Magnetic Resonance Imaging (MRI) | Medical Science Optional Notes for UPSC PDF Download

Magnetic Resonance Imaging (MRI)

Terminology 

• Precession : In addition to alignment of nuclei with a magnetic moment, application of an external magnetic field will produce a secondary spin or wobble (precession) of nuclei around the main or static magnetic field. The precessional path around the magnetic field is circularlike a spinningtop. 
• Larmor frequency : The Larmor or precessional frequency in MRI refers to the rate of precession of the magnetic moment of the proton around the external magnetic field. The frequency of precession is related to the strength of the magnetic field, B0.
• Relaxation: Relaxation is the process in which spins release the energy received from a radiofrequency pulse. MRI signal is influenced, among other factors, by different types of relaxation: T1 relaxation (spin-lattice or longitudinal relaxation) T2 relaxation (spin-spin or transverse relaxation) Relaxation causes MR signal to recover in the longitudinal direction according to the time constantTl or decay in the transverse plane according to the time constant T2.

T1 vs T2 weighted MRI

• The scanner's magnetic fields can be controlled to generate two distinct image types - T1-weighted and T2-weighted. These resulting images reveal various tissue types with differing densities:

T1 vs T2 weighted MRI

Mnemonics for bright or hyperintense Tl-weighted lesions

My Closest Companion is Exceptionally Considerate:

• M: melanin
• B: blood (specifically methaemoglobin in subacute hemorrhage)
• F: fat and slow flow
• P: protein; paramagnetic substances (e.g., manganese, copper); posterior pituitary gland
• C: contrast (gadolinium) and cholesterol

Contraindications to MRI

• Implantable Cardiac Pacemakers or Defibrillators
• Clips for Aneurysms
• Prosthetic Cardiac Valves
• Insulin Pumps or other Infusion Pumps
• Cochlear Implants
• Claustrophobia
• Metallic Foreign Body in the Eye
• Swan-Ganz Catheter
• Vena Cava Filters and Embolization Coils
• Neurostimulators

MRI compatible metals are Titanium, Aluminum and Bras

MRI Compatible devices are

• Sternal wires 
• Heart valves and annuloplasty rings 
• Coronary stents 
• Vena cava filters 
• Orthoepdic implants 
• lUDs 
• Dental implan

Variants of MRI

Magnetic Resonance Spectroscopy can ascertain the precise chemical composition of a sample through either single-voxel spectroscopy or chemical shift imaging.

• Magnetization transfer imaging enables the visualization of protons bound to macromolecules that are typically invisible in standard MR images. This technique facilitates the indirect measurement of protein/lipid components relative to body water.
• Diffusion-weighted imaging (DWI) is employed to capture images of the water component within the brain. Meanwhile, diffusion tensor imaging provides detailed information about water movement at a microscopic, cellular level.
• Fluid-attenuated inversion recovery (FLAIR) is an MRI sequence utilizing inversion recovery to nullify fluid signals. For instance, in brain imaging, FLAIR can be applied to suppress cerebrospinal fluid (CSF) effects in order to enhance the visibility of periventricular hyperintense lesions, such as those associated with multiple sclerosis (MS) plaques.

Indications for MRI of the Chest

• Assessment of a mediastinal mass
• Investigation of suspected Pancoast's (Superior Sulcus) tumor
• Diagnosis of Superior Vena Cava Syndrome
• Staging of lung cancer, including the evaluation of invasion into the heart, great vessels, chest wall, and diaphragm
• Examination for suspected aortic dissection
• Evaluation of the mediastinum and hilum in individuals with allergies to iodinated contrast media
• Diagnosis and assessment of congenital and acquired heart diseases

Cardiac MRI and its limitations

Principle:

• The principle is challenging due to the swift physiological movement of the heart and coronary arteries.
• Electrocardiographic triggering is employed, often during a brief breath-hold of 10-15 seconds, to capture both static and cine images.
• Both black blood and bright blood techniques are utilized.

Clinical Applications:

• It proves valuable in delineating anatomical relationships in patients with complex congenital heart disease and cardiac masses.
• It serves as the examination of choice for determining if a mediastinal or pulmonary mass has invaded the pericardium or heart and is the preferred investigation for arrhythmogenic right ventricular dysplasia.
• Magnetic Resonance Angiogram (MRA) is employed for visualizing the aorta and large blood vessels.
• It is used to evaluate myocardial perfusion and viability, employing specialized pulse sequences to measure blood velocity, enabling the characterization of valvular disease severity.

Limitations:

• Absolute contraindications include the presence of pacemakers, internal defibrillators, or cerebral aneurysm clips.
• Patients with claustrophobia may find the examination challenging.
• Conducting the examination in clinically unstable patients is difficult due to the challenges of close monitoring.
• MRA of coronary arteries is particularly challenging due to their rapid and complex motion.