Neuroimaging Techniques

The functioning of the human mind has been one of the main incentives for philosophers throughout the ages. The need to examine and measure different aspects of the brain anatomy has enhanced the recent developments in neuroimaging techniques. As these techniques have become more affordable and accessible for research, they have allowed an increasingly questioning attitude in making use of neuroimaging methods. Several neuroimaging techniques have provided correlational maps of cognitive processes in the adult human brain at different levels of temporal and spatial detail. Moving beyond a correlational description of the relationship between brain and the behavior was the fresh approach offered by transcranial magnetic stimulation [1], [2],

[3], [4], [5-11]. This chapter gives a brief introduction on how high current magnetic field generators contribute to advanced electromagnetic models and medical applications. The chapter also introduces the ideology behind transcranial magnetic stimulation and circuit requirements.
1.1. What is Transcranial Magnetic Stimulation (TMS)?
Electromagnetic induction described by Michael Faraday is defined by the act of producing a current in a conductive object by using a time-varying magnetic field. It is one of the most experimental observation for magnetic stimulation [12], [13], [14], [15], [21]. Faraday wound two coils on an iron ring and ultimately observed when the coil on one side was connected or disconnected from a power source, an electrical current passed through the coil on the other side. The iron ring acted as a medium linking the magnetic field from the first coil to the second. A change in the magnetic field, related to the changing current in the first coil, induced a current in the second coil. The experiment was enhanced due to the coupling effect given by the iron ring between the two coils. The same principle is applied in non-invasive magnetic stimulation in which the stimulating coil acts as one coil, space as the medium for the flow of the magnetic field and the electrically conductive matter in the human body as the second coil. Over the years there has been an expansion in the development and use of magnetic stimulators worldwide in medical applications. Commercial stimulators produce pulse rates as high as 1250 Hz with pulse stimulation intervals as low as 1 µs (microsecond) with complete digitization. A growth in research interest has produced a marked spread into the clinical areas of diagnosis, and therapy. The ability of magnetic stimulation to induce electrical currents within the body tissue allows influencing of many of the functions in the human body. Additionally it allows to reach deep neural structures such as the motor cortex and spinal nerve without pain and non-intrusion [12], [13], [14], [15], [16-21]. TMS is gaining great interest in psychiatry and is providing an extra boost to the therapeutical treatment methodologies provided by clinicians.
1.2. TMS Fundamentals and Technical Aspects
1.2.1. Circuit requirements
Magnetic stimulators consist of two different parts: a high current pulse generator producing discharge currents of 1000 A (amperes) or more; and a stimulating coil producing magnetic pulses with field strengths of approximately 1 tesla or more with a variable pulse duration.

The discharge current flows through the stimulating coil to generate the necessary magnetic pulse. This pulse induces current in electrically conductive regions of the human body. If the induced current is of high amplitude and duration it will stimulate neuromuscular tissue in the same way as with conventional electrical stimulation. The first commercial magnetic stimulators originated from Sheffield in 1985 [16-21]. A typical magnetic stimulator consists of a capacitor charging or discharging alternatively with the appropriate control and safety electronics. Using the charging circuitry the energy storage system (capacitor) is charged to a level which can be up to a maximum of 3000 volts (kV) depending on the device. When the device receives an input signal as a trigger, the energy stored in the capacitor is discharged into the stimulating coil. The stored energy, with the exception of the energy lost in the wiring and capacitor, is transferred to the coil and then returned to the instrument to reduce coil heating. The discharge system consists of a switch and an electronic device, either a power metal oxide semiconductor field effect transistor (MOSFET) or insulated gate bipolar transistor (IGBT) or silicon controlled rectifier (Thyristor), and is capable of switching large currents in a few microseconds. Power MOSFETs, IGBTs and Thyristor conduct current only in one direction. As indicated in Figure 1 there are two types of waveforms: monophasic or biphasic which is commonly found in magnetic stimulation

devices.

