MAGNETIC RESONANCE IMAGING METHOD FOR IMAGING COMPONENTS WITH SHORT TRANSVERSE RELAXATION TIMES (T2) IN A HUMAN OR AN ANIMAL HEART

A magnetic resonance imaging (MRI) method for imaging components with short transverse relaxation times (T2) is provided, in which a human or an animal heart is subjected to a segmented spoiled gradient echo (SPGR) sequence. Each segment of this SPGR sequence comprises a plurality of basic sequence elements in each of which a radiofrequency (RF) pulse and a frequency encoding gradient moment kx are applied, in order to generate an MRI signal at an echo time TE1. The RF pulses and the frequency encoding gradient moments kx are applied such, that in different basic sequence elements the MRI signal is generated at varying echo times TE1, in order to reduce the effective echo time in the center of k-space. The segments of the SPGR sequence are synchronized with at least one measured cycle indicator reflecting the timing of the cardiac cycles. The MRI signals generated by the SPGR sequence are used for reconstructing at least one first cardiac image.

DESCRIPTION OF PREFERRED EMBODIMENTS

InFIG. 1, an exemplary MRI system is shown which serves to carry out the inventive method for imaging components with short transverse relaxation times (T2) in a human or an animal heart. Preferred embodiments of the inventive method are schematically illustrated inFIGS. 2-4.

The MRI system comprises a main magnet1for producing a main magnetic field B0. The main magnet1usually has the essential shape of a hollow cylinder with a horizontal bore. Inside the bore of the main magnet1a magnetic field is present, which is essentially uniform at least in the region of the isocenter6of the main magnet1. The main magnet1serves to at least partly align the nuclear spins of a sample5arranged in the bore. Of course, the magnet1does not necessarily be cylinder-shaped, but could for example also be C-shaped.

A patient5is arranged in such a way on a moving table4in the bore of the main magnet1, that the heart of the patient5, of which components with short transverse relaxation times (T2) are to be imaged, is arranged approximately in the region of the isocenter6of the magnet1. In order to avoid motion artefacts due to diaphragm movement, the patient5is instructed to hold his breath during the entire image acquisition.

The main magnet1has a z-axis9which coincides with the central longitudinal axis defined by the cylindrical shape of the magnet1. Together with a x-axis7and a y-axis8, which each extend in mutually perpendicular directions with respect to the z-axis9, the z-axis9defines a Cartesian coordinate system of the MRI system, having its origin at the isocenter of the magnet1.

In order to produce a magnetic field which linearly varies in the direction of the x-axis7, the y-axis8and/or the z-axis9, the MRI-system comprises a gradient system2including several coils for producing these varying magnetic fields. A radiofrequency (RF) coil3and a RF transmitter connected with the RF coil3are provided for generating a transmit field B1, in order to repetitively excite the nuclear spins of the patient5by means of RF pulses. The RF coil3is additionally connected with a receiver for the reception of the MR signals measured by the RF coil. Both the RF transmitter and the receiver are controlled by a central control unit.

ECG electrodes11placed on the chest of the patient5are adapted to send electric signals to an ECG analysis unit, in which preferably the timing of the R-waves of the ECG is detected. A corresponding cardiac cycle indicator reflecting the timing of the ECG R-wave is sent from the ECG analysis unit to the central control unit.

The receiver, which constitutes an acquisition module together with the RF coil3, is connected with a reconstruction unit, in which the acquired MR signals are reconstructed into cardiac images. The cardiac images are sent from the reconstruction unit to a user interface10, usually realized by a customary personal computer, in which the images are post processed.

For imaging components with short T2values in the heart of the patient5, a segmented spoiled gradient echo (SPGR) sequence is initiated by an operator by means of a user interface10, which sends the respective instructions to a central control unit of the MRI system (FIG. 1). The central control unit controls a gradient field control unit being connected with the gradient system2as well as the RF transmitter and the receiver both being connected with the RF coil3. The gradient system2and the gradient field control unit together constitute a gradient module of the MRI system. The central control unit controls the gradient field control unit, the RF transmitter and the receiver based on the information received from the ECG analysis unit such, that an ECG-gated segmented SPGR imaging sequence is employed on the MRI system, which allows imaging components with short T2values.

