Patent Publication Number: US-10330760-B2

Title: System and method for assessing T2-relaxation times with improved accuracy

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/968,883, filed Mar. 21, 2014, the disclosure of which is incorporated herein in its entirety by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     This invention was made with government support under R01EB008743-01A2 and K99HL111410-01 awarded by the National Institutes of Health. The government has certain rights in the invention. 
    
    
     BACKGROUND 
     1. Field 
     Apparatuses and methods consistent with exemplary embodiments relate to magnetic resonance imaging (MRI), and, more particularly, to acquiring T 2 -weighted imaging data. 
     2. Description of the Related Art 
     When a substance such as human tissue is subjected to a uniform magnetic field, i.e., a static magnetic field B 0 , the individual magnetic moments of the excited nuclei in the tissue attempt to align with the static magnetic field B 0 , but precess about it in random order at their characteristic Larmor frequency. If the substance is subjected to a magnetic excitation field B 1  that is in the x-y plane and that is near the Larmor frequency, the net magnetization aligned moment M z  may be rotated, i.e., tipped, into the x-y plane to generate a net transverse magnetic moment M t . An MR signal is emitted by the excited nuclei, i.e., spins, after the excitation magnetic field B 1  is terminated, and the MR signal may be received and processed to form an image. 
     In MRI systems, the excited spins induce an oscillating sine wave signal in a receiving coil. The frequency of this signal is near the Larmor frequency, and its initial amplitude is determined by the magnitude of the transverse magnetic moment M t . The amplitude of the emitted MR signal decays exponentially with time. 
     The amplitude of the MR signal is dependent on the spin-lattice relaxation process that is characterized by the time constant T 1 , i.e., a spin-lattice relaxation time. It describes the recovery of the net magnetic moment M to its equilibrium value along the axis of magnetic polarization, i.e., z-magnetization. The difference in T 1  values between tissues can be exploited to provide image contrast. 
     The T 2  time constant is referred to as the spin-spin relaxation constant, or the transverse relaxation constant, and is characterized by a spin-spin relaxation time characterizing the signal decay. The T 2  constant is inversely proportional to the exponential rate at which the aligned precession of the spins would dephase after removal of the excitation magnetic field B 1  in a perfectly homogeneous magnetic field. The T 1  time constant is much longer than T 2  in most tissues of medical interest. 
     The biological tissues have different T 2  values and this property may be exploited to enhance the contrast between the tissues. Accordingly, T 2  serves as an informative MRI parameter, providing non-invasive measurements of tissue status and disease prognosis with respect to a wide range of applications and diseases, including discriminating between acute and chronic myocardial infarction. For example, quantitative T 2  mapping may allow assessment of edema with less variability than T 2 -weighted imaging. 
     In order to quantify T 2 , multiple T 2 -weighted images may be acquired and fitted based on respective echo time (TE) lengths, assuming long repetition time (TR) for complete relaxation. In particular, related art T 2  mapping methods acquire three images with different T 2 -weightings, for example, with T 2  magnetization preparation time of 0 ms, 25 ms, and 55 ms. This data is then fit to a two-parameter model, to generate T 2  maps. However, imperfection in RF pulses of a T 2  magnetization preparation and application of additional RF pulses during imaging are not accounted for in the two-parameter model of the data fit process. Thus, the estimated T 2  times may be inaccurate or not readily reproducible. That is, the two-parameter curve-fitting may mismatch the underlying image acquisition. 
     Accordingly, apparatuses and methods are needed to provide accurate T 2  maps without extensive and/or impractical imaging sequences. 
     SUMMARY 
     Exemplary embodiments address at least the above problems and/or disadvantages and other disadvantages not described above. Also, the exemplary embodiments are not required to overcome the disadvantages described above, and may not overcome any of the problems described above. 
     One or more exemplary embodiments provide apparatuses and methods for providing accurate quantifying of T 2  values. 
