Prospective slice tracking (PST) through interleaved acquisitions of dynamic 2D cardiac MRI

Abstract

To generate MR images of the heart.SOLUTION: A method comprises: acquiring and reconstructing a first set of 2D reference images of the heart during a reference heartbeat, the first set comprising a first reference image acquired along a first reference plane position thorough the heart; and acquiring a second set of 2D images of the heart during a second heartbeat. The step of acquiring the second set comprises: acquiring a first image in the second set acquired along the first reference plane; determining, based on the first reference image and the first image in the second set, a first in-plane displacement of the heart in the image plane of the first reference plane position; determining a shifted plane position for a second image in the second set to be acquired perpendicular to the first image in the second set based on the determined first in-plane displacement; and acquiring and reconstructing the second image along the shifted plane position.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present application relates to a method for generating MR images of the heart, a corresponding MR imaging system, a computer program comprising program code, and a carrier containing the computer program. [Background technology]

[0002] Dynamic 2D cardiac MRI (magnetic resonance imaging) is affected by respiratory motion during dynamic time-series acquisitions. In any 2D MR imaging, the effects of respiratory motion can be divided into two categories: 1) cardiac motion during dynamics within the imaged 2D plane (in-plane) and 2) cardiac motion in and out of the imaged 2D plane (through-plane) between acquisitions. While the impact and visibility of in-plane and through-plane motion can vary depending on the anatomy and a given slice location, both are typically significant in standard slice orientations used in cardiac MRI, particularly short-axis imaging. Furthermore, significant motion in both categories can appear intermittently, i.e., irregularly and unpredictably, during deep breathing.

[0003] While in-plane motion can be corrected retrospectively by image-based motion correction, the problem with through-plane motion is that the acquisition itself no longer contains moving parts, making retrospective image-based motion correction impossible and requiring some form of prospective correction (e.g., slice tracking).Through-plane motion is particularly detrimental for perfusion quantification, which typically involves long dynamic data windows from the same location within the heart.

[0004] Prospective motion correction is often achieved using a separate navigator acquisition before each individual acquisition, where information from the navigator can be used to reject data outside of an acceptance window and / or shift the slice position of the next acquisition according to the motion indicated by the navigator.

[0005] For segmented 3D cardiac imaging, this can be a 1D cone / pencil beam navigator of the diaphragm, estimating motion in each direction and either predictively influencing subsequent image acquisition (i.e., shifting) or retrospectively correcting the data.

[0006] For dynamic 2D imaging, prospective slice tracking with a 1D diaphragmatic navigator is possible, but several problems remain. 1) The assumption of a constant tracking factor between diaphragmatic and cardiac motion, which does not cover intermittent or idiosyncratic deep breathing events, or the need for a potentially inaccurate learning phase. 2) The complex interaction between the 1D navigator and the inversion or saturation preceding pulses typically used in dynamic imaging, necessitating the use of an additional restore pulse that may affect the actual diagnostic image acquisition. 3) When applying diaphragmatic foot head motion to myocardial short-axis slices, its orientation depends on individual anatomy, and the foot head is not necessarily the only contributor to through-plane motion.

[0007] Therefore, there is a need to overcome the above problems and to provide a cardiac MR imaging method that can overcome the occurring through-strain motion in an effective manner. Summary of the Invention

[0008] This problem is solved by the features of the independent claims. Further aspects are set out in the dependent claims.

[0009] According to a first aspect, a method for generating MR images of the heart is provided. The method includes acquiring and reconstructing a first set of 2D reference images of the heart during a reference heartbeat. The first set comprises a first reference image acquired along a first reference plane through the heart. A second set of 2D images of the heart is further acquired during a second heartbeat. The acquisition of the second set includes acquiring a first image in the second set along the first reference plane. A first in-plane displacement of the image plane of the heart at the first reference plane is determined based on the first reference image and the first image in the second set. A shifted plane is then determined for a second image in the second set that intersects with the first image in the second set based on the first in-plane displacement determined from the first reference image and the first image in the second set. The second image is then acquired and reconstructed along the shifted plane. All images in the first set of 2D images and all images in the second set of 2D images are diagnostically relevant.

[0010] Because the second image in the second set intersects with the first image in each set, through-plane motion can be determined for the first image in each set. Because the image planes intersect, any in-plane motion in the corresponding second image in each set becomes through-plane motion. Before the second image in the second set is determined during the second cardiac cycle, in-plane displacement can be determined using various matching algorithms. In the matching algorithm, the first reference image or some anatomical landmark in this image is compared to the first image in the second set, which is acquired along the same plane. In the intersecting direction, this corresponds to through-plane motion in the image orientation of the second image, and therefore, the displacement can be taken into account when determining the position of the second, or shifted, plane relative to the second image in the second set.

