Rationale and Objectives
The use of breast computed tomography (CT) has been limited by radiation hazard and image quality. The objective of this study was to compare radiation doses and image quality for different 64-channel multidetector row CT (MDCT) parameters, and to potentially provide optimal CT parameters for breast imaging.
Materials and Methods
For assessment of radiation doses, CT dose index (CTDI 100 ) values were obtained at various x-ray tube voltages (80, 120, 140 kVp) and currents (30, 50, 100, 150, 200 mAs) using a standard CT body dose phantom. To evaluate image quality, four fresh mastectomy specimens were scanned and three radiologists graded images for overall image quality, glandular tissue-fat conspicuity, and Cooper’s ligament sharpness. Statistically, linear regression analyses and multiple comparisons were used for investigation of the relationship between radiation dose, image qualities, and CT parameters.
Results
CTDI 100 values of ≤6 mGy were obtained at 80 kVp and any mAs, 120 kVp and 30 or 50 mAs, and 140 kVp and 30 or 50 mAs. Image quality at 80 kVp and 200 mAs, 120 kV and 50, 100, 150, or 200 mAs, and 140 kVp and all mAs values tested were significantly superior to those at 80 kVp and 30, 50, 100, or 150 mAs and 120 kV and 30 mAs ( P < .05).
Conclusions
Based on our results, 80 kVp and 200 mAs, 120 kVp and 50 mAs, 140 kVp and 30 mAs, or 140 kVp and 50 mAs can be used for breast MDCT scanning to reduce radiation dose and preserve image quality and 140 kVp at 30 mAs is the optimal setting.
Computed tomography (CT) has been reported to be useful for distinguishing benign breast lesions from cancers , for initial breast cancer staging , and for detecting tumor recurrence after treatment , and recently, multidetector row CT (MDCT) scanners have been purchased by many institutions. MDCT can obtain multiple CT datasets with each x-ray tube rotation, and thus scan times are faster than for conventional spiral CT scanners. Furthermore, MDCT has greater temporal and spatial image resolutions. However, the use of MDCT as a diagnostic tool in the breast is limited by lack of optimum acquisition parameters that yield superior diagnostic image quality at dose levels comparable to two-view mammography.
Breast is one of the radiosensitive organs. The International Commission of Radiological Protection recommended the concept of dose constraints, with the main implication in diagnostic radiology . The potential radiation risks in patients undergoing CT are due to stochastic effects. The probability of stochastic effects depends on the amount of absorbed radiation. The radiation doses used in CT often approach or exceed those levels known to increase the probability of nonfatal and fatal cancers . Risk of breast cancer may correlate with doses less than 100 mGy . The radiation dose values for the female breast in chest CT examinations range from 20 to 31 mGy . The majority of studies of breast CT have been performed using standard doses such as those used for conventional chest CT scans at 120–140 kVp and 200–300 mAs . There are a few studies of breast CT using lower radiation doses, 2.6–4.5 mAs, at 100 kVp and 100 mAs or 120 kVP at 50 mAs .
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Materials and methods
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Phantom Study
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CTDI100=1RPW∫+50−50D(z)dz, C
T
D
I
100
=
1
R
P
W
∫
−
50
+
50
D
(
z
)
d
z
,
where and D( z ) is the absorbed dose at a point z on a line parallel to the rotation axis, and the limits of integration are ±50 mm . The radiation profile width (RPW) is the nominal x-ray beam width (millimeters). In the present study, dependencies between CTDI 100 and tube voltage (kilovoltage peak) and tube current (tube current-time product) were investigated. The dependency of CTDI 100 on tube voltage was assessed by performing measurements at various tube voltage values, namely, 80, 120, and 140 kVp. Milliampere-second values (milliampere second = milliampere × rotation time) were changed by altering tube currents. CTDI 100 values were measured at 30, 50, 100, 150, and 200 mAs. Other scan parameters were kept constant, namely: 512 × 512-image matrix, 16 × 0.625-mm collimation, 3-mm slice thickness, and 34-cm field of view for a single pass. CTDI 100 values were measured three times at each peripheral hole and mean values and standard deviations were calculated.
