Magnetic Resonance Imaging (MRI) of the spine is a non-invasive diagnostic procedure that uses powerful magnets and radio waves to generate detailed images of the spinal column and its surrounding soft tissues. This technique visualizes bones, nerves, cartilage, ligaments, tendons, and muscles, offering a comprehensive view. MRI scans are effective for evaluating soft tissues and identifying various spinal conditions without using ionizing radiation.
Anatomy of the Spine on MRI
The spinal column is composed of 33 individual bones called vertebrae, stacked and cushioned by intervertebral discs. These vertebrae are divided into distinct regions: 7 cervical (neck), 12 thoracic (mid-back), 5 lumbar (lower back), 5 fused sacral, and 4 fused coccygeal (tailbone) vertebrae. On MRI, vertebral bodies, the main weight-bearing parts, appear bright on T1-weighted images due to fatty marrow and have an intermediate signal on T2-weighted images.
Intervertebral discs are positioned between each vertebra, functioning as shock absorbers. A healthy disc consists of a gel-like inner core, the nucleus pulposus, surrounded by a tough, fibrous outer ring, the annulus fibrosus. On T2-weighted MRI, healthy discs appear bright in the center due to high water content, while the fibrous outer ring appears darker.
The spinal cord, a major part of the central nervous system, extends from the base of the skull down to the upper lower back, ending around the L1-L2 level in adults. On T1-weighted MRI, the spinal cord appears with an intermediate signal intensity, while on T2-weighted images, it appears brighter, surrounded by the bright signal of cerebrospinal fluid (CSF).
Nerve roots branch off the spinal cord and exit through small openings between the vertebrae, known as neural foramina. These individual nerves appear as dark structures against the bright background of CSF on T2-weighted images, and their pathways through the foramina are clearly visible. The presence of fat around the nerve roots in the foramina indicates ample space.
Ligaments are strong fibrous bands that connect and stabilize the vertebrae, appearing as dark, linear structures on all MRI sequences. The anterior longitudinal ligament (ALL) runs along the front of the vertebral bodies, while the posterior longitudinal ligament (PLL) runs along the back. The ligamentum flavum, which connects the laminae of adjacent vertebrae, appears as a dark band behind the dura. Paraspinal muscles, which support the spine, show an intermediate signal intensity on MRI.
Cerebrospinal fluid (CSF) is the clear fluid that surrounds and cushions the spinal cord within the spinal canal. On T1-weighted MRI, CSF appears dark, while on T2-weighted images, it has a bright signal. This difference in signal helps in visualizing the spinal cord and nerve roots floating within the CSF.
Understanding Different MRI Views and Sequences
MRI scans produce images in various planes, or “views,” which are important for accurately identifying structures and detecting abnormalities.
The sagittal view offers a side-profile image of the spine, showing the overall curvature and alignment of the vertebral column. This view also allows visualization of disc heights, the spinal canal, and the relationship between the discs and the spinal cord. It helps detect conditions like fractures or abnormal curvatures.
The axial view provides cross-sectional “slices” of the spine, allowing a detailed look at a specific vertebral level. This perspective is helpful for examining the relationship of the intervertebral disc to the spinal cord and nerve roots. It allows visualization of the neural foramina, the openings where nerve roots exit, and identifies disc herniations that might not be as apparent in the sagittal view.
The coronal view presents a front-to-back image of the spine. While less commonly used for routine spinal imaging compared to sagittal and axial views, it is beneficial for identifying scoliosis or detecting certain ligamentous abnormalities. This view allows for a broader assessment of the vertebral bodies and potential lateral issues.
MRI sequences, such as T1-weighted and T2-weighted images, highlight different tissue properties, aiding in distinguishing structures and identifying potential issues. T1-weighted images are generated with short repetition times (TR) and short echo times (TE), making fat appear bright (hyperintense) and fluid, like CSF, appear dark (hypointense). These sequences are preferred for visualizing anatomical structures and assessing bone marrow signal intensity.
T2-weighted images, in contrast, use longer TR and TE times, causing tissues with high water content, such as fluid and areas of inflammation or edema, to appear bright. Fat also appears bright on T2-weighted images, though specialized sequences like STIR (Short-Tau Inversion Recovery) can suppress fat signal to highlight fluid-related abnormalities. T2-weighted sequences are useful for detecting pathology, including disc degeneration, herniations, and spinal stenosis, as they show the spinal cord, CSF, and intervertebral discs.
Key Terms and Common Findings on a Spine MRI Report
Spine MRI reports often contain specific terminology describing common findings.
Disc degeneration refers to age-related changes in the intervertebral discs, which can include loss of disc height and altered signal intensity. On T2-weighted MRI, a degenerated disc appears darker due to reduced water content, a process known as disc desiccation.
A disc bulge describes a generalized outward protrusion of the disc, where the inner gel-like material pushes against the outer fibrous ring, causing it to extend beyond its normal boundaries without a complete rupture of the outer layer. This condition affects a quarter to half of the disc’s circumference. While a bulge may not always cause symptoms, it can sometimes press on adjacent nerves.
Disc herniation is a more focal protrusion where the outer fibrous layer of the disc tears or cracks, allowing the inner jelly-like material to leak out. Unlike a bulge, a herniation involves a rupture or tear in the annulus fibrosus, and the disc material extends beyond its normal confines, potentially compressing spinal nerves or the spinal cord.
Spinal stenosis is characterized by a narrowing of the spinal canal or the neural foramina, the bony openings through which nerve roots exit. On MRI, this narrowing appears as a constricted space around the spinal cord or nerve roots. Causes can include thickened ligaments, bone spurs (osteophytes), or disc issues pushing into the canal.
Spondylosis, also known as osteoarthritis of the spine, refers to age-related degenerative changes in the vertebral bones, discs, and facet joints. On MRI, findings may include bone spurs (osteophytes) forming on the vertebral bodies and changes in the facet joints. These changes can lead to narrowing of the spinal canal or nerve root compression.
Edema or inflammation appears as bright signals on certain MRI sequences, such as T2-weighted images. This increased signal intensity indicates the presence of excess fluid within tissues, such as bone marrow or soft tissues. While non-specific, it suggests an underlying pathological process like trauma, infection, or an inflammatory condition.
Other general terms commonly seen in reports include “alignment,” which refers to the overall curvature and positioning of the spinal column, assessed in sagittal views. “Signal intensity” describes the brightness or darkness of tissues on MRI, reflecting their water and fat content. “Effusion” denotes the accumulation of excess fluid within a joint, such as a facet joint.
Please note that this information is for educational purposes only and should not be used for self-diagnosis. A qualified healthcare professional should always interpret your MRI report in conjunction with your medical history and clinical examination.