Patent Publication Number: US-8115482-B2

Title: Magnetic resonance anatomical image generating method and system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention concerns a method for generation of an anatomical image of an examination area with a magnetic resonance apparatus. 
     The invention also concerns a computer program and a magnetic resonance apparatus for implementation of the method. 
     2. Description of the Prior Art 
     Magnetic resonance (MR) is a known modality with which images of the inside of an examination subject can be generated. Expressed in a simplified way, the examination subject is positioned in a strong, static, homogeneous basic magnetic field (field strengths of 0.2 Tesla to 7 Tesla and more) in an MR such that the nuclear spins of the examination subject orient along the basic magnetic field. 
     To trigger nuclear magnetic resonance signals, radio-frequency excitation pulses are radiated into the examination subject, the triggered nuclear magnetic resonance signals are measured and MR images are reconstructed based thereon. The MR imaging enables image contrasts that result from the combination of multiple parameters. Important MR parameters are, for example, the density of the excited nuclear spins (primarily hydrogen protons); the relaxation times for magnetizations (T 1 , T 2 , T 2 *) of the examined tissue; the magnetization transfer; and diverse additional contrast mechanisms. 
     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 in a k-space matrix as complex number values. An associated MR image can be reconstructed from the k-space matrix populated with values by means of a multi-dimensional Fourier transformation. 
     Depending on the type of the examination and of the examination subject, an acquisition sequence is selected that exhibits those MR parameters that generate an advantageous image contrast for the examination. Value maps in which the distribution of only a single MR parameter is listed make the diagnosis easier for specific examinations. 
     For example, in the functional imaging of cartilage tissue value maps of the relaxation times T 2  and T 2 * have been used for some time in order to monitor the course of a therapy or an illness (such as, for example, osteoarthritis). For this purpose, multi-echo gradient echo sequences or multi-echo spin echo sequences are used for generation of T 2 * maps or T 2  maps, wherein the measured data of the respective multi-echo sequences are fitted to the respective relaxation equations in order to obtain a value map of the corresponding relaxation parameter. 
     Maier et al. describe such a procedure in “T 2  Quantitation of Articular Cartilage at 1.5 T”, Journal of Magnetic Resonance Imaging 17: 358-364 (2003) using a T 2  value map in connection with examinations of patellae. 
     Further application fields of such parameter value maps to support a diagnosis are, for example, the field of liver examinations, in particular for examination and monitoring of an iron uptake of the liver (hemachromatosis) or examination of the nerve bundle at a spinal column. 
     T 2 * and T 2  value maps can also be generated in a known manner without multi-echo sequences when a number of individual measurements of an examination area are implemented in order with the same repetition time but different echo times. The generation of T 1  value maps is likewise known, for example from a series of measurements (at least two) with different repetition times TR but the same echo time. Further measurement sequences are known that are used for a generation of MR parameter value maps, also with regard to other MR parameters. 
     In order to increase the usage of these parameter value maps and in particular to enable a precise localization of the conditions presented in the parameter value map, however, the parameter value maps should be associated with a corresponding anatomical image. However, this procedure requires a high degree of qualification and training since the parameter value maps must be manually adapted to an associated anatomical image. 
     Moreover, specific image information of the anatomical image can interfere with the desired information about, for example, the cartilage tissue in the combined image. For example, osseous tissue that is not important for the monitoring of cartilage tissue but possibly exhibits similar contrasts can optically deflect. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to enable the generation of an image in which both the anatomy and the distribution of relevant MR parameters are recognizable. 
     The above object is achieved according to the invention by a method and MR system for generation of an anatomical image of an examination area wherein at least one image data set of the examination area and a parameter value map are loaded into a processor. The at least one loaded image data set as well as the loaded parameter value map are processed into an anatomical image. The processing includes a weighting of elements of the at least one image data set with a weighting factor. The weighting factor depends on a parameter value of the parameter value map corresponding to the respective element of the image data set. The generated weighted anatomical image is displayed and/or stored. 
     This enables in a simple manner a fast and effective visualization of fluctuations of an MR parameter in the anatomy of the examination area. 
     The at least one loaded image data set was advantageously used for the generation of the parameter value map. In a simple manner it is thus ensured that image data set and value map reproduce the exact same examination area. 
