Brain Coil Setup, Landmarking & Imaging Planes

Patient Preparation & Hardware Setup

Coil Selection & Positioning Mechanics

• Phased-Array Head Coil: Brain MRI utilizes dedicated multi-channel receive-only head coils (typically 16, 32, or 64 channels) configured as a rigid helmet or cradle. These arrays provide exceptional signal-to-noise ratio (SNR) near the skull surface and adequate penetration to deep midline structures.
• Patient Orientation: The patient is positioned head-first supine (HFS) along the longitudinal center of the patient couch.
• Immobilization: The patient's head is gently settled into the posterior base of the coil cradle. Non-conductive, closed-cell foam pads (wedge and rectangular sponges) are firmly but comfortably inserted bilaterally between the patient's temporal regions and the coil inner walls. This immobilization prevents rotational head drift and micro-motion during long acquisitions.
• Supportive Positioning: A contoured positioning sponge or bolster is placed beneath the patient's knees to flatten the lumbar spine against the table, relieving lower back tension and preventing involuntary leg movement.

Claustrophobia Mitigation & Comfort Aids

The rigid enclosure of a head coil can induce acute anxiety or claustrophobia. The Imaging Assistant must execute the following supportive measures:

• Angled Mirror / Prism Glasses: Secure an angled mirror attachment to the upper bridge of the head coil. This allows the patient to look up and see outward through the open bore toward the scan room or control window, effectively dissolving the feeling of confinement.
• Hearing Protection: MRI acoustic noise generated by gradient coil switching during brain protocols frequently exceeds 105 to 115 dBA. Both earplugs (providing a Noise Reduction Rating [NRR] of 29–33 dB) and padded acoustic headset earphones must be provided.
• Pneumatic Squeeze Bulb: Place the emergency squeeze bulb directly in the patient's hand and test it before advancing the table into the bore.
• Lighting and Ventilation: Ensure the bore ventilation fan is active (medium or high airflow across the patient's face) and interior bore lighting is comfortably illuminated.

Anatomical Landmarking & Laser Crosshair Alignment

Precise anatomical landmarking is essential because scanner software uses the laser reference mark to automatically advance the patient table so that the anatomy of interest resides precisely at the magnetic isocenter—the geometric center of the static magnetic field ($B_0$) where magnetic field homogeneity and gradient linearity are at their absolute peak.

Brain Landmarking Workflow:
Supine Head-First ➔ Foam Immobilization ➔ Activate Laser Crosshairs
➔ Align Sagittal Laser to Midline / Coronal to Nasion (or Glabella)
➔ Depress Zero / Landmark Button ➔ Advance Table to Magnetic Isocenter

Primary Landmarks: Nasion vs. Glabella

• Nasion: The depression at the root of the nose, directly between the eyes at the junction of the nasal bones and the frontal bone. This is the commonly used reference landmark for routine cranial imaging.
• Glabella: The smooth, slightly elevated bony prominence situated directly superior to the nasion, between the two superciliary arches (eyebrows). It serves as an equivalent, clinically interchangeable landmark.
• Laser Alignment Steps:
  1. Sagittal Midline: Align the vertical longitudinal laser directly along the sagittal midline of the face (tip of the nose, nasion, philtrum), ensuring no head tilt (lateral rotation or yaw).
  2. Transverse / Coronal Laser: Align the horizontal transverse laser crosshair directly across the nasion (or glabella).
  3. Table Height (Vertical Laser): Adjust the vertical table height so the horizontal laser line passes through the external auditory meatus (EAM) or mid-orbit, placing the center of the brain in the vertical center of the bore.
  4. Zeroing: Press the "Landmark" or "Set Zero" button on the gantry display panel to register table coordinates before driving the couch to the isocenter.

Imaging Planes, Prescriptions & Coverage Boundaries

Routine clinical brain imaging is performed in three orthogonal planes, each aligned to specific internal anatomical reference structures:

1. Axial Plane (AC-PC Line Alignment)

• Prescription Axis: Axial slices are angled parallel to the anterior commissure–posterior commissure (AC-PC) line (also known as the bicommissural line) as visualized on the midline sagittal localizer. Alternatively, slices may be angled parallel to the inferior margin of the corpus callosum or the subcallosal line.
• Superior Boundary: Must completely cover the superior cerebral cortex at the cranial vertex.
• Inferior Boundary: Must extend inferiorly through the foramen magnum, incorporating the cerebellar tonsils and the upper cervical-medullary junction.

2. Sagittal Plane (Falx Cerebri Alignment)

• Prescription Axis: Aligned parallel to the longitudinal cerebral fissure and the falx cerebri on axial and coronal localizers.
• Lateral Boundaries: Slices must extend laterally from the outer cortex of the right temporal lobe to the outer cortex of the left temporal lobe, covering the entire external ears and lateral calvarium.

3. Coronal Plane (Brainstem Alignment)

• Prescription Axis: Angled parallel to the dorsal margin of the brainstem and the floor of the fourth ventricle, perpendicular to the AC-PC line.
• Anterior-Posterior Boundaries: Must extend from the frontal sinus/anterior frontal pole through the posterior occipital protuberance and cerebellar folia.

Applying These Examples Safely

Landmark, coil, arm/leg rotation, support and patient entry direction are examples that vary with the scanner, coil and prescribed examination. Use manufacturer marks and the local protocol; the technologist confirms coverage on localizers. Surface landmarks estimate internal position and are not interchangeable anatomical points. Never force a painful or unstable joint or spine to match a memorized angle. Report limited motion, trauma precautions, altered sensation or pressure immediately. Protocol selection, slice prescription, scanning and interpretation remain with the qualified technologist and radiologist.

Cranial MRA and MRV

Cranial MRA emphasizes arteries, including the circle of Willis, while cranial MRV emphasizes veins and dural venous sinuses. They answer different clinical questions from a routine structural brain examination. Some vascular techniques use IV contrast and others do not; confirm the actual order and preparation rather than assuming all vascular studies require the same injection. Stable supported head position and appropriate head-coil coverage remain important. The technologist selects the sequence, coverage and timing and checks the localizers. The assistant communicates device, contrast or patient-tolerance concerns without independently adding a vascular acquisition.