     One or more exemplary embodiments may provide a three-parameter model that may be used for curve fitting and to generate T 2  maps. 
     One or more exemplary embodiments may provide a pulse sequence to perform imaging after a saturation pulse, which simulates image acquisition for a time equal to infinity. 
     In accordance with an aspect of an exemplary embodiment, an MRI system includes a data processor configured to acquire a first set of T 2 -weighted imaging data and a second set of T 2 -weighted imaging data, from an object disposed in an imaging region; a pulse sequence controller configured to generate a pulse sequence and apply the generated pulse sequence to a gradient coil assembly and RF coil assembly, the generated pulse sequence including: T 2 -preparation modules including T 2 -preparation pulses and associated imaging modules including imaging pulses, to acquire the first set of T 2 -weighted imaging data, and a saturation pulse sequence and an associated saturation imaging module including imaging pulses to acquire the second set of T 2 -weighted imaging data; a curve fitter configured to apply the first set of T 2 -weighted imaging data and the second set of T 2 -weighted imaging data to a three-parameter model for T 2  decay that models an image signal relative to a base intensity parameter, a tissue T 2  value, and an offset parameter to determine a T 2  value at a plurality of locations; and an image processor configured to generate a T 2  map of the object based on the T 2  value determined at the plurality of locations. 
     In accordance with an aspect of an exemplary embodiment, an MRI method includes: (a) applying a T 2 -preparation module including T 2 -preparation pulses to a region of interest (ROI) of an object; (b) subsequent to an application of the T 2 -preparation module, applying an associated imaging module including imaging pulses, to acquire a first T 2 -weighted data from the ROI; (c) repeating applications of the T 2 -preparation module followed by the associated imaging module a plurality of times to acquire a number of sampling points on a T 2 -decay curve; (d) applying a saturation pulse sequence to the ROI after a last iteration of the steps (a), (b), and (c); (e) applying a saturation imaging module including imaging pulses following the saturation pulse sequence, to acquire a second T 2 -weighted data from the ROI with a substantially complete T 2  decay; and (f) generating a T 2  map of the ROI based on the first T 2 -weighted data acquired as a result of iterations of steps (a) and (b) and on the second T 2 -weighted data acquired in step (e) using a T 2 -relaxation model that includes more than two parameters. 
     In accordance with an aspect of an exemplary embodiment, a non-transitory computer-readable storage medium having recorded thereon computer instructions that, when executed by a processor, cause the processor to execute a method including: assessing T 2 -weighted imaging data acquired from an object at a plurality of times to include data at a number of sampling points on a T 2 -decay curve; processing the T 2 -weighted imaging data using a three-parameter model for T 2  decay; and using the processed T 2 -weighted imaging data to generate a T 2  map of the object. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and/or other aspects will become more apparent by describing certain exemplary embodiments with reference to the accompanying drawings, in which: 
         FIG. 1  is a block diagram of an MRI apparatus, according to an exemplary embodiment; 
         FIG. 2  is a diagram of a pulse sequence for acquiring T 2 -weighted data, according to an exemplary embodiment; 
         FIG. 3  is a diagram of a pulse sequence according to an exemplary embodiment; 
         FIG. 4A  is a diagram of a pulse sequence according to an exemplary embodiment; 
         FIG. 4B  is a diagram of a pulse sequence according to an exemplary embodiment; 
         FIG. 5  is a table of T 2  measurements based on phantom imaging; 
         FIG. 6  illustrates a portion of an MRI apparatus, according to an exemplary embodiment; and 
         FIG. 7  is a flowchart of a method, according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Certain exemplary embodiments are described in greater detail below with reference to the accompanying drawings. 
     In the following description, like drawing reference numerals are used for like elements, even in different drawings. The matters defined in the description, such as detailed construction and elements, are provided to assist in a comprehensive understanding of the exemplary embodiments. However, it is apparent that the exemplary embodiments can be practiced without those specifically defined matters. Also, well-known functions or constructions are not described in detail since they would obscure the description with unnecessary detail. 
