Abstract:
In a method and magnetic resonance apparatus to acquire and present calibration images of a periodically moving organ with the use of magnetic resonance technology, calibration images are acquired by acquiring measurement data for multiple calibration images during one continuous period of the organ movement, the multiple calibration images differing in their offset frequency and/or in their spatial position in the organ to be examined, and the calibration images in a presentation manner that, from the visual quality of the respective images, allows the user to select (identify) the image acquired with the offset frequency that should then be used to acquire the diagnostic image are displayed to a user.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention concerns a method to acquire and display calibration images in a periodically moving organ with the use of magnetic resonance technology. Furthermore, the invention concerns a magnetic resonance apparatus to implement such a method 
     2. Description of the Prior Art 
     Calibration images are used (among other things) in magnetic resonance imaging before diagnostically significant images of a subject to be examined are acquired and serve for the adjustment or, respectively, the optimization of the subsequent acquisition of the measurement data from which the diagnostically significant images are generated. 
     Magnetic resonance technology (in the following the term “magnetic resonance” is also shortened to MR) is thereby a technique that has been known for several decades with which images of the inside of an examination subject can be generated. Described in a significantly simplified way, the examination subject is positioned in a relatively strong, static, homogeneous basic magnetic field (field strengths of 0.2 Tesla to 7 Tesla or more) so that nuclear spins in the subject orient along the basic magnetic field. Radio-frequency excitation pulses are radiated into the examination subject to excite nuclear magnetic resonances, the resonant nuclear spin signal is measured, and MR images are reconstructed based thereon. For spatial coding of the measurement data, rapidly switched gradient fields are superimposed on the basic magnetic field. The acquired measurement data are digitized and stored as complex numerical values in a k-space matrix. By means of a multidimensional Fourier transformation, an associated MR image can be reconstructed from the k-space matrix populated with such values. 
     MR signal generation and acquisition are sensitive to small errors and inaccuracies in the technique that is used. Depending on the measurement sequence that is used, artifacts in particular increasingly occur at high field strengths of 3 Tesla or more, which are increasingly being used for diagnostic imaging. 
     Effects known as the off-resonance effects are one cause of image artifacts. These effects occur when the resonance frequency (Larmor frequency) of the nuclear spins to be excited differs slightly from the frequency with which the excitation pulses are radiated. For example, this slight difference can be the consequence of an insufficient shim or an insufficient frequency adjustment. In the reconstructed image, this can often appear as band-shaped artifacts, which can considerably hinder an evaluation of measured data. The TrueFISP sequence (True fast imaging with steady state precession) is one example of a known and established sequence that is sensitive to off-resonance effects. 
     The use of calibration images to select a suitable offset frequency is presently known in order to combat this problem. The offset frequency specifies how significantly the actual excitation frequency of an excitation pulse deviates from a previously selected excitation frequency. Such calibration images are also known as “frequency scouts”. One image per heartbeat is acquired with a TrueFISP sequence over a breath-hold phase. The value for the offset frequency is varied in equal steps in a suitable frequency range across the heartbeats. The image with the best image quality can then be selected from the image series that is acquired in this way. The offset frequency associated with this image can then be used in the following TrueFISP measurement in which the diagnostic image data are then acquired with an advantageous artifact response. 
       FIG. 2  schematically shows the chronological workflow of this acquisition. Trigger points  47  that respectively identify the beginning of a cardiac cycle are determined between the beginning  41  and end  43  of a breath-hold phase based on the EKG signal  45 . A calibration image  51  . . .  54  is respectively acquired at a defined time interval at these trigger points  47 , wherein the calibration images differ in their offset frequency. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method for acquisition and display of calibration images that optimizes the discovery (identification) of the ideal offset frequency so that a subsequent acquisition of measurement data leads to images with higher quality with simultaneously faster acquisition of the calibration images. Furthermore, it is an object of the invention to provide a magnetic resonance apparatus for implementation of such a method. 
