Converting dose-length product to effective dose at CT

Abstract

Purpose: To determine effective dose (ED) per unit dose-length product (DLP) conversion factors for computed tomographic (CT) dosimetry.

Materials and methods: A CT dosimetry spreadsheet was used to compute patient ED values and corresponding DLP values. The ratio of ED to DLP was determined with 16-section CT scanners from four vendors, as well as with five models from one manufacturer that spanned more than 25 years. ED-to-DLP ratios were determined for 2-cm scan lengths along the patient axis, as well as for typical scan lengths encountered at head and body CT examinations. The dependence of the ratio of ED to DLP on x-ray tube voltage (in kilovolts) was investigated, and the values obtained with the spreadsheet were compared with those obtained by using two other commercially available CT dosimetry software packages.

Results: For 2-cm scan lengths, changes in the scan region resulted in differences to ED of a factor of 30, but much lower variation was obtained for typical scan lengths at clinical head and body imaging. Inter- and intramanufacturer differences for ED/DLP were generally small. Representative values of ED/DLP at 120 kV were 2.2 microSv/mGy x cm (head scans), 5.4 microSv/mGy x cm (cervical spine scans), and 18 microSv/mGy x cm (body scans). For head scans, ED/DLP was approximately independent of x-ray tube voltage, but for body scans, the increase from 80 to 140 kV increased the ratio of ED to DLP by approximately 25%. Agreement in ED/DLP data for all three software packages was generally very good, except for cervical spine examinations where one software package determined an ED/DLP ratio that was approximately double that of the other two.

Conclusion: This article describes a method of providing CT users with a practical and reliable estimate of adult patient EDs by using the DLP displayed on the CT console at the end of any given examination.

Figure 1:

Mathematical phantom used with ImPACT software to compute patient doses at CT. Superimposed on the phantom is the extent of a chest CT examination, with z ranging from 35 to 70 as indicated on the vertical scale. (Reprinted, with permission, from references and .)

Figure 2:

Graphs of ED/DLP values as a function of z-axis location for head scans (2-cm scan length) obtained with ImPACT software for four commercial 16-section CT scanners. (Male and female ED/DLP values were identical.)

Figure 3:

Graphs of ED/DLP values as a function of z-axis location for body scans (2-cm scan length) obtained with ImPACT software for four commercial 16-section CT scanners. Differences between male ED/DLP (open circles) and female ED/DLP (closed circles) values arise because of differences in dose to the testes and ovaries (see Table 1).

Figure 4:

Graphs of ED/DLP values as a function of z-axis location averaged across four commercial 16-section scanners. Left: Data obtained with head filter and 16-cm-diameter acrylic dosimetry phantom. Right: Data obtained with body filter and 32-cm-diameter acrylic dosimetry phantom. Table 1 lists the locations of radiosensitive organs, which explains the observed peaks. Error bars = standard deviation.

Figure 5:

Graphs of ED/DLP values as a function of x-ray tube voltage obtained with ImPACT software for four commercial 16-section CT scanners (head filter and 16-cm-diameter acrylic dosimetry phantom). Solid circles (solid lines) are head scans, and open circles (dashed lines) are cervical spine scans, with these scans defined in Table 1. Lines are least-square fits to a second-order polynomial.

Figure 6:

Graphs of ED/DLP values as a function of x-ray tube voltage obtained with ImPACT software for four commercial 16-section CT scanners (body filter and 32-cm-diameter acrylic dosimetry phantom). Solid circles (solid line) are chest scans, open circles (dashed lines) are abdominal scans, and triangles (dotted lines) are pelvic scans, with these scans defined in Table 1. Each line is obtained from a least-square fit.