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Long-T2 suppression based on saturation and diffusion in a steady-state 3D-UTE sequence
Lucas Soustelle1, Julien Lamy1, Paulo Loureiro de Sousa1, François Rousseau2, and Jean-Paul Armspach1

1Université de Strasbourg, CNRS, ICube, FMTS, Strasbourg, France, 2Institut Mines Télécom, Télécom Bretagne, INSERM LaTIM, Brest, France

Synopsis

A new method for long-T2 suppression in a prepared steady-state 3D-UTE sequence is introduced. The method is based on long-T2 signal behavior in steady-state as the diffusion-inducing spoiling gradients are modified, giving a theoretical signal cancellation using appropriate coherence combinations. At the same time, short-T2 signal quantity is optimized, offering a positive contrast over this component. Imaging experiments over a Lego brick soaked in doped water show an excellent agreement with theoretical predictions.

Purpose

Imaging of the very-short T2 tissues is challenging in that the signal decays very rapidly (T2 < 1ms), as well as its signal quantity being often overwhelmed by long-T2 components (fat, free-water). Nevertheless, direct imaging of such components may significantly improve specificity in tissues evaluation. Numerous methods to highlight these species exist using a proper preparation (Inversion-Recovery modules1, saturation module2 or more complex and specific long-T2 suppression pulses3) or a specific excitation pattern4 to either null the undesired signal or to selectively excite the component of interest. In this work, we explore a novel method for long-T2 suppression in a steady-state 3D-UTE sequence taking advantage of diffusion and coherence effects while acquiring short-T2 components. Simulations and analysis were performed using the Extended Phase Graph (EPG) formalism5,6.

Theory

The pulse sequence employed (fig. 1) consists in a long (Tshort2) rectangular saturation pulse followed by a short one (α2 90°), whose flip angle will be computed to maximize the short-T2 signal. It’s very similar to the Actual Flip Angle sequence7, except that α1α2. Gradient spoiling, RF spoiling and delays are optimized to ensure a steady-state of the long-T2 component to be suppressed, and a minimal impact of potential static gradients (e.g. B0 inhomogeneities)8.

First, the short-T2 transverse magnetization can be tracked using the Bloch equations in steady-state:

Mxy=M0(1Es)+Es(1Ew)fz11fz1fz2EsEwfxy,

with Es=eTR1/T1,Ew=eTR2/T1,

fxy=eτ2/2T2α2sinc(α22(τ2/2T2)2),

fzi=eτi/2T2[cos(α2i(τi/2T2)2)+τi/2T2sinc(α2i(τi/2T2)2)] (τi pulse duration) as described in [9]. These quantities account for the signal loss during a RF pulse caused by T2 relaxation.

The short-T2 signal can therefore be maximized with respect to α2 (given a first flip angle α1 = 90°) with ^α2=argminα2(M0Mxy(Es,Ew,fz1,fz2,fxy)).

Then, with α2 set, we take advantage of the steady-state to suppress the water signal. Using the expression of configuration states in [5], the signal to be suppressed can be written:

F+0=cos(α2/2)2F0+e2iΦsin(α2/2)2F0ieiΦsin(α2)Z0,

with F0 and Z0 being functions of α1,α2, RF-phase Φ,n=TR2/TR1,TR2,TR1,Tlong1,Tlong2 and diffusion coefficient D. In this case, having |F+0|=0 would imply a complete long-T2 suppression. Since no trivial analytical expression exists for the F0 and Z0 states in steady-state, we numerically explored the tissues and sequence parameters space in order to assess whether the diffusion effect induced by the spoiling gradients would combine the F0 and Z0 states in order to satisfy |F+0|=0. Using the EPG formalism, we have shown that this condition can be met for sets of parameters and RF-phase increment Φ0=k×360/(n+1) (kN) (fig. 2), offering a signal falling to 0 (referred hereafter to as “signal pit”).

