What is Magnetic Resonance Imaging (MRI)?
MRI is a non-invasive imaging technique that uses a strong magnetic field and radiofrequency pulses—rather than ionizing radiation—to generate detailed multiplanar images of internal body structures, particularly excelling at soft tissue visualization. 1
Physical Principles
MRI exploits the magnetic properties of hydrogen protons, which are abundant throughout the human body 1:
The strong magnetic field in the MRI scanner bore causes hydrogen protons to align parallel to the field direction, precessing around their axis at a specific frequency (the Larmor frequency: 42.58 MHz per Tesla, meaning 63 MHz at 1.5T and 128 MHz at 3T) 1
A radiofrequency (RF) pulse is then applied at this resonant frequency to "spin" the protons out of equilibrium by rotating the net magnetization vector by a specific "flip angle" 1
After the RF pulse stops, the protons release energy as they realign with the magnetic field—this relaxation process emits radio signals that are detected by receiver coils 1
Advanced computational tools process the collected data (including the amount of energy released and the time required for proton realignment) to generate cross-sectional images 1
Image Contrast Mechanisms
Multiple imaging parameters can be adjusted to provide different "weighting" that highlights specific tissue characteristics 1:
T1 relaxation describes how the magnetization vector component parallel to the main field slowly returns to equilibrium by interacting with surrounding molecules 1
T2 relaxation represents the more rapid recovery of the vector component transverse to the field 1
Images can be weighted toward T1, T2, or proton density to derive signal differences intrinsic to tissues, helping characterize pathology 1
Key Hardware Components
An MRI system consists of four major components 2:
- Superconducting magnet generating a homogenous magnetic field (typically 1.5T or 3.0T) 2
- Gradient coils that create linear magnetic field gradients to spatially encode the MR signal 2
- Radiofrequency coils that transmit RF energy and detect the emitted signals 2
- Computer systems that reconstruct images from the collected data 2
Clinical Advantages Over Other Modalities
Compared to CT, MRI provides superior soft tissue contrast without ionizing radiation 1:
Bone marrow assessment: MRI detects early metastatic seeding through bone marrow edema that remains invisible on CT, which requires substantial cortical destruction before lesions become visible 1
Prostatic imaging: MRI superbly delineates prostatic zonal anatomy not clearly visualized on other modalities, establishing itself as the imaging tool of choice for primary prostatic tumors using multiparametric approaches combining anatomic (T1- and T2-weighted) and functional (diffusion-weighted and dynamic contrast-enhanced) sequences 1
Liver metastases: MRI with diffusion sequences and gadoxetate disodium demonstrates higher sensitivity than CT for detecting liver metastases from pancreaticobiliary malignancies 1
Practical Limitations
Several factors constrain MRI's clinical utility 1:
Longer examination times: Pelvic MRI typically requires 20-30 minutes; whole-body MRI takes approximately twice as long (compared to CT whole-body scans completed in seconds) 1
Limited availability: Interpretation expertise is not as widely available as for CT 1
Lymph node assessment: MRI shares CT's limitations for detecting lymph node metastases, relying heavily on morphological features (size, shape, borders) with reported sensitivity of only 39% and specificity of 82% for nodal metastases 1
Claustrophobia: Approximately 5% of patients experience claustrophobia during MRI examinations 3
Safety Considerations
MRI is generally safe but requires rigorous pre-screening to identify absolute contraindications 3:
Cardiac pacemakers and implantable cardioverter-defibrillators are generally contraindicated due to risk of heating of pacemaker leads 4
Ferromagnetic cerebral aneurysm clips pose significant risk of movement near vital brain structures 4
Steel fragments near brain or retina can cause catastrophic injury if displaced 4
Coronary artery stents are MRI-safe at field strengths up to 3 Tesla with no waiting period required 4
Gadolinium contrast should only be used when benefits outweigh risks, given increasing evidence of gadolinium deposition in normal tissues (particularly brain) with unknown long-term health implications 1
Common Clinical Pitfall
Do not confuse vascular stents with cardiac pacemakers—they have completely different safety profiles. Coronary stents are MRI-safe and cause only local artifacts (an image quality issue, not a safety concern), whereas pacemakers remain contraindicated unless specifically labeled MRI-conditional 4.