Contrast injection protocols
High injection rates and sufficient iodine concentration are required for quality visualization of vascular structures. Today, dual-phase injection (biphasic injection) is considered the gold standard. First, a dual-head automatic injector delivers the contrast agent, followed by a saline chaser.
This approach dilutes the contrast in peripheral veins. It prevents streaking and streak and beam-hardening artefacts in the brachiocephalic veins, which often occur during monophasic injection due to excessive contrast density. In addition, the saline chaser makes the enhancement profile more uniform.
For specific tasks, such as triple-rule-out examinations, three-phase protocols are used, where a 50:50 mixture of contrast and saline is administered in the intermediate phase.
Post-processing and reconstruction methods
The introduction of multislice systems has made it possible to obtain truly isotropic images, where each voxel has equal dimensions in all three dimensions. This enables the physician to reformat slices in any arbitrary plane. The main image processing algorithms are:
- MPR (multiplanar reconstruction): evaluation of large bronchi and detection of pulmonary embolism (PE). Helps avoid errors due to the partial volume averaging effect.
- MIP (maximum intensity projection): visualization of blood vessels and the distribution of small nodules. The algorithm displays only structures with density values above a specified threshold.
- MinIP (minimum intensity projection): highlights areas of low density. Ideal for finding areas of emphysema and air trapping.
- VRT (volume rendering): preserves 3D spatial relationships and depth information, used in angiography.
- SSD (shaded surface display): constructs tissue boundaries, used to assess airway anomalies.
- Virtual bronchoscopy: an endoscopy simulation that allows one to "navigate" through an area of bronchial obstruction and examine the distal parts, which is impossible during a real procedure.
Dual-energy CT (DECT)
Dual-energy CT technology is based on material decomposition theory. Tissues with a high atomic number (iodine, calcium) absorb X-rays differently at various photon energies due to the photoelectric effect. By analyzing this difference, the scanner can programmatically differentiate the materials.
Clinical advantages of the method:
- Creation of virtual unenhanced images (virtual unenhanced image) through programmatic iodine subtraction. This eliminates the need for a separate native phase, reducing radiation dose and eliminating misregistration artifacts.
- Generation of pulmonary blood volume maps ("iodine-only" images) to detect perfusion defects in acute and chronic pulmonary embolism. Results are comparable to perfusion scintigraphy data.
- Assessment of true contrast uptake in pulmonary nodules for the differential diagnosis of malignant and inflammatory lesions.
Radiation dose management
The X-ray beam profile consists of a useful full-shadow zone (umbra) and a useless half-shadow (penumbra). The latter represents a "lost" dose that irradiates the patient but is blocked by collimators and does not contribute to image formation. In modern 64-slice scanners, geometric efficiency is high, and the lost dose effect is minimized.
The following strategies are used for additional patient protection:
- Tube current reduction (mAs): a 50% decrease in current halves the effective dose. For chest CT, the current can be safely lowered to 40–70 mAs.
- Voltage adaptation (kV): 120 kV is standard, but for children and thin patients, the voltage is reduced to 100 kV.
- Automatic exposure control (AEC): the system individually adjusts the current to the patient's dimensions, changes it along the table movement axis (Z-axis), and reduces it in angular projections (X- and Y-axes) where tissue density is lower. This saves up to 36% of the dose.
- Iterative reconstruction (IR): modern mathematical algorithms effectively suppress quantum noise. Model-based IR reduces noise levels by approximately 50% compared to traditional filtered back projection (FBP), enabling high-quality imaging at minimal radiation levels.
Briefly: answers to topical questions
What tasks does the three-phase contrast protocol solve in chest CT?
The three-phase contrast protocol is used for Triple-rule-out studies.
Features of the injection phases:
- First phase — administration of undiluted contrast agent.
- Second phase — administration of a contrast and saline mixture (e.g., 50:50 ratio).
- Third phase — flushing with pure saline.
What are the indications and specific features of virtual bronchoscopy?
Virtual bronchoscopy is used to precisely determine the localization, length, and degree of airway stenosis.
Method features:
- Technology — uses surface and volume rendering to create endoscopic simulations.
- Advantage — allows virtual "navigation" through an obstructing lesion to visualize distal segments, which is not possible with conventional bronchoscopy.
- Limitation — rarely used, as it seldom provides information that cannot be obtained using multiplanar reconstruction (MPR).
How does iterative reconstruction (IR) differ from traditional filtered back projection (FBP)?
Iterative reconstruction (Iterative reconstruction, IR) differs from filtered back projection (Filtered back projection, FBP) in its data processing principle, lower noise levels, and the ability to reduce radiation dose.
| Characteristic | Filtered back projection (FBP) | Iterative reconstruction (IR) |
|---|---|---|
| Principle of operation | Traditional mathematical algorithm: solves multiple simultaneous equations of ray sums at various angles to calculate density in HU for each voxel | Projection images are created based on an assumed distribution and compared with real projections; the process is repeated until an acceptable difference is achieved |
| Artifacts and noise | Susceptible to noise and artifacts from dense structures; struggles with noise in low-dose CT | Reduces noise levels and provides a smoother image |
| Radiation dose | Limited in low-dose CT due to noise | Allows radiation dose reduction; hybrid IR algorithms preserve the detail of low-contrast structures while reducing the dose by at least 30% compared to FBP |
To easily remember the difference between projections: MIP (Maximal) — looking for Maximally dense structures (vessels, nodules), MinIP (Minimal) — looking for Minimally dense structures (air, emphysema).
Class and exam questions
Why is it not recommended to reduce tube voltage to 80 kV during chest CT?
Such low tube voltage causes unacceptable beam-hardening artifacts even in patients with minimal body weight and in pediatric practice.
How does increasing the pitch affect radiation dose in modern multidetector CT scanners?
In systems using the "effective mAs" parameter, the scanner automatically adjusts the tube current when the pitch changes. Therefore, the radiation dose remains unchanged, unlike older single-slice units.
Why save the thinnest slices if raw data are deleted?
Raw data takes up too much space on servers. However, without saved thin slices in the PACS system, a physician will not be able to build quality multiplanar reconstructions in the future.
What else to study on this topic
- Computer-Aided Detection (CAD) systems for automated detection of pulmonary nodules and PE
- Quantitative lung parenchyma assessment (densitometry and density masks in emphysema)
- Technical implementation variants of dual-energy CT (dual-source, fast kVp switching, dual-layer detector)
- The role of ultrasound examination (ultrasonography) in the diagnosis of pleural pathology and lung cancer staging
- Impact of collimation width and detector gaps on the geometric efficiency of the scanner
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