Contrast-Enhanced TwinBeam Dual Energy Scanning: Visualization and Quantification
Principle of the TBDE technology, clinical cases in contrast-enhanced examination using TBDE and comparison with other DECT approach.
White Paper Contrast-Enhanced TwinBeam Dual Energy Scanning: Visualization and Quantification Ahmed F. Halaweish, PhD; Andrew N. Primak, PhD Computed Tomography, Siemens Healthineers SIEMENS Healthineers siemens.com/healthineers White Paper | Contrast-Enhanced TwinBeam Dual Energy Scanning: Visualization & Quantification Contrast-Enhanced TwinBeam Dual Energy Scanning: Visualization and Quantification Dual Energy CT (DECT) is an imaging modality based on a different technical approach was introduced in 2005 on the acquisition of two CT datasets corresponding to two the first generation Dual Source CT (DSCT). With DSCT, different X-ray spectra (i.e., spectra with different effective the two tubes can operate simultaneously at different kVp energies). Since CT attenuation for a given material is a values (Figure 1a) resulting in a DECT approach known as function of both material density (electron density ρ) and Dual Source Dual Energy (DSDE). Siemens Healthineers material composition (effective atomic number Zeff) as has pioneered and continuously developed DSDE, well as a function of the effective energy, two materials equipping four scanners with this capability (SOMATOM of different composition can have a different relative Definition, SOMATOM Definition Flash, SOMATOM Drive, change in their attenuation between the two spectra, and SOMATOM Force). Since the introduction of the even if their CT values with one spectrum are the same. SOMATOM Definition Flash, the DSDE solution achieves This difference is exploited by DECT to characterize superior spectral separation by applying an additional tin materials of different composition, provided there is a (Sn) filtration to the tube operated at high kVp. The Tin sufficient separation between the two spectra1 Therefore, Filter predominantly removes low energy photons from . DECT is capable of providing the information about the high kVp spectrum, shifting the mean energy higher tissue composition in addition to the information about and therefore improving the spectral separation to the low tissue density (morphology) provided by conventional kVp spectrum significantly. However, DECT is not limited single-energy CT (SECT). to the Dual Source configuration and can also be imple- DECT was first introduced by Siemens Healthineers in mented utilizing a single X-ray source. Several single source the mid-1980s on the SOMATOM® DR3 scanner, which Dual Energy (SSDE) solutions, from different vendors, was capable of rapid kVp switching. However, since this exist in the market1 The initial Siemens Healthineers SSDE . technology had several serious limitations, it was not implementation is based on two successive, automatically accepted for clinical routine and DECT was forgotten until coupled spiral scans, performed at low (80 kV) and high (130 kV or 140 kV) energies. 2 Contrast-Enhanced TwinBeam Dual Energy Scanning: Visualization & Quantification | White Paper 1a 1b O 140 kV 80 kV Dual Source Dual Energy kVp switching single source Dual Energy 1c 1d Low density scintillator Low energy raw data Photo diode High density scintillator High energy raw data Different Dual Photo diode O 140 kV Integrating ASIC 80 kV Energy solutions Dual-layer detector-based Slow kVp switching single source Dual Energy single source Dual Energy The application of a non-rigid registration between the two This technology has recently become available on the image datasets compensates for potential patient motion SOMATOM Definition AS+, SOMATOM Definition Edge, between the two scans. This offering is currently available SOMATOM Edge Plus5 and SOMATOM go.Top5 scanners. on the SOMATOM Definition Edge, SOMATOM Definition AS, The goal of this white paper is to help readers understand SOMATOM Perspective, and SOMATOM Scope Power the TBDE technology by providing information about scanners. Commercially available Dual Energy solutions useful technical details and relevant clinical applications. from other vendors include: 1) a fast kVp switching SSDE solution, with kVp switching from 80 to 140 kVp between Principles of the TBDE technology two consecutive projections (Figure 1b); 2) a dual-layer detector-based SSDE solution, with the top layer detecting The main concept, upon which TBDE was built, is X-ray the low-energy X-ray photons, and the bottom layer beam splitting—such that a single X-ray tube can simulta- detecting the high-energy photons that have penetrated neously produce two different spectra with low- and undetected through the top layer (Figure 1c); and 3) a slow high-effective energy. The beam-splitting concept was first kVp switching SSDE solution, where the tube potential introduced in 1980 for splitting a 2D fan-shape X-ray alternates between high and low kVp with each gantry beam into two “half-fan” beams with different spectra². rotation in either a single low-pitch spiral acquisition or However, in modern multi-slice CT scanners with wide (in in axial scanning (Figure 1d). the z-direction) detectors, the X-ray beam has a 3D cone shape. Therefore, for the TBDE solution, the beam splitting In continuous efforts to advance the software and hard- concept is applied in the z-direction instead of the axial ware capabilities of our CT scanners, the latest develop- plane, with a movable “split-filter” being used to split a ments at Siemens Healthineers have introduced another single-kVp 3D cone beam into two “half-cone” beams with SSDE approach known as TwinBeam Dual Energy (TBDE). different spectra (Figure 2). 