Abstract:
An imaging technique is disclosed to reduce ringing artifacts from amplitude decay in MR multi-echo acquisition. A flip angle train is determined to match scan parameters for an MR scan to acquire MR data from a given tissue. Reducing the effects of amplitude decay in the echo signal reduces ringing artifacts and thereby improves image quality.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to MR imaging and, more particularly, to a method and apparatus to reduce ringing artifacts in multi-echo acquisition using a non-discrete flip angle train. The present invention is particularly applicable with fast spin echo (FSE) imaging. 
   When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B 0 ), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B 1 ) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, M Z , may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B 1  is terminated and this signal may be received and processed to form an image. 
   When utilizing these signals to produce images, magnetic field gradients (G x , G y , and G z ) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. 
   As is well-known, reducing the duration of an MR scan has a number of advantages. For example, as scan time is reduced, patient throughput increases. An increase in patient throughput allows more subjects to be imaged in a given period of time or support more comprehensive scans without a time penalty. Additionally, it is generally well-known that some subjects, and in particular, children, the elderly, and those that are claustrophobic, are prone to movement during the data acquisition process. This movement can introduce motion artifacts in the final reconstructed image thereby jeopardizing the diagnostic value of the final image. Accordingly, by reducing the duration of the scan it is possible to reduce the likelihood of subject motion induced artifacts in the reconstructed image. 
   FSE is an imaging technique that has been developed and widely known to reduce scan time. FSE is widely used for spin—spin relaxation weighted imaging, proton density imaging, and spin-lattice relaxation weighted imaging in relatively short periods of time. Moreover, FSE imaging may be implemented for neural imaging, body imaging, and extremity imaging. 
   FSE imaging utilizes a multi-echo, spin-echo pulse sequence where different parts of k-space are acquired by different spin echoes. For example, a four echo spin-echo sequence may be applied such that k-space is segmented into four sections. As such, the first echo may be used to fill a center of k-space, the second echo for k-space adjacent to the center, and so forth, with the data from the last spin-echo used to fill the outermost regions of k-space. Since four echoes rather than one are used to fill k-space, scan time, in this example, may be reduced four-fold. 
   In FSE imaging with long echo trains, the observed echo amplitudes typically exhibit an amplitude decay based on tissue T1 and T2 values. This decay can result in undesirable ringing artifacts, particularly in long echo train images. Additionally, with high field acquisitions, e.g. 3T and above, maintaining RF power deposition (SAR) in the imaging bore during the acquisition process is critical. This is particularly critical for multi-echo acquisitions such as FSE. Several strategies have been developed to address both signal amplitude decay and RF power deposition during multi-echo acquisitions. 
   One known strategy utilizes a variable flip angle train to improve image quality and maintain RF deposition within prescribed limits. However, current state of the art techniques using this strategy provide discrete flip angle trains that are not flexible to take into account prescription parameters such as echo train length, echo spacing times. As such, these techniques yield flip angle trains that are not matched to the parameters of the prescription. Also, these techniques generally fail to take into account changes in tissue type when determining the respective flip angles of the data acquisition pulses of the multi-echo acquisition. 
   It would therefore be desirable to have a method capable of automatically determining the flip angles for each data acquisition pulse of a multi-echo acquisition that is based on tissue type as well as scan prescription parameters on a per-scan basis. 
   BRIEF DESCRIPTION OF THE INVENTION 
   The present invention provides a method and apparatus of automatically determining the flip angle train for a multi-echo acquisition of data from a given tissue type to reduce ringing artifacts that overcome the aforementioned drawbacks. 
   An imaging technique, that is particularly applicable with FSE imaging, to reduce ringing artifacts associated with amplitude decay in multi-echo acquisitions is disclosed. Flip angles for each data acquisition pulse of a multi-echo acquisition are determined for a given tissue type and desired scan parameters on a per-scan basis to maintain a relatively constant target amplitude of each echo during sampling. The flip angle for each data acquisition pulse may be determined from a polynomial expression recalled from a database of stored flip angle polynomial expressions closely matching both the tissue type and desired scan parameters. However, the flip angles may also be determined on-the-fly for a given scan prescription. 