The applied segmented SPGR sequence is illustrated inFIGS. 2 and 3. The R-wave of the ECG of the patient5is detected and used as a cardiac cycle indicator for the synchronization of the SPGR sequence with the heartbeat of the patient5. During each heartbeat one segment of the SPGR sequence is applied with a trigger delay of 400 ms after the R-wave of the ECG, i.e. at end diastole. The duration TR′ of one entire segment of the SPGR sequence is here 240 ms.

As shown inFIG. 2, each segment of the applied SPGR sequence comprises a preparation part followed by a plurality of consecutive basic sequence elements (line1, line2, line3etc.). In the preparation part, a fat presaturation pulse in the form of a FAT SAT pulse is applied, in order to suppress the magnetic resonance (MR) signals of the fat protons in the entire segment. Further preparation steps are conceivable to be performed in each or in a part of the segments.

Following the fat presaturation pulse, a plurality of basic sequence elements is applied, in order to acquire multiple lines of the first cardiac image in k-space (line1, line2, line3etc.). Each line corresponds to one phase encoding step. Within one heartbeat (segment) only a fraction of the total number of lines used for reconstruction of the cardiac image can be acquired. After a certain number of heartbeats, however, sufficient lines are acquired, in order to obtain the cardiac image reflecting the components with short T2 values.

FIGS. 3A,3B and3C shows three basic sequence elements (A, B, C) of the SPGR sequence, in each of which one line of the first cardiac image is acquired in k-space at an echo time TE1. The acquisition of lines A and B, and C as shown inFIGS. 3A,3B and3C, can be carried out in the same segment or in different segments of the SPGR sequence. During each RF pulse a gradient moment is applied in slice encoding (or slice selection, SS) direction, in order to only excite the nuclear spins of a certain slice, in particular of a short axis or long axis slice through the heart of patient5. The RF pulse is followed by a gradient moment ky applied in phase encoding direction for encoding the spatial signal location along directions perpendicular to the frequency encoding direction. At the same time as the gradient moment ky, a gradient is applied on the z axis for the rephasing of the spins dephased by the slice selection gradient (rewinding gradient). The phase encoding gradient moment ky is increased in a plurality of incremental steps from ky=0 for the acquisition of line A to ky=kminfor acquiring line B. kminis a gradient moment in the range between 0 and the maximum phase encoding gradient moment kmaxapplied in the SPGR sequence. After the acquisition of line B, the phase encoding gradient moment is further increased to ky>kmin.

The duration of the phase encoding gradient is constant as long as ky≦kmin, because the variation of the gradient moments in the respective basic sequence elements is achieved by variations in the magnitude and/or the slew rate of the gradients and gradient time is bounded by the minimum achievable duration of the rewinding gradient. The gradient moment applied in frequency encoding (also referred to as readout, RO) direction is applied as soon as possible and immediately after the application of the RF pulse and of the phase encoding and slice encoding gradients. Since the duration of the RF pulse as well as the durations of the phase encoding and the slice encoding gradients is constant for ky≦kmin, the timing of the frequency encoding gradients is identical in all of the respective basic sequence elements. Hence, the MR signal is generated and acquired at a constant echo time TE1=TEminin these basic sequence elements with ky≦kmin(seeFIG. 4, part A).

For the acquisition with ky=kmin, the magnitude and the slew rate of the phase encoding gradient are maximal due to limitations set by the MRI system or due to constraints with regard to peripheral nerve stimulation. Thus, for ky>kminthe variation of the phase encoding gradient moment is achieved by varying the duration of the respective gradients. As a result, the frequency encoding gradient applied in readout (RO) direction needs to be shifted in time, such that the corresponding MR signals are generated an acquired at an echo time TE1>TEmin. As a consequence, the echo time TE1becomes a linear function dependent on the phase encoding gradient moment ky for ky>kmin(seeFIG. 4, part C). For ky=kmax, a maximal echo time TE1=TEmaxresults.

