**4.2 Principles and technical designs**

For somatosensory mapping, well-controlled and reproducible stimuli are required. Principally, this can be achieved by using pneumatic, piezoceramic and electromechanical devices. Concerning MR compatibility and safety, pneumatic devices are the best choice. The hardware of a pneumatic stimulation device typically consists of a pressure source, a valve for converting the air stream into the desired pressure oscillations and a vibrotactile display to deliver the stimuli to the skin surface. As vibrotactile probe, a latex balloon, a pickup with an integrated rubber membrane (Briggs et al., 2004), or an injector element to produce air puffs was described (Huang and Sereno, 2007). The pressure oscillations are transmitted to the vibrotactile display via long plastic tubes so that all other components of the device can be operated outside of the MR scanner room. However, pneumatic systems have the disadvantage of limited vibration frequencies. Due to the mechanical damping of the pressure oscillations in the plastic tubes the stimulus frequency is limited to about 30 Hz. Higher stimulation frequencies can be achieved by using nonmagnetic valves, suited for operation inside the MR scanner room. Multi-channel stimulation designs are feasible with multiple valves and pickups (Wienbruch et al., 2006). Pneumatic devices have shown to cause somatosensory brain activation, but failed to additionally activate motor cortical areas in somatosensory paradigms.

Piezoceramic devices provide a wide range of frequencies, but only have small displacement amplitudes, which limits their application to the skin receptors. Stimulation frequencies up to 1000 Hz can be obtained. The vibration amplitude achieved by these devices is limited to some hundred µm and even for this relative high operation voltages (up to 200 Volts) are necessary. Because these devices are nonmagnetic, they can be operated inside the MR scanner room. Basically, bar- and disk-like actuators as well as piezomotors are available. The bar- and the disk-like actuators directly convert the

Fig. 1. Overview on MR-compatibility (GE Medical Systems. MR safety and MR compatibility. http://www.ge.com/medical/mr/iomri/safety.htm; 1997).

*200 Gauss Line*

For somatosensory mapping, well-controlled and reproducible stimuli are required. Principally, this can be achieved by using pneumatic, piezoceramic and electromechanical devices. Concerning MR compatibility and safety, pneumatic devices are the best choice. The hardware of a pneumatic stimulation device typically consists of a pressure source, a valve for converting the air stream into the desired pressure oscillations and a vibrotactile display to deliver the stimuli to the skin surface. As vibrotactile probe, a latex balloon, a pickup with an integrated rubber membrane (Briggs et al., 2004), or an injector element to produce air puffs was described (Huang and Sereno, 2007). The pressure oscillations are transmitted to the vibrotactile display via long plastic tubes so that all other components of the device can be operated outside of the MR scanner room. However, pneumatic systems have the disadvantage of limited vibration frequencies. Due to the mechanical damping of the pressure oscillations in the plastic tubes the stimulus frequency is limited to about 30 Hz. Higher stimulation frequencies can be achieved by using nonmagnetic valves, suited for operation inside the MR scanner room. Multi-channel stimulation designs are feasible with multiple valves and pickups (Wienbruch et al., 2006). Pneumatic devices have shown to cause somatosensory brain activation, but failed to additionally activate motor cortical areas

EFFECTS ON IMAGING

mechatronic devices with ferromagnetic elements

• induced static magnetization • induced currents (dB/dt & HF) • torque and translational force

Piezoceramic devices provide a wide range of frequencies, but only have small displacement amplitudes, which limits their application to the skin receptors. Stimulation frequencies up to 1000 Hz can be obtained. The vibration amplitude achieved by these devices is limited to some hundred µm and even for this relative high operation voltages (up to 200 Volts) are necessary. Because these devices are nonmagnetic, they can be operated inside the MR scanner room. Basically, bar- and disk-like actuators as well as piezomotors are available. The bar- and the disk-like actuators directly convert the

