MECHANICAL ACTUATOR AND A METHOD FOR MAGNETIC RESONANCE ELASTOGRAPHY USING CENTRIFUGAL FORCE

Abstract

A mechanical actuator for Magnetic Resonance Elastography (MRE) and a method for inducing shear waves for MRE as well as respective system and method for MRE using the principle of centrifugal force for wave induction is disclosed. The mechanical actuator comprises a passive driver including a rotational turbine vibrator having an eccentric weight, the turbine vibrator being powered by a fluid (e.g. compresses air or water), and an active driver configured to control the pressure of the fluid powering the turbine vibrator.

Claims

1. A mechanical actuator for Magnetic Resonance Elastography comprising: a passive driver including at least one rotational turbine vibrator comprising a housing, a turbine rotor arranged in the housing and an eccentric weight, said turbine vibrator being powered by a fluid, and an active driver configured to control the pressure of the fluid powering the turbine vibrator.

2. The mechanical actuator of claim 1, wherein the components constituting the passive driver are of non-magnetic and/or non-metallic material or materials.

3. The mechanical actuator of claim 2, wherein the housing, the turbine rotor, and the unbalance are made of polymer and/or are 3D printed.

4. The mechanical actuator of claim 3, wherein the polymer is any one of polyamide, polyurethanes, polypropylene, polycarbonate and acrylonitrile butadiene styrene.

5. The mechanical actuator of claim 1, wherein the passive driver comprises an adaptor plate connected to or merged with the rotational turbine vibrator, said adaptor plate configured to be positioned on and/or or fixed to a surface of a region of an elastic body to be examined.

6. The mechanical actuator of claim 1, wherein the generated centrifugal force is in the range of 0.1 to 50 N.

7. The mechanical actuator of claim 1, wherein the active driver comprises a proportional pressure regulator configured to control the pressure of the fluid powering the rotational turbine vibrator by adjusting a control voltage.

8. The mechanical actuator of claim 7, wherein the proportional pressure regulator is configured to stop the inflow of a fluid in the passive driver when (i) no control voltage is applied; and/or (ii) in case of power loss: and/or (iii) upon user input; and/or wherein the proportional pressure regulator is configured to increase the control voltage gradually or stepwise to a predetermined value during a start-up phase; and/or wherein the active driver comprises a manual valve configured to stop the inflow of a fluid in the passive driver.

9. The mechanical actuator of claim 1, wherein the passive driver comprises a plurality of said rotational turbine vibrators connected in series through a common axle.

10. The magnetic resonance elastography system comprising the mechanical actuator of claim 1 and a magnetic resonance imaging apparatus.

11. A method for inducing shear waves in an elastic body for magnetic resonance elastography comprising: providing the mechanical actuator of claim 1; positioning the passive driver of the mechanical actuator on a region of the elastic body to be examined, controlling, by the active driver of the mechanical actuator, the pressure of the compressed fluid powering the rotational turbine vibrator, thereby generating, by the rotational turbine vibrator, a vibration force having a predetermined frequency and/or magnitude.

12. The method of claim 11, wherein the control voltage is adjusted to increase gradually or stepwise up to a predetermined value during a start-up phase; and/or wherein the inflow of fluid in the passive driver is stopped when no control voltage is applied and/or in case of power loss and/or upon user input.

13. A method for magnetic resonance elastography comprising: inducing shear waves in a region of an elastic body to be examined according to the method of claim 11; imaging the propagation of the induced shear waves, thereby obtaining at least one wave image; and obtaining elasticity data of the region of the elastic body to be examined based on the at least one wave image.

14. The method of claim 13, wherein the shear waves are induced synchronously at a plurality of locations within said region of the elastic body to be examined.

