Introduction:
Histological examination of pathological changes in human tissues is an integral part of clinical routine. A myriad of staining protocols and immunohistochemical applications have been developed for histology, allowing for identification of specific cell types, subcellular structures, substrates, and disease biomarkers, making this approach extremely versatile. Histology can be divided into soft-tissue histology and hard tissue histology.
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Soft-Tissue Histology - Samples are typically embedded in paraffin and sectioned on a microtome using a static blade.
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Hard-Tissue Histology - The samples are embedded in resin, cut with a diamond-coated instrument, and polished into discs thin enough to be stained and imaged under a microscope.
In both scenarios, sectioning is usually done without prior knowledge of the location of the region of interest (ROI). This limits the prognostic value of histological analysis, or results in very time-consuming serial sectioning of specimens. In hard-tissue histology, this situation is further complicated by the fact that specimens are usually opaque, and cutting and grinding techniques result in the loss of a significant proportion of material. Because classical histology relies on the microscopic evaluation of micron-thin tissue sections, biopsies are usually sparsely sampled and run the risk of missing important critical aspects.
What Are Micro-CT and Micro-MRI?
These are diagnostic methods used for viewing the internal structures of the body that are not visible on the outside.
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Micro-Computed Tomography (Micro-CT or μCT) - It is a non-destructive imaging tool for producing high-resolution three-dimensional (3D) images consisting of two-dimensional (2D) transverse projections or slices of a target sample. A Micro-CT device consists of several key components. Camera with X-ray tube, radiation filter, a collimator (focusing beam shape into either fan or cone beam projection), sample stage, and phosphor detector/charge-coupled device.
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Micro-Magnetic Resonance Imaging (Micro-MRI or μMRi) - This is a non-invasive technique that creates detailed three-dimensional (3D) images of tissue without the use of harmful radiation. It is used in the diagnosis and treatment of various diseases. Micro MRI is useful in vivo 3D imaging of microstructures in small tissue structures and small animals. The MRI system has magnets that create a magnetic field around the sample tissue. With the aid of a computer, an image of the sample tissue is generated from the collected resonance data.
What Are the Benefits of Micro-CT?
The benefits of micro-CT are listed below:
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It delivers results within 40 minutes to 12 hours.
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It is very sensitive to bone and lung tissue.
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It provides high-resolution images and contrast agents can be used to further enhance the resolution.
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It does not destroy target tissue.
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Easy image reconstruction and analysis.
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Easy to interpret results in 2D and 3D formats.
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Micro-CT scanners are inexpensive compared to other systems that use similar imaging tools.
What Are the Weaknesses of Micro-CT?
The drawbacks of micro-CT are listed below:
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Use of high radiation can be harmful.
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Radiation exposure can manipulate tumor size and alter outcomes.
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Some types of fabric cannot be dyed.
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Requires good IT infrastructure and data pipelines.
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Not suitable for identifying similar tissue types.
What Are the Benefits of Micro-MRI?
The benefits of micro-MRI are listed below:
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Micro MRI is a non-destructive technique.
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A very good spatial resolution down to 25 µm is obtained when a high-strength magnetic field is applied.
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MRI gives a really good contrast resolution and helps distinguish between normal and diseased tissue.
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It is safer than other imaging modalities such as CT and PET (positron emission tomography) because it uses a magnetic field rather than ionizing radiation.
What Are the Weaknesses of Micro-MRI?
The disadvantages of micro-MRI are listed below:
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MRI is a relatively expensive technique for routine use. High magnetic field strength systems cost a lot.
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Acquiring high-resolution micro-MRI data is time-consuming, which can pose problems for in vivo imaging when the animal needs to be anesthetized for a long time to complete the imaging.
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Micro-MRI systems are not ideal for use in real-time studies of parameters such as blood flow.
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Although micro-MRI systems have an excellent spatial resolution, they cannot match or yet replace micro-CT systems.
What Are the Various Approaches for Image Fusion of Histological Data With CT and MRI?
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Multimodal Image Registration:
The data set (3D data from CT or MRI) is considered the ground truth, which is not changed throughout the process and is usually called a still image. A second data set (tissue section or set of tissue sections) is deformed during the process to best match the underlying static image. This image is usually called a floating image.
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Slice-to-Volume Registration (2D to 3D):
Frame-to-frame registration, in the context presented, refers to the alignment of tissue sections to the optimal plane within the volume. Concerns about geometric integrity and optimal fitting of in-plane 3D objects are therefore eliminated. However, after accurately registering both modalities, efforts can be made to reconstruct the virtual tissue model.
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Volume-to-Volume Registration (3D to 3D):
Perfect registration of two 3D models represents the most difficult approach to multimodal image fusion and usually consists of a multi-step workflow in which low-level matching methods are applied repeatedly. Consistency in the resulting model's geometry integrity or geometry coherence is a frequent problem, visually noticeable on curved objects with layer displacement in the z-axis, also known as the banana effect.
Conclusion:
Micro-computed tomography (micro-CT) and micro magnetic resonance imaging (micro-MRI) is an indispensable standard tool for quantifying structure-function relationships, disease progression, and regeneration in preclinical models, enabling numerous scientific and biotechnological advances. Further advances in this field may enable researchers to view the microstructures of the histological sections in a 3-dimensional view.
