Magnetic resonance imaging (MRI) is a technique that generates images by exploiting the nuclear magnetic behavior of different atoms in a sample tissue placed in a magnetic field. It is a non-invasive technique that produces detailed 3-dimensional (3D) images of tissues without the use of harmful radiation. It finds applications in diagnostics and in the treatment of various diseases.
Micro-MRI helps with the in vivo 3D imaging of microstructure in small animals. Results of micro-MRI analysis are often compared against high-resolution micro-computed tomography or micro-CT and are found to correlate well.
Commercially available micro-MRI systems offer preclinical imaging solutions for small animal-based experiments. Some of these systems use cryoprobes and radiofrequency (RF) coils, along with high field magnets and advanced software, to offer high-resolution cellular and molecular level imaging of small animals, in life science or biomedical research.
Principles of micro-MRI
MRI systems have magnets that create magnetic fields around the sample tissue. Paramagnetic atoms such as gadolinium and hydrogen present in the tissue are influenced by this magnetic field and align in a magnetic dipole.
The RF coils that generate the magnetic field are deactivated for a short period of time and the atoms are allowed to relax into their normal alignment. The relaxation or resonance characteristics of different tissue types are captured by the system. With the help of a computer, an image of the sample tissue is generated based on the resonance data collected.
Applications of micro-MRI
Micro-MRI has various applications including functional, anatomical, and molecular imaging of small animals. Different fields of study which utilize microMRI include pharmacology, brain mapping, neurodegenerative disease, and psychiatry.
The MRI technique, in general, is particularly suitable for imaging soft tissues in the human body. Since it does not use damaging radiation, it is a preferred technique for imaging the brain, nerves, and spinal cord. MRI also provides higher resolution images of ligaments, muscles, and tendons when compared to X-rays or computed tomography (CT), and hence is preferred in case of shoulder and knee injuries.
MRI can distinguish between the white and grey matter in the brain, and is therefore useful in the diagnosis of tumors and aneurysms. Functional MRI is used to study the link between different areas of the brain and cognitive tasks. This helps monitor the neurological status of an individual.
Strengths of micro-MRI
- Micro-MRI is a non-destructive technique
- It has very good spatial resolution – up to 25 µm - when magnetic fields of high strength are applied
- MRI offers good contrast resolution and helps distinguish normal tissue from diseased tissue
- It uses magnetic fields and not ionizing radiation, and is therefore safer than other imaging tools such as CT and PET.
Weaknesses of micro-MRI
- MRI is a relatively expensive technique for routine use. Systems with high magnetic field strength cost a lot.
- Obtaining high-resolution micro-MRI data takes a long time, which might be a problem during in vivo imaging, where animals need to be anesthetized for extended periods for imaging to be completed.
- Micro-MRI systems are not ideal for real-time studies of parameters such as blood flow.
- Although micro‐MRI systems have good spatial resolution, they are not comparable with micro‐CT systems, or cannot replace them yet.
Preclinical Magnetic Resonance Imaging