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TRANSCRIPT
Prototype fMRI compatible actuators
Authors D. Chapius, R. Gassert, V. Hartwig, N. Vanello, A. Bicchi
Date 30th May 2004
Del./Task Identifier D4.11/T4.5
Work Package WP4: New generation of force feedback devices
Partner(s) UNIPI
Work Package Leader UNIPI
Confidentiality Level Public
Abstract:
The purpose of this document is to describe an fMRI (functional Magnetic Resonance Imaging)
compatible haptic interface to investigate the mechanisms of tactile perception within an MR
environment. This interface allows one translation with force-feedback along a horizontal axis as
well as one rotation about a vertical axis linked to the translation. It can be used to move and orient
various objects below the finger. The interface and its components MR compatibility properties are
analyzed in this report.
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TABLE OF CONTENTS
PROTOTYPE FMRI COMPATIBLE ACTUATORS 1
1 INTRODUCTION 3
2 2DOF MRI/FMRI COMPAT IBLE HAPTIC INTERFACE WITH PASSIVE AND ACTIVE
ACTUATION 4
2.1 Concept 4
2.2 MR safety and compatibility 4
2.3 Passive and active sensing and actuation 4
2.4 Working principle of the 2DOF MR compatible haptic interface 5
2.4.1 Actuation/Transmission 5 2.4.2 Sensing 6 2.4.3 Control 6 2.4.4 Safety 6
3 COMPATIBILITY TEST AND RESULTS 8
3.1 Preliminary compatibility test 8
3.1.1 Results 10
3.2 Haptic interface compatibility test 16
3.2.1 Results 16
3.3 Discussion 19
4 CONCLUSIONS 21
5 REFERENCES 22
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1 Introduction Functional brain exploration methodologies, such as functional Magnetic Resonance Imaging
(fMRI), are at present used to study perceptual and cognitive processes. To develop more complex
experimental fMRI paradigms, researchers are interested in realizing active interfaces, using
electrically powered actuators and sensors to be used inside the MRI environment. The use of non-
ferromagnetic metals with higher stiffness and rigidity compared to plastic facilitates the design of
smaller devices. Several reports provide criteria for MR compatible devices[1][2].
Robotic systems working within an MR environment require the development of MR safe and
compatible actuators and sensors. There must be no mutual interference between the materials,
sensors and actuators and the scanner. Only non-ferromagnetic materials can be used, as these
would pose a severe safety threat within the strong static magnetic field of the scanner. Conductive
materials may be used if placed at suitable positions [3].
In this work we describe an fMRI compatible haptic interface to investigate the brain mechanisms
of tactile perception using active as well as passive MR compatible actuators and sensors: the
compatibility of these devices was studied in the D. 4.10 and here is further analyzed using a new
statistical test.
The compatible haptic interface was developed to investigate the mechanisms of tactile perception
within an MR environment. This interface allows one translation with force-feedback along a
horizontal axis as well as one rotation about a vertical axis linked to the translation. It can be used to
move and orient various objects below the finger.
This report describes the preliminary compatibility tests carried out at UNIPI, the MRI/fMRI
compatible haptic device that was developed based on these tests and discusses the MR safety and
compatibility of this device. Moreover we tested the final prototype of the haptic interface to
analyze its compatibility with the MR environment and we reported all results in this paper.
The MRI/fMRI compatible haptic interface was developed by EPFL as subcontractor of ETH.
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2 2DOF MRI/fMRI compatible haptic interface with passive and active actuation
2.1 Concept The MRI/fMRI compatible interface developed in this project is based on the MR compatible robot
technology introduced in [4]. This technology assures a maximum level of MR safety and
compatibility, which is critical as this device will be used by a human subject while inside a
magnetic resonance imaging (MRI) scanner.
2.2 MR safety and compatibility Robotic systems working within an MR environment require the development of MR safe and
compatible actuators and sensors. There must be no mutual interference between the materials,
sensors and actuators and the scanner. Only non-ferromagnetic materials can be used, as these
would pose a severe safety threat within the strong static magnetic field of the scanner. Conductive
materials may be used if placed at suitable positions.
2.3 Passive and active sensing and actuation While the technology presented in [4] uses solely passive actuators and sensors inside the scanner
room (a master-slave system with a commercial DC torque motor placed outside the scanner room
actuating an MR compatible slave over a hydrostatic transmission, and sensors based on intensity
measurement of reflected light), the new interface presented here also uses active components.
