Method for magnetic resonance imaging

Abstract

The present invention relates to a method for Magnetic Resonance Imaging to depict an object by an image having pixels representing volume element of the object. The method comprises: Immobilizing the object and acquiring a reference image at a first echo time immediately following an excitation, wherein said reference image is complex-valued, with a reference magnitude value and a reference phase value for each pixel; acquiring a target image of the object with said receiver coil at a pre-selected second echo time, wherein said target image is complex-valued, with a target magnitude value and a target phase value for each pixel; subtracting, pixel by pixel, the reference phase value from the target phase value to obtain a corrected phase value for each pixel; and obtaining said image from said target magnitude values and said corrected phase values.

Claims

1. A method for Magnetic Resonance Imaging to depict a 3-dimensional object by an image having pixels representing volume elements of the object, comprising: immobilising the object and acquiring a reference image of the object with a receiver coil at a first echo time immediately following an excitation by a transmitter coil, wherein said reference image is complex-valued, representing each volume element by a pixel with a reference magnitude value and a reference phase value; keeping the object immobilised and acquiring a target image of the object with said receiver coil at a pre-selected second echo time, longer than said first echo time, following the same or another excitation by said transmitter coil, wherein said target image is complex-valued, representing each volume element by a pixel with a target magnitude value and a target phase value; subtracting, pixel by pixel, the reference phase value from the target phase value to obtain a corrected phase value for each pixel; and obtaining said image from said target magnitude values and said corrected phase values, wherein the method is applied to each of a plurality of receiver coils arranged around said immobilised object to obtain a respective plurality of said images, followed by the step of calculating a combined phase image, pixel by pixel, according to ${}_S=\left(\underset{p}{.Math.}{m}_{T,p}.Math.e^{-i\left({}_{T,p}-{}_{R,p}\right)}\right)$ with <() denoting the four-quadrant tangent inverse operator, m.sub.T,p being the target magnitude value of a pixel, representing a volume element of the object, in a target image acquired with the p.sup.th receiver coil, .sub.T,p being the target phase value of said pixel, .sub.R,p being the reference phase value of a pixel, representing said volume element, in a reference image acquired with the p.sup.th receiver coil, and .sub.S being the phase value of a pixel, representing said volume element, in the combined phase image.

2. The method of claim 1, wherein said first echo time is less than 1 ms.

3. The method of claim 1, wherein said first echo time is less than 100 s.

4. The method of claim 1, wherein the reference image is acquired at a lower pixel resolution than the target image and, prior to said subtracting, is upscaled to the pixel resolution of the target image.

5. The method of claim 1, wherein the respective reference or target images acquired with said plurality of receiver coils are all acquired following one and the same excitation.

6. The method of claim 1, wherein the method is applied to each of a plurality of receiver coils arranged around said immobilised object to obtain a respective plurality of said images, followed by the step of calculating a combined magnitude image, pixel by pixel, according to $m_S=.Math.\underset{p}{.Math.}{m}_{T,p}.Math.e^{-i\left({}_{T,p}-{}_{R,p}\right)}.Math.$ with || denoting the magnitude operator, m.sub.T,p being the target magnitude value of a pixel, representing a volume element of the object, in a target image acquired with the p.sup.th receiver coil, .sub.T,p being the target phase value of said pixel, .sub.R,p being the reference phase value of a pixel, representing said volume element, in a reference image acquired with the p.sup.th receiver coil, and m.sub.S being the magnitude value of a pixel, representing said volume element, in the combined magnitude image.

7. The method of claim 1, wherein the method is applied to each of a plurality of receiver coils arranged around said immobilised object to obtain a respective plurality of said images, followed by the step of calculating a combined magnitude image, pixel by pixel, according to $m_W=\sqrt{.Math.\underset{p}{.Math.}{m}_{T,p}^2.Math.e^{-i\left({}_{T,p}-{}_{R,p}\right)}.Math.}$ with || denoting the magnitude operator, m.sub.T,p being the target magnitude value of a pixel, representing a volume element of the object, in a target image acquired with the p.sup.th receiver coil, .sub.T,p being the target phase value of said pixel, .sub.T,p being the reference phase value of a pixel, representing said volume element, in a reference image acquired with the p.sup.th receiver coil, and m.sub.W being the magnitude value of a pixel, representing said volume element, in the combined magnitude image.

