WO2014178091A1

WO2014178091A1 - Integrated unit for audio-visual stimulation and eye movement recording for functional magnetic resonance imaging

Info

Publication number: WO2014178091A1
Application number: PCT/IT2014/000134
Authority: WO
Inventors: Paolo Alessandrini; Steve GAZZITANO; Emiliano MACALUSO
Original assignee: I.R.C.C.S. Fondazione Santa Lucia, Istituto Di Ricovero E Cura A Carattere Scientifico
Priority date: 2013-05-03
Filing date: 2014-04-29
Publication date: 2014-11-06
Also published as: EP2992343A1; ITRM20130261A1

Abstract

The object of the present invention is an integrated unit for audio-visual stimulation and eye movement recording for magnetic resonance imaging which comprises two parts: a stereoscopic 3D projection part, far from the magnet and a compact part integral with the coil, which includes: a screen for stereoscopic vision, piezoelectric loudspeakers for a five channel listening, a video camera-IR illuminator assembly for bright pupil tracking at 40 cm from the subject. The system allows to use the entire width of the tracking field and thus to present stimuli with a wide visual field. The system is compatible with the high magnetic field environment of MR tomographs and with the use of coils having a particularly narrow radio frequency.

Description

INTEGRATED UNIT FOR AUDIO-VISUAL STIMULATION AND EYE MOVEMENT RECORDING FOR FUNCTIONAL

MAGNETIC RESONANCE IMAGING

State of the art In functional magnetic resonance imaging (MRI), the subject is subjected to a series of sensory stimuli, mainly visual and auditory stimuli, to record the corresponding brain activity. Besides the devices which produce such stimulations, there exists another class of auxiliary apparatuses, able to record various types of responses of the subject, particularly the eye movement detector devices. Audio-visual stimulators and eye movement detectors currently used are:

1. Video projector

It is usually placed outside the MRI room to avoid interferences and the projection beam passes through the screened wall through, purposely provided waveguide openings. Appropriate optics of the projector allows a little image to be formed on a rear projection screen in correspondence of the mouth of the magnet, inside which the lying subject can view the image by means of a mirror placed above his eyes. In some cases the video projector, suitably screened, is placed inside the MRI room.

2. Active glasses

They are provided with video screens and optics suitable for short distance vision, worn by the subject and receive the signal via magnetic resonance (MR)-eompatible cable. The viewers are constructed with low electromagnetic (e.rn.) emission electronics and suitable shielding.

3. Fiber optic glasses The zone above the subject^'s eyes is e!eetronles-lree and the image arrives, through the fibers, from a projector which is far from the coil. A lower risk of e.rn. interferences is guaranteed.

4. R-eompati bie monitor

Such monitors are of specially designed construction and suitably shielded. They can be placed at the mouth of the magnet and viewed by the subject with a mirror.

With respect to acoustic stimuli, the typologies available are the following:

5. Acoustic tubes

Sounds are carried through flexible tubes from loudspeakers, far from the magnet, to the subject's ears.

6. P s ezoelectric head pho ne s

They are MR-eompatible and receive the electric signal from the outside through shielded cables.

Eye movement recorders or eye trackers (abbreviated as e.t.) are typically used along with the above examined stimuli. An e.t. is usually made up of an infrared (IR) illuminator directed towards the subject's eye, and a video camera which frames the eye itself, sending the associated video signal to a processing unit, which extracts therefrom the information on gaze direction, which is in turn recorded for postprocessing. Depending on relative distances between the components of the system and the interposed optical means, four typologies of devices can be pinpointed, which are listed below,

7. E.T, with a distant video camera

^'The video camera, provided with a long-focus lens, frames the eye from far away (meters), through a mirror. The illuminator is constructed together with the video camera. They both have to be pointed on the subject's eye during the preliminary operations,

8. Bright pupil E.T.

Bright pupil, eye trackers are typically constructed with the criteria of the E.T. with distant video camera, but they are constructional!}' distinguished because the illumination beam is collimated with that of the video camera's field of view. The functional result is that the eye's pupil appears bright, to the video camera and the monitoring is more efficient than that of other "dark pupil" systems.

9. E.T. with a close video camera

Both the video camera and the illuminator are miniaturized and inserted within special glasses, or installed on the MRI coil with supports,

10. E.T. with fiber optic video camera

The end of an optical fiber connected to a monitoring video camera is placed on the subject's eye. The fiber also carries the beam of the IR illuminator. It is a configuration that guarantees a complete electromagnetic separation.

