Preclinical Imaging: Micro MRI (Magnetic Resonance Imaging)
The observation of changes either at the cell, tissue, organ or at molecular level need to be visualized during research clinical work, diagnosis of diseases or during drug development. These changes may be due to environmental factors or other clinical status of body. These image modalities are very important and are known as preclinical imaging. The study of longitudinal section of animal models that are non invasive or in vivo has been one of the very important part of research and drug development studies.

These imaging system are divided in to two major parts or types. The primarily molecular imaging and another is morphological or anatomical. In these methods, many techniques like magnetic resonance imaging commonly known as MRI, high frequency micro ultrasound and CT that is computed tomography are used. These are specially used for anatomical imaging. While for molecular visualization, positron emission tomography (PET), single photon emission computed tomography (SPECT), Fluorescence and bioluminescence are used. Today with the development of science many systems are having both modals that is anatomical modalities like MRI, CT as well as the functional imaging of SPECT and PET.

One of the imaging techniques is micro -MRI which is abbreviation of Magnetic Resonance imaging. The principle of this technique is that it uses the nuclear magnetic alignment of different atoms which are present inside a magnetic field which results in generation of images. The machine MRI generates large magnet field around the target of analysis. These filed develop paramagnetic atoms like gadolinium, hydrogen and manganese which align themselves in dipole of magnet along the magnetic fields, created by radio frequency source or coils present inside the MRI machine.

When the RF pulse is temporarily ceased, the machine captures the relaxation of atoms when they return to their normal position or alignment. Thus basis resonance characteristics of different tissue type, the computer will give or will generate an image of the subject or target place like cell, tissue, or organs. The most advantage of micro MRI over other techniques of imaging is that it has very fine spatial resolution up to 100 micro meters and even 25 micro meters in high magnetic field strength. It distinguishes normal and pathological tissues due to its excellent contrast resolution ability. It is used in all forms of imaging like molecular, functional and anatomical. Also it is safer for its use than radiation based imaging like Micro CT and Micro PET.

Apart from these micro MRI has few disadvantages or weakness. The biggest drawback of micro-MRI is its big prices; its cost. System which uses animal imaging between 1.5 and 14 teslas in magnetic flux density ranges from dollar one million to around dollar six million. Many system average costing is dollar 2 million. Also the other disadvantage of micro MRI is that the image acquisition span is long, it taking time from many minutes and even hours. This may affect patients or animals under test which are anesthetized for longer periods. It is also like snap shots and it is not able to study blood flow. This is having difficulties in analysis the important information like blood flow velocity quantification. The most important use of Micro MRI is due to its ability to non-invasively penetrate the skull and this is helping to image the brain for cancer related or other brain related diseases and in its diagnosis. Due to its high resolution, micro MRI can easily diagnose or detect early small sized tumors. When it is used in combination with other techniques like micro PET and micro SPECT, it can be effectively used for molecular expressions. This is done with nanoparticles with which antibodies are bound and it helps in increasing resolution and to visualize molecular level expression.

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