Abstract:
A method of designing quiet variable-rate MRI slice-select pulses includes creating discretized first slice-select constant-amplitude gradient and RF waveforms, associating discretized time points having a first constant time increment with the first waveforms, selecting a scaling function that smooths the gradient waveform when multiplied together, multiplying the gradient and RF waveforms by the corresponding value of the scaling function to create second gradient and RF waveforms, dividing the time increment between the discretized time points by the corresponding value of the scaling function to create a remapped time increment, cumulatively summing the remapped time increments to create a remapped time scale, interpolating the second gradient and RF waveforms along the remapped time scale to form final gradient and RF waveforms, and providing the final gradient and RF waveforms for incorporation into an MRI pulse sequence. A system implementing the method and a non-transitory computer-readable medium are disclosed.

Description:
BACKGROUND 
       [0001]    Variable-rate selective excitation (VERSE) is a pulse reshaping technique that is applied to the RF and gradient slice-select waveforms used in magnetic resonance imaging (MRI) to reduce the RF power of the slice-select pulses. VERSE allows for a variable trade-off between RF and gradient amplitudes and sample duration for each time sample of the pulse. This results in an RF waveform whose peak region is truncated in amplitude and stretched in time, while the corresponding portions of the gradient waveform experience a dip. This reshaping produces a reduction in peak magnitude and specific absorption rate (SAR) of the RF pulses while preserving MR excitation profiles on-resonance. 
         [0002]    In some applications, a VERSE algorithm has been applied to multidimensional and parallel-transmit selective excitation pulses, reshaping the RF and gradient waveforms to take advantage of available gradient amplitude and slew rate, to produce the shortest pulse possible without changing the two-dimensional excitation profile. This results in, e.g., two-dimensional selective excitation pulses with optimal bandwidth, and identical two-dimensional excitation profiles to the original pulses on-resonance. 
         [0003]    Slice-select pulses can be made quieter by de-rating them—i.e., by reducing gradient slew rate and/or amplitude (along with RF bandwidth), but both of these measures produce longer pulses and thus increase minimum echo time. Other approaches smooth the corners of various gradient waveforms to make them quieter. For instance, gradient crushers around RF refocusing pulses can be smoothed and lengthened while modifying readout bandwidth in fast spin echo sequences. However, these approaches do not change gradients that are played out during an RF pulse, and are not applicable during RF excitation, inversion, or refocusing pulses. 
         [0004]    The pulsing of gradients in MRI pulse sequences can be loud, leading to patient discomfort and impaired communication with scan operators. There are pulse sequences that reduce the acoustic noise. Because of difficulty in quieting the slice-select pulse, these sequences are generally volumetric sequences. While slice select gradient pulses can be de-rated to reduce loudness, a longer minimum echo time can be required, and sometimes a narrower pulse bandwidth is also necessary. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  depicts an MRI system in accordance with some embodiments; 
           [0006]      FIG. 2  schematically depicts an RF signal and gradient signals in accordance with some embodiments; 
           [0007]      FIG. 3  depicts a quiet slice-select pulse designed in accordance with some embodiments; 
           [0008]      FIG. 4  depicts a variable-rate pulse schedule in accordance with some embodiments; and 
           [0009]      FIG. 5  depicts a variable-rate process for designing quiet slice-select pulses in accordance with some embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    In accordance with embodiments, a variable-rate principle is employed to design slice-select pulses with improved acoustic signatures while maintaining identical on-resonance slice profiles. RF and gradient waveforms are scaled down together during the beginning and end of the pulse (with time increments broadened proportionately), and scaled up in the middle of the pulse (with time increments narrowed proportionately). The result is a quieter slice-select pulse with rounder gradient corners, and an unchanged slice profile for on-resonance spins. 
         [0011]    In accordance with implementations using this variable-rate design, the RF and gradient waveforms are reshaped together in such a way that the magnetization vector on-resonance follows the same trajectory (but at varying rates) over the course of the excitation, resulting in an identical slice profile for on-resonance spins. 