In the context of functional magnetic resonance imaging, the use of the aforedescribed video-audio stimulators and eye movement detectors causes limitations in the stimuli and consequently in the responses of the subjects being tested. A brief overview of the limitations deriving from using video-audio stimulators and eye movement detectors is reported below.

The characteristic limitation of image presentation systems is that of the field of view. Apparatuses of type "2", "3" and. ^w4" also allow the vision of stereoscopic 3D images, but the field of view is limited to about 30°, The only system that allows the field of view to be widened, by moving the screen closer to the subject, is of type " \ however there exist no MRI proj ctor-based systems able to project 3D images.

A limitation of current audio systems "5" and "6" is that of the used number of channels: it is common, to use stereophonic apparatuses with only two acoustic signals that are directly presented, to the subject's ears. This reduces the spatial perception of auditory stimuli which is instead a typical characteristic of five-channel audio systems, with sources positioned externally around the listener.

A typical limitation of e. systems is that of the non-centering of the tracking camera with respect to the subject^'s field of view, due to the need of positioning the camera at the side of the screen,. Such configuration reduces by 50% the field of view usable for monitoring eye movements. Since the tracking field is usually 40-50°, the usable field of view is reduced to about 20-25°.

Systems of type "8'" (bright pupil) have excellent tracking abilities, but they require that the camera be not closer than 60 cm from the subject's face. Above this distance, the light scattered from surfaces around the pupil is sufficiently attenuated, while the beam reflected through the pupil (i.e. the signal, of interest) keeps substantially the same intensity. It is thus obtained a sufficient rati between the intensity of the pupil and the background, which is instead very difficult for lesser distances. In such apparatuses, the intensity of infrared light coming back from the pupil varies with the fourth power of the diameter thereof. Further, as the light intensity of the image being viewed by the subject, changes, significant variations i the pupil diameter occur, with limitations of tracking abilities when the stimulus is brighter.

A drawback of devices of type "7" and "8" is the laborious tine-tuning of the system to each subject vol unteer, due to the numerous components characterizing such systems (mirrors, screens, fillers, etc.). This is even more critical in tomographs having a coil with a particularly narrow radio frequency, where there is room for only one mirror that serves both for the vision of images and for the tracking; this leads to further difficulties in the relative position of the various components and makes the adjustment for each subject volunteer harder, all the more considering tliat only by introducing him into the magnet it is possible to check that the various parts are working properly.

Devices which allow the subjeci of the magnetic resonance imaging to be subjected to sensory stimuli are described for example in the following documents.

US 2006/209257 (Dl) describes a video projector device, having a video camera as regards the presentation of the image to the subject and incorporating fiber optics as regards the eye tracker (e.t), in addition the device uses a mirror transparent to infrared and reflective in visible light (co!d mirror) to enable the subject to see the screen; such innovation, along with positioning the end of the optical fibers of the video camera and illuminator above the mirror in the central direction of the image reflected in visible light, guarantees the centering of the e.i. with respect to the subject^' s field of view. The device farther allows the visio of wide angle images, by simply moving the screen closer to the subject's nape. However, the system does not contemplate the possibility of forming on the screen a 3D image which can be transmitted to the subject with an appropriate quality of both the rear projection screen and the cold mirror.

In Kerstin Paschke et al "Mirrored or identical is the role of visual perception underestimated in the mental rotation process of 3D-objects" in i europsyeho!ogia, Pergamon Press, Oxford, vol. 50, no. 8, April 2012 (02), there is discussed the application of 3D visual stimuli in the context of RI, but the purpose is achieved by using active glasses which, however, are not integrable into the video projector-based system.

In Dubowith et al "A simple set-up for tracking eye position during fMRI", Proceedings of the International Society for Magnetic Resonance in Medicine ISM M, 7* Scientific Meeting and Exhibition, Philadelphia, USA, 24 May 1999 (D3). there is examined a configuration in which the eye tracker's beam is reflected by a mirror totally reflective in JR. and independent from the mirror by which the subject views the screen, but this solution is totally absent from the video projector-based system which uses optical fibers to guide the e.t. towards the subject's eye.

From US 5 877 732 A (Ziarati Mokhtar US (1)4) it can be seen that the provision of piezoelectric loudspeakers in the context, of fMRl is known, but it is confined to the use of stereo loudspeakers, i.e. two channel systems and there is no indication of five channel loudspeakers.

In Frederic Gougoux et al "A l-^'unetionai euroimaging Study of Sound Localization: Visual Cortex Activity Predicts Performance in Early-Blind individuals", Plos Biology, vol, 3, no. 2 (DS), there are shown loudspeaker arrays for multi-channel stimulation, but the shown loudspeakers are not piezoelectric, and therefore there is no direct indication about how to export such, stimulation into fMRI.