     In the method according to the invention for acquisition and display of calibration images in a periodically moving organ with the aid of magnetic resonance technology, calibration images are acquired in a first step in that measurement data for multiple calibration images are acquired during a continuous period of the organ movement, wherein the multiple calibration images differ in their offset frequency and/or in their spatial position in the organ to be examined, and the calibration images are displayed to a user in a second step in a presentation manner that, from the visual quality of the respective images, allows the user to select (identify) the image acquired with the offset frequency that should then be used to acquire the diagnostic image. 
     In contrast to known methods with which measurement data for a calibration image are acquired only in a specific phase per cardiac cycle, the method according to the invention enables the acquisition of more calibration images in the same amount of time, which distinctly increases the flexibility in the design of the method. For example, more offset frequencies can be tested, and/or calibration images from many different slices can be acquired, without distinctly increasing the measurement time. The calibration images acquired in one period differ, for instance in their spatial position and/or in their offset frequency. 
     As used herein “acquisition of an image” means the acquisition of magnetic resonance measurement data that are associated with this image and with which the image can be reconstructed in a further processing step. 
     Starting from the known method, in which one TrueFISP image was respectively acquired in the same phase of the cardiac cycle per heart beat (one TrueFISP image for each heart beat), the presentation is based on the insight that the optimal offset frequency that is determined in this manner (thus an offset frequency that initially applies only for this phase of the cardiac cycle) is applied uniformly to all cardiac phases in a subsequent measurement, and nevertheless good, satisfactory results are provided. Furthermore, it has been recognized that this situation in reverse also means that the measurement of different offset frequencies that are acquired in different phases of an organ movement still provide a sufficient information base for the optimization of the offset frequencies. This is even the case when the acquired images for selection of the offset frequency are associated with different phases of the organ movement, and therefore do not a priori necessarily need to be comparable with one another. 
     It has also been recognized that the phase of an organ movement has only a slight influence (if any at all) on the selection of the matching offset frequency. This allows not only a single calibration image (that is associated with a defined phase of the organ movement) but rather a plurality of calibration images to be acquired in a period of the organ movement, without the quality of the selection of the matching offset frequency being too significantly negatively affected. 
     Measurement data for multiple calibration images are now acquired in a period of the organ movement. This acquisition can occur in blocks, meaning that the measurement data for the different calibration images are sequentially acquired one after another. 
     The acquired calibration images can be two-dimensional calibration images whose spatial position in the organ to be examined is associated with a slice through the organ to be examined. 
     As used herein, a “periodically moving organ” means an organ exhibiting a movement pattern that repeats. The movement pattern does not have to repeat exactly over time and does not have to repeat identically. A certain range of variations typically always occurs in the movement pattern in a living subject. Organs with a periodic movement are typically the heart, the lungs, the peristalsis in the gastrointestinal tract, the pulsing of vessels, etc. 
     In an embodiment of the method, the calibration images for which the measurement data are acquired during a period of the organ movement differ only in terms of their offset frequency. This means that the calibration images for the different offset frequencies are acquired for the same slice during the one period. Calibration images for a different slice can then be acquired during the next period etc. 
     In a preferred embodiment variant, those calibration images which differ in their offset frequencies are displayed in a movie-like presentation in the display of the calibration images. For example, the calibration images that belong to one slice and to different offset frequencies can be shown sequentially one after another so that, by mere observation of the movie-like sequence, the user is able to compare images that do not belong to different offset frequencies and to determine the matching offset frequency. 
     In another embodiment, those calibration images that differ in their spatial position relative to the subject to be examined are displayed in parallel (next to one another) in the display of the calibration images. In particular, a combination of these two embodiment variants results in an advantageous presentation. In this case, multiple slices are presented in parallel, and the movie presentation enables a sequential consideration of the different offset frequencies. It is thus simple for a user to identify the matching offset frequency. 
     In the method, the periodically moving organ is advantageously the heart. Embodiments of the invention have a particularly advantageous effect in the imaging of the heart since here off-resonance effects which can often only be insufficiently combated with conventional, standard adjustment methods can occur due to a movement caused by blood flow in the circulatory system. 
     The method is particularly advantageous when a TrueFISP sequence which is particularly susceptible to off-resonance effects is used as the sequence for measurement data acquisition. 
