The previous blog post briefly mentioned dual energy computed tomography (DECT) as a recommended further examination in the investigation of renal trauma (Kozar et al., 2018). This post aims to explore the concept and the benefits of it further. DECT, or ‘spectral imaging’ uses different photon spectra energies to acquire two CT datasets simultaneously (Johnson, 2012). Usually, the higher energy used is 140 kVp and the lower is 80 kVp. The acquisition of more than one energy allows the differentiation of the chemical composition, producing additional physiological information compared to conventional (single energy) CT. This is useful for studies such as lung ventilation and perfusion (Petersilka et al.,2008). Several techniques have been developed to produce these images, including dual source, fast-switching, and the dual layer systems, as shown by Figure 1 (Gupta, 2015).

Figure 1

 

Diagram A shows dual source imaging uses two x-ray tubes and two detectors perpendicular to each other. Petersilka et al., (2008) and Donnino et al. (2009) mention CT scanners as having 62 and 64 slices respectively. In 2015, only 26% of CT equipment in the UK had below 64-slice capacity, with some having up to 320 slices which demonstrates that the sources are becoming dated but are still relevant (Royal College of Radiologists, 2015). Dual kilo-voltage, or ‘fast-switching’ DECT involves the acquisition of two images at different energies using one x-ray tube which rapidly switches between the higher and lower kilo-voltages.

The simultaneous acquisition of perpendicular data effectively cancels out motion artifact, making it a valuable tool in cardiac imaging (Silva et al., 2011). Donnino et al. (2009) demonstrated that even for patients with fast heart rates (over 65 bpm) without beta-blocker use, dual source CT scans had greater diagnostic quality than single source CT. Considering beta-blockers have several contraindications, including asthma and chronic obstructive pulmonary disease, acquisition of high-quality cardiac images without their use is valuable (Koplay et al., 2016).

In an article for the Radiological Society of North America, Silva et al. (2011) point out that conventional CT uses a polychromatic beam which is susceptible to beam hardening which can produce image artefact. DECT simulates a monochromatic x-ray beam, eliminating lower energies, therefore producing a more accurate anatomical image by cancelling out the effects of beam hardening. The reduction of the effects of metal artifacts by DECT is demonstrated in Figure 2.

Figure 2

The key benefit of DECT is that the linear attenuation coefficients gained from imaging at different energies allow the analysis of materials (Alkadhi and Leschka, 2013). This is particularly useful for areas of the body, such as the abdomen, which contain materials with similar attenuation values and, therefore, very similar Hounsfield units. Structures may appear to be the same shade of grey despite their tissues being made up of different elements. Colour coding can be added to further enhance attenuation differences. An interesting example of this is Figure C, which demonstrates dual energy CT’s ability to differentiate between heroin and cocaine compared to conventional CT. Leschka et al. (2013) produced a phantom containing packs of cocaine and heroin submerged in water to recreate drugs in the abdomen. An experiment was performed to test the ability of DECT to distinguish between the drugs. The results demonstrate that low-dose DECT scan has the potential to be more valuable than x-ray in determining the treatment needed for a patient who has ingested drugs.

Figure 3

Single-phase contrast-enhanced DECT can provide information about lesion characterisation and vascularity, by the comparison of water material density and iodine material density images (Silva et al. (2011). The ability to distinguish iodine allows its subtraction from contrast-enhanced images to produce virtual plain CT images. By eliminating the need for a plain scan, using DECT could reduce patient dose (Alkadhi and Leschka, 2013).

Patient dose is an important consideration when using DECT. The dose acquired from CT is ultimately dependent on factors such as the current, energy and pitch, but is also equipment dependent (Coursey et al., 2010). Fast-switching DECT requires the number of projections per rotation to be doubled to produce enough data (Alkahdi and Lesckha, 2013), yet, according to Silva et al. (2011), the patient radiation dose is not doubled. A UK scanning centre notes that the Siemens Force Scanner is considered “dose neutral” (Paul Strickland Scanner Centre, 2018). This is supported by Zhu et al.’s conclusion following their experiment (2016) using a Siemens CT scanner at a paediatrics hospital. However, it is important to note that doses within the region of 10% above the dose acquired from single energy CT were included as ‘dose neutral’. This is study is based on the equipment at one hospital alone, therefore, doses may vary between scanners. The threshold for the definition of dose neutrality for the Siemens Force Scanner is unknown. Another article by the RSNA explains that while DECT can produce a dose similar to single-energy CT, this is achieved by using a low energy which compromises image quality (Coursey et al., 2010). Therefore, the findings of these studies cannot be generalised to all examinations.

The worth of DECT as a routine modality is dependent upon how accessible it is to patients. Many dual energy research sources are from the USA, demonstrating that overseas healthcare systems are more advanced in this area.  The Paul Strickland Scanner Centre (2018) explains that there are currently very few centres in the UK using dual-energy CT as a routine modality. While DECT is clearly a modality that could have many valuable applications in clinical practice, it is currently limited as a resource in the UK.

In conclusion, dual energy CT could become a valuable modality in several patient pathways. It can provide more accurate anatomical information and material differentiation than single-source CT, as well as eliminate the effects of motion artefact and beam hardening. While it is important to consider the potentially higher doses, in some cases DECT could arguably result in a lower overall patient dose. All things considered, the benefits clearly outweigh the risks. Looking towards the future, medical physicists have already begun exploring multi-energy computed tomography (MECT) (Li et al., 2016).