Description of the invention

According to the present invention, the solution to the problems described above is provided by the selection of innovative components and by their integration into compact structures thanks to a series of conceptual innovations and constructional measures which are developed ad hoc to be M -compatible. New functions and adjustment possibilities, so far not used in the field, have thus been obtained. All the structural supports and MR-compatible adjusting mechanisms, which were not commercially available, have been constructed in. an original fashion.

The object of the present invention is an integrated unit, i.e. a single compact structure, for audio-visual stimulation and eye movement recording for magnetic resonance imaging which comprises two parts: a stereoscopic 3D projection part, far from the magnet and a compact part integral with the coil, which includes: a screen for stereoscopic vision, piezoelectric loudspeakers for a five channel listening, a video camera-iR illuminator assembly for bright pupil tracking at 40 cm from the subject. The system allows to use the entire width, of the tracking .field and thus to present stimuli with a wide visual field. The system is compatible with the high magnetic field environment of MR tomographs and with the use of coils having a particularly narrow radio frequency.

The main purpose of the invention is to present to the subject a stereoscopic 3D image on a screen at a short distance from the eyes (about 25-30 cm) to assure a wide field of view (about 50°), and to simultaneously send to the eye the beam of a bright pupil eye tracker which has an apparent direction coming from the screen center, so as to obtain the maximum tracking accuracy wherever the subject's gaze is directed; a maximum tracking angle (about 25°) is thus obtained in all directions with respect to the screen center.

To that end, a cylindrical supporting structure has been provided, which can be partially inserted into the RF (radio frequency) coil and fixed by appropriate joints thereto. This structure supports, at the zone above the subject's face, the mirror by which he can view the screen and a second mirror ori which the beam of the e.t. reflects to arrive to the eye. The infrared beam of the e.t, passes through the viewing screen because this is constituted of an IR (infrared) filter able to reflect visible light used to present visible stimuli, but -· at the same time - to let Infrared light for the e.t. pass through. A similar function can be achieved by using a beam splitter having suitable characterisiics to reflect the screen and let the eye-tracker's beam pass through to an acceptable exte t.

With this structure, another important purpose of this invention has been achieved: having all the components of the system, except the video projector, in an easily installable compact structure and with the possibility of mailing the various adjustments even with the subject, lying on the couch outside the magnet. When the subject is then introduced into the magnet no further adjustments are required.

The video camera-lR illuminator assembly has been constructed according to the already known constructive technique to obtain the "bright pupil" at long distances (illuminator having a concentrator lens and a beam splitter to reflect the illumination beam coaxially to the video camera's field of view). Since, with this compact, structure, the video camera-illuminator assembly is placed at 40 cm from the subject's eyes, the !^'R reflection scattered from surfaces around the pupil is too strong as compared, to the intensity of reflection of the pupil. Therefore, the innovation of applying in front of the video camera's lens a diaphragm having an aperture of 8 mm. has been introduced, the diaphragm enabling the col!imated and concentrated beam coming back from the pupil and passing in the central zone of the lens to pass therethrough, while reducing the effect of scattered light that evenly hits the front lens element of the video camera's lens. It is understood that, in order to protect the present invention, other means having the same effect, such as the use of long-focus and low- luminosity optics, or other lens types which reduce the passage of scattered light are not excluded.

What has been briefly described so far will be better understood by reading the following description of an embodiment, given by way of example and without limitation with reference to the accompanying drawings, wherein:

Fig. 1 shows a schematic view of an MR.! room with the magnet, the radio frequency coil and a person lying on the couch with his head inside the coil; there is also shown a wail of the shielded room with a waveguide which allows the light beam of the projector Al to pass through.

Fig. 2 shows a schematic plan view and a right side view of the loudspeakers used in an embodiment.

Fig. 3 shows a detailed schematic view of the video camera-i lluminator assembly used in an embodiment.

Fig. 4 shows a partially exploded perspective view of only the supporting structure. With reference to said figures, fig. 1 shows a vertical longitudinal section of the MRI room with the magnet, the radiofrequency coil and a person lying on the couch with his head inside the coil. There is also shown a wall of the shielded room with a waveguide which allows the light beam of the projector Al to pass through. There are also schematically shown some elements of the supporting structure, which are indicated by Dl (base of the structure which is mounted to and made integral with the RF coil), D2 (upper plane which supports the video camera-illuminator assembly), D3 (mirror-carrying slide which supports the mirror A4 for viewing the image that forms on the rear projection screen A3 and the mirror C2 on which the e.t. beam reflects, passing through A4 that is transparent to infrared).

