Impedance Imaging of the Thorax: Why it s difficult, and what we are doing about it?

Impedance Imaging of the Thorax: Why it s difficult, and what we are doing about it? Biomedical Engineering Research Centre (BIRC) Western Univerity, London, ON, 6 May 2015 Andy Adler Professor & Canada Research Chair in Biomedical Engineering Systems and Computer Engineering, Carleton University, Ottawa Impedance Imaging, Andy Adler, 2015/05/06 1 / 39

Electrical Impedance Tomography 10-day old healthy baby with EIT electrodes Source: eidors3d.sf.net/data contrib/ifneonate-spontaneous Impedance Imaging, Andy Adler, 2015/05/06 2 / 39

Electronics Block Diagram Impedance Imaging, Andy Adler, 2015/05/06 3 / 39

Current Propagation Healthy Adult Male CT slide at heart Source: eidors3d.sf.net/tutorial/netgen/extrusion Impedance Imaging, Andy Adler, 2015/05/06 4 / 39

Current Propagation I Healthy Adult Male CT slide at heart Source: eidors3d.sf.net/tutorial/netgen/extrusion Impedance Imaging, Andy Adler, 2015/05/06 4 / 39

Finite Element Modelling Impedance Imaging, Andy Adler, 2015/05/06 5 / 39

Finite Element Modelling Simulated Voltages Voxel Currents Impedance Imaging, Andy Adler, 2015/05/06 6 / 39

Thorax Propagation I CT Slice with simulated current streamlines and voltage equipotentials Impedance Imaging, Andy Adler, 2015/05/06 / 39

Changing Conductivity I Heart receives blood (diastole) and is more conductive Impedance Imaging, Andy Adler, 2015/05/06 8 / 39

Changing Conductivity I Heart receives blood (diastole) and is more conductive Impedance Imaging, Andy Adler, 2015/05/06 9 / 39

Changing Conductivity Impedance Imaging, Andy Adler, 2015/05/06 10 / 39

Application: Breathing Chest images of tidal breathing in healthy adult Impedance Imaging, Andy Adler, 2015/05/06 11 / 39

Application: Heart EIT Signal in ROI around heart (and ECG) Impedance Imaging, Andy Adler, 2015/05/06 12 / 39

Mechanical Ventilation Mechanical Ventilator with EIT monitor Source: Swisstom.com Impedance Imaging, Andy Adler, 2015/05/06 13 / 39

Acute Respiratory Distress Syndrome (ARDS) Impedance Imaging, Andy Adler, 2015/05/06 14 / 39

EIT + Lung State A B C D E F G Impedance Imaging, Andy Adler, 2015/05/06 15 / 39

EIT + Lung State A B C D E F G Overdistension A Collapse Impedance Imaging, Andy Adler, 2015/05/06 15 / 39

EIT + Lung State A B C D E F G Overdistension A B Collapse Impedance Imaging, Andy Adler, 2015/05/06 15 / 39

EIT + Lung State A B C D E F G Overdistension A B C Collapse Impedance Imaging, Andy Adler, 2015/05/06 15 / 39

EIT + Lung State A B C D E F G Overdistension A B C D Collapse Impedance Imaging, Andy Adler, 2015/05/06 15 / 39

EIT + Lung State A B C D E F G Overdistension A B C D Collapse E Impedance Imaging, Andy Adler, 2015/05/06 15 / 39

EIT + Lung State A B C D E F G Overdistension A B C D Collapse E F Impedance Imaging, Andy Adler, 2015/05/06 15 / 39

EIT + Lung State A B C D E F G Overdistension A B C D Collapse E F G Impedance Imaging, Andy Adler, 2015/05/06 15 / 39

EIT for Non-Invasive Blood Pressure Pulse transit time from heart to descending aorta using EIT Source: Sola et al, Med. Biol. Eng. Comput., 2011 Impedance Imaging, Andy Adler, 2015/05/06 16 / 39

