We designed and tested a unique triple-modality nanoparticle that is, to our knowledge, the first to allow combined MRI, Photoacoustic and Raman imaging. The MPRs described here could enable radiologists and neurosurgeons to “see” the same probe before and during surgery, thus allowing more accurate brain tumor resection by exploiting the complementary strengths of each modality.
The excellent MRI detectability of the MPRs in the picomolar range is a direct result of their very high longitudinal relaxivity of 3.0 × 106 mM–1 s–1. To our knowledge, this represents the highest relaxivity of a nanoparticle reported to date.
The second modality, Photoacoustic imaging, is a relatively new technique that allows deeper tissues to be imaged with higher spatial resolution compared to most optical techniques25–27
. The exceptionally high optical absorbance coefficient of the MPRs is over 200-fold higher than, for example, previously reported Photoacoustic imaging agents based on carbon nanotubes28,29
. In conjunction with its three-dimensional capabilities, Photoacoustic imaging could guide the more gross resection steps and even identify tumor tissue residing under the surface of normal brain tissue. Then, to completely remove microscopic tumor deposits, Raman imaging with its superior sensitivity could be employed.
Raman spectroscopy in conjunction with MPRs offers ultrahigh sensitivity in the picomolar range as opposed to the nanomolar sensitivity achievable with fluorescence imaging of quantum dots13,17,18,20
. Raman imaging of MPRs, in contrast to other optical imaging techniques, does not suffer from autofluorescence or background signal because the MPR spectral signature is highly amplified and unique (“fingerprint”). While the main limitation of Raman imaging is its limited penetration depth, tumor visualization was achieved in our study through the intact skin and skull in live mice (depth of 2–5 mm). This result is a combination of the design of the nanoparticle with its gold core producing a surface plasmon resonance for Raman signal enhancement; the Raman substrate used; and the number of nanoparticles accumulating within the tumor. Raman nanoparticles are inherently insensitive to photodestruction, which represents a known problem of organic fluorochromes. Furthermore, unlike most quantum dots, which are cytotoxic30,31
, MPR nanoparticles are based on inert gold and silica and thus may have a better chance for clinical translation. Gold and gold-silica nanoparticles have excellent cytotoxicity profiles, as illustrated by detailed toxicity studies in animals32–34
and several clinical trials21
. The design of the MPRs would also allow for multiplexing20
with the potential to detect multiple biomarkers simultaneously in vivo
In addition, MPRs have a unique advantage over conventional low molecular weight contrast agents. For example, low molecular weight Gd-chelates or fluorochromes accumulate in the extracellular space, where blood-brain-barrier breakdown has occurred, and then undergo both rapid diffusion through the interstitium and renal clearance. These low molecular weight agents are therefore unable to delineate tumors for the time period spanning the resection procedure, let alone for the entire period between pre-operative planning and surgical intervention. This diffusion process also introduces imprecision of probe localization, requires repeated contrast administration (e.g. Gd-chelates during intra-operative MRI), and can cause false positive results due to surgically induced contrast enhancement. In contrast, the in vivo
kinetic studies performed with the MPRs here demonstrate that the particle is being retained in the tumor, allowing repeated imaging as required without the need for repeated injection. This contrast agent behavior may also be useful for distinguishing tumor recurrence from non-specific treatment-related effects. As the MPR approach relies on the EPR effect, it could potentially be applied to image other cancer types with intrinsic EPR effect including: lung cancer, melanoma, renal cancer, hepatoma, and many others 35
. Finally, the long intratumoral retention of the MPRs could also be exploited for drug delivery or photothermal therapy.
Novel instrumentation, including endoscopic and intra-operative Photoacoustic and Raman imaging devices required for clinical translation of the MPR approach, are currently under development36,37
. Ideally, a combination of both devices integrated in one handheld probe would be desirable in the operating room. In particular, such endoscopes should be designed for easy intra-operative navigation and enable real-time imaging. Further development of instrumentation could lead to improved brain tumor surgery and patient outcome in the future.
For additional discussion, please refer to the Supplementary Discussion section in the Supplementary Information