Cell culture mapping system

In a recent project, electrophysiology researchers at Westmead Hospital in Sydney required a system to acquire and analyse signals measured using a micro-electrode array. In this work the researchers grow a single layer of heart cells in a culture on the electrode array. As these living cells beat, it is possible to obtain valuable information related to heart rhythm disorders from the electrical signals measured in the cell culture.

Apart from the fact that software was not available 'off-the-shelf' to provide the specialised analysis functions the researchers required, the system hardware requirements posed several technology challenges given the available budget.

The basic hardware requirements for the system were:

• Simultaneous sampling of 64 channels at a high data rate (up to 70 kHz) with possible future expansion of channels;
• Storage of data to hard disk at the same time as display (minimum 10 MBps);
• High quality graphics for data visualisation.

The need for high-speed disk storage and advanced graphics for data visualisation led systems integration company, Madry Technologies to select a Dell dual Xeon processor system with fast SCSI Ultra160 hard disks as the computer platform.

National Instruments' Programmable eXtensions for Instrumentation (PXI) platform was the choice for the data acquisition system. Most modern PCs only provide a small number of internal slots for plug-in cards. PXI forms an extension of the PC's internal PCI bus and allows a large number of slots to be used for instrument hardware. One of the benefits of PXI is the ability to upgrade to more channels in the future using either a larger chassis (with up to 18 slots) or multiple chassis.

[image] Analysis funtion being carried out.

Four PXI 6070 1.25 MSamples/s E series multi-function DAQ cards allowed 64 channels to be sampled at up to 70 kHz per channel. The E series multi-channel analog input cards successively sample input channels to each card using a multiplexer and a single high quality instrumentation amplifier and analog-to-digital converter. The time between these samples can be controlled via the software. For bandwidth limited signals this allows relative channel timing to be recovered accurately.

It was necessary in this project to be able to lock the timing signals of the individual cards together so that the relative timing between all channels in the system was known.

The software architecture used set one of the boards as a master and the others as slaves with acquisition timing signals locked to the master. The PXI backplane provides advanced timing and triggering functions for this purpose.

The timing signals can be routed using LabVIEW software from National Instruments. Madry tested the synchronisation of the cards by feeding a sine wave of a known frequency into all channels. By measuring the phase between the sampled channels using an application developed using LabVIEW Madry was able to check the sampling time delay between any two channels in the system. This showed that users were able to synchronise cards together within the accuracy specified for the hardware.

[image] Heart cell culture being observed through a microscope while simultaneously acquiring signals from a micro-electrode array using a PXI-based measurement system.

Using NI's MXI-3 PCI to PCI bridge technology with an optical fibre link from the desktop PC to the PXI chassis allowed the analysis computer to be located some distance away from the electrode array and data acquisition system with this link being immune to electrical interference.

A software application using LabVIEW was developed to provide the display and data capture facilities required by the researchers. Using a dual monitor graphics card allowed simultaneous display of live data and analysis of snap shot data. LabVIEW also provided the benefit of being an ActiveX container (using the Windows operating system) allowing specialised visualisation functions to be incorporated into the application.

The electrophysiology researchers call this 'cardiac mapping' where the relative timing of the electrical signals measured by the array is displayed.

The system is now in use and it is envisioned the same hardware system will be reconfigured for other research applications. The National Instruments PXI hardware platform allows reconfiguration to be performed simply. For example, it is envisioned that in the future an IMAQ camera image acquisition system will be incorporated into the system.

As biomedical researchers endeavour to obtain groundbreaking results new analysis tools are required. In this application LabVIEW saved considerable software development time in the development of a new tool.

The hardware provided a platform for the researchers to expand in the future thus making the best use of much sought after research funding.