     In the imaging of the heart it is particularly advantageous to select the spatial orientation of the calibration images is selected so that the heart is scanned in a series of short axis slices. 
     The magnetic resonance apparatus according to the invention has a control device that is fashioned to implement the method as described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates the basic components design of a conventional magnetic resonance apparatus. 
         FIG. 2  illustrates a scheme for acquisition of calibration images as it is known in the prior art. 
         FIG. 3  illustrates a scheme for acquisition of calibration images according to one embodiment of the method. 
         FIG. 4  shows a user-friendly presentation of the acquired calibration images. 
         FIG. 5  is a schematic overview of the basic steps in an embodiment of the inventive method. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  schematically shows the design of a magnetic resonance apparatus  1  with its basic components. In order to examine a body by means of magnetic resonance imaging, different magnetic fields matches to one another as precisely as possible in terms of their temporal and spatial characteristics are radiated into the body. 
     A strong magnet (typically a cryomagnet  5  with a tunnel-shaped opening) arranged in a measurement chamber shielded against radio frequencies generates a strong, static basic magnetic field  7  that is typically 0.2 Tesla to 3 Tesla or more. A body or a body part (not shown here) to be examined is placed on a patient bed  9  and is positioned in a homogeneous region of the basic magnetic field  7 . 
     The excitation of the nuclear spins of the body ensues via magnetic radio-frequency excitation pulses that are radiated via a radio-frequency antenna (shown here as a body coil  13 ). The radio-frequency excitation pulses are generated by a pulse generation unit  15  that is controlled by a pulse sequence control unit  17 . After an amplification by a radio-frequency amplifier  19 , they are conducted to the radio-frequency antenna. The radio-frequency system shown here is merely schematically indicated. Typically, more than one pulse generation unit  15 , more than one radio-frequency amplifier  19  and multiple radio-frequency antennas are used in a magnetic resonance apparatus  1 . 
     Furthermore, the magnetic resonance apparatus  1  possesses gradient coils  21  with which magnetic gradient fields for selective slice excitation and for spatial coding of the measurement signal are radiated in a measurement. The gradient coils  21  are controlled by a gradient coil control unit  23  that, like the pulse generation unit  15 , is connected with the pulse sequence control unit  17 . 
     The signals emitted by the excited nuclear spins are received by the body coil  13  and/or by local coils  25 , amplified by associated radio-frequency preamplifiers  27  and further processed and digitized by an acquisition unit  29 . 
     If a coil is used that can be operated both in transmission and in reception mode, for example the body coil  13 , the correct signal relaying is regulated via an upstream transmission/reception diplexer  39 . 
     From the measurement data, an image processing unit  31  generates an image that is presented to a user via an operator console  33  or is stored in a memory unit  35 . A central computer  37  controls the individual system components. 
     Such an MR apparatus corresponds to an MR apparatus as it is known in the prior art. 
     The computer  37  (and, if necessary, additional components for controlling the MR apparatus) can be configured (programmed) so that the method according to the invention can be implemented with the MR apparatus, as is subsequently explained in detail. 
       FIG. 2  shows the scheme of the chronological workflow of the acquisition of calibration images as it is known in the prior art. A detailed description of  FIG. 2  has already been provided. 
       FIG. 3  schematically shows the chronological workflow in an acquisition according to one embodiment of the invention. Here as well, trigger points  47  between the beginning  41  and end  43  of a breath-hold phase are determined based on the EKG signal  45 , which trigger points  47  respectively identify the beginning of a cardiac cycle. 
     A plurality of calibration images  61  . . .  66 ,  71  . . .  76 ,  81  . . .  86 ,  91  . . .  96  is acquired in every cardiac cycle at a defined time interval relative to these trigger points  47 . The acquisition of the measurement data for the calibration images  61  . . .  66 ,  71  . . .  76 ,  81  . . .  86 ,  91  . . .  96  thereby ensues with a single shot TrueFISP sequence. 
     The individual calibration images  61  . . .  66 ,  71  . . .  76 ,  81  . . .  86 ,  91  . . .  96  in a cardiac cycle thereby belong to a short axis slice  60 ,  70 ,  80 ,  90  of the heart  49 . The calibration images  61  . . .  66 ,  71  . . .  76 ,  81  . . .  86 ,  91  . . .  96  of a cardiac cycle differ in their offset frequency. 
