Dual-Energy CT: The Technological Approaches

Transcription

1 Dual-Energy CT: The Technological Approaches Dushyant Sahani, M.D Director of CT Associate Professor of Radiology Massachusetts General Hospital Harvard Medical School

2 Disclosure Research Grant Support GE Healthcare BD Consultant, Bracco Diagnostics

3 Single Energy CT Standard FBP Fast Image Data Space Raw data recon Raw Data Space Single Source CT One X-Ray Tube and Detector Assembly X-Ray Tube kvp 80, 100, 120 and 140

4 Photon energy Challenges with Single Energy CT Tissue characterization Lesion detection (small lesion) Image noise and quality at low kv scanning Complex scanning protocols (multi-phase imaging) Radiation Dose kvp define upper limit for Polychromatic X ray kv 140 kv spectrum

5 Challenges in Lesion Characterization I- I+ I- I+ Mass Enhancement RFA

6 Attenuation on CT Material differentiation on CT is based on X-ray attenuation Attenuation is caused by absorption and scattering of radiation Two main mechanisms Compton scatter and Photon effect Compton effect is energy independent Photo effect is strongly energy-dependent Changing the kv setting results in an alteration of photon energy Johnson et al Eur Rad 2007

7 DE CT - Background Attenuation is energy-dependent At 80kVp Iodine signal is 2x of 140 kvp CT numbers do not vary with beam energy for soft tissues but do for high z materials By analyzing absorption properties of different material at different x-ray energies, materials can be distinguished (Base material decomposition)

8 80kV 140kV Bone 670 HU Iodine 296 HU Bone 450 HU Iodine 144 HU 80kV 140kV

9 Stone composition Element composition Uric Acid Stone Light Elements (H, C, N, O) Non uric acid stone Heavy Elements (P, Ca, S) Attenuation at 80 kvp Lower HU Higher HU Attenuation at 140 kvp Higher HU Lower HU Calcium 710 HU Uric Acid 290 HU Calcium 480 HU Uric Acid 315 HU 80kV 140kV

10 Material Attenuation NIST* Curves cm 2 /g X-Ray Mass Attenuation Coefficients Iodine bone water Photon Energy, kev Note: Bone attenuation curve may be represented by a linear combination of iodine and water. X-ray mass attenuation coefficients *National Institute of Standards and Technology US Dept. of Commerce

11 u(e) (cm2/mg) Material Attenuation Iodine Material Attenuation Coefficient Material Attenuation Coefficient Monochromatic 1 Bone Water (kvp) Photon Energy (kev) 80 kvp 140 kvp 70 KeV 100 KeV

12 Liver Fat quantification- CT Dual Energy CT As the kvp increases, the fat attenuation increases Steatotic liver exhibits greater change in attenuation between 80kVp and 140kVp than normal liver Liver Fat Index = HU at 140 kvp - HU at 80 kvp Liver Fat Index >10HU suggests HS >25%

13 DE-CT: Liver Fat Quantification Courtsey: Saad Sirhoi GE

14 # X-rays # X-rays # X-rays Dual-Energy CT-Approaches Siemens Dual-Source Energy GE Fast Switching (Gemstone) Energy Philips Dual-Layer Energy

15 Dual Source Dual Energy Scanning Tube A Tube B B A 50 cm FOV 26/33 cm FOV Both tubes can be operated independently or in unison with respect to their kv and ma settings Coursey C A et al. Radiographics 2010;30:

16 Dual Source Single Energy 120 Dual Energy Both tubes at same kvp Both tubes at different kvp Faster image acquisition Higher power (160kW) Improved temporal resolution (83ms) for cardiac imaging Improved image quality In Obese patients 500 lbs (220kg) Tissue material Differentiation

17 DS-DECT Image Processing Algorithm 80kVp 140kVp Weighed120kVp Iodine Map 3 material Decomposition Algorithm Soft tissue, Fat & Iodine Virtual NECT

18 Dual Source Dual Energy CT Data from the two acquisitions (80 and 140 kvp) Three material decomposition algorithm Soft tissue, Fat, Iodine Water, Uric acid, calcium Calcium stone Iodine Distribution Image Virtual Un-enhanced Image Stone Composition

19 Iodine (perfusion) Imaging Qualitative Quantitative RM mg/cc RP mc/cc

20 Calcium stone Uric Acid stone Mixed stones, struvite stones and cystine stones show a mixture of red and blue color Stolzman et al Urol Res 2008, Graser et al 2008 Invest Radiol, Graser et al 2009 Eur Radiol, Thomas et al Eur Radiol 2009

21 80kVp 140kVp Limitations Limited field of view of B tube than the standard field of view with the A tube. FOV limitation will not be a problem for the aorta, pancreas, adrenal glands or small bowel Dual energy acquisition are ineffective in obese patients or in patients with very large abdomens (BMI>30).