[0011] Preferably, if the second set includes N images, the positions of the planes of the additional images 2 through N in the second set are parallel to each other and intersect with the position of the first reference plane. In this case, the positions of the planes of the additional images can be determined based on the first in-plane displacement. All of the additional images in the second set are parallel planes through the heart, e.g., short-axis views of the heart viewed from different positions through the heart. For all of these images, the in-plane displacement determined from the corresponding first image serves to set and match the image planes at the second heartbeat, so that substantially the same anatomical regions are shown in the images compared to the first set of images.

[0012] In another aspect, the first set can be acquired and reconstructed in a manner that includes acquiring and reconstructing N reference images during a reference cardiac beat. In this case, the second set of images also consists of N images. For both sets, the location of the image plane of image n can intersect the location of the image plane of image n+1 in both sets, where n is an integer between 1 and N-1. In this case, the location of the shifted plane is determined for image n+1 in the second set based on the corresponding in-plane displacement determined for image n in the first set and image n in the second set. This allows each image of the second set in the second cardiac beat to be corrected based on the preceding images in the current and immediately preceding cardiac beats. This is possible because each successive image has an image plane that intersects with the immediately preceding image. That is, if the first image plane is a short-axis view, the subsequent image plane is a long-axis view, or vice versa.

[0013] Here, the second image in the first set and the second image in the second set can both have a plane location along the same image plane, such as along the long axis of the heart or the short axis of the heart, in which case the images therebetween have intersecting image planes.

[0014] It may also be said that the images in each of the first and second sets with odd-numbered image numbers represent images of the heart along the long axis of the heart, while the images in each of the first and second sets with even-numbered image numbers represent images of the heart along the short axis of the heart. Similarly, the images with odd-numbered image numbers may be short-axis views, and the images with even-numbered image numbers may be long-axis views.

[0015] Preferably, the second image and the additional reference image in the second set have substantially the same anatomical location within the heart, which is possible because the in-plane motion determined from the corresponding first image serves to obtain the position of the image plane and the through-plane motion relative to the second image in the intersecting plane.

[0016] The method may also include reconstructing a final version of each of the images acquired in the first and second sets, where the final version has higher resolution than the first image and the first reference image used to determine the in-plane displacement, respectively. Because the reconstruction of the first image in each set and the determination of the in-plane displacement must occur during the second cardiac cycle, allowing the second image to be acquired in the second set of images, a fast imaging sequence and reconstruction are required. For example, the imaging sequence may be one in which the MR signals used to reconstruct each image are acquired after one excitation RF pulse. That is, the imaging sequence may be a single-shot imaging sequence, preferably a gradient echo imaging sequence.

[0017] A matching process can also be used on the first reference image and the first image, in which at least a subset of the first reference image is compared with at least a subset of the first image to determine an in-plane displacement between the two images, which corresponds to the through-plane displacement of the subsequent image.

[0018] The process of acquiring a second set of 2D images during the second heartbeat, and the entire sequence of processes, can be repeated for any number of successive heartbeats. Additionally, the processes of the method can be applied during administration of an MR contrast agent, with the first and second sets of acquired images representing the inflow of the MR contrast agent into the heart.

[0019] A corresponding MR imaging system is also provided, which is configured to generate MR images of the heart, where the imaging system includes a control unit operable as described above or in further detail below.

[0020] Also provided is a computer program comprising program code which, when executed by a control unit of an MR imaging system, causes the MR imaging system to perform the method described above or in further detail below.

[0021] Also provided is a carrier for the computer program, the carrier being one of an electronic signal, an optical signal, a radio signal, and a computer-readable storage medium. [Brief explanation of the drawings]

[0022] These and other features and advantages of the present invention will become apparent from the following detailed description taken in conjunction with the following drawings, in which like reference numerals represent like elements and in which: [Figure 1] Schematic diagram of an MR system capable of generating cardiac MR images with optimal correction for the through-strain motion that occurs in cardiac MR imaging. [Figure 2] 1 is a schematic diagram of a method for determining the position of a subsequent image plane to intersect with a previous image plane during the same cardiac cycle. [Figure 3] First schematic diagram of one long-axis image acquisition followed by three short-axis image acquisitions. [Figure 4] Schematic showing how images from different heartbeats are acquired and processed to account for through-train motion. [Figure 5]Schematic representation of a flow chart including several steps required to determine cardiac MR images while avoiding through-train artifacts. DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Of course, the description of the embodiments described below should not be construed as having a limiting intent. The scope of the present invention should not be limited by the embodiments or drawings described herein, which are merely examples.