3 White Paper | Contrast-Enhanced TwinBeam Dual Energy Scanning: Visualization & Quantification Spectrum before filter Spectrum after filter Low Energy -120KV 120KM AU120KM Moveable split-filter intensity intensity 0 20 40 00 80 100 120 140 20 40 60 8O 1.00 enery bey energy [kev] 120 140 o High Energy -120KV Sn120kV intensity 0 20 40 60 80 energy [kevi 100 120 140 2 TwinBeam single source Dual Energy To achieve an adequate spectral separation between the acceptable image noise for a reasonable range of patient two beams, the materials used in the split-filter should sizes; and (2) avoiding excessive attenuation that would satisfy two different requirements: (1) on one half of the prohibit the use of TBDE for larger patient sizes. Finally, filter strong beam hardening characteristics to shift the overall performance of the split-filter was optimized the effective energy of the single-kVp spectrum to higher for a single kVp value, producing the best results at keV values, thus generating a high-energy spectrum; 120 kVp using 0.6 mm of Sn and 0.05 mm of Au for the and (2) on the other half a weak beam hardening and high- and low-energy spectra, respectively. high attenuation at high keV characteristics to shift the effective energy of the single-kVp spectrum to lower Spiral scanning is mandatory for TBDE, ensuring full values, generating a low-energy spectrum. Gold (Au) coverage of the scan volume with both beams and possesses a K-edge at 81 keV, thus providing attenuation sufficient overlap of the acquired data for optimal of higher keV photons while maintaining the lower kV performance. Hence, the pitch values are limited to the spectra. The gold filter is also needed to balance the dose 0.25–0.45 range. At first glance, these pitch values might between the strongly filtered high-energy spectrum and appear small, but this translates to a table speed up the low energy spectrum. Combining this knowledge to ~60 mm/s, which is feasible thanks to the fast gantry with our prior use of Tin Filters (Sn) to improve spectral rotation6 For example, the scan time for the TBDE default . separation for DSDE gives us the two desired materials lung protocol is ~9 s, which is acceptable for the vast to generate both high- and low-energy spectra from a majority of patients. single-kVp beam. The thickness of the two materials was One more important detail related to the TBDE design is optimized to facilitate the two conditions: (1) generation that the split-filter absorbs about 2/3 of the photon flux of diagnostic-quality high- and low-energy spectra, with (i.e., scanner dose output) before it reaches the patient. 4 Contrast-Enhanced TwinBeam Dual Energy Scanning: Visualization & Quantification | White Paper Routine dose-neutral DE scanning with TBDE ADMIRE (iterative reconstruction - IR) • can be used in conjunction with TBDE datasets to reduce noise. CARE Dose4D™ (automatic exposure • control) is available for TBDE scanning. iMAR (iterative metal artifact reduction) • can be used in conjunction with TBDE This does not affect the dose to the patient because it can to reduce/eliminate streaking artifacts be maintained at the SECT level by increasing the Quality caused by metal structures within the Ref. mAs but limits the maximum dose (CTDIvol) that can imaged field of view. be delivered to a patient in the TBDE mode. However, a powerful 100 kW X-ray tube combined with a low pitch value, still allows the achievement of CTDIvol values up to ~22.5 mGy that are sufficient for TBDE imaging of larger patients. To generate dose-efficient images that serve as a substitute for conventional SECT images, all the projection data acquired in the TBDE mode is included in the reconstruction process by applying optimal weighting functions to the projections corresponding to the high- and low-energy radiation dose, TBDE had less image noise vs. 120 kVp SECT beams. This dedicated reconstruction algorithm generates up to the phantom size of 40 cm. One of the first clinical a so-called “composed” image that utilizes the full radiation studies³ reported no statistically significant difference in dose of the TBDE scan and provides non-DE-specific infor- image noise and liver CNR between TBDE and SECT in mation on tissue density similar to SECT. The DE-specific spite of 17% lower average size-specific dose estimate for information is contained in the high- (H) and low-energy (L) TBDE