A Lumbar Magnetic Resonance Imaging (L-MRI) scan uses powerful magnetic fields and radio waves to create detailed, cross-sectional pictures of the lower back. This non-invasive procedure visualizes soft tissues, nerves, and intervertebral discs, which are often the source of lower back and leg pain. Unlike X-rays or CT scans, the L-MRI excels at showing water content and subtle changes in soft tissues, making it an invaluable diagnostic tool.
Preparing for the Scan and the Procedure
Before an L-MRI, patients must undergo screening for safety risks associated with the magnetic field. Disclose any metal implants, such as pacemakers, neurostimulators, or surgical clips, as these are often incompatible. Patients must remove all metallic items, including jewelry and watches, and change into a hospital gown.
The scan involves lying still on a narrow table inside a tube-shaped scanner for 30 to 90 minutes. The machine generates loud thumping and humming noises, so earplugs or headphones are provided. For patients with anxiety or claustrophobia, sedation or an “open” MRI machine may be alternative options.
Some L-MRI scans require a Gadolinium-based contrast agent, injected intravenously. This material enhances the visibility of structures like tumors, inflammation, or scar tissue, making them appear brighter. If contrast is used, patients may be asked to fast and must inform the technologist of any kidney problems, as Gadolinium concerns those with impaired renal function.
Understanding Normal Lumbar Anatomy on MRI
Interpreting an L-MRI begins with recognizing the appearance of a healthy lumbar spine. The five vertebral bodies (L1 through L5) appear as stacked rectangular blocks separated by intervertebral discs. T1-weighted images show a bright signal in the vertebral bodies due to fatty marrow, while the bone signal is less prominent on T2-weighted images.
Intervertebral discs consist of the tough outer annulus fibrosus and the gel-like nucleus pulposus. A healthy, hydrated nucleus pulposus appears bright white (high signal intensity) on T2-weighted images because this sequence highlights water content. The surrounding cerebrospinal fluid (CSF) also appears bright white on T2 images, contrasting with the darker spinal cord tissue.
The spinal cord is visible in the mid-sagittal view, ending as the conus medullaris near L1. Below L1, the nerve roots continue downward as the cauda equina, floating within the bright CSF. The nerve roots exit the spinal canal through openings called the neural foramina. This consistent appearance establishes the baseline for measuring pathology.
Identifying Common Spinal Conditions
Pathology is detected by deviations from the normal signal intensity or shape of spinal structures. Degenerative disc disease involves disc desiccation, where the nucleus pulposus loses water and appears dark on T2-weighted images. This leads to reduced disc height and the formation of bony outgrowths, called osteophytes, along the vertebral edges.
A disc herniation occurs when the nucleus pulposus extends beyond the annulus fibrosus, categorized by displacement degree. A disc bulge is a broad, symmetrical extension. A protrusion involves a focal extension, and an extrusion is a severe form where the material is forced out, potentially compressing adjacent nerve roots.
Spinal stenosis refers to the narrowing of the bony canals housing the neural elements. Central stenosis narrows the main spinal canal, crowding the cauda equina. Foraminal stenosis describes the narrowing of the neural foramina, often caused by facet joint hypertrophy or disc material, impinging upon the exiting nerve root.
Infections like discitis or osteomyelitis may show abnormally high signal intensity on T2-weighted images within the disc space and adjacent vertebral bodies. This is often paired with a decreased signal on T1-weighted sequences, indicating edema and inflammation.
Decoding the MRI Report Jargon
The written report uses specific language, starting with the planes of imaging. Sagittal images provide a side-view, while axial images offer a cross-sectional view. The report references T1 and T2 weighting, which determine tissue brightness; T2 is water-sensitive and T1 is fat-sensitive.
“Signal intensity” is used instead of “brightness” or “darkness.” A structure losing fluid, such as a desiccated disc, is described as having low signal intensity on T2. “Effacement” indicates that a normal space, like the area around a nerve root, has been partially or completely obliterated by a herniated disc or thickened ligament.
The report may mention Modic changes, which are degenerative alterations in the bone marrow adjacent to the endplates. These changes are classified into types: Type 1 indicates inflammation, and Type 2 represents fatty replacement. Severity is indicated using terms like “mild,” “moderate,” or “severe” to qualify the degree of stenosis or nerve root compression. Understanding this vocabulary connects the visual image with the clinical description of spinal health.