Rationale and Objectives
To evaluate the effect of automatic tube current modulation on radiation dose and image quality for low tube voltage computed tomography (CT) angiography.
Materials and Methods
An anthropomorphic phantom was scanned with a 64-section CT scanner using following tube voltages: 140 kVp (Protocol A), 120 kVp (Protocol B), 100 kVp (Protocol C), and 80 kVp (Protocol D). To achieve similar noise, combined z-axis and xy-axes automatic tube current modulation was applied. Effective dose (ED) for the four tube voltages was assessed. Three plastic vials filled with different concentrations of iodinated solution were placed on the phantom’s abdomen to obtain attenuation measurements. The signal-to-noise ratio (SNR) was calculated and a figure of merit (FOM) for each iodinated solution was computed as SNR 2 /ED.
Results
The ED was kept similar for the four different tube voltages: (A) 5.4 mSv ± 0.3, (B) 4.1 mSv ± 0.6, (C) 3.9 mSv ± 0.5, and (D) 4.2 mSv ± 0.3 ( P > .05). As the tube voltage decreased from 140 to 80 kVp, image noise was maintained (range, 13.8–14.9 HU) ( P > .05). SNR increased as the tube voltage decreased, with an overall gain of 119% for the 80-kVp compared to the 140-kVp protocol ( P < .05). The FOM results indicated that with a reduction of the tube voltage from 140 to 120, 100, and 80 kVp, at constant SNR, ED was reduced by a factor of 2.1, 3.3, and 5.1, respectively, (P < .001).
Conclusions
As tube voltage decreases, automatic tube current modulation for CT angiography yields either a significant increase in image quality at constant radiation dose or a significant decrease in radiation dose at a constant image quality.
Within the last decade, multidetector row computed tomography (CT) angiography has become a major diagnostic imaging tool to evaluate noninvasively the arterial system of the brain, heart, and body, including the extremities. The main shortcoming of CT angiography remains patient exposure to ionizing radiation. To reduce radiation dose to the patient, various authors have suggested lowering the peak tube voltage to 100 or 90 kVp for CT angiography of the pulmonary arteries, aortoiliac system, and circle of Willis (1–4). Besides decreasing radiation dose, the low tube voltage technique improves vascular enhancement because iodinated contrast material demonstrates higher attenuation levels at lower x-ray tube voltages owing to a greater photoelectric effect (closer to the k edge of iodine) and decreased Compton scattering. The disadvantage of low tube voltage CT angiography is its greater image noise, although this can be offset by increasing the tube current to maintain image quality.
Some of the published studies on low tube voltage CT angiography relied on the manual adjustment of the tube current setting by the operator . However, current state-of-the-art multidetector row CT scanners include automatic tube current modulation, which modulates the tube current according to the x-ray attenuation of the patient section being scanned to keep the radiation exposure as low as possible and to obtain images with a constant, prespecified image noise. The technique also automatically increases the tube current to match lower tube voltages to maintain a constant image noise. To date, very limited data on low tube voltage CT angiography with automatic tube current modulation are available . To optimize low tube voltage CT angiography protocols using automatic tube current modulation, the maximum efficacy of the technique on radiation dose and image quality has to be assessed. These data can be only acquired in an in vitro study because of the high risk of accumulated radiation dose to investigated patients.
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Materials and methods
Anthropomorphic Phantom
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CT Scanning
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Radiation Dose Assessment and Statistical Analysis
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Image Quality Assessment and Statistical Analysis
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SNR=ROIIS/noise SNR
=
ROI
IS
/
noise
where ROI IS is the mean attenuation value of the iodinated solution, and noise is the mean image noise.
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FOM=SNR2/ED FOM
=
SNR
2
/
ED
for each of the three iodinated solutions at each of the four tube voltages.
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Results
Effective Dose
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Table 1
Radiation Dose, Image Noise, Mean Tube Current, Range of Tube Current, and Signal-to-noise Ratio
Protocol A (140 kVp) Protocol B (120 kVp) Protocol C (100 kVp) Protocol D (80 kVp) Effective dose (mSv) 5.38 ± 0.33 4.06 ± 0.61 3.89 ± 0.46 4.15 ± 0.30 Image noise (HU) 14.0 ± 0.6 13.8 ± 0.3 14.2 ± 0.6 14.9 ± 0.5 Mean tube current (mA) 132.2 ± 50.3 173.0 ± 66.7 293.4 ± 112.1 538.1 ± 172.9 Tube current range (mA) 59–179 76–235 131–398 270–670 SNR: Tube 1 (5 mg I/mL) 3.2 ± 0.2 4.0 ± 0.1 5.3 ± 0.9 7.0 ± 0.6 Tube 2 (7.5 mg I/mL) 6.7 ± 0.3 8.6 ± 0.3 10.5 ± 1.1 13.7 ± 1.0 Tube 3 (10 mg I/mL) 10.1 ± 0.3 12.6 ± 0.1 15.7 ± 1.3 20.0 ± 0.9
Note.—Data are mean values ± standard deviation of the means.
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Image Quality Assessment
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Discussion
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