Purpose and BackgroundLab ProcedureOrganizationLab exercisesVisualizing the dipole and measured potentialsForward computation of the body surface potentialsInverse localization of the dipoleLab ReportECG SimulationRob MacLeod and Dave WeinsteinMarch 27, 2003Contents1 Purpose and BackgroundPurposeThis lab continues our examination of electrocardiography by means of SCIRun, the visualizationand simulation program you used in the second lab for this course. The goal here is to comparebody-surface ECGs simulated using a dipole source with those you measured in the lab.The hypothesis to keep in mind as you go through the lab is whether or not a single dipole isan adequate representation of the electrical activity from the heart.BackgroundDave and I provided some of the background for this lab in the lectures and our slides are on thewebsite for your viewing pleasure. The rest of the background comes from the lecture notes ondipole sources and the material on SCIRun available from the SCI website.We expect you to have completed the tutorial on SCIRun using the computers in the CADElab.2 Lab Procedure2.1 OrganizationThis lab will happen in the CADE lab, located on the second floor of the EMCB building, wherewe met for the second lab of the seme ste r. Each of you should sit at one of the computers that runthe Linux operating system, which are gathered in the northwest corner of the lab. Obviously thispresupposes you have an account on the system, which you should all have by now.To assist with the lab, we will have a team of develop ers of SCIRun present. These are thefolks who design and implement the software and Dave is the author of many of the modules andall the networks you will use.2.2 Lab exercisesTo set up all required variables and paths, please execute the following command from the systemprompt when you have logged into the CADE lab computer:source ~macleod/srsetup1The lab is organized into three separate tasks, each with its own network in SCIRun.Each network should load all the necessary data files when it starts but it will still be necessaryto launch the network, which you can do by executing any module in the network. Another efficientway to start the network is to push the “Step” button on the MatrixSelectVector user interface.Either way, when the network starts, you will see modules change color, which indicates thatSCIRun is beginning to run.2.2.1 Visualizing the dipole and measured potentialsLaunch this network with the command:net1With this first network, you will be able to visualize the measured potentials you recorded fromSung Moon as a function of time over a single heart beat. The surface over which you will see thepotentials displayed is not Sung’s but a ge neric body surface based on a model of another person.It is very s mooth and somewhat stylized but contains the shape of the torso. The measurementelectrodes were actually a 32-lead subset of the 192 locations you will se e in the visualization andwe carried out an estimation to the larger set from the me asureme nts according to the schemedescribed in the last lab manual. The 192 measurement leads are, in turn, a subset of the completetorso surface geometry, which contains 771 nodes.In the visualization, you will also see a dipole computed from these data and be able to observehow it shifts direction and magnitude with each time instant. Initially, the Frank-lead dipole seriesis loaded with the network. By opening the MatrixReader user interface (not the top one, but thenext one down), you can change this back and forth between the Frank-lead dipole series and thelimb lead dipole series. You will not be able to shift the location of the dipole but you can move aprobe anywhere on the surface of the body to se e the time signal measured at that location. Thetime signal will appear in the bottom panel of the ErrorMetric module. To shift the probe:1. In one of the Viewer windows, move the cursor over the sphere that marks the probe location.2. Hold down the shift key and then click on the sphere and drag it to a new location withinthat Viewer window.3. When you release the mouse, the display of the time signal should update immediately.4. If you have two Viewer windows, you can update the probe location in either one; it will notappear updated in the other window, however, until you click in that window and induce aredraw of its c ontents.Experiment with the display and with the MatrixSelectVector module, which allows you toselect a particular time instant to visualize and also to step through a sequence of time steps andmake animations of a series of time instants.2.2.2 Forward computation of the body surface potentialsnet2With the second network you will be able to move the dipole anywhere within the torso geometryand then have SCIRun compute the resulting body surface potentials. By displaying both thecomputed and measured potentials, you can attempt to place the dipole so that it produces thebest estimate of the measured potentials.To visualize both the measured and the simulated potentials, SCIRun renders them differently.2Measured potentials: the measured values will appear as color coded disks at all the measure-ment nodes in the torso model. The color shows the potential value at that location accordingto the display bar that you will see at the side of the Viewer window.Computed potentials: the computed values for any dipole location, orientation, and magnitudedetermine the color of the triangle mesh that identifies the surface of the torso geometry.Once the network launches and the Viewer window loads, find the location of the dipole inthe display and use the same Shift-Click-Drag method to move it around. When you release themouse, the new body surface potentials will appear in the Viewer. You can change the location ofthe dipole but its orientation in fixed by the values extracted from the measured data using theFrank electrodes. As with the probe, you can update the dipole in either Viewer window but it willnot up date in the second window until you click on that window. Use the different views in bothwindows to locate the dipole in three-dimensional space.In this network, you will not be able to play through multiple time instants but you can selectany single instant and also move through them manually by pushing the “Step” button on theMatrixSelectVector module.Try and pick an interesting time instant and then move the dipole so that the resulting computedpotential field matches