Abstract:
A system and method a for local coil system for performing a magnetic resonance imaging (MRI) process using an MRI system is provided. The local coil system includes a conductor extending in a spiral path along a single plane and forming a variable spiral extending from inner spirals proximate a center of the spiral path having a first density to outer spirals proximate a perimeter of the spiral path having a second density. The first density and the second density are different densities.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on, claims priority to, and incorporates herein by reference in its entirety, U.S. Provisional Application Ser. No. 61/953,366, filed Mar. 14, 2014, and entitled “SYSTEM AND METHOD FOR MRI IMAGING USING A VARIABLE DENSITY SPIRAL PLANAR COIL.” 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    This invention was made with government support under W81XWH-11-2-076 awarded by the Department of Defense. The government has certain rights in the invention. 
     
    
     BACKGROUND 
       [0003]    The present disclosure relates to systems and methods for magnetic resonance imaging (MRI). 
         [0004]    When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B 0 ), the individual magnetic moments of the excited nuclei 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, M z , may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M t . A signal is emitted by the excited nuclei or “spins”, after the excitation signal B 1  is terminated, and this signal may be received and processed to form an image. 
         [0005]    When utilizing these “MR” 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 MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. 
         [0006]    Surface coils are a technology that is used for signal enhancement in conventional high field MRI instruments. Specifically designed for localized body regions, surface coils provide high magnetic sensitivity close to the sample resulting in high signal to noise ratios (SNR). However, their poor homogeneity generally makes them unsuitable for RF excitation, in particular when MRI sequences rely on flip angle homogeneity like steady state based sequences. Their sensitivity can be extended to larger areas when combined in multiple channel arrays, but only for tissue adjacent to the coil, and in conjunction with advanced decoupling strategies. 
         [0007]    While many local, surface coils are available, great room exists for continued design improvements and further specialization of the coil for particular ROIs. 
       SUMMARY 
       [0008]    The present disclose provides systems and methods that overcome the aforementioned drawbacks using a homogeneous single channel surface coil for both transmit and receive operations. In particular, the coil may be advantageously used at very low magnetic fields. 
         [0009]    In accordance with one aspect of the disclosure, a magnetic resonance imaging (MRI) system is disclosed that includes a magnet system configured to generate a static magnetic field about at least a region of interest (ROI) of a subject arranged in the MRI system and a plurality of gradient coils configured to establish at least one magnetic gradient field with respect to the static magnetic field. The system also includes a radio frequency (RF) system including a local coil. The local coil includes a conductor extending in a spiral path along a single plane and forming a variable spiral extending from inner spirals of a first density and proximate a center of the spiral path to outer spirals of a second density and proximate a perimeter of the spiral path. The first density and the second density are different densities. 
         [0010]    In accordance with another aspect of the disclosure, a coil system is disclosed for performing a magnetic resonance imaging (MRI) process using an MRI system. The coil system includes a conductor extending in a spiral path along a single plane and forming a variable spiral extending from inner spirals proximate a center of the spiral path having a first density to outer spirals proximate a perimeter of the spiral path having a second density. The first density and the second density are different densities. 
         [0011]    The foregoing and other advantages of the invention will appear from the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a block diagram of an MRI system. 
           [0013]      FIG. 2  is a block diagram of an RF system of an MRI system. 
           [0014]      FIG. 3  is a schematic diagram of a low-field MRI system in accordance with the present disclosure. 
           [0015]      FIG. 4  is an image of a variable-density spiral (VDS) local coil in accordance with the present disclosure that may be used with the systems of  FIGS. 1-3 . 
           [0016]      FIG. 5A  is a plot of a B1 field in the variable density spiral coil of  FIG. 4  with a 20 turn loop. 
           [0017]      FIG. 5B  are plots of B1 field in the variable density spiral coil of  FIG. 4  with a 30 turn loop. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    Referring particularly now to  FIG. 1 , an example of a magnetic resonance imaging (MRI) system  100  is illustrated. The MRI system  100  includes an operator workstation  102 , which will typically include a display  104 , one or more input devices  106 , such as a keyboard and mouse, and a processor  108 . The processor  108  may include a commercially available programmable machine running a commercially available operating system. The operator workstation  102  provides the operator interface that enables scan prescriptions to be entered into the MRI system  100 . In general, the operator workstation  102  may be coupled to four servers: a pulse sequence server  110 ; a data acquisition server  112 ; a data processing server  114 ; and a data store server  116 . The operator workstation  102  and each server  110 ,  112 ,  114 , and  116  are connected to communicate with each other. For example, the servers  110 ,  112 ,  114 , and  116  may be connected via a communication system  117 , which may include any suitable network connection, whether wired, wireless, or a combination of both. As an example, the communication system  117  may include both proprietary or dedicated networks, as well as open networks, such as the internet. 
         [0019]    The pulse sequence server  110  functions in response to instructions downloaded from the operator workstation  102  to operate a gradient system  118  and a radiofrequency (“RF”) system  120 . Gradient waveforms necessary to perform the prescribed scan are produced and applied to the gradient system  118 , which excites gradient coils in an assembly  122  to produce the magnetic field gradients G x , G y , and G z  used for position encoding magnetic resonance signals. The gradient coil assembly  122  forms part of a magnet assembly  124  that includes a polarizing magnet  126  and a whole-body RF coil  128  and/or local coil, such as a chest or hand coil  129 . 
         [0020]    RF waveforms are applied by the RF system  120  to the RF coil  128 , or a separate local coil  129 , in order to perform the prescribed magnetic resonance pulse sequence. Responsive magnetic resonance signals detected by the RF coil  128 , or a separate local coil, such as the local coil  129 , are received by the RF system  120 , where they are amplified, demodulated, filtered, and digitized under direction of commands produced by the pulse sequence server  110 . The RF system  120  includes an RF transmitter for producing a wide variety of RF pulses used in MRI pulse sequences. The RF transmitter is responsive to the scan prescription and direction from the pulse sequence server  110  to produce RF pulses of the desired frequency, phase, and pulse amplitude waveform. The generated RF pulses may be applied to the whole-body RF coil  128  or to one or more local coils or coil arrays, such as the local coil  129 . 
         [0021]    The RF system  120  also includes one or more RF receiver channels. Each RF receiver channel includes an RF preamplifier that amplifies the magnetic resonance signal received by the coil  128 / 129  to which it is connected, and a detector that detects and digitizes the I and Q quadrature components of the received magnetic resonance signal. The magnitude of the received magnetic resonance signal may, therefore, be determined at any sampled point by the square root of the sum of the squares of the I and Q components: 
         [0000]        M =√{square root over ( I   2   +Q   2 )}  (1);
 
