Korean J Radiol.  2018 Feb;19(1):23-31. 10.3348/kjr.2018.19.1.23.

Comparison of Chest Pain Protocols for Electrocardiography-Gated Dual-Source Cardiothoracic CT in Children and Adults: The Effect of Tube Current Saturation on Radiation Dose Reduction

Affiliations
  • 1Department of Radiology and Research Institute of Radiology, University of Ulsan College of Medicine, Asan Medical Center, Seoul 05505, Korea. ghw68@hanmail.net

Abstract


OBJECTIVE
To compare radiation doses between conventional and chest pain protocols using dual-source retrospectively electrocardiography (ECG)-gated cardiothoracic computed tomography (CT) in children and adults and assess the effect of tube current saturation on radiation dose reduction.
MATERIALS AND METHODS
This study included 104 patients (16.6 ± 7.7 years, range 5-48 years) that were divided into two groups: those with and those without tube current saturation. The estimated radiation doses of retrospectively ECG-gated spiral cardiothoracic CT were compared between conventional, uniphasic, and biphasic chest pain protocols acquired with the same imaging parameters in the same patients by using paired t tests. Dose reduction percentages, patient ages, volume CT dose index values, and tube current time products per rotation were compared between the two groups by using unpaired t tests. A p value < 0.05 was considered significant.
RESULTS
The volume CT dose index values of the biphasic chest pain protocol (10.8 ± 3.9 mGy) were significantly lower than those of the conventional protocol (12.2 ± 4.7 mGy, p < 0.001) and those of the uniphasic chest pain protocol (12.9 ± 4.9 mGy, p < 0.001). The dose-saving effect of biphasic chest pain protocol was significantly less with a saturated tube current (4.5 ± 10.2%) than with unsaturated tube current method (14.8 ± 11.5%, p < 0.001). In 76 patients using 100 kVp, patient age showed no significant differences between the groups with and without tube current saturation in all protocols (p > 0.05); the groups with tube current saturation showed significantly higher volume CT dose index values (p < 0.01) and tube current time product per rotation (p < 0.001) than the groups without tube current saturation in all protocols.
CONCLUSION
The radiation dose of dual-source retrospectively ECG-gated spiral cardiothoracic CT can be reduced by approximately 15% by using the biphasic chest pain protocol instead of the conventional protocol in children and adults if radiation dose parameters are further optimized to avoid tube current saturation.

Keyword

Dual-source CT; Cardiothoracic CT; ECG-gating; Chest pain; Child; Adults

MeSH Terms

Adult*
Chest Pain*
Child*
Cone-Beam Computed Tomography
Electrocardiography
Humans
Methods
Retrospective Studies
Thorax*

Figure

  • Fig. 1 Retrospectively ECG-gated spiral cardiothoracic CT using dual-source technique in 23-year-old woman with repaired tetralogy of Fallot.A. Anteroposterior scout image of conventional CT protocol shows tube current modulation curve in green throughout whole longitudinal scan range indicating no tube current saturation. In other words, tube current time product per rotation remains below maximum value of 370 over entire scanning range. Of note, tube current modulation curve is flat on top indicating no change in tube current time product per rotation over whole scan range in this protocol. B. Anteroposterior scout image of uniphasic chest pain CT protocol demonstrates tube current modulation curve in green over whole longitudinal scan range. In contrast to conventional CT protocol (A), tube current time product per rotation is modulated in this protocol according to patient-specific attenuation information acquired from scout image. C. Anteroposterior scout image of biphasic chest pain CT protocol also displays tube current modulation curve in green over whole longitudinal scan range. In this protocol, cardiac region is defined by user as green dotted lines and radiation dose is subsequently reduced to half (arrows) in upper and lower parts of scan range outside cardiac region because one of two X-ray tubes in dual-source CT system is switched off in these regions. D. From axial CT image acquired at slice position (orange line in A–C) approximately 2.5 cm above liver dome for bolus tracking, cross sectional area and mean body density was measured as 597 mm2 and −147 Hounsfield unit, respectively, and optimal volume CT dose index for non-ECG-synchronized cardiothoracic CT was calculated as 3.8 mGy (for one slice) that was increased to 4.8 mGy (for whole scan range) after application of automatic tube current modulation. Axial CT image looks noisy because low radiation dose setting (80 kVp and 25 mA) was used to minimize patient radiation exposure. CT = computed tomography, ECG = electrocardiography

  • Fig. 2 Retrospectively ECG-gated spiral cardiothoracic CT using dual-source technique in 19-year-old man with repaired double-outlet right ventricle and subaortic ventricular septal defect.A. Anteroposterior scout image of conventional CT protocol shows upper part of tube current modulation curve in yellow throughout whole longitudinal scan range indicating tube current saturation. It means that tube current time product per rotation exceeds maximum value of 370 over entire scanning range. Of note, tube current modulation curve is flat on top indicating no change in tube current time product per rotation over whole scan range in this protocol. B. Anteroposterior scout image of uniphasic chest pain CT protocol demonstrates upper part of tube current modulation curve in yellow in upper thorax (yellow arrow) and lower thorax (yellow arrow) exceeding maximum tube current time product per rotation of 370. In contrast, tube current time product per rotation is modulated below maximum value of 370 in cardiac region (green arrow). C. Anteroposterior scout image of biphasic chest pain CT protocol shows that tube current time product per rotation is reduced (arrows) in upper and lower parts of scan range outside cardiac region defined by user as green dotted lines. Nonetheless, yellow tube current saturation is still shown in upper thorax. D. From axial CT image acquired at slice position (orange line in A–C) approximately 2.0 cm above liver dome for bolus tracking, cross sectional area and mean body density was measured as 522 mm2 and −240 Hounsfield unit, respectively, and optimal volume CT dose index for non-ECG-synchronized cardiothoracic CT was calculated as 2.8 mGy (for one slice) that was increased to 5.1 mGy (for whole scan range) after application of automatic tube current modulation. Axial CT image looks noisy because low radiation dose setting (80 kVp and 25 mA) was used to minimize patient radiation exposure.


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