Method

Experiments were conducted on a 7T BioSpec 70/30 USR small animal MRI system (Bruker BioSpin MRI GmbH, Ettlingen, Germany). The phantom was composed of a Lego brick (T1/T2 500/0.5 ms at 7T) immersed in a 1 mM Ni2+,2Cl- solution (D = 1.81.10-9 m²/s, T1/T2 = 550/290 ms). Sequence parameters were: repetition time = 31.07 ms (TR1/TR2 = 5/25 ms (n = 5), τ1 = 1 ms, τ2 = 70 μs), TE = 50 μs, tspoil = 2/10 ms, α1/α2 = 90°/64°, RF phase increment Φ0 = 0°, receiver bandwidth = 150 kHz, matrix size = 96x96x96, voxel dimension = 0.26 mm isotropic, number of radial lines = 28733, dummy scans = 184, acquisition time/scan = 14min52s. To confirm the diffusion effect induced by the spoiling gradients, 23 amplitude values linearly spaced from 44 to 336 mT/m were used for the latter. Scans were performed using a 86 mm diameter transmitter and a mouse surface coil for reception.

Results

Fig. 3 shows doped water and Lego brick signals evolution along Gspoil amplitude, and demonstrates the existence of a signal decrease for the long-T2 component, obviously capped to the noise level. While the long-T2 signal is being nulled, there’s no impact over the short-T2 component, consequently offering a good contrast. Fig. 4 shows slices highlighting this effect, where a positive contrast over the Lego brick is made visible.

Conclusion

We demonstrated the possibility of an efficient long-T2 signal cancellation by using a saturation-based sequence along with an appropriate damping induced by spoiling gradients in a prepared 3D-UTE imaging experiment, allowing for short-T2 components highlighting. The short scan duration seems promising for a whole brain short-T2 tissues exploration.

Acknowledgements

No acknowledgement found.

References

1. Du, J. et al., Dual inversion recovery, ultrashort echo time (DIR UTE) imaging: Creating high contrast for short-T2 species, MRM 2010; 63:447-455

2. Du, J. et al., Short T2 contrast with three-dimensional ultrashort echo time imaging, MRM 2011; 29:470-482

3. Larson, P. et al., Designing long-T2 suppression pulses for ultrashort echo time imaging, MRM 2006; 56:94-103

4. Deligianni, X. et al., Water-selective excitation of short-T2 species with binomial pulses, MRM 2014; 72:800-805

5. Weigel, M., Extended phase graphs: Dephasing, RF pulses, and echoes - pure and simple, JMRI 2015; 41:266-295

6. Weigel, M. et al., Extended phase graphs with anisotropic diffusion, JMR 2010; 205:276-285

7. Yarnykh, V. et al., Actual flip-angle imaging in the pulsed steady state: A method for rapid three-dimensional mapping of the transmitted radiofrequency field, MRM 2007; 57:192-200

8. Nehrke, K., On the steady-state properties of actual flip angle imaging (AFI), MRM 2009; 61:84-92

9. Sussman, M., Design of practical T2-selective RF excitation (TELEX) pulses, MRM 1998; 40:890-899

Figures

Pulse sequence

Simulated signal vs. spoiling gradient amplitude. Explored parameters are T2, T1, D, TR and B1, with TR2/TR1=5 and tSpoil=2/10 ms. Signal pit occurrences have a monotonic behavior with respect to the value of every parameter. As T2 is increasing, the diffusion effect prevails, making the pit position shifts less and less. T1 doesn’t have a significant impact in the tested range, unlike D as the latter controls the damping. TR also has an impact due to its involvement in the steady-state evolution through relaxation effects. B1 deviations also shift the pit, and even multiply occurrences for low gradient amplitudes.

Long/Short-T2 signals and noise evolution vs. Gspoil extracted in the ROIs displayed on Fig. 4.

Axial views from a doped water soaked Lego brick using the proposed method. Maximal contrast between long- and short-T2 components was obtained for intermediary spoiling gradient amplitudes.

Proc. Intl. Soc. Mag. Reson. Med. 25 (2017)
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