8. A method for Magnetic Resonance Imaging to depict a 3-dimensional object by an image having pixels representing volume elements of the object, comprising: immobilising the object and acquiring a reference image of the object with a receiver coil at a first echo time immediately following an excitation by a transmitter coil, wherein said reference image is complex-valued, representing each volume element by a pixel with a reference magnitude value and a reference phase value; keeping the object immobilised and acquiring a target image of the object with said receiver coil at a pre-selected second echo time, longer than said first echo time, following the same or another excitation by said transmitter coil, wherein said target image is complex-valued, representing each volume element by a pixel with a target magnitude value and a target phase value; subtracting, pixel by pixel, the reference phase value from the target phase value to obtain a corrected phase value for each pixel; and obtaining said image from said target magnitude values, and said corrected phase values, wherein the method is applied to each of a plurality of receiver coils arranged around said immobilised object to obtain a respective plurality of said images, followed by the step of calculating a combined magnitude image, pixel by pixel, according to $m_S=.Math.\underset{p}{.Math.}{m}_{T,p}.Math.e^{-i\left({}_{T,p}-{}_{R,p}\right)}.Math.$ with || denoting the magnitude operator, m.sub.T,p being the target magnitude value of a pixel, representing a volume element of the object, in a target image acquired with the p.sup.th receiver coil, .sub.T,p being the target phase value of said pixel, .sub.R,p being the reference phase value of a pixel, representing said volume element, in a reference image acquired with the p.sup.th receiver coil, and m.sub.S being the magnitude value of a pixel, representing said volume element, in the combined magnitude image.

9. The method of claim 8, wherein the respective reference or target images acquired with said plurality of receiver coils are all acquired following one and the same excitation.

10. The method of claim 8, wherein said first echo time is less than 1 ms.

11. The method of claim 8, wherein said first echo time is less than 100 s.

12. The method of claim 8, wherein the reference image is acquired at a lower pixel resolution than the target image and, prior to said subtracting, is upscaled to the pixel resolution of the target image.

13. A method for Magnetic Resonance Imaging to depict a 3-dimensional object by an image having pixels representing volume elements of the object, comprising: immobilising the object and acquiring a reference image of the object with a receiver coil at a first echo time immediately following an excitation by a transmitter coil, wherein said reference image is complex-valued, representing each volume element by a pixel with a reference magnitude value and a reference phase value; keeping the object immobilised and acquiring a target image of the object with said receiver coil at a pre-selected second echo time, longer than said first echo time, following the same or another excitation by said transmitter coil, wherein said target image is complex-valued, representing each volume element by a pixel with a target magnitude value and a target phase value; subtracting, pixel by pixel, the reference phase value from the target phase value to obtain a corrected phase value for each pixel; and obtaining said image from said target magnitude values, and said corrected phase values, wherein the method is applied to each of a plurality of receiver coils arranged around said immobilised object to obtain a respective plurality of said images, followed by the step of calculating a combined magnitude image, pixel by pixel, according to $m_W=\sqrt{.Math.\underset{p}{.Math.}{m}_{T,p}^2.Math.e^{-i\left({}_{T,p}-{}_{R,p}\right)}.Math.}$ with || denoting the magnitude operator, m.sub.T,p being the target magnitude value of a pixel, representing a volume element of the object, in a target image acquired with the p.sup.th receiver coil, .sub.T,p being the target phase value of said pixel, .sub.R,p being the reference phase value of a pixel, representing said volume element, in a reference image acquired with the p.sup.th receiver coil, and m.sub.W being the magnitude value of a pixel, representing said volume element, in the combined magnitude image.

14. The method of claim 13, wherein the respective reference or target images acquired with said plurality of receiver coils are all acquired following one and the same excitation.