22 Single Source DE CT Single Source - Dual Energy CT SSDE CT

23 GSI Acquisition and Projection Based Material Decomposition GSI Data Acquisition Interleaved High- and Low-kVp Projections Attenuation-to-material density transformation iodine water 2 2 P1 ( i) 1( i) Pl o( wi ) 1( i) Ph i g( ih ) 1P l o( wi ) 1P h i g( ih ) 1P l o( wi ) Ph i g( ih ) P2 ( i) 2( i) Pl o( wi ) 2( i) Ph i g( ih ) 2P l o( wi ) 2Ph i g( ih ) 2Pl o( wi ) Ph i g( ih )... Iodine Projections Water Projections split Low kvp Projections High kvp Projections Image Reconstruc tion Image reconstruction MD Iodine MD Water Monochromatic Generation 140 kvp

24 cm 2 /g Monochromatic Image Generation Water (mg/ml) Iodine (mg/ml) p ( E) d ( E) d Monochromatic iodine E 70 kev (HU) water kev

25 Flux Polychromatic vs Monochromatic images 70 kev MONOchromatic Xray beam 70 ke V Polychromatic Xray beam 80 kvp 140 kvp Energ y kvp kev specifies defines the the upper photon limit energy x-ray for a polychromatic monochromatic Xray x-ray beam source 80 kvp 140 kvp kvp ~ KeV

26 Single Source DE CT Dual Energy Acquisition 80/140 kvp datasets Material Density images Effective Z Monochromatic images( kev) Water Image Iodine Image

27 SS DECT in differentiating material using spectral attenuation curve. The figure shows spectral attenuation curve of three different materials (contrast in abdominal aorta, fat and soft tissue - corresponding ROI marked). The spectral attenuation curve may be a signature sign of individual material and facilitate differentiation of tissues having similar attenuation values on single energy CT. 27

28 Effective Z (Zeff) MD Water MD Iodine High and low kvp Rapid kv twitching Uric Acid stone = MD Water +, MD Iodine Non-Uric Acid stone = MD Water +, MD Iodine + Effective Z Image Effective z number scatter plot

29 FEATURES DS DECT SS DECT X Ray tube Two Tube mounted at 90 degrees Single X-ray tube & Gemstone detector with fast processing speed Tube potential Two Tube A (140kV, 50cm FOV) & Tube B (80/100kV, only 33cm FOV). Single X-ray tube with 50cm FOV capable of ultrafast kv switching between 80 and 140kV. ma ma on each tube can be controlled ma is fixed for both 80 and 140kV acquisition cycle. Reconstruction technique Post processing of data Image space based Three material decomposition Soft tissue, fat, Iodine Iodine map and virtual non contrast DE data (80/100kV kv) water, uric acid, calcium Stone analysis Monochromatic Images ( kev) Contrast exploitation from Lower kev images Projection space based Monochromatic Images ( kev) Contrast exploitation from Lower kev images DE data (80kV + 140kV) Material density images from two material decomposition Allows separation of different material/tissue Effective Z image Distinguishing tissues based on effective atomic number

30 DECT Spectrum Monochromatic VNC MD Water MD Iodine Iodine map Effective Z Exploitation of contrast Replacement to unenhanced CT Material differentiation and replacement to unenhanced CT Material differentiation, qualitative and quantitative iodine uptake assessment Assessment of iodine uptake both qualitative and quantitatively Material differentiation based on effective atomic number

31 Why Dual Energy CT? Challenges with Single energy CT Advantages of DECT MD Water MD Iodine Tissue composition Uric acid (UA) or non UA stone Better material differentiation enhancing tissue characterization Stone seen on MD water image only suggesting UA stone HU value reliability More consistent HU values on Monochromatic images Beam hardening and metal artifact Reduction of Beam hardening and metal artifact True vs pseudo enhancement HU is 33 True /pseudenhancement More Reliable detection of enhancement with Iodine map Iodine content 0.83mg/cc which is very negligible iodine uptake suggesting benign cyst on Iodine image Protocol complexity Simplification of multiphase protocol TNC VNC DE Arterial Single DE acquisitions giving both phases

32 Scanner Accessibility Cost Hardware Reconstruction time Technologist Training for post processing dual energy data Work Flow Challenges PACS Large data DE soft ware are not integrated on PACS Quantification of material images Radiologist Training Familiarity of different data sets More image series for analysis Standard analysis

33 Summary DECT is an exciting technology that provides information about how substances behave at different energies Enabling improved detection of iodinecontaining substances on low-kvp/kev images Material differentiation The ability to generate virtual unenhanced images These capabilities are promising for improved detection and characterization Evaluation of vascular structures. Work-flow challenges Validation