Magnetic resonance elastography imaging method and brain and abdomen region imaging actuator

Abstract

A device to estimate mechanical properties of brain and abdomen organs undergoing magnetic resonance elastography (MRE) includes electromagnetic actuators, mechanical wave generation mechanisms, and a control unit to generate oscillatory motion signals in synchronization with the MR scanner. Preserving only the shear wave component, a local fitting algorithm is used to estimate the viscoelastic properties of soft tissues. The device is portable and easy to implement in clinical diagnostics, and can be modified to measure other soft materials.

Claims

1. An actuator for magnetic resonance elastography imaging of the abdomen region, consisting of: a base (1′), a lifting base (2′) that is placed on the base (1′) and adapted for sliding relative to the base (1′), a first locking mechanism placed between the base (1′) and the lifting base (2′) for fixing a relative position of the base (1′) and the lifting base (2′), a mounting base (3′) that is installed on a side of the lifting base (2′), a sliding base (4′) installed on the mounting base (3′) that is adapted for sliding relative to the mounting base (3′), a second locking mechanism installed between the mounting base (3′) and the sliding base (4′) for limiting the motion between the sliding base (4′) and the mounting base (3′), and a supporting rod (5′) hinged on the sliding base (4′), wherein a rotating shaft (41′) is installed on top of the sliding base (4′), and the supporting rod (5′) is hinged to the rotating shaft (41′); wherein the actuator for magnetic resonance elastography imaging also includes an electromagnetic coil (6′) installed on a first end of the supporting rod (5′) that is connected to the sliding base (4′), a compressing plate (7′) installed at a second end of the supporting rod (5′), a hinge axis of the supporting rod (5′) is perpendicular to an axis of the supporting rod (5′), and the lifting base (2′) has curved grooves on top for placing the supporting rod (5′); wherein the hinge axis of the supporting rod (5′) is an axis of the rotating shaft (41′); wherein the lifting base (2′) connects to the base (1′) and the first locking mechanism includes threaded bolts between the lifting base (2′) and base (1′); wherein the mounting base (3′) has two parallel sliding grooves (31′), the sliding base (4′) has a positioning block that is adapted for engaging the sliding grooves (31′), and the second locking mechanism includes threaded bolts between the mounting base (3′) and the sliding base (4′); wherein the electromagnetic coil (6′) is connected to an amplifier, a sinusoidal signal with a frequency of ω is produced by using a signal generator, and an amplifier output is adjusted; and wherein the actuator is adjustable for the patients with different sizes and weights.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

Brief Descriptions of the Drawings

(1) FIG. 1 is a flow chart of the procedure of the magnetic resonance elastography method of the present invention.

(2) FIG. 2 is a displacement distribution map of the wave propagation inside brain tissue at a vibration frequency of 60 Hz (units in μm).

(3) FIG. 3 is distribution maps of U1′ (left) and U1″ (right) from FIG. 2.

(4) FIG. 4 is estimated G′ (left) and G″ (right) values using least square fitting based on the wave propagation equation from FIG. 3. (units in Pa)

(5) FIG. 5 is a schematic view of the magnetic resonance elastography actuator for brain tissue of the present invention.

(6) FIG. 6 is a schematic view of the actuator for brain tissue mounted on the head coil.

(7) FIG. 7 is a schematic view of the head coil in FIG. 6.

(8) FIG. 8 is a schematic view of the magnetic resonance elastography actuator for abdomen region.

(9) FIG. 9 is a schematic view of the implementation of the abdomen actuator in use.

(10) FIG. 10 is a sequence map of one magnetic resonance elastography.

(11) Notations: 1. Head coil; 11. inner space of the head coil; 2. Base; 3. Pressing device; 31 U-shape plate; 32. Compressing rod; 33. Compressing plate; 4. Sliding base; 41. Rotating shaft; 5. Supporting rod; 51. Upper part of the supporting rod; 52 lower part of the supporting rod; 6. Coil; 7. Compressing plate; 1′. Base; 2′. Lifting base; 3′. Installation base; 31′. Sliding grooves; 4′. Sliding base; 41′. Rotating shaft; 5′. Supporting rod; 6′. Coil; 7′. Compressing plate.