[0024] The drawings should be construed as schematic, diagrammatic representations, and the elements shown are not necessarily drawn to scale. Rather, the various elements are depicted so that their function and versatility are apparent to those of ordinary skill in the art. Connections and couplings between components of functional blocks, devices, physical or functional units shown in the drawings and described herein may also be implemented as indirect connections and couplings. Couplings between components may be established through wired or wireless connections. The functional blocks may be implemented in hardware, software, firmware, or a combination thereof.

[0025] FIG. 1 shows a schematic diagram of an MR system 1 equipped with a magnet 10 that generates a polarization field B0. A subject 12 lying on a table 11 is moved to the center of the MR system 1, and MR signals after RF excitation are detected by a receive coil 2. The receive coil 2 is composed of individual coil sections, each of which is associated with a corresponding detection channel 3. By applying RF pulses and magnetic field gradients, nuclear spins in the subject 12, particularly in a region located within the receive coil 2 (the heart in this example), can be excited, position-encoded, and relaxation-induced currents can be detected. Methods for generating MR images and detecting MR signals using sequences of RF pulses and magnetic field gradients are known in the art and will not be described in detail here.

[0026] The MR system includes a control module 13 used to control the MR system. The control module 13 includes a gradient control unit 14 that controls and switches magnetic field gradients and an RF control unit 15 that controls and generates RF pulses for imaging sequences. An image sequence control unit 16 is provided that controls the sequence of applied RF pulses and magnetic field gradients to control the gradient control unit 14 and the RF control unit 15. Computer programs required for the operation of the MR system and imaging sequences required to generate MR images can be stored in a memory 17, along with the generated MR images. The generated MR images can be displayed on a display 18, and an input unit 19 is provided and used by a user of the MR system to control the functions of the MR system. A processing unit 20 manages the operation of each functional unit shown in FIG. 1 and includes one or more processors capable of executing instructions stored in the memory 17. The memory contains program code executed by the processing unit 20. The processing unit can reconstruct MR images based on the detected images.

[0027] In the following disclosure, methods are described that allow for through-train motion to be taken into account so that the same image plane of the heart can be acquired even when motion is occurring. In particular, control module 13 is configured to operate as described below.

[0028] Instead of performing dynamic 2D image scans of parallel planes in the same orientation one after the other in time, it is proposed to alternate the acquisition of an orientation of the subject, such as a short axis image, with the acquisition of a diagnostically relevant image orientation that intersects this orientation of the subject.

[0029] FIG. 2 shows a diagram of successive cross-perfusion slices. As an example of the first image in an image set, image 22 shows two or three ventricles along the long axis, i.e., a long-axis view, followed by image 23 showing a short-axis view, and then image 24 of four ventricles at the long-axis slice location and short-axis image 25, acquired one after the other in a time window during the same heartbeat. In each image, the location of the plane of the next subsequent image is indicated; in image 22, the plane location is represented by line 31, which indicates the image plane of image 23. Similarly, in image 23, a plane location (line) 32 is indicated, representing the location of the plane of the third image 24. In image 24, plane location 33 is indicated, and in image 25, plane location 34 is indicated, which may be acquired in the next heartbeat in the same plane indicated in image 22.

[0030] In this manner, by reconstructing a subsequent cross-oriented image, aligning it with some arbitrary reference of the same orientation from the preceding heartbeat, and then prospectively applying the resulting shift to the next cross-oriented acquisition, a prospective slice correction for slice position can be obtained for the subsequent heartbeat. This is described in more detail below with reference to FIG. 4. In FIG. 2, the two arrows 28 and 29 in the first image represent the projection of cross-motion onto the subsequent short-axis slice (image plane 31, used to adjust the slice position of image 23 to the selected reference). In this manner, the short-axis views displayed in image 23 are acquired at the same anatomical location throughout the entire examination, allowing retrospective correction for any remaining in-plane motion. Although the time between successive slice acquisitions is short, the time required for preliminary, low-resolution reconstruction of the previous slice and matching it to the reference to adjust the subsequent slice position is made possible by the typically employed saturation recovery (SR) preparation, leaving approximately 50 ms or more before the start of the next data acquisition. The crossing direction can be applied in several different ways.

