A system for performing spectral analysis and determining a safety marker that is representative of a state of destabilisation of microbubbles contained in a region of a soft vascularised biological tissue, said microbubbles being subjected to an ultrasonic excitation signal at a predetermined emission frequency f 0 to induce localised and reversible opening of the biological barriers in said region, and said state of destabilisation of the microbubbles being detrimental to the biological tissue, said ultrasonic excitation signal being formed by an ultrasonic sequence composed of a predetermined integer number Nb, higher than or equal to 1, of wave train(s), called "shot(s)"; and the system for performing spectral analysis and determining a safety marker being characterised in that it is configured to, after each shot Bb is triggered, b being between 1 and Nb: - receive a response signal detected by a passive cavitation detector having a predetermined detection passband, - regularly measure (106, 108), during the shot Bb, at a series of times ta, the temporal variation in the spectral lines corresponding to the subharmonic and ultra-harmonic frequencies of the received acoustic-response signal of the microbubbles, and - determine (110, 112), by quantifying it, the temporal variation, over the times ta, of a safety marker that is defined, at each time ta, by a number MDD a equal to the ratio of the sum of the surface areas of the spectral lines, measured at the time ta and corresponding to the subharmonic and/or ultra-harmonic frequencies of the received acoustic-response signal of the microbubbles, to the sum of the surface areas of the spectral lines, measured at the first time t1 and corresponding to the subharmonic and/or ultra-harmonic frequencies of the acoustic-response signal of the microbubbles in their initial state. said ultrasonic excitation signal being formed by an ultrasonic sequence composed of a predetermined integer number Nb, higher than or equal to 1, of wave train(s), called "shot(s)"; and the system for performing spectral analysis and determining a safety marker being characterised in that it is configured to, after each shot Bb is triggered, b being between 1 and Nb: - receive a response signal detected by a passive cavitation detector having a predetermined detection passband, - regularly measure (106, 108), during the shot Bb, at a series of times ta, the temporal variation in the spectral lines corresponding to the subharmonic and ultra-harmonic frequencies of the received acoustic-response signal of the microbubbles, and - determine (110, 112), by quantifying it, the temporal variation, over the times ta, of a safety marker that is defined, at each time ta, by a number MDD a equal to the ratio of the sum of the surface areas of the spectral lines, measured at the time ta and corresponding to the subharmonic and/or ultra-harmonic frequencies of the received acoustic-response signal of the microbubbles, to the sum of the surface areas of the spectral lines, measured at the first time t1 and corresponding to the subharmonic and/or ultra-harmonic frequencies of the acoustic-response signal of the microbubbles in their initial state. - receive a response signal detected by a passive cavitation detector having a predetermined detection passband, - regularly measure (106, 108), during the shot Bb, at a series of times ta, the temporal variation in the spectral lines corresponding to the subharmonic and ultra-harmonic frequencies of the received acoustic-response signal of the microbubbles, and - determine (110, 112), by quantifying it, the temporal variation, over the times ta, of a safety marker that is defined, at each time ta, by a number MDD a equal to the ratio of the sum of the surface areas of the spectral lines, measured at the time ta and corresponding to the subharmonic and/or ultra-harmonic frequencies of the received acoustic-response signal of the microbubbles, to the sum of the surface areas of the spectral lines, measured at the first time t1 and corresponding to the subharmonic and/or ultra-harmonic frequencies of the acoustic-response signal of the microbubbles in their initial state.