Method for acquiring T2* and vascular images from magnetic resonance imaging system

Abstract

According to the present invention, accurate T2* and vascular images are concurrently acquired by acquiring a T2* image without a flow compensation and a T2* image with a flow compensation and subtracting the two images to reconstitute an image showing the flow phenomenon. Furthermore, an accurate T2* image can be acquired by using the readout gradient without the flow compensation and also the accurate T2* and vascular images can be concurrently acquired. The clinical judgment for blood flow rate of the blood vessel and the clinical judgment for acute stroke can be concurrently made, and so the present invention can be widely utilized in clinical practice.

Claims

1. A method for producing at least one T2* weighted image and a vascular image from a magnetic resonance imaging (MRI) system, the method comprising: performing a scan with the MRI system including repeatedly alternatively using a first readout gradient which corresponds with a T2* weighted image without a flow compensation and a second readout gradient which corresponds with a T2* weighted image with a flow compensation, with a desired resolution of image, to acquire a signal; reconstituting the signal to produce the T2* weighted image without a flow compensation and the T2* weighted image with a flow compensation; subtracting, the T2* weighted image without a flow compensation from the T2* weighted image with a flow compensation, or the T2* weighted image with a flow compensation from the T2* weighted image without a flow compensation to acquire a third image; and producing the vascular image based at least on the third image.

2. The method according to claim 1 wherein an echo time (TE) to apply the first and second readout gradients is more than 15 msec.

3. The method according to claim 1 wherein the first readout gradient is applied in a negative direction and then applied in a positive direction so as to be two times the area of the gradient applied in the negative direction.

4. The method according to claim 1 wherein the second readout gradient is applied in a positive direction in a constant amount, applied two times in a negative direction in the constant amount and then applied in a positive direction so as to be two times the area of the constant amount.

5. The method according to claim 1 wherein the T2* weighted image without a flow compensation is a minimized flow phenomenon image and the T2* weighted image with a flow compensation is a weighted flow phenomenon image.

6. The method according to claim 5, further comprising outputting the minimized flow phenomenon image and the third image or a MIP (Maximum Intensity Projection) image of the third image as the result.

7. The method according to claim 1, wherein the phase without a flow compensation is expressed as the following equation (3): $\begin{array}{cc}\begin{array}{c}=-{}_0^{+2+2}{G}_0{a}_{0}dt-{}_0^{+2}{G}_0{a}_{1}tdt-{}_{+2}^{+2+}\\ \left(G_0-G_r\right)a_{1}tdt-{}_{+2+}^{+2+2}\left(G_0+G_r\right)a_{1}tdt\\ =-\left[G_0{a}_0\left(+2+2\right)+G_r{a}_1{}^2-\frac{1}{2}{G}_0{a_1\left(+2+2\right)}^2\right].\end{array}& \left[\mathrm{Formula}3\right]\end{array}$

8. The method according to claim 7, wherein the phase with a flow compensation is expressed as the following equation (4): $\begin{array}{cc}\begin{array}{c}=-{}_0^{+2+2}{G}_0{a}_{0}dt-{}_0^{}{G}_0{a}_{1}tdt-{}_{}^{+}\left(G_0+G_r\right)\\ a_{1}tdt-{}_{+}^{+2+}\left(G_0-G_r\right)a_{1}tdt-{}_{+2+}^{+2+2}\left(G_0+G_r\right)\\ a_{1}tdt\\ =-[G_0{a}_0\left(+2+2\right)+G_r{a}_1\left({}^2-{}^2\right)-\frac{1}{2}{G}_0{a}_1\\ {\left(+2+2\right)}^2].\end{array}& \left[\mathrm{Formula}4\right]\end{array}$

9. The method according to claim 1, wherein a certain interval not applied by the readout gradient is comprised between the use of the first readout gradient and the use of the second readout gradient.

10. The method according to claim 1, further comprising the step of performing MIP (Maximum Intensity Projection) on the third image to reconstitute a vascular image.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 represents a pulse sequence of FLASH (Fast Low Angle Shot) used to obtain a conventional MRA imaging.

