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Uniform Image Quality Achieved by Tube Current Modulation Using SD of Attenuation in Coronary CT Angiography

Gham Hur1, Seong Wan Hong1, Su Young Kim2, Yong Hoon Kim1, Yoon Joon Hwang1, Won Ro Lee3 and Soon Joo Cha1

1 Department of Radiology, Inje University Ilsanpaik Hospital, Daewha-dong, Ilsan-seogu, Goyang-si, Gyuanggi-do 411-706, South Korea.
2 Department of Radiology, Inje University Ilsanpaik Hospital, Seoul, South Korea.
3 Department of Cardiology, Inje University Ilsanpaik Hospital, Goyang-si, Gyuanggi-do 411-706, South Korea.


Figure 1
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  		Fig. 1A —Examples of discrepancy between body weight, body mass index (BMI), and noise at level of heart. All images are axial unenhanced images obtained at 300 mA. Insets in top right-hand corner show anteroposterior topograms. SD represents noise level and has exponential relationship with attenuation coefficient of scanning region. SD values were obtained from centrally located left atrium that contains homogeneous fluid (blood). Unenhanced image of 74-year-old woman with large breasts who weighs 67 kg (BMI = 27.88) (A) shows higher noise (SD of CT attenuation values = 19.65 H) than unenhanced image of 66-year-old man with small breasts who weighs 76 kg (BMI = 27.25) (B) with SD of 12.70 H.

 

Figure 2
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  		Fig. 1B —Examples of discrepancy between body weight, body mass index (BMI), and noise at level of heart. All images are axial unenhanced images obtained at 300 mA. Insets in top right-hand corner show anteroposterior topograms. SD represents noise level and has exponential relationship with attenuation coefficient of scanning region. SD values were obtained from centrally located left atrium that contains homogeneous fluid (blood). Unenhanced image of 74-year-old woman with large breasts who weighs 67 kg (BMI = 27.88) (A) shows higher noise (SD of CT attenuation values = 19.65 H) than unenhanced image of 66-year-old man with small breasts who weighs 76 kg (BMI = 27.25) (B) with SD of 12.70 H.

 

Figure 3
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  		Fig. 1C —Examples of discrepancy between body weight, body mass index (BMI), and noise at level of heart. All images are axial unenhanced images obtained at 300 mA. Insets in top right-hand corner show anteroposterior topograms. SD represents noise level and has exponential relationship with attenuation coefficient of scanning region. SD values were obtained from centrally located left atrium that contains homogeneous fluid (blood). Unenhanced image of 60-year-old woman who weighs 48 kg (BMI = 21.33) has high noise level (SD = 16.02 H) because of cardiomegaly and pericardial effusion.

 

Figure 4
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  		Fig. 2A —Nomograms for radiation dose (volume CT dose index [CTDIvol]), noise level in SD of CT attenuation values, and size of phantom scanned with precontrast (A) and postcontrast (B) parameters. Each exponential curve shows relationship between phantom size and SD value at given tube current. Radiation dose (CTDIvol) in parentheses increases linearly with tube current; however, data are scanner-specific (Aquilion 64, Toshiba Medical Systems) and other scanners may produce different radiation doses with same parameters. Nomogram obtained using precontrast parameters: 120 kV, 180-mm field of view, 4 x 3 mm collimation, sequential, prospective ECG gating, and half exposure.

 

Figure 5
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  		Fig. 2B —Nomograms for radiation dose (volume CT dose index [CTDIvol]), noise level in SD of CT attenuation values, and size of phantom scanned with precontrast (A) and postcontrast (B) parameters. Each exponential curve shows relationship between phantom size and SD value at given tube current. Radiation dose (CTDIvol) in parentheses increases linearly with tube current; however, data are scanner-specific (Aquilion 64, Toshiba Medical Systems) and other scanners may produce different radiation doses with same parameters. Nomogram obtained using postcontrast parameters: 120 kV, helical, retrospective ECG gating, 64 x 0.5 mm collimation, pitch of 0.204, and half reconstruction.

 

Figure 6
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  		Fig. 3 —Graph illustrates how we created tube current modulation table. For example, to reach target SD of 12 H in patient with SD of 10 H on unenhanced images, we placed transverse line (line A) to pass through point of intersection between 300-mA exponential curve and SD value equal to 12 H (red circle), desired target noise level; second, we placed a vertical line (line B) at point where 300-mA exponential curve crosses SD value of 10 H (blue circle), which is SD value obtained from unenhanced images of patient; third, we read crossing point of vertical and transverse lines (green circle) that lies between exponential lines of 200 and 250 mA, closer to 200 mA. Positive slope of line A shows increasing difference of SD values between unenhanced and contrast-enhanced images as phantom size increases from 180 to 300 cm, and it reflects difference in radiation dose between two parameters on reconstructed images.

