Pulse sequences and basic modes
A standard T2-weighted image (T2-WI) provides maximal contrast between the spinal cord, nerve roots, and cerebrospinal fluid (CSF). However, gradient echo — GRE T2 — is frequently used for examining the cervical and thoracic spine. This mode features a shorter echo time (*TE) and minimizes CSF pulsation artifacts. It also has drawbacks: low intra-medullary lesion contrast, inferior signal-to-noise ratio (SNR) compared to fast spin-echo (FSE), and extreme sensitivity to magnetic field inhomogeneities.
Another crucial tool is FLAIR (fluid-attenuated inversion recovery). This is a heavily T2-weighted sequence that completely eliminates the CSF signal (causing a "signal drop-out" effect, where fluid appears black). In the spine, this eliminates distortions related to CSF motion while keeping pathological lesions hyperintense.
To acquire thin slices without loss of SNR, three-dimensional (3D) imaging is employed. For example, 3D FSE allows for scanning segments with 1–2 mm slices and reduces CSF "flow void" artifacts. 3D GRE requires a short repetition time (TR), but remains highly vulnerable to phase shifts caused by patient motion.
Fat-suppression technologies
Suppressing the high MR signal from lipids (e.g., in bone marrow) is critical for identifying pathologies. Several main approaches are utilized:
- Frequency-selective fat saturation (FS): Employs a radiofrequency pulse at the lipid resonant frequency. Provides complete fat suppression and is highly useful in T2-WI for detecting occult vertebral fractures. Drawbacks: requires a high-field scanner, increases specific absorption rate (SAR), and prolongs scan time.
- STIR (Short Tau Inversion Recovery): A technique based on a short inversion time (TI). It is extremely sensitive to musculoskeletal pathologies (metastases, trauma, Modic changes, Baastrup's disease) because it synergistically prolongs T1 and T2 in affected tissues. The main advantage of STIR is its complete insensitivity to magnetic field inhomogeneity, making it applicable on low-field scanners. However, it increases scan time and is contraindicated for the thoracic spine.
- SPAIR and SPIR: Combine FS and STIR techniques. For the spine, SPAIR is preferred as it provides high SNR and faster scanning, though it operates exclusively on high-field scanners.
Diffusion-weighted imaging (DWI)
Performing spinal DWI is associated with technical difficulties. This is due to high magnetic inhomogeneity in this region, small anatomical structures, low SNR, and involuntary patient motion (respiration, vascular pulsation).
Nevertheless, DWI is actively used for:
- Diagnosing spinal cord infarction (restricted diffusion).
- Detecting active multiple sclerosis plaques.
- Evaluating post-traumatic changes and compression.
- Searching for drop metastases in hypercellular pediatric tumors.
Specific artifacts in spine MRI
Artifacts are distorted signals lacking a true anatomical basis. It is important to remember that 3T scanners generate more artifacts than standard 1.5T units.
- Magnetic susceptibility artifacts: Occur at tissue interfaces. They are especially pronounced following surgeries involving metallic instrumentation. The most sensitive modes to these artifacts are EPI and GRE. To minimize metal-induced distortion, SE or FSE sequences should be used.
- Motion artifacts: Caused by swallowing (in the cervical spine) or cardiac contractions (in the thoracic spine). Minimized using spatial presaturation techniques.
- Truncation artifacts: Associated with imperfections in mathematical Fourier transforms. They appear as bright or dark lines parallel to sharp signal intensity transitions (e.g., at the spinal cord-CSF boundary). Can lead to overdiagnosis of spinal stenosis. Eliminated by increasing spatial resolution (matrix size).
- CSF pulsation artifacts: Cerebrospinal fluid motion during systole produces linear phase-encoding directional ghosts. In the thoracic spine, turbulent flow zones associated with arachnoid trabeculae can visually mimic intradural mass lesions.
Briefly: answers to topical questions
What are the absolute contraindications to spinal MRI?
Absolute contraindications to MRI include the presence of certain electronic implantable devices and ferromagnetic foreign bodies.
- Electronic implantable devices — pacemakers, middle ear implants, insulin pumps.
- Ferromagnetic foreign bodies — metal fragments inside or near the eyeball, cerebral aneurysm clips.
At the same time, modern models of certain implantable devices may be MRI-conditional, making the examination feasible.
What are the Modic types of degenerative bone marrow changes identified on MRI?
Three types of Modic degenerative bone marrow changes are distinguished on MRI.
- Type 1 changes (Type 1 changes) — bone marrow oedema (bone marrow oedema) due to acute or subacute inflammation. Characterized by low signal on T1-weighted images and high signal on T2-weighted images.
- Type 2 changes (Type 2 changes) — fatty degeneration, characterized by the replacement of normal bone marrow with adipose tissue. Yields high signal on T1-weighted images and isointense to hyperintense signal on T2-weighted images.
- Type 3 changes (Type 3 changes) — reactive osteosclerosis (reactive osteosclerosis). Manifests as decreased signal intensity on both T1-weighted and T2-weighted images.
To remember the sensitivity of pulse sequences to metal artifacts (from strongest to weakest), use the acronym E-G-S-F: EPI > GRE > SE > FSE (Echo Planar Imaging is the most vulnerable, Fast Spin Echo is the most robust).
Class and exam questions
Why is the GRE sequence not used in patients with spinal metal implants?
Due to the high sensitivity of GRE to local magnetic field inhomogeneities, pronounced signal loss occurs around metallic hardware.
How to distinguish a truncation artifact from a true spinal cord pathology on MRI?
The truncation artifact appears as bright or dark lines running strictly parallel to the boundary between the spinal cord and cerebrospinal fluid. To verify and eliminate this artifact, the physician increases the spatial resolution (matrix size).
What is the main clinical advantage of the STIR sequence over the FS method?
The STIR sequence is virtually insensitive to magnetic field inhomogeneities. Consequently, it can be successfully applied on low-field MRI scanners, whereas fat saturation (FS) requires high field strength and homogeneity.
What else to study on this topic
- Assessment of spinal cord infarction and restriction of diffusion on DWI
- Differential diagnosis of CSF pulsation artifacts and intradural tumors
- Physical principles of truncation artifact formation (Fourier transform)
- Differences and clinical applications of SPIR and SPAIR technologies
- Three-dimensional imaging (3D FSE and 3D GRE) in spinal neuroradiology
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