     Referring to  FIG. 1 , an example of an MRI apparatus  100  is illustrated. The MRI apparatus  100  includes a magnet assembly  124 , an operator workstation  102 , including a display  104 , one or more input devices  106 , such as a keyboard, mouse, microphone, joystick, etc., and a processor  108 . The operator workstation  102  provides the operator interface that enables scan orders to be entered into the MRI apparatus  100 . For example, the operator workstation  102  may be coupled to at least one of a pulse sequence controller  110 , a data buffer  112 , a data processor  114 , a data storage server  116 , a curve fitter  150 , and an image processor  152 , which may be interconnected with one another via a communication apparatus  117 , which may include any suitable network interface, to provide a connection wirelessly and/or by wire. As an example, the communication apparatus  117  may include a proprietary network, a dedicated network, and/or an open network, such as the Internet. 
     The magnet assembly  124  includes a main magnet  126 , a gradient coil assembly  122 , and an RF coil assembly  128  which are sequentially arranged in that order from outermost side to the bore  115 . The object  119  is located on the cradle  121  which is moved to the bore  115  of the magnet system  124 , such that a magnetic field may be applied to the object  119 . The main magnet  126  may be an open magnet. 
     The main magnet  126  generates a static magnetic field B 0  in the bore  115  of the magnet system  124 . A direction of the static magnetic field B 0  may be parallel or perpendicular to a body axis  270  of the object  119 , i.e., to a longitudinal direction of the object  119 . 
     The pulse sequence controller  110  functions in response to instructions received from the operator workstation  102  to operate a gradient controller  118  and an RF transceiver  120 . 
     Gradient waveforms to perform the prescribed scan are generated and applied to the gradient controller  118 . The gradient controller  118  is connected with gradient coils of a gradient coil assembly  122 , and outputs signal pulses to form the magnetic field gradients. The gradient controller  118  may include driving circuits corresponding to X, Y, and Z gradient coils of the gradient coil assembly  122  that respectively generate the magnetic field gradients in X-axis, Y-axis, and Z-axis directions that are orthogonal to each other and are used for position encoding and slice selection. 
     The RF transceiver  120  is connected with the RF coil assembly  128  to apply an RF pulse and/or a signal related to application of the RF pulse to the RF coil assembly  128 . As illustrated in  FIG. 1 , the RF coil assembly  128  may include a whole-body coil which may serve as a transmit/receive coil. Additionally or optionally, the RF coil assembly  128  may include a local RF coil or coils which may be configured to transmit the RF pulse to and/or receive the MR signals from the object. For example, the RF transceiver  120  may include an RF transmitter  123  which transmits the RF pulse sequence to the whole-body coil or the local coil of the RF coil assembly  128 , to apply RF pulses to the object, to perform the prescribed magnetic resonance pulse sequence. The MR signals from the object may be detected by the whole-body coil or the local coil of the RF coil assembly  128  and may be received by an RF receiver  125  of the RF transceiver  120 , where they are amplified, demodulated, filtered, and digitized based on commands received from the pulse sequence controller  110 . The RF transmitter  123  may generate a wide variety of RF pulses used in MRI pulse sequences. In response to the scan prescription and control of the pulse sequence controller  110 , the RF transmitter  123  may generate RF pulses of desired frequency, phase, and pulse amplitude. 
     The RF receiver  125  may include one or more RF receiver channels. Each RF receiver channel may include an associated RF preamplifier that amplifies the MR signal received by the RF coil assembly  128 , and a detector that detects and digitizes the in phase and quadrature components of the received MR signal. The magnitude of the received MR signal may, therefore, be determined at any sampled point by the square root of the sum of the squares of the in phase and quadrature components, i.e., I and Q channels:
 
