Learning Objectives

By the end of this section, the learner should be able to:

Explain the physical principles underlying MRI image formation.
Identify the main advantages and limitations of MRI compared with other imaging modalities.
Correctly orient MRI images in sagittal, coronal, and axial planes.
Differentiate between T1-weighted and T2-weighted MRI images.
Apply MRI principles to common clinical scenarios such as spinal cord compression.

Overview

Magnetic resonance imaging (MRI) is one of the most sophisticated imaging techniques used in clinical medicine. With advances in technology and decreasing costs, MRI has become increasingly accessible and widely used. MRI provides excellent soft-tissue contrast without exposing patients to ionizing radiation, making it invaluable in neurological, musculoskeletal, oncologic, and cardiovascular imaging.

Basic Principles of MRI

MRI exploits the magnetic properties of hydrogen protons, which are abundant in the human body due to its high water content.

Magnetic Alignment and Excitation

When a patient is placed within the strong magnetic field of an MRI scanner, hydrogen protons align with the magnetic field.
A radiofrequency (RF) pulse is then applied, which excites these protons and causes them to change orientation (“spin”).
When the RF pulse is switched off, protons return to their original alignment through a process known as relaxation.

Relaxation and Signal Generation

Different tissues relax at different rates due to variations in their molecular environment.
These differences in relaxation times generate contrast between tissues and allow detailed image formation.

Key concept: MRI images are based on proton excitation and relaxation, not radiation.

Clinical Uses of MRI

MRI produces high-resolution, multiplanar images, particularly suited for soft tissue evaluation.

Common Indications

Brain and spinal cord pathology (tumors, stroke, demyelination)
Musculoskeletal injuries (ligaments, cartilage, discs)
Tumor detection and characterization
Functional assessment using gadolinium-based contrast agents

Advantages and Limitations of MRI

Advantages

Excellent soft tissue contrast
Multiplanar imaging capability
No ionizing radiation

Limitations

Long scan times (30–45 minutes)
High cost
Limited use in emergencies and trauma
Contraindicated in patients with certain ferromagnetic or electronic implants

MRI Safety Considerations

Although MRI does not use ionizing radiation, safety is critical due to the strong magnetic field:

Ferromagnetic objects may become projectiles
Contraindications include some pacemakers, defibrillators, and older implanted devices
Strict safety screening is essential before scanning

Interpreting an MRI Scan

Image Orientation

MRI images are typically displayed in three anatomical planes:

Sagittal
Coronal
Axial (transverse)

Axial images are viewed from the feet upward, meaning the left side of the image corresponds to the patient’s right side.

MRI Image Weighting

MRI images are commonly classified as T1-weighted or T2-weighted, based on relaxation properties.

Key Differences

Feature	T1-Weighted	T2-Weighted
Bright (White)	Fat	Water, edema, inflammation
Intermediate	Gray matter	Gray matter
Dark (Black)	Bone, air, fluid	Bone, air, fat
Clinical use	Anatomy	Pathology

Memory aid: T2 = H₂O → Water is White

Clinical Relevance: Spinal Cord Compression

MRI is the imaging modality of choice for evaluating suspected spinal cord compression due to:

Disc herniation
Spinal stenosis
Tumors
Cauda equina syndrome

On T2-weighted sagittal MRI, cerebrospinal fluid appears bright, allowing easy visualization of compression or interruption by disc material.

Comparison with Other Imaging Modalities

Factor	CT	MRI	X-ray	Ultrasound
Duration	3–7 min	30–45 min	2–3 min	5–10 min
Cost	Moderate	High	Low	Low
Dimensions	3D	3D	2D	2D
Soft tissue	Moderate	Excellent	Poor	Poor
Bone	Excellent	Poor	Excellent	Poor
Radiation	Yes	None	Yes	None