Due to the application of the SPGR sequence with varying echo times TE1as shown inFIGS. 3A,3B,3C and4, the effective echo time in the center of k-space is significantly reduced, as compared to a SPGR sequence in which the MR signals are generated and acquired in all basic sequence elements at the same echo time TE1=TEmax.

Based on the MR signals acquired at TE1, a first cardiac image is reconstructed, in which components with short T2values, in particular components with T2>TE1, are visible. For the reconstruction, the MR signals of all segments are combined, in order to yield a fully sampled image in the spectral spatial frequency domain, i.e. k-space, which is then transferred to image space by means of an (inverse) Fourier transformation. The person skilled in the art is well acquainted with performing such image reconstructions.

As can be seen fromFIGS. 3A, B and C, a second MR signal is generated and acquired in each basic sequence element at an echo time TE2=5.64 ms. The echo time TE2is constant throughout the entire SPGR sequence. The MR signal at echo time TE2is generated by means of additional gradient moments applied in frequency encoding direction, in order to induce a gradient echo at TE2. Based on the MR signals acquired at TE2, a second cardiac image is reconstructed, in which components with short T2values are visible to a much less extent as compared to the first cardiac image due to their natural T2-decay. Components with long T2values, however, appear with nearly the same signal intensities in both the first and the second cardiac image.

In order to obtain an image free of components with long T2values, the signal intensities of the second cardiac image is subtracted from the corresponding signal intensities of the first cardiac image, which is usually carried out in the user interface10(FIG. 1). The difference image can then be used for the detection of myocardial fibrosis.

In a concrete measurement, a two-dimensional SPGR sequence with variable echo times was applied on a Siemens Magnetom Espree 1.5T MRI scanner within one breath hold in an ECG-gated mode and with data acquisition at end diastole. A rectangular field of view (FOV) of 342 mm×267 mm was defined for image acquisition yielding a voxel size of 1.8×1.8×5.5 mm (192×150 base image matrix, bandwidth per pixel=960 Hz). The duration of each of the 35 segments was 270 ms. The minimum TE1of the basic sequence elements for the acquisition of the first cardiac image was 0.79 ms, and the TE2for the acquisition of the second cardiac image was 5.64 ms. The RF pulse resulted in a flip angle of 15° and had a duration was 320 μs with a time-bandwidth product of 1.1. An asymmetric sampling of 29% was applied in frequency encoding direction and maximum gradient magnitudes and slew rates were applied. Three averages were acquired with identical sequence parameters, resulting in a total scan time of 15 s.

FIG. 5Ashows the obtained first cardiac image based on the MR signals acquired at TE1, andFIG. 5Bshows the obtained second cardiac image based on the MR signals acquired at TE2.FIG. 5Cshows the differential image obtained by subtracting the signal intensities of the image shown inFIG. 5Bfrom the image shown inFIG. 5A. Please note that for representation purposes, brightness and contrast levels have been adjusted independently in all images. The images shown inFIGS. 5A-5Cwere acquired in a healthy human volunteer. No components with short T2values are visible within the myocardium indicating that no myocardial fibrosis is present in the heart of this healthy volunteer.

FIGS. 6A and 6Bshow short axis gadolinium late enhancement images for comparison purposes acquired on different levels of the heart of a patient with suspected myocardial infarction. Both images were acquired using a standard inversion recovery sequence after administration of a contrast agent. Late enhancement occurs within the myocardium at the positions indicated by the arrows, indicating the presence of myocardial fibrosis at these positions.

FIG. 7shows the difference image obtained using the segmented SPGR sequence and after subtracting the second cardiac image (TE2) from the first cardiac image (TE1). Short T2components are visible within the myocardium at the positions indicated by the arrows, which is in good agreement with the positions of myocardial fibrosis as detected in the late enhancement images shown inFIGS. 6A and 6B.

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