Synopsis

Cardiac balanced SSFP using simultaneous multi-slice acquisition is limited by RF pulse duration and SAR. The interplay of these constraints means that the minimum pulse duration does not usually yield the minimum TR. We show that multiband RF pulses designed using VERSE to directly minimize TR can yield a reduced TR. The result is demonstrated in a multiband-2 sequence on a 3T clinical system where the TR is only 15% longer than for single-slice excitation and the time per slice is reduced by 44%.

Purpose

Cardiac bSSFP sequences are widely used for high SNR and pronounced blood-myocardial contrast. This imaging technique is often hampered by long uncomfortable breath-holds and banding artefacts due to $$$B_0$$$ inhomogeneity, both of which are reduced with shorter TR.

Minimum-duration RF pulses can in fact result in a longer TR due to SAR restrictions. Minimum-TR RF pulse design reformulates the pulse design problem to consider minimum-TR directly¹. Furthermore, this technique makes use of VERSE² to design time-variable gradients, which can lead to slice-profile distortions due to the finite temporal bandwidth of gradient systems³. This effect includes eddy currents, and can be accurately modelled by a Gradient Impulse Response Function (GIRF)⁴.

In this work we demonstrate how minimum-TR RF pulse design can be used in a multiband cardiac bSSFP acquisition^5–7, with the additional use of GIRF-corrected VERSE RF pulses⁸ to overcome slice-profile distortions due imperfect gradient systems³.

Methods

The minimum-TR framework¹ designs RF pulses that minimise a sequence TR using two constraints. Firstly, an SAR constraint is given by

$$ TR_{min} = E/SAR_{lim} \times \alpha$$

where $$$ E_{RF} $$$ is RF pulse energy, $$$SAR_{lim}$$$ is a SAR limit and $$$ \alpha $$$ is a conversion factor in $$$ W/kg/\mu T^2 $$$ determined from a SAR model. Secondly, time-constraints either on the total time to perform all sequence elements, or a duty-cycle limit, lead to

$$ TR_{min} = max(T_{enc} + T_{pulse}, T_{pulse}/\delta ) $$

where $$$T_{encoding}$$$ is the encoding time, $$$T_{pulse}$$$ is the pulse duration and $$$ \delta $$$ is an RF duty cycle limit. Figure 2b shows these two constraints for conventional and VERSE pulses for $$$SAR_{lim}$$$=20W/kg, encoding time $$$ T_{encoding} $$$= 1.786ms and $$$ \delta =50\%$$$. The minimum TR is found at the intersection of the curves defined by these equations. Use of VERSE to reshape the RF pulse changes the shape and alters the SAR constraint, resulting in a different optimum.

Time-variable selection gradients also lead to fidelity issues due to limited temporal bandwidth of gradient systems, which can be modelled and potentially corrected for by using a Gradient Impulse Response Function (GIRF)^3,4,8. Our measured system GIRF³ roughly resembles a Lorentzian and so we approximated the GIRF using a simple Lorentzian model$$ H(t) = \frac{1}{\tau}e^{-t/\tau} $$where $$$\tau=42\mu s$$$ was determined empirically.

Time-variable selection gradients also require time-variable modulation for carrier for off-center slice excitation. For a slice-offset $$$ \Delta x $$$ the phase modulation is found by$$e^{i \theta(t)} = e^{i \gamma \Delta x \int_t^TG(s)ds}$$

For improved results, the gradient term was also found using the GIRF.

Experiments

Vendor RF pulses with time-bandwidth product 2.13 were scaled to flip angles 25 to 90 degrees. Time-optimal VERSE² was applied to create VERSE singleband pulses. VERSE multiband (vMB) 2-3 pulses were created by applying a time-dilated multiband modulation function to the VERSE SB pulses³.Slice profiles were measured in a gadolinium-doped phantom using a 2D gradient-echo sequence, on a clinical Philips 3T Achieva system. On the same system, multiband 2 cardiac bSSFP images were acquired in a single healthy volunteer using blipped-SSFP^5,9. MB2 reconstruction used a SENSE-based algorithm in ReconFrame (Gyrotools, GmbH, Zurich). To overcome a scanner software issue SAR limit was set to 18.6 W/kg for phantom work.

Results & Discussion

Figure 3a shows the minimised TR for different flip-angles. The TR can always be reduced by using time-variable selection gradients, for multiband factors 1-3. Figure 3b shows the same results, expressed as time spent per slice, which directly relates to scan time reduction. When accelerating a singleband acquisition with conventional multiband 2, an average of 36% of reduction in time per slice can be achieved. However this acceleration comes at an average increase of 29% in TR, making breath-holding more challenging and increasing vulnerability to artefacts. A VERSE MB2 implementation reduces the time-per-slice by 43% with only an average increase of 15% in TR, improving banding artefacts.

Figure 4 shows off-center slice-profile measurements using MB2 and vMB2 pulses, without and with GIRF correction. GIRF-corrected RF pulses result in reduced slice sidelobes and were used for in-vivo imaging.

Figure 5 shows in-vivo images of cardiac bSSFP multiband acquisition with and without the minimum TR framework. A minimum-TR RF pulse design approach results in the optimal reduction in TR, reducing banding artefacts and resulting in shorter breath-holds.

Conclusion

Acknowledgements

References

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