 

References:

Alkadhi, H., and Leschka, S. (2013) Dual Energy CT: Principles, Clinical Value and potential applications in forensic imaging. Journal of Forensic Radiology and Imaging [online]. pp.180-185. Available from: https://doi.org/10.1016/j.jofri.2013.07.003 [Accessed 31 October 2018].

Coursey, C., Nelson, R., Boll, D., Paulson, E., Ho, L., Neville, A., Marin, D., Gupta, R., and Schinedra, S. (2016) Dual-Energy Multidetector CT: How Does It Work, What Can It Tell Us, and When Can We Use It in Abdominopelvic Imaging? RadioGraphics [online]. Available from: https://pubs.rsna.org/doi/pdf/10.1148/rg.304095175 [Accessed 5 November 2018].

Donnino, R., Jacobs, J., Doshi, J., Hecht, E., Kim, D., Babb, J., and Srichai, M. (2009) Dual-Source Versus Single Source Cardiac CT Angiography: Comparison of Diagnostic Image Quality. American Journal of Roentgenology [online]. 192 (4), pp. 1051-1056.           Available from: https://www.ajronline.org/doi/pdf/10.2214/AJR.08.1198 [Accessed 5 November 2018].

Johnson, T. (2012) Dual Energy CT: General Principles. American Journal of Roentgenology [online]. 199 (5), pp. S3-S8. Available from: https://www.ajronline.org/doi/full/10.2214/AJR.12.9116 [accessed 2 November 2018].

Kozar, R., Crandall, M., Shanmuganathan, K., Zarzaur, B., Coburn, M., Cribari, C., Kaup, K., Schuster, K., and Tominaga, G. (2018) Organ Injury Scaling 2018 Update: Spleen, Liver, and Kidney. Journal of Trauma and Acute Care Surgery [online]. Available from: https://www1.uwe.ac.uk/library [Accessed 23 October 2018].

Lam, S., Gupta, R., Kelly, Curtin, H., Forghani, R. (2015) Multiparametric Evaluation of Head and Neck Squamous Cell Carcinoma Using a Single-Source Dual-Energy CT with Fast kVp Switching: State of the Art. Cancers [online]. 7(4), pp. 2201-2216. Available from: https://www.researchgate.net/publication/283683844_Multiparametric_Evaluation_of_Head_and_Neck_Squamous_Cell_Carcinoma_Using_a_Single-Source_Dual-Energy_CT_with_Fast_kVp_Switching_State_of_the_Art [Accessed 9 November 2018].

Leschka, S., Fornaro, J., Larberke, P. Blum, S., Hatem, A., Niederer, I., Miele, C., Hibbeln, D., Hausmann, R., Wildermuth, S. and Eisenhart, D. (2013) Differentiation of cocaine from heroine body packs by computed tomography: Impact of different tube voltages and the dual-energy index. Journal of Forensic Radiology and Imaging [online]. 1 (2), pp.46-50. Available from: https://doi.org/10.1016/j.jofri.2013.03.041 [Accessed 7 October 2018].

Li, B., Shen, C., Ouyang, L., Yang, M., Zhou, L., Jiang, S. and Jia, X. (2016) Multi‐Energy CT Reconstruction with Spatial Spectral Nonlocal Means Regularization. The International Journal of Medical Physics research and Practice [online]. Available from: https://doi.org/10.1118/1.4957948 [Accessed 2 November 2018].

Paul Strickland Scanner Centre (2018) Dual Energy CT scanning. Available from: https://www.stricklandscanner.org.uk/for-health-professionals/ct-scans [Accessed 8 November 2018].

Petersilka, M., Bruder, H., Krauss, B., Stierstorfer, K., and Flohr, T. (2008) Technical principles of dual source CT. European Journal of Radiology [online]. 68 (3), pp. 362-368. Available from: https://doi.org/10.1016/j.ejrad.2008.08.013 [Accessed 2 November 2018].

Silva, A., Morse, B., Hara, A., Paden, R., Hongo, N. and Pavlicek, W. (2011) Dual-Energy (Spectral) CT: Applications in Abdominal Imaging. RadioGraphics [online]. 31 (4), pp. Available from: https://doi.org/10.1148/rg.314105159 [Accessed 7 November 2018].

The Clinical Imaging Board (2015) CT Equipment, Operations, Capacity and Planning in the NHS. The Royal College of Radiologists, the Society and College of Radiographers, and the Institute of Physics and Engineering in Medicine. Available from: https://www.rcr.ac.uk/sites/default/files/ct_equipment_in_the_nhs_report_cib_may_2015_v2_final240615.pdf [Accessed 5 November 2018].

Zhu, X., McCullough, W., Mecca, P., Servaes, S. and Darge, K. (2016) Dual-energy compared to single-energy CT in pediatric imaging: a phantom study for DECT clinical guidance. Pediatric Radiology [online]. 46 (12), pp. 1671-1679. Available from: https://link.springer.com/content/pdf/10.1007%2Fs00247-016-3668-x.pdf [Accessed 9 November 2018].