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 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 (SAR) 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 (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 Resonance
of the Knee, which exploits the phenomenon of the Pseudo-Steady-State (PSS), leading to a noticeable SAR reduction
without loss of SNR and also providing excellent contrast resolution.