High-resolution tomographic imaging of a human cerebellum: comparison of absorption and grating-based phase contrast

Abstract

Human brain tissue belongs to the most impressive and delicate three-dimensional structures in nature. Its outstanding functional importance in the organism implies a strong need for brain imaging modalities. Although magnetic resonance imaging provides deep insights, its spatial resolution is insufficient to study the structure on the level of individual cells. Therefore, our knowledge of brain microstructure currently relies on two-dimensional techniques, optical and electron microscopy, which generally require severe preparation procedures including sectioning and staining. X-ray absorption microtomography yields the necessary spatial resolution, but since the composition of the different types of brain tissue is similar, the images show only marginal contrast. An alternative to absorption could be X-ray phase contrast, which is known for much better discrimination of soft tissues but requires more intricate machinery. In the present communication, we report an evaluation of the recently developed X-ray grating interferometry technique, applied to obtain phase-contrast as well as absorption-contrast synchrotron radiation-based microtomography of human cerebellum. The results are quantitatively compared with synchrotron radiationbased microtomography in optimized absorption-contrast mode. It is demonstrated that grating interferometry allows identifying besides the blood vessels, the stratum moleculare, the stratum granulosum and the white matter. Along the periphery of the stratum granulosum, we have detected microstructures about 40 mm in diameter, which we associate with the Purkinje cells because of their location, size, shape and density. The detection of individual Purkinje cells without the application of any stain or contrast agent is unique in the field of computed tomography and sets new standards in non-destructive three-dimensional imaging.

Trimodal low-dose X-ray tomography

“…From its first implementations up to this day, grating interferometry has been mainly exploited for imaging biological soft-tissue samples (8,9,16,17,21). However, other classes of samples, such as paleontological specimens, can also benefit from the high sensitivity of the phase and darkfield signals provided by this technique.…”

“…The δ resolution in the standard and SWI volumes was very similar despite the drastic difference in dose and number of images: δ resolutions of 4.7 × 10 −10 and 4.9 × 10 −10 were obtained, respectively, with the standard and SWI acquisition schemes (note that similar values have been measured in ref. 9). δ values can be converted into Hounsfield units for the phase signal (HU-P) as described in ref.…”

“…It gives access to differential phase contrast (DPC) at ultrahigh sensitivity (density differences down to 0.5 mg∕cm 3 can be routinely resolved) (8,9). XGI also provides dark-field images of the sample that reveal the presence of scattering structures on the nanoscale (10).…”

X-ray microscopy of soft and hard human tissues

“…The choice of the photon energy for heterogeneous tissue using phase-contrast CT, rather than absorption contrast, is easier, as the real part of the refractive index varies much less with elemental composition than the absorbance [3]. Therefore, both absorption and phase-contrast imaging provide complementary information [4,5]. Other techniques including magnetic resonance microscopy and histology are also complementary [6].…”

Visualization and Segmentation of Cells in Unstained Paraffin-Embedded Cerebral Tissue

“…These imaging techniques yield only information on surface-near regions and volumes restricted to cubic micrometers. In the last decade, the tissue preparation and high-resolution hard X-ray tomography has been advanced to reveal individual unstained cells in cubic millimeter brain volumes [1,2] with phase contrast. More recently, sufficient contrast for cell imaging in single distance propagation mode [3] or even with lab-based conventional approaches [4] was obtained by embedding brain specimens in paraffin.…”