Active components require electric energy to be sent into the MR room to generate mechanical
motion or power a sensor that will produce an electric output signal. We have investigated the
following additions with respect to [4]:
• Non-ferromagnetic metals for the slave piston. This helps reducing friction in the
hydrostatic transmission.
• An ultrasonic motor to power the rotary degree of freedom of the haptic device.
• A linear potentiometer to measure the position of the slave piston.
All of these additions have one thing in common: they require compatibility testing; as the metals
are electrical conductors and the ultrasonic motor and linear potentiometer require electric and
electronic signals from the control room into the MR room and back. In the case of the non-
ferromagnetic metals, eddy currents will be induced during motion in the fringe field of the scanner
or by the switching magnetic field gradients within the scanner bore. This leads to thermal and
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mechanical effects and can disturb the imaging. The electric cabling required for the ultrasonic
motor and the linear potentiometer acts as an antenna and can pick up the radio frequency (RF)
signals emitted by the MR scanner or disturb the imaging. Therefore, these components can only be
used in specific locations (e.g. at a minimum distance from the scanner bore), and require MR
compatibility testing. The tests carried out to verify the compatibility of these components are
described in section 3.
2.4 Working principle of the 2DOF MR compatible haptic interface
2.4.1 Actuation/Transmission For this interface, a master-slave system with hydrostatic transmission is used to actuate the linear
degree of freedom. This concept was introduced in [4] and its performance was analyzed in [5]. The
original transmission was miniaturized and now uses single ended pistons made from brass. The
slave cylinder can be disconnected from the master cylinder to allow easy installation of the
interface within the MR facility. The transmission length is eight meters. The master hydraulic
cylinder is actuated by a DC torque motor over a differential belt and pulley system (Figure 1). The
rotary degree of freedom is actuated by an ultrasonic motor (USR60-E3N, Shinsei Corp., Japan) [6].
The slave system in integrated into a glass fiber box (Figure 2).
Figure 1 The master actuator: DC torque motor (a), master hydraulic cylinder (b), hydraulic pump (c) and circuitry (d) as well as disconnectable hydrostatic transmission (e) and security switches (f)
a b
e
c
d
f
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Figure 2 The slave module: A) open version showing the slave hydraulic piston (a) and the tactile pad (b). The ultrasonic motor is located beneath the tactile pad. B) The closed module showing the touch-pad (c) and the
disconnectable (d) hydrostatic transmission
2.4.2 Sensing The master DC torque motor is equipped with a commercial quadrature encoder with 5000
increments per revolution. The displacement of the slave piston is measured using a linear
potentiometer mechanically linked to the slave piston. The ultrasonic motor is also equipped with a
quadrature encoder.
2.4.3 Control The interface is controlled by a commercial PC running Windows and developed control software
created with LabWindows from National Instruments. The hardware is controlled over a
Multifunction DAQ card from National Instruments connected to a custom designed
interface/security PCB. This PCB holds the interfacing electronics that convert the encoder signals
to the requirements of the acquisition card as well as the logic circuits that monitor the security
hardware. Power sources, safety hardware and the interface/security PCB are contained in a master
rack (Figure 3 A). Control is done at 500 Hz, which is sufficient for control of the used transmission
[5].
2.4.4 Safety As this interface will be used by a human subject within an MR environment, safety is a crucial
factor. The developed interface presents the safety features proposed in [4], including:
A
b
a
B
c
d
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• A master emergency button (which can be disconnected from the master rack) for the
experiment supervisor to cut power to the DC and ultrasonic motors
• A security bellow for the subject in the MR scanner to disable the DC and ultrasonic motors
and avert the experiment supervisor over a pneumatic hose and switch (Figure 3 B)
• Electrical end-of-travel switches on the master actuator to disable the DC torque motor if the
master piston moves past the desired position (these positions can be adjusted by moving the
switches)
• Mechanical end-of-travel limitations (intrinsic property of the hydraulic cylinder)
• Software security routines which monitor position and speed of the DC and ultrasonic
motors
• A master enable button located on the master rack to disable the DC and ultrasonic motor
If any of these security features are activated, the green enable button on the master rack is
deactivated and a red security lamp lights up to avert the experiment supervisor.