   Therefore, in accordance with one aspect of the present invention, an MRI apparatus includes a magnetic resonance imaging (MRI) system having a plurality of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field and an RF transceiver system and an RF switch controlled by a pulse module to transmit RF signals to an RF coil assembly to acquire MR images. The MRI apparatus also includes a computer programmed to determine, in real-time, a respective flip angle for each data acquisition pulse of a pulse sequence for multi-echo acquisition of MR data matched to a given target tissue and a given scan prescription to reduce ringing artifacts from amplitude decay of the multi-echo acquisition. 
   In accordance with another aspect of the invention, a method is introduced that includes the step of receiving a user input of a given tissue targeted for MR data acquisition and at least one parameter of a scan to acquire MR data. The method also includes the step of determining an echo train duration time for a multi-echo acquisition of MR data for a scan carried out with the at least one parameter to acquire MR data with contrast of the given tissue. The method further includes the steps of determining a target amplitude for a majority of echoes of the multi-echo acquisition and determining a flip angle for each data acquisition pulse of the multi-echo acquisition to acquire MR data such that, for the majority of the echoes of the multi-echo acquisition, a maximum echo amplitude is substantially equal to the target amplitude. 
   In accordance with another aspect of the invention, the invention is embodied in a computer program stored on a computer readable storage medium and having instructions which, when executed by a computer, cause the computer to determine a target amplitude versus echo train time relationship for a multi-echo acquisition of MR data from a given target tissue. The computer is further caused to determine a desired maximum amplitude for a plurality of echoes of the multi-echo acquisition for a user-prescribed MR scan from the target amplitude versus echo train time relationship. The computer is further caused to determine a flip angle for each data acquisition pulse of the prescribed MR scan such that the plurality of echoes has a maximum amplitude substantially equal to the desired maximum amplitude. 
   Various other features and advantages of the present invention will be made apparent from the following detailed description and the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention. 
     In the drawings: 
       FIG. 1  is a schematic block diagram of an MR imaging system for use with the present invention. 
       FIG. 2  is a flow chart setting forth the steps of determining and storing a flip angle train in accordance with the present invention. 
       FIG. 3  is a graph illustrating target amplitudes versus total echo train times for white matter at 3T. 
       FIG. 4  is a graph illustrating a target echo amplitude train for white matter at 3T. 
       FIG. 5  is a graph illustrating a flip angle train determined from the target echo amplitude train of  FIG. 4 . 
       FIG. 6  is a graph illustrating a polynomial expression fit to a non-linear portion of the flip angle train of  FIG. 5 . 
       FIG. 7  is a flow chart setting forth the steps of an FSE imaging technique with a synthesis between flip angle of the data acquisition pulses, scan parameters, and tissue type in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , the major components of a preferred magnetic resonance imaging (MRI) system  10  incorporating the present invention are shown. The operation of the system is controlled from an operator console  12  which includes a keyboard or other input device  13 , a control panel  14 , and a display screen  16 . The console  12  communicates through a link  18  with a separate computer system  20  that enables an operator to control the production and display of images on the display screen  16 . The computer system  20  includes a number of modules which communicate with each other through a backplane  20   a . These include an image processor module  22 , a CPU module  24  and a memory module  26 , known in the art as a frame buffer for storing image data arrays. The computer system  20  is linked to disk storage  28  and tape drive  30  for storage of image data and programs, and communicates with a separate system control  32  through a high speed serial link  34 . The input device  13  can include a mouse, joystick, keyboard, track ball, touch activated screen, light wand, voice control, or any similar or equivalent input device, and may be used for interactive geometry prescription. 