         [0012]      FIG. 1  depicts components of MRI system  10  in accordance with some embodiments. System  10  can be controlled from operator console  12 , which can include a keyboard or other input device  13 , a control panel  14 , and a display  16 . Console  12  communicates through communication network  18  with a computer system  20 . Computer system  20  can include computer executable instructions that when executed enable an operator to control the production and display of images on screen  16 . Computer system  20  can include a number of modules which communicate with each other through a backplane  20   a . These modules can include image processor module  22 , control processor unit (CPU)  24 , and 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 (i.e., computer executable instructions). The computer system can communicate MRI imaging system control  32  through network  18  and/or high speed serial link  34 . Input device  13  can include a mouse, joystick, keyboard, track ball, touch screen, light wand, voice control, or similar device, and may be used for interactive geometry prescription. 
         [0013]    MRI imaging system control  32  can include a set of modules connected together by backplane  32   a . These modules can include control processor unit (CPU)  36  and pulse generator module  38  which connects to the operator console through serial link  40 . System control  32  receives commands through serial link  40  from the operator. These commands can indicate, for example, the scan sequence that is to be performed. 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 connects to a set of gradient amplifiers  42 , to indicate the timing and shape of the gradient pulses that are produced during the scan. Pulse generator module  38  also receives patient data from 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. Further, 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 scan room interface circuit  46  that a patient positioning system  48  receives commands to move the patient to the desired position for the scan. 
         [0014]    The gradient waveforms produced by the pulse generator module  38  are applied to gradient amplifier system  42  having Gx, Gy, and Gz, amplifiers. Each gradient amplifier excites a corresponding physical gradient coil in an assembly generally designated  50  to produce the magnetic field gradients used for spatially encoding acquired signals. Gradient coil assembly  50  forms part of magnet assembly  52  which includes polarizing magnet  54  and whole-body RF coil  56 . Transceiver module  58  in system control  32  produces pulses which are amplified by RF amplifier  60  and coupled to RF coil  56  by transmit/receive switch  62 . The resulting signals emitted by the excited nuclei in the patient may be sensed by the same RF coil and coupled through the transmit/receive switch to preamplifier  64 . The amplified MR signals are demodulated, filtered, and digitized in the receiver section of transceiver  58 . The transmit/receive switch is controlled by a signal from pulse generator module  38  to electrically connect RF amplifier  60  to coil  56  during the transmit mode and to connect preamplifier  64  during the receive mode. The transmit/receive switch also enables a separate RF coil (for example, a surface coil) to be used in either transmit or receive mode. 
         [0015]    The MR signals picked up by the RF coil  56  are digitized by the transceiver module  58  and transferred to memory module  66  in the system control  32 . This raw k-space data can be rearranged into separate k-space data arrays for each image to be reconstructed, and each of these is input to array processor  68  which can execute instructions to perform the embodying methods described herein. The image data can be conveyed through the serial link to computer system  20  where it is stored in disk memory  28 . In response to commands received from operator console  12 , image data may be archived on tape drive  30 , or it may be further processed by image processor  22  and presented on display  16 . 
         [0016]    Embodying systems and methods are suitable for use with the above-referenced MR system, or any similar or equivalent system for obtaining MR images. 
         [0017]    In accordance with some embodiments, quieter slice-select pulses with rounder gradient corners, and an unchanged slice profile for on-resonance spins is designed by scaling down both RF and gradient waveforms during the beginning and end of the pulse (with time increments broadened proportionately), and scaling up in the middle of the pulse (with time increments narrowed proportionately). In accordance with an implementation, a subsequent slice-select gradient refocusing lobe, which is not simultaneous with any RF pulse, can be independently reshaped. 
         [0018]      FIG. 2  schematically depicts original RF waveform  210  and original gradient waveform  220  in accordance with some embodiments. In accordance with one implementation, the scaling of waveforms is achieved by selecting scaling function R  240  that varies in time over the course of the pulse. 