         [0022]    and the phase of the received magnetic resonance signal may also be determined according to the following relationship: 
         [0000]    
       
         
           
             
               
                 
                   ϕ 
                   = 
                   
                     
                       
                         tan 
                         
                           - 
                           1 
                         
                       
                        
                       
                         ( 
                         
                           Q 
                           I 
                         
                         ) 
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0023]    The pulse sequence server  110  also optionally receives patient data from a physiological acquisition controller  130 . By way of example, the physiological acquisition controller  130  may receive signals from a number of different sensors connected to the patient, such as electrocardiograph (“ECG”) signals from electrodes, or respiratory signals from a respiratory bellows or other respiratory monitoring device. Such signals are typically used by the pulse sequence server  110  to synchronize, or “gate,” the performance of the scan with the subject&#39;s heart beat or respiration. 
         [0024]    The pulse sequence server  110  also connects to a scan room interface circuit  132  that 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  132  that a patient positioning system  134  receives commands to move the patient to desired positions during the scan. 
         [0025]    The digitized magnetic resonance signal samples produced by the RF system  120  are received by the data acquisition server  112 . The data acquisition server  112  operates in response to instructions downloaded 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 some scans, the data acquisition server  112  does little more than pass the acquired magnetic resonance data to the data processor server  114 . However, in scans that require information derived from acquired magnetic resonance data to control the further performance of the scan, the data acquisition server  112  is programmed to produce such information and convey it to the pulse sequence server  110 . For example, during prescans, magnetic resonance data is acquired and used to calibrate the pulse sequence performed by the pulse sequence server  110 . As another example, navigator signals may be acquired and used to adjust the operating parameters of the RF system  120  or the gradient system  118 , or to control the view order in which k-space is sampled. In still another example, the data acquisition server  112  may also be employed to process magnetic resonance signals used to detect the arrival of a contrast agent in a magnetic resonance angiography (MRA) scan. By way of example, the data acquisition server  112  acquires magnetic resonance data and processes it in real-time to produce information that is used to control the scan. 
         [0026]    The data processing server  114  receives magnetic resonance data from the data acquisition server  112  and processes it in accordance with instructions downloaded from the operator workstation  102 . Such processing may, for example, include one or more of the following: reconstructing two-dimensional or three-dimensional images by performing a Fourier transformation of raw k-space data; performing other image reconstruction algorithms, such as iterative or backprojection reconstruction algorithms; applying filters to raw k-space data or to reconstructed images; generating functional magnetic resonance images; calculating motion or flow images; and so on. 
         [0027]    Images reconstructed by the data processing server  114  are conveyed back to the operator workstation  102  where they are stored. Real-time images are stored in a data base memory cache (not shown in  FIG. 1 ), from which they may be output to operator display  112  or a display  136  that is located near the magnet assembly  124  for use by attending physicians. Batch mode images or selected real time images are stored in a host database on disc storage  138 . When such images have been reconstructed and transferred to storage, the data processing server  114  notifies the data store server  116  on the operator workstation  102 . The operator workstation  102  may be used by an operator to archive the images, produce films, or send the images via a network to other facilities. 
         [0028]    The MRI system  100  may also include one or more networked workstations  142 . By way of 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, such as a different healthcare institution or clinic. 