   The system control  32  includes a set of modules connected together by a backplane  32   a . These include a CPU module  36  and a pulse generator module  38  which connects to the operator console  12  through a serial link  40 . It is through link  40  that the system control  32  receives commands from the operator to indicate the scan sequence that is to be performed. The pulse generator module  38  operates the system components to carry out the desired scan sequence and produces data which indicates the timing, strength and shape of the RF pulses produced, and the timing and length of the data acquisition window. The pulse generator module  38  connects to a set of gradient amplifiers  42 , to indicate the timing and shape of the gradient pulses that are produced during the scan. The pulse generator module  38  can also receive patient data from a physiological acquisition controller  44  that receives signals from a number of different sensors connected to the patient, such as ECG signals from electrodes attached to the patient. And finally, the pulse generator module  38  connects to a scan room interface circuit  46  which receives signals from various sensors associated with the condition of the patient and the magnet system. It is also through the scan room interface circuit  46  that a patient positioning system  48  receives commands to move the patient to the desired position for the scan. 
   The gradient waveforms produced by the pulse generator module  38  are applied to the gradient amplifier system  42  having G x , G y , and G z  amplifiers. Each gradient amplifier excites a corresponding physical gradient coil in a gradient coil assembly generally designated  50  to produce the magnetic field gradients used for spatially encoding acquired signals. The gradient coil assembly  50  forms part of a magnet assembly  52  which includes a polarizing magnet  54  and a whole-body RF coil  56 . A transceiver module  58  in the system control  32  produces pulses which are amplified by an RF amplifier  60  and coupled to the RF coil  56  by a transmit/receive switch  62 . The resulting signals emitted by the excited nuclei in the patient may be sensed by the same RF coil  56  and coupled through the transmit/receive switch  62  to a preamplifier  64 . The amplified MR signals are demodulated, filtered, and digitized in the receiver section of the transceiver  58 . The transmit/receive switch  62  is controlled by a signal from the pulse generator module  38  to electrically connect the RF amplifier  60  to the coil  56  during the transmit mode and to connect the preamplifier  64  to the coil  56  during the receive mode. The transmit/receive switch  62  can also enable a separate RF coil (for example, a surface coil) to be used in either the transmit or receive mode. 
   The MR signals picked up by the RF coil  56  are digitized by the transceiver module  58  and transferred to a memory module  66  in the system control  32 . A scan is complete when an array of raw k-space data has been acquired in the memory module  66 . This raw k-space data is rearranged into separate k-space data arrays for each image to be reconstructed, and each of these is input to an array processor  68  which operates to Fourier transform the data into an array of image data. This image data is conveyed through the serial link  34  to the computer system  20  where it is stored in memory, such as disk storage  28 . In response to commands received from the operator console  12 , this image data may be archived in long term storage, such as on the tape drive  30 , or it may be further processed by the image processor  22  and conveyed to the operator console  12  and presented on the display  16 . 
   The present invention is directed to an imaging technique to carry out a multi-echo acquisition of MR data such that ringing artifacts associated with signal amplitude decay between echoes of the multi-echo acquisition are reduced. The present invention may be carried out with a system such as that described in  FIG. 1  or equivalent thereof. The present invention is particularly applicable for acquisition of MR data at field strengths of 1.5 T and higher. 
   Referring now to  FIG. 2 , a flow chart setting for the steps of a technique  70  for determining and storing a flip angle train for a multi-echo acquisition is shown. Technique  70  is preferably carried out as a set of instructions of a computer program. Further, the computer program may be bundled within a computer data signal that may be embodied in a carrier wave for uploading/downloading to an MR scanner. 
   Technique  70  is carried out to generate a variable flip angle train for a multi-echo acquisition. Preferably, the technique is repeatably carried out for different tissue types and scan parameters to populate a database with coefficient values of a polynomial expression of a flip angle train. As will be described, the coefficient values may be accessed to generate a flip angle train matched to a target tissue identified for inspection and the parameters of a scan to acquire data with contrast of the target tissue. Technique  70  may be used to determine and store values for coefficients for a multi-order flip angle polynomial expression for use in 2D or 3D MR data acquisition. Technique  70  preferably begins at step  72 , and an identification of input parameters including tissue type, tissue T1 and T2 values, and Echo Spacing Time (EST) among others occurs at step  74 . An amplitude versus total echo train time relationship is determined at  76 . The amplitude versus total echo train time relationship is determined through a numerical solution of Bloch Equations such as: 
   