     In an embodiment of the invention the parameter value map was generated from at least two image data sets of the examination area, with only echo signals of a specific echo time of measurement data of the examination area being used for a generation of every single image data set, and with the echo times of the echo signals that differ for different image data sets. The use of multiple image data sets in the generation of the parameter value map also allows the use of multiple (at least two) image data sets in the processing into an anatomical image, which offers advantages by averaging. 
     In a further embodiment, the examination area encompasses cartilage. Parameter value maps are directly suitable for a diagnosis of cartilage tissue. 
     In another embodiment the parameter is a time constant of the transversal magnetic relaxation (T 2 ) or a time constant of the true decay of the transversal magnetization (T 2 *) or a time constant of the longitudinal magnetic relaxation (T 1 ). Particularly simple value maps can be created for the time constants of the magnetic resonance technique. The diagnostic use is simultaneously very high. 
     The above object also is achieved in accordance with the present invention by a computer-readable medium encoded with programming instructions that cause a processor, loaded with the aforementioned image data set of the examination area and the aforementioned parameter value map, to implement the processing described above in connection with the inventive method and system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates a magnetic resonance apparatus. 
         FIG. 2  is a flowchart of an exemplary embodiment of a method for generation of an anatomical image of an examination area with a magnetic resonance apparatus in accordance with the invention. 
         FIG. 3  shows an example of a measurement sequence suitable for the inventive method. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  schematically shows the basic components of a magnetic resonance apparatus  1 . In order to examine a body by means of magnetic resonance imaging, various magnetic fields matched to one another as precisely as possible in terms of their temporal and spatial characteristics are radiated at the body. 
     A strong magnet (typically a cryomagnet  5  with a tunnel-shaped opening) arranged in a radio-frequency-shielded measurement chamber  3  generates a static, strong basic magnetic field  7  that is typically 0.2 Tesla to 7 Tesla and more. A body or a body part to be examined (not shown here) is borne on a patient bed  9  and 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 control unit  17 . After an amplification by a radio-frequency antenna  19  they are conducted to the radio-frequency antenna. The radio-frequency system shown here is merely schematically indicated. More than one pulse generation unit  15 , more than one radio-frequency amplifier  19  and multiple radio-frequency antennas are typically used in a magnetic resonance apparatus  1 . 
     Furthermore, the magnetic resonance apparatus  1  has 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 processed further and digitized by an acquisition unit  29 . 
     Given a coil that can be operated both in transmission mode and in acquisition mode (such as the body coil  13 , for example), the correct signal relaying is regulated by an upstream transmission-reception diplexer  39 . 
     An image processing unit  31  generates from the measurement data an image that is presented to a user via a control console  33  or is stored in a memory storage unit  35 . 
     A central computer  37  controls the individual system components, for example during the acquisition of the measurement data. The computer  37  is fashioned such that the inventive method can be implemented with the computer  37  together with the pulse sequence controller unit  17  controlled by the computer  37  and with the image processing unit  31 . For example, for this purpose an inventive computer program can be executed on the computer  37  and possibly also installed on the image processing unit  31 . 
     The computer  37  can also be formed of multiple sub-units, of which at least one can also be operated independently of a magnetic resonance apparatus  1 . 
       FIG. 2  shows a workflow diagram of an exemplary method for generation of an anatomical image of an examination area with a magnetic resonance apparatus. 
     Measurement data of at least one measurement sequence with which the desired MR parameters can be shown are thereby acquired in a first step  10 . A series of suitable sequences are listed at the top right in  FIG. 2 , for example (multi-echo) gradient echo sequences for the T 2 * parameter or (multi-echo) spin echo sequences for the T 2  parameter. The multi-echo sequences generate at least two echo signals at echo times TE i  (iεN) after an excitation pulse. It thereby applies that: TE i ≠TE j  if i≠j (jεN). The acquired measurement data of each echo signal are digitized in a known manner and stored in a k-space matrix as complex number values, wherein a separate k-space matrix exists for each echo time TE i . 
     In a further step  12  an image data set I i (x,y) is generated by means of known transformation techniques from at least one of the k-space matrices populated with values. I i (x,y) is hereby the intensity in the image element with the coordinates (x,y) of the image data set belonging to the echo time TE i . 