15. A method for producing a mechanical actuator for Magnetic Resonance Elastography according to claim 1, comprising producing the housing, the turbine rotor and the unbalance and/or a customized adaptor plate by 3D printing.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0052] FIG. 1A is a plan view showing the structure and components of the pneumatic turbine;

[0053] FIG. 1B is a side view showing the structure and components of the pneumatic turbine;

[0054] FIG. 1C shows schematically adaptor plates of varying designs;

[0055] FIG. 2 shows schematically a MRE system including a pneumatic turbine;

[0056] FIG. 3 shows the force generated by the pneumatic turbine as a function of the frequency of vibration;

[0057] FIG. 4 shows a coronal view of a cylindrically shaped gelatin phantom;

[0058] FIG. 5 illustrates the generation of a given force with turbines having different geometries;

[0059] FIG. 6 illustrates the generation of a force in a direction parallel to the body surface (left hand side) and in a direction perpendicular to the body surface (right hand side);

[0060] FIG. 7 shows schematically an exemplary passive driver including a plurality of turbine vibrators.

DETAILED DESCRIPTION

[0061] The mechanical actuator according to an example comprises a passive driver including or constituted by a pneumatic turbine (also referred to as a pneumatic vibrator or rotational pneumatic vibrator) 10 powered by compressed air, as illustrated in FIGS. 1A and 1B. In this examples, compresses air is used, however other gases or liquids may be used for driving the pneumatic turbine 10.

[0062] The pneumatic turbine comprises a housing 16 having an interior rotor chamber 17, which may have a substantially cylindrical form. A turbine rotor 18 having a central driving shaft (axle) 19 is arranged within the interior rotor chamber 17. The rotor 18 is rotatably supported within the chamber by means of bearings, for example spherical rolling bearings 21. E.g. compressed air is supplied to the pneumatic turbine 10 via an air inlet 14 and exits the pneumatic turbine 10 through a sound absorber 22. The input and output air streams are directed to and out of the rotor chamber 17 via conduits formed in the housing (not shown in FIGS. 1A and 1B). In this example a pneumatic fitting (for example a pneumatic fitting M022A0605 of Norgren GmbH, Germany) serves as the air inlet 14 for inflow of compressed air into the turbine. The sound absorber 22, which serves as an air outlet, may be made of plastic (for example M/1545, Norgren GmbH, Germany).

[0063] The turbine rotor 18 is provided with an eccentric weight (unbalance) 20 arranged in an opening thereof between the axle 19 and the rotor blades, which causes a rotary vibration. Preferably, the unbalance 20 is exchangeable, which enables adapting the turbine 10 for different levels of force generation. The unbalance 20 may be made of polyamide or other suitable polymers. The unbalance may for example have a half-cylindrical shape having for example a height h in the range of 2 mm to 50 mm, more specifically in the range of 5 mm to 40 mm; and a weight m in the range of 2 g to 30 g, more specifically in the range of 3 g to 15 g. The distance r.sub.ecc of the center of mass of the unbalance 20 to the rotational center of the turbine rotor 18 may be for example in the range of 1 mm to 15 mm, more specifically in the range of 3 mm to 10 mm. Thus, for vibration (actuation) frequencies ranging from 10 Hz to 400 Hz, more specifically in the range of 30 Hz to 120 Hz the generated forces may be in the range of 0.01 N to 150 N more specifically between 0.01 N and 10 N.

[0064] The geometry of the housing the turbine may be designed in such a way that it encloses the turbine rotor and provides a connective part at the bottom such that an adaptor plate can be fixed to the housing. The turbine rotor may have a diameter in the range of 10 mm to 100 mm, more specifically in the range of 20 mm to 50 mm. The length of the turbine may be in the range of 10 mm to 100 mm, more specifically in the range of 20 mm to 50 mm. It may also be possible to design a rotor, where the length of the cylindrical shape is larger compared to its diameter. The number of rotor blades may range from 6 to 50, more specifically in the range of 10 to 20 blades. The blades may be distributed symmetrically around the rotational center.

