1. David Tso, BSc; Nima Kashani, BSc; Arash Eftekhari, MD; Anja Reimann, MD; Chris Davison, MD; Ahmed Albuali, MD; Steven Co, MD; Savvas Nicolaou, MD Vancouver General Hospital University of British Columbia

2. Objectives  To review the principles of dual energy CT (DECT)  To discuss novel techniques implemented using DECT  Material characterization  Bone vessel subtraction  Virtual Non-contrast  Iodine distribution maps  To explain the utility of DECT in acute care settings  Cardiac perfusion  Aortic dissection  Peripheral run-off  Pulmonary embolism  Renal stones  Gout

3. What is Dual Energy CT?  2 x-ray sources and 2 data acquisition systems mounted on same x-ray gantry  Each x-ray source has independent high voltage generator  Allows for independent control of both potential and current  E.g. 100 kV & 140 kV  Simultaneous low & high energy images can be reconstructed with comparable image noise levels Remy-Jardin et al. Radiol Clin North Am. 2010 Jan;48(1):193-205.

4. Dual Energy Bone Subtraction  Problem with single source CT in evaluating arteries with calcified plaque or vessels located next to the skull bone  Vasculatures cannot be unambiguously distinguished from surrounding bony or calcified structures  DECT is able to distinguish iodine from bone or calcifications using attenuation difference between the two energies Watanabe Y et al. Eur Radiol. 2009 Apr;19(4):1019-24.

5. Dual Energy Bone Removal Tool

6. Material Characterization  Materials better differentiated by applying two X-ray spectra and analyzing attenuation differences.  Technique works well for compounds with large atomic numbers such as calcium (photoelectric effect ~Z 3 ).  Reconstructed images can be processes with 3-material decomposition algorithm  Typical CT numbers of 3 materials of known density are plotted on a graph  Y-axis is CT number at 140 kV  X-axis is CT number at 80kV  3 materials should be sufficiently different as to create a triangle  E.g. Fat, Tissue, Iodine Johnson T et al. Eur Radiol. 2007; 17:1510-7.

7. MaterialDEI Bone0.1148 Liver0.0011 Lung-0.0021 Soft Tissue-0.0052 Skin-0.0064 Proteins-0.0087 Fat-0.0194 Gall fluid-0.0200 Dual Energy Index Depends on chemical composition and not on radiodensity Greater the difference in DEI values, better the separation

8. 8 Material characterization  Modified 2-material decomposition: Characterization of kidney stones Calcified stones are BLUE (heavy ions)  Uric acid stones are RED (Nitrogen, Oxygen containing) HU at 140 kV HU at 80 kV high Z low Z

9. Virtual non-contrast imaging  DECT allows for a virtual non-enhancing scan by identifying iodine contrast and subtracting it from the image  DECT can eliminate need of non-contrast scan and decrease additional radiation exposure with another scan  Able to look at calcium in vessels  Able to visualize hematomas and aortic dissections  Decreases amount of contrast needed for exam Nicolaou et al. Eur J Radiol. 2008 Dec;68(3):398-408.

10. 100% Virtual Non-contrast100% Iodine Overlay

11. CT Angiography  CT angiography is able to accurately detect significant CAD  Non-invasive visualization of coronary arteries  High spatial resolution  Ability to visualize extracardiac findings  Can rule out CAD for low to intermediate probability of CAD  Unfortunately, CT angiography fails to assess significance of occlusions  No info on perfusion Appropriateness criteria for cardiac CT and cardiac MR. J Am Coll Radiol. 2006 Oct;3(10):751-71.

12.  Myocardial blood pool analyzed by assessing iodine content within myocardium  Using unique X-ray absorption characteristics of iodine at different kV levels  Color-coded “iodine maps” represent myocardial blood pool  Perfused myocardium contains iodine versus an infarct which will not have iodine uptake DECT – Heart Perfused Blood Volume Ruzsics et al. Eur Radiol. 2008 Nov;18(11):2414-24.

13. DECT + adenosine stress  Using adenosine stress protocol can allow for comparison of rest and stress DECT in detecting perfusion deficits  Protocol allows for quantification of iodine  DECT adenosine stress protocol enables examination of anatomy and function in a single investigation  Well tolerated  Radiation exposure equivalent to SPECT

14. Dual Energy – perfused blood volume defects Rest Perfusion Stress Perfusion

15. Quantification of Iodine Iodine Quantification Maps

16. DECT in assessing pulmonary embolism  Determining the severity of pulmonary embolism (PE) is necessary for optimal patient care  Pulmonary CT angiography is currently imaging of choice  CT methods require an unenhanced and iodine enhanced image for comparison  Using DECT, pulmonary vasculature can be assessed  Degree of iodine enhancement representing lung perfusion can be seen on an image without having to compare it with an unenhanced CT image Chae et al. AJR 2010; 194:604–610

17. DECT – Pulmonary embolism

18. Iodine map showing segmental defect

19. Iodine Quantification

20. DECT – Detecting Aortic Aneurysms  DECT temporal resolution of 83 ms reduces motion artifact and decreases false positive aortic dissections  False positive aortic dissections or indeterminate scans require additional imaging (MRI) or repeat scans with gating at expense of increased radiation and time Nicolaou et al. Eur J Radiol. 2008 Dec;68(3):398-408.

