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Chest CT Performed with 3D and z -Axis Automatic Tube Current Modulation Technique

Posted Mar 31, 2009 Updated Apr 7, 2023
By Kosuke Matsubara PhD , Tadanori Takata , Kichiro Koshida PhD , Kimiya Noto MSc , Tetsunori Shimono PhD , Junsei Horii , Tomoyuki Yamamoto , Osamu Matsui MD PhD
7 min read

Rationale and Objectives

Chest computed tomographic (CT) scans are the most effective examinations for detecting lung cancer at an early stage. In chest CT examinations, it is important to consider the reduction of radiation dose, particularly to the mammary gland. The objective of this study was to assess breast doses and effective doses on chest CT examinations between three-dimensional and z -axis automatic tube current modulation (ATCM) techniques.

Materials and Methods

Absorbed dose to the breast, lung, mediastinum, and skin was evaluated with an anthropomorphic phantom and radiophotoluminescence glass dosimeters using two different CT scanners. The dosimeters were placed inside and outside the phantom. The phantom was scanned using three-dimensional and z -axis ATCM techniques after scanning localizer radiographs from the horizontal and vertical directions. After scanning, each organ dose was calculated. Moreover, the dose-length product recorded in the dose reports was examined, and each effective dose was calculated.

Results

Compared with z -axis ATCM, three-dimensional ATCM reduced breast dose by 0.7% to 18.6% and effective dose by 4.9% to 10.2%. In particular, three-dimensional ATCM reduced frontal breast dose. For other organs, three-dimensional ATCM reduced absorbed doses by 3.4% to 13.6% compared to z -axis ATCM.

Conclusion

Three-dimensional ATCM can reduce absorbed doses to the breast and other organs, in addition to reducing effective dose, compared to z -axis ATCM.

Computed tomographic (CT) scans are the most effective examinations for detecting lung cancer at an early stage . With increased numbers of detector rows and faster gantry rotation times, the time required to scan the entire chest has been reduced to less than 5 seconds . Although multidetector-row computed tomography has been associated with higher radiation dose than conventional computed tomography, the introduction of automatic tube current modulation (ATCM) has facilitated the effective reduction of patient radiation dose while maintaining optimal image quality .

In this technique, tube current is automatically adjusted in the x and y planes (angular ATCM), the z plane ( z -axis ATCM), or both (three-dimensional ATCM) by inputting an appropriate noise value . Previous studies have shown that three-dimensional ATCM is the most effective in reducing patient radiation dose and could reduce the absorbed dose, particularly in the anterior and posterior parts of the body . Later, we envisioned the breast as one of the organs located in the anterior or posterior part of the body. The recently recommended tissue weighting factor for the breast reported in an International Commission on Radiological Protection (ICRP) publication is higher than that previously recommended in an earlier ICRP publication . This implies that overexposure to the breast should be avoided, and three-dimensional ATCM can be used to achieve this objective.

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Materials and methods

CT Scanners

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Phantom

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Figure 1, Female anthropomorphic chest phantom. The phantom was sliced into 14 pieces of 25 mm thickness, and each slice was drilled with holes to enable the insertion of the dosimeters in the position of each organ. The phantom was equipped with standard, close-fitting Mix D plugs and inserted in the holes. Each breast was molded into 20-mm-thick sections, and each slice was drilled with holes to enable the insertion of dosimeters or close-fitting Mix D plugs.

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Dose Calibration

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Measurement of Organ Dose

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Table 1

Parameters Used for Dose Measurement on Chest Computed Tomography

Parameter LightSpeed VCT ∗ Aquilion 64 † Collimation (mm) 40 32 Tube voltage (kV) 120 120 Pitch 0.984:1 0.828:1 Scan length (cm) 30.0 30.0 Scan time (s) 4.4 6.7 Tube current (mA), range 10–700 10–600 Noise value (Hounsfield units) 10.0 10.0 Gantry rotation time (s) 0.5 0.5 Section thickness (mm) 2.5 3.0 Reconstruction kernel Standard FC13

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Effective Dose Estimation

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effective dose=k×DLP, effective dose

=

k

×

DLP,

where k is a weighting factor (in milliSieverts per milligrays per centimeter) that is a function of body region; 0.014 was used as the weighting factor according to the ICRP .

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Statistical Analysis

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Results

Organ Doses on Two Different Scanners

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Figure 2, Organ doses with 3D mA and Auto mA scanned using the LightSpeed VCT (GE Healthcare, Milwaukee, WI). The phantom was positioned head first. Bars represent the average dose of multiple radiophotoluminescence glass dosimeters at each data point. Error bars are 2 ± standard deviation. L, left; N.S., not significant; R, right.

Table 2

Coefficients of Variation (positioned with head first)

Coefficient of Variation (%) Organ Auto mA ( z -Axis ATCM) ∗ 3D mA (Three-Dimensional ATCM) ∗ Real EC ( z -Axis ATCM) † Volume EC (Three-Dimensional ATCM) † Right frontal breast 3.8 3.6 14.5 11.7 Right internal breast 10.1 6.9 9.7 6.3 Left frontal breast 14.4 25.8 7.5 10.7 Left internal breast 9.8 14.2 12.3 13.5 Lung 4.2 7.0 9.6 7.1 Mediastinum 7.0 4.2 9.0 5.4 Skin 16.8 11.0 12.3 16.1

ATCM, automatic tube current modulation.

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Figure 3, Organ doses with Volume EC and Real EC scanned using the Aquilion 64 (Toshiba Medical Systems, Tokyo, Japan). The phantom was positioned head first. Bars represent the average dose of multiple radiophotoluminescence glass dosimeters at each data point. Error bars are 2 ± standard deviation. L, left; N.S., not significant; R, right.

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Breast Dose (feet first)

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Figure 4, Breast doses with 3D mA and Auto mA when the phantom was positioned feet first and scanned using the LightSpeed VCT (GE Healthcare, Milwaukee, WI). Bars represent the average dose of multiple radiophotoluminescence glass dosimeters at each data point. Error bars are 2 ± standard deviation. L, left; N.S., not significant; R, right.

Table 3

Coefficients of Variation (positioned with feet first)

Coefficient of Variation (%) Organ Auto mA ( z -Axis ATCM) ∗ 3D mA (Three-Dimensional ATCM) ∗ Real EC ( z -Axis ATCM) † Volume EC (Three-Dimensional ATCM) † Right frontal breast 21.6 20.8 12.7 16.5 Right internal breast 13.1 10.5 8.0 5.3 Left frontal breast 4.2 3.2 12.7 11.1 Left internal breast 5.7 4.8 12.0 9.5

ATCM, automatic tube current modulation.

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Figure 5, Breast doses with Volume EC and Real EC when the phantom was positioned feet first and scanned using the Aquilion 64 (Toshiba Medical Systems, Tokyo, Japan). Bars represent the average dose of multiple radiophotoluminescence glass dosimeters at each data point. Error bars are 2 ± standard deviation. L, left; R, right.

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Effective Dose

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Discussion

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Academic Radiology, Volume 16, Issue 4
Journals General Radiology
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