The pressure of crack-shaped cavities formed in silicon upon implantation with helium and subsequent annealing is quantitatively determined from the measurement of diffraction contrast features visible in transmission electron micrographs taken under well-defined dynamical two-beam conditions. For this purpose, simulated images, based on the elastic displacements associated with a Griffith crack, are matched to experimental micrographs, thus yielding unambiguous quantitative data on the ratio p of the cavity pressure to the silicon matrix shear modulus. Experimental results demonstrate cavity radii of some 10 nm and p values up to 0.22, which may be regarded as sufficiently high for the emission of dislocation loops from the cracks.