Advances in the Fusion of Histologic Data With CT and MRI

Introduction:

Histological examination of pathological changes in human tissues is integral to clinical routine. Different staining methods and immunohistochemistry procedures are used in histology, among them the identification of certain types of cells, parts of cells, substrates, and diseases, making this technique quite useful. Recent advancements in medical imaging have greatly improved the incorporation of histological information with CT and MRI (Computed tomography and magnetic resonance imaging). Histology can be divided into soft-tissue histology and hard-tissue histology.

• Soft-Tissue Histology: Samples are typically embedded in paraffin and sectioned on a microtome using a static blade.

• 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. This situation is further complicated in hard-tissue histology because 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 risk missing important critical aspects.

What Are Micro-CT and Micro-MRI?

These are procedures used to assess an organism's internal characteristics that cannot be seen from the outside. Micro-CT and micro-MRI are innovations in medical diagnostics that enable high-resolution imaging, facilitating the combination of histologic data with more sophisticated imaging.

• 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. The camera has an X-ray tube, a radiation filter, a collimator (focusing beam shape into either fan or cone beam projection), a sample stage, and a phosphor detector/charge-coupled device.

• 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:

• It delivers results within 40 minutes to 12 hours, providing improved diagnostic accuracy.

• It is very sensitive to bone and lung tissue, which enhances imaging biomarkers in pathology.

• It provides high-resolution images, and contrast agents can be used to enhance the resolution further.

• It does not destroy target tissue.

• Easy image reconstruction and analysis.

• Easy to interpret results in 2D and 3D formats.

• 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:

• Use of high radiation can be harmful.

• Radiation exposure can manipulate tumor size and alter outcomes.

• Some types of fabric cannot be dyed.

• Requires good IT infrastructure and data pipelines.

• As for the second application of feature fusion, the adopted scope is unsuitable to filter out similar tissue types, which hampers the fusion of histologic data.

What Are the Benefits of Micro-MRI?

The benefits of micro-MRI are listed below:

• Micro MRI is a non-destructive technique.

• An excellent spatial resolution down to 25 µm is obtained when a high-strength magnetic field is applied.

• MRI gives an excellent contrast resolution and helps distinguish between normal and diseased tissue.

• 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.

• MRI provides very high contrast sensitivity or the ability to differentiate one type of tissue from another characteristic of pathological or healthy tissues, which is critical in diagnosing and imaging personalized medicine.

What Are the Weaknesses of Micro-MRI?

The disadvantages of micro-MRI are listed below:

• MRI is a relatively expensive technique for routine use. High magnetic field strength systems cost a lot.

• Acquiring high-resolution micro-MRI data is time-consuming, posing problems for in vivo imaging when the animal must be anesthetized for a long time to complete the imaging.

• Micro-MRI systems are not ideal for real-time studies of parameters such as blood flow.

• While micro-MRI systems offer high spatial resolution, they are unable to provide one-to-one replacement with micro-CT systems, especially in multimodal modalities.

What Are the Various Approaches for Image Fusion of Histological Data With CT and MRI?

Histological data is registered to CT and MRI, and multimodal image registration is important in this task. This method improves the knowledge of the structural and functional organization of tissues and shows that state-of-the-art imaging methods can also be useful in medicine.

• 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 match the underlying static image best. This image is usually called a floating image.

• Slice-to-Volume Registration (2D to 3D):

In the context presented, frame-to-frame registration refers to the alignment of tissue sections to the optimal plane within the volume. Therefore, concerns about geometric integrity and optimal fitting of in-plane 3D objects are eliminated. However, after accurately registering both modalities, efforts can be made to reconstruct the virtual tissue model.

• Volume-to-Volume Registration (3D to 3D):

Perfect registration of two 3D models represents the most challenging 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 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 CT and MRI have become standard measures for examining structure functional mapping, disease evolution, and healing in experimental animals. These enhance many scientific and biotechnological applications and promise future advancements in integrating histologic data with imaging processes. Further advances in this field enable researchers to view the microstructures of the histological sections in a 3-dimensional view.