[0065] As explained above, the pneumatic turbine 10 is a rotational pneumatic vibrator creating a centrifugal force by the eccentric weight (unbalance) 20 within the turbine rotor 18. The generated force F depends on the weight of the unbalance m.sub.ecc, the distance r.sub.ecc of the center of mass of the unbalance to the rotational center of the turbine 10 and more specifically to the rotational center of the turbine rotor 18 as well as the rotational speed of the turbine rotor 18. The generated force F can be computed via the formula:

F=m.sub.eccr.sub.ecc.sup.2

Consequently, a larger force can be generated by one or more of 1) using materials with higher densities for the unbalance, 2) changing the geometry such that the distance of the unbalance's center of mass to the rotational center increases, 3) enlarging the volume of the unbalance, and 4) increasing the frequency of the turbine. For sufficiently large but tolerable wave actuation, the ratio of the weight of unbalance to the size of turbine may be optimized.
Accordingly, a pre-set force F.sub.n and wave actuation frequency f.sub.n may be generated through the variation of a number of parameters (individually or in combination).

[0066] For example, a pre-set (nominal) force may be generated with housings having different geometries. As shown in FIG. 5, a turbine vibrator with a small diameter but large cylindrical height can exert the same force as a turbine vibrator with a large diameter but small cylindrical height. Accordingly, for a given force, the size and geometry of the housing and thus the size and geometry of the turbine rotor can be adapted to the outer geometric restrictions on the patient and in the MR-bore. In particular, the housing geometry may be selected such as to fit in holes of body, flex or other coils (vendor independent) and still exert the same nominal force. Examples include a housing with a constant width and variable depth for flat designs; a housing with variable width and small depths for small but high designs, etc. In contrast, the geometry of a ball vibrator disclosed in above mentioned patent document 1 is highly dependent on the radius of the ball and increases proportionally in height and width and depth with increasing ball size.

[0067] Further, different forces may be generated by employing unbalances having different weight and/or arrangement. The turbine vibrator may be configured such that the unbalance is removable and replaceable with another unbalance, for example an unbalance being made of the same material, but having a different volume (for example by changing the cylindrical height of the unbalance) or by an unbalance having the same geometry, but being made of a material with higher or lower density. The turbine rotor may for example exhibit one or more unbalance accommodating portions (e.g. bores or recesses), wherein the unbalance is releasably accommodated in the respective unbalance accommodating portions.

[0068] At least some of the parts of the pneumatic turbine 10, such as the housing 16 and/or the turbine rotor 18 and/or the unbalance 20 may be 3D printed. The 3D printed parts can be designed with a suitable CAD software (for example Autodesk Inventor, Autodesk GmbH, Germany) and may be made by selective laser sintering (for example using 3D printers of Materialise GmbH, Germany).

[0069] Since the pneumatic turbine 10 is located within a scanner room where MRI imaging is carried out, it is exposed to high magnetic fields, for example 1 T, in particular about 1.5 to 3 T. Thus, preferably no magnetic components are used, i.e. the components constituting the pneumatic turbine 10 (as wells as the adaptor plate 12) are preferably made of non-magnetic and/or non-metallic materials, such as plastic, glass, non-ferromagnetic metals (such as brass) and/or combinations thereof. For any static part, non-magnetic materials are sufficient. For any rotating part (such as turbine, unbalance, bearings) the material should be non-metallic as well.

[0070] For example polyamide (PA) may be used for all 3D printed parts. Other non-magnetic material such as for example polyurethanes (PU), polypropylene (PP), polycarbonate (PC) acrylonitrile butadiene styrene (ABS) or other (thermoplastic) polymers may be used. The spherical rolling bearings may be made of polyoxymethylene (a thermoplastic) and glass (based on DIN 625-626). The sound absorber 22 may be made of plastic. In this example, the pneumatic fitting that serves as the inlet 14 for compressed air into the turbine is the only metal-containing part. However, the pneumatic fitting is made of non-magnetic brass and does not experience forces during MR measurements. Other suitable non-magnetic materials may be used, such as for example polymers (PA, PU, PP, PC, ABS, epoxy resins) or non-magnetic ceramics.

