Article Series: Dual Energy CT – Scientific Evidence and Clinical Application (1/7) – Practical Technical Aspects

This article is part of the seven-article series on “Dual Energy CT – Scientific Evidence and Clinical Application” and covers the practical technical aspects of Dual Energy.

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Scan protocols

The concept of Dual Source Dual Energy CT implies separating the spectrum into a high energy and a low energy part and to emit these two parts from different x-ray tubes [1]. This means that the dose should be the same as in a corresponding single energy exam.

Regarding normal CT protocols which require a certain contrast to noise ratio (CNR) as reference, there are two studies which have shown that DECT does work without additional dose and can provide equivalent CNR for a certain dose or even improve CNR with intelligent postprocessing [2-3]. With the first generation Dual Source CT, dose neutrality and cross scatter correction required compromises in collimation, i.e. to double the collimation in the trunk of the body to achieve the same dose efficiency per detector element as with single energy CT [4-5]. The reason was that in some areas of the body only such a small part of the 80kVp spectrum was transmitted that the noise properties of the detector became relevant. With the second generation Dual Source CT, an additional filter of 0.1 mm tin has been integrated which improves the spectral separation. Now, it is possible to work with a filtered 140kVp and a 100kVp spectrum so that sufficient transmission is achieved even in large patients and with 0.6mm collimation [6-7].

Another aspect that limited the routine clinical application in the first generation DSCT scanner was the field of view of the smaller detector which was limited to about 27cm diameter and thus unable to cover the whole body of most patients. This implied some limitations in evaluating lesions of the liver, both kidneys, or peripheral parts of the lung [8-11]. With the second generation system, the smaller detector provides 33cm diameter which is sufficient to cover the diameter of thorax and abdomen in most patients. If some subcutaneous adipose tissue is not covered with Dual Energy information, this is usually not of clinical relevance.

Postprocessing

Right from the start it was clear that Dual Energy CT would primarily need to provide normal CT images in order to allow routine clinical application. Therefore, the manufacturer implemented the generation of weighted average images from both Dual Energy datasets which utilize the full dose and provide optimal contrast to noise and spectral properties similar normal CT images obtained at 120kVp [12]. Depending on the tube voltages and filters used for primary acquisition, this partially requires the application of a weighting factor. This factor can be altered, e.g. if iodine contrast is the main diagnostic feature of the CT protocol like in CT angiography [13-14].

Additionally, advanced algorithms can be applied to generate optimized images which intelligently integrate the advantages of both the high and the low energy acquisition in one image set. With a sigmoidal blending algorithm, both the high contrast of the low energy acquisition and the low noise of the high energy acquisition can be combined in one dataset [15-16].
With these techniques, the contrast to noise ratio can be significantly improved, which may even make it possible to reduce the dose in certain instances [16]. Also, the two acquired datasets can be used to either eliminate iodine-related density or to extrapolate to a dataset as if it had been acquired at much higher energy, e.g. 511keV. These datasets may be used for an improved attenuation correction in PET-CT, rendering additional unenhanced low-dose CT or transmission scintigraphy dispensable [17-18].

