A diagnostic imaging device includes a probe that uses both an ultrasound transducer and frequency-domain diffuse optical imaging (FD-DOI) to assist with locating and diagnosing sub-tissue anomalies. According to one aspect, the diagnostic imaging device relates to a clip-on cap that can be utilized with existing ultrasound transducers. The diagnostic imaging device described herein can be utilized for image-guided needle biopsy to regions where prostate tissues are highly suspicious for high-grade cancer, as well as for image guided interventions, such as cryotherapy, photodynamic therapy, and brachytherapy for early-stage or localized prostate cancer.