     
       
         
           
             
               
                 
                   
                     
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   Referring to  FIG. 3 , an exemplary graph illustrating a target amplitude versus total echo train time relationship is shown. Curve  97  shows the relationship between target amplitude and total echo train time for white matter at 3 Tesla with a T1=1000 ms and a T2=100 ms and EST equal to 4 milliseconds. From this target amplitude and EST relationship, a given target amplitude and a corresponding total echo train duration time may be found, and vice-versa. 
   Referring back to  FIG. 2 , total echo train duration time for the input parameters is selected at  78  as is the target amplitude corresponding to the echo train duration from curve  97 . An echo target train is created at step  82 . The echo target train defines the amplitudes of the echoes to be sampled during the total echo duration time. To reduce ringing artifacts, it is preferred to acquire MR data when the maximum amplitude for the echoes is substantially uniform, i.e. equals the target amplitude. 
   An exemplary echo target train is shown in  FIG. 4 . Echo target train  98  has a target amplitude of 0.2 for a majority of echoes of a multi-echo acquisition for white matter at 3 Tesla. Segment  100  represents target amplitudes of the first three echoes. The first echo has an amplitude set to 0.6. The second echo has an amplitude set to a value half way between 0.6 and the target echo amplitude, e.g. 0.2. In the illustrated example, the second echo has an amplitude of 0.4. The third echo has an amplitude set to the target echo amplitude of 0.2. Segment  102 , which comprises a majority of the echoes, represents echoes desired to have amplitudes equal to the target echo amplitude of 0.2. The duration time of segment  102  is determined from the total echo train duration time selected at step  78 . Segment  104  represents echoes having amplitudes less than the target echo amplitude. 
   Referring again to  FIG. 2 , the echo target train created at step  82  is used to determine a flip angle train at step  84 . The flip angle train corresponds to the values of the flip angles of a multi-echo acquisition that yields echo amplitudes provided in the echo target train created at step  82 . The flip angles determined for each echo in the echo target train may be selected to maintain cumulative RF deposition during data acquisition within a prescribed level. 
     FIG. 5  illustrates a graph of a flip angle train determined from the echo target train  98 . Flip angle train  106  is determined using a numerical solution of Bloch Equations and known gradient search algorithms. Therefrom, the flip angle for each data acquisition pulse in the echo target train that yields the target amplitude at each echo is determined. Flip angle train  106  has a linear segment  108 , a non-linear segment  110 , and a constant segment  112 . Segments  108 ,  110 , and  112  correspond to flip angles determined for the echoes in segments  100 ,  102 , and  104  of echo target train  98 , respectively. As described below, a polynomial fit is applied to segment  110  to determine a polynomial expression or expression that substantially describes segment  110 . The polynomial expression provides values for coefficients of a known expression used to define the flip angle of each data acquisition pulse of a pulse sequence such that ringing artifacts from amplitude decay are reduced. 
   Referring back to  FIG. 2 , a polynomial expression is fit at step  86  to a portion of the flip angle train determined at step  84 . The polynomial expression is fit to the portion of the flip angle train corresponding to the flip angles that are determined to yield echo amplitudes substantially equal to the target amplitude. In particular, the polynomial fit is applied to the non-linear segment  110  of flip angle train  106  to yield flip angle values set forth in curve  114  of  FIG. 6 . 
   Referring now to  FIG. 6 , a graph is illustrated of a polynomial expression fit to segment  110  of flip angle train  106 . Curve  114  graphically represents a sixth order polynomial expression fit to segment  110  of flip angle train  106 . It is recognized that polynomials of less or more than sixth order are possible and contemplated. Coefficients a0, a1, a2, a3, a4, a5, and a6 for curve  114  are 18.59, −132.79, 1241.31, −4486.70, 8368.94, −7667.48, and 2775.30, respectively. Flip angles for data pulses yielding echo amplitudes substantially equal to the target amplitude may be determined from the following sixth order polynomial expression:
 
flip angle( i )= a   6   x   6   +a   5   x   5   +a   4   x   4   +a   3   x   3   +a   2   x   2   +a   1   x+a   0  
 
(Eqn. 4), where
 
           x   =           (     i   -   2     )     ×   actual   ⁢           ⁢   echo   ⁢           ⁢   spacing   ⁢           ⁢   time       default   ⁢           ⁢   echo   ⁢           ⁢   spacing   ⁢           ⁢   time       .           
The default echo spacing time represents the echo spacing time used during the creation of echo target train at step  82  of  FIG. 2 . The actual echo spacing time represents the echo spacing time determined from the given parameters, i.e., TR, TE, and number of echoes of the to-be-executed MR scan. Horizontal axis  116  of the graph represents the echo number of the echoes in segment  102  of echo target train  98 . The flip angle, in degrees, is represented by vertical axis  118 .
 