Figure 3 A) Master control rack with emergency button and B) MR compatible safety bellow
A
B
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3 Compatibility test and results
3.1 Preliminary compatibility test
Tests were carried out at UNIPI, in order to evaluate the compatibility of the materials and the
devices that can be used to realize the haptic interface and to provide criteria for its design: the
influence of various metal pipes (non-ferromagnetic metals as aluminum, copper and brass,) placed
just outside and inside the scanner bore (moving in translation as well as resting) were tested.
Moreover, we tested a shielded cable that is intended to be used with a linear potentiometer and two
types of motor: a DC motor and an ultrasonic motor.
Likewise, the ultrasonic motor was placed at the scanner bore entry and tested at rest and while it
was actuating a mechanical brake, i.e. while it was loaded and producing high power.
Across all experiments, we scanned (by means a scanner Signa Horizon 1.5T, GE Medical Systems)
a spherical phantom of CuSO4 solution, using a GE-EPI (gradient echo, echo planar imaging) with
the following parameters: TE/TR 40/3000 msec, bandwidth 62.5 kHz, FOV 24 cm, resolution
64x64 pixels, Flip angle 90°, Slice thickness 5 mm, number of slices 25, 25 time frames long
sequences acquired.
The Signal to Noise Ratio (SNR):
SDcornerPcentreSNR /= (1)
where Pcentre is the mean value of a 11x11 pixels area at the centre of the image, and SDcorner is
the standard deviation of a 10x10 pixels area at the higher right corner [7], and the standard
deviation (SD) of each voxel signal in time domain were calculated.
We used a t-test in order to look for differences between the above parameters regarding image sets
acquired in various experimental conditions and image sets acquired with no device (reference
images)[8] .
The Student t-test is a parametric one (the hypothesis are based on the distribution parameters, i.e.
mean and variance) that assesses whether the means of two groups are statistically different from
each other.
The hypothesis can be summarized this way:
-null hypothesis 210 µµ −=H
-monodirectional alternative hypothesis 211 µµ >=H
-bidirectional alternative hypothesis 211 µµ ≠=H
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where 1µ and 2µ are the means of the two populations.
If the number of elements 1n and 2n of the two samples are smaller than 30 (in our case the
number of slices for each acquisition sequence is 20) the equation for the t-test, under the a priori
hypothesis of equal means, is the following:
+−+
+
−=
21
21
21
222
211
21
2
)(
nnnn
nnsnsn
xxt (2)
where 1x and 2x are the means of the two samples and 21s and 2
2s their variances.
In the previous equation, the upper part of the ratio is just the difference between the two means or
averages. The lower part is a measure of the variability or dispersion of the scores.
This indicator has 221 −+ nn degrees of freedom.
Equation (x) is valid when the two statistical populations have the same variance. This condition
may be not fulfilled in our case, thus the denominator of the equation must be replaced with:
11 2
22
1
21
−+
− ns
ns
(3)
The t-value will be positive if the first mean is larger than the second and negative if it is smaller.
In order to decide whether the null hypothesis is true or not we must fix a critical value for the
indicator t: it is possible to associate the critical value for the probability of the null hypothesis to be
true, given the number of degrees of freedom and the kind of alternative hypothesis (in our case the
alternative hypothesis is bidirectional ), with the critical value for t.
If the estimated t-value is larger than the critical one then you can conclude that the difference
between the means for the two groups is different.
If we choose that the significance level equals to 0.05 (this means that five times out of a hundred
you would find a statistically significant difference between the means even if there was none) and
considering that in our case the number of degrees of freedom is 38, we get a t critical value of
2.024.
If we apply the t test in order to evaluate significant differences between acquired sequences with
the devices under test and reference images and we find a t value greater than 2.024, we can assert
that the relative experiment created significant artefact in the images.
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We hypothesized that the parameters’ variance in each set could be different due to the motion of
the objects, or to the motors or currents being turned on and off to simulate the devices’ working
conditions.
The t test has supplied results more stable and coherent than ones used previously and this allowed
to obtain information also about the stability of the measurement system during the same session of
tests and sessions made in several days: we acquired in fact several reference sequences (same
investigation conditions) and compared them using the statistical test so as to highlight possible
differences owed to the system itself. Distinguishing the artefact owed to the devices under test by
the ones due to instability of the scanner in a less ambiguous way is possible so.
