Computed Tomography |
63 |
Reid’s baseline
Orbitomeatal line
Figure 2-1. Direction of the x-ray beam in the axial plane. The orbitomeatal line extends from the lateral canthus to the center of the external auditory meatus and is the reference plane for positive (+) and negative (–) angulation. Reid’s anatomic baseline extends from the inferior orbital rim to the upper margin of the external auditory meatus. Axial head scans are performed with positive angulation and may include the medulla oblongata and the base of the frontal lobe on the lowest slice; the orbit will be missed. Axial orbit scans are performed with negative angulation. Separate orbit and head sequences are usually required.
is the plane of the optic canal, which is −10° to RBL and −20° to OML. Orbits are usually scanned parallel to RBL or −10° to OML to achieve an axial-plane angle parallel to the orbital floor. For intracranial structures, angulation between 0° and +25° to OML is useful, with less positively angulated images preferred for the sellar region, the middle range preferred for the cerebral hemispheres, and the most positively angulated images preferred for the posterior fossa.
2-5 MULTIPLANAR RECONSTRUCTION
The newer multislice scanners have the ability to obtain isotropic images, so multiplanar reconstructions with good quality can be obtained. This feature eliminates the need for direct coronal images and decreases the total radiation dose for the patient.
2-6 COMPUTER ANALYSIS
CT scans can be oriented to slice or volume. Slice-oriented scanning uses single sections for diagnostic value (Figures 2-2 to Figure 2-34). Thin sections and proper alignment of the plane of scanning are crucial to achieving maximal resolution.
(text continued on page 79)
Figure 2-2. Axial CT scan near the cervicomedullary junction demonstrating normal anatomy. CT, cerebellar tonsil; LPM, lateral pterygoid muscle; MO, medulla oblongata; TM, temporalis muscle.
Figure 2-3. Axial CT scan of the normal medulla oblongata (MO) and cerebellar hemispheres (Cb). ES, ethmoid sinus; G, globe; L, lens; TM, temporalis muscle; SS, sphenoid sinus.
64
Figure 2-4. Axial CT scan demonstrating normal anatomy at the level of the pituitary gland (PiG). Cb, cerebellum; G, globe; ON, optic nerve; P, pons; SOF, superior orbital fissure; SS, sphenoid sinus; TL, temporal lobe.
Figure 2-5. Axial CT scan demonstrating normal anatomy in the region of the sella turcica. AC, anterior clinoid process; BA, basilar artery; Cb, cerebellum; DS, dorsum sella; FS, frontal sinus; ICA, internal carotid artery; OF, optic foramen; P, pons; SOV, superior ophthalmic vein; TL, temporal lobe; V4, fourth ventricle.
65
Figure 2-6. Axial CT scan demonstrating normal anatomy at the level of the upper pons (P). BA, basilar artery; Cb, cerebellum; DS, dorsum sella; FL, frontal lobe; FS, frontal sinus; ICA, internal carotid artery; PC, posterior clinoid process; TL, temporal lobe; V4, fourth ventricle.
Figure 2-7. Axial CT scan demonstrating normal suprasellar anatomy. Cb, cerebellum; FL, frontal lobe; MB, midbrain; OC, optic chiasm; PS, pituitary stalk; TL, temporal lobe; V4, fourth ventricle.
66
Figure 2-8. Axial CT scan at the level of the midbrain (MB) demonstrating normal anatomy. CA, cerebral aqueduct; FL, frontal lobe; ICi, interpeduncular cistern; MaB, mamillary body; OT, optic tract; TL, temporal lobe; V, vermis of cerebellum.
Figure 2-9. Axial CT scan at the level of the third ventricle (V3) demonstrating normal anatomy. FL, frontal lobe; HCN, head of caudate nucleus; LV, lateral ventricle; OL, occipital lobe; QCi, quadrigeminal cistern; V, vermis of cerebellum.
67
Figure 2-10. Axial CT scan demonstrating normal anatomy at the level of the internal capsule (InC). FL, frontal lobe; HCN, head of caudate nucleus; LV, lateral ventricle; OL, occipital lobe; PG, pineal gland; Pu, putamen; SP, septum pellucidum; T, thalamus.
Figure 2-11. Axial CT scan at the level of the body of the lateral ventricle (LV) demonstrating normal anatomy. CR, corona radiata; FC, falx cerebri; FL, frontal lobe; OL, occipital lobe; PL, parietal lobe; SP, septum pellucidum.
68
Figure 2-12. Axial CT scan demonstrating normal anatomy of the centrum semiovale (CeS). FC, falx cerebri; FL, frontal lobe; PL, parietal lobe; SSS, superior sagittal sinus.
Figure 2-13. Axial CT scan demonstrating normal anatomy at the level of the upper calvarium. The central sulcus (CS) separates the precentral motor cortex (single dot) from the postcentral sensory cortex (double dots). FC, falx cerebri.
69
Figure 2-14. Axial bone window CT scan demonstrating normal anatomy at the level of the C1 vertebrae. MS, maxillary sinus; NLD, nasolacrimal duct; Np, nasopharynx; OP, odontoid process; Pt, pterygoid plates of sphenoid bone; ZB, zygomatic bone.
Figure 2-15. Axial bone window CT scan of the skull base demonstrating normal anatomy. ES, ethmoid sinus; IOF, inferior orbital fissure; MaC, mandibular condyle; OcC, occipital condyle.
70
Figure 2-16. Axial bone window CT scan of the skull base demonstrating normal anatomy. C, clivus; CaC, carotid canal; ES, ethmoid sinus; FO, foramen ovale; FSp, foramen spinosum; JF, jugular foramen; LP, lamina papyracea; SS, sphenoid sinus.
