Introduction
Brain neoplasm refers to any abnormal growth of tissues in the brain. It can be either benign or malignant. Benign lesions are noncancerous growth, whereas malignant lesions are cancerous. About 3,00,000 people are diagnosed with brain neoplasms worldwide. The survival rate is only five percent though various treatment techniques are available. Physicians cannot rely on conventional MRI (magnetic resonance imaging). The updated magnetic resonance imaging version is diffusion MRI.
What Is Diffusion MRI?
Diffusion MRI or diffusion-weighted MRI produces images with contrast determined by Brownian movement (zig-zag movements) of the water molecules at the microscopic level of the tissues. A tissue can be defined as a group of cells performing a specific function. Each cell contains organelles and is covered by a cell membrane. Cell membranes act as barriers and impede the movement of water molecules across the cells. The diffusivity of water molecules across the cell membrane varies with each cell type and the cellular population in tissues. Diffusion MRI can differentiate tissue containing dense cells from sparse cells. It can help in differentiating brain abscesses from cystic and necrotic neoplasms. Pus in brain abscess impedes the movement of the water molecules and, thus, low apparent diffusion coefficient (ADC) and has a bright signal in diffusion MRI. On the other hand, cystic and necrotic tissues have an increased ADC and low signal on diffusion MRI images, similar to that of cerebrospinal fluid, with margins having either the same or less signal intensity on diffusion MRI.
What Are the Uses of Diffusion MRI?
Diffusion-weighted MRI provides data about the cellularity of brain tumors and is thus useful in grading tumors. However, there is a limitation in determining the grades of the tumor since overlapping values are obtained in low and high grades. Lymphomas are highly cellular tumors; they exhibit low diffusion coefficients and bright signals on diffusion-weighted MRI. Meningiomas also show low ADC values, yet they are difficult to diagnose by diffusion MRI. Diffusion MRI is effective in differentiating arachnoid cysts from epidermoid tumors. T1 and T2 conventional MRI produces arachnoid cysts and epidermoid tumor images with the same signal intensity as cerebrospinal fluid. Epidermoid tumors are solid with low ADC and hyperintense (bright) on diffusion MRI. Arachnoid cysts are hypointense on diffusion MRI. The ADC value of an epidermoid cyst is the same as that of the brain; the ADC value of arachnoid cysts is the same as that of cerebrospinal fluid (CSF). Tumors with high vascularity (cerebellar hemangioblastoma) have high ADC and are hypointense in diffusion MRI.
What Is Diffusion Tensor MRI?
Diffusion tensor imaging (DTI) is a type of diffusion-weighted imaging that helps to map axonal fibers in the white matter of the central nervous system. The body has different cell types containing distinct intracellular and extracellular components. Most of the cells in the body exhibit isotropic movement of water, which means the diffusion of water is the same in all directions (for example, water diffuses equally in all directions in cerebrospinal fluid). On the other hand, brain cells in white matter exhibit anisotropic movement of water molecules, meaning water diffusion is more along the axonal structures than perpendicular to them. Gray matter in the brain is less anisotropic when compared to white matter. The two parameters used in diffusion tensor imaging are:
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Apparent Diffusion Coefficient (ADC) - The apparent diffusion coefficient (ADC) refers to the average diffusion coefficient.
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Fractional Anisotropy (FA) - Fraction anisotropy (FA) is the fraction of the total magnitude of diffusion anisotropy. The value of FA ranges between 0 and 1. FA of cerebrospinal fluid is 0 as it is isotropic.
How Does Diffusion Tensor MRI Work?
In diffusion tensor imaging, data acquisition is made using pairs of 90-degree and 180-degrees radiofrequency impulses instead of repeated 180-degrees. This method of data acquisition is called spin-echo echo-planar imaging (SE-EPI). DTI also produces color-coded images that represent the orientation of different nerve fibers. The axonal fibers in the brain's white matter are classified as follows:
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Association Fibers- They interconnect cortical areas in each hemisphere.association fibers that are found in DTI color maps. They are cingulum, occipitofrontal fasciculi, uncinate fasciculus, superior and inferior longitudinal fasciculi.
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Projection Fibers - They connect cortical areas with the brain stem, cerebellum, and spinal cord. Projection fibers found in DTI color maps include corticospinal tracts, corticobulbar tracts, and corticopontine tracts.
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Commissural Fibers - They interconnect similar cortical areas between the hemispheres. Commissural fibers found in color DTI include the corpus callosum and anterior commissure.
The optic tract, fornix, tapetum, and many brain stem and cerebellum fibers are rarely identified in color DTI.
What Is the Color Coding of Nerve Fibers in DTI?
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Red- transverse fibers.
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Green-anteroposterior fibers.
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Blue-craniocaudal fibers.
Oblique fibers are presented with secondary colors in DTI as follows
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Magenta (red + blue).
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Yellow (green + red).
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Cyan (green + blue).
What Is the Role of Diffusion Tensor MRI in Pre-surgical Evaluation?
The principal aim in the surgical resection of brain tumors is the complete removal of tumor cells to avoid recurrences while maintaining the surrounding functional brain fibers intact to prevent postoperative complications. Conventional MRI fails to provide the exact extent of brain lesions. Diffusion tensor MRI produces the relationship of tumors with nerve fibers. Thus DTI helps in the pre-surgical planning of brain tumors. DTI detects tumor cells involving white matter fibers by fraction anisotropy maps and tractography (3D image processing of neural tracts)
DTI produces five different patterns of white matter involvement by tumor cells as follows:
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Displaced - Normal anisotropy relative to the contralateral tract in the corresponding location but situated in an abnormal T2-weighted signal intensity area or presented with an abnormal orientation.
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Invaded - Slightly reduced anisotropy without displacement of white matter architecture, remaining identifiable.
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Infiltrated - Reduced anisotropy but remained identifiable.
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Disrupted - Marked reduced anisotropy and unidentifiable.
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Edematous - Normal anisotropy and typically oriented but located in an abnormal T2-weighted signal intensity area.
What Are the Limitations of DTI?
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DTI mapping cannot identify synapses.
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It cannot distinguish efferent from afferent nerves.
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It cannot determine the function of the nerves.
Conclusion
Though diffusion MRI produces promising data about the tumors near vital brain structures, proper interpretation of data is required. Intracranial neoplasms involve both cortex and its corresponding white fibers. In such cases, the combination of functional MRI (fMRI) and diffusion MRI are advantageous. However, technical developments are required to reduce the duration of the scan and other limitations.
