METHOD AND SYSTEM FOR CAPTURING AND RECONSTRUCTING IMAGES OF A HEART DURING FREE BREATHING

Abstract

The invention relates to a method for reconstructing an image of a patient's heart using a magnetic resonance imaging device (MRI1) that generates a magnetic field, which method comprises: - Acquiring the electrical activity of the patient's heart; - Generating an MRI sequence and acquiring a 2D dark-blood image; - Generating an MRI sequence and acquiring a 2D bright-blood image; - Applying a non-rigid registration algorithm to each subset of dark-blood images for each section of the heart; - Applying a non-rigid registration algorithm to each subset of bright-blood images for each section of the heart; - Generating a set of 2D images for each section plane, each image corresponding to a combination produced between each pair of dark-blood and bright-blood images.

Description

[0001] Description

[0002] METHOD AND SYSTEM FOR ACQUIRING AND RECONSTRUCTING IMAGES OF A FREE-BREATHING HEART

[0003] Field of invention

[0004] The invention relates to the field of magnetic resonance imaging, in particular cardiac magnetic resonance imaging. It relates more particularly to late gadolinium enhancement or LGE magnetic resonance imaging or MRI. It applies to cardiac imaging, but also to angiography. The field of the invention relates more particularly to imaging methods and devices for detecting and locating a scar in myocardial tissue.

[0005] State of the art

[0006] The gold standard for assessing regional scar formation and myocardial fibrosis is white blood late gadolinium enhancement (BR-LGE) imaging. In this type of imaging, viable myocardial signal cancellation is induced using inversion-recovery pulses, allowing scars to be visualized with high contrast between healthy myocardial tissue and scars. However, for small scars or myocardial scars adjacent to the blood chambers of the heart, the high signal intensity from the blood prevents clear visualization and precise delineation of scars, especially subendocardial scars. To circumvent this problem, black-blood LGE (BL-LGE) imaging techniques have been proposed.They allow simultaneous cancellation of signals from healthy myocardium and blood, thus providing high contrast both between scars and between blood and between scars and healthy myocardium.

[0007] Blood signal cancellation, to obtain a "black blood" contrast, is achieved by applying, to an area to be imaged of a patient, a radiofrequency (RF) inversion-recovery sequence with a 180° pulse followed by a preparatory module and a reading module. The time between the preparatory module which occurs just after the emission of a magnetization inversion radiofrequency pulse and the reading sequence is called inversion time Tl.

[0008] The aim is to acquire the signals to generate the image of the area to be imaged in order to locate and detect a scar present in the myocardial tissues.

[0009] A disadvantage of prior art solutions known as black blood - white blood acquisition is that they require a significant acquisition time due to the 3D acquisition involving numerous calculations. The patient is in these conditions in free breathing. A consequence is having to apply image correction algorithms requiring large computational resources involving computation times extending the acquisition sequence and a difficulty in applying them in real time. However, when the acquisition time is extended, the variability of the consequences of breathing results in the production of image processing artifacts altering the readability of the latter. These artifacts accentuate the difficulty of reconstructing clear and precise images in order to locate and detect the scar. Furthermore, long MRI acquisitions are uncomfortable for the patient.A duration of 10 to 20 min is considered a very long duration and it is difficult for the patient to remain within the MRI without moving.

[0010] Today, these sequences require an acquisition time of more than 15 min. The acquired images are not satisfactory in many cases, because the artifacts of an image extracted from a section plane of the 3D image result in cases in which it is impossible to discriminate the presence of a potential scar from the presence of blood located near the muscle. Indeed, in some cases, the scar is so close to the blood, it is called subendocardial, that it is difficult to know if it is a scar, blood or an artifact of the image. A problem is to base a clinical decision leading to a diagnostic error on the presence of a scar or not.

[0011] Another solution is to make a rapid acquisition in apnea, but this solution requires reducing the acquisition time from a few seconds to a few minutes and often involves having to reduce the area to be imaged, for example by limiting it to a 3D portion. However, it is nevertheless necessary to obtain a minimum number of images to reconstruct a 3D portion. One problem is that heartbeats also influence the measurement and require acquisitions to be synchronized with the heartbeat. This constraint requires reducing the number of potential acquisitions over a given portion of time. Indeed, it is necessary to sequence the acquisitions on the heart rate, it is therefore necessary to spread these acquisitions over a minimum number of beats. Acquisitions in apnea severely constrain the acquisition capacity.

[0012] Finally, there are 2D acquisition methods, but these do not provide sufficient resolution and contrast to detect and locate scars. It would be necessary to perform several 2D acquisitions of the same area to disambiguate certain cases, but variations in respiration and heartbeat over time do not allow 2D images to be produced on identical section planes of the organ over time.