 

Figure 7
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  		Fig. 4A —SD of CT attenuation values before and after use of adjusted tube current (mA) in 55-year-old man weighing 84 kg who presented with atypical chest pain and hypertension. Unenhanced image obtained with tube current of 300 mA at level of right pulmonary vein shows SD value of 14.12 H (average of four images = 14.62 H).

 

Figure 8
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  		Fig. 4B —SD of CT attenuation values before and after use of adjusted tube current (mA) in 55-year-old man weighing 84 kg who presented with atypical chest pain and hypertension. Unenhanced image obtained with tube current of 380 mA, which is adjusted mA according to table, shows SD value of 12.61 H (average of four images = 12.34 H).

 

Figure 9
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  		Fig. 5A —Coronary CT angiography (CTA) images that were used as samples to illustrate each grade of image quality: grade 1 = high mottle, 2 = medium mottle, 3 = low mottle, and 4 = minimal or no mottle. Left anterior oblique views of normal coronary CTA show samples characterized as high quality (A) (grade 4, no mottle) and samples considered low quality (B) (grade 1, high mottle).

 

Figure 10
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  		Fig. 5B —Coronary CT angiography (CTA) images that were used as samples to illustrate each grade of image quality: grade 1 = high mottle, 2 = medium mottle, 3 = low mottle, and 4 = minimal or no mottle. Left anterior oblique views of normal coronary CTA show samples characterized as high quality (A) (grade 4, no mottle) and samples considered low quality (B) (grade 1, high mottle).

 

Figure 11
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  		Fig. 5C —Coronary CT angiography (CTA) images that were used as samples to illustrate each grade of image quality: grade 1 = high mottle, 2 = medium mottle, 3 = low mottle, and 4 = minimal or no mottle. Selected normal coronary vessels (C) and normal internal mammary arteries (D) of 1.5 mm in diameter are shown in color-coded volume-rendered images (upper row) and curved multiplanar reconstructions (bottom row). From left (grade 1) to right (grade 4), sample image of each quality grade is shown.

 

Figure 12
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  		Fig. 5D —Coronary CT angiography (CTA) images that were used as samples to illustrate each grade of image quality: grade 1 = high mottle, 2 = medium mottle, 3 = low mottle, and 4 = minimal or no mottle. Selected normal coronary vessels (C) and normal internal mammary arteries (D) of 1.5 mm in diameter are shown in color-coded volume-rendered images (upper row) and curved multiplanar reconstructions (bottom row). From left (grade 1) to right (grade 4), sample image of each quality grade is shown.

 

Figure 13
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  		Fig. 6 —Correlation chart shows tube currents determined by different parameters and by corrected SD of CT attenualtion values. Patients are sorted by weight (red line). SD values of unenhanced images obtained at corrected tube current are displayed at bottom of chart (transverse line at bottom). Blue line indicates actual tube current modulated by SD values and shows wide range of differences from fixed tube current at 400 mA (transverse line at top) and tube currents that would have been modulated by patient weight (transverse line with three steps to indicate 10-kg increments). Graph indicates poor correlation between is and SD values in patients at all levels of body weight. Using fixed mA of 400 mA or modulating mA by patient body weight are recommended by manufacturer (Aquilion 64, Toshiba Medical Systems), mA SD = mA modulated by SD values, mA weight = mA modulated by body weight, Fixed mA = mA fixed at 400 mA. SD correct = SD values measured from images obtained with adjusted mA.

 

Figure 14
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  		Fig. 7A —Scatterplots between SD of CT attenuation values, body weight, and body dimensions. Body dimension from topogram (product of anteroposterior and transverse diameters) shows better correlation with SD of CT attenuation values than with body weight. Scatterplot between SD of CT attenuation values and body weight: Pearson's correlation coefficient = 0.692 (R2 = 0.479).

 

Figure 15
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  		Fig. 7B —Scatterplots between SD of CT attenuation values, body weight, and body dimensions. Body dimension from topogram (product of anteroposterior and transverse diameters) shows better correlation with SD of CT attenuation values than with body weight. Scatterplot between SD of CT attenuation values and body dimensions: Pearson's correlation coefficient = 0.771 (R2 =0.595).

 

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