Purpose: To optimize contrast to noise ratio (CNR) in magnetic resonance imaging (MRI) of prostate cancer using at 3T. Methods: CNR was expressed as a difference in MR signals of two samples. Amulti-echo spin-echo (MESE) pulse sequence was used. The theoretical value of the maximum CNR was obtained using the derivative of CNR with echo time (TE) as a variable. The T₁ relaxation time was ignored as repetition time (TR) was assumed to be very long (TR >> T₁). The theoretical calculations were confirmed with in vitro and in vivo experiments. For in vitro experiments we used samples with different T₂ values using various concentrations of super paramagnetic iron oxide (SPIO) and for in vivo experiments we used an animal model of prostate cancer. Results: CNR was maximized by selecting the optimum TE for a multi-echo spin-echo (MESE) pulse sequence based on theoretical predictions. MR images of prostate cancer at 3T were obtained and showed maximum CNR at the predicted TE. Conclusions: It was possible to maximize CNR of prostate tumour by selecting the optimal TE based on simple theoretical calculations. The proposed method can be applied to other pulse sequences and tissues. It can be applied to any MRI system at any magnetic field. However the method requires knowledge of T₂ relaxation times.