 M =√{square root over ( I   2   +Q   2 )}  Equation (1)
 
     The phase of the received MR signal may be determined as: 
     
       
         
           
             
               
                 
                   φ 
                   = 
                   
                     
                       tan 
                       
                         - 
                         1 
                       
                     
                     ⁡ 
                     
                       ( 
                       
                         Q 
                         I 
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     2 
                     ) 
                   
                 
               
             
           
         
       
     
     The pulse sequence controller  110  may optionally receive patient data from a physiological acquisition controller  130 . For example, the physiological acquisition controller  130  may receive physiological information signals from different sensors connected to the object  119 , such as electrocardiograph (ECG) signals and/or respiratory signals indicating a respiratory expansion from respiratory bellows or other respiratory monitoring device. The physiological information signals may be used by the pulse sequence controller  110  to synchronize, or gate, the execution of the scan with the object&#39;s heart beat and/or respiration. 
     The pulse sequence controller  110  may be connected to a scan room interface  132  that receives signals from various sensors associated with the condition of the object  119  and the magnet system. For example, the scan room interface  132  provides commands to a patient positioning system  134  to move the object  119  on the cradle  121  to desired positions during the scan. 
     The digitized MR signal samples generated by the RF transceiver  120  are received by the data buffer  112 . The data buffer  112  operates in response to instructions received from the operator workstation  102  to receive the real-time magnetic resonance data and provide buffer storage, such that no data is lost by data overrun. 
     In the scans that require information derived from acquired magnetic resonance data to control the further performance of the scan, the data buffer  112  may be controlled to generate such information and convey it to the pulse sequence controller  110 . For example, during prescans, magnetic resonance data may be acquired and used to calibrate the pulse sequence performed by the pulse sequence controller  110 . As another example, navigator signals may be acquired and used to adjust the operating parameters of the RF transceiver  120  and/or the gradient controller  118 , or to control the view order in which k-space is sampled. As another example, the data buffer  112  may process MR signals used to detect the arrival of a contrast agent, for example, in an MR angiography (MRA) scan. For example, the data buffer  112  acquires magnetic resonance data and processes it in real-time to generate information that is used to control the scan. 
     The data processor  114  receives magnetic resonance data from the data buffer  112  and processes it in accordance with instructions downloaded from the operator workstation  102 . The data processor  114  may obtain image data sets having different MR parameter values to generate an MR parameter map. The MR parameter map may include at least one of a T 1  map, a T 2  map, etc. An image processor  152  may form the MR parameter map based on the obtained data set. For example, the image processor  152  may perform at least one of reconstructing two-dimensional (2D) or three-dimensional (3D) images by performing a Fourier transformation of raw k-space data, performing image reconstruction algorithms, such as iterative or backprojection reconstruction algorithms, applying filtering to the raw k-space data or to the reconstructed image data, generating functional magnetic resonance (fMR) images, calculating motion or flow images, and so on. 
     Although the curve fitter  150  and the image processor  152  are illustrated as components separate from the data processor  114  in  FIG. 1 , the curve fitter  150  and/or the image processor  152  may be incorporated within the data processor  114 . Also, at least one of the curve fitter  150  and the image processor  152  may be omitted and the data processor  114  may perform functions at least one of the curve fitter  150  and the image processor  152 , in accordance with exemplary embodiments. 
     Images reconstructed by the image processor  152  may be transferred to the operator workstation  102  and/or stored. Real-time images may be stored in a database memory cache (not shown in  FIG. 1 ), from which the images may be output to operator display  112  or a display  136  that is located near the magnet assembly  124 . Batch mode images or selected real time images may be stored in a host database on disc storage  138  or on a remote server (not shown). When the images have been reconstructed and transferred to storage, the image processor  152  may notify the operator workstation  102 , i.e., a user. The operator workstation  102  may be used by an operator to archive the images, generate films, or send the images via a network to other facilities. 
     The MRI apparatus  100  may include one or more networked workstations  142 . For example, a networked workstation  142  may include a display  144 , one or more input devices  146 , such as a keyboard and mouse, and a processor  148 . The networked workstation  142  may be located within the same facility as the operator workstation  102 , or in a different facility, for example, a different healthcare institution or a clinic. 
