Reduction of Contrast Agent Dose at Low kV Settings - USA

Whitepaper Reduction of contrast agent dose at low kV settings Christian Canstein, MSc Johannes G. Korporaal, PhD siemens-healthineers.us/computed-tomography SIEMENS Healthineers White Paper · Reduction of contrast agent dose at low kV settings mA Scientific background The actual basis for low kV imaging is the mass attenuation coefficient, which is a property that depends on the chemical composition and density of a material and determines the specific absorption of X-ray beams by this material. Figure 2 shows that for lower photon energies, the mass attenuation coefficient of iodine increases, whereas soft tissue is less energy-dependent. This means that with kV low kV imaging, and thus lower average photon energy, the iodine to soft tissue contrast in the CT image (in Hounsfield units, HU) will increase. This can be easily verified by scanning a static phantom with different Introduction iodine concentration samples at different tube voltages Automated tube voltage selection enabled by Siemens (Figure 3). Healthineers CARE kV* can be used for multiple clinical Due to the linear relationship between iodine applications, with one of the most beneficial being concentration and enhancement, Figure 3 shows how contrast-enhanced scans with lower radiation dose but much iodine can be saved if the attenuation is kept with consistent image quality. constant. For example, if an iodine enhancement of 300 This is an established method that is currently being HU is desired, the iodine concentration at 70 kV can be used in clinical routine and that can help to reduce the ~50% lower than the iodine concentration at 120 kV. radiation dose required. Overall, tube potentials between 70 and 100 kVp have been shown as most effective for clinical contrast CT examinations, depending on the specific situation. 40 Iodine With the advent of CT scanners with X-ray tubes powerful 35 Soft tissue enough to produce high mA at low kV (e.g. SOMATOM go. (ICRU-44) Platform, SOMATOM Edge Plus, SOMATOM Drive and 30 SOMATOM Force), the same physical behavior of X-rays and iodine contrast agent can be used in clinical routine 25 to reduce the amount of contrast agent instead of 20 radiation dose. This white paper briefly summarizes the scientific 15 background and explains the principle of saving contrast 10 agent dose instead of radiation dose. 5 0 Mass attenuation coefficient (cm2/g) 40 60 80 100 120 140 160 Photon energy (keV) Figure 2. Mass attenuation coefficients for soft tissue and iodine at different photon energies. At lower energies, the difference in absorption between soft tissue and iodine increases, which results in a higher iodine contrast in the image. The K-edge of iodine leads to the spike visible at 33.2 keV. Data from NIST tables *Not available on the SOMATOM go.Now and SOMATOM go.Up 2 White Paper · Reduction of contrast agent dose at low kV settings 70 kV When comparing the relative iodine enhancement 700 between all different kVs from 70 to 150 kV with each 80 kV other, it is possible to calculate the potential change in 600 100 kV iodine concentration with a decrease in kV while yielding 500 120 kV the same iodine enhancement in HU (Figure 4). 400 [Iodine] 300 reference Change kV new kV mg/mL 200 120 120 0 100 Iodine enhancement (HU) 120 110 -0.10 0 0 3 6 9 12 15 120 100 -0.20 Iodine concentration (mgl/mL) 120 90 -0.30 Figure 3. With static iodine phantoms, the increase in iodine enhancement (in HU) at a lower kV can be verified [data from iodine 120 80 -0.40 samples in 20 cm water phantom]. The arrow indicates how much iodine can potentially be saved in constant enhancement when going from 120 to 70 kV. 120 70 -0.49 - New/Alternative kV 70 kV 80 kV 90 kV 100 kV 110 kV 120 kV 130 kV 140 kV 150 kV 70 kV 1 +0.18 +0.37 +0.58 +0.77 +0.97 +1.14 +1.36 +1.54 80 kV -0.15 1 +0.16 +0.33 +0.50 +0.67 +0.81 +0.99 +1.15 90 kV -0.27 -0.14 1 +0.15 +0.29 +0.44 +0.56 +0.72 +0.85 100 kV -0.37 -0.25 -0.13 1 +0.12 +0.25 +0.36 +0.49 +0.61 110 kV -0.44 -0.33 -0.23 -0.11 1 +0.11 +0.21 +0.33 +0.44 Reference kV 120 