SETTING EMG STIMULATION PARAMETERS BY MICROCONTROLLER MSP430

Overview

This article deals with basic techniques of neurophysiologic monitoring and stimulation. The described practices are used in medical diagnosis, surgery and treatment. Different devices with many functions and properties are used in stimulation. A system for electromyographic monitoring (EMG) and stimulation is proposed. The stimulating part of the prepared device described in this article is based on signal processing using an MSP430 microcontroller. This should certainly prove to be beneficial for future use, in a similar manner to the benefits bestowed by some current parts of modern, smaller and cheaper medical devices related to EMG.

Keywords:
EMG, stimulation, monitoring, PWM, microcontroller

Introduction

Modern medical practice is based on high-tech technology utilization. To this end, the most modern technology is required to ensure continued progress in the development of medical devices. Many forms of bio-electric phenomena can be recorded with relative ease. These include measurement, stimulation and recording such as in the neurophysiologic intraoperative monitoring (NIOM) used in medical monitoring, treatment and surgery.

Techniques based on monitoring include: electromyography (EMG), somato-sensory evoked potentials (SEPs), motor evoked potentials (MEPs), brainstem auditory evoked potentials (BAEPs), electroencephalography (EEG), electrocardiography (ECG) and electroneurography (ENG). Electrical stimulation is required to record some of these signals, with the stimulating and recording device parameters dependent on the expected frequency range and signal intensity [1]-[4] (Tab. 1,Tab. 2).

This paper proposes a system for stimulation and EMG signal recording based on the MSP430 microcontroller, which is currently one of the most advanced, fastest, ultra-low-power and most compact processors.

The first two sections of this article discuss medical measurement techniques for selected bioelectric phenomena, and the design of the proposed system is then described

Intra-operative electromyography

Intra-operative electromyography (EMG) provides useful diagnostic and prognostic information in spinal and peripheral nerve surgery [1]. Basic techniques include free-running EMG, stimulus-triggered EMG and intra-operative nerve conduction studies. These techniques can be used to monitor the following; (1) nerve roots during spinal surgery; (2) the facial nerve during cerebellopontine angle surgery and (3) peripheral nerves during brachial plexus exploration and repair. However, there are a number of technical limitations which can cause false-positive or falsenegative results, and these must be recognized and avoided wherever possible [2].

EMG can be monitored in any muscle accessible to a needle, wire, or surface electrode, so that mechanical irritation of peripheral nerves or nerve roots results in muscle activity in the corresponding musculature.

MEPs Techniques

Recording and Stimulation

Trans-cranial stimulation activates spinal cord motor fibers. It is important to localize motor tract deficits by choosing appropriate muscles to record. The most convenient recording is performed from at least two muscles on either side below the surgical level, and from one muscle above it which serves as a control signal. The precise methodology involved in the choice of stimulated muscles and instigated medical processes is beyond the scope of this article [2].

Trans-cranial activation of subcortical motor tracts is elicited most efficiently by anodal stimulation. Figure1 shows a typical electrical recording from a single nerve fiber, including the dc offset potential (resting potential) which occurs on membrane penetration. It also shows the transient disturbance of membrane potential (the action potential) when an adequate stimulus is applied. 

Conduction velocity in a peripheral nerve is measured by stimulating a motor nerve at two points a known distance apart along its course [3]. Subtraction of the shorter latency from the longer one gives the conduction time along the segment of nerve between the stimulating electrodes (Fig. 2). The conduction velocity of the nerve can be determined when the separation distance is known. This has great potential clinical value, especially where conduction velocity in a regenerating nerve fiber is slowed following nerve injury. 

Characterization and Interpretation

In order to understand the level of clinical significance represented by a pattern of EMG activity, the activity must be characterized beyond a simple burst or train description [3]. The most important feature suggesting significance is its relationship to the surgical events at that time. In addition, a number of electrical features of EMG activity can suggest greater or lesser degrees of irritation and therefore greater or lesser clinical significance.

The following EMG patterns are highlighted in Figure 3: (A) a minor burst of activity occurring as a lumbar root is manipulated; (B) a more intense burst occurring on the background of an ongoing train of activity; (C) intense ongoing trains of activity from multiple motor units, denoting asynchronous activity. (D) a residual train of activity as the effect of nerve root irritation wanes and (E) an interference pattern in the left gastrocnemius muscle after inadvertent trauma to the corresponding nerve root. 

System Design

The proposed EMG system comprises recording and stimulating parts. The digital part of the system is based on MSP430 microcontroller utilization. Here, a pulse width modulation (PWM) signal generated by a timer and D/A converter in the microcontroller is used for stimulation.

Theory of PWM signals

Pulse width modulation (PWM) is a powerful technique for controlling analog circuits with processor digital outputs. PWM is employed in a wide variety of applications, ranging from measurement and communication to power control and conversion.