     The networked workstation  142  may gain remote access to the data processor  114 , curve fitter  150 , image processor  152 , and/or data storage server  116  via the communication apparatus  117 . In this manner, magnetic resonance data, reconstructed images, or other data may be exchanged with the networked workstations  142 , such that the data or images may be remotely processed by a networked workstation  142 . The data may be exchanged in any suitable format, such as in accordance with the transmission control protocol (TCP), the Internet protocol (IP), or other suitable protocols. 
     Referring to  FIG. 6 , the pulse sequence controller  110  may include a T 2 -preparation sequence generator  602  and an imaging sequence generator  604  to control an execution of a T 2  weighted pulse sequence. 
     Referring to  FIG. 2 , a T 2  weighted imaging process  200  is illustrated. The process  200  may be a gated acquisition, for example, using an ECG signal as a trigger  202  used to determine a period in the cardiac cycle, such as a diastolic period  204 . The T 2 -preparation sequence generator  602  may execute a T 2 -preparation module  206  based on the trigger  202  after a time delay lapses, and, consequently, the imaging sequence generator  604  may execute an imaging sequence and a single image may be acquired, in a single-shot image acquisition  208 . 
     Referring to  FIG. 3 , the T 2  weighted pulse sequence of  FIG. 2  may be adapted in accordance with an exemplary embodiment to create a new pulse sequence  300 , by the pulse sequence controller  110 . As illustrated, N image acquisitions I 1 , I 2 , and I 3  through I N  may be performed, which, for example, may be single-shot image acquisitions, to acquire multiple single-shot images using ECG signal as a trigger. As a non-limiting example, N may range from 3 to 9, to acquire a corresponding number of images. 
     Each of the image acquisitions is timed relative to a trigger signal  302   1 ,  302   2 , and  302   3  through  302   N  and may be acquired with different T 2 -preparation modules  304   2  and  304   3  through  304   N , i.e., T 2 -preparation pulse sequences with differently set parameters, for example, with differently set time echo lengths TE T2P , executed by the T 2 -preparation sequence generator  602 . The sequence of RF pulses of the T 2 -preparation module may include, for example, a 90° pulse, followed by one or more of 180° and/or −180° pulses. A −90° RF pulse may conclude the T 2 -preparation module. However, this is not limiting. In an exemplary embodiment, no T 2 -preparation module is used for the image acquisition I 1 , i.e., TE T2P  is equal to 0. 
     The T 2 -preparation modules  304   2  through  304   N  may be designed to include pulses that allow for a dense number of sampling points on the T 2 -decay curve. Also, by increasing N, for example to around 9, more images may be acquired to have more samples on the T 2 -decay curve. Increasing the number of samples increases the accuracy of the T 2  maps. 
     As illustrated in  FIG. 3 , each trigger signal  302   2  through  302   N  is followed by each of the T 2 -preparation modules  304   2  through  304   N  executed with a time delay after each trigger signal  302   2  through  302   N . The T 2 -preparation modules  304   2  through  304   N  are followed by respective imaging modules  306   2  through  306   N  which include pulse sequences to acquire the MR image data of the object. Because in the image acquisition I 1  no T 2 -preparation module is performed, the imaging module  306   1  is executed after the trigger signal  302   1 , with a time delay. For example, the time delay may be adjustable with respect to the imaging modules  306   1  through  306   N , to ensure the MR signal readout at the same phase of a cardiac cycle. For example, the imaging modules may include imaging sequences executed by the imaging sequence generator  604 . 
     Rest periods  308   1  and  308   2  with no RF pulses may follow all or some of the imaging modules, for example, the imaging modules  306   1  and  306   2 . The rest periods  308   1  and  308   2  may last Δt rest  seconds after an execution of the imaging module, to control any T 1  recovery effect on the next image acquisition. For example, Δt rest  may be from 0 to 10 seconds and, as a non-limiting example, around 6 seconds. 
     At the end of the sequence  300 , a saturation sequence generator  606  may execute a saturation (SAT) pulse  310   SAT , with a time delay after a trigger signal  302   SAT  to perform a single saturation image acquisition I SAT . For example, the imaging sequence generator  604  may control an execution of the imaging module  306   SAT  to acquire a saturation image data after an execution of a preceding SAT pulse  310   SAT . For example, the time delay may be adjustable with respect to the imaging module  306   SAT , to ensure the MR signal readout at the same phase of a cardiac cycle as in the acquisition of the imaging modules  306   1  through  306   N . The saturation image corresponding to the image acquisition I SAT  is used for the new fitting model, as described in detail below. The image acquisition I SAT  does not use a preceding rest period since the SAT pulse  310   SAT  aims to destroy all magnetization along the z-axis. For example, the SAT pulse  310   SAT  may include 90° RF pulses at relatively short repetition times, followed by a spoiling gradient, but this is not limiting. 
     Referring to  FIG. 4A , a pulse sequence section  400  is illustrated. For example, the pulse sequence section  400  may be a section of the pulse sequence  300  described above with reference to  FIG. 3  which may include a plurality of such pulse sequence sections. The pulse sequence section  400  may correspond to an image acquisition I k  which represents one of the image acquisitions I 1  through I N . 