         [0019]    RF waveform  210  and gradient waveform  220  are discretized into time increments i, where i ranges from 1 to N points. For point i  202 , the RF waveform has an amplitude B 1   org   i    212 , the gradient waveform has amplitude Gz orig   i    222 , and the time increment dt orig   i    232  is equal to T/(N−1), where T is the length in time of the waveforms  210  and  220 . For each point i, a scaling factor R i    242  is chosen, where the scaling factor varies with i in such a way as to round off the corners of gradient waveform  220  to reduce acoustic noise. For example, the scaling factor may be proportional to the square root of a half-cycle sine: 
         [0000]        R   i =1.4*sqrt(sin(θ i ))  (EQ. 1)
 
         [0020]    where θ i =((i−1)/(N−1))*π. 
         [0021]    For each time increment i  202 , the scaling factor R i    242  is used to scale B 1   orig   i    212  to create a new RF amplitude B 1   new   i    262 . R i  also scales Gz orig   i    222  to create a new gradient amplitude Gz new   i    272 ; and inversely scale time increment dt orig   i    232  to create a new time increment dt new   i    282 . New RF amplitude can be expressed as: 
         [0000]        B 1 new   i   =R   i   *B 1 orig   i   (EQ. 2)
 
         [0000]        Gz   new   i   =R   i   *Gz   orig   i   (EQ. 3)
 
         [0000]        dt   new   i   =dt   orig   i   /R   i   (EQ. 4)
 
         [0022]    The new waveform amplitudes B 1   new   i , Gz new   i , and corresponding time increments dt new   i  are calculated for all points i between 1 and N. The time t new   i  for point i is the cumulative sum of all dt new   i  for all preceding points. Because the time increments dt new   i  vary in width for each value of i, the waveforms must then be interpolated onto a new grid having constant time increments, using a reasonable number of points. This results in final remapped waveforms B 1   rem  and Gz rem , which can be stored as external waveforms to be imported by the MR pulse sequence. 
         [0023]    Curve  240  is a plot of scaling function R over time, where when the function is greater than one the waveform amplitudes are increased and their timing is compressed; and when the function is less than 1 the waveform amplitudes are decreased and their timing is stretched. As illustrated by scaling-function curve  240 , both the RF and gradient waveforms are scaled down during the beginning and end of the pulse (with time increments broadened proportionately), and scaled up in the middle of the pulse (with time increments narrowed proportionately). In some embodiments the average value of R i , for i from 1 to N, is 1. 
         [0024]      FIG. 3  depicts a quiet slice-select pulse designed in accordance with some embodiments, using a scaling factor R that is the square root of a half-cycle sine. Waveforms  310  and  312  are conventional RF and gradient slice-select waveforms, respectively. Waveform  314  is a conventional gradient refocusing lobe. The amplitude of waveform  314  has been chosen so that the area under waveform  314  is the negative of one half the area under waveform  312 , to insure that the phase of the magnetization is essentially constant across the slice profile. 
         [0025]    Waveforms  320  and  322  show the remapped RF and gradient waveforms, respectively for the quiet slice-select pulse. Waveform  324  is a gradient refocusing waveform reshaped using conventional methods. Its shape is a half-cycle sine, and its amplitude was chosen so that the area under waveform  324  is the negative of one half the area under waveform  322 . Note that since the area under waveform  322  is slightly less than the area under waveform  312 , waveform  324  has a correspondingly smaller area than waveform  314 . 
         [0026]    Slice profiles corresponding to these pulses were calculated using a Bloch simulator, for on-resonance spins. Slice profile  316  corresponds to the conventional slice-select pulse comprising components  310 ,  312 , and  314 . Slice profile  326  corresponds to the remapped quiet slice-select pulse comprising components  320 ,  322 , and  324 . The slice profiles  316 ,  326  are essentially indistinguishable. The slice-select pulse sequences were played out on a commercial 3T MRI scanner with a repetition time of 34 msec, and sound levels were recorded using a microphone placed in the scanner bore. An overall reduction in sound levels of roughly 7 dBA was recorded for the quiet pulse relative to the conventional pulse. Furthermore, as is evident by the conventional and quiet slice-select waveforms as presented in  FIG. 3 , the improvements provided by the embodying variable-rate approach are achieved without increasing the pulse length. 
