We investigated a high sensitive collimator (registered collimator) design and associated image reconstruction methods. Its opening as designed to align with the CZT pixel by having same pitch and shape. By the combination of wide opening holes and low septa height, thesensitivity of the collimator can increase dramatically. However, it may also lead to loss of system resolution. The goal of this study is 1) to determine the optimal parameters of the collimator which can increase sensitivity and decrease the charge sharing effect, and 2) to develop system matrix for iterative image reconstruction that can adequately model the proposed collimation scheme to achieve high resolution recovery in spite of using wider collimation holes. Thickness and height of septa in the registered collimator were analytically determined to 1 mm which blocks 97% of 140keV gamma rays and 5.48 mm which indicates 0.2 % efficiency that equivalently means about 10 times higher counts than low energy high resolution(LEHR) collimator. Then 20 fan beam-based geometrical response function(GRF) was developed to model the wide opening holes in the registered collimator. The GRF was included in the system matrix of ordered subset expectation maximization(OSEM) method to compensate the loss of resolution. For performance comparison, GATE simulation was performed using a 5 mm diameter hot sphere phantom, which is located at 1 cm off-centered radial position, and a 40.36 by 40.36 mm CZT detector module. The preliminary results showed that the proposed collimator achieved about 13 times more counts and improved image resolution. In the future, we will keep our experiments with30 cone beam model as well as 20 fan beam model using more realistic phantoms.