21. DE Bone Removal - Aortic Aneurysm

22. 120 kV 100% Iodine Overlay Virtual Non-contrast Utility of non contrast for intramural hematoma

23. Visualizing Endoleaks with DECT  DECT can generate virtual nonenhanced image which may obviate routine acquisition of true nonenhanced data  Reduce exposure to radiation by needing only one acquisition  Use of low-peak-voltage techniques is associated with increased image noise  Small endoleaks potentially can be better visualized when data are evaluated at lower photon energy levels  DECT scan performed during the delayed phase with reconstruction of virtual nonenhanced images is able to detect endoleaks after endovascular AAA repair with high accuracy Chandarana H, et al. Radiology. 2008 Nov;249(2):692-700. Stolzmann P., et al. Radiology. 2008 Nov;249(2):682-91.

24. Imaging Peripheral Arteries with DECT  Simultaneous dual energy acquisition with DECT can identify bone and plaque and to subtract them from the enhanced dataset  Better definition of bone edge and a vessel running in close proximity  Relevant vessel erosion of calf arteries running close to bone substantially less frequent and less severe using DE bone subtraction  Anterior tibial artery  Peroneal artery  Dorsalis pedis artery Meyer BC, et al. Eur J Radiol. 2008 Dec;68(3):414-22.

25. MIP Runoff – Plaque off MIP Runoff – Plaque on VRT Runoff – Plaque ONVRT Runoff – Plaque OFF

26. Challenges of CTA near the skull  Conventional CTA has limited role in evaluating cerebrovascular function near base of skull due to difficulty of separating vessels from bony structure  Traditional subtraction methods require enhanced and unenhanced data  Potential for error in registration between two acquisitions Zhang et al. AJR 2010; 194:23–30

27. MIP Carotid Bone Removed (Plaque On) VRT Carotid Bone Removed (Plaque On) MIP Carotid Bone Removed (Plaque Off) VRT Carotid Bone Removed (Plaque Off)

28. Utility of renal stone identification  Identification of renal calculi can avoid extensive metabolic investigations  Composition of stone influences course of treatment  Medical dissolution therapy for uric acid and cystine calculi  ESWL and more invasive therapies for struvite and calcium oxalate calculi  Treatment and prophylaxis with antibiotics for struvite stones for causative organism Moe OW. Lancet. 2006 Jan 28;367(9507):333-44.

29. DECT and calculi  Differences in HU more dramatic for calcium- based calculi than non-calcified calculi  Uric acid and cystine – light chemical elements (H, C, N, O)   high attenuation at higher kVp than at lower kVP  Calcium oxalate and struvite – heavier chemical elements (P, Ca, S)   high attenuation at lower kVP than at higher kVP Primak AN et al. Acad Radiol. 2007; 14:1441-7.

30. DECT in detecting renal calculi Red = Uric acid

31. Diagnosis of Gout  Diagnosis commonly dependent on clinical assessment and response to medications  Gold standard for the diagnosis of gout is arthrocentesis  Only performed on 11% of patients Difficulties of joint aspiration include: Inadequate fluid Inflamed joints Challenging anatomic sites e.g. spine, SI joints Nicolaou et al. AJR 2010; 194:1072–1078

32.  Three materials are displayed in different colors in the result image  Bone in blue  Uric acid in green  Iodine/Trabecular Bone in pink 1.The colorcoded results are independent of the absolute HU-value 2.It only depends on the difference in absorption at 140kV and 80kV

33. Benefits of DECT in Gout  Confirming the diagnosis of gout in complex sites  Detect subclinical cases  Accurately delineating anatomic distribution of disease  Monitoring response to treatment  Shorter scan time  ~15 min for all peripheral joints  Simultaneous scanning of multiple joints  Colour-differential display of crystals  Lower cost compared to MRI  Obviates need for arthrocentesis  Total for all peripheral joints ranges from 2 to 3 mSv  Target image areas for gout are radio-insensitive peripheral joints Choi et al. Ann Rheum Dis. 2009 Oct;68(10):1609-12.

34. DECT demonstration of uric acid deposition with tophus formation in a destructive arthropathy centered at the DIP joint of the right second toe DECT findings in keeping with gout arthropathy

35. Conclusions  DECT can improve workflow, efficiency, and costs associated with triaging emergency patients  DECT has the ability to combine imaging of anatomy with function as in perfusion studies as in myocardial and pulmonary investigations  DECT can improve work efficiency and ease diagnosis by improving workflow in vascular studies with pure vessel bone subtraction techniques and calcium separation from peripheral arteries in run off studies  DECT can alter the course of treatment with determination of material composition of renal stones  DECT provides a reliable and non-invasive means for detecting gouty arthropathy