Korean J Radiol.  2009 Dec;10(6):568-574. 10.3348/kjr.2009.10.6.568.

Method for Automatic Tube Current Selection for Obtaining a Consistent Image Quality and Dose Optimization in a Cardiac Multidetector CT

• Qi W
• Li J
• Du X

• KMID: 1102559
• DOI: http://doi.org/10.3348/kjr.2009.10.6.568

Abstract

OBJECTIVE
To evaluate a quantitative method for individually adjusting the tube current to obtain images with consistent noise in electrocardiogram (ECG)-gated CT cardiac scans. MATERIALS AND METHODS: The image noise from timing bolus and cardiac CT scans of 80 patients (Group A) who underwent a 64-row multidetector (MD) CT cardiac examination with patient-independent scan parameters were analyzed. A formula was established using the noise correlation between the timing bolus and cardiac scans. This formula was used to predict the required tube current to obtain the desired cardiac CT image noise based on the timing bolus noise measurement. Subsequently, 80 additional cardiac patients (Group B) were scanned with individually adjusted tube currents using an established formula to evaluate its ability to obtain accurate and consistent image noise across the patient population. Image quality was evaluated using score scale of 1 to 5 with a score of 3 or higher being clinically acceptable. RESULTS: Using the formula, we obtained an average CT image noise of 28.55 Hounsfield unit (HU), with a standard deviation of only 1.7 HU, as opposed to a target value of 28 HU. Image quality scores were 4.03 and 4.27 for images in Groups A and B, respectively, and there was no statistical difference between the image quality scores between the two groups. However, the average CT dose index (CTDIvol) was 30% lower for Group B. CONCLUSION: Adjusting the tube current based on timing bolus scans may provide a consistent image quality and dose optimization for cardiac patients of various body mass index values.

Keyword

Computed tomography (CT); Coronary artery imaging; Consistent image quality; Dose reduction and optimization; Automatic tube current modulation

MeSH Terms

Female
Heart Diseases/*radiography
Humans
Male
Mathematics
Middle Aged
Radiation Dosage
Radiographic Image Interpretation, Computer-Assisted
Retrospective Studies
Tomography, X-Ray Computed/*instrumentation

Figure

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  Fig. 1 Noise correlation between coronary CT angiography (CCTA) and timing bolus (TB) scans, both acquired at constant tube currents, showing good linear relationship.

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  Fig. 2 Required tube current as function of timing bolus noise for same patient to obtain preset image noise levels of 28 HU and 30 HU.

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  Fig. 3 Scatter plot showing relationship between coronary CT angiography (CCTA) image noise and patient body mass index (BMI) for patients in Group A with constant mAs. Image noise in this group had wide distribution: 56 mAs to 245 mAs.

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  Fig. 4 Scatter plot showing relationship between coronary CT angiography image noise and patient body mass index (BMI) for patients in Group B with adaptive tube current setting. Image noise deviated very little from mean value of 28.6 HU.

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  Fig. 5 Effective dose distributions for patients in groups A with constant mA and B, and with adaptive mA.

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  Fig. 6 Example of coronary CT angiography images with fixed (650 mA, left) and adaptive mA (330 mA, right) for patients from Groups A and B, respectively. Body mass index and image noise were 30.1 kg/m2 and 28.5 HU for Group A and 23.4 kg/m2 and 26.9 HU for Group B. Both images were rated excellent (score of 5). However, for patient in Group B, where tube current was adjusted based on timing bolus noise measurement, dose saving of about 50% was achieved compared to when fixed mA was used.

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