     The pulse sequence section  400  may include a navigator pulse  402  preceding a T 2 -preparation module  412   1 , according to an exemplary embodiment. For example, the data acquired using the navigator pulse  402  may be used to gate each of subsequent image acquisitions I 1  through I N . For example, the data acquired using the navigator pulse  402  may be used to track breathing. In the illustrated example of tracking breathing, a position of the diaphragm of the object being imaged may be tracked over time, as illustrated by a positional graph  404 . The positional information on the positional graph  404  may be compared to a set of positional thresholds  406 ,  408  that indicate a range  410  of accepted diaphragm positions. 
     As illustrated in  FIG. 4A , the navigator pulse  402  is applied before the T 2 -preparation module  412   1  and before an associated imaging module  414   1 , which are timed to be within the range  410  of the accepted diaphragm positions. That is, if the position of the tracked physiological movement as determined by the image data acquired using the navigator pulse  402  is within the accepted range  410 , then the T 2 -preparation module  412   1  is applied and the imaging module  414   1  is performed thereafter. 
     After the image acquisition is completed, the sequence may include a pause, i.e., a rest period  416 , for the time period Δt rest , to allow for full T 1  recovery from the last acquisition. However, if the position of the navigator pulse  401  is outside of the range  410  of the accepted diaphragm positions, no T 2 -preparation pulses or imaging pulses are applied and no recovery period is applied. Thus, the imaging time may be shortened. The magnetization remains undisturbed and the image data is acquired in the next RR interval by executing the navigation pulse and the image acquisition. The navigation pulses may be executed by a navigation sequence generator  608  and the navigation image data may be acquired and processed by the data processor  114 . 
     Referring to  FIG. 4B , a pulse sequence section  418  is illustrated. For example, the pulse sequence section  418  may be a section of the pulse sequence  300  described above with reference to  FIG. 3 . A portion of the pulse sequence section  418  may be substantially the same as the pulse sequence section  400  described above with reference to  FIG. 4A . In addition, the pulse sequence section  418  includes a navigator pulse  403  which precedes a SAT pulse  420   SAT . 
     Although the navigator image data acquired using the navigator pulse  403  indicates that the position of the tracked physiological movement is within the range  410 , the T 2 -preparation module is not applied and the imaging module  414   SAT  is performed after application of SAT pulse  420   SAT . Further, no rest period is applied after the last image acquisition I N  which immediately precedes the saturation image acquisition I SAT . 
     In  FIG. 6 , the T 2 -preparation sequence generator  602 , the imaging sequence generator  604 , the saturation sequence generator  606 , and the navigation sequence generator  608  are illustrated as being incorporated into the pulse sequence controller  110 . However, the pulse sequence controller  110  may omit some of the illustrated components or may have a greater number of components. Further, one or more of the T 2 -preparation sequence generator  602 , the imaging sequence generator  604 , the saturation sequence generator  606 , and the navigation sequence generator  608  may be components separate from the pulse sequence controller  110 . 
     With reference to  FIG. 7 , in a navigator-gated acquisition, an ECG signal is detected in operation S 10 . The NAV pulse is applied in operation S 20 , to immediately precede the T 2  preparation module. In operation S 30 , it is determined whether the NAV signal is outside the gating window  410 , for the acquisition of the kth image in the kth image acquisition Ik. If it is determined that the NAV signal is outside the gating window  410 , no T 2  preparation or imaging pulses are applied, and the acquisition of the kth image is performed in the next R-R interval. If it is determined that the NAV signal is within the gating window  410 , it is determined whether all of the T 2  preparation images are acquired (operation S 40 ). If it is determined that not all of the T 2  preparation images are acquired, the kth image with the desired T 2  preparation time is acquired (operation S 50 ), followed by a rest period for magnetization recovery (operation S 60 ). If it is determined that all of the T 2  preparation images are acquired, the SAT pulse is applied and a saturation-prepared (SAT) image is acquired in operation S 70 , which immediately follows the acquisition of a last T 2 -prepared image without a rest period. 
     The above-described respiratory tracking is only an example. The above-described systems and methods are applicable to cardiac applications and, as a non-limiting example, may track respiratory and/or cardiac cycles. In cardiac applications, the exemplary sequence may be used in multiple varieties. For example, when Δt rest  is 6 seconds, and three images are acquired, the sequence may readily fit in one breath-hold scan, for example, for approximately 12 seconds of a breath-hold. This greatly helps to avoid any mis-registration between the images due to the breathing. If the patient has difficulties in holding breathe, the navigator gating may be enabled to get the same images in a free breathing mode. For a typical navigator gating efficiency of 50 percent, the sequence will be longer by only 2-4 seconds. 
     Also, the saturation image acquisition I SAT  may be appended with only the cost of one more cardiac cycle (nearly 1 second) of the scan duration. In this case, the sequence still fits in one breath hold but the T 2 -maps may be estimated using both the two-point fit model, as well as using a three-point fit model, as will be described below. 
     That is, the related art T 2  maps are generated by curve-fitting using the following two-parameter equation to corresponding pixels from each of, for example, three images: 
     