What is shown in fig. 1 can be more easily understood front fig. 4 which shows a partial iy exploded perspective view of only the supporting structure, wherein there are illustrated the same elements as shown in fig. 1, however, with the addition of upper sides D6, D7 on which the mirror-carrying slide D3 slides, the latter being also shown in detail. It is possible to appreciate the adjustments that can be made to adapt the apparatus to the subject; the mirror-carrying slide D3 can. be moved to position the mirrors above the eyes; the mirror A4 is rotated about a horizontal axis passing through, its center such that the subject can view the entire height of the screen; the mirror C2 is oriented such that the video camera views the pupil at the center (this is controllable through a monitor that the operator can look at); the screw adjustment D5 allows the mirror€2 to be moved forward or backward until the central direction of the video camera's field of view passes through half the height of the subject's field of view (this is controllable because the corneal reflection moves to the central zone of the pupil).

A feature of the construction of upper sides D6 and D7 allows the comfortable entrance of the subject's head into the structure: thanks to the articulation of the group formed by wings D6 and 7 (together with the slide D3) with their sides, about two pins projecting into 1⁾1, near the screen A3» the ends D6, D7 can be lifted by 20 cm and the head can be comfortably introduced. The structure is then re-closed and the upper half of the RF coil is lowered thereon, as normally happens when there is not the structure. The vision system is made up of a conventional polarization-based stereoscopic 3D projection apparatus, on the principle of those used in movie projection theaters, where spectators wear passive polarizer glasses to separate the two images, with the innovative solution of moving the screen much closer to the subject's eyes (up to 30- 40 em), thus obtaining a field of view of the order of S0°-6O% never used so far in MRL The screen A3 and the mirror A4 are non-depolarizing, i.e. they have a polarization contained in the limits within which it is normally accepted in polarization-based D vision.

As mentioned above, figure 1 shows a 120Hz video projector Al for presenting the two left-hand and right-hand images in quick sequence, the polarization modulator Λ2 which polarizes them along two 90° directions, the rear projection screen A3 without alteration of the polarization, the mirror A4 which allows the vision by the lying subject, the passive polarization glasses A5. As can be seen, the distance of the screen A3 from the subject's eyes is very short and the mirror A4 is transparent in the infrared, so as to allow the passage of the infrared beam of the e.t.

Figure 3 shows the detail of the video camera-illuminator assembly. The video camera CI a frames the eye through the beam splitter O e that reflects, in the same direction, the i frared beam produced by a led source Cle and concentrated by the lens Cld. Thanks to the appropriate adjustments of the position of the beam splitter, of the position of the source and of the source-to-lens distance, it is possible to obtain an illumination beam which is coaxial with that of the video camera's field of view and which uniformly lights the eye zone. The result is that the pupil appears bright to the video camera and the tracking is effective. The diaphragm Ob placed in. front of the front lens element of the video camera lens has a hole with a diameter of about 8 mmn the center thereof to limit scattered light.

With respect to sound diffusion, around the subject having his head within the F coil, in a preferred embodiment, there are five piezoelectric loudspeakers being arranged three in -front of and two behind the (ace. as in conventional apparatuses out of MRL Unlike conventional MR! acoustic equipment in which piezoelectric stereo headphones are used, it is obtained a l ve- channel spatial distribution of sound, while the function of attenuating the noise of the scanner is carried out by common soundproofing headphones or earplugs. To compensate for the strong sound attenuation due to ear protection, loudspeakers need to have a high power capacity and be controlled by appropriate amplifiers.

With reference to figure 2, the loudspeakers used in the prototype are shown in a plan view and in a right side view. Loudspeakers Bldx and Blsx are electrically connected in parallel and diffuse the "center" audio channel; they are two, given the presence of the central bar of the RF coil and given that they are supported by the supporting structure exactly in the spaces between the bars. B2dx and B2sx are supported at mid- height laterally to the coi I and diffuse the "front right" and "front left" channels. B3dx and B3sx are on the bottom of the coil, near the nape and diffuse the "rear right" and "rear left" channels. Shielded electrical cables of loudspeakers and e.t. are collected at the low part of the structure and led out from the rear of the magnet. Good results have been achieved with piezoelectric loudspeakers having a diameter of 10 centimeters, a power capacity of 50 W, a lower cut-off frequency of 500 Hz.

In figure 4, there is shown the calotte- shaped structure that supports the four upper loudspeakers, two of which are visible; this structure is rested on the RF coil, whose upper half is re-closed onto the head of the subject, once the latter is lying on the couch.

All modifications dictated by practical implementation and by those skilled in the art can be made to what has been described and illustrated so far without departing from the scope of the invention as defined in the accompanying claims.

