Susceptibility-weighted imaging (SWI) is a novel neuroimaging technique that utilizes tissue magnetic susceptibility differences to generate unique contrast, distinct from spin density, T1, T2, and T2*. This review, the first of two parts, provides an overview of the technical background of SWI. It discusses the concept of gradient-echo imaging and how local changes in susceptibility can be measured. The steps to transform original magnitude and phase images into SWI data are introduced, along with the use of SWI-filtered phase to visualize and potentially quantify iron in the brain. The article also offers advice on interpreting SWI data and presents recommended sequence parameters for different field strengths. SWI has shown promise in enhancing contrast in MR imaging, particularly in detecting hidden information in phase data. The review concludes by discussing the current technology, post-processing methods, and future research directions in SWI.Susceptibility-weighted imaging (SWI) is a novel neuroimaging technique that utilizes tissue magnetic susceptibility differences to generate unique contrast, distinct from spin density, T1, T2, and T2*. This review, the first of two parts, provides an overview of the technical background of SWI. It discusses the concept of gradient-echo imaging and how local changes in susceptibility can be measured. The steps to transform original magnitude and phase images into SWI data are introduced, along with the use of SWI-filtered phase to visualize and potentially quantify iron in the brain. The article also offers advice on interpreting SWI data and presents recommended sequence parameters for different field strengths. SWI has shown promise in enhancing contrast in MR imaging, particularly in detecting hidden information in phase data. The review concludes by discussing the current technology, post-processing methods, and future research directions in SWI.