Dual Histology Stain/MRI Contrast Agents for Molecular Imaging of the Brain

 

We are currently working on developing novel magnetic resonance (MR) techniques and contrast agents for molecular imaging of the brain. In particular, we are interested in developing novel MR contrast agents that will help generate improved models of brain cytoarchitecture. These studies are being performed at high magnetic field strengths (9.4 and 14 Tesla), where very high resolution (20-40 um) images can be obtained of ex vivo brain specimens from humans and mice. Validation of the MR molecular images by comparison with histology images is desirable, however, the comparison of MR images of intact brain specimens with histology images is complicated by the deformations/distortions caused by the histology sample preparation. We are therefore working on synthesizing histology stains tagged with MRI contrast agents that allow for: (1) molecular imaging of ex vivo brain specimens, (2) co-registration of ex vivo MRI and histology brain images, and (3) increased signal-to-noise ratios (SNR) which are required to obtain the high resolution (20-100 mm) images needed for developing improved cytoarchitectural brain models.

 

 

Luxol Fast Blue

Initial studies have investigated the use of the myelin stain Luxol Fast Blue 38 (LFB-38) as a dual histology/MRI stain for myelinated structures in the brain. The molecular structure of LFB-38, shown in Figure 2 below, is a sulfonated copper phthalocyanine compound with a paramagnetic Cu2+ ion and should therefore act as an MRI relaxation agent.

 

Figure 1: Chemical structure of Luxol Fast Blue 38, where X is SO3-, used for staining myelin. The copper ion is in the +2 oxidation state and is paramagnetic and should act as a MRI relaxation agent.

 

Experiments performed at 14 Tesla indicate that LFB-38 shortens the average T1 of rat brain hippocampal slices from 2.0 seconds for unstained specimens to 1.5 seconds for the stained specimens. T2 relaxation times were only slightly shortened in LFB-38 stained specimens with T2 = 24 ms for stained specimens and T2 = 27 ms for unstained specimens. 

 

Gd-DOTA Tagged Histology Agents

The use of molecular agents that act as both histology stains and MR molecular imaging contrast agents provides a powerful tool not only for the co-registration of MR and histology images but also for improving the MRI signal-to-noise ratio (SNR) and highlighting specific brain cytoarchitectural features. While our initial studies have involved examining the potential of the myelin stain LFB-38 to act as a myelin specific contrast agent in ex vivo brain studies, we are also working on attaching Gd-DOTA MRI contrast agents to other histology stains, such as thionin or cresyl violet (see Figure 4) used for Nissl staining.

 

 

Figure 2: Chemical structures of two common Nissl stains used for staining neurons.