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(TSE) or Fast Spin Echo (FSE) are the most commonly used sequences in Magnetic Resonance as they provide anotable 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 witha complete recovery of the Longitudinal Magnetization. Finally, the multiple 180 ° pulses of these sequences reducethe inhomogeneities of the Magnetic Field by minimizing the phenomena of magnetic susceptibility. However, theirapplication, especially in scanners with a high intensity of static magnetic field B0 (1.5T and/or 3T) is prevented bythe deposition of RF due to the long echo trains, which sometimes involve exceeding the limits of the specific absorption rate (SAR) for patient safety. Over time, a common solution to the SAR problem has been the use of refocusingangles smaller than 180 ° (160 °-140 °-120 °), which lead to its reduction at the cost, however, of an obvious penaltyin terms of signal-to-noise ratio (SNR). 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 Resonanceof the Knee, which exploits the phenomenon of the Pseudo-Steady-State (PSS), leading to a noticeable SAR reductionwithout loss of SNR and also providing excellent contrast resolution.