         [0029]    The networked workstation  142 , whether within the same facility or in a different facility as the operator workstation  102 , may gain remote access to the data processing server  114  or data store server  116  via the communication system  117 . Accordingly, multiple networked workstations  142  may have access to the data processing server  114  and the data store server  116 . In this manner, magnetic resonance data, reconstructed images, or other data may exchanged between the data processing server  114  or the data store server  116  and the networked workstations  142 , such that the data or images may be remotely processed by a networked workstation  142 . This data may be exchanged in any suitable format, such as in accordance with the transmission control protocol (TCP), the internet protocol (IP), or other known or suitable protocols. 
         [0030]    With reference to  FIG. 2 , the RF system  120  of  FIG. 1  will be further described. The RF system  120  includes a transmission channel  202  that produces a prescribed RF excitation field. The base, or carrier, frequency of this RF excitation field is produced under control of a frequency synthesizer  210  that receives a set of digital signals from the pulse sequence server  110 . These digital signals indicate the frequency and phase of the RF carrier signal produced at an output  212 . The RF carrier is applied to a modulator and up converter  214  where its amplitude is modulated in response to a signal, R(t), also received from the pulse sequence server  110 . The signal, R(t), defines the envelope of the RF excitation pulse to be produced and is produced by sequentially reading out a series of stored digital values. These stored digital values may be changed to enable any desired RF pulse envelope to be produced. 
         [0031]    The magnitude of the RF excitation pulse produced at output  216  is attenuated by an exciter attenuator circuit  218  that receives a digital command from the pulse sequence server  110 . The attenuated RF excitation pulses are then applied to a power amplifier  220  that drives the RF transmission coil  204 . 
         [0032]    The MR signal produced by the subject is picked up by the RF receiver coil  208  and applied through a preamplifier  222  to the input of a receiver attenuator  224 . The receiver attenuator  224  further amplifies the signal by an amount determined by a digital attenuation signal received from the pulse sequence server  110 . The received signal is at or around the Larmor frequency, and this high frequency signal is down converted in a two step process by a down converter  226 . The down converter  226  first mixes the MR signal with the carrier signal on line  212  and then mixes the resulting difference signal with a reference signal on line  228  that is produced by a reference frequency generator  230 . The down converted MR signal is applied to the input of an analog-to-digital (“ND”) converter  232  that samples and digitizes the analog signal. The sampled and digitized signal is then applied to a digital detector and signal processor  234  that produces 16-bit in-phase (I) values and 16-bit quadrature (Q) values corresponding to the received signal. The resulting stream of digitized I and Q values of the received signal are output to the data acquisition server  112 . In addition to generating the reference signal on line  228 , the reference frequency generator  230  also generates a sampling signal on line  236  that is applied to the ND converter  232 . 
         [0033]    Referring to  FIG. 3  a system  300  is illustrated that, instead of a 1.5 T or greater static magnetic field, utilizes a substantially smaller magnetic field. That is, as a non-limiting example, the system  300  may be less than 10 mT, such as a 6.5 mT, electromagnet-based scanner that is capable of imaging objects up to, as a non-limiting example, 15.6 cm in diameter. The system  300  may use a local coil  302  such as will be described. 
         [0034]    In particular, referring to  FIG. 4 , the above-mentioned local coil  400  may have variable-density spiral (VDS) design designed to improve B1 signal amplitude and homogeneity. In the illustrated design, the coil  400  forms a variable-density spiral that extends along a common or single plane  402 . Across that single plane  402 , the spiral is defined by a coil that moves from inner spirals  404  of a first density to outer spirals  406  of a second, different density. That is, inner spirals  404  nearer the center  408  of the coil  400  are of the first density (D1) and outer spirals  406  near a perimeter  410  of the coil  400  are of the second, different density (D2). In one configuration, the first density D1 of the inner spirals  404  may be less than the second density D2 of the outer spirals  406 . In alternative designs, the first density D1 of the inner spirals  404  may be greater than the second density D2 of the outer spirals  406 . 