3.1.1 Results The following tables show the results obtained by the statistical test used for last analysis (t test) for
the three metal pipes, the ultrasonic motor, the DC motor, the cable and the linear potentiometer,
and finally, the scanner temporal stability.
Statistically significant results (with t value greater than 2.024) were obtained in the following
cases:
Copper tube
Dimensions: outer diameter 35.2mm, inner diameter 32, length 140mm
Following results regard the copper tube into the bore, fixed or in movement (z-transl).
Copper fixed tube bore entrance
Slice n° 1 5 10 20
t SNR 2.334 -0.2844 -0.0649 -0.5053
t SD -0.545 0.0411 -0.6422 -0.3686
Copper tube in movement (z-transl)
Slice n° 1 5 10 20
t SNR 1.1636 0.6517 -0.4203 -1.2636
t SD -0.2386 -0.2449 -0.274 0.3341
Aluminium tube
Dimensions: outer diameter 23.2mm, inner diameter 20, length 250mm.
Following results regard the aluminium tube into the bore, fixed or in movement (z-transl or x-
transl).
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Aluminium tube fixed bore entrance
Slice n° 1 5 10 20
t SNR 1.6348 -1.9616 -1.7200 0.7207
t SD -0.4615 -0.2243 -0.1466 0.1019
Aluminium tube in movement (z-transl)
Slice n° 1 5 10 20
t SNR 0.3155 -1.6846 -0.2238 -1.1829
t SD -0.0745 -0.2647 -0.2989 -0.0223
Aluminium tube in movement (x-transl)
Slice n° 1 5 10 20
t SNR 3.0601 -0.2022 -1.5856 2.1266
t SD 0.2262 0.1240 -0.1184 -0.0077
Brass tube
Dimensions: outer diameter 20.2mm, inner diameter 16, length 105.5mm.
Following results regard the brass tube into the bore, fixed or in movement (z-transl or x-transl).
Brass tube fixed bore entrance
Slice n° 1 5 10 20
t SNR 0.1085 -0.3448 0.4823 1.3954
t SD -0.0606 -0.4061 -0.1016 -0.5743
Brass tube in movement (z-transl)
Slice n° 1 5 10 20
t SNR 0.1755 -0.6812 -0.0245 -07212
t SD -0.0903 0.1819 0.4089 0.0625
Brass tube in movement (x-transl)
Slice n° 1 5 10 20
t SNR 1.0047 -0.2243 -0.8560 -0.7447
t SD -0.0610 -0.1845 -0.2487 -0.3723
The t test shows the compatibility of these three materials with the MR environment, confirming the
previous tests of [3].
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Ultrasonic Motor
The piezo-motor we used is a Shinsei USR60-E3N (non-magnetic with encoder) [6].
The following results regard the experiment with the piezo motor on with high or low load at
several distances from the bore entrance and motor off in z-translation.
Piezo motor z translation bore entrance
Slice n° 1 5 10 20
t SNR -0.2963 -0.9721 -0.2835 -0.1105
t SD -1.1647 -0.0765 0.2433 0.0453
Piezo motor 50 cm bore entrance
Slice n° 1 5 10 20
t SNR -0.6649 1.8194 1.2143 0.2233
t SD 0.3545 -0.1398 0.2426 0.1668
Piezo motor bed feet 15 sec on – 15 sec off
Slice n° 1 5 10 20
t SNR 1.3636 0.7301 0.0356 2.3461
t SD -0.8989 -0.6253 -0.2339 -0.3833
Piezo motor on bore entrance
Slice n° 1 5 10 20
t SNR 0.1071 0.5632 0.6427 0.8979
t SD -0.7972 -4.8244 -0.3595 0.0356
Piezo motor on bore entrance high load
Slice n° 1 5 10 20
t SNR -0.3799 -0.0254 -1.15 0.0652
t SD -0.5689 0.0053 0.1244 -0.0286
The results show the compatibility of the piezo motor with the MR environment; in fact the t values
are smaller than the critical t value in each case (a t value greater than the critical value occurs in
only one case but it’s not significant because it’s for a border line image where are possible
artefacts due to instability of the magnetization).
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DC motor
The DC motor we used is a Maxon Re 40 24V [9].