(2) FIGS. 2(a) to 2(c) represent the images acquired using pulse sequences of FLASH and show images of vein, artery and vein & artery, respectively.

(3) FIG. 3 represents the pulse sequences used in the method concurrently acquiring T2* and vascular images according to an embodiment of the present invention.

(4) FIG. 4 is a view illustrating the theoretical background of the pulse sequences used in the method for concurrently acquiring T2* and vascular images according to an embodiment of the present invention.

(5) FIGS. 5 and 6 represent the phase without a flow compensation and the phase with a flow compensation, respectively, in the method for concurrently acquiring T2* and vascular images according to an embodiment of the present invention.

(6) FIGS. 7(a)-7(c) and FIGS. 8(a)-8(c) represent the images acquired using pulse sequences according to an embodiment of the present invention, which show an image with a flow compensation and an image without a flow compensation, respectively.

(7) FIGS. 9(a)-9(c) represents images obtained from the subtraction of the images shown in FIGS. 7a-7(c) and FIGS. 8(a)-8(c).

(8) FIGS. 10(a)-10(c) shows MIP (Maximum Intensity Projection) images of the images shown in FIGS. 9(a)-9(c).

DETAILED DESCRIPTION FOR CARRYING OUT THE INVENTION

(9) Hereinafter, preferred embodiments of the method for acquiring images through the magnetic resonance imaging system according to the present invention are described in detail with reference to the attached drawings. However, description concerning well-known functions and configurations which can make the subject matter of the present invention unnecessarily vague will be omitted.

(10) The configurations of the MRI system as applied to the present invention are widely known in the art and so the description thereof is omitted.

(11) FIG. 3 represents the pulse sequences used in the method for concurrently acquiring T2* and vascular images according to an embodiment of the present invention.

(12) In FIG. 3, Z-gradient, Y-gradient and X-gradient represent a slice selection, a phase decoding and a readout gradient, respectively.

(13) As shown in FIG. 3, the readout gradient without a flow compensation is applied in a negative direction and then applied in a positive direction so as to be two times the area of the gradient applied in the negative direction.

(14) Further, the readout gradient with a flow compensation is not applied for a certain time within echo time (TE), then applied in a positive direction, applied in a negative direction, again applied in a negative direction and then again applied in a positive direction. The amount of area of the readout gradients thus applied is (1, 1, 1, 2) when the amount of area of the readout gradient applied first is 1.

(15) The signal acquired by the readout gradient without a flow compensation and the signal acquired by the readout gradient with a flow compensation are repeatedly acquired with a desired resolution of images to reconstitute the images, thereby acquiring two images that reflect different flow phenomenon from each other. The two images thus acquired are the minimized flow phenomenon first T2* image and the weighted flow phenomenon second T2* image.

(16) By acquiring T2* image according to the increase and decrease of such flow phenomenon, one minimized flow phenomenon T2* image (first T2* image), and two acquired images (first and second T2* images) can be subtracted to further obtain a third image showing the blood flow part. Then, MIP on the third image is conducted to reconstitute the vascular image.

(17) The images acquired through T2* and vascular images according to one embodiment of the present invention are a minimized flow phenomenon T2* image acquired using the readout gradient without a flow compensation, and a vascular image obtained from the subtraction between two T2* images. According to this method, the T2* image and the vascular image can be concurrently acquired from one scan and also a more accurate image can be obtained even with respect to the T2* image. These will be described later.

(18) The embodiment of the present invention described with reference to FIG. 3 uses the pulse sequence in the form of first applying the readout gradient without a flow compensation, and then applying the readout gradient with a flow compensation. It does not matter if the order thereof is changed.

(19) FIG. 4 is a view illustrating the theoretical background of the pulse sequences used in the method for concurrently acquiring T2* and vascular images according to an embodiment of the present invention. In order to acquire the T2* image, TE time must be more than 15 msec. Given these circumstances, the modeling is made assuming offset gradient (Go). The formula must account for this.