[0013] Summary of the Invention and Advantages

[0014] According to a first aspect, the invention relates to a method for reconstructing an image of a patient's heart from a magnetic resonance imaging device generating a magnetic field comprising:

[0015] - A first phase of image generation including:

[0016] ■ Acquisition of the electrical activity of the patient's heart;

[0017] ■ Generation of a first 180° inversion radiofrequency signal to switch a longitudinal magnetization of tissues of an imaged area, said first inversion radiofrequency signal being generated between two QRS complexes of the acquired electrical activity, called the first inter-beat phase;

[0018] ■ Generation of a first magnetization preparation comprising a set of pulses following the generation of the first inversion radio frequency signal in the same first inter-beat phase;

[0019] ■ Acquisition of a first 2D image by a magnetization measurement of the imaged area, said acquisition being carried out after a first predefined duration following the generation of the first magnetization preparation, said acquisition being synchronized with the heart rate at a first time marker of the same inter-beat phase;

[0020] ■ Generation of a second magnetization preparation comprising a set of pulses in a second interbeat phase following the first interbeat phase;

[0021] ■ Acquisition of a second 2D image by a magnetization measurement of the imaged area, said acquisition being carried out after a second predefined duration following the generation of the second magnetization preparation, said acquisition being synchronized with the acquisition of a heart rate at the same first time marker of the second inter-beat phase as the first marker of the first inter-beat phase;

[0022] - Repeating the first phase to generate a plurality of first images and a plurality of second images for a plurality of slice planes of the heart in interbeat phases succeeding the first and second interbeat phases, each pair of first image and second image being associated with a slice plane;

[0023] Generating a set of third 2D images for each cutting plane, each third image corresponding to a first combination made between each pair of first merged images and second merged images for each cutting plane.

[0024] This aspect of the invention concerns the case where one image per section plane is acquired. This possibility is offered for rapid acquisitions requiring around thirty beats to cover around fifteen section planes with one black blood image and one white blood image per section. However, this sequence can be improved by a free-breathing acquisition by acquiring several images per section plane for the black blood on the one hand and for the white blood on the other hand.

[0025] According to another aspect, the invention relates to a method of reconstructing an image of a patient's heart from a magnetic resonance imaging device generating a magnetic field comprising:

[0026] - A first phase of image generation including:

[0027] ■ Acquisition of the electrical activity of the patient's heart;

[0028] ■ Generation of a first 180° inversion radiofrequency module so as to tilt a longitudinal magnetization of an imaged area, said first inversion radiofrequency signal being generated between two QRS complexes of the acquired electrical activity, called the first inter-beat phase;

[0029] ■ Generation of a first magnetization preparation comprising a set of pulses following the generation of the first inversion radiofrequency module in the same first inter-beat phase;

[0030] ■ Acquisition of a first 2D image by a magnetization measurement of the imaged area, said acquisition being carried out after a first predefined duration following the generation of the first magnetization preparation, said acquisition being synchronized with the heart rate at a first time marker of the same inter-beat phase;

[0031] ■ Generation of a second magnetization preparation comprising a set of pulses in a second inter-beat phase following the first inter-beat phase;

[0032] ■ Acquisition of a second 2D image by a magnetization measurement of the imaged area, said acquisition being carried out after a second predefined duration following the generation of the second magnetization preparation, said acquisition being synchronized with the acquisition of a heart rate at the same first time marker of the second inter-beat phase as the first marker of the first inter-beat phase;

[0033] - Repeating the first phase to generate a plurality of first images and a plurality of second images for a plurality of slice planes of the heart in interbeat phases succeeding the first and second interbeat phases, each pair of first image and second image being associated with a slice plane;

[0034] - Repetition of the first phase to generate a plurality of first images and second images of the same section plane of the heart in inter-beat phases succeeding the first and second inter-beat phases.

[0035] In other words, the method comprises repeating the first phase, to generate a subset of first images and a subset of second images of each of a plurality of slice planes of the heart, in inter-beat phases succeeding the first and second inter-beat phases.

[0036] The method also includes:

[0037] - Application of a non-rigid registration algorithm to each subset of first images of the same cutting plane to register said first images with each other;

[0038] - Application of a non-rigid registration algorithm to each subset of second images of the same section plane to register said second images with each other;

[0039] - Merging on the one hand the first images aligned with each other to produce a first merged image and on the other hand the second images aligned with each other to produce a second merged image;

[0040] - Generation of a set of third 2D images for each cutting plane, each third image corresponding to a first combination made between each pair of first merged images and second merged images for each cutting plane.

[0041] One advantage is that it allows the production of short-duration free breathing acquisitions that do not require the calculation times of a 3D acquisition.

[0042] Advantageously, the method comprises the following steps:

[0043] - merging, for each of the section planes, on the one hand the first images aligned with each other to produce a first merged image and on the other hand the second images aligned with each other to produce a second merged image; - generation, for said section planes, of a set of third 2D images, each third image corresponding to a first combination made between the first merged image and the second merged image produced for one of the section planes.

[0044] Advantageously:

[0045] - for each cutting plane, a non-rigid registration algorithm is applied to the first images of the cutting plane to register said first images of the subset of first images with each other;

[0046] - for each cutting plane, a non-rigid registration algorithm is applied to the first images of the cutting plane to register said first images of the subset of first images with each other.

[0047] According to one embodiment, the first duration being determined such that the first acquired images are images having a first contrast making it possible to display black blood images and the second duration being determined such that the second acquired images are images having a second contrast making it possible to display white blood images.

[0048] According to one embodiment, the recalibration step is followed by the steps of:

[0049] - Averaging of the first acquired images realigned to produce a first merged image for each cutting plane;

[0050] - Averaging of the second acquired images realigned to produce a first merged image for each cutting plane.

