**3. Functional Magnetic Resonance Imaging**

Somatosensory stimuli were applied in many functional magnetic resonance imaging (fMRI) studies. Especially light stimulation using air puffs (Stippich et al., 1999b) or other tactile stimuli (Hodge et al., 1998;Moore et al., 2000;Rausch et al., 1998;Servos et al., 1998), scratching of the hand palm (Hoeller et al., 2002), vibration (Gelnar et al., 1998b;Golaszewski

Somatosensory Stimulation in Functional Neuroimaging: A Review 339

amplitude of vibrotactile stimuli delivered to the volar surface of the right index finger and BOLD activity in the primary somatosensory area that persisted during an attentiondemanding tactile tracking task (Nelson et al., 2004). The secondary somatosensory cortex did not show any clear relationship with the vibration amplitude, but was more often activated during the attention demanding tracking task compared with passive vibration. Responses in secondary areas seem to be less influenced by these variables, but are probably more dependant of the level of attention directed to the stimulus (Apkarian et al., 2000;Backes et al., 2000;Peyron et al., 2000;Takanashi et al., 2001) and on whether stimulation is delivered uni- or bilaterally (Apkarian et al., 2000;Backes et al., 2000;Disbrow et al., 2001;Peyron et al., 2000;Takanashi et al., 2001). Regarding attentional phenomena interfering with somatosensory processing, tactile processing while varying the focus of attention was studied. Activations were contrasted between attend and ignore conditions, both of which employed identical stimulation characteristics and an active task. Random effects analysis revealed significant attention effects in the primary somatosensory area. The blood oxygenation level-dependent response was greater for attended than for ignored stimuli. Modulations were also found in the secondary somatosensory cortex and the middle temporal gyrus. These findings suggest that the stimulus processing at the level of the primary representations in the primary

Somatosensory stimulation has the advantage of not requiring movement which may cause artifacts. With somatosensory stimulation (repetitive brushing of the hand palm) in brain tumor patients, a lower incidence of severe movement artifacts was found compared to an active motor paradigm (finger-to-thumb-tapping), however, the motor paradigm elicited a significantly higher percentage of signal increases. (Apkarian et al., 2000;Backes et al., 2000;Disbrow et al., 2001;Hoeller et al., 2002;Peyron et al., 2000;Takanashi et al., 2001). Several fMRI studies discovered a similar functional localisation comparing somatosensory stimulation and active motor paradigms (Golaszewski et al., 2002;Golaszewski et al., 2006;Golaszewski et al., 2002;Lee et al., 1998). Lee et al. (1998) demonstrated in an fMRI study similar results with active and passive activation tasks by comparing palm-finger brushing with sponge-squeezing and active finger movements according to their functional localisation. The sensorimotor and somatosensory BOLD responses were located to a large extent in the postcentral gyrus, and their spatial locations were not significantly different. Golaszewski et al. showed largely similar functional maps by active finger-to-thumb tapping and vibration of the hand palm (Golaszewski et al., 2002a;Golaszewski et al., 2002b). In patients who are physically unable to perform active finger-to-thumb-tapping, hand-squeezing or fist clenching as sensorimotor activation tasks the vibration of the hand palm can be regarded as a proper paradigm in presurgical

In an fMRI study with a piezoelectric vibration device Francis et al. found a frequency dependence of the primary and secondary somatosensory area (Francis et al., 2000). With both frequencies applied to the index finger during the same scanning session, an increase in the vibration frequency from 30 to 80 Hz showed a significant increase of the BOLD response within the secondary somatosensory area and the posterior insula, while the

Moreover, functional imaging studies are important for the monitoring of rehabilitation and the understanding of motor recovery after cortical strokes (Cramer et al., 2000). Functional MRI was used to compare sensory and motor maps obtained in normal controls with

number of pixels activated in the primary somatosensory area declined.

somatosensory area is modulated by attention (Sterr et al., 2007).

fMRI mapping of the sensorimotor hand area.