15. The method of claim 13, wherein said first echo time is less than 1 ms.

16. The method of claim 13, wherein said first echo time is less than 100 s.

17. The method of claim 13, wherein the reference image is acquired at a lower pixel resolution than the target image and, prior to said subtracting, is upscaled to the pixel resolution of the target image.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described in further details by means of exemplary embodiments thereof under reference to the enclosed drawings, in which:

(2) FIG. 1 shows the generation of a complex-valued image in Magnetic Resonance Imaging according to the state of the art;

(3) FIG. 2 shows a flow chart of different embodiments of the method of the invention for Magnetic Resonance Imaging;

(4) FIG. 3 depicts a timing diagram of an exemplary acquisition cycle used in the method of FIG. 2; and

(5) FIGS. 4a to 4d depict exemplary reference (FIG. 4a) and target images (FIG. 4b) acquired and complex-valued images (FIG. 4c) obtained according to the method of FIG. 2, and a magnitude image and phase image (FIG. 4d) calculated according to the method of FIG. 2.

DETAILED DESCRIPTION

(6) Magnetic Resonance Imaging (MRI) is used in radiology to visualize soft tissues, non-invasively and in vivo. The process of generating an image of a patient usually consists of the following steps: creating a bulk (longitudinal) magnetisation in the tissue by placing and immobilising the patient inside a powerful static magnetic field; creating regional variation in this magnetic field, and thereby in the resonant frequency and phase of the nuclei, with three comparatively small, linear perpendicular magnetic fields (gradients); disturbing the magnetisation with one or more pulses of radio-frequency (RF) electromagnetic radiation (excitation) applied at the resonant frequency by one or more transmitter coils, tipping the magnetisation into the transverse plane (which is perpendicular to the static magnetic field); and acquiring the RF signals emitted by the tissues as the magnetisation relaxes to the longitudinal direction, by one or more receiver coils.

(7) In 2-dimensional tomographic imaging, space encoding of the signal works as follows. The first gradient field (slice select) is applied during RF excitation, so that only spins in a narrow section of tissue are excited. The second (readout) is applied while the signal is being acquired, so that spins along the readout axis are encoded by their resonant frequency. A number of such excitation-readout steps are acquired with differing applications of the third (phase-encode) gradient, which encodes the signal along that gradient direction according to a dephasing rate. In 3-dimensional imaging, slice encoding is replaced by a second loop of phase-encoding steps in the slice gradient direction.

(8) The RF signals emitted by the patient are captured as echoes at a certain time after excitation, by one or more receiver coils. Fourier-transforming the acquired MR-signals generates images of the patient, which consist of a large number of pixels representing volume elements, reflecting the local proton density and magnetic properties of the tissue. The acquired MR signals are complex-valued; images of the patient, as the Fourier-transform of the acquired signal, are therefore likewise complex-valued. That is, image signals consist of a magnitude value and a phase value and can be represented in conventional complex number notation.

(9) Some MRI methods use only the magnitude of the MR-signal. Nevertheless, the phase value contains additional information, which can be clinically useful. While the magnitude value of the signal decays exponentially with echo time, the phase value evolves linearly, and reflects local deviation from the main magnetic field strength. The sensitivity of phase to local magnetic field also allows local iron (which is highly paramagnetic) to be imaged. Phase values can be used, in combination with magnitude values, e.g. to depict veins, due to the iron content of the deoxyhemoglobin iron, in a technique known as Susceptibility-Weighted Imaging. These techniques benefit from high static magnetic field, which provides enhanced magnetic susceptibility effects and higher quality images due to increased signal-to-noise ratio (SNR).

(10) As shown in FIG. 1, captured signals of an acquired complex-valued signal image 1 of an object to be investigated cannot be interpreted straightforwardly. Fourier-transforming image 1 results in a complex-valued image 2 which can be separated on a pixel-by-pixel basis into a magnitude image M and a phase image .

(11) However, the phase image suffers from a conceptual ambiguity: As adding 2 to the phase of a signal results in the same measured phase value, the encoding range in captured phase values is effectively limited to 2 radians. Variations in phase values of an object when passing through 2 lead to discontinuities in the phase image known as phase wraps 3, which distort the readability and obscure interesting phase features. Different algorithms are known in the art to remove such phase wraps 3 from a phase image .

(12) Moreover, the phase values of the phase image contain a time-independent phase offset, which, inter alia, depends on the position of the receiver coil of the MRI machine relative to the object to be examined and, to a certain extent, on the individual volume element to be examined. Phase images acquired by different receiver coils which generally are arranged as phased array coil elements around the 3-dimensional object can therefore not be combined with ease.