Figure 1 The TMS applied field strength gradient to human brain and example of monophasic (Top) and biphasic (Bottom) pulse profile [22]
Standard monophasic stimulators generate an over damped sine current pulse and dissipate the whole pulse energy as heat. In biphasic devices, the oscillation is underdamped, which leads to an almost sinusoidal waveform, and recovery of a substantial amount of the pulse energy [16-19]. Monophasic discharge currents reduce heat dissipation in the coil, and increase the accuracy in stimulation. Additionally monophasic pulse allows for a better understanding of the mechanisms involved in magnetic stimulation. The waveforms of coil current for a typical coil when used in monophasic and biphasic configuration are shown in Figure 1. In general the total energy stored in magnetic stimulators ranges from 500 J to 10 kJ, which is coherent with the standards for obtaining an effective magnetic stimulation based on maximizing the peak coil energy. The high power level can be achieved by using a large energy storage capacitor or by having an efficient energy transfer from the capacitor to the coil. Around 500 J of energy is needed to be transferred from the storage capacitor into the stimulating coil in few microseconds. The impulse power output of a typical magnetic stimulator during the discharge phase is approximately 10 MWatts. During the discharge, energy initially stored in the capacitor in the form of electrostatic charge is converted into magnetic energy in the stimulating coil in approximately 400 μs or longer based on the device characteristics. A fast rate of energy transfer is necessary to achieve a rapid rate of rise of magnetic field. It also needs to be noted that since the magnetic field strength falls off with distance from the stimulating coil, the stimulus strength is at its highest close to the coil surface. The stimulation characteristics of the magnetic pulse, such as depth of penetration, strength and accuracy, depend on the rise time, peak magnetic energy transferred to the coil and the spatial distribution of the field [20-21].
1.2.2. Coil requirements
The stimulating coil is generally enclosed in a plastic cover, which consists of multiple tightly wounded, well insulated copper coils together with other electronic circuitry such as temperature sensors and safety switches. Up to now circular coils with a diameter of 80-100 mm have been the most widely used for magnetic stimulation. In the case of circular coils, the induced tissue current is zero on the central axis of the coil and increases to a maximum in a ring approximately under the diameter of the coil. Stimulation is likely to occur under the winding and not under the coil center. During the stimulating phase, when the magnetic field is increasing from zero to its maximum, the induced current generated in the tissue flows in the opposite direction to the coil current. Although the circular coil is a very useful general purpose coil, the site of stimulation is not well defined. The most notable advancement in coil design has been the double coil (also termed butterfly or figure of eight coil). Double coils utilize two windings normally placed side by side [23], [24], [25], [26-30].
Magnetic field strength is the most widely used figure of merit for the output of magnetic stimulators. Although an important parameter, magnetic field strength is not an accurate measurement of magnetic stimulator performance. Magnetic field strength is defined as the magnetic flux density, however does reflect the total magnetic flux produced by the stimulating coil over its total area. In a small coil, the magnetic flux is concentrated in a small area, the magnetic field intensity will be higher comparatively in a larger coil, but the field decrease much more rapidly with distance. Hence a small coil is somewhat more powerful in the stimulation of superficial nerves and a large coil is more suitable for structures at depth. The amplitude, waveform and spatial characteristics of the induced current are major parameters in magnetic stimulation. A more accurate indicator of the stimulating power output is the induced charge density per phase defined as the integral of the induced current density during the rise time of the magnetic field. It provides a better indicator of output by taking into account the effects of both amplitude and duration of the induced stimulating current [26-30]. Different areas, such as bone structure (grey matter, fat, white matter) with differing conductivities affect the induced current and its path which in turn is based on a choice of coil on magnetic field strength and induced current with a suitability for its intended application must be taken into account [26-30].

1.2.3. Human anatomy
Magnetic stimulation’s most attractive attributes are safety and ease in stimulating most neural structures, unimpeded by fat and bone and without discomfort. The majority of the clinical applications for the technique are for the non-invasive stimulation of the peripheral and central motor pathways. Other uses include stimulation of the left and right prefrontal cortex, visual cortex, language center, cerebellum and peripheral sensory nerves [26-30].

1.3. Conclusion
The main goal of this chapter is to explore through research and understanding of the necessary background information and concept required to design and offer different platform for magnetic stimulation.