Neonatal Breathing Preterm newborns have complex, unstable physiology Ventilatory support is often essential Currently, no adequate monitors of breathing These data are from a lamb model of neonates Source: D. Tingay et al, In Press Impedance Imaging, Andy Adler, 2015/05/06 1 / 39

EIT for Brain Imaging Applications: Epileptic foci Stroke (Ischaemic vs. Haemoragic) Fast Neural Imaging Source: Holder, www.ucl.ac.uk/medphys/ research/eit/pubs/brain EIT overview.pdf Impedance Imaging, Andy Adler, 2015/05/06 18 / 39

EIT for Cancer Imaging: Breast/Prostate Cancerous tissue has different electrical properties Image tissue Image increased vascularization Source: Khan, Mahara, Halter et al, Conf. EIT, 2014 Impedance Imaging, Andy Adler, 2015/05/06 19 / 39

Non-medical applications Flow in pipes Mixing tanks Imaging metalic ores Hydro-geology Figure shows resistivity in a cross-section of La Soufrière de Guadaloupe volcano. Source: N. Lesparre et al, Conf. EIT, 2014 Impedance Imaging, Andy Adler, 2015/05/06 20 / 39

Why is EIT hard? Impedance Imaging, Andy Adler, 2015/05/06 21 / 39

Inverse Problems... Plato s cave Impedance Imaging, Andy Adler, 2015/05/06 22 / 39

Plato s cave... Shadows on the wall Source: iamcriselleeee.files.wordpress.com/2013/11/cave-2.jpg Impedance Imaging, Andy Adler, 2015/05/06 23 / 39

Inverse Problems Forward Problem: Forms Shadows Impedance Imaging, Andy Adler, 2015/05/06 24 / 39

Inverse Problems Forward Problem: Inverse Problem: Forms Shadows Shadows Forms Impedance Imaging, Andy Adler, 2015/05/06 24 / 39

Inverse Problems Forward Problem: Inverse Problem: Forms Shadows Shadows Forms Ill-conditioned Sensitivity to some movements is low Impedance Imaging, Andy Adler, 2015/05/06 24 / 39

Inverse Problems Forward Problem: Inverse Problem: Forms Shadows Shadows Forms Ill-conditioned Sensitivity to some movements is low Ill-posed Some movements don t change shadows Impedance Imaging, Andy Adler, 2015/05/06 24 / 39

Inverse Problems Techniques: to calculate stable & meaningful parameters in the presence of inversion difficulties Examples Image deblurring / restoration Medical imaging Geophyical imaging Model parameter fitting Reconstruction with incomplete/noisy data Impedance Imaging, Andy Adler, 2015/05/06 25 / 39

Forward Model (linearized) Reconstruction in Pictures Forward Problem Measurements (difference) Image (difference) = + noise Jacobian Impedance Imaging, Andy Adler, 2015/05/06 26 / 39

Forward Model (linearized) Reconstruction in Pictures Forward Problem Measurements (difference) Image (difference) = + noise Regularized linear inverse model Jacobian Linear Solution: Minimize norm 2 + Penalty Function Norm weighted by measurement accuracy Impedance Imaging, Andy Adler, 2015/05/06 26 / 39

Electrode errors: Zero bad data Idea #1: Reconstruction with Data Errors Traditional Solution Error Here Replace With zero 2-2 σ 1-2 σ 2-2 σ 3-2 σ 4 Impedance Imaging, Andy Adler, 2015/05/06 2 / 39

Electrode errors: Zero bad data Idea #1: Reconstruction with Data Errors Traditional Solution Error Here Replace With zero 2 Error Model Solution -2 σ 1-2 σ 2-2 σ 3-2 σ 4 Electrode Error: Regularized imaging Error Here 2-2 σ 1-2 σ 2 Low SNR here Replace With zero -2 σ 3-2 σ 4 Impedance Imaging, Andy Adler, 2015/05/06 2 / 39

Electrode Error compensation Offline compensation using jack-knife approach (2005) A B EIT images in anaesthetised, ventilated dog A: uncompensated, B: compensated. Left: ventilation Centre: saline (right lung) Right: ventilation and saline Automatic detection (via reciprocity comparison) (2009) New work to speed online calculation & use data quality Impedance Imaging, Andy Adler, 2015/05/06 28 / 39