vs. SECT (9.7 vs. 11.7 mGy). The only image quality images reconstructed from the projection data acquired parameter that was better for SECT was vascular CNR. by the detector rows irradiated by the corresponding high- This happened because the SECT imaging was done and low-energy beams. Similar to the other Siemens with an automatic kVp selection (CARE kV), and 56% Healthineers DECT solutions, DE postprocessing of the of patients were scanned with 100 kVp resulting in a TBDE data is done in the image domain. higher average iodine signal as expected with low kVp The TBDE approach is fully capable of producing the acquisitions. same image quality at equal radiation dose compared to Now that we have covered the basic principles of the conventional SECT. A major factor responsible for TBDE TBDE technology, let us evaluate its application in dose neutrality is the use of 120 kVp for the primary (pre- contrast-enhanced examinations. Between the two splitting) beam. For a fair comparison, the image quality SSDE solutions (Dual Spiral DE and TBDE) available between TBDE and SECT should be compared using the in the Siemens Healthineers portfolio, TBDE is definitely composed (C) image that utilizes the full radiation dose of a preferred choice for contrast-enhanced exams due to a the TBDE scan. A phantom evaluation of the image quality much smaller temporal offset between the acquisition achievable with TBDE demonstrated that, at equivalent of the high- and low-energy datasets. 5 White Paper | Contrast-Enhanced TwinBeam Dual Energy Scanning: Visualization & Quantification 3a O HUI Monoenergetic Plus @ 50 keV VNC + iodine overlay Virtual noncontrast (VNC) A Courtesy of Universitätsspital Basel, Basel, Switzerland Contrast-enhanced examinations using TBDE Figure 3a Body Imaging Collimation 64 x 0.6 Dose-neutral routine body imaging can be easily performed using TBDE on the SOMATOM Definition AS+, Effective mAs 355 SOMATOM Definition Edge, SOMATOM Edge Plus5 and Tube potential 120 kV + AuSn SOMATOM go.Top5 scanners. Doing these exams in the Pitch 0.3 TBDE mode can help improve visualization of existing image content (e.g., with Monoenergetic Plus images), Rotation time 0.33s create new image content (e.g., with virtual noncontrast7 Scan length 650 mm or bone-removed images), or provide an opportunity for radiation dose reduction (e.g., by eliminating the Scan time 18 s non-contrast phase in multiphase exams). In the two CTDIvol 7.6 mGy abdominal8 examples illustrated in this white paper (Figure 3a and 3b), contrast-enhanced body scans (60- DLP 508 mGy cm 65 cm scan range) were performed in 18 and 20 seconds, respectively. These scan times are within a normal breath-hold for an average patient. The radiation dose with CTDIvol of 7.6 and 10.1 mGy, respectively, was on the lower side of the dose range for state-of-the-art SECT body imaging. The composed (C) images, utilizing the full radiation dose, provided a high-quality 120 kV equivalent dataset for routine reading purposes, while the high (H) and low (L) energy images, in combination with DE postprocessing, were used to generate virtual noncontrast and iodine-only datasets for visualization 6 Contrast-Enhanced TwinBeam Dual Energy Scanning: Visualization & Quantification | White Paper 3b Figure 3b Monoenergetic Plus @ 50 keV Virtual noncontrast (VNC) VNC + iodine overlay Collimation 64 x 0.6 Effective mAs 473 Tube potential 120 kV + AuSn Pitch 0.25 Rotation time 0.33s Scan length 602 mm Scan time 20 s CTDIvol 10.1 mGy 628 mGy cm Courtesy of Universitätsspital Basel, Basel, Switzerland DLP and quantification of the contrast enhancement9 Head and neck CTA imaging (carotids/bone removal) . Another DE-specific dataset, known as Monoenergetic Plus, facilitated improved iodine contrast-to-noise ratio Head and neck CTA is conceptually similar to body CTA (CNR) throughout the entire enhanced region of interest. but requires a different acquisition protocol due to the A combination of these advanced imaging datasets nature of the anatomy being imaged. For example, was useful in distinguishing between benign cysts and penetrating dense bone of the skull can be problematic liver metastases. The first clinical study of TBDE body in certain acquisition modes and can limit the diagnostic imaging reported a very good agreement between quality of the generated data. With TBDE, head and neck VNC and true noncontrast images, suggesting that the CTAs can be performed routinely, as demonstrated in the true noncontrast phase might be omitted from various example below, once again providing additional diagnosti- multiphase abdominal exams3 cally useful information from a Dual Energy scan, such as . VNC, iodine-only, Monoenergetic Plus and bone-removed Body examinations are not just limited to abdominal datasets. Automated bone removal with no or little user contrast-enhanced acquisitions. The combination of the interaction is one of the advantages of DECT in head and split-filter technology, the fast table speeds, and the high- neck exams because it facilitates stenosis