et al., 2002;Golaszewski et al., 2006;Golaszewski et al., 2002;Hodge et al., 1998), electrical stimulation (Arthurs et al., 2000;Backes et al., 2000;Korvenoja et al., 1999;Krause et al., 2001;Kurth et al., 1998;Takanashi et al., 2001), noxious stimuli (Apkarian et al., 2000;Peyron et al., 2000), and proprioception induced by passive joint movement (Rausch et al., 1998) were used. Usually, the primary somatosensory cortex in the postcentral gyrus and the secondary somatosensory cortex in the parietal operculum, insula, and more posterior ventral parietal areas are activated. A clear somatotopic organization in the primary somatosensory cortex could be demonstrated, whereas this somatotopic organization could not be clearly shown in the secondary somatosensory area (Disbrow et al., 2000;Gelnar et al., 1998b;Hodge et al., 1998;Krause et al., 2001;Kurth et al., 1998;Servos et al., 1998). An evident somatotopy also in the secondary somatosensory area was demonstrated in a study by Gelnar et al. (Gelnar et al., 1998c). A vibratory stimulus was applied to an individual digit tip (digit 1, 2, or 5) on the right hand of healthy adults which led to a BOLD response in cortical regions located on the upper bank of the Sylvian fissure, the insula, and the posterior parietal cortices. Multiple digit representations were observed in the primary somatosensory cortex, corresponding to the four anatomic subdivisions areas 3a, 3b, 1, and 2. There was no simple medial to lateral somatotopic representation in individual fMRI maps but a clear spatial distance between digit 1 and digit 5 was seen on the cortex in both the primary and secondary somatosensory regions. Ruben et al. was able to demonstrate a somatotopic organization of the secondary somatosensory area with electrical stimulation of the right hallux, the index and the fifth finger (Ruben et al., 2001). They were not able to observe separate representations of digit 2 and 5 in the secondary somatosensory area, but a somatotopic representation between the fingers and the hallux could be detected bilaterally within the secondary somatosensory region. Kurth et al. demonstrated a somatotopy in the primary somatosensory cortex by using electrical finger stimulation (Kurth et al., 2000). Functional MRI detected separate representations for all five fingers in the primary somatosensory cortex. Responses were located in the posterior wall of the deep central sulcus (corresponding to Brodmann area 3b), and the anterior (Brodmann area 1) or the posterior crown of the postcentral gyrus (Brodmann area 2) with rare activations in Brodmann area 3a and 4. In Brodmann area 3b, a regular somatotopic mediolateral digit arrangement for fingers 5 to 1 with a mean Euclidean distance of 16 mm between fingers 1 and 5 was found. In contrast, Brodmann area 1 and 2 showed a greater number of adjacent activation foci with a significantly greater overlap and partly even reversed ordering of the neighboring fingers. This paradigm can be used to localize the central sulcus preoperatively (Kurth et al., 1998) and it is applicable even in patients with severe hemiparesis without severe hemianesthesia.

In many studies investigating the primary somatosensory cortex, only a contralateral BOLD response could be elicited, whereas the secondary somatosensory areas were activated bilaterally (Backes et al., 2000;Disbrow et al., 2001;Korvenoja et al., 1999). It is still uncertain, what stimulus leads to the most robust BOLD response within the somatosensory cortex. There is evidence that pain stimuli are less reliable than vibrotactile or electrical stimuli for evoking primary somatosensory activation (Backes et al., 2000;Disbrow et al., 2001;Korvenoja et al., 1999;Peyron et al., 2000). Activation magnitude in the primary somatosensory cortex depends on the intensity of stimulation (Arthurs et al., 2000;Krause et al., 2001), the size of the stimulated body surface (Apkarian et al., 2000;Peyron et al., 2000), and the rate of stimulation (Apkarian et al., 2000;Peyron et al., 2000;Takanashi et al., 2001). Nelson et al. demonstrated an increasing stimulus-response relationship between the