DETAILED DESCRIPTION OF THE INVENTION

Description of the Preferred Embodiments

Embodiments (I)

(12) A magnetic resonance elastography method for measuring the elastic properties of soft tissues, wherein the method contains the following steps:

(13) {circle around (1)} induce mechanical waves in soft tissues with a frequency of ω;

(14) {circle around (2)} measure the phase of the image which corresponds to the wave displacement;

(15) {circle around (3)} calculate displacement u(t) with respect to time using,

(16) $\stackrel{.fwdarw.}{u}=\frac{\omega \stackrel{.fwdarw.}{\phi }}{\mathrm{\gamma \pi }{\mathrm{NG}}_0}$
where {right arrow over (u)} is the displacement vector and {right arrow over (ω)} is the phase value.

(17) {circle around (4)} apply Fast Fourier Transform to u(t), calculate the first principal component U.sub.1.

(18) {circle around (5)} apply spatial filtering to U.sub.1, and calculate the curl value of the filtered U.sub.1. In this way, only the shear wave component was kept.

(19) {circle around (6)} use least square fitting to estimate the shear modulus at each pixel point, calculate the storage modulus G′ and loss modulus G″ to get a distribution map of the modulus of the soft tissue.

(20) where ω is the vibration wave frequency, γ is the gyroscopic ratio, G.sub.0 is the magnitude of the motion encoding gradient, N is the number of motion encoding cycles of applying the motion encoding gradient.

(21) The displacement of the wave propagation {right arrow over (u)}(t) in a linear elastic material satisfies

(22) $\rho \frac{{\partial }^2\stackrel{.fwdarw.}{u}}{\partial t^2}=\mu {\nabla }^2\stackrel{.fwdarw.}{u}+\left(\lambda +\mu \right)\nabla \left(\nabla .Math.\stackrel{.fwdarw.}{u}\right).$

(23) If considering only the shear wave propagation, the equation could be simplified as

(24) $\rho \frac{{\partial }^2\stackrel{.fwdarw.}{u}}{\partial t^2}=\mu {\nabla }^2\stackrel{.fwdarw.}{u},$

(25) and in terms of ω,
−ρω.sup.2U.sub.1=μ∇.sup.2U.sub.1,

(26) where ρ is the density of the soft tissue, μ is shear modulus, λ is lame's constant.

(27) Substitute μ and U.sub.1 with complex variables μ=μ′+iμ″ and U.sub.1=U.sub.1′+iU.sub.1″ to get the equation of estimating shear modulus μ

(28) $-{\mathrm{\rho \omega }}^2\left[\begin{array}{c}{U}_1^{\prime }\\ U_1^{″}\end{array}\right]=\left[\begin{array}{cc}{\nabla }^2{U}_1^{\prime }& -{\nabla }^2{U}_1^{″}\\ {\nabla }^2{U}_1^{″}& {\nabla }^2{U}_1^{\prime }\end{array}\right]\left[\begin{array}{c}{\mu }^{\prime }\\ {\mu }^{″}\end{array}\right]$
where ρ is the density of the soft tissue, ω is the vibration frequency, U.sub.1′ is the first principal component of the filtered and curled U.sub.1 value, U.sub.1″ is the corresponding imaginary component, μ′ is the storage modulus, μ″ is the loss modulus.

(29) The least square fitting method of step {circle around (6)} is: calculate U.sub.1′ and U.sub.1″ values of each pixel point, then estimate the shear modulus μ using a patch of pixels surrounding the target pixel, the fitting window size could be 3×3, 5×5, or 7×7.

(30) The filtering algorithm in step {circle around (5)} is selective based on the dimension of {right arrow over (u)}(t). If {right arrow over (u)}(t) is a 3D array, the mean filtering is applied; when {right arrow over (u)}(t) is a 2D vector, ideal filter or Butterworth filter is applied.

(31) Displacement of the wave propagation in brain tissue with an actuation frequency of 60 Hz is shown in FIG. 2. FIG. 3 is distribution maps of U.sub.1′ (left) and U.sub.1″ (right) from FIG. 2. FIG. 4 is estimated G′ (left) and G″ (right) values using least square fitting based on the wave propagation equation from FIG. 3.

Embodiments (II)

(32) A magnetic resonance elastography imaging apparatus for brain tissue, wherein the apparatus comprises the base 2 placed on top of the head coil 1; two clamps 3 for holding the base 2 on top of head coil 1; a positioning adjustment slider 4 that mounted on base 2, which can slide along the grooves of the base 2; a place holding device that can fix the position of slider 4 on top of base 2; a vibration plate 7 that connects to a supporting rod 5 mounted on the slider 4, placed at the end of the supporting rod 5. Both the head coil 1 and the base 2 have open grooves for allowing motions of the supporting rod 5. The end of the supporting rod 5 that connected to the vibration plate was placed inside of the head coil 1, in the open space of 11.

