Magnetic resonance imaging (MRI) has revolutionized the field of medical imaging and diagnosis since its introduction in the 1970s. An MRI scanner uses strong magnetic fields and radio waves to generate detailed images of the inside of the human body. MRIs can visualize soft tissues and organs with exceptional clarity, allowing physicians to identify tumors, tears, bleeding, infections, and other abnormalities.
While MRIs provide unparalleled views of many anatomical structures and pathological processes, there are some limitations to what this technology can detect. In some cases, alternative tests may be required to fully evaluate a medical condition. Understanding the capabilities and constraints of MRI scanning is important for managing patient care and expectations.
Anatomical Limitations
MRI technology has difficulty imaging certain parts of the body. Bones and lungs, which contain little water or fat, do not show up well on MRI. Imaging small, intricate structures like the inner ear can also be challenging. Here are some examples of what MRIs cannot adequately show:
- Inside bones – X-rays or CT scans better evaluate fractures or bone tumors
- Lung parenchyma – Chest X-ray or CT better visualizes lung tissue
- Middle/inner ear – CT or specialized MRI sequences needed to see delicate structures
- Heart – Echocardiogram or cardiac MRI better for cardiovascular imaging
While MRI can suggest potential problems in these areas, other tests are required for confirmation and clear visualization. The radiologist may need to recommend additional imaging based on a patient’s symptoms and the MRI findings.
Metal Objects
The strong magnets used in MRI scanning cause safety issues and image artifacts from metallic objects in or on the patient. Items containing ferromagnetic metals cannot be brought into the MRI room because they could turn into dangerous projectiles.
On the imaging side, metal near or in the region of interest will disrupt the magnetic fields and show up as black spots or cause distortions on the scan. Examples include:
- Orthopedic implants – screws, rods, artificial joints
- Dental work – fillings, braces, retainers
- Vascular stents, filters, or coils
- Surgical clips, staples, or wire sutures
- Shrapnel or bullet fragments
Radiologists must carefully scrutinize MRIs that show hardware or metal in the body. The images may not fully reflect the underlying pathology due to obscuration or artifacts. Other imaging modalities, like CT or X-ray, can better visualize tissues near metal.
Soft Tissue Contrast
While MRI excels at differentiating between varieties of soft tissue, there are certain structures and pathologies it cannot easily distinguish:
- Cervical mucus versus cervical discharge indicative of infection
- Adenomyosis of the uterus versus endometrial masses/polyps
- Benign versus malignant soft tissue masses
- Abscess fluid versus hematoma fluid
Finding the borders between some organs can also be difficult, like the gallbladder and liver. Clinical information and other test results may be needed to clarify the diagnosis.
Physiological Processes
MRI provides a static structural view of anatomy but does not show real-time functional processes in the body. Important activities going on at the cellular and metabolic level cannot be directly visualized on MRI, including:
- Blood flow and perfusion
- Oxygenation status
- Inflammation and edema
- Neuronal depolarization activity
- Cerebral glucose metabolism
- Hepatic and renal function
Advanced MRI techniques like fMRI, perfusion imaging, and MR spectroscopy can quantify some of these physiological parameters. But other modalities like PET, SPECT, EEG, MEG provide more direct functional information.
Bone Marrow Abnormalities
MRIs can detect bone marrow replacement processes but cannot fully characterize them. Cancers like leukemia and lymphoma infiltration, as well as bone marrow scarring or infarction, will show up bright on T2 MRI sequences. However, the appearance does not indicate the specific pathology or stage of disease activity.
A bone marrow biopsy is required for definitive diagnosis and typing of marrow disorders found on MRI. More advanced MRI methods may provide additional details in the future but cannot yet replace pathological examination.
Causes of Pain, Numbness, or Weakness
MRIs are commonly performed to look for pinched nerves, disc herniations, muscle tears, and other anatomical sources of pain or neurological symptoms. However, finding structural abnormalities does not necessarily identify them as the definitive cause of a patient’s symptoms.
For example, a disk bulge seen on MRI may be asymptomatic while the real pain source is subtle nerve inflammation. MRIs do not detect radiculopathy, neuropathy, complex regional pain syndrome, or other micropathology. Clinical correlation is required to determine if MRI findings directly explain the patient’s complaints.
Certain Cancers and Tumors
MRI can detect masses, but it cannot definitively diagnose cancer or determine the grade of malignancy. Some examples include:
- Brain tumors – biopsy establishes cancer type and grade
- Adrenal masses – biopsy and hormone tests diagnose adrenocortical carcinoma
- Gynecologic tumors – pap smears and biopsies determine if cervical, uterine, or ovarian cancers are present
- Prostate cancer – MRI unreliable for diagnosing small or early-stage prostate cancers
Since malignant tumors enhance differently than benign masses, MRI has some utility for suggesting likelihood of cancer. However, pathological confirmation is still required for definitive diagnosis in many cases.
Certain Infections and Inflammation
MRIs can only suggest infection or inflammation. It cannot identify specific infectious organisms or definitively differentiate infectious versus inflammatory processes. Some examples include:
- Meningitis – MRI cannot differentiate between bacterial, fungal, or viral causes
- Encephalitis – MRI alone cannot diagnose if viral encephalitis is herpes simplex vs other viruses
- Osteomyelitis – MRI cannot distinguish bacterial joint infection from inflammatory arthritis
- Pelvic inflammatory disease – MRI appearance similar for gonorrhea, chlamydia, etc
Cultures, serological studies, or tissue biopsies are required to definitively diagnose the type of infection when MRI shows evidence of active inflammatory processes.
Certain Metabolic, Toxic, and Radiation Injuries
MRI findings of tissue edema and swelling from metabolic problems, toxins, or radiation exposure are nonspecific. Examples requiring other diagnostic tests include:
- Hepatic encephalopathy – blood tests indicate liver failure
- Toxic leukoencephalopathy – lab studies confirm toxin exposure
- Radiation necrosis – history of radiation treatments
Clinical history and lab work are needed to pinpoint the precise etiology of the MRI abnormalities in these situations.
Conclusion
MRI is one of the best imaging tools available to visualize soft tissue structures and identify areas of pathology in the human body. However, there are limitations to what MRI alone can diagnose. In many cases, correlating MRI results with other tests and clinical information is necessary to reach the correct diagnoses.
Being aware of the constraints of MRI technology helps medical professionals use the modality appropriately and interpret the findings accurately. Combining MRI with other imaging, laboratory, pathological, and clinical data allows the care team to build the clearest possible picture of a patient’s health status.