Figure 2-17. Axial bone window CT scan of the skull base demonstrating normal anatomy. C, clivus; ES, ethmoid sinus; GWS, greater wing of sphenoid bone; LP, lamina papyracea; MAC, mastoid air cells; SS, sphenoid sinus.
71
Figure 2-18. Axial bone window CT scan of the skull base demonstrating normal anatomy. FS, frontal sinus; MAC, mastoid air cells; SOF, superior orbital fissure.
Figure 2-19. Axial bone window CT scan of the skull base demonstrating normal anatomy. AC, anterior clinoid process; DS, dorsum sella; FS, frontal sinus; ICA, internal carotid artery; MAC, mastoid air cells; OR, orbital roof; PA, petrous apex of temporal bone.
72
Figure 2-20. Axial bone window CT scan of the skull base at the level internal auditory canal (IAC) demonstrating normal anatomy. CN7, seventh cranial nerve; Co, cochlea; I, incus; M, malleus; MAC, mastoid air cells; ScC, semicircular canal; TyC, tympanic cavity; Ve, vestibule.
Figure 2-21. Axial CT scan of the mid-orbit demonstrating normal anatomy. CaS, cavernous sinus; ES, ethmoid sinus; G, globe; L, lens; LR, lateral rectus muscle; MC, Meckel cave; MR, medial rectus muscle; ON, optic nerve; SOF, superior orbital fissure; SS, sphenoid sinus.
73
Figure 2-22. Axial CT scan of the mid-orbit demonstrating normal anatomy.CaS, cavernous sinus; DS, dorsum sella; G, globe; LG, lacrimal gland; OCa, optic canal; ON, optic nerve; PiG, pituitary gland; SOF, superior orbital fissure.
Figure 2-23. Axial CT scan of the superior orbit demonstrating normal anatomy. AC, anterior clinoid process; G, globe; PC, posterior clinoid process; SOV, superior ophthalmic vein.
74
Figure 2-24. Axial bone window CT scan of the mid-orbit demonstrating normal anatomy. ES, ethmoid sinus; GWS, greater wing of sphenoid bone; LP, lamina papyracea; MAC, mastoid air cells; NS, nasal septum; PA, petrous apex of temporal bone; SOF, superior orbital fissure; SS, sphenoid sinus.
Figure 2-25. Axial bone window CT scan of the mid-orbit demonstrating normal anatomy. CG, crista galli; DS, dorsum sella; GWS, greater wing of sphenoid bone; OCa, optic canal; SOF, superior orbital fissure.
75
Figure 2-26. Axial postcontrast CT scan demonstrating normal cavernous sinus (CaS) anatomy. BA, basilar artery; G, globe; PiG, pituitary gland; SOF, superior orbital fissure; TC, tentorium cerebelli.
Figure 2-27. Axial postcontrast CT scan demonstrating normal anatomy of the midorbit. AC, anterior clinoid process; BA, basilar artery; G, globe; ICA, internal carotid artery; MR, medial rectus muscle; OA, ophthalmic artery; OCa, optic canal; ON, optic nerve; PC, posterior clinoid process; PCA, posterior cerebral artery; PCoA, posterior communicating artery; SOF, superior orbital fissure.
76
Figure 2-28. Axial postcontrast CT scan demonstrating normal anatomy of the superior orbit. MCA, middle cerebral artery; SOV, superior ophthalmic vein.
Figure 2-29. Coronal soft tissue window (left) and bone window (right) CT scans demonstrating normal anatomy at the anterior globe (G). FB, frontal bone; FS, frontal sinus; L, lens; LS, lacrimal sac; MPL, medial palpebral ligament; MxB, maxillary bone; NS, nasal septum; SOT, superior oblique muscle tendon.
Figure 2-30. Coronal soft tissue window (left) and bone window (right) CT scans demonstrating normal anatomy at the mid-globe (G). CG, crista galli; IO, inferior oblique muscle; LG, lacrimal gland; LPS, levator palpebrae superioris muscle; MR, medial rectus muscle; MS, maxillary sinus; NLD, nasolacrimal duct; SR, superior rectus muscle.
77
Figure 2-31. Coronal soft tissue window (left) and bone window (right) CT scans demonstrating normal orbital anatomy. FB, frontal bone; ICa, infraorbital canal; IR, inferior rectus muscle; IT, inferior turbinate; LP, lamina papyracea; LPS, levator palpebrae superioris muscle; LR, lateral rectus muscle; MR, medial rectus muscle; MS, maxillary sinus; NS, nasal septum; ON, optic nerve; SO, superior oblique muscle; SOV, superior ophthalmic vein; SR, superior rectus muscle; ZB, zygomatic bone.
Figure 2-32. Coronal soft tissue window (left) and bone window (right) CT scans demonstrating normal orbital apex anatomy. GWS, greater wing of sphenoid bone; IR, inferior rectus muscle; IOF, interior orbital fissure; LPS, levator palpebrae superioris muscle; LR, lateral rectus muscle; LWS, lesser wing of sphenoid bone; MR, medial rectus muscle; MS, maxillary sinus; ON, optic nerve; SR, superior rectus muscle.
Figure 2-33. Coronal soft tissue window (left) and bone window (right) CT scans of the optic nerve (ON) within the optic canal (OCa) demonstrating normal anatomy. AC, anterior clinoid process; IOF, inferior orbital fissure; PMF, pterygomaxillary fissure; SOF, superior orbital fissure; SPF, sphenopalatine foramen; SS, sphenoid sinus.
78