(33) The supporting rod 5 comprises an upper part 51, a sliding hole is inside the upper part 51, a lower part 52 that can slide inside the sliding hole, and a locking part that can fix the relative position between the upper part 51 and lower part 52.

(34) The adjustment slider 4 has a rotating shaft 41 on top, the supporting rod 5 is hinged to the rotating shaft 41. The axial direction of the rotating shaft 41 is perpendicular to the adjustment slider 4.

(35) Clamps 3 comprises two U-shaped plates 31 that are arranged on both sides of the base 2 and are connected between the magnetic resonance head coil 1 and the base 2, two fixing bars 32 screwed on each of the U-shaped plates 31 respectively, and two compressing plates 33 at the ends of the fixing bars 32. When in a compressed state, the compressing plates 33 press on the upper surface of the base 2.

Embodiments (III)

(36) As shown in FIG. 8, an actuator for magnetic resonance elastography imaging of the abdomen region, wherein it comprises of a base 1′, a lifting base 2′ that is placed on the base 1′ and can sliding relative to the base 1′, a locking mechanism I placed between bases 1′ and lifting base 2′ for fixing the relative position of the base 2′ and lifting base 2′, a mounting base 3′ that is installed on the side of the lifting base 2′, a sliding base 4′ installed on the mounting base 3′ that can slide relative to the mounting base 3′. A locking mechanism II installed between the mounting base 3′ and the sliding base 4′ for limiting the motion between the sliding base 4′ and the mounting base 3′, and the supporting rod 5′ hinged on the sliding base 4′. The actuator for magnetic resonance elastography imaging also includes a electromagnetic coil 6′ installed on the supporting rod 5′ that is closed to the mounting base 3′, a compressing plate 7′ installed at the end of the supporting rod 5′. The hinge axis of the supporting rod 5′ is perpendicular to the axis of the supporting rod 5′, and close to the coil 6′. The lifting base 2′ has curved grooves on top for placing the supporting rod 5′.

(37) The rotating shaft 41′ is installed on top of the sliding base 4′, the supporting rod 5′ is hinged to the rotating shaft 41′.

(38) The lifting base 2′ connects to the base 1′ via sliding block module, the locking mechanism I are threaded bolts between the lifting base 2′ and base 1′.

(39) The mounting base 3′ has two parallel sliding grooves 31′, the sliding base 4′ has positioning block that matches the sliding grooves 31′, the locking mechanism II are a threaded bolts between the mounting base 3′ and the sliding base 4′.

(40) When using the actuator for magnetic resonance elastography imaging of the abdomen region, first place the base 1′ on top of the patient bench. An elastic belt is used to fix the actuator on the side of the patient. Adjust the position of the lifting base 2′ relative to base 1′, and adjust the position of sliding base 4′ in the grooves of 31′, so that compressing plate 7′ is placed right on the desired examination position, then lock the positions using locking mechanism I and II. As shown in FIG. 9, connect the actuator coil 6′ to an amplifier, and produce a sinusoidal signal with a frequency of ω using the signal generator. Adjust the amplifier output so that the vibration magnitude is amiable for the patient.

(41) As shown in FIG. 10, add motion-encoding gradient to imaging sequences such as gradient-echo sequence. The motion-encoding gradient can be either sinusoidal or trapezoidal. Set the frequency of the motion-encoding gradient to the same as the actuator vibration frequency, ω. The motion-encoding gradient can be applied to either X, Y, or Z direction of the gradient, for measuring the displacement in the designated direction. Produce the actuation signal from the signal generator, triggered by the sequence output from the magnetic resonance scanner. The signal is amplified and output to the actuator coil 6.

(42) Take an example using the motion-encoding gradient G.sub.z in the Z direction. Add motion-encoding gradient with a frequency of ω in the Z direction. Set up the trigger signal so that the motion-encoding gradient G.sub.z has a phase shift θ of relative to the actuator frequency. Sample four temporal points

(43) $\left(\theta =0,\frac{\pi }{4},\frac{\pi }{2},\frac{3\pi }{4}\right)$
for measurement, and acquire images at each temporal point. By combining the sampling information, the mechanical shear wave propagation in the soft tissue can be acquired base on the images.

(44) The above preferred embodiments are described for illustration only, and are not intended to limit the scope of the invention. It should be understood, for a person skilled in the art, that various improvements or variations can be made therein without departing from the spirit and scope of the invention, and these improvements or variations should be covered within the protecting scope of the invention.