The following results regard the experiment with the DC motor with high or low load and with or
without RF shield in the MR scanner room, at about 3 meters from the bore entrance.
The previous analysis evidenced the presence of artefacts in the images that caused a SNR decrease
[3] ; the results of the new methods confirm this data.
DC motor off
Slice n° 1 5 10 20
t SNR 0.1293 0.9994 0.6059 1.0641
t SD -0.6049 -0.0781 0.0997 -0.4407
DC motor on with low load
Fetta n° 1 5 10 20
t SNR 3.2886 3.5989 3.0607 4.2183
t SD -3.9957 -3.5301 -3.3858 -3.5823
DC motor on with high load
Fetta n° 1 5 10 20
t SNR -3.2284 3.3528 3.7272 3.7317
t SD -20.1791 -14.0852 -20.9847 -18.1793
DC motor on with high load and RF filter
Fetta n° 1 5 10 20
t SNR 0.8382 2.2447 1.8499 3.3204
t SD -2.5062 -7.1358 -3.2805 -3.3176
The DC motor placed about 3 meters from the bore entrance (inside the MR scanner room) causes
highly significant artefacts in the images, as appears from the previous table (in which the t value,
especially for the deviation standard analysis, is much larger than the critical value).
The use of the RF filter seems to improve the compatibility results.
However, it is impossible to use the DC motor in the MR scanner room. This is why in our system
the DC motor is placed in the console room and power is transmitted using a hydrostatic connection
[4][5]. In this way we avoid most compatibility problems.
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Scanner stability in the same day
The follow tables show sequences acquired during the same day under the same test conditions
(phantom with any device into the scanner: baseline).
First baseline – second baseline
Slice n° 1 5 10 20
t SNR -0.4121 -1.783 -1.635 -1.050
t SD -0.1371 -0.3484 -0.4076 0.183
First baseline – last baseline
Slice n° 1 5 10 20
t SNR -0.3401 0.0470 1.3858 -1.4092
t SD -0.1263 -0.7271 -0.070 -0.2401
The t value is always smaller than the critical t value, which means that there is no significant
difference between two series of images acquired under the same test condition (baseline: phantom
with no devices inside the scanner) and in the same day: this means that there are no statistically
significant artefacts caused by system instability in the same day .
So, if we compare a sequence regards a test with an under test device into the bore with one of these
baseline and we found artefacts, these are caused by the non perfect compatibility of the device.
Scanner stability in several days
The following tables compare results of baseline measurements obtained on several days under the
same test conditions.
Baseline 12/07/2003 – Baseline 10/07/2003
Slice n° 1 5 10 20
t SNR 7.8168 15.1287 14.2822 12.1362
t SD 1.2378 0.8222 0.3028 0.5409
Baseline 12/07/2003 – Baseline 09/07/2003
Slice n° 1 5 10 20
t SNR -3.0988 -1.0982 1.5479 4.0722
t SD -0.3153 0.0027 0.3682 0.1614
Baseline 10/07/2003 – Baseline 09/07/2003
Slice n° 1 5 10 20
t SNR -10.1395 -14.388 -10.5120 -8.6411
t SD -0.8449 -0.7125 -0.2443 -0.3806
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There are significant differences between baselines acquired in several days. Therefore the device
analysis must be performed relative to the baseline of the corresponding day.
Shielded cable and linear potentiometer
The following results refers to the experiment with the electric cable that we tested with or without
linear potentiometer in two cases: current flow and no current flow through the cable at several
distances from the bore and with or without RF shield.
Shielded cable with no current Slice n° 1 5 10 20
t SNR 0.4927 -1.2331 -0.1442 -1.6831
t SD 0.2585 0.1568 0.1539 -0.2677
Shielded cable with linear potentiometer and no current Slice n° 1 5 10 20
t SNR 1.0668 2.9799 2.4716 2.8136
t SD -1.8813 -1.4334 -1.3499 -1.6861
Shielded cable with linear potentiometer and current Slice n° 1 5 10 20
t SNR 1.3769 2.9576 2.2219 0.2269
t SD -0.8236 -0.9341 -1.0097 -1.1117
Shielded cable with no current 40 cm inside the bore Slice n° 1 5 10 20
t SNR 0.5827 1.0836 0.6218 1.2294
t SD -0.5548 -0.1926 -0.4883 -0.6074
Shielded cable with linear potentiometer and current at 80 cm from the bore Slice n° 1 5 10 20
t SNR 4.7915 5.111 5.5723 3.1689
t SD -2.1682 -2.1252 -2.1469 -2.4087
Shielded cable with linear potentiometer, current and RF filter at the bore entrance Slice n° 1 5 10 20
t SNR 0.2782 2.7140 2.1476 0.3566
t SD -0.6390 -0.8781 -1.1157 -1.1544
Shielded cable with linear potentiometer and current at the bore entrance
Slice n° 1 5 10 20
t SNR 0.2599 2.2259 1.8817 1.6065
t SD -0.7341 -0.5845 -1.1468 -1.2426
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The previous tables show that there are some problems when the linear potentiometer is connected
to the shielded cable with the current flows through it. This problem may be due to the last piece of
shielded cable, constituted by two not shielded wires that may create artefacts on the image.