   Referring back to  FIG. 2 , the first and second discrete flip angles of segment  108  of flip angle train  106  and the coefficients of the polynomial expression fit  114  to the non-linear segment  110  of the flip angle train at step  86  are stored in a database at step  88  and may be recalled prior to an MR scan, as will be described. The database may be accessed for determination of a flip angle train matching desired parameters of a to-be-implemented scan. It is also contemplated that the flip angle expression may be determined on-the-fly. In this manner, it is not necessary to recall the flip angle polynomial expression from the database. At step  90 , a decision is made to repeat steps  74 – 88  for a different set of scan parameters and target tissue. In this manner, coefficient values for a number of polynomial expressions may be stored and recalled for an MR scan to provide flip angles matched to a tissue type and specific prescription parameters. If the decision is made at step  92  to repeat steps  74 – 88 , the process returns to step  74  and continues as described above for steps  74 – 88 . Otherwise, the process ends  96 . 
   In  FIG. 7 , the steps of a method of MR imaging using stored coefficient values according to one aspect of the present invention are shown. Method  120  is preferably carried out as a set of instructions of a computer program of an MR scanner system. Method  120  preferably begins with receiving input parameters defining an MR scan at step  122 . The input parameters may include tissue type, TE, TR, T1 and T2 values, EST, the number of echoes to be sampled, sequence type, field-of-view, and receiver bandwidth, among others. Total echo train time is then determined from the number of echoes to be sampled, TE, and EST at  124 . From the total echo train time, a target echo amplitude is then automatically determined at step  126 . A database of stored flip angles and polynomial expression coefficients is accessed and queried to determine the flip angle train corresponding to the target amplitude and total echo train time at  128 . In this manner, the identified flip angle train is the most optimal of the stored flip angle trains for the given input parameters to reduce ringing artifacts and to reduce noise in the scan image. 
   At step  130 , a flip angle train for an imaging scan, such as a FSE scan, is determined for use during the imaging scan. The imaging scan is initiated at step  132 , and MR data is acquired during data acquisition at step  134  and, preferably, with the amplitude of a majority of the echoes substantially equal to the target amplitude to reduce ringing artifacts. 
   Therefore, in accordance with one embodiment of the present invention, an MRI apparatus includes a magnetic resonance imaging (MRI) system having a plurality of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field and an RF transceiver system and an RF switch controlled by a pulse module to transmit RF signals to an RF coil assembly to acquire MR images. The MRI apparatus also includes a computer programmed to determine, in real-time, a respective flip angle for each data acquisition pulse of a pulse sequence for multi-echo acquisition of MR data matched to a given target tissue and a given scan prescription to reduce ringing artifacts from amplitude decay of the multi-echo acquisition. 
   In accordance with another embodiment of the invention, a method is introduced that includes the step of receiving a user input of a given tissue targeted for MR data acquisition and at least one parameter of a scan to acquire MR data. The method also includes the step of determining an echo train duration time for a multi-echo acquisition of MR data for a scan carried out with the at least one parameter to acquire MR data with contrast of the given tissue. The method further includes the steps of determining a target amplitude for a majority of echoes of the multi-echo acquisition and determining a flip angle for each data acquisition pulse of the multi-echo acquisition to acquire MR data such that, for the majority of the echoes of the multi-echo acquisition, a maximum echo amplitude is substantially equal to the target amplitude. 
   In accordance with another embodiment of the invention, the invention is embodied in a computer program stored on a computer readable storage medium and having instructions which, when executed by a computer, cause the computer to determine a target amplitude versus echo train time relationship for a multi-echo acquisition of MR data from a given target tissue. The computer is further caused to determine a desired maximum amplitude for a plurality of echoes of the multi-echo acquisition for a user-prescribed MR scan from the target amplitude versus echo train time relationship. The computer is further caused to determine a flip angle for each data acquisition pulse of the prescribed MR scan such that the plurality of echoes has a maximum amplitude substantially equal to the desired maximum amplitude. 
   The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.