[0071] At the bottom, the pneumatic turbine 10 and more specifically the housing 16 can be attached to an adaptor plate 12 of varying geometries. FIG. 1C shows schematically adaptor plates 12 of varying designs that can be attached to the housing 16. For example, the housing 16 may include a connector part for connecting the housing to adaptor plates with different geometries. The adaptor plate 12 is configured to be placed on the surface of the volume/region of interest of a human or animal body. The size and/or geometry of the turbine 10 and/or the surface geometry of the adaptor plate 12 may be optimized for in-vivo imaging. The adaptor plate may be optimized to fit the geometry of the location on the body surface for wave actuation. For example, if the examined body part is femoral, slightly curved adaptor plates may used. If the examined body part is an abdomen, the adaptor plate may have a substantially plane surface. Circular plates with a diameter for example in the range of 20 mm to 200 mm, more specifically in the range of 50 mm to 150 mm, or rectangular plane or bend plates with an area in the range of 4 cm.sup.2 to 225 cm.sup.2, more specifically in the range of 9 cm.sup.2 to 100 cm.sup.2 may be used.

[0072] The connecting part of the turbine 10 to the adaptor plate 12 may be designed to fit in one of the pockets of a commercially available body coil 24 for fixation (for example Body coil 18 from Siemens, Erlangen, Germany) and, therefore, does not need additional mounting support. However, an additional mounting support may also be provided.

[0073] FIG. 2 shows schematically a MRE system comprising a mechanical actuator using centrifugal force. The mechanical actuator comprises a passive driver including a pneumatic turbine 10 (which may be the above described pneumatic turbine) and an active driver 30. The pneumatic turbine 10 is positioned (via an adaptor plate 12) on the surface of the region of interest of a patient 26. For example, the pneumatic turbine may be arranged under or within one of the pockets of a body coil 24 for fixing the patient 26.

[0074] The air inlet 14 of the pneumatic turbine 10 is connected by a conduit (pipe) 28 and via a pressure regulator 32 to a pressure hose 40. The pressure hose is typically available in all scanner rooms of a medical clinic in accordance with the norm DIN 13260-2. The pressure hose 40 typically supplies compressed air with a nominal pressure of p.sub.hose=5 bar (5*10.sup.5 Pascal).

[0075] All magnetic (i.e. MR-unsafe) and active electronic parts of the MRE system are located in a control room 50 (i.e. are placed outside of the scanner room 52). These parts comprise the active driver unit/system for regulating the pressure of the e.g. compressed air powering the turbine such that a specific actuation frequency is achieved.

[0076] The active driver unit (active driver) 30 comprises a proportional pressure regulator 32 having a controllable resistance 33 (such as for example VPPM-6L-L-G18-0L6H-V1N-S1, Festo Vertrieb GmbH & Co. Kg, Germany) connected to the in-house pressure hose 40 and the air inlet 14 via the conduit 28. The compressed air from the pressure hose 40 is fed to the proportional pressure regulator 32, which regulates the output pressure of the compressed air via a control voltage. The proportional pressure regulator 32 sets the output air pressure by adjusting the control voltage, for example by adjusting the control voltage within a range of 0 V to 10 V (corresponding to minimal/maximum pressure output). During start up, the control voltage, and according the output pressure, may increase gradually or stepwise until the nominal frequency of the pneumatic turbine 10 is reached.

[0077] To ensure controlled output of fluid, several safety features may be implemented alone or in combination. For example, the pressure regulator 32 may be designed such as to be normally closed, meaning that if no control voltage is applied to the system, or in case of a power loss, the valve of the pressure regulator closes and no fluid is fed into the turbine. Further, the control voltage may be controlled to increase stepwise during start up, such that a constant communication with the patient can be maintained to ensure that the induced vibration level is tolerable for the patient. Thirdly, an emergency stop button (for example an emergency stop button on a graphical user interface 37 of the active driver) can be implemented, configured to instantly set the control voltage to 0 V and stop the output of the fluid. Lastly, a manual stop valve 38 may be placed between the in-house pressure hose 40 and the pressure regulator 32. This allows an operator to manually stop the inflow of the fluid in the driver system.