et al., 2002;Golaszewski et al., 2006;Golaszewski et al., 2002;Hodge et al., 1998), electrical stimulation (Arthurs et al., 2000;Backes et al., 2000;Korvenoja et al., 1999;Krause et al., 2001;Kurth et al., 1998;Takanashi et al., 2001), noxious stimuli (Apkarian et al., 2000;Peyron et al., 2000), and proprioception induced by passive joint movement (Rausch et al., 1998) were used. Usually, the primary somatosensory cortex in the postcentral gyrus and the secondary somatosensory cortex in the parietal operculum, insula, and more posterior ventral parietal areas are activated. A clear somatotopic organization in the primary somatosensory cortex could be demonstrated, whereas this somatotopic organization could not be clearly shown in the secondary somatosensory area (Disbrow et al., 2000;Gelnar et al., 1998b;Hodge et al., 1998;Krause et al., 2001;Kurth et al., 1998;Servos et al., 1998). An evident somatotopy also in the secondary somatosensory area was demonstrated in a study by Gelnar et al. (Gelnar et al., 1998c). A vibratory stimulus was applied to an individual digit tip (digit 1, 2, or 5) on the right hand of healthy adults which led to a BOLD response in cortical regions located on the upper bank of the Sylvian fissure, the insula, and the posterior parietal cortices. Multiple digit representations were observed in the primary somatosensory cortex, corresponding to the four anatomic subdivisions areas 3a, 3b, 1, and 2. There was no simple medial to lateral somatotopic representation in individual fMRI maps but a clear spatial distance between digit 1 and digit 5 was seen on the cortex in both the primary and secondary somatosensory regions. Ruben et al. was able to demonstrate a somatotopic organization of the secondary somatosensory area with electrical stimulation of the right hallux, the index and the fifth finger (Ruben et al., 2001). They were not able to observe separate representations of digit 2 and 5 in the secondary somatosensory area, but a somatotopic representation between the fingers and the hallux could be detected bilaterally within the secondary somatosensory region. Kurth et al. demonstrated a somatotopy in the primary somatosensory cortex by using electrical finger stimulation (Kurth et al., 2000). Functional MRI detected separate representations for all five fingers in the primary somatosensory cortex. Responses were located in the posterior wall of the deep central sulcus (corresponding to Brodmann area 3b), and the anterior (Brodmann area 1) or the posterior crown of the postcentral gyrus (Brodmann area 2) with rare activations in Brodmann area 3a and 4. In Brodmann area 3b, a regular somatotopic mediolateral digit arrangement for fingers 5 to 1 with a mean Euclidean distance of 16 mm between fingers 1 and 5 was found. In contrast, Brodmann area 1 and 2 showed a greater number of adjacent activation foci with a significantly greater overlap and partly even reversed ordering of the neighboring fingers. This paradigm can be used to localize the central sulcus preoperatively (Kurth et al., 1998) and it is applicable even in patients with severe hemiparesis without

In many studies investigating the primary somatosensory cortex, only a contralateral BOLD response could be elicited, whereas the secondary somatosensory areas were activated bilaterally (Backes et al., 2000;Disbrow et al., 2001;Korvenoja et al., 1999). It is still uncertain, what stimulus leads to the most robust BOLD response within the somatosensory cortex. There is evidence that pain stimuli are less reliable than vibrotactile or electrical stimuli for evoking primary somatosensory activation (Backes et al., 2000;Disbrow et al., 2001;Korvenoja et al., 1999;Peyron et al., 2000). Activation magnitude in the primary somatosensory cortex depends on the intensity of stimulation (Arthurs et al., 2000;Krause et al., 2001), the size of the stimulated body surface (Apkarian et al., 2000;Peyron et al., 2000), and the rate of stimulation (Apkarian et al., 2000;Peyron et al., 2000;Takanashi et al., 2001). Nelson et al. demonstrated an increasing stimulus-response relationship between the

severe hemianesthesia.

