4.1 Brain and Head/Neck Protocols

Brain and Head/Neck Protocols

MRI of the brain and head/neck region requires high-resolution imaging, excellent soft-tissue contrast, and precise slice planning. The technologist must optimize the signal-to-noise ratio (SNR) while managing artifacts from CSF flow, vascular pulsation, swallowing, and involuntary eye movements.

Patient Positioning and Coil Selection

The patient is positioned supine and head-first in a dedicated multi-channel phased-array head coil. Proper immobilization of the head using foam pads is essential to prevent motion artifacts. The patient's head must be positioned such that the interpupillary line is parallel to the table, preventing tilt. The laser light is centered at the glabella (midway between the eyebrows) for brain, pituitary, orbits, and TMJ protocols. For orbits or nasopharyngeal protocols, the nasion (bridge of the nose) may be used. For soft tissue neck protocols, a neurovascular or dedicated neck coil is used, with the laser centered at the mentum (chin) or thyroid cartilage.

Brain MRI Protocols

A standard brain MRI protocol consists of multiplanar sequences to evaluate anatomy and pathology:

• Sagittal Localizer: Aligned along the falx cerebri (midline of the brain) to plan subsequent axial and coronal scans.
• Axial-Oblique Slices: Aligned parallel to the anterior commissure-posterior commissure (AC-PC) line (also known as the subcallosal line). This alignment standardizes slice orientation, enabling comparison across sequential exams. Slice coverage must extend from the foramen magnum through the vertex of the skull.
• Coronal Slices: Planned perpendicular to the AC-PC line, covering from the frontal sinuses through the occipital protuberance.

Key sequences include T1-weighted Spin Echo (SE) (high spatial resolution for anatomy), T2-weighted Fast Spin Echo (FSE) (highly sensitive to pathology/edema), Fluid Attenuated Inversion Recovery (FLAIR) (utilizing an inversion time [TI] of ~2000–2500 ms to null cerebrospinal fluid [CSF] signal to highlight periventricular lesions like demyelinating plaques in multiple sclerosis), and Diffusion-Weighted Imaging (DWI) (using b-values of 0 and 1000 s/mm²) to identify acute ischemic stroke by displaying restricted water diffusion (bright on DWI, dark on ADC maps).

Pituitary Gland Protocol & Dynamic Contrast Timing

Pituitary imaging focuses on the sella turcica, requiring high-resolution, thin slices (2–3 mm) and a small field of view (FOV) of 12–16 cm.

• Slice Alignment: Sagittals are planned parallel to the midline of the brain. Coronals are planned perpendicular to the AC-PC line (or parallel to the brainstem/pituitary stalk), covering from the posterior clinoid processes to the optic chiasm.
• Dynamic Contrast Enhancement (DCE): Pituitary microadenomas (<10 mm) are often isointense to normal gland tissue on baseline scans. To visualize them, a rapid dynamic T1-weighted sequence is performed immediately after a bolus injection of a gadolinium-based contrast agent (GBCA), with images acquired every 15–30 seconds for 2–3 minutes. The normal pituitary gland enhances rapidly and intensely because it lacks a blood-brain barrier. Microadenomas enhance more slowly, appearing as hypointense (dark) nodules against the brightly enhancing normal gland during the early dynamic phases.

Orbits and Internal Auditory Canals (IAC) Protocols

• Orbits: Axial slices are planned parallel to the optic nerve using the sagittal localizer. Coronal slices are planned perpendicular to the optic nerve. Slices are thin (2–3 mm) with a small FOV. Post-contrast T1-weighted sequences must include fat suppression (fat saturation). Retro-orbital fat is normally bright on T1. Suppressing this signal is critical to allow abnormal enhancement of the optic nerve (e.g., in optic neuritis) or orbital tumors to stand out clearly.
• IAC: This protocol evaluates the cerebellopontine angle (CPA) and cranial nerves VII (facial) and VIII (vestibulocochlear) for acoustic neuromas (vestibular schwannomas). Axial slices (1.5–2 mm) are planned parallel to the IACs (perpendicular to the brainstem), covering from the superior margin of the petrous ridge to the inferior margin of the mastoid process. High-resolution 3D T2 sequences like FIESTA, CISS, or SPACE utilize sub-millimeter isotropic voxels to display the cranial nerves as dark structures surrounded by bright CSF. Post-contrast thin-slice T1-weighted images with fat suppression are used to identify small intracanalicular tumors.

Soft Tissue Neck and TMJ Protocols

• Soft Tissue Neck: Axial slices are planned parallel to the vocal cords or hard palate, covering from the clivus (skull base) to the thoracic inlet (clavicles/aortic arch). Coronal slices are aligned parallel to the cervical spine. Post-contrast T1-weighted images are acquired with a delay of 60 to 90 seconds. This delay allows contrast to distribute throughout the soft tissues, lymph nodes, and venous system, optimizing the differentiation between pathologically enhancing neck masses/nodes and adjacent normal structures.
• TMJ: Evaluates the positioning and mobility of the articular disc. Sagittal-oblique slices are planned perpendicular to the long axis of the mandibular condyle. Coronal-oblique slices are planned parallel to the long axis of the mandibular condyle. Thin slices (1.5–2 mm) are performed in both closed-mouth and open-mouth positions (using a non-magnetic bite block). Normally, the posterior band of the articular disc sits at the 12 o'clock position relative to the mandibular condyle in the closed-mouth position, and translates anteriorly between the condyle and articular eminence during jaw opening.

Summary of Protocol Requirements