[0051] One benefit is to increase the signal-to-noise ratio, also called SNR.

[0052] According to one embodiment, the first duration is the inversion time in inversion-recovery.

[0053] Advantageously, the first duration is defined so that the longitudinal magnetization of the blood and that of the healthy myocardium cancel each other out at the same time during the acquisition of the first 2D image.

[0054] According to one embodiment, the determination of the first duration is automatically calculated from a computer-implemented method for calculating an optimal inversion time carried out from processing at least one image acquired from an MRI pre-sequence. This calculation is for example carried out by the computer-implemented method for determining the optimal inversion time described in patent application FR2203766. The MRI pre-sequence comprises the acquisition of a plurality of MRI images by acquisition sequences having distinct respective inversion times. The inversion time is calculated from these images.

[0055] According to one embodiment, the method comprises a step of colorizing the pixels of each first merged image having a luminance greater than a given threshold, the colorization step being carried out prior to the first combination. In the example case, the colored images correspond to the black blood images.

[0056] An advantage is that it allows the location and identification of a scar near the myocardium and blood volume(s) in a safe, i.e. unambiguous manner.

[0057] According to one embodiment, the first combination is a superposition of the first image on the merged second image. That is, a superposition of the colored black blood image on the colored white blood image.

[0058] In a particular embodiment, the first image is the first merged image.

[0059] According to one embodiment, each new generation of a plurality of first images and second images of the same cutting plane is carried out after each repetition of the first phase to generate a plurality of first images and a plurality of second images for a plurality of cutting planes of the heart.

[0060] According to one embodiment, each generation of a new first image and a new second image in a new section plane of the heart is carried out after all the repetitions of the first phase to generate a plurality of first images and second images of a previous section plane of the heart.

[0061] An advantage is to produce all the images of the same section plane in a short time interval to avoid drifts of image shifts in the same section plane when they are interlaced after the acquisition of different images of other section planes. According to one embodiment, the method comprises:

[0062] ■ a display of a set of first merged images for each cutting plane;

[0063] ■ a display of a set of second merged images for each cutting plane;

[0064] ■ a display of a set of third 2D images for each cutting plane;

[0065] ■ each first image of a given cutting plane being displayed in the immediate vicinity of the second image and the third image of the same given cutting plane.

[0066] Advantageously, the first displayed image and the second displayed image for a section plane are respectively the first merged image and the second merged image.

[0067] According to one embodiment, the first merged image for each section plane corresponds to an image of a section of the heart in black blood, the second merged image for each section plane corresponds to an image of a section of the heart in white blood, the third image for each section plane corresponding to an image of a section of the organ on which a scar, if any, is displayed in color.

[0068] According to one embodiment, the first and second magnetization preparation is a T1 weighting in T1 rho, said method being carried out during late enhancement after injection of Gadolinium.

[0069] According to one embodiment, the first magnetization preparation is a T1 rho type preparation, said T1 rho preparation being defined by the sequence of a configuration of the magnet defined as follows: 90x-SLy-180y-SL-y-SLy-180-y-SL-y-90-x.

[0070] According to another example of T1 rho type preparation, another magnetization configuration can be for example 90x-Sly-90-x.

[0071] According to one embodiment, the first magnetization preparation is a T2prep type preparation or an MTC type preparation.

[0072] According to one embodiment, the generation of a second magnetization preparation comprising a set of pulses in an inter-beat phase is followed by a step of filtering the acquired images corresponding to fatty areas of the imaged organ. According to one embodiment, the first images and the second images are acquired successively in a synchronized manner with the electrical activity of the heart between two QRS complexes for several minutes in free breathing during a single examination.

[0073] According to one embodiment, the first images and the second images are acquired on a set of 8 to 20 section planes of the organ, the phase of repeating the acquisitions of a plurality of first images and second images in the same section plane making it possible to acquire between 2 and 10 images of first images and between 2 and 10 images of second images per section plane.

[0074] According to another aspect, the invention relates to a magnetic resonance imaging system comprising a magnetic field generator and a radiofrequency device, an electrocardiograph and a processing device, a display for displaying the generated images, said system being configured to implement the method of the invention.

[0075] One advantage is that it allows images to be generated in which it is possible for a radiologist to reliably locate and detect the scar in myocardial tissue.

[0076] One advantage is that it allows an area of ​​interest such as a scar to be displayed in color.

[0077] Another advantage is that it allows for rapid acquisition of images in free breathing, which does not require the patient to carry out the imaging in apnea.

[0078] Furthermore, the method according to the invention is simple, fast and inexpensive in terms of calculations. It can be easily integrated into an existing MRI device without modification.

[0079] An advantage is that for each given slice of the heart, a plurality of images are collected, which allows for noise reduction, or increasing the signal-to-noise ratio and reducing residual artifacts.

[0080] An advantage of using a non-rigid algorithm is that it overcomes motion artifacts related to breathing variation. Implementing such an algorithm increases the signal-to-noise ratio.