kV -0.49 -0.40 -0.30 -0.20 -0.10 1 +0.09 +0.19 +0.29 130 kV -0.53 -0.45 -0.36 -0.26 -0.17 -0.08 1 +0.10 +0.19 140 kV -0.58 -0.50 -0.42 -0.33 -0.25 -0.16 -0.09 1 +0.08 150 kV -0.61 -0.53 -0.46 -0.38 -0.30 -0.22 -0.16 -0.07 1 Figure 4: Table displaying the relative change in iodine concentration necessary when either lowering or increasing the tube voltage for constant HU enhancement from a vial of iodine within a 30 cm of water phantom. (Assuming a 1 mg/mL value of iodine as the baseline for each kV.) Data from phantom experiments with static iodine concentration, patient results may vary depending on patient size. 3 White Paper · Reduction of contrast agent dose at low kV settings 120 kV, 80 kV, 120 mL contrast 40 mL contrast Figure 1: Clinical examples of two patients scanned with a default 120 kV protocol with 120 mL of contrast agent (left) and with a reduced kV protocol (80 kV) with only 40 mL on SOMATOM Force. Courtesy of Radiologie Karlsruhe Karlstrasse and Universitätsklinikum Mannheim, Germany 4 White Paper · Reduction of contrast agent dose at low kV settings The principle of saving contrast agent dose As the iodine contrast increases, noise can be allowed to The main aim of Siemens Healthineers CARE kV* is to increase by the same extent to maintain the CNR. Then maintain image quality when changing the kV, with for each kV level, the respective tube current (mAs) image quality defined in terms of the contrast-to-noise required to reach the desired noise level is determined, ratio (CNR). Since it is directly dependent on enhancement, as well as the corresponding radiation dose (CTDIvol). the CNR increases for lower kVs if radiation and iodine dose Finally, the kV level yielding the lowest radiation dose are fixed (Figure 5). This fact can be utilized to either save that can be achieved by the system will be chosen. radiation dose or iodine dose while keeping the CNR This methodology is schematically illustrated in Figure 6a. constant; however, it is dependent on patient size. Before Saving contrast agent dose explaining the principle of saving contrast agent dose, the If the aim is to reduce the contrast agent dose instead of principle of saving radiation dose will briefly be reviewed. the radiation dose, the same steps need to be performed Saving radiation dose as when saving radiation dose. However, these four steps The starting point for saving radiation dose is the should be taken in the opposite order, as described on assumption that the same injection, protocol (amount, the next page and as illustrated in Figure 6b. concentration and timing) of the reference protocol (R) would be used for all kVs, which inherently leads to an increase in iodine attenuation with decreasing kV. CNR 100 Child Adult 80 60 40 20 0 Voltage / kV 60 80 100 120 Figure 5: CNR increases for lower kV settings at fixed CTDIvol and fixed iodine dose. *Not available on the SOMATOM go.Now and SOMATOM go.Up 5 White Paper · Reduction of contrast agent dose at low kV settings Aim: Constant image quality (CNR) for all tube voltages Contrast-to-noise ratio 70 80 100 120 140 kV a) Save radiation dose (CARE kV) b) Save contrast agent dose Step 1 Starting point: Contrast agent dose is fixed Starting point: Radiation dose is fixed R R Radiation dose Contrast agent dose 70 80 100 120 140 kV 70 80 100 120 140 kV Step 2 How much iodine enhancement do you get? How much image noise do you get? R R Enhancement Image noise 70 80 100 120 140 kV 70 80 100 120 140 kV How much iodine enhancement is needed to Step 3 How much noise is needed to obtain required CNR? obtain required CNR? R R Image noise Enhancement 70 80 100 120 140 kV 70 80 100 120 140 kV Which kV has the lowest radiation dose Which kV has the lowest contrast agent dose Step 4 for the required noise? for the required iodine enhancement? R R Radiation dose Contrast agent dose 70 80 100 120 140 kV 70 80 100 120 140 kV Figure 6: Low kV imaging at constant contrast-to-noise ratio: a) Principle of saving radiation b) Principle of saving contrast agent dose R denotes the reference protocol. 