Refocus flip angle modulation on the pd tse sequences in the magnetic resonance imaging of the knee, for the evaluation of meniscal injuries

Abstract

The Radiofrequency Refocused Echo Spin-Echo Echo-Train Rapid-Acquisition sequences, known as Turbo Spin Echo or Fast Spin Echo are the most commonly used sequences in Magnetic Resonance as they provide a notable contribution in morphological and anatomical terms, thanks to their high spatial and contrast resolution. Furthermore, they allow a higher signal noise ratio than the other families of sequences thanks to the numerous refocusing pulses and thanks to a TR so long as to allow sampling as many K-space phase encodings as possible and with a complete recovery of the Longitudinal Magnetization. Finally, the multiple 180 ° pulses of these sequences reduce the inhomogeneities of the Magnetic Field by minimizing the phenomena of magnetic susceptibility. However, their application, especially in scanners with a high intensity of static magnetic field B0(1.5T and/or 3T)is prevented by the deposition of RF due to the long echo trains, which sometimes involve exceeding the limits of the specific absorption rate for patient safety. Over time, a common solution to the SAR problem has been the use of refocusing angles smaller than 180°(160°-140°-120°),which lead to its reduction at the cost, however, of an obvious penalty in terms of signal-to-noise ratio. In this study we present a modulation method of the Refocus Flip-Angle applied to the DP-TSE sequences in the evaluation of meniscal lesions in the study protocol of the Magnetic Resonance of the Knee, which exploits the phenomenon of the Pseudo-Steady-State(PSS), leading to a noticeable SARreduction without loss of SNR and also providing excellent contrast resolution