CT dose index and patient dose: they are not the same thing

Abstract

Estimates of individual patient risk, and epidemiologic studies assessing potential late effects, must use patient size–specific dose estimates—they cannot use only scanner output (volume CT dose index or dose-length product).

Figure 1:

Equipment typically used to measure CTDI100 includes an integrating electrometer (black arrow), a 100-mm-long CTDI ionization chamber (white arrow), and a CTDI phantom made of polymethylmethacrylate (arrowhead). The phantom is placed with its long axis perpendicular to the plane of the transverse CT scan and the ion chamber placed in one of the holes through the phantom. CTDI100 is obtained by integrating the dose over 100 mm from a single transverse scan and dividing it by the nominal beam width. (Reprinted, with permission, from reference .)

Figure 2a:

(a) Radiation dose profile along a line perpendicular to the scan plane shows a peak dose level at the center of the primary beam and long dose tails caused by scattered radiation. NT = nominal beam width. (b) The radiation dose profiles from nine adjacent transverse CT scans along a line perpendicular to the transverse scans, when summed, produce the MSAD profile. The value of MSAD is the average value of this profile over one scan interval in the central portion of the profile. (Reprinted, with permission, from reference .)

Figure 2b:

(a) Radiation dose profile along a line perpendicular to the scan plane shows a peak dose level at the center of the primary beam and long dose tails caused by scattered radiation. NT = nominal beam width. (b) The radiation dose profiles from nine adjacent transverse CT scans along a line perpendicular to the transverse scans, when summed, produce the MSAD profile. The value of MSAD is the average value of this profile over one scan interval in the central portion of the profile. (Reprinted, with permission, from reference .)

Figure 3:

Graph shows relative dose (mean patient dose per 1 mGy of scanner output, CTDI_vol) for an abdominal CT scan and different patient sizes (here represented by the sum of anteroposterior [A/P] and lateral dimensions). Over the range of patient sizes from a newborn to a large adult, relative dose is exponentially related to patient size. For a patient with an anteroposterior dimension of 30 cm and a lateral dimension of 40 cm, the anteroposterior + lateral value would be 70 cm and the mean patient dose in the center of the scan range would be approximately equivalent to the CTDI_vol value reported on the console. For a neonate having both anteroposterior and lateral dimensions of 10 cm, the anteroposterior + lateral value would be 20 cm and the mean patient dose in the center of an abdomen scan would be about 2.3 times the displayed CTDI value, for body CTDI_vol measurements made by using a 32-cm phantom. CTDI_vol measurements made on the basis of 16-cm phantoms would require a different scale factor.