6 White Paper · Reduction of contrast agent dose at low kV settings Vectron Straton MX Sigma Athlon Chronon Step 1 Step 2 When reducing the contrast agent dose (Figure 6b), the For each kV and mAs combination, the resulting image new starting point is the radiation dose (CTDIvol) of the noise can be calculated using the topogram of the reference protocol (R), which should be kept constant for patient. It is important to stress that a specific radiation all kV levels. This can be achieved by adjusting the tube dose will not result in the same image noise at the current (mA). Until now this step had not been possible different tube voltages. Particularly at a lower kV, in clinical routine due to certain technical limitations of the image noise will be higher for a given CTDIvol. CT systems: the X-ray tubes of existing CT systems were Although this effect is small, it reduces the contrast not powerful enough to produce sufficient tube currents agent reduction potential that would be possible if one at low kV settings to maintain the CTDIvol of the reference were to consider only the increased iodine enhancement protocol. Consequently, measures to save iodine dose at a lower kV. were only feasible in pediatric or very thin patients. However, with the Vectron™ tube with vastly increased Step 3 tube power at low kVs (2 x 120 kW, 2 x 1,300 mA at 70–90 kV, as integrated in SOMATOM Force) it is now With the expected image noise calculated in step 2, possible to scan even obese patients at low tube voltages the iodine enhancement that is required to preserve the desired CNR can be calculated for each kV. with sufficiently high mA and thus offer the possibility of contrast agent dose reduction. SOMATOM Drive with the Straton MX Sigma tube provides 2 x 100 kW and 2 x 650 Step 4 mA at 70 kV, and 2 x 750 mA at 80 and 90 kV. SOMATOM If the required iodine enhancement is known, the Edge Plus with the Straton MX Sigma tube provides 100 corresponding contrast agent dose at each kV can be kW and 650 mA at 70 kV, and 750 mA at 80 and 90 kV. calculated. Based on this, the tube voltage setting that SOMATOM go.Top and go.All with the Athlon tube requires the lowest iodine contrast agent dose to reach provides 75 kW and 825 mA at 70 and 80 kV and 800 mA this enhancement level can be selected. at 90 kV. SOMATOM go.Up and go.Now with the Chronon tube provides 32 kW and 400 mA at 80 kV. 7 White Paper · Reduction of contrast agent dose at low kV settings This principle works on every CT scanner; however, for Low kV contrast CT scanning is feasible in clinical routine meaningful savings of contrast agent dose a significant and, depending on user preferences, it can be used decrease in the kV is required (Figure 4). In the past, it directly to reduce either radiation or iodine dose. has not been possible to keep the dose constant at all kV It is possible to use low kV imaging to save both levels, as the X-ray tubes were not powerful enough to radiation and iodine dose at the same time; however, produce sufficient tube current (mA) at low kV levels. the configuration of a suitable protocol is not easy and New X-ray tubes powering the new generation of the reduction potential for both radiation and iodine scanners, like SOMATOM Force, SOMATOM Drive, is significantly less compared to the two methods SOMATOM Edge Plus and SOMATOM go. Platform, now described above. have sufficient power to fulfill this condition (Figure 7). An additional technical enhancement that enables the success of this principle are the availability of iterative reconstruction methods (such as ADMIRE and SAFIRE) and integrated circuit detectors (Stellar detector) which help to reduce image noise at low kV scans and therefore allow for a diagnostic image quality even at low kV settings. The increase in iodine enhancement > at lower kV can be utilized to reduce the contrast agent dose, while maintaining the image quality (CNR) at a given radiation dose. max mA 1,600 Conventional tube VectronTM tube (SOMATOM Force) 1,200 800 required mA values for large adult required mA values for adult 400 required mA values for child 0 70 80 100 120 Voltage / kV Figure 7: Maximum tube output at different kVs: With conventional tubes, sufficiently high mAs output at low kVs is only possible for pediatric examinations (black arrows). New CT tube generations, however, can overcome this limitation. This enables a significant reduction in the kV, not only in protocols for children, but also for large adult scans (gray arrow). 