Non-invasive 3D imaging in high resolution

Now possible: characterization of surface to buried structures at resolutions from mm to 50 nm, with little to no sample preparation. When talking about high resolution imaging, there are several techniques out there which do a good job. However, there are very few non destructive techniques which can characterize a sample’s surface and its internal structures, or its porosity and connectivity in 3D and visualize them at high resolution. Moreover, there are no imaging techniques which could easily do this for samples at multi-length scales – from meso- to nano-scale.

Fig 1: Distribution of hepatocytes cells in tissue engineered scaffolds.

Fig 2: Human laryngeal nerve fiber

Conventional imaging modalities

Visible, scanning electron microscopy (SEM) and atomic force microscopy (AFM) for instance are surface visualization tools. Transmission Electron Microscopy (TEM) on the other hand requires specimens to be ultrathin. In the majority of the cases, destructive sample preparation through physical or chemical cross section or staining must be carried out. This approach can be tedious and introduces artifacts. Optical and confocal microscopy suffer from diffraction limits with spatial resolution being no better than 150 nm. While electron microscopy can achieve spatial resolutions in the nm or Å scale, sample preparation can be very elaborate, including the need for vacuum compatibility and electrical conductivity. Moreover, conventional imaging modalities are in 2D, and it is difficult to characterize functional and structural changes of materials and sensors in 3D. Doing so at a multi-scale level is downright impossible.

X-ray Computed tomography (CT)

X-rays on the other hand have the advantage that they interact weakly with matter and can penetrate deeply into materials – whether they are gaseous, fluid or hard opaque materials. The material world has been using x-rays in NDT (non-destructive testing), while in the medical community, CTs (computed tomography scanners) have been successfully deployed since the 1960’s. Medical CTs can provide resolutions in the mm or sub mm. Conventional micro CTs have resolutions from tens of microns to a few microns, opening up a number of applications in biomedical, semiconductor, building materials, or geological research.

Fig 3: Defect characterization in semiconductor package

Fig 4: Self assembled nano magnetic particles @ 50 nm resolution

Fig 5: TiSiC composite materials for aircraft frame @ 1.5 µm resolution

Fig 6: Modeling pores & connectivity in oil sandstone

Limitations related to resolution and contrast

Nevertheless, for a number of emerging applications such as tissue engineering, alternative energy (such as fuel cell) research, advanced composites, MEMs, semiconductor and nanotechnology, conventional micro CTs lack the necessary resolving power to visualize structures or defects which are a micron or smaller. Moreover, many biological materials, polymers and low Z composites have very low x-ray absorption, hence the imaging contrast for these materials is very poor, even at low resolution.

Novel micro and nano CT and applications

To fill the gaps in resolution and contrast in conventional CTs, we are pleased to announce a suit of novel lab-based micro and nano CT systems from Xradia Inc, USA. With the microXCT, imaging resolution of 1 micron or better is possible even with low-contrast biological and soft materials. A high resolution can be achieved even with relatively large specimens, often without having to reduce the sample dimension. With phase enhanced optics, it is also possible to image inherently low-contrast materials, such as cells within tissue engineered scaffolds; bone-cartilage interface without sample staining, or to visualize small-density differences within polymer composites and to resolve cracks, voids, pore dimensions and connectivity within porous materials. For nanoscale imaging, the nano XCT extends the imaging capability with resolutions to sub 50 nm. These high-resolution and high-contrast capabilities open up an exciting imaging possibility for a variety of research areas, from biomedical to material science – especially where samples need to be modeled accurately and without the need for invasive or destructive sample preparation.

Author: S H Lau, Vice President, Business Development, Xradia Inc, Concord, CA, USA

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