[0081] Finally, 2D acquisition allows for faster and more robust acquisition than 3D acquisition. Brief description of the figures

[0082] Other characteristics and advantages of the invention will emerge from reading the detailed description which follows, with reference to the appended figures, which illustrate:

[0083] Fig. 1: an exemplary embodiment of a system of the invention comprising an MRI device, an electrocardiograph, a calculation for carrying out the operations of processing the acquired images and a human-machine interface for viewing the images;

[0084] Fig. 2: A schematic representation of a portion of the sequence generated by an MRI device over two heartbeats,

[0085] Fig. 3: a schematic representation of an MRI acquisition phase in black blood and white blood carried out on a plurality of heartbeats, Fig. 4: a schematic representation of a 3D heart illustrating the different cutting planes and the images acquired in each cutting plane, Fig. 5: a flowchart of the steps of the method according to one embodiment of the invention.

[0086] Description of invention

[0087] MRI system

[0088] Figure 4 schematically represents an imaging system Si comprising a magnetic resonance device MRh according to the invention comprising a set of excitation and measurement equipment, a Ki calculator for carrying out processing on the acquired images and an IHMi display for displaying the generated images.

[0089] In a manner known per se, the set of excitation and measurement equipment comprises an MRI imaging device comprising a static magnetic field generator GEN_Bo comprising a main polarization magnet, a gradient generator GEN_GRAD, and a radiofrequency device DISPO_RF comprising a radiofrequency signal generator and a radiofrequency antenna.

[0090] The static magnetic field generator GEN_Bo comprises a magnet intended to generate a static magnetic field of substantially uniform polarization in a polarization zone (generally a tunnel) intended to comprise the area to be imaged of the patient, for example the heart. The gradient generator GEN_GRAD comprises three gradient coils or solenoids arranged and configured to vary the intensity of the magnetic field in the polarization zone along respective orthogonal axes conventionally denoted x, y and z fixed relative to the polarization zone. The choice of the intensities circulating in these coils makes it possible to select, from several possible ones, a thickness and a cutting plane in which the magnetization of the area to be imaged of the patient received in the polarization zone will be measured.

[0091] The GEN_RF radiofrequency device comprises coils or solenoids and is capable of generating MRI acquisition sequences comprising sequences for preparing the magnetization of the area to be imaged and sequences for reading signals from the area to be imaged. Each acquisition sequence comprises at least one radiofrequency pulse of predetermined and controllable frequency, shape, duration, phase, and amplitude.

[0092] The preparatory module, also called preparation or preparation phase, is configured to excite, i.e. modify the direction of magnetization of the tissues in the area to be imaged, i.e. modulate their magnetization. It is noted TAprep or TBprep or T1 rho in the rest of the description.

[0093] The reading module is configured to measure the magnetization of the area to be imaged resulting from the preparation sequence. The reading module is also called the acquisition module or reading phase or reading window.

[0094] In other words, the images are acquired by magnetization measurements of the area to be imaged.

[0095] The assembly advantageously includes an ELCi electrocardiograph intended to acquire an ECGi electrocardiogram of the patient.

[0096] The imaging system comprises a processing device Ki configured to perform calculations, in particular image registration and averaging of said images. The processing device also optionally makes it possible to color the images.

[0097] The imaging system S1 further comprises a display for displaying the two-dimensional grayscale and / or color images of the area to be imaged from the signals measured by the RF magnetic field generator and processed by the processing device. According to one embodiment, the reconstructed image is a grayscale and color matrix image. It comprises a set of pixels each characterized by an intensity I capable of taking a set of M values ​​(M being a finite integer greater than 1) corresponding to M grayscale levels ranging from 0 and M-1. For example, this value can take 256 values ​​between 0 and 255. This value can be higher in other embodiments. According to one embodiment, the images can be colored in part for a selection of pixels having a luminance greater than a given threshold.

[0098] Representations of the images generated by the Ki processing device are intended to be displayed on a screen of the human-machine interface HMI1 of the Si imaging system.

[0099] Acquisition sequence

[0100] Figure 2 represents an example of a portion of a SEQi sequence of MRI acquisition in black blood in white blood of an image of the myocardium as well as the ECGi electrical activity represented on two consecutive beats. An ECGi electrocardiogram acquired by the ECLi electrocardiograph during two heartbeats as well as the SEQi acquisition sequence applied to the area to be imaged are superimposed in order to better represent the temporal characteristics linked to each other of each of these curves. The acquisition of the ECG is a step noted ACQo. The lower part of Figure 2 represents the variation of the longitudinal magnetization Mz of the tissues of the area to be imaged as a function of time t as well as two images IMi and IM2 reconstructed from signals measured during the sequence.

[0101] Preferably, the invention makes it possible to synchronize the excitation and MRI acquisition sequences on the ECGi electrocardiogram. This synchronization makes it possible, on the one hand, to take measurements at times when the heart is most stable and, on the other hand, to take measurements at equivalent times between two same positions of the heart in order to compare images of the same slice. One way of synchronizing these excitations and acquisitions is to trigger the measuring device on the QRS complex.

