Introduction

MRI can generate various contrasts to accentuate aspects of tissue composition, physiology, and structure. A series of RF and magnetic field gradient pulses is played out in sequence, with specific timings, to manipulate contrast and encode spatial information for image generation, often sequentially. Examples are inversion recovery or spin-echo (SE) contrast preparation, which are followed by a signal encoding/collection phase. This allows excellent contrast control but can be time-inefficient. 'Patterned Multislice Excitation' (PME) MRI^[1] is a new class of imaging methods that combines contrast generation and signal collection phases to increase efficiency. Application to the measurement of water diffusion^[2] and tissue pulsations^[3] is demonstrated in human brain at 3 T.

Theory

PME accelerates 2D imaging techniques that require more than one RF pulse per slice by changing each RF pulse's excitation pattern to not only act on the current slice, but to also manipulate magnetization of upcoming slices. The slice pattern targeted with each RF pulse, together with slice acquisition order, dictates the sequence and timing of RF pulses that each slice experiences. Each RF pulse is followed by signal acquisition, increasing time-efficiency.
Figure 1 compares PME-SE with conventional-SE MRI. The latter requires two RF pulse intervals to acquire data for one slice: an excitation pulse and subsequent refocusing pulse, followed by signal acquisition (Figure 1a). With PME-SE, each RF pulse simultaneously excites one slice and refocuses a previously excited one (Figure 1b). A gradient crusher following the RF pulse serves to dephase signal originating from outside the acquisition slice, while refocusing SE signal excited in a previous interval. Pulse shapes, flip angles or slice thicknesses can vary for pulse components. Pulse pattern and crusher gradients can be chosen to extend TE over multiple slice TR (sTR). More than two slices can be excited simultaneously, e.g. to achieve stimulated echo (STE) PME (Figure 2). PME combines a slice-select gradient with a frequency-modulated pulse similar to simultaneous multislice (SMS) MRI^[4]. PME and SMS can be combined.

Methods

PME MRI was evaluated on a 3 T Prisma (Siemens Healthineers, Erlangen, Germany) equipped with 32-channel head coil and AS82 gradient set (maximum amplitude: 80 mT⋅m⁻¹; slew rate: 200 T⋅m⁻¹⋅s⁻¹). A pulse-oximeter (MP150, Biopac, Goleta, CA, USA) provided cardiac phase for brain pulsation analysis. Scans were performed on healthy volunteers (n=19, 14 female, aged 29.4±8.8 years) under IRB-approved protocol.
PME-SE and PME-STE implementations used an EPI readout after the crusher, which also served to generate tissue displacement or diffusion contrast. Interleaved slice acquisition with alternating slice select gradient polarity for lipid suppression was used. At slice stack edge, PME pulses were adjusted to cyclically prepare slices at the other end for subsequent volume acquisitions. SE-PME RF pulses were generated by simple addition of 90° excitation and 180° refocusing sub-pulses.
Cardiac-induced vascular pulsations cause subtle brain-tissue movement, which can be measured with SE-based phase-contrast velocity-encoded MRI^[5], which PME can accelerate. PME-SE was used with: 150 ms⋅mT⋅m⁻¹ crusher gradients; 4 ms duration RF pulse; single-shot EPI; 2-mm isotropic resolution; rate-3 SENSE; 49 axial-oblique slices (along AC-PC); 25% slice gap; 63-ms TE; 40-ms sTR; 1960-ms volume TR (vTR); 200 repetitions; separate experiments for 3 velocity encoding directions.
Using rate-2 SENSE and otherwise similar parameters^[1], PME-SE and PME-STE (TM=5 sTR, Figure 2) were compared to conventional-SE for diffusion imaging with twelve (2.0 and 2.5 mm isotropic resolution) or six (3 mm isotropic resolution) diffusion directions, and two b-values (b~1100 and b<25 s⋅mm⁻²). For PME-STE, crusher orientation was modulated over sTR to eliminate SE signal. For PME-SE, crusher varied over vTR. TE for PME-SE/PME-STE/conventional-SE was 66.5/50.4/59.8 (3-mm) and 84.0/68.3/70.2 (2-mm) ms; vTR was 1800/1376/3800 (3-mm) and 2444/2068/4900 (2-mm) ms. See Table 1 in ^[1] for additional details.

Results

Sample results for tissue displacement and water diffusion imaging are shown in Figures 3 and 4&5, respectively. vTR was reduced by 53/52/50% (PME-SE) and 64/61/58% (PME-STE) compared to conventional-SE at 3.0/2.5/2.0-mm resolution. Compared to conventional-SE, relative temporal SNR per unit time at 3.0/2.5/2.0-mm resolution was, for PME-SE: 1.25±0.02/1.16±0.05/1.13±0.03 (@low-b) and 1.38±0.08/1.26±0.08/1.18±0.02 (@high-b); for PME-STE: 0.79±0.03/0.86±0.02/0.82±0.01 (@low-b) and 0.81±0.04/1.00±0.06/0.95±0.02 (@high-b). PME led to increased SAR (due to scan time reduction) and RF pulse amplitude.

Discussion & Conclusion

PME allowed 2-3 fold accelerated SE and STE multi-slice MRI. As with SMS, PME acceleration increases peak RF amplitude and power deposition approximately linearly with acceleration factor. At field strengths of 3 T and above, this may limit the ultimately achievable acceleration.

Acknowledgements

This work was supported by the intramural research program of the National Institute of Neurological Disorders and Stroke, National Institutes of Health.