grading, speeds power X-ray tube give way to whole-body CTA/runoff up image analysis, and provides 3D rendered vascular examinations with relative ease. The composed (C) images, models for the referring physician. The first clinical study of utilizing the full radiation dose, can be used for routine head and neck CTA in 50 patients confirmed that automated reading purposes (120 kV equivalent), while the high (H) bone removal with TBDE is feasible with no or moderate and low (L) energy datasets, in combination with dedi- user interaction4 Note that CTDIvol for the TBDE protocol . cated DE postprocessing algorithms, can be used to was 25% smaller compared to the routine SE CTA protocol generate diagnostic task-specific datasets to enhance (6.2 vs. 8.3 mGy). the visualization and quantification of the contrast inflow and runoff through the vasculature. Bone removal, in combination with maximum intensity projections, can facilitate a good overview of the contrast enhancement, unobstructed by the bone, projected into one plane. 7 White Paper | Contrast-Enhanced TwinBeam Dual Energy Scanning: Visualization & Quantification Advantages of TBDE vs. other single source Dual Energy approaches Better radiation dose efficiency by using • CARE Dose4D™ (AEC) and dose neutrality compared to SECT Same spatial resolution compared to SECT • No trade-off between choosing the best spectral • separation and the best dose efficiency Wide range of clinically relevant applications • Can be utilized for both contrast-enhanced • and noncontrast studies Streamlined acquisition and postprocessing • (PACS ready) Comparison against other DECT approaches the fast kV switching cannot operate at CTDIvol values To date, the best DECT solution has been the Dual Source below ~6 mGy, making it impossible to achieve dose approach combined with Tin Filter and the dedicated neutrality with SECT for smaller patients. This is not set of kVp pairs (up to four with the third generation of a problem for TBDE. For example, the default TBDE_ DSDE). The full use of tube power for both spectra in Thorax_Analysis_IR protocol on a SOMATOM Definition combination with improved spectral separation results Edge scanner has CTDIvol of 4.5 mGy. Finally, with fast in best DECT performance needed for more advanced kV switching, only half-projection views per rotation are functionality and a widespread adoption of DECT into used to reconstruct the 80 kV and 140 kV images. This routine clinical use. limits the achievable image quality in terms of spatial In comparison with the fast kV switching SSDE solution, resolution compared to conventional SECT images that TBDE offers better radiation dose efficiency at compa- use the full number of projection views per rotation. On rable spectral separation. The fast kV switching is oper- the contrary, TBDE uses the full number of projection ated at constant mA settings and cannot be combined views per rotation to reconstruct the high- and low- with automatic exposure control (AEC), a standard dose energy images resulting in the same spatial resolution reduction feature on modern CT scanners. The lack of compared to SECT. AEC is a clear disadvantage compared to TBDE that offers In comparison to the dual-layer detector SSDE solution, a full 3D dose modulation and saves patient’s radiation TBDE also results in comparable DECT performance dose by adjusting the tube current both along the z-axis for the recommended TBDE applications. However, a (e.g., going from the lungs to the liver) and in the axial scanner equipped with the TBDE technology can offer plane (e.g., going from the lateral to AP views in the more clinically relevant features. This technology has shoulders). In addition, to the best of our knowledge, recently become available on the SOMATOM Definition AS+, SOMATOM Definition Edge, SOMATOM Edge Plus* *This feature is pending 510(k) clearance, and is not yet commercially available in the United States. 8 Contrast-Enhanced TwinBeam Dual Energy Scanning: Visualization & Quantification | White Paper 4a 4b Low energy Au High energy Sn Bone-removed MIP Composed MIP Fig. 4a and 4b: Courtesy of Universitätsspital Basel, Basel, Switzerland and SOMATOM go.Top5 scanners but it is not a desirable Conclusion mode of operation on a dual-layer detector scanner because it is designed and optimized for the “spectral” Combining the TBDE acquisition mode with DE post- processing facilitates generation of additional DE-specific mode operated either at 140 or 120 kVp. Due to its limited features and a relatively small (70 cm) gantry size, this information and can provide incremental diagnostic scanner might be more suitable as a pure research value to conventional SECT imaging. Routine contrast- scanner rather than a scanner for routine clinical imaging enhanced abdominal, whole-body runoff, and head and that is also capable of DECT imaging. Regarding DECT neck TBDE imaging can be easily performed, in a dose performance of the dual-layer detector SSDE solution, neutral fashion, while providing virtual noncontrast, the best