In future tests we will test the linear potentiometer with particular attention to the connection with
the power cable.
The use of the RF filter seems to minimize these artefacts (the difference is evident analyzing the t
values regarding the SD analysis).
3.2 Haptic interface compatibility test In the final parts of this work we tested the complete prototype of the haptic interface in the MR
environment in order to evaluate its compatibility such as the absence of artefacts in the images.
We used the methods described in the section 3.1: we acquired the images using a spherical
phantom and a GE-EPI (gradient echo, eco planar imaging) sequence in the following cases:
- baseline: phantom with no devices inside the scanner
- device off ( see Figure 2 B ) inside the bore
- device inside the bore, rotation parts on (see Figure 2 A) (U-motor on, hydraulics part and
DC motor off)
- device inside the bore, rotation and translation parts on (U motor on, hydraulics part and DC
motor on) with sensor part on (see Figure 1 ): complete operation mode.
The hydraulics part and the master rack (electronic driver and the emergency button, see Figure 3)
were positioned in the console room and the electric cables and the hydraulic ones passed through
the wires guides in the Faraday’s shield. The electric cables are shielded and, moreover, we used a
RF filters on the plug- in with the master rack.
3.2.1 Results
The following tables show the results obtained by the statistical test used for last analysis (t test) for
the cases described previously; there are statistically significant results with a t value greater than
2.024.
Baseline: scanner stability in the same day
We acquired several baseline images in order to evaluate the scanner temporal stability: the
following tables report the t values regard the differences between different baselines acquired in
the same day but in different moments.
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First baseline – second baseline
Slice n° 1 5 10 20
t SNR -0.8060 0.6737 -0.7408 -1.6288
t SD -0.2332 0.2367 0.9364 0.1043
First baseline – last baseline
Slice n° 1 5 10 20
t SNR 1.2075 -0.3686 -1.1303 0.2727
t SD -0.3681 -0.0361 0.2637 -0.2083
It’s possible to note that in the same testing session there aren’t problem due to the scanner
environment stability.
Baseline: scanner stability in several days
The following tables compare results of baseline measurements obtained on several days under the
same test conditions.
First baseline 02/04/2004 – First baseline 03/04/2004
Slice n° 1 5 10 20
t SNR 1.4001 -0.9679 0.2825 -8.4784
t SD -0.3090 -0.0667 0.6230 0.3312
First baseline 02/04/2004 – Last baseline 03/04/2004
Slice n° 1 5 10 20
t SNR -2.9608 -1.5047 1.3336 -3.4352
t SD 0.6711 -0.1039 0.6927 0.0626
First baseline 02/04/2004 – Last baseline 04/04/2004
Slice n° 1 5 10 20
t SNR -0.4946 0.7838 2.0114 -3.7732
t SD 0.3662 -0.1074 0.6634 0.4013
In this new test session there weren’t artifacts due to MR environment instability: in fact statistical t
test for differences between baselines acquired in different days, don’t show a t value greater than
the critical t value (a t value greater than the critical value occurs in some case but it’s not
significant because it’s for border line images where are possible artefacts due to instability of the
magnetization).
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Device off inside the bore
Device was positioned inside the bore in the right position for the experiment with a subject (near to
his right hand); power was off both for the two motors and for the sensor part.