[0031] A first example is shown in FIG. 3, and another example is shown in FIG. 4. In both examples, N 2D slice images are acquired per heartbeat in a dynamic perfusion experiment. FIG. 3 shows a first image 41, which is a long-axis view of two, three, or four ventricles, etc., followed by N-1 short-axis slices, which are prospectively shifted according to the same first long-axis slice at each heartbeat. This means that before images 42, 43, and 44 are acquired, image 41 is compared with the same image acquired at the preceding heartbeat, and based on the comparison of these two images, in-plane motion can be determined with respect to the position of this image plane. The next plane, indicated by line 47, is intersected, and the slice positions can be adapted accordingly.

[0032] FIG. 4 shows a kind of flowchart that explains how the different images are acquired and how post-processing is performed. In the first cardiac cycle, images 51 to 54 are acquired. The image slice positions 56 to 59 are such that subsequent image slices intersect with each other. In the illustrated example, image 51 is a long-axis view, image 52 is a short-axis view, image 53 is a long-axis view, and image 54 is a short-axis view. That is, the intersecting positions are acquired in an alternating manner. After this first step, in a second step, these images are also reconstructed as reference images for the first cardiac cycle at the initial position. Images 51 to 54 in the first image set serve as the reference image set.

[0033] In the third step, the same slice positions are used in the same order as in the reference image set for all remaining heartbeats. At the beginning of this third step, in step 3-a, image 61 is acquired at a potentially shifted slice position relative to the previous acquisition at the immediately preceding heartbeat, taking into account the patient's respiratory motion, and is preferably reconstructed at a lower resolution. Before subsequent images 62-64 are acquired, image 61 is matched with image 51, as indicated by arrow 80 in step 3-b. This matching can be based on the complete image 51 and image 61, or a subset of these images. In the next step 3-c, an in-plane motion displacement corresponding to the through-plane motion for the subsequent slice position is calculated, and in step 3-d, the next subsequent slice position, i.e., the plane position 65 used to generate image 62, is shifted to match the reference position of steps 3-b and 3-c. The same steps 3-a-d described above are applied to the subsequent images. That is, image 62 is compared with image 52 to determine the in-plane motion shown in these images, which corresponds to the through-plane motion with respect to image 63, so that the position of the image plane can be adapted before image 63 is acquired. Steps a-d are repeated for each of the arrows shown between images 62 and 64 in FIG. 4. The same process can be applied to the image slice of the third cardiac beat, with the image of the second cardiac beat serving as the reference image. Alternatively, the image of the first cardiac beat serves as the reference image.

[0034] Applied to the example shown in Figure 3, images 51-54 can be acquired as shown in Figure 4, or only image 51 can be acquired, and in the second cardiac cycle, image 41 shown in Figure 3 can be determined and steps a-d can be performed before different short-axis views 42, 43, or 44 are acquired as symbolized in Figure 3.

[0035] As a further step, after steps a-d of step 3, in step 4, full resolution versions of all images for each slice and heartbeat can be reconstructed at the end of the entire acquisition cycle. Deferring the final diagnostic image reconstruction is beneficial because the full resolution reconstruction and matching required before acquiring image 62 would take too long in real time.

[0036] Motion is estimated across 2D image slices from images of the heart itself, rather than from a diaphragm surrogate, allowing for more accurate determination of through-plane motion than is possible with a 1D navigator. Because the prior, fully diagnostic acquisition is the basis for navigation, no additional data acquisition is required. This further avoids potential interference of the navigator restore pulse during actual perfusion acquisition and involves no learned or a priori assumptions regarding the relationship between diaphragm motion and cardiac motion.

[0037] FIG. 5 summarizes some of the above-described steps. In a first step S81, a first set of 2D reference images of the heart is acquired and reconstructed as shown by images 51-54, providing at least a first reference image 51. In step S82, a second set is acquired, which may be images 41-44 shown in FIG. 3 or images 61-64 shown in FIG. 4. This process of acquiring the second set includes step S83, in which a first image in the second set is acquired along a first reference plane, i.e., a first image during a second heartbeat is acquired along the same image plane as the first image in the reference set. In step S84, a first in-plane displacement is determined based on the first reference image and the first image in the second set, i.e., based on images 61 and 51 or a subset of these images. Based on this first in-plane displacement, it is possible to determine the position of a shifted plane for a second image in the second set to be acquired intersecting the first image in the second set. This is step S85, and in the next step S86, a second image can be acquired and reconstructed along the shifted plane position. All images in the first set of 2D images and all images in the second set of 2D images are diagnostically relevant images.