       
         
           
             
               
                 
                   
                     S 
                     = 
                     
                       A 
                       
                         ( 
                         
                           
                             - 
                             
                               TE 
                               
                                 
                                   T 
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     where S is a signal intensity at a given location, 
     A is a base parameter that, for example, may include the equilibrium magnetization and local receiver coil gain, and 
     TE T     2     P  is T 2  preparation echo time. 
     In accordance with an exemplary embodiment, the T 2  maps are generated by a curve fitter  150  using a three-parameter fit. When the images with different T 2 -weights are acquired, several imaging pulses are applied until the center of k-space is acquired. These imaging pulses cause the magnetization to reach a steady state that is different than the equilibrium magnetization. The difference may be characterized using a three-parameter model in accordance with an exemplary embodiment that takes the form:
 
 S ( t )= Ae   (−t/T     2     )   +B   Equation (4),
 
     where S is a signal intensity at a given location, 
     A is a base parameter that, for example, includes the equilibrium magnetization and local receiver coil gain, 
     t is a T 2 -preparation echo time, and 
     B is a new model offset parameter due to a T 1  recovery effect during an image acquisition window introduced in accordance with an exemplary embodiment. 
     The curve fitter  150  processes the acquired T 2 -weighted images, i.e., images weighted with different T 2  echo times, and the saturation-prepared image, to fit the T 2 -decay curve by using the three-parameter model at each pixel, to yield a T 2  map, which may be displayed as an image to a user. For example, a displayed image may be a color image. 
     In particular, the use of the SAT pulses in the exemplary sequences allows for an accurate fitting of a B offset, which captures the effect of the imaging pulses. Imaging after a SAT pulse simulates an acquisition where all magnetization history is erased, i.e., a complete T 2  decay, followed by the imaging pulses in the saturation imaging acquisition. That is, to estimate the offset value B, the above-described sequences and similar sequences include an imaging module executed after a saturation pulse SAT, which simulates acquisition at a very long TE T2P , i.e., an equivalent of an image with TE T2P  equal to infinity, to obtain a saturation-prepared image which captures the effect of the imaging pulses on the magnetization curve and improves the estimation of the offset parameter B. 
     Thus, the use of the above-described pulse sequences or similar sequences and the three-parameter model of Equation 4 creates a fitting construct that allows for accurate characterization of T 2  times, which is independent of a number of echos and the T 2  preparation echo time used, unlike the two-parameter fit of the related art. 
     Further, the two-parameter model may be inadequate for most single-shot acquisitions used in practice, as the signal regrows during imaging pulses. However, the above-described systems and methods of exemplary embodiments may overcome the shortcomings of the related art. For example, a number of sequences have been proposed for myocardial T 2  mapping, with reported healthy myocardium T 2  values ranging between 40-60 ms. These sequences sample the T 2  relaxation curve based on a variety of contrast mechanisms, echo types, k-space orderings and trajectories, and segmented/single-shot acquisitions. In all of these studies, however, a two-point model for T 2  decay, such as described above, is used to generate the T 2  data regardless of the details of the sequence. A study was done to compare the above-described two-point and three-point models for T 2  mapping for different k-space orderings using steady-state free precession (SSFP) and gradient recalled echo (GRE) imaging pulse sequences. 
     Suppose a T 2  preparation is performed such that the magnetization at the start of imaging is: 
     
       
         
           
             
               
                 
                   
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                           TE 
                           
                             
                               T 
                               2 
                             
                             ⁢ 
                             P 
                           
                         
                       
                       / 
                       
                         T 
                         2 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     5 
                     ) 
                   
                 
               