[0102] In the remainder of the description, the first image IM1 will refer to an image acquired in black blood and the second image IM2 will refer to an image acquired in white blood. According to one example, throughout the sequence, from the first beat during which the image acquisitions are carried out, the odd beats are used to acquire the images in black blood and the even beats are used to acquire the images in white blood. In the remainder of the description, this example will be more detailed. Other configurations are however conceivable for executing the method of the invention, for example associating the odd beats with the acquisition of images in white blood and the even beats with the acquisition in black blood.

[0103] The invention also applies in the case where the white blood images are acquired before the black blood images. The invention also applies in the case where the images acquired in black blood and white blood are not immediately acquired on two consecutive beats, but for example spaced by one or more beat(s) without measurement. However, an advantage of acquiring these images on two consecutive beats is to minimize the acquisition time and therefore to minimize the effects of breathing and therefore image artifacts on a time scale of two beats.

[0104] For MRI acquisition in black blood and white blood, a Gadolinium-based contrast agent is advantageously injected intravenously into the patient, 10 to 15 minutes before the application of the acquisition sequences in order to obtain images with maximum contrast between scars and healthy tissues and blood. In the heart, the contrast is rapidly eliminated from healthy myocardium, poor in interstitial tissue, but accumulates for a prolonged period in myocardial scars. The effect of gadolinium has the effect of shortening the T1 relaxation time of the tissues where it accumulates. The relaxation of the scar magnetization following a magnetization pulse is thus faster than that of blood and healthy myocardium.

[0105] First, we try to generate images with a black blood contrast. In an image of this type, the intensity of the pixels corresponding to blood and healthy muscle is zero (black pixels) or substantially zero because it is acquired when the longitudinal magnetization of the blood is zero.

[0106] In order to generate such an IMi image, the DISPO_RF device generates an inversion-recovery acquisition sequence. This acquisition sequence includes a 180° inversion pulse denoted Rfi in Figure 2, which flips the longitudinal magnetization of the tissues in the imaged area in the opposite direction, i.e., which reverses the longitudinal magnetization. In Figure 2, we see that the magnetization of the area to be imaged changes from Mz to -Mz under the effect of the inversion pulse Rfi. Due to the longitudinal relaxation, the longitudinal magnetization of the different tissues present in the area to be imaged increases to return to its initial value, passing through the zero value. Naturally, the reversal speeds of the magnetizations of the different tissues are different.

[0107] In a manner known per se, the preparation sequence then includes a preparatory module, for example an adiabatic preparation in T 1 rho, noted T 1 rho or TAprep in figure 2 of duration TSL, designating in Anglo-Saxon terminology “Spin Lock Time”.

[0108] Other preparations, such as a T2prep preparation or an MT magnetization transfer preparation can be used.

[0109] It is noted in the present application that the preparation can sometimes be defined as the T1 rho preparation or sometimes be defined as the T 1 rho preparation preceded by the 180° Rfi inversion sequence. The second IM2 image being acquired without a 180° inversion pulse, but only with a T1 rho preparation, the preparation in this case is limited to the T1 rho preparation.

[0110] The preparation sequence (Rfi(180°), T1 rho) is configured so that the longitudinal magnetization of the blood and that of the healthy myocardium cancel each other out at the same time Tl, see the position of the point NP of the curve A(blood) and A(Musc). The inversion time Tl corresponds to a time measurement between the emission of the signal Rf at 180 and the acquisition module ACQi. The time Tl corresponds to the duration noted Di in the presence request. Indeed, the invention also applies to cases in which the measurement of the magnetization would not necessarily be calculated at the inversion time, but possibly at another time depending on the desired result.

[0111] An acquisition window or a reading window is noted indifferently for each beat, they are noted ACQi for the reading windows relating to the acquisition in black blood and ACQ2 the reading windows relating to the acquisition in white blood. We note that in figure 2, at this same instant Tl, the longitudinal magnetization of the scars A(Cica) is clearly greater than zero, in fact the point PP is above zero. By acquiring the signals from the area to be imaged at this instant Tl, we obtain an image presenting a very high contrast between the pixels corresponding to the blood and the healthy myocardium which are black and the scars, globally white.

[0112] The inversion-recovery sequence then includes an ACQi read sequence comprising a 90° pulse and a read gradient to read the transverse magnetization of the area to be imaged.

[0113] The inversion time Tl is the time separating the 180° pulse from the ACQi acquisition window. The time marker of the ACQi acquisition window is imposed upstream in the ECG, generally in diastole. It is chosen as the instant when the muscle and blood signals cancel each other, i.e. where the longitudinal magnetizations of the blood and the myocardium cancel each other in order to generate the image with the best contrast. In order to obtain the best contrast between the myocardial scars and the blood and the healthy myocardium, we try to start the inversion signal Rf at an earlier instant to cause the cancellation of the signals at a duration Tl.

[0114] The sequence is followed by a white blood acquisition phase performed at the beat following the black blood acquisition. In this phase, a second IM2 image is acquired. The preparation phase is denoted TBprep in Figure 2. According to one embodiment, the TBprep preparation could be different from that of the TAprep preparation used to image the heart area in black blood, however it is preferable that the TAprep black blood and TBprep white blood preparations are identical. The invention therefore relates to other preparations, however in the remainder of the description, the white blood acquisition phase is described with the same white blood preparation as the black blood preparation, for example a T1 rho preparation.