amplitude of vibrotactile stimuli delivered to the volar surface of the right index finger and BOLD activity in the primary somatosensory area that persisted during an attentiondemanding tactile tracking task (Nelson et al., 2004). The secondary somatosensory cortex did not show any clear relationship with the vibration amplitude, but was more often activated during the attention demanding tracking task compared with passive vibration. Responses in secondary areas seem to be less influenced by these variables, but are probably more dependant of the level of attention directed to the stimulus (Apkarian et al., 2000;Backes et al., 2000;Peyron et al., 2000;Takanashi et al., 2001) and on whether stimulation is delivered uni- or bilaterally (Apkarian et al., 2000;Backes et al., 2000;Disbrow et al., 2001;Peyron et al., 2000;Takanashi et al., 2001). Regarding attentional phenomena interfering with somatosensory processing, tactile processing while varying the focus of attention was studied. Activations were contrasted between attend and ignore conditions, both of which employed identical stimulation characteristics and an active task. Random effects analysis revealed significant attention effects in the primary somatosensory area. The blood oxygenation level-dependent response was greater for attended than for ignored stimuli. Modulations were also found in the secondary somatosensory cortex and the middle temporal gyrus. These findings suggest that the stimulus processing at the level of the primary representations in the primary somatosensory area is modulated by attention (Sterr et al., 2007).

Somatosensory stimulation has the advantage of not requiring movement which may cause artifacts. With somatosensory stimulation (repetitive brushing of the hand palm) in brain tumor patients, a lower incidence of severe movement artifacts was found compared to an active motor paradigm (finger-to-thumb-tapping), however, the motor paradigm elicited a significantly higher percentage of signal increases. (Apkarian et al., 2000;Backes et al., 2000;Disbrow et al., 2001;Hoeller et al., 2002;Peyron et al., 2000;Takanashi et al., 2001). Several fMRI studies discovered a similar functional localisation comparing somatosensory stimulation and active motor paradigms (Golaszewski et al., 2002;Golaszewski et al., 2006;Golaszewski et al., 2002;Lee et al., 1998). Lee et al. (1998) demonstrated in an fMRI study similar results with active and passive activation tasks by comparing palm-finger brushing with sponge-squeezing and active finger movements according to their functional localisation. The sensorimotor and somatosensory BOLD responses were located to a large extent in the postcentral gyrus, and their spatial locations were not significantly different. Golaszewski et al. showed largely similar functional maps by active finger-to-thumb tapping and vibration of the hand palm (Golaszewski et al., 2002a;Golaszewski et al., 2002b). In patients who are physically unable to perform active finger-to-thumb-tapping, hand-squeezing or fist clenching as sensorimotor activation tasks the vibration of the hand palm can be regarded as a proper paradigm in presurgical fMRI mapping of the sensorimotor hand area.

In an fMRI study with a piezoelectric vibration device Francis et al. found a frequency dependence of the primary and secondary somatosensory area (Francis et al., 2000). With both frequencies applied to the index finger during the same scanning session, an increase in the vibration frequency from 30 to 80 Hz showed a significant increase of the BOLD response within the secondary somatosensory area and the posterior insula, while the number of pixels activated in the primary somatosensory area declined.

Moreover, functional imaging studies are important for the monitoring of rehabilitation and the understanding of motor recovery after cortical strokes (Cramer et al., 2000). Functional MRI was used to compare sensory and motor maps obtained in normal controls with

Somatosensory Stimulation in Functional Neuroimaging: A Review 341

response. Second, by selecting the site and frequency of the stimulus, the different receptor types (cutaneous mechanoreceptors, proprioceptors, thermo receptors) can be specifically excited and their functional integration at the cortical level can be studied. Third, the stimulus response underlies adaptation which can be used to analyze the somatosensory information processing, its influence to cortical structures, and the modulation by other brain regions (Giabbiconi et al., 2007). On the other hand, a cortical response may be affected adversely by somatosensory adaptation phenomena. This has to be considered when