             
           
         
       
     
     For convenience of description, the following will focus on a balanced SSFP (bSSFP) acquisition. If there are n RF pulses to the center of k-space, the signal intensity may be expressed by Equation (6):
 
 S ( n )=[sin(α/2) M ( TE   prep )− M   SS ]λ n   +M   SS  
 
     where α is a flip angle, 
     M SS  is steady-state magnetization, and 
     λ and M SS  may be explicitly written in terms of exp (−TR/T 1 ), exp (−TR/T 2 ), and α. 
     The Equation (6) may be re-arranged for S(n), as Equation (7):
 
 S ( n )= c ( n ) M ( TE   prep )+ d ( n ) α′( n )exp(− TE   prep   /T   2 )+ d ( n )
 
     where α′(n) and d(n) are constants depending on n which is the number of RF pulses to the center of k-space. 
     Since S(n) corresponding to the center of k-space has the highest influence in determining the contrast of the sampled points on the T 2  curve, for accurate T 2  estimation, a three-point fit, such as described above, is needed for linear k-space ordering, while a two-point fit or a three-point fit may be used for centric ordering. 
     Phantom imaging of NiCl 2  doped agarose vials with different concentrations was performed using a T 2  mapping sequence including a GRE with centric ordering, GRE with linear ordering, and a SSFP with linear ordering. For linear ordering there were 37 pulses to the center of k-space, and eight TE prep  of T 2  preparation were used at 0, 25, 35, 45, 55, 65, 75, and 85 ms. Spin echo sequences were acquired to generate reference T 2  and T 1  values. For T 2  mapping, T 2  estimation was performed offline using MATLAB (v7.6, by MathWorks Inc., of Natick, Mass.). For each acquisition, both a two-point fit and a three-point fit were performed. An ROI analysis was performed on T 2  maps, where ROIs were drawn on each vial, and the mean value and standard deviation in the ROI was recorded for each acquisition. 
     The results of the T 2  estimation are shown in the table of  FIG. 5 , where T 2  values outside 10 percent of the reference value are indicated in italics. The T 1  values for the vials were 752, 1185, 1182, and 1180 ms, respectively, with the T 1  values for vials 2, 3, and 4 being similar to that of the healthy myocardium. When a linear k-space ordering is utilized, the two-point fit overestimates the T 2  values by 20 to 60 percent (range: 13.1 ms to 28.1 ms), as compared to the corresponding reference values. The overestimation is worse for shorter T 1  values, where the longitudinal magnetization recovers more rapidly during the 37 imaging pulses applied until the center of k-space is acquired, causing a larger shift in S(n). 
     The three-point fit leads to an accurate T 2  estimate, as does the centric ordering with a two-point fit. 
     Thus, parameter fitting with the two-point model for T 2  mapping overestimates the T 2  values when linear k-space ordering is utilized in the acquisition. On the other hand, the three-point fit of an exemplary embodiment may be used with centric or linear k-space ordering to generate accurate T 2  maps. 
     Although one or more exemplary embodiments are described above as using a cardiac gated acquisition and a navigator pulse to gate the heartbeat and/or breathing, the exemplary embodiments are not limited thereto. For example, the described-above is applicable in imaging of organs and tissues which do not require the cardiac gating and/or the breathing gating. For example, an ECG signal and/or breathing signal may be omitted from the described above sequencing and other physical, hardware, or software signal may be used as a trigger and/or for gating. 
     Exemplary embodiments may be implemented by software or hardware components such as a field-programmable gate array (FPGA) or an application-specific integrated circuit (ASIC). The hardware component may include a storage medium capable of addressing, or may be configured to be executed by one or more processors. Software component may include object-oriented software components, class components, and task components, and processes, functions, attributes, procedures, subroutines, segments of a program code, drivers, firmware, a micro code, a circuit, data, a database, data structures, tables, arrays, and variables. Functions provided by different components may be combined into a smaller number of components or may be further separated into additional components. 
     The foregoing exemplary embodiments and advantages are merely exemplary and are not to be construed as limiting. The present teaching can be readily applied to other types of apparatuses. Also, the description of the exemplary embodiments is intended to be illustrative, and not to limit the scope of the claims, and many alternatives, modifications, and variations will be apparent to those skilled in the art.