[0115] The D2 duration separating the time marker of the AQ2 acquisition window, which is fixed for each beat and the time marker corresponding to the TBprep preparation in white blood can be adjusted to optimize the contrast in white blood in order to best image the contours of the muscle vis-à-vis the blood.

[0116] The magnetization gradient measurement is configured for an ACQ2 acquisition of the white blood image at the same interbeat marker as the black blood image. Thus, in Figure 2, the ACQ2 readout phase is located at a duration of the preceding QRS complex identical to the duration between the QRS of the beat preceding the ACQ1 readout phase.

[0117] When reading the magnetizations at this same time, it is possible to obtain an image in which the pixels of the blood areas are represented in white, i.e. with a high luminance, and in which the pixels of the myocardial tissue are a little less luminous than those of the blood. Thus, it is possible to obtain the 2D topology of the section allowing the blood and myocardium areas to be visualized in relation to each other.

[0118] It is understood that by combining the IM1 and IM2 images it will be possible to locate and detect the scar present if necessary.

[0119] To this end, the invention makes it possible to carry out successive measurements of the sequence of these two images IM1 in black blood and IM2 in white blood on a plurality of sections of the heart to study the entire heart and taking into account several images per section in order to make the analysis of the images more robust.

[0120] Figure 3 represents an example of a sequence of these phases noted Phi, with i = 1 to N, over a plurality of consecutive pairs of beats. The PHi phases comprise each time over two beats:

[0121] - First beat: a generation GEN1 of an inversion RF signal, a preparation phase generated and noted GEN2 and a reading phase noted ACQ1;

[0122] - Second beat: the generation of a GEN3 preparation phase and a reading phase noted ACQ2.

[0123] Figure 4 represents a 3D view of a heart comprising a plurality of K section planes denoted PCk, with k = 1 to K. Figure 4 makes it possible to represent a plurality of section planes each comprising a plurality of images acquired using the method of the invention. The images of the section plane PCk are represented, the others are not represented in Figure 4. The images are denoted respectively IMi k (i) for the i èmeimage acquired from the PCk section, with i = 1 to N.

[0124] According to one embodiment, a first acquisition of the image of the heart makes it possible to evaluate the number of cutting planes and therefore the number of acquisition repetitions. If the heart is considered small, between 7 and 9 cutting planes can be defined, if the heart is larger, between 14 and 20 slices can be defined. If the heart is of average size, the number of slices can be defined between 10 and 13 slices.

[0125] This estimate can be made at the start of the exam, for example during a pre-sequence.

[0126] According to another example, the evaluation of the size of the heart is determined automatically during the first acquisitions of the method of the invention, during the first beats. This size can automatically allow a calculation to be carried out dynamically so as to acquire images according to a given number of slices.

[0127] According to an example of a complete 3-minute sequence with a pulse of 60 beats per minute, considering a configuration of 15 cutting planes for the organ, we would have a total of 3x60 = 180 images, including 90 images in black blood and 90 images in white blood. Such a configuration allowed the acquisition of 6 images {IMi k (1 ), IMi k (2), IMi k (3), IMi k (4), IMi k (5), IMi k (6)} by black blood section plane and 6 acquired images {IM2 k (1), IM2 k (2), IM2 k (3), IM2 k (4), IM2 k (5), IM2 k (6)} by black blood section plan.

[0128] An advantage of the method of the invention is that it can be carried out during free breathing. For this purpose, it is necessary that the images acquired in the same slice are processed so that they are not altered by the effects of free breathing. The rhythm of breathing is a priori different from the heart rate. Even if correlations exist between these two rhythms, it is possible that breathing accelerates while the heartbeat remains stable or vice versa.

[0129] The problem is that during the second ACQ2 reading, breathing may have significantly changed the position of the heart, which means that images acquired at another beat in the same slice may be out of sync with an image acquired previously in the same slice.

[0130] According to one embodiment, the images of each slice resulting from the acquisition in black blood are registered using a non-rigid image registration algorithm. The images acquired in white blood for each slice are registered identical to the method chosen in black blood. It is possible to choose a different algorithm for processing the images in black blood and in white blood, but it is preferable to choose the same algorithm for ease of implementation. The registration significantly improves the image that will be merged from a plurality of images acquired in the same slice, because the contrast is better and the breathing artifacts could be compensated thanks to the non-rigid algorithm.

[0131] According to a first example, a non-rigid algorithm is an algorithm based on the method of mutual information between images based on statistical relations. The function to be optimized can be implemented by a statistical similarity criterion. An advantage of this method is that the matching between homologous attributes of images of the same section is independent of their geometric position.

[0132] In a second example, a non-rigid algorithm based on a transformation model is implemented. The transformation model makes it possible to determine functions to minimize the difference between two images. The difference can be translated by a geometric error to be minimized. Different approaches can be used, such as those based on the extraction from each of the images of geometric primitives or shape descriptors such as salient points, shape singularities or contours. A parametric or non-parametric approach can be used.

[0133] As an example of optimizing a transformation model or a similarity criterion, the least squares method can be used.

[0134] Other optimization methods can be implemented such as gradient descent. However, this latter method applies more particularly to image intensities and is not optimal in the context of the invention since the aim is to optimize the sharpness and contrast of the merged image. Nevertheless, the invention includes this embodiment.