In clinical routine, the vibrotactile sense is assessed by brushing on a certain body region (Frey hair) or by using a tuning fork. These manual stimulations were used in the earlier studies of somatotopic mapping (Polonara et al., 1999). However, for more complex stimulation designs, it is more convenient to use quantitative testing equipment. Within the past ten years, various prototypes of stimulation devices have been tested for somatotopic mapping. Among these devices, pneumatically driven air bags were introduced (Gelnar et al., 1998b;Golaszewski et al., 2002a;Stippich et al., 1999b), as well as piezodisks (Harrington et al., 2000b;Maldjian et al., 1999b), cable driven rotating masses (Golaszewski et al., 2002b) and even coil designs using the static magnetic field of an MR scanner (Graham et al., 2001). As most of these devices were used in fMRI-paradigms, the interactions between MRI and the certain stimulation device must be considered. In this chapter, we first focus on the MR compatibility and the MR safety and subsequently give an overview on the different types

According to the safety guidelines by General Electric (GE) Medical Systems (GE-Medical Systems, 1997), a device is considered to be MR save, if it can be demonstrated that it does not lead to an increased safety risk towards the patient and the staff, when the device is introduced or used in the MR scanner room. For a certain device to be labeled MR compatible, it has to be demonstrated that it performs in its intended function without performance degradation. For the MR compatibility, effects on the devices and effects on the imaging have to be differentiated (Chinzei et al., 1999). These devices are influenced by induced static magnetization as well as torque and translational forces (see Figure 1). Both effects influence the performance of devices containing ferromagnetic materials. Standard springs made of metal do not function as expected. According to the guidelines mentioned above, devices containing ferromagnetic materials should be operated behind the 20-mT line. In this zone, the effects on the devices are irrelevant. However, the risk that such a device is pulled towards the scanner bore (projectile effect) still is high. For safety reasons, not permanently fixed electromagnetic devices should be operated only behind the 5-mT line. The imaging quality is degraded by field inhomogenities and RF (radio frequency) emission (see Figure 1). Static field inhomogenities come from the ferromagnetic materials contained in the devices, but in most cases the image quality can be restored after shimming the magnet. RF is typically produced by pulsed electronics and the digital hardware emitted by the cables of the device. As MRI is highly sensitive to RF noise, such devices have to be operated outside the MR scanner room. On the other hand, small amplitude electromagnetic fields up to some hundred Hz, as produced by some vibrotactile stimulation devices, only

designing a specific stimulation protocol.

**4.1 MR compatibility and MR safety** 

showed minor effects on imaging.

of devices.

functional maps from two patients with good recovery six months after a cortical stroke. Cortical map reorganization along the detected infarct rim might be an important contributor to recovery of motor and sensory function after stroke. Moreover, functional imaging studies with somatosensory stimulation are also important for the monitoring of the rehabilitation after extremity transplantations (Piza, 2000). A close relationship between the intensity of phantom limb pain in amputees and the amount of reorganization of the somatosensory cortex was reported in fMRI studies (Flor et al., 2001;Hamzei et al., 2001;Koppelstaetter et al., 2007).

Functional MRI was also used to investigate brain activations underlying menthol-induced cold allodynia (Seifert and Maihöfner, 2007). Healthy volunteers were investigated using a block-design fMRI approach. Brain activity was measured during application of innocuous cold stimuli (5°C above cold pain threshold) and noxious cold stimuli (5°C below cold pain threshold) to the skin of the forearm using a peltier-driven thermostimulator. The stimuli were adjusted to the individual cold pain threshold. Cold allodynia was induced by topical menthol and cortical activations were measured during previously innocuous cold stimulation (5°C) that was at this situation perceived as painful. On a numeric rating scale for pain (0-10) innocuous cold, cold pain and cold allodynia were rated. Sensory and affective components of allodynia and cold pain were equal in the McGill pain questionnaire (Roelcke et al., 1997b). All tested conditions (innocuous cold, noxious cold and cold allodynia) led to significant activations of the bilateral insular cortices, the bilateral frontal cortices and the anterior cingulate cortex. When compared with innocuous cold, noxious cold led to significantly more activations of the posterior insula and to less activations of the ipsilateral insular cortex.

