2002 MIRL Symposium

1. 2002 MIRL Symposium Craig Goodrich 2002 symposium

2. COMPUTER SIMULATION CAROTID ATHEROSCLEROTIC PLAQUEIMAGING

3. Topics of Discussion • Why do a computer simulation? • Carotid anatomy • Atherosclerotic disease • Magnetic resonance imaging • Implementation of model • Results 2002 symposium

4. Why do a computer simulation? • Truth standard difficult for human imaging • Animal models • User determines truth with computer model 2002 symposium

5. Arterial wall anatomy • Tunica Internalayer of endothelial cells and elastic fibers • Tunica Mediasmooth muscle cells (SMC) in framework of loose connective tissue • Tunica Externa (adventitia) connective tissue sheath mostly collagen, with scattered elastin and SMC – blends with adjacent tissues anchoring vessel to them 2002 symposium

6. Atherosclerotic development • High lipid levels in artery -> monocytes become filled with lipids (now called foam cells) • Foam cells attach to endothelium and release growth factors causing SMC to grow. • Monocytes invade, migrating between endothelial cells. Monocytes, SMC, endothelial cells begin phagocytization of fat resulting in plaque (which projects into lumen). • Endothelial cells become swollen with lipids and gaps appear in endothelial lining. Platelets begin sticking to exposed collagen fibers which leads to clot. 2002 symposium

7. atherosclerotic disease 2002 symposium

8. Magnetic Resonance Imaging • Proton magnetic moment • Precession depends on field strength • Appropriate manipulation of field (i.e. linear field gradients) -> image • Image contrast depends on proton density, relaxation, acquisition timing • Motion degrades image quality 2002 symposium

9. Example MRA images Time-of-flight (TOF) Proton density weighted (PD) T1 weighted 2002 symposium

10. 20 phase/heartbeat phase contrast MRA 2002 symposium

11. Modeled anatomy • Lumen – determined from phase contrast images • Endothelium – 1 pixel layer • Tunica Interna – 6 pixel layers • Tunica Media - 21 pixel layers • Tunica Externa – 9 pixel layers(probably should be thicker) 2002 symposium

12. Proton Density Anatomical Computer model 2002 symposium

13. Modeling disease • Foam cells • Lipid layer • Fibrous plaque • Thrombus 13.5% 62% 132% 2002 symposium

14. MRI simulationusing IDL • Read imaging parametersTR, TE, K_fill, fnecho_lim, theta • Read tissue parametersT1, T2, PD • Read masks for tissue typesbackground (assume muscle)vessel wall components • Determine type of imaging sequence and call subroutine 2002 symposium

15. Spin echo subroutine 1. T1 &T2 correct each tissue mask 2. multiply each mask by PD 3.For loop for each time phase a) cut hole in neck mask for vessel structure b) add all tissue masks c) FFT assembled anatomy d) truncate 40962 array to 5122 array (simulates continuous -> discrete) e) store in temporary Kspace array (kx,ky,t) 4. Sequentially fill Kspace as function of TR & HR 5. Add T2* attenuation 6. Inverse FFT for final image and return image 2002 symposium

16. Results Anatomy model Simulated PD images Simulated phase artifact 2002 symposium

17. Future Work • Determine thickening threshold for plaque detection • Other contrast models T2 weighted T1 weighted TOF • Add other anatomy (including movement) • Add blood flow • Add other k-space trajectories • Coil sensitivities • Noise, better PD, T2, T1 estimates ……….. Etc. 2002 symposium