**4.2 Principles and technical designs** 

• static field inhomogenity (dephasing, signal voids) • emissions from DC-DC converters (low S/R-ratio)

EFFECTS ON DEVICE

**MRT**

in somatosensory paradigms.

electrical signal into bending motions (Piezomechanik Gmbh, 2002). For stimulation applications, these devices can either be held between the fingertips (Harrington et al., 2000b) or touched by the fingertips (Maldjian et al., 1999b). Functional MRI with piezoceramic vibrators showed brain activation within the somatosensory cortex but not within motor cortical areas (Harrington et al., 2000a). It is important to avoid loops in the cables, because this may lead to currents from the RF- and gradient coils. These may cause heat and even fire hazard. There is less data with piezomotors. Basically, piezomotors are well suited for construction of MR compatible robotic stimulation devices, for example to induce passive limb motions. For their operation, high driving frequencies (> 40 kHz) are necessary, therefore effects on the MR imaging have to be considered, when using such devices (Chinzei et al., 1999).

Electromagnetic stimulation devices may be classified into three groups depending on the vicinity to the MR scanner at their operation. In the first group, there is common standard equipment containing motors or actuators with pulsed electronics. Such equipment causes RF-emission and therefore has to be operated outside the shielded area of the MR scanner room. For vibrotactile stimulation long cables are needed to transmit the stimulus from the outside to the subject. Cable driven rotating eccentric masses are an example for such type of stimulation device. A frequency range between 1–130 Hz and displacement amplitudes up to 4 mm can be reached. In an fMRI study implementing this technique, BOLD responses within the somatosensory as well as the motor cortical areas could be demonstrated (Golaszewski et al., 2002b). The second group consists of non-switched moving magnet, and moving coil devices, which can be operated inside the scanner behind the 20-mT line (Golaszewski et al., 2006;Gallasch et al., 2006). With this technique, the parameters of a stimulus (amplitude, frequency, waveform) can be selected within a wide range, which is advantageous for basic investigations. On the other hand, these devices also need some mechanics for translating the stimulus to the subject under investigation. When these mechanic parts are made of metallic materials, the device will also influence the imaging and itself be influenced by the magnetic field. In the third group of somatosensory stimulation, the devices comprise coil actuators utilizing the static magnetic field of the MR scanner. By applying currents to a coil, Lorenz forces generate vibration (Graham et al., 2001), as well as load and movement (Riener et al., 2005). This type of actuator-stimulator is suited for the operation inside the MR scanner, but it is important to be careful in order to prevent heating of the coils due to induced currents.

#### **4.3 Device for stimulation of the foot sole**

A recently developed stimulator for the foot sole is described here as an example for an electromechanical device to be operated inside the scanner room (Gallasch et al., 2006). It consists of two moving magnet actuators rigidly connected on a platform by two nonmagnetic adjustable stands (see Figure 2). To preserve MR compatibility (operation behind the 20-mT line) the foot sole is contacted via long indentors (30 cm). Further, to avoid effects on imaging, the actuators are powered by non-pulsed servo amplifiers. All other components containing pulsed electronics (digital controller and PC) are operated outside the MR scanner room. For stimulation of slowly and rapid adapting mechanoreceptors a mixed open and closed loop control scheme was implemented. Slowly adapting receptors respond to nearly static loading (0 - 1 Hz). This is achieved by

Somatosensory Stimulation in Functional Neuroimaging: A Review 345

Fig. 3. Drawing of finger cuff with inflatable air bladder (1), flexible Welco strips (2, 3) and

Fig. 4. Stimulator system consisting of twin finger cuff, valve box and microprocessor unit.

air connector (4)

an open loop programming of the contact force (0- 20 N). Rapid adapting receptors respond to vibration, which is achieved by the closed loop control scheme. With the implemented controller arbitrary vibration waveforms within the frequency band of 20 to 100 Hz can be generated. A computer is used for stimulus synthesis, sequencing of the stimuli and synchronization with the MR scans. The first MRI studies with this device show that specially designed electromagnetic devices are well suited for somatotopic mapping.