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Specimen Study
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Table 1
Criteria Used to Grade Image Quality
Grade Overall Image Quality Glandular Tissue-fat Conspicuity Cooper’s Ligament Sharpness 1 Poor Indistinguishable between glandular tissue and fat Indistinguishable ligaments 2 Moderate Distinguishable between glandular tissue and fat Distinguishable but not continuous and not sharp ligaments 3 Satisfactory Clear conspicuity between glandular tissue and fat within abundant fatty portion and unclear conspicuity within abundant glandular portion Continuous but not sharp ligaments 4 Excellent Clear conspicuity between glandular tissue and fat within both abundant glandular and fatty portions Continuous and sharp ligaments
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Statistical Analysis
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Results
Phantom Study
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Table 2
The Radiation Doses According to Tube Voltages and Currents at the Four Peripheral Holes in the CT Body Dose Phantom
CTDI 100 ∗ at Four Holes of the CT Body Dose Phantom
(mGy) Tube Voltage (kVp) Tube Current
(mAs) 11 o’clock † 1 o’clock † 7 o’clock † 5 o’clock † 80 30 0.79 ± 0.09 0.73 ± 0.01 0.58 ± 0.04 0.60 ± 0.07 80 50 1.31 ± 0.04 1.35 ± 0.13 0.99 ± 0.09 1.01 ± 0.11 80 100 2.52 ± 0.01 2.65 ± 0.20 2.02 ± 0.15 2.16 ± 0.18 80 150 3.88 ± 0.09 3.93 ± 0.27 3.00 ± 0.26 2.93 ± 0.03 80 200 5.19 ± 0.21 5.05 ± 0.17 3.88 ± 0.07 3.91 ± 0.06 120 30 2.57 ± 0.09 2.62 ± 0.29 2.09 ± 0.11 2.00 ± 0.02 120 50 4.26 ± 0.07 4.08 ± 0.01 3.36 ± 0.03 3.62 ± 0.30 120 100 8.75 ± 0.47 8.69 ± 0.88 6.72 ± 0.06 6.85 ± 0.18 120 150 12.63 ± 0.20 13.86 ± 1.26 10.14 ± 0.12 10.27 ± 0.21 120 200 19.44 ± 0.06 16.55 ± 0.12 13.56 ± 0.12 13.59 ± 0.04 140 30 3.59 ± 0.03 3.59 ± 0.09 3.10 ± 0.08 3.20 ± 0.26 140 50 6.16 ± 0.07 6.24 ± 0.18 5.12 ± 0.26 5.27 ± 0.42 140 100 12.21 ± 0.09 12.06 ± 0.05 10.58 ± 0.33 10.57 ± 0.95 140 150 19.08 ± 1.23 18.98 ± 1.15 14.94 ± 0.02 15.03 ± 0.03 140 200 25.91 ± 2.26 24.24 ± 0.10 20.78 ± 1.50 21.54 ± 1.65
CT, computed tomography; CTDI 100 , CT dose index.
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Specimen Study
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Table 3
Image Quality Means as Assessed by the Three Radiologists
Image Quality ∗ Tube Voltage (kVp) Tube Current
(mAs) Overall Image Quality Glandular Tissue-fat Conspicuity Cooper’s Ligament Sharpness 80 30 2.0694 2.5556 1.5833 80 50 2.2361 2.7361 1.7917 80 100 3.2639 3.3750 2.7222 80 150 3.4306 3.4583 3.0417 80 200 3.8333 3.8333 3.6250 120 30 3.5417 3.6528 3.0694 120 50 3.8333 3.7500 3.4444 120 100 3.9306 3.7500 3.6111 120 150 3.9583 3.8056 3.6389 120 200 3.9722 3.8750 3.6667 140 30 3.7500 3.7639 3.5278 140 50 3.8889 3.7639 3.5972 140 100 3.9722 3.8750 3.6667 140 150 3.9722 3.8611 3.7222 140 200 3.9722 3.8889 3.7222
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Discussion
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Conclusion
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