Is It Better to Enter a Volume CT Dose Index Value before or after Scan Range Adjustment for Radiation Dose Optimization of Pediatric Cardiothoracic CT with Tube Current Modulation?

Abstract

Objective: To determine whether the body size-adapted volume computed tomography (CT) dose index (CTD_vol) in pediatric cardiothoracic CT with tube current modulation is better to be entered before or after scan range adjustment for radiation dose optimization.

Materials and methods: In 83 patients, cardiothoracic CT with tube current modulation was performed with the body size-adapted CTDI_vol entered after (group 1, n = 42) or before (group 2, n = 41) scan range adjustment. Patient-related, radiation dose, and image quality parameters were compared and correlated between the two groups.

Results: The CTDI_vol after the CT scan in group 1 was significantly higher than that in group 2 (1.7 ± 0.1 mGy vs. 1.4 ± 0.3 mGy; p < 0.0001). Image noise (4.6 ± 0.5 Hounsfield units [HU] vs. 4.5 ± 0.7 HU) and image quality (1.5 ± 0.6 vs. 1.5 ± 0.6) showed no significant differences between the two (p > 0.05). In both groups, all patient-related parameters, except body density, showed positive correlations (r = 0.49-0.94; p < 0.01) with the CTDI_vol before and after the CT scan. The CTDI_vol after CT scan showed modest positive correlation (r = 0.49; p ≤ 0.001) with image noise in group 1 but no significant correlation (p > 0.05) in group 2.

Conclusion: In pediatric cardiothoracic CT with tube current modulation, the CTDI_vol entered before scan range adjustment provides a significant dose reduction (18%) with comparable image quality compared with that entered after scan range adjustment.

Keywords: Cardiac CT; Child; Image quality evaluation; Radiation dose optimization; Tube current modulation.

Fig. 1. Axial CT image obtained approximately 1–2 cm above dome of liver in which X-ray output in CTDI_vol based on 32-cm phantom was individually determined.

To measure area and mean density, CT technologist draws region of interest to include entire patient cross-section with upper (50000 HU) and lower (−900 HU) limits of CT numbers. A. On same axial CT image with mediastinal window setting, AP was measured from most anterior body surface to most posterior body surface (vertical arrow) and LAT was measured from most right lateral body surface to most left lateral body surface (horizontal arrow). B. On same axial CT image with lung window setting, additional radiolucent pad (white arrow) is shown to be placed on CT table (black arrows) to adjust patient's vertical position at isocenter. Blanket wrapping around patient and patient cloth are shown in lung window. AP = anteroposterior diameter, CT = computed tomography, CTDI_vol = volume CT dose index, HU = Hounsfield units, LAT = lateral diameter

Fig. 2. Comparison of user-determined radiation dose in pediatric cardiothoracic CT with tube current modulation in two groups.

A. CT scout image shows slice position (horizontal orange line) of axial CT image for determination of CTDI_vol that was entered after scan range adjustment (transparent purple rectangle) in group 1. B. CT scout image shows slice position (horizontal orange line) of axial CT image for determination of CTDI_vol that was entered with minimal longitudinal range at same position (transparent purple rectangle) in group 2. C. In group 2, scan range was subsequently extended longitudinally (arrows) to full scan range (transparent purple rectangle) of cardiothoracic CT scanning. As result, initially entered CTDI_vol value with minimal longitudinal range was automatically changed to new value after scan range adjustment, based on body attenuation information obtained from CT scout image in group 2. Of note, both arms are raised up on CT scout image in both cases.

Fig. 3. Variable arm positions on CT scout images.

A. CT scout image shows that both arms are horizontal, which is greatly disadvantageous in terms of image quality and radiation dose, as compared with both arms raised up. B. CT scout image shows that both arms are down beside body trunk, which may slightly degrade image quality and increase radiation dose, as compared with both arms raised up. C. CT scout image shows that right arm is raised up and left arm is horizontal, which may cause asymmetric image quality degradation in left shoulder region and increased radiation dose, as compared with both arms raised up.

Fig. 4. Subjective image quality grading of pediatric cardiothoracic CT.

Axial (A) and coronal (B) CT images show excellent image quality (grade 1) without substantial artifacts in 6 months-old female infant whose arm were raised up during CT scanning. Axial CT image (A) at same slice position, where CTDI_vol value was calculated, was used to measure CT densities in aorta (1), spinal muscles (2), and air (3) by placing rectangular regions of interest in areas showing homogeneous attenuation as much as possible. Axial (C) and coronal (D) CT images show severely degrade subjective image quality (grade 3) in posterior thoracic inlet and left shoulder regions due to horizontal position of left arm during CT scanning. Metal artifacts from electrocardiography electrode located at right upper chest are also noted on axial CT image (C).

Fig. 5. Scatter plots demonstrating correlations between radiation dose and image noise of cardiothoracic CT.

A. In scatter plot, group 1 illustrates higher correlation (r = 0.52; p = 0.0004) between CTDI_vol before CT scan and slice thickness-normalized image noise than that (r = 0.37; p = 0.02) in group 2. B. In scatter plot, group 1 illustrates modest correlation (r = 0.49; p = 0.001) between CTDI_vol after CT scan and slice thickness-normalized image noise. In contrast, group 2 shows no significant correlation (r = 0.20; p = 0.2) between them that may be attributed to reduction of CTDI_vol after CT scan, graphically recognized as leftward shift of red triangular points of group 2 in X-axis, compared with blue rhomboid points of group 1.