     In a first cardiac cycle, the first calibration image  61  is acquired with, for example, a first offset frequency v 1 , the second calibration image  62  is acquired with a second offset frequency v 2  etc. The offset frequencies v 1 , v 2  . . . vn can thereby be gradually increased from calibration image to calibration image so that the frequency range of interest for the short axis slice  60  is already covered in a few cardiac cycles. 
     In a second cardiac cycle, the first calibration image  71  is in turn acquired with the first offset frequency v 1 , the second calibration image  72  is acquired with the second offset frequency v 2  etc. 
     A different offset frequency is thus associated with every time phase of the cardiac cycle. 
     A number of calibration images  61  . . .  66 ,  71  . . .  76 ,  81  . . .  86 ,  91  . . .  96  that belong both to different offset frequencies and to different short axis slices  60 ,  70 ,  80 ,  90  of the heart can be acquired in a single breath-hold phase in this way. It is even possible that all slice geometries (which typically must be covered for a left ventricular function analysis, for example)—i.e. 8 to 12 short axis slices—are measured in a single breath-hold phase and subsequently are optimized with regard to the offset frequency. 
     The sorting of the calibration images with regard to the cardiac cycles that is described using  FIG. 3  is one advantageous possibility for the sorting of the calibration images. Other possibilities are also conceivable, for example in that calibration images with different spatial positions that, however, belong to a single offset frequency are acquired in one cardiac cycle, and calibration images with the same different spatial positions that then belong to a different offset frequency are acquired in a next cardiac cycle etc. A combination of the sorting just described and the sorting described using  FIG. 3  is also possible. 
     The calibration images are presented to a user after all of the calibration images  61  . . .  66 ,  71  . . .  76 ,  81  . . .  86 ,  91  . . .  96  have been acquired. 
     An advantageous presentation of the calibration images is explained using  FIG. 4 . 
     The calibration images  61  . . .  66 ,  71  . . .  76 ,  81  . . .  86 ,  91  . . .  96  are thereby displayed in parallel in groups  103  . . .  108  on a display device  101 . The calibration images of different offset frequency that belong to one of the short axis slices  60 ,  70 ,  80 ,  90  of the heart  49  are thereby respectively combined into one group  103  . . .  108 . For presentation of the different offset frequencies, the calibration images are thereby displayed in a movie-like presentation, wherein the individual offset frequencies are shown in chronological order in the manner of a movie (indicated by the arrows in the drawing). This way a user can quickly get an overview of the quality of the individual calibration images per slice and for different offset frequencies. The user can thereby affect the movie-like presentation in that, for example, he can scroll forward and back, can pause the movie-like presentation, etc. 
     This allows a user to also quickly assess the calibration images  61  . . .  66 ,  71  . . .  76 ,  81  . . .  86 ,  91  . . .  96  with regard to stripe artifacts for different slice orientations and to select the matching offset frequency for a subsequent acquisition. The user thus can determine the matching offset frequency separately for every slice  60 ,  70 ,  80 ,  90 ; however, it is also possible to determine a common offset frequency that represents a compromise for the stripe artifacts with regard to all slices. 
       FIG. 5  schematically shows individual method steps of an embodiment of the method. 
     After beginning of a breath-hold phase (Step  111 ), the periodic organ movement of the organ to be examined is monitored (Step  113 ). The acquisition of multiple calibration images (Step  115 ) ensues in one period of the organ movement. This calibration images differ with regard to their spatial position in the organ to be examined and/or with regard to their offset frequency. The acquisition of the calibration images is respectively continued in the following periods of the organ movement until the end of the breath-hold phase (Step  117 ) is reached. In the event that it is necessary, the reintroduction of a breath-hold phase and an additional acquisition of calibration images ensue. After all desired calibration images have been acquired, the calibration images are shown to a user (Step  119 ). This presentation allows the user to select the matching offset frequency for the subsequent acquisition of the diagnostically relevant images (Step  121 ). 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.