(13) With reference to FIGS. 2 to 4, different embodiments of a method for MRI accommodating these phase offsets shall now be described.

(14) According to FIG. 2 in a first step 4 of said method a first or reference image S.sub.R of the object, here: the reference image S.sub.R,p of an exemplary receiver coil from a multitude of receiver coilswith indices p=1, 2, 3, . . . of the MRI machine (see FIG. 4a), is acquired at a first or reference echo time TE.sub.R (FIG. 3). Said reference echo time TE.sub.R immediately follows the respective excitation of the object by the transmitter coil of the MRI machine. In fact, said reference echo time TE.sub.R may be the shortest echo time selectable for said receiver coil; it can be less than 1 ms or even less than 100 s. The shortest echo time selectable, inter alia, depends on the MRI machine and the measurement sequence applied, e.g. on the sequence for space encoding, or on whether or not the receiver coil is also used as a transmitter coil, etc.

(15) As can be seen from the schematic representation of FIG. 3, acquiring each reference image S.sub.R or S.sub.R,p, respectively, takes a certain, finite acquisition period 5; similarly, the excitation is conducted by a pulse 6 of finite length. The excitation could alternatively be conducted by a predefined series of pulses, each pulse 6 for acquiring one or more volume elements step-by-step. Any excitation, whether a single pulse or a series of pulses, may be generated by a single or a plurality of transmitter coils, as known in the art.

(16) As shown in FIG. 4a, said reference images S.sub.R,1, S.sub.R,2, . . . , S.sub.R,p areas a Fourier-transformation of complex measured datacomplex-valued, similarly to the image 2 of FIG. 1. Each of the complex-valued reference images S.sub.R,p can therefore be separated into a reference magnitude image M.sub.R,p and a reference phase image .sub.R,p, in which the volume elements of the object are represented by complex-valued pixels comprising each a reference magnitude value m.sub.R,p and a reference phase value .sub.R,p, respectively (shown without index for the respective pixel in FIG. 4a).

(17) The phase value .sub.R,p acquired in a receiver coil at an echo time TE.sub.R depends both on the local deviation from the static magnetic field B.sub.0 and the phase offset .sub.o,p for that receiver coil according to .sub.R,p=2.Math.B.sub.0.Math.TE.sub.R+.sub.o,p (neglecting phase wraps.) Hence, in the limit TE.sub.R.fwdarw.0, a phase value .sub.R,p approximates the phase offset .sub.o,p.

(18) Reverting to FIG. 2, in a step 7which can be performed using the same excitation pulse 6 or pulses as in step 4, or using a different excitation pulse 6 or pulsesa second or target image S.sub.T of the object (here: the target image S.sub.T,p of the receiver coil with index p, see FIG. 4b), is acquired at a pre-selected second or target echo time TE.sub.T over a finite acquisition period 8 (FIG. 3), while keeping the object immobilised with respect to step 4. The target echo time TE.sub.T is usually significantly longer than said reference echo time TE.sub.R.

(19) The reference echo time TE.sub.R and/or the target echo time TE.sub.T may follow the same excitation for a multitude of receiver coils; alternatively, for each receiver coil different reference and/or target echo times TE.sub.R,p, TE.sub.T,p, following the same or a separate excitation for each coil could be used.

(20) The target images S.sub.T,p are complex-valued, representing the volume elements of the object selected by pixels comprising each a target magnitude value m.sub.T,p and a target phase value .sub.T,p, respectively (pixel index not shown in FIG. 4b).

(21) As shown in FIGS. 4a and 4b, both the reference images S.sub.R,p and the target images S.sub.T,p can be separated into reference magnitude images M.sub.R,p and reference phase images .sub.R,p, as well as target magnitude images M.sub.T,p and target phase images .sub.T,p, respectively.

(22) According to an optional embodiment of the method, in step 4 the reference images S.sub.R,p may be acquired at a lower pixel resolution than the target images S.sub.T,p in step 7. In this case, the reference images S.sub.R,p are each upscaled in a step 9 to the pixel resolution of the respective target image S.sub.T,p. Such an upscaling step 9 can be based on generally known numeric algorithms for interpolation and/or extrapolation.

