Lymph node staging using dedicated magnetic resonance contrast agents--the accumulation mechanism revisited

When diagnosing cancer, assessing the nodal stage is tremendously important in determining the patient's prognosis. Computed tomography (CT) and magnetic resonance (MR) imaging (MRI) assessments of the regional lymph node (LN) size and shape are currently used for the initial nodal staging in clinical settings, although this approach has a rather low sensitivity, and biopsy often leads to restaging of the LNs. Acknowledging the great medical need to accurately stage LNs, scientists and clinicians have been working since the late 1980s on MR contrast agents that provide more reliable staging results. Different types of molecules (i.e., iron oxide nanoparticles and Gd-based contrast agent) have shown promising LN accumulation and imaging results, but no clinically approved, dedicated LN staging contrast agent is currently available. The literature describes a mechanism of contrast agent accumulation in the LNs that considers some but not all published experimental evidence. However, confidence in the mechanism of LN accumulation is a prerequisite for the directed synthesis of compounds for accurate and sensitive LN staging. To improve our understanding of the LN contrast agent accumulation mechanism, we reviewed the published data on the enrichment of colloidal MR contrast agent candidates in LNs, and we suggest an extended mechanism for contrast agent enrichment in LNs. For further clarification, physiology and results from drug targeting studies are considered where applicable.

Interstitial T1-weighted MR lymphography: lipophilic perfluorinated gadolinium chelates in pigs

PURPOSE

To investigate the enhancement of the regional lymph nodes, lymphatic vessels, and thoracic duct after interstitial administration of lymphotropic perfluorinated gadolinium chelates at magnetic resonance (MR) imaging.

MATERIALS AND METHODS

Two perfluorinated gadolinium chelates, gadofluoramide and gadofluorine 8, were injected subcutaneously into the hind legs of 10 pigs, respectively. Both contrast media were studied at doses of 10 and 25 micromol per kilogram of body weight. T1-weighted three-dimensional gradient-echo and maximum intensity projection images were obtained at 1.5 T between 1 and 210 minutes and 24 hours after injection. The contrast agents were qualitatively compared regarding enhancement and depiction of the regional lymph nodes, lymphatic vessels, and thoracic duct.

RESULTS

The inguinal and iliac lymph nodes and lymphatic vasculature enhanced substantially within 10 minutes after subcutaneous administration of both lymphotropic contrast agents. Gadofluorine 8 showed a lymphographic effect superior to that of gadofluoramide. The paraaortic lymph nodes and thoracic duct were best visualized 10--50 minutes after injection of 25 micromol/kg of gadofluorine 8. Lymphatic system enhancement diminished after 2 hours, and the liver and bowel tract enhanced within 24 hours.

CONCLUSION

Interstitial administration of perfluorinated gadolinium chelates offers great potential for T1-weighted MR lymphography with positive enhancement of the lymph nodes and lymphatic vasculature.

Interstitial MR lymphography with a conventional extracellular gadolinium-based agent: assessment in rabbits

PURPOSE

To evaluate gadoterate meglumine as a contrast agent for interstitial magnetic resonance (MR) lymphography in combination with an adapted fast three-dimensional (3D) MR sequence.

MATERIALS AND METHODS

In 12 New Zealand White rabbits, 0.5 mL of undiluted gadoterate meglumine was injected subcutaneously into the dorsal foot pad (n = 9) or the foreleg (n = 3) bilaterally. Immediately after administration, a slight massage was performed at the injection site. Imaging was performed with a 3D spoiled gradient-recalled echo sequence (6.7/1.6 [repetition time msec/echo time msec]; field of view, 28.0 x 19.6; two signals acquired) similar to that used for 3D MR angiography. Thus, 3D maximum intensity projection images could be obtained. Images were obtained before injection and 5, 15, 30, 60, and 120 minutes after injection.

RESULTS

In the hind legs, as many as four successive lymph node groups were depicted with maximum enhancement after 5-15 minutes for the popliteal lymph node group, 15-30 minutes for the inguinal lymph group, and 30-60 minutes for the iliac-paraaortal lymph node group; the iliac-paraaortal lymph node group was not consistently enhanced. In the forelegs, four successive lymph node groups, including axillary and mediastinal lymph node groups, showed marked gadolinium uptake, with maximum enhancement 5-15 minutes after injection.

CONCLUSION

As a widely tested positive-enhancing T1 contrast agent with favorable safety features, gadoterate meglumine allows the depiction of three to four successive lymph node groups early after subcutaneous injection. With the sequence used, 3D MR lymphangiograms can be obtained.