Thoracic & Lumbar/Sacral Spine MRI Positioning

Thoracic & Lumbar/Sacral Spine MRI Positioning

Regional Anatomy: Thoracic, Lumbar, Sacral & Coccygeal Spine

1. Thoracic Spine (T1–T12)

• Vertebral Morphology: Twelve thoracic vertebrae featuring heart-shaped vertebral bodies, circular spinal canals, and long, slender spinous processes that project obliquely downward.
• Normal Curvature: Exhibits a natural posterior convexity (thoracic kyphosis).
• Rib Articulations: Uniquely characterized by costal facets (demifacets) on the vertebral bodies and transverse processes that articulate with the ribs (costovertebral and costotransverse joints). These rib cages impart rigid structural stability, making disc herniations less frequent in the thoracic spine than in the cervical or lumbar regions.
• Thoracic Spinal Cord: Contains the thoracic segment of the spinal cord. Because the spinal canal is narrow relative to the cord diameter, thoracic disc bulges or compression fractures carry a high risk of direct cord compression (myelopathy).

2. Lumbar Spine (L1–L5)

• Vertebral Morphology: Five massive, kidney-shaped vertebral bodies designed to support heavy axial weight-bearing loads, flanked by stout pedicles, broad horizontal spinous processes, and strong laminae.
• Normal Curvature: Displays an anteriorly convex curvature (lumbar lordosis).
• Intervertebral Discs: Large fibrocartilaginous discs composed of an outer fibrous ring (annulus fibrosus) and an inner gelatinous core (nucleus pulposus). Discs at L4–L5 and L5–S1 bear the greatest mechanical torque in the human spine and represent the most common sites of disc degeneration, bulges, and extruded herniations.

3. Neurological Termination: Conus Medullaris & Cauda Equina

Understanding where the spinal cord ends is one of the most critical concepts in neuroimaging:

• Conus Medullaris: The tapered, conical terminal lower end of the spinal cord. In normal adults, the conus medullaris terminates at the L1–L2 vertebral interspace (occasionally between lower T12 and upper L2). A conus terminating below L2 suggests an abnormally low, tethered spinal cord.
• Cauda Equina ("Horse's Tail"): Below L1–L2, the thecal sac contains no solid spinal cord. Instead, it is populated by a collection of descending lumbosacral spinal nerve roots suspended within CSF.
• Filum Terminale: A delicate fibrous strand extending from the apex of the conus medullaris down to the coccyx, anchoring the cord.
• Thecal Sac (Dural Sac) Termination: The fluid-filled thecal sac terminates inferiorly at the level of the S2 vertebra.

Spinal Cord Craniocaudal Termination:
Cervical / Thoracic Cord ➔ Conus Medullaris (Terminates at L1-L2)
➔ Descending Lumbosacral Nerve Roots inside Thecal Sac ➔ Thecal Sac Terminates at S2

4. Sacrum (S1–S5) & Coccyx

• Sacrum: A triangular bone formed by the fusion of five sacral vertebrae (S1–S5). Connects the axial spine to the pelvic girdle via the bilateral sacroiliac (SI) joints. Transmits the sacral nerve roots through anterior and posterior sacral foramina.
• Coccyx: Three to five fused rudimentary coccygeal vertebrae articulating with the sacral apex.

Patient Positioning & Coil Selection

Coil Configuration: Phased-Array Spine Matrix

• Modern clinical MRI systems utilize multi-channel phased-array spine matrix coils embedded directly beneath the surface pad of the patient couch.
• Software automatically selects and energizes the specific coil elements corresponding to the targeted spinal segment (e.g., thoracic array vs. lumbar array), ensuring optimal signal-to-noise ratio (SNR) without manual coil repositioning.

Patient Orientation & The Knee Elevation Bolster

• Orientation: Patients may be scanned head-first supine (HFS) or feet-first supine (FFS). Scanning feet-first is particularly beneficial for lumbar spine studies, as it leaves the patient's head near or outside the bore entrance, dramatically reducing claustrophobia.
• The Knee Wedge / Bolster (Mandatory Practice):
  • Placing a large firm triangle sponge or cylindrical bolster beneath the patient's flexed knees is often helpful when tolerated and permitted by the protocol.
  • Biomechanical Rationale: Elevating and flexing the knees flexes the hips, which rotates the pelvis posteriorly and flattens the natural lumbar lordosis against the table.
  • Clinical Benefits:
    1. Closer Coil Contact: Minimizes the gap between the lumbar skin surface and the embedded spine coil elements, optimizing SNR.
    2. Pain Relief: Relieves severe mechanical tension on the psoas muscles, sciatic nerve, and posterior lumbar facet joints.
    3. Motion Reduction: A patient lying flat on an unpadded table experiences excruciating lower back spasm within minutes, resulting in severe motion artifacts. Knee elevation allows patients to remain comfortable and motionless throughout the scan.

Arm Positioning

• Patients should rest their arms across their upper chest or place them alongside their torso with foam separation pads preventing direct skin-to-bore contact. Arms folded across the chest keep the elbows and forearms out of the sagittal spine FOV, avoiding phase wrap-around artifacts.

Anatomical Landmarking & Laser Crosshair Centering

Precise surface landmarking aligns the targeted spinal segment with the magnetic isocenter:

1. Thoracic Spine Landmarking

• Primary Landmark: The sternal angle (Angle of Louis / manubriosternal junction). This palpable bony ridge where the manubrium meets the sternal body corresponds internally to the T4–T5 vertebral interspace.
• Alternative Landmark: Centering midway between the suprasternal (jugular) notch and the xiphoid process.
• Artifact Mitigation: Respiration and cardiac motion generate ghosting across the thoracic cord. Technologists apply anterior saturation bands across the anterior chest wall and heart, or utilize respiratory gating/triggering.

2. Lumbar Spine Landmarking

• Primary Landmark: The iliac crests. Palpating the superior margin of the bilateral iliac crests identifies the horizontal plane corresponding internally to the L4–L5 intervertebral disc space (or L4 vertebral body).
• Centering Point: The transverse laser crosshair is aligned either directly at the level of the iliac crests (L4–L5) or approximately 1 to 2 inches (2.5–5 cm) superior to the iliac crests to place the mid-lumbar spine (L3) directly at the magnetic isocenter.
• Midline Alignment: Align the vertical sagittal laser along the anterior midline (linea alba / umbilicus).

3. Sacrum & Coccyx Landmarking

• Centering Point: Position the transverse laser crosshair midway between the anterior superior iliac spine (ASIS) and the pubic symphysis, centering directly over the sacral promontory and pelvic midline.

Imaging Prescriptions & Coverage Boundaries

• Sagittal Prescriptions: Must span transversely from the right pedicle / neural foramen through the left pedicle / neural foramen, capturing the entire vertebral body width and transverse processes.
  • Thoracic Coverage: Must include C7/T1 through T12/L1.
  • Lumbar Coverage: Must include T12 superiorly down through the sacrum (S1–S2) inferiorly to verify the position of the conus medullaris and visualize the thecal sac termination.
• Axial Prescriptions: Typically angled parallel to each individual intervertebral disc space (angled stacks through L3–L4, L4–L5, and L5–S1) from the pedicles above to the pedicles below, evaluating neural foraminal patency and thecal sac impingement.