(20) As shown in FIG. 4, the entire echo time is configured as in the following equation (1):
TE=+2+2[Formula 1]

(21) The movement of the blood flow can be expressed as the following equation (2):
Flow motion=a.sub.0+a.sub.1t

(22) wherein a.sub.0 is a static component and does not influence the phase due to G.sub.r in t=TE. Hence, considering only the a.sub.1t component is enough, and the phase disagreement due to an offset gradient (G.sub.0) should be considered even at a.sub.0 component. For the first compensation, =.

(23) The phase without a flow compensation is expressed as the following equation (3):

(24) $\begin{array}{cc}\begin{array}{c}=-{}_0^{+2+2}{G}_0{a}_{0}dt-{}_0^{+2}{G}_0{a}_{1}tdt-{}_{+2}^{+2+}\\ \left(G_0-G_r\right)a_{1}tdt-{}_{+2+}^{+2+2}\left(G_0+G_r\right)a_{1}tdt\\ =-\left[G_0{a}_0\left(+2+2\right)+G_r{a}_1{}^2-\frac{1}{2}{G}_0{a_1\left(+2+2\right)}^2\right]\end{array}& \left[\mathrm{Formula}3\right]\end{array}$

(25) The ordinary vascular image acquired by weighting the T1 value of the blood usually uses a flow compensation, but the method according to the embodiment of the present invention does not use a flow compensation in acquiring the T2* image, through which it has been found that a more accurate T2* image can be acquired.

(26) Meanwhile, the phase with a flow compensation is expressed as the following equation (4):

(27) $\begin{array}{cc}\begin{array}{c}=-{}_0^{+2+2}{G}_0{a}_{0}dt-{}_0^{}{G}_0{a}_{1}tdt-{}_{}^{+}\left(G_0+G_r\right)\\ a_{1}tdt-{}_{+}^{+2+}\left(G_0-G_r\right)a_{1}tdt-{}_{+2+}^{+2+2}\left(G_0+G_r\right)\\ a_{1}tdt\\ =-[G_0{a}_0\left(+2+2\right)+G_r{a}_1\left({}^2-{}^2\right)-\frac{1}{2}{G}_0{a}_1\\ {\left(+2+2\right)}^2]\end{array}& \left[\mathrm{Formula}4\right]\end{array}$

(28) FIGS. 5 and 6 represent the phases without a flow compensation and with a flow compensation according to the equation above.

(29) FIGS. 7(a)-7(c) and 8(a)-8(c) represent the images acquired using pulse sequences according to an embodiment of the present invention, which show an image with a flow compensation and an image without a flow compensation, respectively.

(30) FIGS. 9(a)-9(c) represents images obtained from the subtraction of the images shown in FIGS. 7(a)-7(c) and FIGS. 8(a)-8(c). FIGS. 10(a)-10(c) shown MIP (Maximum Intensity Projection) imaging of the imaging shown in FIGS. 10(a)-10(c).

(31) As shown in FIGS. 9 and 10, in the case of using the pulse sequence according to one embodiment of the present invention, accurate T2* and vascular imaging can be concurrently acquired. That is, the difference between the gradient with a flow compensation and the gradient without a flow compensation can be used to identify the flow phenomenon of the blood in the blood vessel. In the flow compensation method used for an ordinary vascular image which is acquired by weighing the T1 value of the blood, it has been analyzed and identified that, in the case of acquiring T2* images, it is useful to not perform a flow compensation in the acquisition of the more accurate image.

(32) Using such an image, the clinical judgment for the blood flow rate of the blood vessel and the clinical judgment for acute stroke can be concurrently made, and so the present invention can be widely utilized in a clinical practice.

(33) Furthermore, as shown in FIGS. 9(a)-9(c) and FIGS. 10(a)-10(c), the method for concurrently acquiring T2* and vascular images according to one embodiment of the present invention can ensure the quality of image which is comparable with that of images obtained by established protocols, and shorten the total acquisition time by 40% as compared with the case of sequentially acquiring traditional T2* weighted image and MRA images.

(34) Although the forgoing is described with reference to the preferred embodiments of the present invention, it will be appreciated by those of ordinary skill that various changes and modifications can be made to the present invention without departing from the sprit and scope of the invention as set forth in the claims below.