Variable Refocusing Flip Angle Single-Shot Imaging for Sedation-Free Fast Brain MRI

BACKGROUND AND PURPOSE

Conventional single-shot FSE commonly used for fast MRI may be suboptimal for brain evaluation due to poor image contrast, SNR, or image blurring. We investigated the clinical performance of variable refocusing flip angle single-shot FSE, a variation of single-shot FSE with lower radiofrequency energy deposition and potentially faster acquisition time, as an alternative approach to fast brain MR imaging.

MATERIALS AND METHODS

We retrospectively compared half-Fourier single-shot FSE with half- and full-Fourier variable refocusing flip angle single-shot FSE in 30 children. Three readers reviewed images for motion artifacts, image sharpness at the brain-fluid interface, and image sharpness/tissue contrast at gray-white differentiation on a modified 5-point Likert scale. Two readers also evaluated full-Fourier variable refocusing flip angle single-shot FSE against T2-FSE for brain lesion detectability in 38 children.

RESULTS

Variable refocusing flip angle single-shot FSE sequences showed more motion artifacts (P < .001). Variable refocusing flip angle single-shot FSE sequences scored higher regarding image sharpness at brain-fluid interfaces (P < .001) and gray-white differentiation (P < .001). Acquisition times for half- and full-Fourier variable refocusing flip angle single-shot FSE were faster than for single-shot FSE (P < .001) with a 53% and 47% reduction, respectively. Intermodality agreement between full-Fourier variable refocusing flip angle single-shot FSE and T2-FSE findings was near-perfect (κ = 0.90, κ = 0.95), with an 8% discordance rate for ground truth lesion detection.

CONCLUSIONS

Variable refocusing flip angle single-shot FSE achieved 2× faster scan times than single-shot FSE with improved image sharpness at brain-fluid interfaces and gray-white differentiation. Such improvements are likely attributed to a combination of improved contrast, spatial resolution, SNR, and reduced T2-decay associated with blurring. While variable refocusing flip angle single-shot FSE may be a useful alternative to single-shot FSE and, potentially, T2-FSE when faster scan times are desired, motion artifacts were more common in variable refocusing flip angle single-shot FSE, and, thus, they remain an important consideration before clinical implementation.

Characterization and reduction of the transient response in steady-state MR imaging

Refocused steady-state free precession (SSFP) imaging sequences have recently regained popularity as faster gradient hardware has allowed shorter repetition times, thereby reducing SSFP's sensitivity to off-resonance effects. Although these sequences offer fast scanning with good signal-to-noise efficiency, the "transient response," or time taken to reach a steady-state, can be long compared with the total imaging time, particularly when using 2D sequences. This results in lost imaging time and has made SSFP difficult to use for real-time and cardiac-gated applications. A linear-systems analysis of the steady-state and transient response for general periodic sequences is shown. The analysis is applied to refocused-SSFP sequences to generate a two-stage method of "catalyzing," or speeding up the progression to steady-state by first scaling, then directing the magnetization. This catalyzing method is compared with previous methods in simulations and experimentally. Although the second stage of the method exhibits some sensitivity to B(1) variations, our results show that the transient time can be significantly reduced, allowing imaging in a shorter total scan time. Magn Reson Med 46:149-158, 2001.

Linear combination steady-state free precession MRI

A new, fast, spectrally selective steady-state free precession (SSFP) imaging method is presented. Combining k-space data from SSFP sequences with certain phase schedules of radiofrequency excitation pulses permits manipulation of the spectral selectivity of the image. For example, lipid and water can be resolved. The contrast of each image depends on both T1 and T2, and the relative contribution of the two relaxation mechanisms to image contrast can be controlled by adjusting the flip angle. Several potential applications of the technique, referred to as linear combination steady-state free precession (LCSSFP), are demonstrated: fast musculoskeletal, abdominal, angiographic, and brain imaging.

Fluctuating equilibrium MRI

A new fast, spectrally selective imaging method called fluctuating equilibrium magnetic resonance is presented. With all gradients refocused over a repetition interval, certain phase schedules of radiofrequency excitation pulses produce an equilibrium magnetization that fluctuates from excitation to excitation, thus permitting simultaneous acquisition of several images with different contrast features. For example, lipid and water images can be rapidly acquired. The effective echo time can be adjusted using the flip angle, thus providing control over the T(2) contribution to the contrast. Several applications of the technique are presented, including fast musculoskeletal, abdominal, breast, and brain imaging, in addition to MR angiography. A technique for combining lipid and water images generated with this sequence for angiography is described and other potential applications are suggested. Magn Reson Med 42:876-883, 1999.

Prospective MR signal-based cardiac triggering

A cardiac motion compensation method using magnetic resonance signal-based triggering is presented. The method interlaces a triggering pulse sequence with an imaging sequence. The triggering sequence is designed to measure aortic blood velocity, from which cardiac phase can be inferred. The triggering sequence is executed repeatedly and the acquired data processed after each sequence iteration. When the desired phase of the cardiac cycle is detected, data are acquired using the imaging sequence. A signal-processing unit of a conventional scanner is used to process the triggering data in real time and issue triggering commands. Alternatively, a workstation, with a bus adaptor, can access data as they are acquired, process and display the data, and issue triggering commands. With a graphical user interface, the triggering pulse sequence and data-processing techniques can be modified instantaneously to optimize triggering. The technique is demonstrated with coronary artery imaging using both conventional two-dimensional Fourier transform scans and spiral trajectories.