Features of Spinal CT
Computed tomography is indispensable for evaluating posterior vertebral structures, diagnosing facet joint pathologies, and identifying Baastrup's disease. In the postoperative period, CT is used to visualize implanted surgical materials and assess their potential instability.
- Patient positioning: The procedure is performed in the supine position. If the patient has no injuries, the neck is slightly flexed — this helps reduce the natural cervical lordosis.
- Scanning process: Modern multislice scanners provide fast and continuous data acquisition. Prior to the main phase, a topogram (Scout image / Localiser) is acquired, which is a lateral, anteroposterior, or combined digital radiograph. It is used to select the scan volume and retrospectively localize slice levels. Notably, breath-holding is not required for spinal CT.
- Data reconstruction: Slices are formatted in the sagittal and axial planes (strictly parallel to the intervertebral discs). Specialized algorithms for both bone and soft tissues are applied during processing.
Types of CT Reconstructions
Multislice data acquisition opens up wide possibilities for post-processing. The physician can perform curved planar reformations (especially relevant in scoliosis) and create volumetric 3D models.
- Axial slice: Viewed in a soft-tissue window. Ideal for evaluating the structure of the spinal canal and adjacent soft tissues.
- Sagittal slice: Evaluated in a bone window. Allows for the study of the bony structure of vertebral bodies, spinous processes, and the height of intervertebral disc spaces.
- Three-dimensional (3D) reformation: Generates a volumetric external model of the examined segment and pelvis for spatial evaluation of deformities.
- 3D reformation with a virtual cut: Makes it possible to "slice" the 3D model (e.g., perform a left cut) to clearly assess the spinal canal (spinal canal) lumen and internal cancellous bone structure.
Spinal MRI: Protocols and Artifacts
MRI is the method of choice due to the absence of ionizing radiation. The method is based on aligning the body's protons along the magnetic field lines of a strong magnet. Radiofrequency pulses disrupt this equilibrium, allowing the differentiation of water, fat, and bone based on their distinct resonance.
The standard protocol includes a multi-directional localiser, used to plan sagittal planes, which in turn are used to plan high-resolution axial slices. The routine set includes sagittal T1, T2, fat-suppressed T2 (such as STIR), and axial T2. For the lumbar spine, an axial T1 is mandatory.
To suppress motion artifacts, a spatial presaturation slab (spatial presaturation slab) is used:
- At the cervical level: positioned above the larynx and carotid artery.
- At the thoracic and lumbar levels: applied to mitigate artifacts from respiration, aortic and cardiac pulsation.
Difference Between T1- and T2-Weighted Images
T1-weighted images (Spin-Echo T1) Contrast depends on the rate of longitudinal magnetization recovery (T1). It uses a short repetition time (TR 300–700 ms) and a short echo time (TE <30 ms). Tissues with a long T1 (CSF / CSF) do not have time to relax and appear dark. Tissues with a short T1 (fat) have a bright signal. T1 provides excellent anatomical detail of bone marrow and discs.
T2-weighted images (SE and fast FSE / TSE) Are based on transverse relaxation time (T2). They require a long TR (2000–3000 ms) and a long TE (80–120 ms). The main T2 marker is bright cerebrospinal fluid (CSF). The spinal cord and nerves have an intermediate signal. The method is extremely sensitive to pathologies with increased extracellular fluid. Important nuance: On fast spin-echo (FSE), fat appears brighter than on conventional SE, which can sometimes mask vertebral metastases.
Application of Contrast Enhancement
Paramagnetic agents (gadolinium chelates - Gd) are administered intravenously as contrast at a dose of 0.1 mmol/kg. Gadolinium shortens the T1 and T2 of surrounding molecules, which manifests as increased signal intensity on T1.
Contrast enhancement in intra-axial (intra-axial) lesions occurs due to blood-brain barrier (BBB) disruption, while in extra-axial (extra-axial) lesions it is due to hypervascularity. In spinal surgery, contrast is critical for the differential diagnosis of scar tissue (postoperative fibrosis) versus recurrent disc herniation.
Briefly: answers to topical questions
How does normal and pathological bone marrow appear on T1- and T2-weighted images?
In health and disease, the signal intensity of bone marrow on MRI depends on its composition and the nature of the lesion. Normally, fatty bone marrow has a high (bright) signal on T1-weighted images (T1WI) and appears brighter on fast spin-echo (FSE) T2WI. With age, hyperintense foci appear. In pathology, the picture changes:
- Modic type 1 changes (edema) — decreased signal on T1WI and increased on T2WI.
- Modic type 2 changes (fatty degeneration) — increased signal on T1WI, isointense or hyperintense on T2WI.
- Modic type 3 changes (osteosclerosis) — decreased signal on T1WI and T2WI.
- Leukemia and reconversion — diffuse or focal signal decrease on T1WI and hyperintensity on fat-suppressed T2WI.
What are the absolute contraindications for spinal MRI?
Sources explicitly indicate the presence of metallic foreign bodies as an absolute contraindication to MRI. Cardiac pacemakers are listed as a contraindication, and claustrophobia as a limitation for the procedure.
What anatomical structures form the facet (zygapophyseal) joint?
The facet (zygapophyseal) joint is formed by the articular processes of adjacent vertebrae and is a true synovial joint. Its structure includes the following elements:
- Superior articular process (Processus articularis superior) — features a concave articular facet.
- Inferior articular process (Processus articularis inferior) — features a convex articular facet.
- Articular cartilage — hyaline cartilage covering the articular surfaces of the processes.
- Articular capsule — attached along the periphery of the cartilage. On the posterolateral side, it is represented by a robust fibrous capsule with a synovial membrane, whereas on the anterior side, the fibrous capsule is absent.
Class and exam questions
Does the patient need to hold their breath during a spinal CT scan?
No, breath-holding is not required during spinal CT. Special spatial presaturation slabs are used to eliminate respiratory artifacts in MRI.
How to distinguish T1 from T2 on MRI scans?
The main landmark is cerebrospinal fluid (CSF). On T1-weighted images, CSF appears dark, whereas on T2-weighted images it exhibits a characteristic bright signal.
Why is fat suppression used after contrast administration?
This helps differentiate contrast-enhancing scar tissue from epidural fat and postoperative blood, which is particularly important in adult patients with fatty degeneration of the bone marrow.
What else to study on this topic
- Gradient echo (GRE) images and blood flow visualization
- Differential diagnosis of herniation recurrence and fibrosis using contrast
- Physics of artifact formation in fast spin-echo (FSE) compared to SE
- Postoperative MRI protocols and metal artifact suppression
- Proton relaxation mechanisms (longitudinal and transverse magnetization)
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