 0.3 weightedaverage dataset (30% from 80-kVp data and 70% from 140-kVp data) is the one usually used for routine diagnostic purposes because it simulates a standard 120-kVp acquisition [7, 15] (Figs. 1 and 2). In addition to the aforementioned image datasets (80 kvp, 140 kvp, and 0.3 weightedaverage), with the use of work stations equipped with dual-energy software, any other desired weighted-average mixture from the two acquisitions can be generated and result in significant differences in attenuation and image noise [16] (Fig. 1). To take advantage of the plentiful available weighted-average mixtures, it is possible to manually adjust the weighting factor of high- and low-energy acquisitions until the best image quality and contrast for an individual scan are reached. Graser et al. [17] reported that with manual adjustment of the weighting factors for ET of the abdomen, image quality can be improved for some patients when a 0.5 weighting factor is set rather than the usual 0.3 weighting factor. Unfortunately, manual adjustment is time-consuming and can only be done at a dedicated workstation. nother approach is that multiple image datasets at different weighting factors are reconstructed at the time of acquisition and sent to the PS. This process is also time-consuming, overloads the PS, and increases interpretation time. nother, perhaps more practical, approach is to reconstruct a predetermined single best weighting factor at the time of T acquisition. Tawfik et al. [18] compared multiple weighting factors in ET of the head and neck and observed significantly improved image quality using a weighting factor of 0.6 (60% from 80 kvp, 40% from 140 kvp with a tin filter). Those authors also reported increased contrast-to-noise ratios and subjective lesion delineation of squamous cell carcinoma of the upper aerodigestive tract. They recommended that a 0.6 weighted-average image dataset be reconstructed for ET of the head and neck in addition to the aforementioned routine datasets. The weighted-average image datasets are mixtures of the data acquired at low and high tube voltages that simulate standard single-energy acquisitions at the equivalent photon energies. This process is called linear image fusion. lternatively, a nonlinear algorithm for blending low- and high-voltage data has been described that differs from the linear blending method that entails use of the same mixing ratio across all the pixels. In nonlinear blending, a different mixing ratio is calculated at each pixel according to a specific formula with the attenuation value of the pixel as a variable [19, 20]. ccordingly, voxels with low attenuation in muscle and homogeneous organs are preferentially obtained from the high-voltage data to minimize noise, whereas voxels with Fig. 2 68-year-old man with right oropharyngeal carcinoma (arrow). xial T images comparing linear and nonlinear dual-energy image fusion., 140-kVp image., 80-kVp image., 0.3 weighted-average linear fusion image (30% from 80 kvp and 70% from 140 kvp)., Nonlinear fused image. Nonlinear image fusion provides high lesion and vascular contrast enhancement (weighted toward 80 kvp) and very low noise (weighted toward 140 kvp). S36 JR:199, November 2012
Head and Neck ual-energy T Fig. 3 34-year-old man with right peritonsillar abscess (star). xial T images show iodine characterization., 0.3 weighted-average image., Subtraction of iodine from contrast-enhanced dual-energy scan results in virtual unenhanced image., Iodine distribution map resulting from readdition of iodine. Fig. 4 57-year-old woman with left retromolar carcinoma (arrow, ). xial dual-energy T images., 0.3 weighted-average map., Nonlinear fused image., Iodine distribution map., Iodine distribution map with region of interest (1) inserted in lesion for quantification of net enhancement. Iodine overlay is 81 HU, and iodine content is 3 mg/ml. JR:199, November 2012 S37
Vogl et al. high attenuation are extracted from the lowkilovoltage data to maximize contrast enhancement. Therefore, theoretically, nonlinear blending can be used to maintain the better contrast of the low-voltage scan while the noise characteristics of the high-voltage scan are retained. y use of nonlinear fusion of low- and highvoltage data, T images with better contrast-tonoise values and improved image quality may be produced [19, 20] (Fig. 2). Material haracterization in the Head and Neck With material characterization algorithms, iodine can be differentiated from other tissues on a contrast-enhanced dual-energy scan. The dual-energy software then subtracts iodine from all regions of the image, generating a virtual unenhanced image (Fig. 3). On this virtual unenhanced image, enhancing lesions Fig. 5 86-year-old man with squamous cell carcinoma infiltrating right auricular soft tissue (arrow, ). xial dual-energy T images., 0.3 weighted-average image., Nonlinear fused image., Iodine distribution map., Iodine distribution map with region of interest (1) inserted in lesion. Iodine overlay is 43 HU, and iodine content is 2.5 mg/ml. can be differentiated from calcification and other high-attenuation lesions without having the patient undergo scanning before contrast administration [21]. Material-specific datasets are volumetric and therefore can be evaluated as reconstructed axial images or after processing with conventional 3 applications such as multiplanar reconstruction, maximum intensity projection, and volumerendered reformation [12]. nother application of dual-energy technique is the generation of iodine distribution images or maps on which the calculated iodine distribution on an image is color coded and superimposed on the virtual unenhanced images [22, 23]. For imaging of the head and neck, use of the color-coded map is thought to increase visual detection of lesions. ecause color is superimposed on original T images, excellent anatomic detail is preserved, and lesions can be easily delineated from their surroundings (Fig. 4). lthough the iodine distribution map is referred to as a perfusion map, especially in the lung, it is neither a first-pass nor a dynamic study and hence cannot be used for calculation of semiquantitative (peak enhancement) or quantitative perfusion parameters (e.g., blood flow, blood volume, transit time) [24]. n iodine distribution map, however, is indirectly linked to perfusion because it reflects both intravascular and extravascular iodine concentrations in tissues. The intravascular component depends on regional blood volume, and the extravascular component depends on the permeability of the capillaries to contrast medium [25]. Unfortunately, the two compartments cannot be separated with this method. part from perfusion parameters, which can be obtained only with dynamic (timeresolved) imaging [3], the iodine distribution map allows calculation of the net enhancement value of a lesion (i.e., the difference between unenhanced and enhanced images) by simple insertion of a region of interest (ROI). This quantitative net enhancement value is called the iodine overlay (measured in HU). nother, more or less similar, value also obtained is the iodine content (measured in milligrams per milliliter) (Figs. 4 and 5). This method eliminates the need for an unenhanced scan for subtraction, so the radiation dose is markedly reduced. Errors in insertion of ROIs between unenhanced and enhanced scans are avoided because in dual-energy scanning, both values are obtained by insertion of only one ROI [26, 27]. onclusion lthough the literature is still accumulating, early results of ET of the head and neck are promising. The main advantage of ET is that several additional datasets are obtained without a radiation dose penalty. Improved image quality, better lesion detection, and quantitative calculation of the degree of enhancement are immediate well-recognized benefits. Further research is required to make the most of such new technology. References 1. Yeh M, Shepherd J, Wang ZJ, et al. ual-energy and low-kvp T in the abdomen. JR 2009; 193:47 54 2. Kang MJ, Park M, Lee H, et al. ual-energy T: clinical applications in various pulmonary diseases. RadioGraphics 2010; 30:685 698 3. Thieme SF, Johnson TR, Reiser MF, Nikolaou K. S38 JR:199, November 2012
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