[0135] Registration can be performed by choosing a reference image and determining a transformation function for the other images of the same section with respect to this image. Each image is then registered by optimizing a transformation to obtain the reference image according to a geometric criterion from the image considered.

[0136] When a plurality of images are aligned with each other in a section plane, it is possible to carry out operations aimed at merging these images in order to produce a single merged image per section plane. According to one embodiment, a step of averaging the images of the same section is carried out so as to reduce the noise and increase the signal-to-noise ratio.

[0137] Averaging has the advantage of preserving image detail, as it increases the signal-to-noise ratio (SNR). This technique smooths out noise to reduce residual image artifacts. In addition, averaging improves the bit depth of the digital image beyond what is possible with a single image.

[0138] One advantage of the averaging step of images taken from the same slice is to reduce the maximum deviation. The noise amplitude decreases as the square root of the number of images used, i.e. with only 4 images, the noise amplitude can be reduced by a factor of two. Using an example of a 2-minute free-breathing acquisition, it is possible to collect 4 to 5 images per slice plane, which provides good noise reduction performance.

[0139] Each set of images acquired in black blood and white blood are respectively merged to produce a single IMiF black blood image. k and an IM2F white blood image k unique by PCk cutting plane.

[0140] In other words, a single fused IMiF black blood image is produced k and a fused IM2F white blood image k by section plane PCk as visible in figure 5.

[0141] According to one embodiment, the method of the invention then comprises a step of coloring the brightest pixels of the black blood image. The brightest pixels correspond in particular to the scar given the fact that the signals from the muscle and the blood cancel each other out and are represented by low intensity pixels, i.e. black. Coloring the scar makes it possible to identify and represent the outline of the latter.

[0142] One advantage of coloring is that it allows the scar to be clearly identified and used in combination with the white blood image, which shows the contours of the muscles. Thus, such a combination allows the scar to be identified and located, particularly in relation to the muscle and blood near the scar.

[0143] The COMB1 combination of IMiF black blood images k and white blood IM2F kcan be performed for each cutting plane PCk. Thus, the same combination operation makes it possible to obtain a plurality of combined black blood - white blood images to produce a single IM3 image k by PCk section plane.

[0144] The COMB1 combination of images can be achieved in different ways. In a first example, the images are superimposed on each other. The superimpositions may include taking into account a variation in the opacity of the black blood image in order to correctly visualize the contours of the white blood image. The opacity of the black blood images can be configured to have a value between 40% and 90%, for example 75%. In a second example, the pixels of the scar that have a luminance greater than a given threshold are extracted to be integrated into the white blood image.

[0145] Other image combination methods can be used to produce a single image from the black blood image and the white blood image by section.

[0146] In other words, for each cutting plane PCk, a single combined image IM3 is produced. k from the merged IMiF black blood image k and the merged white blood image IM2F k .

[0147] An advantage of the method of the invention is to generate a plurality of reconstructed images per section plane from a black blood image and a white blood image. Depending on the size of the heart, between 8 and 20 section planes can be selected. The method of the invention makes it possible to display to a user, more particularly a radiologist, a reconstructed image for each section plane. Each of the images makes it possible to display the presence or absence of a scar by locating it precisely in relation to the anatomy of the heart, the myocardium and the volumes of blood surrounding the imaged area.

