What is Magnetic Resonance Imaging (MRI)?

What is Magnetic Resonance Imaging (MRI)?

MRI is a non-invasive cross-sectional imaging technique that uses strong magnetic fields and radiofrequency waves to generate detailed three-dimensional images of the body, particularly excelling at soft tissue visualization without ionizing radiation. 1

Physical Principles

MRI operates through a sophisticated physical process:

• The strong magnetic field in the MRI scanner causes hydrogen protons (naturally abundant in the human body) to align in its direction. 1
• A radiofrequency (RF) pulse is then applied to "spin" the body's protons out of equilibrium. 1
• Advanced computational tools generate images based on the energy released and the time it takes protons to "realign" with the magnetic field once the RF pulse is turned off. 1
• Multiple imaging acquisition parameters can be modified to provide different "weighting" of the images (e.g., T1, T2 weighting), each providing different degrees of visualization representing different physical and biologic properties of tissues. 1

Key Clinical Advantages

MRI offers several distinct advantages over other imaging modalities:

• MRI produces multiplanar images without potentially harmful ionizing radiation. 1
• MRI has become indispensable for diagnosing neurological, musculoskeletal, and oncological conditions since its development in the 1980s. 1
• MRI is particularly useful in assessing bone marrow, as bone marrow edema resulting from early metastatic cancer foci, not visible on CT, can be depicted on MRI. 1
• MRI superbly delineates prostatic zonal anatomy not clearly visualized on other modalities. 1
• MRI offers superior soft tissue contrast compared to CT, with excellent ability to characterize different tissue types. 1

Important Limitations and Practical Considerations

Scan Duration

• MRI examination times are significantly longer compared with CT (pelvic MRI typically 20-30 minutes; whole-body MRI approximately twice as long). 1
• MRI typically requires imaging acquisitions of 30 minutes for abdominal studies. 1

Claustrophobia

• Severe claustrophobia affects 3-7% of patients, with up to 37% experiencing moderate to severe anxiety that necessitates procedure termination in 5-10% of cases. 2
• The lengthy scan time significantly exacerbates claustrophobic symptoms. 2
• Oral benzodiazepines are first-line anxiolytics for claustrophobic patients. 2

Contrast Considerations

• Due to increasing evidence of gadolinium deposition in normal tissues (e.g., brain) and lack of knowledge of the long-term health implications, intravenous gadolinium for MRI is only used when potential benefits outweigh risks. 1
• Severe allergy to gadolinium contrast agents prevents contrast-enhanced MRI. 2
• Renal insufficiency or compromised renal function contraindicates gadolinium administration due to nephrotoxicity risk. 2

Absolute Contraindications

Patients with cardiac pacemakers, implantable cardioverter-defibrillators (ICDs), and certain ferromagnetic metallic implants cannot undergo MRI due to risk of device malfunction, heating, and potential movement or dislodgment of metallic objects. 2, 3

• Ferromagnetic cerebral aneurysm clips pose a significant risk of movement near vital brain structures and are contraindicated for MRI. 3
• Steel fragments near the brain or retina can cause catastrophic injury if displaced and are contraindicated for MRI. 3
• Stainless-steel or non-titanium implants may preclude MRI, particularly when located near the area being imaged. 2

Common Pitfalls to Avoid

• Do not confuse vascular stents with cardiac pacemakers, as they have completely different safety profiles—coronary artery stents are MRI-safe at field strengths up to 3 Tesla with no waiting period required. 3
• Recognize that artifact does not equal danger; coronary stents cause local artifacts but this is an image quality issue, not a safety concern. 3
• Calcification detection remains problematic with MRI. 2

Mandatory Pre-MRI Screening

Use a standardized written questionnaire to cover implants, ports, catheters, metallic implants, vascular stents, coils, active devices, cardiac pacemakers, shrapnel, and claustrophobia before MRI. 3

• Obtain and verify the patient's implant pass to determine the device manufacturer, specific model, material composition, and MRI safety classification. 3
• Check online resources, such as https://mrisafety.com/, to confirm the implant's safety level if documentation is unavailable. 3
• Patients with metal fragments from metal-working occupations require orbital radiographs to exclude intraocular foreign bodies before scanning. 3