Imaging Electrode Movement Idea #2: Electrode movement Sensitivity to sensor movement = + noise Jacobian now includes measurement change due to movement image now includes x,y sensor movement Impedance Imaging, Andy Adler, 2015/05/06 29 / 39

Imaging Electrode Movement Idea #2: Electrode movement Sensitivity to sensor movement = + noise Adapted penalty function Jacobian now includes measurement change due to movement Penalty:Image and movement Expected image Expected movement 2 1 -½ -½ 1 -½ -½ 1 -½ image now includes x,y sensor movement -½ 1 -½ -½ 1 -½ -½ 1 Unlikelyhood of movement and image co-variance Unlikelyhood of movement Impedance Imaging, Andy Adler, 2015/05/06 29 / 39

Electrode movement compensation Source: Gómez-Laberge et al, Phys. Meas., 2006 Impedance Imaging, Andy Adler, 2015/05/06 30 / 39

Idea #3: Data Quality Impedance Imaging, Andy Adler, 2015/05/06 31 / 39

Idea #3: Data Quality Depth Sounder with analog and digital guages Impedance Imaging, Andy Adler, 2015/05/06 31 / 39

What s the problem? With strong priors and complex algorithms, algorithms give us pretty pictures, even when they are irrelevant. Question: how can we know when to trust a pretty picture? how can we know when the data are junk? Impedance Imaging, Andy Adler, 2015/05/06 32 / 39

Data Quality Measure: Concept Concept: High Quality Data are Consistent Idea: Use IP to predict each data point from all others Original d Remove i d (i) Solve ˆm (i) Predict ˆd (i) Calculate error ɛ i = d i ˆd (i) i Impedance Imaging, Andy Adler, 2015/05/06 33 / 39

Normalized data quality values Example: Data quality measures R1 R2 R3 R4 T1 T2 T3 T4 A Patient 1 Patient 0.95 0.9 B 0.85 0.8 0.5 0. 0.65 0.6 Patient1 Patient R1 R2 R3 R4 T1 T2 T3 T4 Protocol steps Clinical data and data quality metric for each stage of the protocol (R1 R4 recruitment: PEEP, T1 T4 titration: PEEP ). A: EIT images B: Calculated data quality. Impedance Imaging, Andy Adler, 2015/05/06 34 / 39

Perspectives Data analysis is hard powerful algorithms are useful we live in a world of big data complex systems fail in complex ways users like pretty pictures So... the situation will get worse Impedance Imaging, Andy Adler, 2015/05/06 35 / 39

Solutions? Impedance Imaging, Andy Adler, 2015/05/06 36 / 39

Solutions? Thus, we need Impedance Imaging, Andy Adler, 2015/05/06 36 / 39

Solutions? Thus, we need Open Data Impedance Imaging, Andy Adler, 2015/05/06 36 / 39

Solutions? Thus, we need Open Data Open source analysis Impedance Imaging, Andy Adler, 2015/05/06 36 / 39

For EIT... Impedance Imaging, Andy Adler, 2015/05/06 3 / 39

Traffic jam on the way to Carleton Impedance Imaging, Andy Adler, 2015/05/06 38 / 39

Abstract for talk: Impedance Imaging of the Thorax: Why its difficult and what we are doing about it Abstract: Electrical impedance tomography (EIT) uses body surface electrical current stimulation and measurements to generate images of the internal tissue electrical impedance. Currently, the most successful application of EIT is for imaging the thorax, where the movement on conductivity contrasting air and blood can be imaged over time. The generation of EIT images requires solving an inverse problem, which is ill-conditioned because of the diffuse nature of current propagation. The technology is thus sensitive to electrode properties, data quality, and patient movement. To address these issues, several innovative strategies to analyze and interpret these data have been developed. This talk will explain our recent progress in imaging the chest with EIT, and the image generation and interpretation strategies that are required. Impedance Imaging, Andy Adler, 2015/05/06 39 / 39