1. A method of estimating an unknown image from a plurality of cone-beam computed tomography (CBCT) image projections, the method comprising: estimating the unknown image by solving for a minima of a function comprising a fidelity term that is based on the plurality of image projections and a regularization term that is responsive to a sparsity of the CBCT image projections; wherein the step of solving comprises: iteratively estimating the minima of the function by: calculating a gradient of the function; determining a step size based on a Barzilai-Borwein (BB) formulation; and adjusting the minima estimate using the gradient and the step size.