Scanning and reconstruction parameters
Computer tomography is based on the passage of X-rays through the patient's body with data registration by detectors. Standard scanning parameters for adults include a tube voltage of 120 kV (kV) and a tube current of 200–400 mA (mA). The gantry rotation time ranges from 0.5 to 0.75 seconds.
The quality of Z-axis data acquisition determines the spiral pitch (pitch) — the ratio of the table feed per gantry rotation to the beam collimation. Too low a pitch increases radiation exposure, while a high pitch leads to data gaps.
Various algorithms are used for the mathematical construction of images:
- Filtered back projection (Filtered back projection, FBP): A traditional method calculating density for each voxel. It is fast, but prone to noise from dense structures.
- Iterative reconstruction (Iterative reconstruction): A modern method in which projections are repeatedly compared with calculated data. This significantly reduces noise levels and radiation dose, making the image smoother.
Contrast enhancement in neuroimaging
The use of contrast agents significantly expands the diagnostic capabilities of the method, allowing for the visualization of vascular and tissue pathologies.
- Intravenous administration: Used to detect pathologies with a compromised blood-brain barrier (blood-brain barrier), such as tumors (e.g., glioblastomas) or abscesses. For CT angiography, the dose is increased to 60–70 mL followed by a rapid saline flush (saline flush).
- Intrathecal administration: Contrast is injected into the subarachnoid space for CT myelography. The method allows for the detection of CSF leakage (CSF leak) and a detailed study of the cranial nerves, optic chiasm, and basal foramina.
Image evaluation: Hounsfield scale
Each voxel (volume element) of the image has a numerical density value measured in Hounsfield units (Hounsfield Units, HU). The scale is normalized to distilled water at 0°C.
The physician can adjust image brightness and contrast by regulating the window width (window width) and window level (window level). There are highly specialized "stroke windows" (stroke windows) for detecting early ischemia and bone algorithms for fracture detailing.
| Tissue / Medium | Density (HU) |
|---|---|
| Air (Air) | -1000 |
| Fat (Fat) | -100 to -50 |
| Water (Water | 0 |
| Cerebrospinal fluid | 8–15 |
| White matter | 20–30 |
| Blood and Gray matter | 30–45 |
| Bone tissue (Bone) | 700 to 3000 |
Main indications for the procedure
CT is the first-line method in emergency medicine due to its high scanning speed. The main urgent indications are:
- Acute traumatic brain injury: Complete replacement of skull radiography, rapid detection of fractures and hematomas (often combined with cervical spine CT).
- Acute stroke: Non-contrast CT rules out hemorrhage, while CT perfusion and CT angiography evaluate the arterial bed and ischemic changes to determine the feasibility of thrombolysis.
- Intracranial hemorrhage: Detection of blood and diagnosis of aneurysms.
- Herniation syndromes and rapidly progressive hydrocephalus.
In situations where magnetic resonance imaging (MRI) is unavailable or contraindicated, CT is used for seizures, syncope, ataxia, and suspected infections.
Post-processing methods and common artifacts
Data acquisition with thin collimation allows for multiplanar reformations (MPR), creating coronal and sagittal projections (e.g., for assessing the pituitary gland). Maximum intensity projections (MIP) and volume rendering (Volume rendering) are used for surgical planning.
The most common image distortions are:
- Motion artifacts (Patient motion artefact): Occur due to breathing or in confused patients, leading to image blurring.
- Beam hardening (Beam hardening): Low-energy photons are absorbed by massive bone structures. The remaining hard rays create hypoattenuation bands in brain tissue, most commonly between the tips of the temporal bone pyramids.
Briefly: answers to topical questions
What spiral pitch value is considered optimal to prevent data gaps and excessive radiation exposure?
The source indicates that the spiral pitch (pitch) ranges from 1 to 2. A low pitch increases the radiation dose, while a high pitch leads to gaps in tissue irradiation and Z-axis data collection. The specific optimal value is not stated in the source.
Hounsfield scale baseline points: Air is minus one thousand (void), Water is exactly zero, Bone is from one thousand and above (solid). Blood and gray matter lie close together in a narrow range of 30–45 HU.
Class and exam questions
Why is CT chosen in acute head trauma?
CT is performed significantly faster and possesses the highest accuracy in visualizing skull bone fractures, while also enabling the emergency detection of life-threatening intracranial hemorrhages.
What is the difference between filtered back projection and iterative reconstruction?
Filtered back projection is a traditional algorithm that is faster but produces more noise and artifacts. Iterative reconstruction mathematically recalculates data multiple times, reducing radiation dose and image noise.
How to eliminate motion artifacts in neuroimaging?
In CT, motion minimization is achieved by shortening the scan time. In MRI (used as an alternative), special correction algorithms can be used, such as PROPELLER, which hardware-compensate for patient movement.
What else to study on this topic
- Technique and parameters of brain CT perfusion
- Differential diagnosis of glioblastomas based on contrast-enhanced CT data
- Specialized artifact suppression algorithms in MRI (PROPELLER)
- ALARA principles and pediatric radiation dose calculation
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