OtherCONTINUING EDUCATION

Principles of CT: Radiation Dose and Image Quality

Lee W. Goldman
Journal of Nuclear Medicine Technology December 2007, 35 (4) 213-225; DOI: https://doi.org/10.2967/jnmt.106.037846

Article Figures & Data

Figures

  • FIGURE 1. 
    FIGURE 1. 

    Standard CT dosimetry phantoms consist of cylindric acrylic phantoms with holes for dosimeter insertion at various locations. The 2 sizes are 16 cm in diameter to represent heads and small pediatric bodies and 32 cm in diameter to represent adult bodies. (Courtesy of Lawrence Rothenberg.)

  • FIGURE 2. 
    FIGURE 2. 

    Phantom insert to hold TLDs used to measure dose profiles. TLDs are close together within x-ray primary beam and are farther apart to measure profile tails.

  • FIGURE 3. 
    FIGURE 3. 

    Example of doses measured by TLDs inside phantom for a 10-mm slice thickness, plotted as function of TLD position along z-axis. This type of plot is referred to as a dose profile. Slice thickness for single-slice CT scanners is usually equivalent to full width at half maximum of dose profile.

  • FIGURE 4. 
    FIGURE 4. 

    Cumulative dose from a series of contiguous slices is known as multislice average dose (MSAD). Dose-profile tails extend quite far from center of x-ray beam and thus contribute radiation dose to nearby slices. Total dose for a procedure consists of several contiguous slices and is typically 25%–40% higher than single-slice dose.

  • FIGURE 5. 
    FIGURE 5. 

    (A) Dose to slice 1 from scanning of slice 1. (B) Dose to slice 1 from scanning of slice 2 equals dose to slice 2 from scanning of slice 1. (C) Dose to slice 1 from scanning of slice 3 equals dose to slice 3 from scanning of slice 1. (D) Dose to slice 1 from scanning of all slices equals dose to all slices from scanning of slice 1, or total area under dose profile, which is measured using long ionization chamber.

  • FIGURE 6. 
    FIGURE 6. 

    Example of 100-mm-long CT ionization chamber for measuring CTDI.

  • FIGURE 7. 
    FIGURE 7. 

    Typical central and peripheral doses (CTDI) in head and body phantoms. Central dose is about equal to peripheral dose in head phantom and is more than half the peripheral dose in body phantom. Weighted sum of central and peripheral doses, known as CTDIw, is single-number estimate of patient radiation dose to scanned volume.

  • FIGURE 8. 
    FIGURE 8. 

    CT spatial resolution phantom, consisting of 4–12 line-pairs per centimeter (from American College of Radiology accreditation phantom), reconstructed using standard (A) and bone (B, high-resolution) filters.

  • FIGURE 9. 
    FIGURE 9. 

    CT resolution is limited by sampling—size and spacing of measurements (samples) used to form image. (A) Pattern is unresolved because sample size (aperture) is too large. (B) Pattern is unresolved because samples are too far apart. (C) Aperture size and sample spacing are adequate to resolve pattern. (D) “Effective” resolution may be lower than expected because of position of samples relative to pattern. (E) In aliasing, pattern seems to be resolved but with incorrect number of bars.

  • FIGURE 10. 
    FIGURE 10. 

    CT image noise depends on number of x-ray photons contributing to image. (A and B) Comparison of noise from scans using 270 mAs (typical clinical value) and 100 mAs. (C) Appearance of image noise is strongly affected by reconstruction filter; sharp filter such as bone also sharpens (enhances) appearance of noise.

  • FIGURE 11. 
    FIGURE 11. 

    Low-contrast phantom to test CT performance in presence of typical image noise levels (from American College of Radiology accreditation phantom). Five-millimeter rods are visible, but smaller ones are obscured by noise.

  • FIGURE 12. 
    FIGURE 12. 

    Beam-hardening artifact caused by unusually severe hardening of x-rays passing though thick bone.

  • FIGURE 13. 
    FIGURE 13. 

    Partial-volume streaks are caused by opposing x-ray beams, which nominally pass through the same voxels but actually sample slightly different cone-shaped tissue volumes as a result of beam divergence. Small structure, such as piece of bone, is detected by beam from one direction but is missed by opposing beam. Resulting inconsistency leads to streak artifact.

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