Fig. 2. Example of an electromagnetic vibrotactile stimulation system (Gallasch et al., 2006).

#### **4.4 Perspectives**

Recently, various types of stimulation devices were evaluated for somatotopic mapping. Although substantial physiological results have been obtained with some of these devices, this technology still needs to be improved. Clinicians expect equipment for quantitative sensory testing, which is safe and simple to use. Other systems will be needed for stimulation of the entire spectrum of somatosensory fibers. These are the large diameter Abeta fibers mediating touch and vibration, the smaller A-delta fibers mediating cool sensation and the first signs of pain, and the small diameter C-fibers mediating sensation of heat and pain. We therefore suggest a bimodal stimulation system to deliver with both vibrotactile and temperature stimuli. For the sole of the foot, such a system may have an arrangement as shown in Figure 2 with additional Peltier elements on the tip of the indentors, however with pneumatic actuators instead of the electromagnetic ones. For hand and fingers wearable stimulation devices are prospective, e.g. pneumatic finger or toe cuffs (Gallasch et al., 2010;Figure 3, 4, 5) or some kind of stimulation glove with pressurized sections at the fingertips including flat shaped heat pipes for quick cooling and warming. For the usage as a clinical tool, further multicenter studies with standardized stimulation protocols have to be carried out. Such studies are necessary to establish stable stimulusresponse relationships independent of a certain scanner type.

an open loop programming of the contact force (0- 20 N). Rapid adapting receptors respond to vibration, which is achieved by the closed loop control scheme. With the implemented controller arbitrary vibration waveforms within the frequency band of 20 to 100 Hz can be generated. A computer is used for stimulus synthesis, sequencing of the stimuli and synchronization with the MR scans. The first MRI studies with this device show that specially designed electromagnetic devices are well suited for somatotopic

Fig. 2. Example of an electromagnetic vibrotactile stimulation system (Gallasch et al., 2006).

Recently, various types of stimulation devices were evaluated for somatotopic mapping. Although substantial physiological results have been obtained with some of these devices, this technology still needs to be improved. Clinicians expect equipment for quantitative sensory testing, which is safe and simple to use. Other systems will be needed for stimulation of the entire spectrum of somatosensory fibers. These are the large diameter Abeta fibers mediating touch and vibration, the smaller A-delta fibers mediating cool sensation and the first signs of pain, and the small diameter C-fibers mediating sensation of heat and pain. We therefore suggest a bimodal stimulation system to deliver with both vibrotactile and temperature stimuli. For the sole of the foot, such a system may have an arrangement as shown in Figure 2 with additional Peltier elements on the tip of the indentors, however with pneumatic actuators instead of the electromagnetic ones. For hand and fingers wearable stimulation devices are prospective, e.g. pneumatic finger or toe cuffs (Gallasch et al., 2010;Figure 3, 4, 5) or some kind of stimulation glove with pressurized sections at the fingertips including flat shaped heat pipes for quick cooling and warming. For the usage as a clinical tool, further multicenter studies with standardized stimulation protocols have to be carried out. Such studies are necessary to establish stable stimulus-

response relationships independent of a certain scanner type.

mapping.

**4.4 Perspectives** 

Fig. 3. Drawing of finger cuff with inflatable air bladder (1), flexible Welco strips (2, 3) and air connector (4)

Fig. 4. Stimulator system consisting of twin finger cuff, valve box and microprocessor unit.

