A soft X-ray imaging system consisting of three arrays of silicon surface barrier diodes is applied to the tomographic analysis of internal plasma perturbations in the T-l0 tokamak (R0=1.5 m, a=0.32 m). It is found that density limit disruptions in a plasma with a high safety factor at the edge (qa=3.5-4.5) are associated with joint rotation of the m=2, n=1 and m=1, n=1 modes, overlapping at the energy quench stage. Electron cyclotron resonance heating is used to prevent density limit disruptions or to recover stable operation of the discharge after the energy quench at the density limit