[0078] A probe/sensor 34 providing a feedback on the actuation frequency of the pneumatic turbine 10, such as an MR-safe fiber-optic sensor (for example WLL180T, Sick AG, Germany), may be attached to the housing of the turbine 10 to provide feedback on the actuation frequency to the pressure regulator 32. The optical signal(s) detected by the sensor 34 is/are converted into electrical signal(s) indicative of the rotational speed of the turbine by a suitable electro-optical interface 36. The measured rotational speed of the turbine may be evaluated, for example by using a suitable electronic circuitry and may be fed into a feedback loop that regulates the control voltage, i.e. the output pressure, of the proportional pressure regulator 32. The electronic circuitry may be for example a general-purpose or dedicated electronic circuitry configured to implement a software package, such as Labview, NI USB-6525, National Instruments Germany GmbH, Germany, for processing the detected actuation frequency. The electronic circuitry may be connected to a user interface 37.

[0079] The fiber-optic sensor 34 may be configured to detect one or more signals per rotation of the turbine. For example, optical markers may be placed on one or more of the rotor blades. The optical marker(s) is/are detected by the optical sensor 34 during the turbine rotation, thereby producing the one or more signals. The detection of more than one optical signal per rotation increases the temporal resolution and/or shortens the measurement intervals. In contrast, the ball vibrator disclosed in the above mentioned Patent Document 1 is capable of detection of only one signal per rotation. This means, that the temporal resolution is quite restricted. One has to measure for a long time to receive a sufficiently accurate temporal resolution of the actuation frequency (with little information about fluctuation in the signal frequency).

[0080] The number of rotations may be counted over a time interval of 1000 ms and subsequently the frequency is calculated, updated in the active driver system and displayed in the user interface 37. During the technical evaluation of the actuator, nominal frequencies may be set between 15 Hz and 60 Hz with a step width of 5 Hz for a time interval of 30 seconds each. The stability of each frequency may be recorded and evaluated with suitable software programs, for example Matlab of The MathWorks, Inc. USA.

[0081] The force generated by the pneumatic turbine 10 may be measured for example by a load detector (not shown in FIG. 2), for example a precision miniature load cell such as 9206-V0001 of Burster Przisionstechnik GmbH & Co. KG, Germany, connected to the bottom of the turbine 10.

[0082] The feasibility of using the mechanical actuator for wave actuation during MRE was experimentally confirmed on a cylindrically shaped phantom 42 having elasticity similar to that of soft tissue. This was achieved by adding 7.5% gelatin (240 bloom) to 1000 ml distilled water. A cubical inclusion 44 (side length 50 mm) made of distilled water with 1.0 agarose was placed in the center of the phantom 42. A concentration of 0.03% sodium azide acting as a preservative was added to both materials. All chemicals were acquired from Carl Roth GmbH & Co. KG (Germany). The gelatin-agarose phantom was placed in a 3 T whole-body scanner (Magnetom Skyra, Siemens, Germany). Imaging was performed using an eight-channel receive-only phase-based array coil. A gradient-echo based sequence was employed (TE/TR=20/50 ms, matrix size=25660, FOV=450 mm, slice thickness=5 mm).

[0083] Three 3D printed unbalances made of polyamide with varying cylinder heights (h.sub.1=9.5 mm, h.sub.2=19.0 mm, h.sub.3=38.0 mm) were designed to examine the influence of varying eccentric weights upon the generated forces. The weights of the 3D-printed unbalances were m.sub.1=4.5 g, m.sub.2=8.7 g and m.sub.3=17.4 g, respectively. The calculated distance of the center of mass of the unbalance to the rotational center of the turbine was r.sub.ecc=8.8 mm. Thus, for frequencies ranging from 15 Hz to 60 Hz, the theoretically generated forces are expected to be between 0.41 N to 1.6 N for the smallest and 6.50 N to 26.0 N for the largest unbalance (see FIG. 3).