Somatosensory Stimulation in Functional Neuroimaging: A Review 347

In the studies of Gelnar, Harrington, and Stippich et al., brain activation within the postcentral gyrus and superior and inferior parietal lobule have been found (Gelnar et al., 1998a;Harrington et al., 2000a;Stippich et al., 1999a). Furthermore, brain activation within Brodmann area 3a was detected due to somatosensory stimulation (Geyer et al., 1999;Geyer et al., 2000;Kurth et al., 2000), which can be explained by the fact that Brodmann area 3a receives input from the deep and from the proprioceptive receptors (Ibanez et al., 1989;Iwamura et al., 1993;Kaas et al., 1979;Maldjian et al., 1999a;Recanzone et al., 1992;Tharin and Golby, 2007). BOLD response in the primary motor cortex due to vibrotactile stimulation is an important finding, because the stimulation does not require the collaboration of the subject under examination. In an fMRI study with mechanical vibration, BOLD response in primary sensorimotor cortex was found in all of the investigations (Golaszewski et al., 2002a,b). Motor cortical activation caused by vibration, is presumably based on the co-stimulation of cutaneous mechanoreceptors and muscle spindles that requires sufficient displacement amplitudes and vibration frequencies. Similar to the fingerto-thumb-tapping paradigm, vibration led to contralateral brain activity in postcentral gyrus in ten out of ten subjects. Vibration stimulation failed to consistently activate supplementary motor area and anterior cingular cortex since it represents a passive paradigm that does not involve motor cortical areas for planning of volitional movements. Vibratory stimuli are transmitted via the large afferents of the dorsal column to the thalamus and are relayed there to the brain cortex. This "information" originates from the extra personnel space that might be an explanation, why Brodmann area 9 in superior frontal gyrus responds with

In functional brain imaging with certain somatosensory stimulation protocols the whole sensorimotor cortex can be addressed for functional brain mapping that offers the possibility of several clinical applications for somatosensory paradigms in Neuroradiology. Somatosensory paradigms can be used for preoperative functional brain mapping of the sensorimotor cortex in patients with perirolandic lesions. Further applications include the investigation of brain plasticity and reorganization (Pons et al., 1992) and investigation of

Apkarian, A.V., Gelnar, P.A., Krauss, B.R., and Szeverenyi, N.M. (2000). Cortical responses

Arthurs, O.J., Williams, E.J., Carpenter, T.A., Pickard, J.D., and Boniface, S.J. (2000). Linear

current amplitude and selective attention. Clin Neurophysiol *111*, 1738-44. Bittar, R.G., Olivier, A., Sadikot, A.F., Andermann, F., Comeau, R.M., Cyr, M., Peters, T.M.,

amplitude in human somatosensory cortex. Neuroscience *101*, 803-6. Backes, W.H., Mess, W.H., van Kranen-Mastenbroek, V., and Reulen, J.P. (2000).

to thermal pain depend on stimulus size: a functional MRI study. J Neurophysiol

coupling between functional magnetic resonance imaging and evoked potential

Somatosensory cortex responses to median nerve stimulation: fMRI effects of

and Reutens, D.C. (1999a). Localization of somatosensory function by using

patients in comatose and vegetative state (Kampfl et al., 1998).

**5. Perspective of the application of somatosensory stimulation within the** 

**clinical environment** 

activation in some cases.

**6. References** 

*83*, 3113-22.

Fig. 5. Single subject analysis: fMRI maps of eight single subjects (1-8) applying pneumatic cuff somatosensory finger stimulation with fixed (fixed simulation FS, green) and random (random stimulation RS, red) presentation of vibrotactile stimuli with a mean frequency of 4 Hz over all blocks. Yellow spots represent activation overlap between FS and RS maps

Fig. 6. Patient in vegetative state 14 days post hypoxia. Vibration stimulation with a moving magnet actuator system delivered to the sole of the left foot (Gallasch et al., 2006;Golaszewski et al., 2006) elicits brain activation contra- and ipsilaterally within the primary and secondary sensorimotor cortex and especially within the premotor cortex, the center for predefined movement loops, and the supplementary motor area that represents the superior center for motor planning. Functional brain mapping in this patient proved an intact somatosensory channel to the sensorimotor system for a targeted therapeutic approach in neurorehabilitation.