Claims

CLAIMS A method for reconstructing an image of a patient's heart from a magnetic resonance imaging (MRI-i) device generating a magnetic field (B0) comprising: - A first phase (PHi) of image generation comprising: ■ Acquisition (ACQo) of the electrical activity (ECGi) of the patient's heart; ■ Generation (GENi) of a first 180° radiofrequency reversal signal (Rfi) to reverse a longitudinal magnetization of tissues of an imaged area, said first radiofrequency reversal signal (Rfi) being generated between two QRS complexes of the acquired electrical activity (ECGi), called first inter-beat phase (Tci); ■ Generation (GEN2) of a first magnetization preparation (TAprep) comprising a set of power ({p ) following the generation of the first radio frequency inversion signal (Rfi) in the same first interbeat phase (Tci); ■ Acquisition (ACQ1) of a first 2D image (IM1) by a magnetization measurement of the imaged area, said acquisition (ACQ1) being carried out after a first predefined duration (D1) following the generation of the first magnetization preparation (T pr ep), said acquisition (ACQ1) being synchronized with the heart rate (ECGi) at a first temporal marker (M1) of the same inter-beat phase (Tci); ■ Generation (GEN3) of a second magnetization preparation comprising a set of pumps ({p ) in a second inter-beat phase (TC2) succeeding the first inter-beat phase (Tci); ■ Acquisition (ACQ2) of a second 2D image (IM2) by a magnetization measurement of the imaged area, said acquisition (ACQ2) being carried out after a second predefined duration (D2) following the generation of the second magnetization preparation (TB) pr ep), said acquisition (ACQ2) being synchronized with the acquisition of a heart rhythm (ECGi) at the same first temporal marker (Mi) of the second inter-beat phase (TC2) as the first marker (M1) of the first inter-beat phase (Tci); - Repetition of the first phase (PH1), to generate a subset of first images and a subset of second images of each slice plane (PCk) of a plurality of slice planes of the heart, in inter-beat phases (Tcik) following the first and second inter-beat phases (Tel, Tc2); - Application of a non-rigid registration algorithm (ALG1) to each subset of first images (IMi k (i)) of one of the section planes (PCk) to realign said first images (IM1 k (i)) of the subset of first images among themselves; - Application of a non-rigid registration algorithm (ALG1) to each subset of second images (IM2 k(i)) of one of the cutting planes (PCk) to realign said second images (IM2) k (i)) of the subset of second images relative to each other; - Fusion, for each of the cutting planes, of the first images registered to each other to produce a first fused image (IMiF k ) and on the other hand, second images registered to each other to produce a second fused image (IM2F k ) ; - Generation, for said section planes, of a set of third 2D images, each third image (IM3 k) corresponding to a first combination (COMB1) made between the first fused image and the second fused image produced for one of the cutting planes. Method according to claim 1, wherein the first duration (D1) is determined such that the first images acquired are images having a first contrast allowing the display of black blood images and the second duration (D2) is determined such that the second images acquired are images having a second contrast allowing the display of white blood images. A method according to any one of claims 1 to 2, wherein the recalibration step is followed by the steps of: - Averaging of the first rejected images to produce a first merged image (IMiF k ) for each cutting plane (PCk); - Averaging of the second rejected images to produce a first merged image (IM2F) k) for each slice plane (PCk). A method according to any one of claims 1 to 3, wherein the first duration (Di) is defined such that the longitudinal magnetization of the blood and that of the healthy myocardium cancel each other out at the same instant Tl during the acquisition (ACQi) of the first 2D image. A method according to any one of claims 1 to 4, wherein the determination of the first duration (Di) is automatically calculated from a computer-implemented method of an optimal inversion time obtained from the processing of at least one image acquired from an MRI presequence. A method according to any one of claims 1 to 5, comprising a pixel colorization step for each first fused image (IMiF k) having a luminance greater than a given threshold, the colorization step being carried out prior to the first combination (COMB1). A method according to any one of claims 1 to 6, wherein the first combination (COMB1) is a superposition of the first image (IMiF k ) on the second merged image (IM2F k ). A method according to any one of claims 1 to 7, wherein: ■ Each generation of a new first image and a new second image in a new cross-sectional plane of the heart is performed after: ■ the set of repetitions of the first phase (PHi) to generate a plurality of first images ({IMi k (i)}j[iN]) and of seconds images ({IM2 k (i)}i[iN]) of a previous section plane (PCk) of the core.

9. A method according to any one of claims 1 to 8, comprising: ■ a display of a set of first merged images (IMiFk ) for each cutting plane (PCk); ■ a display of a set of merged second images (IM2F) k ) for each cutting plane (PCk); ■ a display of a set of third 2D images for each cutting plane (PCk); ■ each first image (IMiF k ) of a given cross-section plane (PCk) being displayed in the immediate vicinity of the second image and the third image (IM3F k ) of the same given cutting plane (PCk).

10. A method according to any one of claims 1 to 9, wherein the first fused image (IMiF k ) for each slice plane (PCk) corresponds to an image of a slice of the heart in black blood, the second fused image (IM2F k ) for each slice plane (PCk) corresponds to an image of a slice of the heart in white blood, the third image (IM3F k) for each section plane (PCk) corresponding to an image of a section of the organ on which a scar, if any, is displayed in color.

11. Method for reconstructing an image according to any one of claims 1 to 10, wherein the first and second magnetization preparation (TAprep, TBprep) are T1 rho weightings.

12. Method for reconstructing an image according to any one of claims 1 to 11, wherein the first magnetization preparation and / or the second magnetization preparation (TAprep, TBprep) is a T2prep type preparation or a magnetization transfer (MT) preparation.

13. Method for reconstructing an image according to any one of claims 1 to 12, wherein the generation (GEN3) of a second magnetization preparation (TBprep) comprising a set of pulses ({p ) in an inter-beat phase (TCN) is followed by a step of filtering the acquired images corresponding to fatty areas of the imaged organ.

14. Method for reconstructing an image according to any one of claims 1 to 13, wherein the first images (IM1) and the second images (IM2) are acquired successively in a synchronized manner with the electrical activity of the heart between two QRS complexes for several minutes in free breathing during a single examination.

15. A method for reconstructing an image according to any one of claims 1 to 14, wherein the first and second images are acquired over a set of 8 to 20 cross-sectional planes of the organ, the reiteration phase of acquiring a plurality of first images (IMi) k (i)}j[iN]) and second images (IM2 k (i)}i[iN]) in the same cutting plane (PCk) allowing the acquisition of between 2 and 10 images of first images ({IMi k (i)}j[iN]) and between 2 and 10 images of second images ({IM2 k (i)}i[iN]) by cutting plane.

16. Magnetic resonance imaging (Si) system comprising a magnetic field generator (GEN_Bo, GEN_GRAD) and a radio frequency device (DISPO_RF), an electrocardiograph (ELC1) and a processing device (K1), a display (IHM1) for displaying the generated images, said system being configured to implement the method of any one of claims 1 to 15.