Respect to the first baseline
Slice n° 1 5 10 20
t SNR 0.5100 1.1159 0.8301 -1.3576
t SD 0.0550 0.1766 0.4974 -0.3034
Respect to the last baseline
Slice n° 1 5 10 20
t SNR 1.5131 1.3395 0.6156 -0.4374
t SD 0.5715 0.4300 0.4326 -0.0107
The previous results show the good MR compatibility of the parts of the haptic interface that must
be positioned inside the bore in order to stimulate the subject fingers.
Device inside the bore, rotation part on (U-motor on, hydraulic part and DC motor off)
In this experiment we tested the rotation part of the device: the stimulation pad rotates by mean the
ultrasonic motor inside the scanner. Rotation direction, velocity and temporization are controlled by
a PC with an acquisition card placed inside the console room.
Rotation (sequence: 15 sec on 15 sec off) respect to the first baseline
Slice n° 1 5 10 20
t SNR 2.2342 -0.2353 -0.7089 0.4743
t SD -0.4409 -0.0956 0.5130 -0.2278
Rotation (sequence: 15 sec on 15 sec off) respect to the last baseline
Slice n° 1 5 10 20
t SNR 0.9752 1.4692 -0.8140 -0.4374
t SD 0.5715 0.4300 0.4326 -0.0107
Rotation (sequence: 15 sec on 15 sec off) respect to the first baseline of the last testing day
Slice n° 1 5 10 20
t SNR 1.5622 -1.0791 0.9533 0.7699
t SD 0.3245 -0.4133 0.1965 0.3591
The device rotation don’t act any artefact in images: in fact the good compatibility of the U-motor
was already demonstrated in [3] and in the section 3.1.1 of this report.
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Device inside the bore, complete operation mode
Following tables regard the tests of the device complete operation mode: translation and rotation
parts ON and sensor part (linear potentiometer) ON.
These experiments were performed on a different day respect the previous so the reference series
for the statistical t test are the baselines acquired in that day.
Ultrasonic motor and DC motor ON; potentiometer plug off
Slice n° 1 5 10 20
t SNR 1.1846 -1.0944 0.7291 -1.1248
t SD -0.0789 0.5738 0.2640 0.4482
Ultrasonic motor and DC motor ON (seq: 15 sec on 15 sec off); potentiometer plug off
Slice n° 1 5 10 20
t SNR 2.1843 -0.4931 -0.1762 -0.0538
t SD 0.2312 -0.5531 0.2769 -0.1559
Ultrasonic motor and DC motor ON; potentiometer plug ON
Slice n° 1 5 10 20
t SNR 0.2795 1.1197 -0.0256 -1.0419
t SD 0.4073 0.0864 0.1243 -0.1308
Ultrasonic motor and DC motor ON (seq: 15 sec on 15 sec off); potentiometer plug ON
Slice n° 1 5 10 20
t SNR 1.8152 0.1332 -1.0693 1.3127
t SD -0.2293 -0.1513 -0.1048 -0.0719
During this test session the haptic interface worked completely like as it must be work during a
stimulation experiment on a subject: the previous results show the good compatibility of the device
(a t value greater than the critical value occurs in only one case but it’s not significant because it’s
for a border line images where are possible artefacts due to instability of the magnetization).
3.3 Discussion
Results for aluminium, copper and brass indicate no differences with reference images acquired in
the same day, both for SNR and SD values. Experiments with these materials moved by an operator
led to the same conclusions. Statistical differences were found for the electric cable with the
potentiometer plugged both with current flowing (0.25 mA) and with no current. Experiments were
performed with motors in two conditions: turned alternatively on and off for 15 seconds intervals,
TOUCH-HapSys Prototype fMRI compatible actuators
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and always off. The ultrasonic motor showed no differences with reference images in both
conditions while the DC motor showed significant differences, even if placed in the most distant
corner of the MRI room, at about 3 meters from the scanner bore. Figure 4 shows the homogeneous
phantom image (baseline) and result of the difference between the base image and the image
acquired during the DC motor running. Figure 5 shows the relation between SNR and time during
the DC motor running (sequence: 15 s ON, 15 s OFF).
Figure 4 Homogeneous phantom image (baseline) (left) and result of the difference between the base image and the image acquired during the DC motor running (right).
Figure 5 Relation between SNR and time (continuous line) during the DC motor running (outlined line: motor ON=high level, motor OFF=low level).