[0084] An initial response behavior of the turbine was triggered at 10 Hz. Nominal frequencies were set between 15 Hz and 60 Hz (step width of 5 Hz) during the feasibility study and were kept stable within a range always smaller than 0.3 Hz. As predicted, the generated forces depended on the weight of the unbalance within the turbine and increased from 0.67 N to 3.09 N (4.5 g) and from 2.7 N to 7.77 N (17.4 g) in the analyzed frequency range of 15 Hz to 60 Hz (see FIG. 3). The experimentally obtained force levels matched the theoretical values closely in the lower frequency range, with any observed deviations probably caused by increased friction and additional weight of the turbine. The proposed pneumatic vibrator generated forces that were sufficiently large to penetrate the phantom entirely during the feasibility study. Thus, shear waves propagated through the entire volume of the phantom 42.

[0085] Magnitude and phase images were obtained at a frequency of 60 Hz generated by the proposed pneumatic vibrator. FIG. 4 shows a coronal slice of the cylindrically shaped gelatin phantom acquired with MRI. The left hand side of FIG. 4 shows a magnitude image of the phantom with the propagating shear waves, in which the inclusion is indicated with an arrow. The right hand side of FIG. 4 shows a phase image acquired with a gradient-echo based sequence. The spatial length of the same phase angle is larger within the inclusion compared to the phase angle in the background material (black markers). The stiffer inclusion 44 could be detected by an increased wave length compared to the wave length in the background material, as indicated in FIG. 4. The passive driver did not produce any artifacts in the acquired MR images.

[0086] Thus, the proposed mechanical actuator for MRE has advantages over conventionally applied pneumatic cushions, since using centrifugal forces rather than acoustic pressure levels, it is possible to achieve sufficiently large wave actuation at higher frequencies as compared to air cushions, where the amplitude of sound pressure waves dampens with increasing frequencies. The actuation frequency can be for example regulated smoothly between 10 Hz and 150 Hz and between 10 Hz and 70 Hz for unbalances with a weight of 4.5 g and 17.4 g, respectively. The maximum applicable frequency is only restricted by the available in-house air pressure system but well within the range of currently applied frequencies of wave actuation for MRE imaging.

[0087] The driver is easy to set up and can be incorporated within existing equipment in a medical clinic. The passive driver is MR-safe and does not interfere with the imaging procedure. The design is adaptable and may be easily reproduced through low-cost 3D-printing.

[0088] In operation, the mechanical actuator according to any of the above described examples may be provided and set-up as described above. An exemplary method for inducing vibration in an elastic body for magnetic resonance elastography comprises positioning the passive driver on a region of the elastic body (for example a human or animal body) to be examined and controlling, by the active driver, the pressure of the fluid powering the rotational turbine vibrator. The rotational turbine vibrator generates thereby a vibration force having a predetermined frequency and/or magnitude, which may be controlled by the active driver as described above. The waves propagating through the region to be examined may be detected and evaluated by using conventional MRI/MRE apparatuses and methods. Based on the detected waves, qualitative and/or quantitative data of the elastic properties of various areas of the examined region (such as for example the elastic properties of the various tissues in the examined region) may be determined. On the basis of the information, the areas/tissues within the examined region may be discriminated.

[0089] Further as explained above, several safety features may be implemented alone or in combinations.