         [0027]      FIG. 4  depicts the variable-rate pulse schedule  400  for the embodying variable-rate slice-select pulse approach depicted by the solid curves  320 ,  322 . Pulse schedule  400  was determined by calculating t new   i  from dt new   i , derived from a scaling function R that was proportional to the square root of a half-cycle sine. Both conventional pulses and quiet variable-rate pulses had identical slice profiles on-resonance. For the variable-rate excitation, the peaks of the RF pulse, slice-select gradient lobe, and refocusing lobe had relative amplitudes of about 1.19, 1.19, and 0.68, respectively compared to the standard pulse. Compared to the standard pulse, the variable-rate pulse had overall relative sound levels of about −7 dBA. 
         [0028]    In accordance with one implementation, at 300 Hz off-resonance, the variable-rate pulse experienced a similar average spatial shift to the standard pulse, but with a broadening of the slice profile by about 12%. Use of variable-rate slice-selective excitation pulses can provide added acoustic benefits beyond conventional reshaping and smoothing of gradient pulses such as phase-encodes, crushers, spoilers, and rewinders, although with potentially higher RF power than conventional slice-select pulses. Variable-rate remapping to produce quieter slice-select pulses is not limited to excitation pulses—application to slice-selective inversion pulses and slice-selective refocusing pulses is also available. Pulse reshaping can also be done by pulse scaling functions R other than the exemplary square root of a half-cycle sine. 
         [0029]    Sound level measurements indicate that in one implementation, acoustic noise levels can be reduced by about 7 dB relative to standard slice-select pulses, without any increase in minimum echo time. This reduction can be achieved without any modifications to scanner hardware components and modules. Overall reduction in loudness achieved is a function of the full pulse sequence, including readout, phase-encode, crushers, etc. 
         [0030]      FIG. 5  depicts process  500  for creating variable-rate slice-select pulses for use in an MRI system in accordance with some embodiments. In step  505 , starting discretized slice-select waveforms are created, including a constant-amplitude gradient waveform and a corresponding RF waveform, together with starting constant time increments between each point. In step  510 , a scaling function is created. The gradient waveform can be smoothed by multiplying it by the scaling function. In step  515 , for each instance of time the RF and gradient waveforms are multiplied by the corresponding value of the scaling function to create new RF and gradient waveforms. In step  520 , for each instance of time, the time increment is divided by the corresponding value of the scaling function to create a remapped time increment. 
         [0031]    In step  525 , a remapped time scale is created that is the running cumulative sum of the remapped time increments. In step  530 , the new RF and gradient waveforms together with their remapped time scale are interpolated onto a new constant-increment time scale. In step  535 , the final remapped gradient and RF waveforms are stored as external files for incorporation into the MR pulse sequence. 
         [0032]    In accordance with some embodiments, a computer program application stored in non-volatile memory or computer-readable medium (e.g., register memory, processor cache, RAM, ROM, hard drive, flash memory, CD ROM, magnetic media, etc.) may include code or executable instructions that when executed may instruct and/or cause a controller or processor to perform methods discussed herein such as a method for designing slice-select pulses using a variable-rate to obtain improved acoustic signatures in MRI measurements while maintaining identical on-resonance slice profiles, as described above. 
         [0033]    The computer-readable medium may be a non-transitory computer-readable media including all forms and types of memory and all computer-readable media except for a transitory, propagating signal. In one implementation, the non-volatile memory or computer-readable medium may be external memory. 
         [0034]    Although specific hardware and methods have been described herein, note that any number of other configurations may be provided in accordance with embodiments of the invention. Thus, while there have been shown, described, and pointed out fundamental novel features of the invention, it will be understood that various omissions, substitutions, and changes in the form and details of the illustrated embodiments, and in their operation, may be made by those skilled in the art without departing from the spirit and scope of the invention. Substitutions of elements from one embodiment to another are also fully intended and contemplated. The invention is defined solely with regard to the claims appended hereto, and equivalents of the recitations therein.