Magnetic resonance microscopy approaches to molecular imaging: Sensitivity vs. specificity

Magnetic resonance imaging (MRI) has become a staple of diagnostic radiology. Despite its diagnostic utility the resolving power of typical clinical MRI instruments is only on the order of 1 mm. This has led to the development of magnetic resonance microscopy (MRM), which employs the same physical imaging principals used in MRI, but with instrumentation designed to resolve structural details down to the level of 10-100 microns in samples ranging from less than I mm to several centimeters in size. Until recently, major advancements in MRM have focused on hardware and software developments allowing the detection of radio-frequency signals originating from very small volume elements within the sample. Such high-resolution images have facilitated the early detection of diseased tissue by focusing on submillimeter structural changes induced in the tissue. To sensitize the MRM technique to pathologic tissue changes, investigators have developed techniques, such as chemical shift imaging to detect pre-cancerous changes in tissue metabolism and MR relaxometry to detect changes in tissue composition during the earliest stages of degeneration for diseases such as osteoarthritis or multiple sclerosis. However, such non-specific measurements can only serve as surrogate measures of disease progression and potential measures of treatment efficacy. As disease diagnosis moves from the anatomic to the molecular stage, scientists will require imaging techniques that can detect molecular events deep inside the human body. To meet this goal, MR scientists are working to improve imaging resolutions in vivo and they are developing molecular probes that can dramatically amplify the MR signal in response to specific and highly localized molecular events. This article will identify current trends in the MRM field aimed at meeting the challenges imposed by molecular imaging and areas for future development in this highly promising imaging field.