H. Wayne Slone, MD
CT, or computed axial tomography, is an imaging technique that uses x-rays to obtain cross-sectional images. The appearance of x-ray-imaged structures depends on their density. Water is arbitrarily assigned the value of zero, with denser structures like bone having positive values and less dense tissues such as fat and air having negative values. Today, the most CT scanners are of fourth generation (multislice, volumetric acquisition). Multislice detectors allow faster imaging, acquisition of thinner slices, faster and better reconstructions, and improved image quality. The more the detectors the greater and faster the coverage, allowing for improved CT angiography among a number of improvements. Currently, 16- and 64-slice detector CT scanners are commonly used, with 160- and 256-slice scanners becoming available.
- Conventional CT
- Axial images only, except for head-direct coronal images can be obtained if the patient is able to lay prone with head extended. With modern scanners direct coronal imaging not necessary as reconstructed images are just as good.
- Reconstructions—computer generated: Sagittal, coronal, 3D
- Cisternography and myelography—after the intrathecal administration of contrast
- CT angiography (CTA)—requires contrast with 3D reconstructions
- Circle of Willis
- Carotid arteries
- Functional CT
- CT perfusion—requires contrast, cerebral blood flow imaging
- Xenon CT—cerebral blood flow imaging
- Interventional
- CT fluoroscopy—real-time imaging
- Intraoperative CT, portable CT
Examination of choice for evaluation of acute intracranial hemorrhage, calcifications, and cortical bone:
- Acute intracranial hemorrhage—subdural, epidural, subarachnoid, intraparenchymal, intraventricular
- Mental status/neurological change—rule out (R/O) hemorrhage
- Headache—for “worst headache of my life”; R/O subarachnoid hemorrhage. For chronic headaches MR is preferable, although imaging is generally not indicated.
- Stroke—R/O hemorrhage but also R/O arterial occlusion (CTA) and ischemia/infarction (CT perfusion);
- early CT findings of acute ischemic stroke:
- Hyperdense artery sign: Thrombus seen in 35–50% with clinical signs of acute middle cerebral artery stroke; poor prognostic sign
- Obscuration of lentiform nucleus
- Insular ribbon sign
- Sulcal effacement
- Parenchymal hypodensity
- CT perfusion findings of ischemic stroke:
- Decreased cerebral blood volume
- Decreased cerebral blood flow
- Increased mean transit time
- Increased time to peak
- CT perfusion findings of ischemic stroke:
- Trauma—R/O hemorrhage, edema, herniation, pneumocephalus, fracture
- Any patient with loss of consciousness, neurologic deficit, anisocoria, fixed or dilated pupils, bleeding diathesis, or anticoagulation, and all penetrating head injuries
- Hydrocephalus
- New-onset seizure—R/O hemorrhage or mass
- Postoperative craniotomy—R/O hemorrhage, herniation
- In patients where MR is contraindicated (e.g., pacemaker) for:
- Tumor
- Infection—abscess, empyema, AIDS
- Seizures
- Multiple sclerosis
- Neurodegenerative disorders
- Granulomatous disease
- Trauma—R/O fracture
- Postoperative fusion—metal will cause some artifacts limiting the exam
- Spondylolysis
- Arthritis
- Spinal stenosis (MR is examination of choice)
- Disc disease (MR is examination of choice)
- Cord compression—only postmyelography with injection of intrathecal contrast
- Characterization of an isolated indeterminate bone lesion note on MR or nuclear medicine scan (e.g., hemangioma)
- In patients where MR is contraindicated (e.g., pacemaker) for:
- Tumor—ideally after intrathecal contrast
- Infection—epidural abscess, discitis, osteomyelitis, although even CT with contrast is relatively insensitive
Indications for Contrast with Head CT
- Tumor—(MR is exam of choice)
- Infection—abscess, empyema, AIDS, (MR is exam of choice)
- Seizure—R/O tumor (MR is exam of choice)
- Arteriovenous malformations
- CT angiography/venography
- CT perfusion
Indications for Intravenous Contrast with Spine CT
- Tumor—if MR contraindicated
- Infection—if MR contraindicated
- Disc disease—to enhance the epidural space/veins and better define the margins of the discs, although MR is still best
Indications for Intrathecal Contrast with Spine CT (Postmyelogram CT)
- Cord compression—when MR is contraindicated
- Disc disease, spinal stenosis, radiculopathy—if MR is contraindicated or if MR findings do not correlate with clinical findings.
- Readily available 24/7 even at small hospitals
- Noninvasive
- Fast—ideal for uncooperative and critically ill patients
- Extremely sensitive for acute intracranial hemorrhage
- Ideal for evaluation of calcifications and cortical bone
- Beam-hardening artifacts limit posterior fossa evaluation
- Insensitive to acute ischemia although CTA and CT perfusion improve sensitivity
- Limited spinal cord evaluation
- Limited soft tissue contrast
- Metal streak artifacts
- Radiation dose—current hot topic because as CT scanner capabilities have improved, radiation doses have increased. Especially important in pediatric imaging. Newer techniques such as CT perfusion and CT angiography have significantly increased radiation doses.
