Handheld-probe-based optical imagers are a popular approach toward breast imaging because of their potential portability and maximum patient comfort. A novel handheld-probe-based optical imager has been developed and its feasibility for three-dimensional fluorescence tomographic imaging demonstrated. Extensive tomography studies were performed on large slab phantoms ( $650\mathrm{ml}$ ) to assess the performance limits of the handheld imager. Experiments were performed by using different target volumes ( $0.1–0.45{\mathrm{cm}}^3$ ), target depths ( $1–3\mathrm{cm}$ ), and fluorescence (Indocyanine Green) absorption contrast ratios in a nonfluorescing ( $1:0$ ) and constant fluorescing backgrounds ( $1000:1$ to $5:1$ ). The estimated sensitivity and specificity of the handheld imager are 43% and 95%, respectively.