TOUCH-HapSys Prototype fMRI compatible actuators
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Regarding the temporal stability of scanner measurements, the results indicated that reference
images acquired in the same day showed no statistically significant differences. Images in the same
conditions but acquired on different days showed a statistical difference in SNR values, with no
difference for standard deviations.
Tests regard the complete haptic interface show the good compatibility of this device: the device off
was positioned inside the scanner room on the patient bed in the right position for the stimulation
experiment with a subject. The images acquired in these experimental conditions don’t show
statistical differences with the reference images (baseline: phantom with no device).
The ultrasonic motor is placed inside the scanner on the stimulation part with the linear
potentiometer: power and sensor signals are sent from the console room to the scanner room by
mean shielded cable through the wires guides in the Faraday’s shield. The images acquired during
the rotation movement of the stimulation pad don’t show artifacts so this part of the interface can be
considered MR compatible.
Finally we tested the complete function of the haptic interface: rotation, translation and sensor part
with both motors ON and the linear potentiometer power ON for the pad position detection.
Also in this case the statistical test didn’t show statistical differences between the reference images
and the images acquired in the experimental conditions of subject stimulation so it’s possible to
conclude that the designed haptic interface has a good compatibility with the MR environment and
it’ possible to use it for functional experiment of a volunteer.
In Figure 6 it’s possible to see the result of the difference between two baselines acquired in the
same day but in two different moments and result of the difference between the first baseline and
the image acquired during the haptic interface complete operation mode.
Figure 6 Result of the difference between two baselines acquired in the same day but in two different moments (left) and result of the difference between the first baseline and the image acquired during the haptic interface complete
operation mode (right).
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4 Conclusions We have developed an MR compatible haptic interface with two degrees of freedom to investigate
the brain mechanisms of tactile perception. The interface uses active as well as passive MR
compatible actuators and sensors. In order to study the eventual presence of artifacts in the MR
images due to the device function, a statistical t test was used. In this report results of compatibility
experiments of this interface are reported.
While this prototype concludes the contribution of EPFL to Touch-HapSys, EPFL will continue
collaboration with UNIPI to adapt control of the interface to different studies, as well as to develop
a force sensor to measure direction and amplitude of finger friction force exerted against the haptic
interface and carry out behavioural and fMRI studies using this device in collaboration with UNIPI.
5 References [1] Chinzei, K., Kikinis, R., Jolesz, F.A., MR Compatibility of Mechatronic Devices: Design
Criteria, Proc MICCAI ’99, Lecture Notes in Computer Science, 1999, 1679: 1020-31.
[2] Chinzei K., Miller K., MRI Guided Surgical Robot, Proc. 2001 Australian Conference on
Robotics and Automation, Sidney 14-15 November 2001.
[3] V. Hartwig, N. Vanello, N. Sgambelluri, E. Scilingo, L. Landini, A. Bicchi, D 4.10: Design of
fMRI compatible actuators, http://www.touch-hapsys.org/, September 2003.
[4] R. Moser, R. Gassert, E. Burdet, L. Sache, H. R. Woodtli, J. Erni, W. Maeder and H. Bleuler,
An MR Compatible Robot Technology, Proc. IEEE International Conference on Robotics and
Automation (ICRA), 2003.
[5] G. Ganesh, R. Gassert, E. Burdet and H. Bleuler, Dynamics and Control of an MRI Compatible
Master-Slave System with Hydrostatic Transmission, Proc. IEEE International Conference on
Robotics and Automation (ICRA), 2004
[6] Shinsei Corporation Inc., Ultrasonic motor USR60 series,
http://www.tky.3web.ne.jp/~usrmotor/English/html/
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[7] GE Medical System (ed): MR Safety and MR Compatibility: Test Guidelines for Signa SP TM
Version 1.0, http:// www.gemedicalsystems.com/rad/mri/pdf/safety1.pdf , October 1997.
[8] V. Hartwig, N. Vanello, R. Gassert, D. Chapuis, M.F. Santarelli, V. Positano, E. Ricciardi, P.
Pietrini, L. Landini, A. Bicchi: A compatibility test for tactile displays designed for fMRI
studies, accepted to EuroHaptics 2004, Munich, June 2004.
[9] Maxon DC motor Data Sheet, RE 40 diameter 40 mm, Graphite Brushes, 150 Watt,
http://www.maxonmotorusa.com/files/catalog/2003/pdf/03_082_e.pdf