     In a further step  14 , a spatially resolved parameter value map (for example a value map of the relaxation constant T 2  (x,y) of the transversal magnetization or of the time constant of the free induction decay T 2 *(x,y) or a value map of a further MR parameter) is generated from the acquired measurement data. For example, this occurs by fitting the measurement data of the various echo signals to corresponding relaxation equations. 
     The at least one image data set I i (x,y) and the generated parameter value map are loaded in the steps  16   a  and  16   b . As already noted, there are also other possibilities in order to arrive at the at least one image data set and the parameter value map for the inventive method. The steps  10  through  14  merely provide an example. 
     The at least one image data set I i (x,y) is processed into a relaxation-weighted anatomical image in a processing step  18 . For this the at least one image data set I i (x,y) is weighted depending on the associated relaxation parameter value (for example T 2 (x,y)) and the associated characteristic time value (for example the echo time TE i ). 
     In an embodiment the at least one loaded image data set and the loaded parameter value map are processed according to the following formula: 
     
       
         
           
             
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     I(x,y) is the intensity value of the relaxation-weighted anatomical image at the coordinate (x,y), N is the number of the different image data sets I i (x,y) used for processing, I i (x,y) is the intensity value of the i-th image data set at the coordinate (x,y), T i  is the time value characteristic of the image data set I i (x,y), for example the echo time TE i  (if T rel  is T 2  or T 2 *) or the repetition time TR (if T rel  is T 1 ). T rel (x,y) is the parameter value at the coordinate (x,y). T rel  stands for T 1 , T 2  or T 2 *. It is understood that N cannot be greater than the maximum number of the echo signals after the excitation pulse. 
     The at least one loaded image data set I i (x,y) is thus multiplied per element (i.e. for every possible combination of x with y) with a weighting factor that exponentially decreases with the ratio of respective echo time TE i  of the image data set I i (x,y) to respective parameter value T rel (x,y). If more than one image data set I i (x,y) was loaded, identical elements of the different image data sets I i (x,y) are initially squared after the multiplication with the weighting factor, added, and then the square root is taken. The result is divided by the number of the loaded image data sets N. 
     Possibly only a portion of the elements of the at least one image data set is weighted. For example, only the element with the coordinates (x,y) with xε[x min ; x max ] and yε[y min ; y max ] where an assistance with a diagnosis is desired may be weighted. 
     In a simpler embodiment of the method, the squaring of the weighted intensity values of the image data sets and the subsequent taking of the square root and/or the division by the number N of the various image data sets used for processing are omitted. This procedure also leads to an acceptable result but is not entirely mathematically correct. Moreover, the average achieved with the above, mathematically correct formula can have a positive influence on the end result. 
     The weighted anatomical image generated in the processing step  18  is displayed and/or stored in a step  20 . 
       FIG. 3  shows an example of a measurement sequence suitable for the method according to the invention in an example of a double echo spin echo sequence. 
     An excitation pulse  22  (for example a 90° pulse) excites the spins in the examination area. After half of an echo time TE 1 , an additional pulse  24  (what is known as a rephasing or 180° pulse  24 ) is radiated. This pulse  24  ensures that a first echo signal  30  is generated at the time TE 1 . After a wait time T w  after the first echo signal  30 , a second 180° pulse  26  is radiated that generates a second echo signal  32  after an echo time TE 2 =TE 1 +2*T w . 
     For spatial coding of the magnetic resonance signals, pulse-shaped magnetic gradient fields are generated in the three independent spatial directions. 
     An identical slice-selection gradient G S  for selection of a respectively identical slice in the examination subject is respectively radiated upon the radiation of each pulse  22 ,  24 ,  26 . Phase coding gradients G P  are radiated between slice selection and echo signal. An identical frequency coding gradient G F  is respectively radiated during the readout of the echo signals  30  and  32 . 
     In this case the maximum amplitude of each signal falls exponentially with the ratio of the time to the time constant T 2 . For example, a T 2  value map can thus be calculated from two image data sets that were acquired at the echo times TE 1  and TE 2  in the described manner. 
     The spin echo pulse sequence shown in  FIG. 3  is repeated with various phase coding gradients G P  (indicated by the horizontal lines in the pulses of the phase coding gradients G P ) until the k-space matrix is filled with sufficient values for image reconstruction. 
     For example, multi-echo gradient echo sequences (such as FLASH, for instance) are suitable for the calculation of a T 2 * value map. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.