Multidetector CT/Helical CT and Image Characteristics

Medical Radiation Sciences - Multidetector CT/Helical CT Notes

Overview

  • Source: Loyalist College Medical Radiation Sciences 1 (MRAD 1000)

  • Focus: CT Image Characteristics, Quality, and Technical Parameters.

Review of CT Generations and Key Concepts

  • **Key Topics:

    • CT Generations

    • kVp, mAs, Focal Spot

    • Filtered Back Projection (Filters)

    • X-ray Linear Attenuation Coefficient**

Required Readings

  • Bushong, Chapter 27, Pages 373-377, 380-382, 384-390

Key Terms

  • MSCT: Multi-Slice Computed Tomography

  • Modulation: Adjustment of X-ray tube current in real time during scan

  • Contrast Resolution: Ability of a CT scanner to differentiate between tissue densities

  • Spatial Resolution: Ability of the CT scanner to differentiate between two closely spaced objects

Objectives

  • Describe Multi-Detector CT in 4, 16, and 64 Slice contexts.

  • Apply the advantages of higher slice scanners to clinical applications.

  • Understand technique selection in CT and its effect on patient dose and image quality.

  • Describe CT image characteristics including image matrix, Hounsfield units, WW/WL.

  • Connect various CT settings to resulting image quality regarding spatial resolution, contrast resolution, noise, uniformity, and linearity.

Multi-Detector CT Explained

  • Mechanism: While the x-ray tube rotates, the patient moves through the plane of the rotating x-ray beam. Data is collected continuously allowing reconstruction of an image at any z-axis position along the patient.

Comparison: Multi-Slice vs Single Detector Array

  • Single Slice Detector Array: One-dimensional; consists of many detector elements arranged in a single row.

  • MSCT: Uses multiple rows of detectors allowing collection of more data simultaneously.

Multi-Slice Detector Configurations

  • Configuration Details: Number of data collection channels and section thickness can be configured via electronic binning of detector elements.

    • Example: A 4-slice scanner with a detector width of 0.5 mm can yield various slice thicknesses through binning (e.g., 4 x 0.5 mm, 2 x 1 mm, or 1 x 2 mm).

Data Acquisition Rate

  • Slice Acquisition Rate (SAR): Measures efficiency of MSCT.

    • Formula: SAR=racSlicesextacquiredper360extdegreesRotationexttimeSAR = rac{Slices ext{ acquired per } 360 ext{ degrees}}{Rotation ext{ time}}

  • Larger volumes of tissue can be imaged quickly—possible to image from head to toe in a single breath-hold.

    • Formula for Z-axis coverage: Z−axiscoverage=SARimesWimesTimesBZ-axis coverage = SAR imes W imes T imes B where:

    • $W$ = slice width

    • $T$ = imaging time

    • $B$ = pitch

Multi-Slice Scanner Variants

  • 4-Slice Scanners: Initial limitations allowed collection of 4 slices simultaneously.

    • Flexibility in detector configurations presented by binning.

  • 16-Slice Scanner: Introduced in 2002, capable of capturing thinner slices.

    • The inner 16 detector elements are half the size of the outer, allowing thinner simultaneous acquisitions.

  • 64-Slice Scanner: Introduced in 2005, capable of up to 40 mm wide beams, which can increase scatter reaching detectors and affect low-contrast detection.

Specifications of 64-Detector CT Scanners

  • Example Scanner Specifications:

    • GE LightSpeed VCT: 64 x 0.625 mm, Total z-axis detector length 40 mm, Gantry rotation time 0.35 s

    • Philips Brilliance 64: 64 x 0.625 mm, 40 mm, 0.33 s

    • Siemens Sensation 64: 32 x 0.6 mm, 28.8 mm, 0.33 s

    • Toshiba Aquilion 64: 64 x 0.5 mm, 32 mm, 0.33 s

Advanced Scanners Beyond 64 slices

  • Cardiac Imaging Applications: Short scan times, enabling chest scans in under 5 seconds with devices like 128, 256, and beyond scanners.

  • Common installations in Canada include 128 multi-detector scanners.

Cone Beam Effect (MSCT)

  • Definition: Associated with the divergent nature of the x-ray beam. Wider exit beam width can yield different attenuation measurements across tissues, potentially causing streaking for thicker slices.

Advantages of Higher Slice Scanners (MSCT)

  • **Benefits:

    • Increased speed and volume coverage

    • Improved spatial resolution

    • Efficient use of the x-ray beam

    • Decreased patient dose

    • Enables cardiac CT imaging**

Cardiac Multi-Slice CT

  • Optimal Scanning Time: During motionless heart window (typically diastole). Data collection is electrocardiographically gated to reduce motion artifacts.

  • Gating Mechanisms: Two approaches: retrospectively (higher radiation) and prospectively (reducing dose but reliant on regularity of heart motion).

Technique Selection in CT

  • **Key Parameters:

    • kVp:** Typically set between 80-140 (commonly 100-120).

    • mA: Range from 28 to 500.

    • mAs: Generally varies from 28 to over 1000.

  • Tube Modulation: Automatic Tube Current Modulation (ATCM) for maintaining consistent image quality; includes spatial and temporal modulation.

Pitch and Slice Thickness

  • Pitch: Measure of the relationship between patient couch movement and x-ray beam width expressed as ratios. Increasing the pitch above 1:1 increases tissue volume imaging efficiency.

  • Formula and Practical Examples:

    • Pitch=racCouchextMovementBeamextWidthPitch = rac{Couch ext{ Movement}}{Beam ext{ Width}}

    • Thickness influences the amount of detected x-rays.

  • Example Calculations: Calculating tissue imaged based on parameters with given formulas.

CT Image Characteristics

  • Image Matrix and Hounsfield Units: Each pixel represents a volume (voxel) related to tissue density and brightened via Hounsfield Units, ranging from -1000 for air to +3000 for dense bone.

  • Field of View (FOV): Diameter of image reconstruction, affecting pixel size. Increasing matrix size for a fixed FOV reduces pixel size and enhances detail.

  • Window Width/Level: Post-processing techniques to improve image interpretability.

Assessing CT Image Quality

  • Five Principal Characteristics:

    • Spatial Resolution

    • Contrast Resolution

    • Noise

    • Linearity

    • Uniformity

Spatial Resolution

  • Defined by degree of blurring at sharp interfaces; improved by thinner slice thickness; affected by pixel size.

Contrast Resolution

  • Ability to differentiate small variations in density, beneficial in small structures.

Noise

  • Variability in pixel values inherent in imaging, influenced by factors like kVp, mA, and patient dose.

Conclusion

  • Understanding the detailed mechanisms and parameters involved in multi-slice CT, including technical specifications, imaging characteristics, advantages and disadvantages, is critical for optimizing image quality, enhancing patient safety, and informing clinical decision-making.

Key Takeaways

  • MSCT represents multiple rows of parallel detectors enhancing imaging efficiency.

  • Reconstruction algorithms counteract artifacts resulting from the cone beam effect.

  • Advances in detector counts (4-64 and beyond) have reduced imaging times and improved quality.

References

  • Bushong, S. C. (2021). Radiologic Science for Technologists: Physics, Biology, and Protection. Elsevier.

  • Goldman, L. W. (2008). Principles of CT: Multislice CT. Journal of Nuclear Medicine Technology, 36(2), 57–68.