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Technique for Improving T1 Contrast in Spin-Echo Imaging

IP.com Disclosure Number: IPCOM000012542D
Publication Date: 2003-May-13
Document File: 2 page(s) / 41K

Publishing Venue

The IP.com Prior Art Database

Abstract

In spin echo imaging, T1 contrast is not only due to the primary spin echo. T1 contrast can also be affected by additional spin and stimulated echoes originating from prior RF. This is the finding of a long-standing investigation into the reduction in T1 contrast between the present Lx MRI systems and the older 5.4 systems.

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FIELD OF TECHNOLOGY:

Magnetic Resonance (MR)

INVENTION TITLE:

Technique for Improving T1 Contrast in Spin-Echo Imaging

PROBLEM/BACKGROUND:

In spin echo imaging, T1 contrast is not only due to the primary spin echo, but T1 contrast can also be affected by additional spin and stimulated echoes originating from prior RF.

If care is not taken in the pulse sequence design, these additional echoes can degrade the image contrast. Alternatively they can be used to actually enhance the contrast.

This is the finding of a long-standing investigation into the reduction in T1 contrast between the present Lx MRI systems and the older 5.4 systems.

INVENTION DESCRIPTION:

The following diagram shows a simplified schematic of a spin echo pulse sequence. Nominally the flip angles q1 and q2, of the excitation and refocusing pulses, are set to 90 and 180 degrees, respectively. However at the edge of the selected slice, the actual flip angles can vary considerably from these values.

In any series of RF pulses, a large number of multiple spin and stimulated echoes are potentially present. However most are either eliminated by crushing gradients or considerably reduced by relaxation periods.

For the above spin echo sequence the two main echoes requiring consideration are the primary spin echo and a parasitic stimulated echo.

The Spin Echo

If the echo time Te is much less than the repetition time Tr, the steady state longitudinal magnetization is given by

� .                                � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � [1]

The amplitude of the spin echo, produced pairs of excitation and refocusing pulses, is

� ;                                                                � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � [2]

where PD and T1 are the proton density and T1 relaxation time, respectively, for the tissue under consideration.

Ignoring proton density differences for a moment, the spin echo intensity is lower for long T1 tissue species.

The Stimulated Echo

The stimulated echo occurring concurrently with the spin echo at the nth acquisition is originated by the excitation and refocusing pulses at the (n-1)th acquisition. Then it is attenuated by the excitation pulse at the nth acquisition, and is finally converted back into transverse magnetization by the refocusing pulse in the nth acquisition.

The amplitude of the stimulated echo is

.                                        � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � [3]

Relative to the spin echo, the stimulated echo contrast is modified due to the extra relaxation term. And this extra contrast is opposite in sign to that inherent in the steady state longitudina...