References

1.    Johnson TRC, Kalender WA. Physical Background. In: Johnson TRC, Fink C, Schönberg SO, Reiser MF, eds. Dual Energy CT in Clinical Practice. Heidelberg: Springer, 2010:3-10.
2.    Schenzle JC, Sommer WH, Neumaier K, Michalski G, Lechel U, Nikolaou K, Becker CR, Reiser MF, Johnson TR. Dual energy CT of the chest: how about the dose? Invest Radiol 2010;45:347-353.
3.    Thomas C, Ketelsen D, Tsiflikas I, Reimann A, Brodoefel H, Claussen CD, Heuschmid M. Dual-energy computed tomography: is there a penalty in image quality and radiation dose compared with single-energy computed tomography? J Comput Assist Tomogr 2010;34:309-315.
4.    Guimaraes LS, Fletcher JG, Harmsen WS, Yu L, Siddiki H, Melton Z, Huprich JE, Hough D, Hartman R, McCollough CH. Appropriate Patient Selection at Abdominal Dual-Energy CT Using 80 kV: Relationship between Patient Size, Image Noise, and Image Quality. Radiology 2010
5. Krauss B, Schmidt B, Flohr TG. Dual Source CT. In: Johnson TRC, Fink C, Schönberg SO, Reiser MF, eds. Dual Energy CT in Clinical Practice. Heidelberg: Springer, 2010:11-20.
6.    Primak AN, Giraldo JC, Eusemann CD, Schmidt B, Kantor B, Fletcher JG, McCollough CH. Dual-Source Dual-Energy CT With Additional Tin Filtration: Dose and Image Quality Evaluation in Phantoms and In Vivo. AJR Am J Roentgenol 2010;195:1164-1174.
7.    Primak AN, Ramirez Giraldo JC, Liu X, Yu L, McCollough CH. Improved dual-energy material discrimination for dual-source CT by means of additional spectral filtration. Med Phys 2009;36:1359-1369.
8.    De Cecco CN, Buffa V, Fedeli S, Vallone A, Ruopoli R, Luzietti M, Miele V, Rengo M, Maurizi Enrici M, Fina P, et al. Preliminary experience with abdominal dual-energy CT (DECT): true versus virtual nonenhanced images of the liver. Radiol Med 2010
9.    Graser A, Johnson TR, Hecht EM, Becker CR, Leidecker C, Staehler M, Stief CG, Hildebrandt H, Godoy MC, Finn ME, et al. Dual-energy CT in patients suspected of having renal masses: can virtual nonenhanced images replace true nonenhanced images? Radiology 2009;252:433-440.
10.    Karcaaltincaba M, Karaosmanoglu D, Akata D, Senturk S, Ozmen M, Alibek S. Dual energy virtual CT colonoscopy with dual source computed tomography: initial experience. Rofo 2009;181:859-862.
11.    Thieme SF, Becker CR, Hacker M, Nikolaou K, Reiser MF, Johnson TR. Dual energy CT for the assessment of lung perfusion–correlation to scintigraphy. Eur J Radiol 2008;68:369-374.
12. McCollough CH, Schmidt B, Liu X, Yu L, Leng S. Dual-Energy Algorithms and Postprocessing Techniques. In: Johnson TRC, Fink C, Schönberg SO, Reiser MF, eds. Dual Energy CT in Clinical Practice. Heidelberg: Springer, 2010:43-54.
13.    Yu L, Primak AN, Liu X, McCollough CH. Image quality optimization and evaluation of linearly mixed images in dual-source, dual-energy CT. Med Phys 2009;36:1019-1024.
14.    Behrendt FF, Schmidt B, Plumhans C, Keil S, Woodruff SG, Ackermann D, Muhlenbruch G, Flohr T, Gunther RW, Mahnken AH. Image fusion in dual energy computed tomography: effect on contrast enhancement, signal-to-noise ratio and image quality in computed tomography angiography. Invest Radiol 2009;44:1-6.
15.    Holmes DR, 3rd, Fletcher JG, Apel A, Huprich JE, Siddiki H, Hough DM, Schmidt B, Flohr TG, Robb R, McCollough C, et al. Evaluation of non-linear blending in dual-energy computed tomography. Eur J Radiol 2008;68:409-413.
16.    Apel A, Fletcher JG, Fidler JL, Hough DM, Yu L, Guimaraes LS, Bellemann ME, McCollough CH, Holmes DR, 3rd, Eusemann CD. Pilot multi-reader study demonstrating potential for dose reduction in dual energy hepatic CT using non-linear blending of mixed kV image datasets. Eur Radiol 2010
17.    Noh J, Fessler JA, Kinahan PE. Statistical sinogram restoration in dual-energy CT for PET attenuation correction. IEEE Trans Med Imaging 2009;28:1688-1702.
18.    Rehfeld NS, Heismann BJ, Kupferschlager J, Aschoff P, Christ G, Pfannenberg AC, Pichler BJ. Single and dual energy attenuation correction in PET/CT in the presence of iodine based contrast agents. Med Phys 2008;35:1959-1969.