spectral separation is achieved when it is iodine-only, bone-removed and Monoenergetic Plus operated at 140 kVp. Still, even at 140 kVp, the energy datasets for task-specific reading purposes and enhance- overlap between the high- and low-energy spectra is ment of available diagnostic information. TBDE imaging larger than the overlap between 80 and 140 kVp spectra. is not limited to the contrast-enhanced exams presented This is due to the high-energy tail of the low-energy in this white paper but can also be applied to character- spectrum caused by a portion of the high-energy photons ize kidney stones, visualize iodine uptake of the lung absorbed by the top (low-energy) detector layer. parenchyma in case of suspected pulmonary embolism, Therefore, even the best spectral separation of dual-layer or highlight urate tophi for gout patients. This flexibility detector DE is inferior to dual-spiral DE and comparable brings the TBDE technology to a more routine setting, to TBDE. However, the dose efficiency at 140 kVp is known with simplified acquisition and postprocessing capabili- ties when combined with the FAST DE Results and/or to be worse than at 120 kVp. As a consequence, users have to choose between the best spectral separation at syngo.via Rapid Results technology. 140 kVp and the best dose efficiency at 120 kVp. 9 White Paper | Contrast-Enhanced TwinBeam Dual Energy Scanning: Visualization & Quantification 5 Low energy Au High energy Sn Composed Bone-removed Courtesy of University Hospital Erlangen, Erlangen, Germany Figures 5-7 63-year-old female Collimation 64 x 0.6 Effective mAs 301 Tube potential 120 kV + AuSn Rotation time 0.33s CTDIvol 6.4 mGy DLP 100.8 mGy cm 10 Courtesy of University Hospital Erlangen, Erlangen, Germany Courtesy of University Hospital Erlangen, Erlangen, Germany 6 7 Low energy Au Low Energy Au Plaques on Contrast-Enhanced TwinBeam Dual Energy Scanning: Visualization & Quantification | White Paper Plaques off High energy Sn High Energy Sn Bone-removed 3D rendering Composed Composed 11 On account of certain regional limitations of sales rights References and service availability, we cannot guarantee that all 1 McCollough CH, Leng S, Yu L, Fletcher JG. Dual- and multi-energy products included in this brochure are available through CT: Principles, technical approaches, and clinical applications. the Siemens Healthineers sales organization worldwide. Radiology, 2015;276(3):637-53. Availability and packaging may vary by country and 2 are subject to change without prior notice. Some/All of Rutt B, Fenster A. Split-filter computed tomography: a simple technique for dual-energy scanning. J Comput Assist Tomogr. the features and products described herein may not be 1980 Aug;4(4):501-9. available in the United States. 3 Euler A, Parakh A, Falkowski AL, Manneck S, Dashti D, Krauss B, Szucs-Farkas Z, Schindera ST. Initial Results of a Single-Source The information in this document contains general Dual-Energy Computed Tomography Technique Using a Split-Filter: technical descriptions of specifications and options as Assessment of Image Quality, Radiation Dose, and Accuracy of well as standard and optional features which do not Dual-Energy Applications in an In Vitro and In Vivo Study. Invest always have to be present in individual cases. Radiol. 2016 Aug;51(8):491-8. 4 Kaemmerer N, Brand M, Hammon M, May M, Wuest W, Krauss B, Siemens Healthineers reserves the right to modify the Uder M, Lell MM. Dual-Energy Computed Tomography Angiography design, packaging, specifications, and options described of the Head and Neck With Single-Source Computed Tomography: herein without prior notice. Please contact your local A New Technical (Split Filter) Approach for Bone Removal. Invest Siemens Healthineers sales representative for the most Radiol. 2016 Oct;51(10):618-23. current information. 5 This product is pending 510(k) clearance, and is not yet commercially available in the United States. Note: Any technical data contained in this document 6 Usually the fastest rotation time should be used for TBDE; may vary within defined tolerances. Original images but for heavy patients 0.5 second is preferred. always lose a certain amount of detail when reproduced. 7 Virtual noncontrast (VNC) images are currently not meant as a direct replacement for true noncontrast images. They are meant Please find fitting accessories: to provide the additional noncontrast information in protocols siemens.com/medical-accessories that do not call for a true noncontrast acquisition. 8 For the best abdominal imaging results, patients should be scanned feet first. 9 Optimized contrast enhancement results are achieved through the use of Monoenergetic Plus postprocessed datasets. International version. Not for distribution or use in the U.S. Siemens Healthineers Headquarters Siemens Healthcare GmbH Henkestr. 127 91052 Erlangen, Germany Phone: +49 9131 84-0 siemens.com/healthineers Published by Siemens Healthcare GmbH · 5459 1117 online. · ©Siemens Healthcare GmbH, 2017
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