Minimum-Time VERSE Pulse Correction for Slice Selectivity Improvement in 2D-UTE Imaging

Lucas Soustelle¹, Paulo Loureiro de Sousa¹, Sascha Koehler², Chrystelle Po¹, François Rousseau³, and Jean-Paul Armspach¹

¹Université de Strasbourg, CNRS, ICube, FMTS, Strasbourg, France, ²Bruker BioSpin MRI, Ettlingen, Germany, ³Institut Mines Télécom, Télécome Bretagne, INSERM LaTIM, Brest, France

Synopsis

The Variable-rate Selective Excitation (VERSE) approach allows to achieve very short echo time in 2D-UTE sequences when applied on a selective half-pulse and its paired slice selection gradient. Unfortunately, the latter may suffer from non-linearities and eddy current effects, all the more important on preclinical scanners equipped with strong gradient systems.
An efficient method was implemented on a 7T preclinical scanner to measure the real slice selection gradient profile. A reshaping of the corresponding pulse was made, improving the slice selectivity.

Purpose

The Variable-rate Selective Excitation (VERSE) algorithm is a valuable selective RF pulse reshaping technique¹ to assess ultra-short T₂ content (T₂ < 1 ms) in 2D-UTE sequence² since it allows very short TE (when used in pair with a half-pulse), typically capped to the delay for switching the system from a transmitting to a receiving state. Its design has been improved using a Time-Optimal approach³, offering a simple way to shape a pair of RF pulse and slice selection gradient to obtain a Minimum-Time VERSE⁴ (MT-VERSE) or a Delay-Insensitive VERSE⁵ design. Due to relaxation during excitation, the observed ultra-short T₂ components suffer from an increase in width of their slice profiles as well as an attenuation of their nominal amplitudes if the RF duration is greater or comparable to the acquired T₂⁶. To this end, we use MT-VERSE to shape a short duration selective RF pulse. However, the slice selection gradient may suffer from significant non-linearities and eddy current effects, especially on preclinical systems. Thus, the real gradient shape may present a delay during its ramp down, leading consecutively to a non-respect of the ordered weighting function¹, and degrading the desired slice profile.

We propose here an efficient method for RF pulse reshaping, in the sense of MT-VERSE, which takes into account gradient imperfections while taking advantage of the scanner's limit performances in a preclinical context.

Material & Methods

Figure 1 shows the pipeline used for correction purpose. An initial pair {B₁(t); G(t)} is MT-VERSEd and implemented on the scanner's system. Using a built-in trajectory measurement method⁷, the real gradient profile is calculated, with respect to the pulse and geometry constraints. This yields an efficient correction, since the behavior of the ramp down may vary, e.g. with the maximum amplitude of the requested slice selection gradient. Then, a reshaping of the original MT-VERSEd pulse is done by taking into account the real slice selection gradient waveform undergoing distortions and delays from non-linearities and eddy current effects.

Two phantoms were used to assess the improvement of slice selection after correction: one composed of a piece of eraser (T₂≈ 520 μs at 7T) surrounded by a 1.5% agar gel (T₂= 100 ms at 7T; Sigma-Aldrich), and one plastic (T₂ ≈ 500 μs at 7T; LEGO) surrounded by doped agar with paramagnetic Ni2+ agent.

Experiments were conducted on a 7T BioSpec 70/30 USR small animal MRI system (Bruker BioSpin MRI GmbH, Ettlingen, Germany) equipped with a gradient system providing a maximum amplitude of 442 mT/m and a peak slew rate of 3440 T/m/s. We used a volume transmit/receive resonator of 7.2 cm in diameter, providing a maximum B₁ amplitude of 150 μT. The 2D-UTE Bruker sequence was modified to include a VERSE excitation. A SLR half-pulse was used in the experiments (Time Bandwidth Product = 3, t_RF = 128 μs when MT-VERSEd, Flip Angle = 30°). The followed parameters were used: matrix = 128x128, FOV = 40x40 mm², Slice Thickness = 1 mm (number of radial projections = 402), with TR/TE = 500/0.05 ms and receiver Bandwidth = 100 kHz.

Results

Figure 2 shows the difference between requested and measured slice selection gradient shapes, which emphasizes the need of a correction (delay between respective approximately null amplitude reach Δt ≈ 50 μs). Figure 3 shows slice profiles before and after correction. Figure 4 shows slices before and after correction, and presents the decrease of out-of-slice contamination when using the correction.

Discussion

The use of MT-VERSE allows to excite a broader bandwidth compared to classical VERSE (consisting in a reshaping of B₁ to take into account the ramp down of the slice selection gradient), which represents an advantage when dealing with ultra-short T₂ components, especially at high fields. Preliminary results seem promising and demonstrate the feasibility of using a short selective RF pulse while ensuring good slice selectivity in the presence of a non-ideal and strong slice selection gradient, and offers perspectives for ultra-short T₂ excitation, such as lipidic myelin content assessment using a proper long-T₂ suppression scheme. Nevertheless, the method imposes an extra step (i.e. measuring G’^v(t) and reshaping B₁^v(t)), and relies strongly on system performances.

Conclusion

In this work, an efficient pipeline was implemented for MT-VERSE improvement in preclinical scanner use. The use of the measured slice selection gradient, instead of the theoretical one, highly improves the slice selectivity.

Acknowledgements

The authors thanks Geneviève Guillot and Daniel Gounot for useful discussions.

References

1. Conolly, S. et al., JMR 1988; 78:440-458
2. Pauly, J. et al., SMRM 1989; 28
3. Lee, D. et al., MRM 2009; 61:1471-1479
4. Hargreaves, B. et al., MRM 2004; 52:590-597
5. Kerr, A. et al., ISMRM 2015; 921
6. Robson, M. et al., MRM 2010; 64:610-615
7. Duyn, J. et al., JMR 1998; 132:150-153

Figures

Figure 1: Pipeline for VERSE & correction method. The gradient system response measurement is essential to correct exactly the MT-VERSE pulse, resulting in a stretching of the previously calculated one.

Figure 2: Theoretical vs. Measured slice selection gradient profile (top) & Original vs. Corrected SLR pulse (bottom). To overcome the undesirable effects of the real gradient profile, the original pulse has to be stretched. Thresholded to 2% of the measured gradient’s maximum value, the delay between the measured and the requested gradient is about 50 μs.

Figure 3: Slice selectivities of an uncorrected MT-VERSE excitation (left) vs. corrected (right) of the eraser phantom. An asymmetry appears on the uncorrected profile, as well as close out-of-slice signal. The darker signal at the bottom of both images corresponds to the piece of eraser (ultra-short T₂). Note its dephased state on the uncorrected slice profile (induced by the remaining tail of the real gradient profile after the end of the original excitation).

Figure 4: Slices of the plastic phantom (LEGO - left: uncorrected; right: corrected). Note the slice selectivity improvement with the highly reduced out-of-slice contamination on the corrected slice. Furthermore, the homogeneous short T₂ source is well excited thanks to the broad bandwidth of the used pulses (BW > 16 kHz).

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)

1888