Pulse-width modulation uses a square wave with a modulated duty cycle which results in variation in the average waveform value. When a square waveform f(t) with a low value y_min, a high value y_max and a duty cycle D is considered the resultant average value of the waveform is given by:

As f(t) is a square wave, its value is y_max for 0 < t < D.T and y_min for D.T < t < T . Expression (1) then becomes:

The duty cycle “D” is the time an entity spends in an active state as a fraction of the total time considered.

Microcontroller MSP430

The microcontroller is used as a PWM signal generator. This signal is fully programmable, so that the frequency range, amplitude, and target signal latency are easily set.

MSP430 Microcontrollers (MCUs) from Texas Instruments (TI) are 16-bit, RISC-based, mixed-signal processors designed specifically for ultra-low-power. MSP430 MCUs have the right mix of intelligent peripherals, ease-of-use, low cost and the lowest power consumption for many applications [4].

Since the MSP430 MCU is designed specifically for ultra-low-power applications, its flexible clocking system, multiple low-power modes, instant wakeup and intelligent autonomous peripherals enable true ultralow- power optimization which dramatically extends battery life.

The MSP430 MCU clock system has the ability to enable and disable various clocks and oscillators which allow the device to enter various low-power modes (LPMs). This flexible clocking system optimizes overall current consumption by enabling the required clocks only when appropriate [6].

The architecture, combined with five low power modes is optimized to achieve extended battery life in portable measurement applications. The device features a powerful 16-bit RISC CPU, 16-bit registers, and constant generators that contribute to maximum code efficiency. The digitally controlled oscillator (DCO) allows wake-up from low-power modes to active mode, typically within 3 μs.

The MSP430F563x series are microcontroller configurations with a high performance 12-bit analogto- digital (A/D) converter, comparator, two universal serial communication interfaces (USCI), USB 2.0, hardware multiplier, DMA, four 16-bit timers, a realtime clock module with alarm capabilities and up to 74 I/O pins.

Typical applications for this device include analog and digital sensor systems, digital motor control, remote controls, thermostats, digital timers and handheld meters. The MSP430F5638 is used as a generator of PWM signals where the amplitude, latency and frequency of the EMG stimulating signal can be set.

Timer

Timer_A is a 16-bit timer/counter with up to seven capture/compare registers [5]. This timer can support multiple capture/compares, PWM outputs, and interval timing, and it also has extensive interrupt capabilities. Interruptions can be generated from the counter in overflow conditions and from each capture/compare register.

Timer_A features include:

• A synchronous 16-bit timer/counter with four operating modes
• A selectable and configurable clock source
• Up to seven configurable capture/compare registers Configurable outputs with pulse width modulation (PWM) capability
• Asynchronous input and output latching
• An interrupt vector register for fast decoding of all  Timer_A interrupts

D/A Converter

The DAC12_A module is a 12-bit, voltage output DAC, which can be configured in 8-bit or 12-bit mode and can be used in conjunction with the DMA controller. When multiple DAC12_A modules are present, they can be grouped together for synchronous update operation.

Features of the DAC12_A include:

• 12-bit monotonic output
• 8-bit or 12-bit voltage output resolution
• Programmable settling time vs power consumption
• Internal or external reference selection
• Straight binary or 2's complement data format, right or left justified
• Self-calibration option for offset correction
• Synchronized update capability for multiple DAC12_A modules

Results of System Testing

Figure 5 depicts an example of a PWM signal where the amplitude can be set by a D/A converter. The frequency and latency of generated pulses are set in the time-interrupt routine. There is some noise problem when the device is powered by the USB cable. Although there is no apparent problem when the amplitude is 200 mV, the absolute noise destroys all useful signals at less amplitude.

The comparison of noises for different power supplies at low signal amplitudes is shown in Figure 6.

For further stimulations and measurements, it is necessary to use a battery powered device.

Conclusion

An EMG system for recording and stimulation is proposed in this article, and the stimulating device was developed and tested. For this stimulation, a modern MSP430 microcontroller was used to generate stimulation pulses with the configurable parameters of pulse width and amplitude, the number and frequency of pulses in the burst and the latency between bursts.

Our future task is to develop a wearable neurostimulator based on EMG technology, including an adaptive output control based on on-line signal DSP processing.

Acknowledgement

This material, and the MSP-TS430PZ100USB 100- Pin Socket Target Board and USB Programmer were financially supported by the: AV 4/0012/07 FEI.

The research described in the paper was financially supported by the Slovak Ministry of Education under VEGA Grant No. 1/0987/12.

Ing. Martin Nováček

Institute of Electronics and Photonics

Faculty of Electrical Engineering and Information Technology

Slovak University of Technology in Bratislava

Ilkovičova 3, SK-812 19, Bratislava

Slovak Republic

E-mail: martin.novacek@stuba.sk

Phone: +421-2-602 91 213