Significantly increased activations in bilateral dorsolateral prefrontal cortices and brainstem (ipsilateral parabrachial nucleus) were found during cold allodynia when compared with equally intense cold pain conditions. Cold allodynia led to significantly more activations of the bilateral anterior insula, whereas the activation of the contralateral posterior insula was equal. It was concluded that cold allodynia activates a network similar to that of normal cold pain, but additionally recruits bilateral dorsolateral prefrontal cortex and the midbrain, suggesting that these brain areas are involved in central nociceptive sensitization processes.

In the authors' facility, somatosensory stimulation is also used in the clinical routine to assess patients with chronic disorders of consciousness (unresponsive wakefulness syndrome, minimally conscious state) in fMRI (unpublished data). In a series of 22 consecutive patients with chronic disorders of consciousness, seven patients showed reliable response in typical brain areas using a pneumatic activation device (Figure 3, 4, 6). The above-mentioned somatosensory assessment combined with cognitive testing in functional neuroimaging is routinely acquired in patients with chronic disorders of consciousness for the planning of neurorehabilitation and estimation of prognosis.

### **4. Current devices for somatosensory stimulation**

Next to the classical electrical nerve stimulation, vibrotactile stimulation has become very common in functional brain imaging. Vibrotactile stimulation has several advantages over electrical stimulation. First, the stimuli are not painful and therefore a certain stimulus can be presented over a long time period. This is often necessary to obtain a stable cortical

functional maps from two patients with good recovery six months after a cortical stroke. Cortical map reorganization along the detected infarct rim might be an important contributor to recovery of motor and sensory function after stroke. Moreover, functional imaging studies with somatosensory stimulation are also important for the monitoring of the rehabilitation after extremity transplantations (Piza, 2000). A close relationship between the intensity of phantom limb pain in amputees and the amount of reorganization of the somatosensory cortex was reported in fMRI studies (Flor et al., 2001;Hamzei et al.,

Functional MRI was also used to investigate brain activations underlying menthol-induced cold allodynia (Seifert and Maihöfner, 2007). Healthy volunteers were investigated using a block-design fMRI approach. Brain activity was measured during application of innocuous cold stimuli (5°C above cold pain threshold) and noxious cold stimuli (5°C below cold pain threshold) to the skin of the forearm using a peltier-driven thermostimulator. The stimuli were adjusted to the individual cold pain threshold. Cold allodynia was induced by topical menthol and cortical activations were measured during previously innocuous cold stimulation (5°C) that was at this situation perceived as painful. On a numeric rating scale for pain (0-10) innocuous cold, cold pain and cold allodynia were rated. Sensory and affective components of allodynia and cold pain were equal in the McGill pain questionnaire (Roelcke et al., 1997b). All tested conditions (innocuous cold, noxious cold and cold allodynia) led to significant activations of the bilateral insular cortices, the bilateral frontal cortices and the anterior cingulate cortex. When compared with innocuous cold, noxious cold led to significantly more activations of the posterior insula and to less

Significantly increased activations in bilateral dorsolateral prefrontal cortices and brainstem (ipsilateral parabrachial nucleus) were found during cold allodynia when compared with equally intense cold pain conditions. Cold allodynia led to significantly more activations of the bilateral anterior insula, whereas the activation of the contralateral posterior insula was equal. It was concluded that cold allodynia activates a network similar to that of normal cold pain, but additionally recruits bilateral dorsolateral prefrontal cortex and the midbrain, suggesting that these brain areas are involved in

In the authors' facility, somatosensory stimulation is also used in the clinical routine to assess patients with chronic disorders of consciousness (unresponsive wakefulness syndrome, minimally conscious state) in fMRI (unpublished data). In a series of 22 consecutive patients with chronic disorders of consciousness, seven patients showed reliable response in typical brain areas using a pneumatic activation device (Figure 3, 4, 6). The above-mentioned somatosensory assessment combined with cognitive testing in functional neuroimaging is routinely acquired in patients with chronic disorders of consciousness for

