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AN IMPROVED MAXWELL TERM COMPENSATING METHOD FOR FAST SPIN ECHO MR IMAGES

IP.com Disclosure Number: IPCOM000235438D
Publication Date: 2014-Feb-27

Publishing Venue

The IP.com Prior Art Database

Abstract

The invention provides a technique to optimize echo spacing (ESP) while performing Maxwell term compensation for a FSP MR image. This is mainly because prolong ESP introduce additional T2 decay artifact. The technique allows compensating two gradient axes together with de-rating technique. By flow compensation, the technique allows to compensate Maxwell term for FSE sequence easier. X-axis gradients and Y-axis gradients are considered together while performing Maxwell term compensation. This reduces ESP prolonging while compensating Maxwell term. For instance, a phase error by the Maxwell term from X-axis gradients before refocus radiofrequency (RF) pulse are cancelled by the Maxwell term from Y-axis gradients after the refocus RF pulse due to the phase reversal effect of the refocus RF pulse.

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AN IMPROVED MAXWELL TERM COMPENSATING METHOD FOR FAST SPIN ECHO MR IMAGES

FIELD OF INVENTION

The invention generally relates to magnetic resonance imaging and more particularly to a technique for compensating Maxwell term for the fast spin echo magnetic resonance (FSP MR) images. 

BACKGROUND OF THE INVENTION


A fast spin echo (FSE) sequence is sensitive to a Maxwell term from a linear imaging gradient.  A slice selective gradient (Gy) and read gradient (Gx) in a sagittal scanning produces the Maxwell term magnetic field on physical z axis direction on 1.5T system. The Maxwell field is given by:

Where B0 is a main magnetic field and x, y, z are magnet based physical coordinates. Artifacts caused by the quadratic-cross Maxwell terms that include xz and yz terms are small and are removed by utilizing regular FSE phase correction technique. For the sagittal or coronal scanning, the main Maxwell term is from Z2 term written down as below:

A = 1/ (2B0)*(Gx2+Gy2 ) z2

Large coverage on z direction is mainly due to scanning spines with phase or frequency direction swapping. Z2 has spatial dependence.  The read gradient and slice gradient are contributed to the Z2 Maxwell term together.

The Maxwell field leads to phase accumulation and produces phase error. The spin does not acquire Carr-Purcell-Meiboom-Gill (CPMG) condition with phase error and result in artifacts and shading.

A conventional technique includes reduction of Maxwell term artifact. The gradient is re-shaped, de-rated or quadratic nulled in every gradient axis individually to minimize phase error by Maxwell field.  However, the conventional technique does not perform echo spacing (ESP) optimization.

Hence there exists a need for an efficient technique to optimize echo spacing (ESP) while performing Maxwell term compensation for the fast spin echo magnetic resonance (FSP MR) images.

BRIEF DESCRIPTION OF THE INVENTION

The invention provides a technique to optimize echo spacing (ESP) while performing Maxwell term compensation for FSP MR images. This is mainly because prolong ESP introduce additional T2 decay artifact. The technique allows compensating two gradient axes together with de-rating technique. By flow compensation, the technique allows easy compensation of Maxwell term for FSE sequence.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a technique to optimize echo spacing (ESP) while performing Maxwell term compensation for a FSP MR image. This is mainly because prolong ESP introduce additional T2 decay artifact. The technique allows compensating two gradient axes together with de-rating technique. By flow compensation, the technique allows easy compensation of Maxwell term for FSE sequence .

Z2 Maxwell term is given as:

A = 1/ (2B0)*(Gx2+Gy2 ) z2

X-axis gradients and Y-axis gradients are considered together while performing Maxwell term compensation. This benefits in reducing ESP prolonging while compensating Maxwell term. For instance, a phase error by Maxwell term...