The present invention is directed to a device for visualizing structure located on the interior of a biological substance. The device of the present invention includes a marker member which may be a solid cylinder or lumen having an interior volume. The marker member has a proximal end, and a distal end. The distal end of the device is removably insertable in the biological substance relative to the interior structure to be visualized and is composed of a biologically stable substrate material. An image-enhancing material is contained relative to the marker member in a manner such that the imaging material does not directly contact the biological substance; such imaging material will remain self-contained without causing significant tissue absorption, toxicity or undesired image distortion. The imaging material of choice is one which is capable of producing an emission or signal detectable external to the biological substance by suitable imaging instrumentation. Also disclosed is a method for visualizing critical structures or radiation therapy targets employing the device of the present invention, particularly in imaging processes such as positron emission tomography and/or single photon emission computerized tomography, MRI or ultrasound either used alone or in combination with anatomical imaging processes such as computerized tomography or mammography.