[0090] As explained above, the proposed mechanical actuator for Magnetic Resonance Elastography and the respective methods according to embodiments of the invention may have one or more of the following features and/or advantages: [0091] a) Modular set up, in particular: [0092] Easily adaptable and customizable through simple exchange of components, such as unbalances, adaptor plates, etc.; [0093] Cost efficient, since one turbine can be equipped with variable eccentric weights for different levels of force generation and variable adaptor plates; [0094] Simple and fast production of additional parts (i.e. unbalances, adaptor plates) through 3D printing; [0095] b) Variable geometry of the adaptor plates or housing of the rotational turbine vibrator [0096] Depending on the location to be imaged (thigh, abdomen, liver, head) and patient size (pediatric, small and big adult patient) adaptor plates and housings with varying surface geometries (e.g. circular, rectangular, irregular shaped; plan or bend; see for example FIG. 1C) can be chosen to be optimally attached to the particular body surface; [0097] c) Multi-parametrical configuration of the wave actuation system for a nominal exerted force/mechanical wave amplitude at body surface [0098] As explained above a pre-set force F.sub.n and wave actuation frequency f.sub.n may be generated through the variation of a number of parameters (individually or in combination), including: [0099] variable housing and thus turbine geometry: For example, the size of the housing can be adapted to outer geometric restrictions on the patient and in the MR-bore; [0100] variable weight of the unbalances: For example unbalances made of the same material, but with different volumes (through for example the change in the cylindrical height) or unbalances having the same geometry, but being made of materials with different densities may be employed; [0101] d) Variable direction of force generation [0102] The turbine vibrator can be used (similar to the ball vibrator disclosed in Patent Document 1) to generate a force in a direction parallel to the body surface. However, it is also possible to generate the force in a direction perpendicular to the body surface, as illustrated in FIG. 6. This is the same direction of the wave actuation of the conventionally used air cushions. Hence, the proposed turbine vibrator is compatible with the same MR-motion encoding sequences as used for conventionally used air cushions; [0103] e) More accurate and faster control of the generated vibration force: [0104] As explained above, the rotational velocity of the turbine rotor and thus the generated force may be measured at higher temporal resolution and/or with reduced measurement times by detecting more than one optical signal during one rotation of the turbine rotor. This facilitates the control of the generated vibration force; [0105] f) Better adaptability and control of the force generation over a broad force range: [0106] It is important, in particular for medical applications, to not only create a vibration large enough to obtain a sufficient signal to noise ratio, but also to be able to limit the vibration in the upper regime. For one, there are limits on the vibration (see EU whole-body vibration limit EU 2002/44/EC). For another, there is a limit of the vibration amplitude that is tolerable to the patient during the examination. This limit may vary for different body locations (e.g. head vs liver/lung vs thigh). Since a number of parameters of the proposed turbine vibrator (such as frequency and geometric requirements), may be appropriately selected and altered (individually or in combination), a better and more accurate control of the generated force can be achieved over a broad range of forces. Thus, optimizing the force generation around both the lower limit (to receive sufficient SNR) and around the upper limit can be more easily realized (with additional frequency and geometric requirements); [0107] g) Compact design: [0108] Due to its compact design the passive driver may be placed at various locations of the elastic body to be examined and is easily set up and fixated to a patient using a body coil. Further, more than one passive driver can be placed on a patient for a multi-location multi-wave induction.

[0109] For example, it is possible to combine a plurality of turbine rotors for multi-location actuation. For example, the passive driver may comprise a plurality of turbine vibrators connected in series to another through a common axle (common drive shaft). The active driver synchronously drives the turbine vibrators connected in series. FIG. 7 shows one example of connecting three turbine vibrators 10a, 10b and 10c in series though a common axle 19. The number of connected turbine vibrators is not restricted to three and may vary. The use of multiple synchronously driven mechanical actuators enables wave actuation at multiple locations on the body with synchronized mechanicals waves. This may increase the spatial resolution of the obtained elastograms, thereby enabling better detection of small inclusions, such as small tumors.

[0110] A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made. For example, the steps described can be performed in a different order and still achieve desirable results. Further, the pneumatic turbine may have other dimensions and/or configuration and may be subject to optimization for in-vivo imaging. Accordingly, other embodiments are within the scope of the claims.