[0038] Finally, the invention described above provides an effective method for generating images of the heart at the same slice location, which is useful for perfusion-based imaging as one of its applications.

Claims

1. 1. A method for producing an MR image of the heart, comprising: a step (S81) of acquiring and reconstructing a first set (51, 52, 53, 54) of 2D reference images of the heart during a reference heartbeat, said first set including a first reference image (51) acquired along a position of a first reference plane through the heart and at least one other reference image acquired at a position of a second reference plane intersecting the position of the first reference plane, said other reference images representing a predetermined anatomical region of the heart; and acquiring (S82) a second set of 2D images (41, 42, 43, 44, 61, 62, 63, 64) of the heart during a second heartbeat; The step of collecting the second set comprises: a first step (S83) of acquiring a first image (41) in the second set acquired along the first reference plane; a second step (S84) of determining a first in-plane displacement of the heart in the image plane at the location of the first reference plane based on the first reference image (51) and the first image (41) in the second set; a third step (S85) of determining, based on the determined first in-plane displacement, the position of a shifted plane for a second image (42) in the second set acquired parallel to the position of the second reference plane of the heart; a fourth step (S86) of acquiring and reconstructing the second image (42) along the shifted plane position; A method, wherein all images of said first set (51, 52, 53, 54) of 2D reference images and all images of said second set (41, 42, 43, 44, 61, 62, 63, 64) of 2D images are diagnostically relevant images.

2. the second set includes N images (41, 42, 43, 44, 61, 62, 63, 64); the positions of the planes of the additional images 2 to N in the second set are parallel to each other and intersect with the position of the first reference plane; The method of claim 1 , wherein the position of the plane in each of the additional images is determined based on the first in-plane displacement.

3. 2. The method of claim 1, wherein the second image and the further reference image in the second set (41, 42, 43, 44, 61, 62, 63, 64) have substantially the same anatomical location within the heart.

4. further comprising the step of reconstructing a final version of each of the collected images in the first set and the second set; 2. The method of claim 1, wherein the final version has a higher resolution compared to the first image (41) and the first reference image (51), respectively.

5. The method of claim 1 , wherein each image in the first set and the second set is acquired with a single-shot gradient echo imaging sequence.

6. using a matching process on the first reference image (51) and the first image (41); 2. The method of claim 1, wherein the matching process compares at least a subset of the first reference image (51) with at least a subset of the first image (41) to determine the in-plane displacement.

7. 2. The method of claim 1, wherein the step of acquiring the second set of 2D images (41, 42, 43, 44, 61, 62, 63, 64) of the heart during the second heartbeat, and the first step (S83), the second step (S84), the third step (S85), and the fourth step (S86) included in that step, are repeated for any number of consecutive heartbeats.

8. The method of claim 1 , wherein the method is applied at least in part during administration of an MR contrast agent, and the acquired image data represents the inflow of said contrast agent into the heart.

9. 1. An MR imaging system configured to generate MR images of a heart, the MR imaging system including a control module, The control module includes: acquiring and reconstructing a first set of 2D reference images (51, 52, 53, 54) of the heart during a reference heartbeat; acquiring a second set of 2D images (41, 42, 43, 44, 61, 62, 63, 64) of the heart during a second heartbeat; the first set includes a first reference image (51) acquired along a position of a first reference plane through the heart and at least one other reference image acquired at a position of a second reference plane intersecting the position of the first reference plane, the other reference image representing a predetermined anatomical region of the heart; When acquiring the second set of 2D images of the heart during the second heartbeat, the control module: acquiring a first image (41) in the second set acquired along the first reference plane; determining a first in-plane displacement of the heart in an image plane at the location of the first reference plane based on the first reference image (51) and the first image (41) in the second set; determining a position of a shifted plane for a second image (42) in the second set acquired parallel to the position of the second reference plane of the heart based on the determined first in-plane displacement; configured to acquire and reconstruct the second image (42) along the shifted plane position; An MR imaging system, wherein all images of said first set (51, 52, 53, 54) of 2D reference images and all images of said second set (41, 42, 43, 44, 61, 62, 63, 64) of 2D images are diagnostically relevant images.

10. MR imaging system according to claim 9, further configured to carry out the method according to any one of claims 2 to 8.

11. A computer program comprising program code, which, when executed by a control unit of an MR imaging system, causes the MR imaging system to carry out the method of any one of claims 1 to 8.

12. A computer readable storage medium containing the computer program of claim 11.

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