Cardiac Dual Energy CT: Technique

Transcription

1 RSNA 2013, VSCA51-01, Chicago, Dec. 5, 2013 Cardiac Radiology Series Cardiac Dual Energy CT: Technique Willi A. Kalender, Ph.D. Institute of Medical Physics University of Erlangen Dual Energy Functional Imaging vs. SPECT Iodine concentration in the myocardium depicting perfusion deficit Courtesy of J. Schoepf, Charleston, SC, USA Cardiac Dual Energy CT: Techniques Principles of Cardiac CT Physics of Dual Energy CT Technical Implementations Conclusions Seite 1 1

2 Goals of cardiac CT Image the complete heart free of motion unsharpness at high spatial resolution Allow for angiography Provide quantitative results w.r.t. geometric parameters, calcium etc. U of Erlangen 1998 First satisfactory spiral scans (4-slice scanner) Principles of phase-correlated cardiac CT scanning Record ECG as a synchronization function Select prospective triggering (one heart ph.) or retrospective gating (all heart phases) and choose table feed accordingly Scan in spiral mode (the standard initially) or in sequential mode (a recent alternative) Reconstruct one image volume (prospective) or several phase-selectively (retrospective, 4D) Partial Scan Reconstruction Use one segment of data of phase-coherent data for a selected heart phase Table position 1. Detector 2. Detector 3. Detector 4. Detector Heartbeat 1 Heartbeat 2 Heartbeat 3 Time Effective scan time typ. T eff 150 ms at t rot = s 1 Partial scan data (180 + fan angle) Kachelrieß, Ulzheimer, Kalender, Med. Phys. 27(8): (2000) Seite 2 2

3 Multi-Segment Reconstruction Combine n segments to obtain of phase-coherent data for a selected heart phase Table position 1. Detector 2. Detector 3. Detector 4. Detector Heartbeat 1 Heartbeat 2 Heartbeat Partial scan data (180 + fan angle) Kachelrieß, Ulzheimer, Kalender, Med. Phys. 27(8): (2000) Time Effective scan time T eff 48 ms typ ms at t rot = 0.4 s Cardiac Dual Energy CT: Techniques Principles of Cardiac CT Physics of Dual Energy CT Technical Implementations Conclusions Goals of dual energy CT Provide information on tissue characteristics beyond their Hounsfield units Allow for differentiation of tissues that have similar CT values Seite 3 3

4 Dual Energy CT offers information on tissue composition CT image Calcium density map Soft tissue density map Why is it that we can distinguish fat from muscle and liver in the Ca map? Kalender WA et al. Radiology 1987; 164: Mass Attenuation Coefficients as a Function of Energy µ cm²/g 10.0 K-edge Energy kev Mass attenuation coefficient cm²/g Iodine Water Calcium Iodine Calcium Water significant difference in 60 dependence on energy ,71 kev Energy Principle of Basis Material Decomposition The attenuation coefficient µ of any material x can be represented as a linear combination of two linearly independent functions, e.g. the Compton effect and the photoelectric effect. µ x (E) = c 1 σ(e) + c 2 τ(e) Just the same, attenuation can be assigned to two basis materials, e.g. water and calcium, with sufficiently different atomic numbers Z. µ x (E) = c 1 (µ/ρ) 1 (E) + c 2 (µ/ρ) 2 (E) Alvarez RE, Macovski A. Phys Med Biol 1976; 21: Seite 4 4

5 Principle of Basis Material Decomposition 1. Measure attenuation at two energies 2 values 2. Solve the system of 2 equations for each ray the 2 unknown basis material values δi ρirds Kalender WA et al. Med Phys 1986; 13: Representation of arbitrary materials as vectors in the plane of the two basis materials, in this case for calcium and water Vector for calcium (Z = 20) Higher atomic number Z Vector for iron (Z = 26) Vector for phosphorus (Z = 15) Lower atomic number Z Vector for water (Z = 7.4) Vector for fat (Z = 6.8) Tissue Parameters derived from Basis Material Density values Electron Density Z1 Z 2 ρe ρ1 ρ2 NA A1 A 2 Effective Atomic Number 1/x 1 Z1 Z2 Z1 x Z2 x Zeff ρ1 ρ2 ρ1 Z1 ρ2 Z 2 A1 A 2 A1 A 2 Monoenergetic CT values μ E0 μ/ρ E0 ρ1 μ/ρ E0 ρ2 Kalender WA et al. Digit Bilddiagn 1987; 7: Seite 5 5

6 Cardiac Dual Energy CT: Techniques Principles of Cardiac CT Physics of Dual Energy CT Technical Implementations Conclusions Technical Implementations of cardiac CT 1980/90s: electron beam CT 1990s ff.: slipring-based fast spiral CT 2000s ff.: slipring-based fast sequential CT Demands on the scanner Rotation times: as short as possible Power levels: as high as possible Technical Implementations of DECT 1. Take two separate scans (1970s ff.) 2. Rapid kv-switching (1980s ff.) 3. Sandwich detectors (1990s ff.) 4. Dual Source CT (2000s ff.) 5. Energy discriminating detectors (2010s??) Demands on the scanner Separation of the two detected spectra: as good as possible Power levels: as high as possible for low kv Seite 6 6

7 Dual Source CT (DSCT) System set-up 2 Straton tubes and 2 x 128-slice acquisition with double z-sampling 280 ms gantry rotation 50 and 33 cm field of view X-ray power Acquisition with up to 2 x 100 kw Cardiac CT 75 ms temporal resolution (t rot /4) Dual Energy CT Simultaneous acquisition with 80 kv / 140 KV with 0.5 mm tin filter added for the 140 kv beam DSCT: High-pitch scanning (p=3.4) Flash Cardiac 0.26 s Scan direction 75 ms per slice Scan only for one heart phase and only during one heart beat and at minimum radiation dose!!! Spectral separation for Dual Energy CT Improved X-ray tubes and generators with wider ranges of kv settings New at RSNA 2013: - 70 to 150 kv in steps of 10 kv - 2 x 120 kw generator More energy pairings, e.g. 70 kv and 150 kv plus added filtration allow for better spectral separation Seite 7 7

8 Dual Source Cardiac CT with 70 cm/s table movement at 70 kv and effective dose of 0.22 msv 34 year old male with intermediate cardiac risk profile Courtesy of Stefan Schönberg, U of Mannheim Cardiac Dual Energy CT: Conclusions For cardiac CT, very fast rotation and wide detectors are essential to obtain short effective scan times per slice and short overall scan times. For dual energy CT, separation of the two spectra detected is most important. High power values are essential. Dual source CT systems offer significant advantages for cardiac dual energy CT! Thank you for your kind attention! Handouts will be available on Seite 8 8