8 White Paper · Reduction of contrast agent dose at low kV settings 70 kV, 70 mL contrast Figure 8: Clinical example of a patient scanned with a suggested 70 kV protocol with 70 mL of contrast agent on SOMATOM Edge Plus. 80 kV, 30 mL contrast Figure 9: Clinical example of a TAVR patient scanned with a 80 kV protocol with 30 mL of contrast agent on SOMATOM Force. 9 White Paper · Reduction of contrast agent dose at low kV settings Saving contrast agent in clinical practice Reduction of the IDR can be accomplished by lowering If one changes from a reference scan protocol to a low kV the iodine concentration and/or the volume and injection protocol and one decides to reduce the contrast agent flow rate, which will lead to identical temporal dose, the contrast injection protocol must be adapted to enhancement curves for reference and low kV protocols. reduce the iodine load. As previously demonstrated in a pre-clinical animal study The easiest way to adapt the iodine load seems to be a (Figure 8), peak width, peak height, and peak time reduction of the contrast volume without changing any remain the same with a reduction of the IDR. More other injection parameters. practical details, for instance about setting the CTDIvol of reference protocols for lower kVs and adaptation of the However, a reduction in contrast volume leads to a shorter injection duration and, thus, to significant bolus tracking parameters, can be found in the “How-to” changes in the contrast enhancement over time: peak guide, which provides step-by-step instructions for enhancement occurs earlier, requiring adaptation of the clinical application. scan delay, and scan timing becomes more critical because of a narrower enhancement curve. To avoid these changes in scan timing, the preferred and most robust strategy to achieve contrast dose reduction is to keep the injection duration constant while reducing the iodine delivery rate (IDR). 120 kV, 64 mL at 5.0 mL/s, 300 mgl/mL 70 kV, 32 mL at 2.5 mL/s, 300 mgl/mL 400 400 300 300 200 200 Enhancement (HU) Enhancement (HU) 100 100 de 0 10 20 30 40 50 60 0 10 20 30 40 50 60 Time (s) Time (s) Figure 8: An animal study in pigs demonstrated that an identical temporal enhancement curve for the 120 kV reference protocol (left) and 70 kV protocol (right) can be obtained with constant injection duration and adapted IDR, while saving 50% in contrast agent. Data courtesy of Lell et al. 10 White Paper · Reduction of contrast agent dose at low kV settings References Azzalini L, Abbara S, Ghoshhajra BB. Ultra-low contrast computed Scientific Publications: tomographic angiography (CTA) with 20-mL total dose for Siegel MJ, Schmidt B, Bradley D, Suess C, Hildebolt C. 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Emerg Radiol. 2012 Websites: Oct;19(5):437-45. http://www.nist.gov/pml/data/xraycoef Hough DM, Yu L, Shiung MM, Carter RE, Leng S, Fidler JL, Huprich JE, https://www.healthcare.siemens.com/medical-imaging/low-dose/ Yondal DY, McCollough CH, Fletcher JG. Individualization of order-guide-to-low-dose Abdominopelvic CT Protocols With Lower Tube Voltage to Reduce IV Contrast Dose or Radiation Dose. AJR 2013; 201:147-153. 11 At Siemens Healthineers, our purpose is to enable On account of certain regional limitations of sales rights healthcare providers to increase value by empowering and service availability, we cannot guarantee that all them on their journey toward expanding precision products included in this brochure are available through medicine, transforming care delivery, and improving the Siemens Healthineers sales organization worldwide. patient experience, all enabled by digitalizing healthcare. 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