2002 symposium1 Craig Goodrich 2002 MIRL Symposium

COMPUTER SIMULATION CAROTID ATHEROSCLEROTIC PLAQUE IMAGING

2002 symposium3 Topics of Discussion Why do a computer simulation? Carotid anatomy Atherosclerotic disease Magnetic resonance imaging Implementation of model Results

2002 symposium4 Why do a computer simulation? Truth standard difficult for human imaging Animal models User determines truth with computer model

2002 symposium5 Arterial wall anatomy Tunica Interna layer of endothelial cells and elastic fibers Tunica Media smooth 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 symposium6 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 symposium7 atherosclerotic disease

2002 symposium8 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 symposium9 Example MRA images T 1 weighted Proton density weighted (PD) Time-of-flight (TOF)

2002 symposium10 20 phase/heartbeat phase contrast MRA

2002 symposium11 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 symposium12 Proton Density Anatomical Computer model

2002 symposium13 Modeling disease Foam cells Lipid layer Fibrous plaque Thrombus 13.5% 62%132%

2002 symposium14 MRI simulation using IDL Read imaging parameters TR, TE, K_fill, fnecho_lim, theta Read tissue parameters T 1, T 2, PD Read masks for tissue types background (assume muscle) vessel wall components Determine type of imaging sequence and call subroutine

2002 symposium15 Spin echo subroutine 1. T 1 & T 2 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 array to array (simulates continuous - > discrete) e) store in temporary Kspace array (kx,ky,t) 4. Sequentially fill Kspace as function of TR & HR 5. Add T 2 * attenuation 6. Inverse FFT for final image and return image

2002 symposium16 Results Simulated phase artifact Anatomy model Simulated PD images

2002 symposium17 Future Work Determine thickening threshold for plaque detection Other contrast models T 2 weighted T 1 weighted TOF Add other anatomy (including movement) Add blood flow Add other k-space trajectories Coil sensitivities Noise, better PD, T 2, T 1 estimates ……….. Etc.