Routine Brain and Trauma CT Technique

Acquisition settings and contrast timings below are illustrative adult protocol examples. Select the authorized protocol for the indication, scanner, body size, access device, and clinical condition. Treatment decisions belong to the responsible clinical team.

Clinical Role of Routine Non-Contrast Head CT (NCHCT)

Non-contrast computed tomography of the head (NCHCT) remains the most frequently performed emergency neuroimaging examination in modern medicine. Because of its universal availability, rapid scan acquisition times (often under 5 to 10 seconds on modern multidetector scanners), and exceptional sensitivity for acute blood products and calvarial disruptions, NCHCT functions as the primary diagnostic triaging tool in three critical emergency clinical scenarios:

1. Acute Traumatic Brain Injury (TBI): Identifying acute epidural, subdural, subarachnoid, and intraparenchymal hemorrhages, cerebral contusions, midline shift, mass effect, herniation syndromes, and calvarial or skull base fractures.
2. Acute Altered Mental Status (AMS) & Coma: Rapidly ruling out acute hydrocephalus, substantial intracranial hemorrhage, massive intracranial neoplasms, and global cerebral edema.
3. Acute Ischemic Stroke Triage: The primary and absolute mandate of emergency NCHCT in suspected acute ischemic stroke is to rule out intracranial hemorrhage. Hemorrhage changes the acute stroke pathway and must be promptly assessed by the stroke team. Noncontrast CT evaluates hemorrhage, mass effect and visible ischemic changes; it does not prove baseline tissue viability. CTA and selected perfusion imaging can add vascular and physiologic information under the clinical pathway.

Patient Positioning, Landmark Alignment & Gantry Tilting

Proper patient positioning and scan plane alignment are critical to optimize diagnostic image quality, minimize beam-hardening artifacts, and protect radiosensitive ocular structures.

Positioning Reference Lines: OML vs. IOML

• Orbitomeatal Line (OML): An imaginary reference line connecting the outer canthus of the eye to the center of the external auditory meatus (EAM). The OML represents the traditional standard positioning plane for axial cranial imaging.
• Infraorbitomeatal Line (IOML / Reid's Base Line): A reference line connecting the inferior margin of the bony orbit (infraorbital rim) to the center of the EAM. The IOML forms an angle approximately $9^\circ\text{ to }10^\circ$ steeper (inferior) relative to the OML.
• Clinical Standard: The patient is placed supine on the CT table with the head placed symmetrically within a dedicated head holder. The chin is gently tucked toward the chest, and the head is secured with forehead and chin straps to prevent motion. Alignment lights are utilized to ensure zero lateral head tilt and zero rotation (the sagittal laser light must bisect the nasion, nasal septum, and philtrum).

Lens protection and safe positioning

Reduce direct lens exposure when the clinical question permits, using an acquisition plane and scan range that actually exclude the lenses while covering the required brain. OML or IOML alignment alone is not a guarantee. Inspect the localizer and planned inferior boundary. Changing a reformatted display plane after acquisition does not remove exposure already delivered. Organ-based modulation and other dose strategies depend on the scanner and approved protocol.

Do not flex or tilt a trauma patient's neck without appropriate clearance. Keep immobilization and airway support intact. If safe lens exclusion cannot be achieved, obtain the clinically required coverage with an optimized technique rather than omitting important anatomy. Dense petrous bone can produce posterior-fossa beam hardening; appropriate acquisition and reconstruction reduce the problem without guaranteeing its elimination.

Technical Parameters & Dedicated Display Windows

Sequential (Axial Step-and-Shoot) vs. Helical Acquisition

• The Step-and-Shoot Standard: While helical (spiral) scanning is universally utilized for CT angiography (CTA) and volumetric 3D examinations, sequential axial step-and-shoot mode is one established option for routine non-contrast brain CT. In sequential acquisition, the patient table remains stationary during data acquisition for each rotation, and steps forward only between tube rotations. This reduces helical interpolation artifacts, windmill artifacts, and z-axis cone-beam distortion across the dense calvarium, with performance depending on the scanner and protocol. Validated helical head techniques are also used.
• Exposure Factors: Routine brain CT demands low-contrast detectability to differentiate gray matter (attenuation $+35\text{ to }+40\text{ HU}$) from adjacent white matter ($+25\text{ to }+30\text{ HU}$)—a narrow density differential of merely $5\text{ to }10\text{ HU}$. Consequently, brain protocols employ $120\text{ kVp}$ and relatively high tube current-time products ($250\text{ to }400\text{ mAs}$) to suppress quantum mottle and maintain adequate signal-to-noise ratio (SNR).
• Slice Collimation: Contiguous $2.5\text{ mm}$ images are reconstructed through the posterior fossa (from the foramen magnum through the petrous ridges to the tentorium cerebelli) to mitigate partial volume averaging across complex skull base bone-brain interfaces. Contiguous $5.0\text{ mm}$ images are reconstructed from the tentorium up to the calvarial vertex.

Display Window Settings

To improve task-specific conspicuity, display windowing must be precisely tailored to highlight specific intracranial pathological processes:

• Standard Brain Window (WW 80, WL 40): Window width (WW) of $80\text{ HU}$ centered at a window level (WL) of $+40\text{ HU}$. Spreads the entire grayscale spectrum across the narrow density range of brain tissue ($0\text{ to }+80\text{ HU}$), maximizing visual distinction between normal cerebral cortex, subcortical white matter, deep gray nuclei, and ventricles.
• Narrow Stroke Window (WW 30 to 40, WL 30 to 35): By compressing the grayscale range to just $30\text{ HU}$, subtle $2\text{ to }3\text{ HU}$ attenuation drops caused by early cytotoxic intracellular water accumulation become strikingly conspicuous to the reader.
• Bone Window (WW 2500 to 3000, WL 500 to 700): An extremely broad window width suppresses image contrast, allowing visualization of structural cortical bone architecture from $+500\text{ to }+2000\text{ HU}$. This window can reduce display saturation that obscure subtle non-displaced calvarial linear fractures, depressed skull fractures, and skull base disruptions.
• Subdural / Intermediate Window (WW 150 to 200, WL 75 to 80): On standard brain windows, an acute extra-axial blood clot ($+60\text{ to }+80\text{ HU}$) abutting the inner table of the calvarium ($>+1000\text{ HU}$) blends with bone due to partial volume averaging and display saturation. The subdural window elevates the level and widens the display width, clearly separating the hyperdense rim of an acute subdural hematoma from the hyperdense calvarial bone.