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A METHOD FOR CALCULATING OPTIMUM SLEW RATE FOR GRADIENTS IN MAGNETIC RESONANCE IMAGING

IP.com Disclosure Number: IPCOM000033823D
Publication Date: 2004-Dec-29
Document File: 7 page(s) / 80K

Publishing Venue

The IP.com Prior Art Database

Abstract

In an embodiment, a method for calculating optimum slew rate for gradients in MRI includes starting with maximum value of the slew rate. If the Reilly fraction for the slew rate violates the limit, then a new slew rate is predicted. The new slew rate is determined using the current pulse waveform and the algorithm for calculating dB/dT. Iteration is carried out with different slew rates, changing the timing of the current waveform accordingly. The calculated pseudo waveform is then used as the input to the same algorithm, which is used to calculate the Reilly fraction. The slew rate, which gives the best value of the Reilly fraction, is used as the new slew rate. For the new pulse waveform Reilly fraction is calculated. If there is a violation, the new slew rate is set as the upper limit of slew rate, otherwise, the new slew rate is set it as the lower limit. If the difference between the upper limit and lower limit is within the tolerance limit, the new slew rate is set as the final slew rate, otherwise a new slew rate is predicted. If the new slew rate is not within the upper limit and lower limit, quadratic interpolation is used to calculate the new slew rate.

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A METHOD FOR CALCULATING OPTIMUM SLEW RATE FOR GRADIENTS IN MAGNETIC RESONANCE IMAGING

FIELD OF THE INVENTION

[0001]   This invention relates generally, to a method for calculating slew rate for magnetic field gradients in magnetic resonance imaging (MRI), and more particularly, to a method for calculating optimum slew rate using Reilly’s fraction for gradients in MRI.

BACKGROUND OF THE INVENTION

 

[0002]     In MR pulse sequences, scan time reduction has always been a challenging issue.  In order to achieve reduced scan time, magnetic field  gradients  should slew at the maximum possible rate that the hardware can deliver.  However changing the gradient field causes a corresponding change in electric field which in turn may cause peripheral nerve stimulation.

[0003]    A known method for calculating optimum slew rate includes implementing an iterative binary search algorithm for achieving max dB/dt for a given pulse sequence which would always be less than 66% of the dB/dt amplitude on the Reilly curve.  However, this method is very slow for pulse sequences having many gradients and also results in longer response time for radiologists and clinical technicians during protocol changes.

[0004]     The functional form of the Reilly model is as given below:

             wherein R¥  is the asymptotic value at infinite duration and Treilly is the

 time  constant.

[0005]    If the gradients are modeled using linear segments, then for each of the gradient slopes on each axis,

 

 wherein DGij is the change in the gradient amplitude for the ith segment on axis j, Lj is the effective coil length of the jth axis and Dtiis the delta time for the linear gradient segment.  However, a difficulty exists in using the above equation, especially, for evaluating the delta time.  Considering the sum of time duration of all the neighbouring segments having the same polarity, the Reilly fraction for the segment is given by:

 

where Fij is the Reilly fraction for Ith segment on axis j.  Further magnitude sum of the Reilly fraction for ith segment on all three axes is taken as the resultant Reilly fraction.

 

[0006]   The maximum fraction among all the segments is taken as the limiting Reilly fraction.  If this Reilly fraction is more than the maximum permitted value, then the slew rates are required to be lowered.  However lowering of slew rates means changing of the timing of the pulses, the amplitude and the overlaps. Hence direct computation of the value of new slew rate is not possible and iteration over a range of slew rates is required to find the optimal slew rate.

[0007]    Another known method includes “iterative global slew-rate derating”   proposed by Jason Poltzin in 1999.  In this method, the pulse sequence timing and amplitude points are input into the Reilly model (either rectangular or convolution).  The maximum fraction value is then determined to see if it exceeds the allowed limit (66%).  If it does, then the...