Next to the classical electrical nerve stimulation, vibrotactile stimulation has become very common in functional brain imaging. Vibrotactile stimulation has several advantages over electrical stimulation. First, the stimuli are not painful and therefore a certain stimulus can be presented over a long time period. This is often necessary to obtain a stable cortical

2001;Koppelstaetter et al., 2007).

activations of the ipsilateral insular cortex.

central nociceptive sensitization processes.

the planning of neurorehabilitation and estimation of prognosis.

**4. Current devices for somatosensory stimulation** 

response. Second, by selecting the site and frequency of the stimulus, the different receptor types (cutaneous mechanoreceptors, proprioceptors, thermo receptors) can be specifically excited and their functional integration at the cortical level can be studied. Third, the stimulus response underlies adaptation which can be used to analyze the somatosensory information processing, its influence to cortical structures, and the modulation by other brain regions (Giabbiconi et al., 2007). On the other hand, a cortical response may be affected adversely by somatosensory adaptation phenomena. This has to be considered when designing a specific stimulation protocol.

In clinical routine, the vibrotactile sense is assessed by brushing on a certain body region (Frey hair) or by using a tuning fork. These manual stimulations were used in the earlier studies of somatotopic mapping (Polonara et al., 1999). However, for more complex stimulation designs, it is more convenient to use quantitative testing equipment. Within the past ten years, various prototypes of stimulation devices have been tested for somatotopic mapping. Among these devices, pneumatically driven air bags were introduced (Gelnar et al., 1998b;Golaszewski et al., 2002a;Stippich et al., 1999b), as well as piezodisks (Harrington et al., 2000b;Maldjian et al., 1999b), cable driven rotating masses (Golaszewski et al., 2002b) and even coil designs using the static magnetic field of an MR scanner (Graham et al., 2001). As most of these devices were used in fMRI-paradigms, the interactions between MRI and the certain stimulation device must be considered. In this chapter, we first focus on the MR compatibility and the MR safety and subsequently give an overview on the different types of devices.

#### **4.1 MR compatibility and MR safety**

According to the safety guidelines by General Electric (GE) Medical Systems (GE-Medical Systems, 1997), a device is considered to be MR save, if it can be demonstrated that it does not lead to an increased safety risk towards the patient and the staff, when the device is introduced or used in the MR scanner room. For a certain device to be labeled MR compatible, it has to be demonstrated that it performs in its intended function without performance degradation. For the MR compatibility, effects on the devices and effects on the imaging have to be differentiated (Chinzei et al., 1999). These devices are influenced by induced static magnetization as well as torque and translational forces (see Figure 1). Both effects influence the performance of devices containing ferromagnetic materials. Standard springs made of metal do not function as expected. According to the guidelines mentioned above, devices containing ferromagnetic materials should be operated behind the 20-mT line. In this zone, the effects on the devices are irrelevant. However, the risk that such a device is pulled towards the scanner bore (projectile effect) still is high. For safety reasons, not permanently fixed electromagnetic devices should be operated only behind the 5-mT line. The imaging quality is degraded by field inhomogenities and RF (radio frequency) emission (see Figure 1). Static field inhomogenities come from the ferromagnetic materials contained in the devices, but in most cases the image quality can be restored after shimming the magnet. RF is typically produced by pulsed electronics and the digital hardware emitted by the cables of the device. As MRI is highly sensitive to RF noise, such devices have to be operated outside the MR scanner room. On the other hand, small amplitude electromagnetic fields up to some hundred Hz, as produced by some vibrotactile stimulation devices, only showed minor effects on imaging.

Somatosensory Stimulation in Functional Neuroimaging: A Review 343

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

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

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

devices (Chinzei et al., 1999).

currents.

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

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