(23) In a step 10 following the steps 4 and 7 (and step 9, where applicable) the reference phase values .sub.R,p of a reference image S.sub.R,p are subtracted from the target phase values .sub.T,p of the respective target image S.sub.T,p, pixel by pixel, to obtain a corrected phase value .sub.K,p for each pixel, and a complex-valued image S.sub.K,p is composed from said target magnitude values m.sub.T,p and said corrected phase values .sub.K,p, e.g. according to
s.sub.K,p=m.sub.T,p.Math.e.sup.i(.sup.T,p.sup..sup.R,p.sup.)=m.sub.T,p.Math.e.sup.i(.sup.K,p.sup.)(eq. 1)
with m.sub.T,p being the target magnitude value of a pixel, representing a volume element of the object, in a target image acquired with the p.sup.th receiver coil, .sub.T,p being the target phase value of said pixel, .sub.R,p being the reference phase value of a pixel, representing said volume element, in a reference image acquired with the p.sup.th receiver coil, .sub.K,p being the corrected phase value of a pixel, representing said volume element, in the image S.sub.K,p acquired the p.sup.th receiver coil, and s.sub.K,p being the complex value of said pixel in the image S.sub.K,p.

(24) As shown in FIGS. 4a to 4c, method steps 4, (9), 7 and described so far are applied to each of a plurality of receiver coils in the same way, yielding a respective plurality of said images S.sub.K,p. The images S.sub.K,p may each be separated into a target magnitude image M.sub.T,p and a corrected phase image .sub.K,p, which are formed of the target magnitude values m.sub.T,p and the corrected phase values .sub.K,p, respectively, and which could be evaluated separately. In a preferred embodiment of the method, however, the target magnitude images M.sub.T,p and corrected phase images .sub.K,p acquired with different receiver coils are evaluated in combination, as will now be detailed in the following.

(25) In a first variant, a combined phase image .sub.S can be calculated, pixel by pixel, in a following (optional) step 11 according to

(26) $\begin{array}{cc}{}_S=\underset{p}{.Math.}{m}_{T,p}.Math.e^{-i\left({}_{T,p}-{}_{R,p}\right)}& \left(\mathrm{eq}.2\right)\end{array}$
with <() denoting an angle operator, which, e.g., can be implemented as a four-quadrant tangent inverse operation (generally known as a tan 2-function in trigonometry), and .sub.S being the phase value of a pixel, representing a respective volume element, in the combined phase image .sub.S.

(27) Alternatively or in addition thereto, in an optional step 12, which may be executed prior to, in parallel with, or after step 11, a combined magnitude image M.sub.S can be calculated, pixel by pixel, according to:

(28) $\begin{array}{cc}{m}_S=.Math.\underset{p}{.Math.}{m}_{T,p}.Math.e^{-i\left({}_{T,p}-{}_{R,p}\right)}.Math.& \left(\mathrm{eq}.3\right)\end{array}$
and/or, see optional step 13 in FIG. 2, according to:

(29) $\begin{array}{cc}{m}_W=\sqrt{.Math.\underset{p}{.Math.}{m}_{T,p}^2.Math.e^{-i\left({}_{T,p}-{}_{R,p}\right)}.Math.}& \left(\mathrm{eq}.4\right)\end{array}$
with || denoting the magnitude operator (generally known from absolute value operations), m.sub.S being the magnitude value of a pixel, representing a respective volume element, in the combined magnitude image M.sub.S, and m.sub.W being the magnitude value, based on a root-sum of squares calculation in eq. 4, of a pixel, representing a respective volume element, in the combined magnitude image M.sub.S.

(30) Step 13, when applied, can be executed prior to, in parallel with, or after step 11 and step 12.

(31) As the example of FIG. 4d shows, the combined phase image .sub.S is phase offset-compensated and yields phase values .sub.S for each pixel which represent the respective volume element more reliably than the target phase values .sub.T,p of a single target phase image .sub.T,p; and the combined magnitude image M.sub.Swhether calculated according to eq. 3 or eq. 4shows a substantial increase in SNR as compared to any single target magnitude image M.sub.T,p. Thereby, the structures in a patient's body are depicted in significantly more detail.

(32) The invention is not limited to the embodiments described in detail above, but encompasses all variants and modifications thereof which will become apparent to the person skilled in the art from the present disclosure and which fall into the scope of the appended claims.