Magnetic resonance imaging

ℹ️ About

• Magnetic resonance imaging (MRI) is a valuable imaging technique that does not involve exposure to ionizing radiation, making it safer for patients, especially those requiring repeated imaging.

Method

• The core of any MRI scanner is a powerful magnet, which creates a stable and intense magnetic field that aligns hydrogen nuclei (protons) within the body.
• The MRI machine applies a radio frequency (RF) pulse specific to hydrogen, which temporarily disrupts the alignment of these protons.
• Once the RF pulse is turned off, the hydrogen nuclei return to their original alignment within the magnetic field. This realignment releases energy, producing a signal that is detected by the MRI coils.
• The detected signals are converted into mathematical data and processed using a Fourier transform to produce detailed images of the body's internal structures.
• Contrast agents, such as gadolinium, may be administered to enhance image clarity. Gadolinium alters the local magnetic environment, making tissues and fluids appear particularly bright on T1-weighted images.

The Components

• Most MRI machines use superconducting magnets, with coils of superconductive wire cooled by liquid helium, housed inside a vessel known as a cryostat.
• The radio frequency (RF) system includes an RF synthesizer, power amplifier, and transmitting coil, which generate and transmit RF pulses during the imaging process.
• Gradient coils are critical for spatial encoding, allowing the magnetic field to vary linearly across the imaging volume. This variation enables the precise localization of protons along the x, y, and z axes during image formation.

MRI Modalities

• T1-Weighted Imaging: Provides high-resolution images where fat appears bright, and fluids like cerebrospinal fluid (CSF) appear dark. T1-weighted images are useful for assessing anatomical details and detecting abnormalities like tumours.
• T2-Weighted Imaging: Fluid appears bright (e.g., CSF), while fat appears darker. T2-weighted images are particularly useful for detecting inflammation, edema, and lesions involving water content, such as in multiple sclerosis plaques or strokes.
• FLAIR (Fluid-Attenuated Inversion Recovery): Similar to T2-weighted imaging but suppresses the signal from free fluid like CSF. FLAIR is useful for identifying lesions near fluid-filled spaces, such as periventricular white matter lesions in multiple sclerosis.
• Diffusion-Weighted Imaging (DWI): Measures the movement of water molecules within tissues. It is highly sensitive for detecting acute strokes, where restricted diffusion indicates early ischemic changes.
• Diffusion Tensor Imaging (DTI): A specialized form of DWI that maps the orientation of white matter tracts in the brain. It is valuable in neurological research and assessing conditions like traumatic brain injury and neurodegenerative diseases.
• Gradient Echo Imaging (GRE): Provides high-resolution images and is sensitive to blood products like hemosiderin. GRE is commonly used to detect microbleeds and small vascular malformations.
• Magnetic Resonance Angiography (MRA): Visualizes blood vessels using MRI technology without the need for contrast agents. It is used to assess vascular structures in conditions like aneurysms, stenosis, and arteriovenous malformations.
• Magnetic Resonance Spectroscopy (MRS): Evaluates the chemical composition of tissues. It is useful for assessing brain metabolites in conditions like tumours, metabolic disorders, and infections.
• Functional MRI (fMRI): Measures brain activity by detecting changes in blood flow, typically using the blood oxygen level-dependent (BOLD) signal. fMRI is used in research and pre-surgical planning to map brain function and locate critical areas like language and motor centers.
• Cardiac MRI: Specifically tailored to assess the heart and great vessels. It provides information about cardiac function, structure, and flow dynamics and is used to evaluate cardiomyopathies, congenital heart diseases, and myocardial viability after infarction.

Contraindications

• Ferromagnetic Devices: Devices that can be influenced or moved by the strong magnetic field can pose a significant risk if present in the body.
• Suspected Intraocular Metal Fragments: Patients with a history of potential eye injuries from metal should undergo an orbital x-ray before an MRI to rule out metal fragments.
• Pacemakers: Traditional pacemakers may malfunction in the magnetic field and are a contraindication for MRI. Newer MRI-compatible pacemakers are available but require specific protocols.
• Certain Aneurysm Clips: Some older clips used in brain aneurysm surgeries may be attracted to the magnet, risking displacement or damage.
• Severe Renal Impairment: Use of gadolinium-based contrast agents may increase the risk of nephrogenic systemic fibrosis in patients with severe renal impairment.

Advantages of MRI

• Superior Soft Tissue Contrast: MRI offers better contrast resolution for soft tissues compared to CT, making it ideal for imaging the brain, spinal cord, muscles, and joints.
• No Ionizing Radiation: Unlike CT scans, MRI uses magnetic fields and radio waves, making it safer for repeated imaging, particularly in children and pregnant women.
• Multiplanar Imaging: MRI can acquire images in multiple planes (axial, sagittal, coronal), providing a comprehensive view of complex anatomical structures.

Disadvantages of MRI

• Cost and Availability: MRI is more expensive and less available than other imaging modalities like CT, making access limited in some settings.
• Contraindications: Presence of certain metal implants or devices can prevent patients from undergoing MRI, limiting its use in some cases.
• Length of Scan: MRI scans can take longer than CT scans, and patients must remain still during the scan, which can be challenging for claustrophobic patients or those with severe pain.
• Risk of Gadolinium Use: Although rare, the use of gadolinium contrast can cause adverse reactions and is contraindicated in patients with severe kidney impairment.

References

• Radiopaedia - MRI Overview
• National Institutes of Health (NIH) - Advanced MRI Techniques
• BNF - MRI Guidance
• RadiologyInfo.org - Body MRI