         [0035]    In the illustrated design, the variability of the density may be continuous. That is, each successive turn in the coil  400  may adjust the density. The coil  400  may maintain a constant area between each successive turn. As a non-limiting example, the coil  400  may form a spiral that features 20 turns combined in series with a 20-turn loop around the perimeter to balance the magnetic field near the perimeter  406  of the coil  400 . The illustrated coil  400  covers a 20 cm circular field of view. The coil form was 3D printed in a polycarbonate. 
         [0036]    This design provides particular advantages. For example,  FIG. 5A  show the simulated B1 field in a VDS coil with 30-turns and  FIG. 5B  shows a simulated B1 filed of 20 cm diameter loop coil. Both are relative to a (6×20) cm2 plane set perpendicular to the coil surfaces. As can be seen, the variable-density spiral design shows flat magnetic field lines in the 2 cm range above the surface and maintains below 40 percent Bz deviation at ±7.5 cm along the X axis. The 20 cm loop produces curved magnetic field lines and over 70 percent Bz deviation at ±7.5 cm along the x-axis. 
         [0037]    The VDS coil of the present disclosure was tested in vivo in a human hand at 6.5 mT using the intrinsic 1H NMR signal and a balanced steady state free precession (b-SSFP) pulse sequence. A low-field MRI (IfMRI) scanner with a bi-planar electromagnet (B0) and biplanar gradients was used for all experiments, as previously described. The 3D imaging experiment was performed with Cartesian acquisition of k-space and 50 percent undersampling rate following a Gaussian probability density function. The sequence was set with TR/TE=23.22/11.6 ms, acquisition matrix=(64×64×7), voxel size=(3×3×6) mm3, number of averages (NA)=220. The readout duration was 7.04 ms with 9091 Hz bandwidth. The total acquisition time was 20 min. 
         [0038]    The test demonstrated a variable-density spiral surface coil for use at very-low magnetic field, achieving (3×3×6) mm3 voxel size in a (64×64×7) matrix in 20 min with a maximum image SNR of 22. The non-field cycled results demonstrate up to a factor of 2 in speed, SNR, and voxel size over those obtained with far more complex multi-channel superconducting quantum interference device (SQUID)-detected ultra-low field MRI systems using 30-80 mT prepolarization fields. 
         [0039]    The above-described, variable-density spiral design can provide homogeneous magnetic field and high sensitivity over broad regions of interest when used for either transmit, receive, or both. As opposed to conventional surface coils that provide high sensitivity but suffer from high magnetic field inhomogeneity and need a separate coil for transmit operations, the above-described variable, spiral-design of the present disclosure can be tuned to provide high homogeneity, while maintaining high sensitivity over large field of views in a streamlined design. The coil can be used, therefore, for transmit and receive operations. So, while surface coils and surface coil arrays provide high sensitivity for material/tissue in the close vicinity of the coil, the variable density design of the present disclosure can be tuned to provide high sensitivity in bigger volumes. As opposed to surface coil arrays, the variable density spiral cover a broad field of view but does not require decoupling strategies. 
         [0040]    Accordingly, variable-density spiral coils in accordance with the present disclosure can be used to replace current surface coils and be used without the need of a separate transmit coil, resulting in simpler, open access and lower cost devices. The variable-density coil design of the present disclosure can be advantageously used to perform MRI of the hand and extremities in patients. The variable-density spiral coil can also be used to monitor patient recovery in the case of knee or hip replacement with open access designs. Further still, the open-access, planar-design of the present variable-density spiral coil is compatible with conveyor belts and could be used to perform MRI and NMR spectroscopy for quality control in the food industry and other industrial processes. Furthers still, in the security industry, the variable-density spiral coils can be used to perform MRI and NMR spectroscopy to check for potentially dangerous chemicals in airports and areas with dense population traffic. 
         [0041]    The present invention has been described in terms of one or more embodiments, and it should be appreciated that many equivalents, alternatives, variations, and modifications, aside from those expressly stated, are possible and within the scope of the invention.