Magnetic Resonance Imaging (MRI) of the spine is a sophisticated diagnostic tool that provides detailed pictures of soft tissues, distinguishing it from X-rays or CT scans which primarily image bone. MRI uses strong magnetic fields and radio waves to create cross-sectional images, making it the preferred method for evaluating the spinal cord, nerve roots, and intervertebral discs. Understanding how these images are generated helps a person better grasp the findings in their report. The images reveal the structure of the vertebrae, the health of the cushioning discs, and any potential compression of neural elements. This capacity to visualize soft tissue with high contrast makes the MRI invaluable for diagnosing spinal conditions.
Understanding MRI Imaging Basics
The appearance of different tissues on an MRI depends on their signal intensity (brightness or darkness). Tissues with a strong signal are hyperintense (bright), while those with a weak signal are hypointense (dark). This visual contrast is controlled by two primary sequences: T1-weighted and T2-weighted images.
T1-weighted images are used to display anatomical detail, as they highlight fat content. On a T1 scan, fat, which is abundant in the bone marrow of the vertebrae, appears bright, while water and cerebrospinal fluid (CSF) appear dark. This sequence provides excellent clarity for mapping the overall structure of the spine.
T2-weighted images are sensitive to water and inflammation, making them the sequence of choice for identifying most spinal pathologies. On a T2 scan, water and CSF appear bright, the opposite of T1 images. Areas of inflammation or hydrated tissue will also show up as bright. Conversely, dense structures like cortical bone and air are dark on both T1 and T2 images.
Mapping the Spinal Anatomy
Interpreting a spinal MRI requires understanding how core structures appear using the T1 and T2 contrast principles. The vertebral bodies, the bony segments of the spine, appear bright on T1-weighted images due to their fatty marrow. These segments are surrounded by a thin, dark line representing the cortical bone, which lacks signal on both sequences.
Between each vertebral body sit the intervertebral discs, composed of the soft, gel-like nucleus pulposus and the tough, fibrous outer ring, the annulus fibrosus. A healthy nucleus pulposus is highly hydrated, appearing distinctly bright white on T2-weighted images. The surrounding annulus fibrosus and the bony endplates appear dark on T2 due to their lower water content.
The spinal cord and cerebrospinal fluid (CSF) are found within the central spinal canal. Since CSF is mostly water, it appears dark on T1 images but exceptionally bright on T2, creating a high-contrast background. The cord appears as an intermediate gray structure running through the bright CSF column. Nerve roots exit the spinal canal through small lateral openings called foramina.
Interpreting Image Orientation and Views
Spinal MRI scans are typically acquired in three main planes, or views, to provide a complete three-dimensional perspective of the anatomy. Each orientation offers unique information about the spinal structures and any associated pathology.
Sagittal View
The Sagittal view presents a side profile of the spine. This plane is valuable for evaluating the overall alignment of the vertebral bodies, the height of the intervertebral discs, and the integrity of the spinal cord. It allows for a clear visualization of how far a disc may be extending into the spinal canal, which is a common cause of central cord compression.
Axial View
The Axial view is a cross-sectional image, appearing like a horizontal slice. This view is essential for assessing the diameter of the central spinal canal and the smaller nerve root openings. The axial view is the best way to determine the specific location and size of a disc herniation or bony spur, especially when trying to pinpoint the exact nerve root being compressed.
Coronal View
The Coronal view shows the spine from front to back. It is generally less frequently relied upon for routine spinal evaluation compared to the sagittal and axial planes. The coronal view is sometimes used to assess the overall side-to-side alignment of the spine or to evaluate certain bone marrow abnormalities.
Identifying Common Spinal Findings
Disc degeneration, or desiccation, is a frequently observed finding referring to the normal aging process of the intervertebral discs. As a disc loses water content over time, its nucleus pulposus loses its bright signal on the T2-weighted image, appearing gray or black instead. This loss of T2 signal indicates that the disc has become less hydrated and flexible.
Disc bulge and disc herniation are two related but distinct findings describing the displacement of disc material beyond the normal boundary of the vertebrae. A disc bulge is a generalized, broad-based, and symmetric extension of the disc contour, typically affecting more than half of the disc’s circumference. In contrast, a disc herniation is a focal displacement of the nucleus pulposus, involving a tear in the outer annulus fibrosus, and usually affects less than a quarter of the disc’s circumference.
Spinal stenosis is another common condition, defined as the abnormal narrowing of the spinal canal (central stenosis) or the nerve root openings (foraminal stenosis). On axial T2-weighted images, central stenosis is visible when the bright CSF signal surrounding the nerve roots is partially or completely obliterated by thickened ligaments or disc material. Foraminal stenosis is best seen on sagittal views where the small fat-filled space surrounding the exiting nerve root is compressed by bony overgrowth or disc extension.
Endplate changes, known as Modic changes, are alterations in the bone marrow of the vertebral bodies adjacent to a degenerated disc.
Modic Changes
Modic changes are categorized into three types:
- Modic Type 1 changes represent bone marrow edema and inflammation, appearing dark on T1-weighted images and bright on T2-weighted images.
- Modic Type 2 changes reflect the conversion of inflamed marrow to fatty tissue, presenting as a bright signal on both T1 and T2 images.
- Modic Type 3 changes indicate subchondral sclerosis, where the bone becomes dense, resulting in a dark signal on both T1 and T2 sequences.
It is important to remember that while the MRI provides detailed images of structure, the official interpretation and correlation with a person’s symptoms must be done by a qualified radiologist or physician.