Skull SMV (Cranial Base) is a radiography positioning projection of the Skull. Vertex on IR. IOML parallel to IR. CR perpendicular to IOML. The centering point is located midway between the angles of the mandible, perpendicular to IOML. IOML parallel to IR. The central ray is perpendicular to IOML. Centered midway between the angles of the mandible, about 3/4 inch anterior to the EAMs (1.5 inches below the mandibular symphysis). Image-quality criteria include entire cranial base visible, mandibular symphysis over the frontal bone, mandibular condyles anterior to the petrous pyramids. A sample technique is 85 kVp with DR 14 or CR 28 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Cranial base and foramen magnum.
- Sphenoid sinus and foramina at the base.
- Mastoid processes and petrous pyramids.
- Mandible projected anteriorly over the frontal bone.
- Bony nasal septum and maxillary sinuses.
- Zygomatic arches.
Patient preparation
- Trauma patient: rule out cervical spine fracture or subluxation before attempting this projection.
- Verify patient identity.
- Remove glasses, earrings, dentures, and hairpins.
- Position patient supine or upright with head fully extended.
- Rest vertex of head against the IR.
- IOML (infraorbitomeatal line) parallel to IR.
- MSP perpendicular to IR with no rotation.
- Suspend respiration during exposure.
Position & centering point
Midway between the angles of the mandible, perpendicular to IOML. IOML parallel to IR.
Central ray
Perpendicular to IOML. Centered midway between the angles of the mandible, about 3/4 inch anterior to the EAMs (1.5 inches below the mandibular symphysis).
Exposure / technique
- kVp
- 85
- mAs
- DR 14 / CR 28
- SID
- 40" (102 cm)
- Notes
- Grid required. Head extension critical for IOML parallel to IR. Sample technique: DR 14 / CR 28 mAs at 85 kVp, 40 in SID, for a 23 cm part. Adjust for your equipment.
kVp and mAs are sample values. Use caution when applying them in your department.
Image-quality criteria
- Entire cranial base visible, mandibular symphysis over the frontal bone.
- Mandibular condyles anterior to the petrous pyramids.
- No rotation: MSP parallel to the edge of the IR. No tilt: equal distance from mandibular ramus to lateral cranial cortex.
- Symmetric appearance of petrous pyramids and mastoids.
- Sphenoid sinus and foramen ovale visible.
- Correct extension: mandibular mentum projected anterior to the ethmoid sinuses.
Common errors
- 1 Inadequate neck extension: mandibular mentum not anterior to the ethmoid sinuses.
- 2 Tilt: unequal distance from mandibular ramus to lateral cranial cortex. Rotation: MSP not parallel to the IR edge.
- 3 IOML not parallel: mandibular condyles not anterior to the petrous pyramids.
Clinical indications
- Evaluation of basal skull fracture.
- Assessment of sphenoid sinus pathology.
- Advanced bony pathology of the inner temporal bone structures (skull base).