Claims

L Integrated stimulation unit for functional resonance imaging, comprising: a rear projection screen, a viewing mirror having the necessary transparency to the infrared beam of an eye tracker, a video camera-ilhmiinaior assembly for the bright-pupil eye tracker capable of framing the eye characterized in that, the rear projection screen is the non-depolarizing screen (A3) placed behind the subject's nape, the viewing mirror having the necessary transparency to the infrared beam of the e.t. is the non-depolarizing mirror (A4), the video camera-illuminator assembly for the bright pupil eye tracker is the video camera assembly (CI) capable of framing the eye by reflection on a mirror (C2) which is totally reflective in the infrared, with the purpose of assuring a field of view comprised between 50° and 60° of the 3D image which forms on the screen, while the central axis of the e.t. beam may be adjusted to overlap the central axis of the field of view so as to ensure the maximum tracking angle (25°) in. all directions with respect to the screen center; said integrated unit being further characterized in that it comprises piezoelectric loudspeakers for a II ve-channel listening.

2. Integrated stimulation unit for functional magnetic resonance imaging according to claim 1, characterized in that the mirror (A4) uses an IR-pass filter with a bright surface; the bright surface allows the vision of the screen (A3) by reflection, while maintaining the polarization of visible light and therefore the stereoscopic image; further, as it is an I -pass filter, the function of transparency to the IR beam of the e.t. is performed very well.

3.. .Integrated stimulation unit tor functional magnetic resonance imaging according to claim 1 , characterized by the use for the mirror (A4) of a reflection 30% - transmission 70% beam-splitter (C lc) followed by an IR-pass filter, such that visible light from the screen is .reflected at 30% towards the subject again without destroying the polarization; the rest of the visible light passes and is absorbed by the poorly reflective surfaces of the structure's interior; the IR beam of the e.t. illuminator passes at 70% through the beam splitter and then reflects towards the eye on. C2, in this case a "hot mirror" totally reflective in the IR and with few reflections in visible light; the greater attenuations relative to the previous case may be compensated for by an increase in the illuminator's intensity.

4. integrated stimulation unit for functional magnetic resonance imaging according to claim 1, characterized by the presence of a diaphragm Ob placed in front of the lens of the tracking video camera (CI a) so as to increase the pupil-background contrast for bright pupil e.t.

5. integrated stimulation unit for functional magnetic resonance imaging according to the preceding claims, characterized by the presence of eye tracking components symmetric to those already described to perform a binocular tracking.

6. Integrated stimulation unit for functional magnetic resonance imaging according to the preceding claims, characterized by the presence of an illuminator (C ! e) adjusted with the source's position between the lens (Cl d) and the focus so as to enlarge the image of the source in order to have an apparent size of 2-3° by the subject; this size corresponds to a good compromise between the concentration of the beam returned by the eye and the need not to stress the eye's interior surface with art excessive intensity.

7. Integrated stimulation unit, for functional magnetic resonance imaging according to the preceding claims, characterized by the presence of a cylinderlike structure (Dl ) made of dielectric material which is partially msertable into the Mill RF coil and securable thereto through some joints with a quick operation which does not jeopardize a quick return to the previous conditions for the normal use of the resonance; said structure supports all the components described in the previous claims plus two loudspeakers on the bottom.

8. integrated stimulation unit for functional magnetic resonance imaging according to the preceding claims, characterized by the presence in a single compact structure of the main components of the audio-visual stimulation and e.t. and of their adjustments, capable of operating even outside the magnet and with the subject having his head in the coil, so as to quickly and comfortably carry out. the adaptation of the system to the subject to then insert it into the magnet and move on immediately to perform the functional magnetic resonance imaging test.

9. integrated stimulation unit for functional magnetic resonance imaging according to claim !, characterized by a sound diffusion apparatus for five acoustic cliannels, accomplished with piezoelectric loudspeakers mounted on the coil according to a spatial aixangement which is characteristic of those used in conventional listening spaces.

10. Integrated stimulation unit for functional magnetic resonance imaging according to claim 1 and 9, characterized by the use of high-power piezoelectric loudspeakers connected to audio amplifiers having adequate power and equalization, so as to be well audible by the subject in the coil although he is wearing headphones or other acoustic protections from the loud operation noise of the resonance.

1 1. ntegrated stimulation unit for functional magnetic resonance imaging according to claim 1 , characterized in that the non-depolarizing screen A3 is placed at about 10 cm from the subject's nape.

32. integrated stimulation unit tor functional magnetic resonance imaging according to claim 1 , characterized in that the video camera assembly CI is placed at about 40 cm from the subject' s eyes.