Rationale and Objectives
To compare the image quality and diagnostic confidence of low-dose computed tomography (CT) of urololithiasis using filtered back projection (FBP) and iterative reconstruction techniques (IRT).
Materials and Methods
A 4.8 × 4.3 × 5.2 mm 3 uric acid ureteral stone was placed inside an anthropomorphic Alderson phantom at the pelvic level. Fifteen scans were performed on a 64-row dual-source CT system using different tube voltages (80, 100, and 120 kV) and current-time products (8, 15, 30, 70, and 100 mAs). Image reconstruction using FBP and IRT (iterative reconstruction in image space) resulted in 30 data sets. Objective image quality was evaluated by noise measurements. Effective doses were estimated for each data set with use of an established dosimetry program. Subjective image quality and confidence level were rated by two radiologists.
Results
Noise was systematically lower for images reconstructed with IRT compared to FBP (55 ± 30 vs 65 ± 26 Hounsfield units; P = .004) for volume CT dose index values above about 0.6 mGy (or an effective dose of about 0.4 mSv for both sexes). For the 14 scans rated to have diagnostic image quality, the estimated effective doses ranged from 0.3 to 2.5 mSv for males and from 0.4 to 3.1 mSv for females. Subjective image quality and diagnostic confidence for IRT was not significantly better than those for FBP.
Conclusions
In a phantom study for CT of urolithiasis, IRT improves objective image quality compared to FBP above a certain dose threshold. However, this does not translate into improved subjective image quality or a higher degree of confidence for the diagnosis of high-contrast urinary stones.
Nonenhanced multidetector computed tomography (CT) has widely replaced conventional intravenous urography for the examination of patients who present to the emergency department with symptoms of urolithiasis . Nonenhanced CT takes less time than intravenous urography, especially in patients with obstructing calculi, and reduces the patient’s risk for complications due to application of intravenous contrast material. Moreover, nonenhanced CT allows for an effective detection of conditions known to cause unilateral flank pain besides urolithiasis . In addition, CT has been shown to directly impact patient management and guide further care in the emergency department .
However, despite the advantages of CT for the diagnosis of urolithiasis, the inherent radiation exposure applied to young patients presenting with symptoms of urolithiasis continues to raise concerns, particularly for patients undergoing repetitive studies . Along with the increased availability of CT scanners, the number of emergency department visits in the United States associated with a CT scan increased from 2.7 million to 16.2 million annually between 1995 and 2007 according to a study by Larson et al . In their study, symptoms of side or flank pain accounted for 4.6% of all CT-associated visits . Therefore, while measures to reduce ionizing radiation have been successfully implemented in clinical practice, the topic continues to be of interest to both the radiological community and the general public .
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Materials and methods
Phantom
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CT System and Protocols
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Data Analysis
Subjective Image Quality
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Objective Image Quality and Radiation Dose
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Statistical Analysis
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Results
Subjective Image Quality and Diagnostic Confidence
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Table 1
Scan Parameters, Effective Doses, Noise Levels, and Subjective Scores for 30 Computed Tomography Images Reconstructed with the Standard FBP and the Iterative IRIS Algorithm. Scans 1–14 Were Rated to Have Diagnostic Image Quality (bold).
Voltage (kV) Reference Current–Time Product (mAs) Effective Current–Time Product (mAs) Reconstruction Algorithm CTDI vol (mGy) ED, Male (mSv)
(ICRP 103) ED, Female (mSv)
(ICRP 103) Noise
(HU) Image Quality
Reader 1/2 Confidence Level
Reader 1/2112010086FBP4.122.53.1385/53/3212010086IRIS4.122.53.1265/53/331207060FBP2.881.92.2315/53/341207060IRIS2.881.92.2215/53/3510010079FBP2.341.41.8455/53/3610010079IRIS2.341.41.8335/53/371203024FBP1.210.80.9584/43/381007059FBP1.611.01.2484/43/391007059IRIS1.611.01.2344/33/3101203024IRIS1.210.80.9433/33/3111201512FBP0.620.40.5903/32/21212087FBP0.540.40.4953/32/21312087IRIS0.540.40.4893/32/214803030IRIS0.560.30.41343/32/2 15 100 15 13 FBP 0.54 0.3 0.4 105 3/2 2/2 16 100 15 13 IRIS 0.54 0.3 0.4 101 2/3 2/2 17 120 15 12 IRIS 0.62 0.4 0.5 83 2/3 1/2 18 80 100 71 FBP 1 0.6 0.8 85 2/2 2/2 19 80 100 71 IRIS 1 0.6 0.8 78 2/2 2/2 20 80 70 52 FBP 0.72 0.4 0.6 103 2/2 2/2 21 80 70 52 IRIS 0.72 0.4 0.6 100 2/1 2/1 22 100 30 23 FBP 0.71 0.4 0.5 86 1/1 1/1 23 100 30 23 IRIS 0.71 0.4 0.5 80 1/1 1/1 24 80 30 30 FBP 0.56 0.3 0.4 134 1/1 1/1 25 80 15 15 FBP 0.29 0.2 0.2 199 1/1 1/1 26 80 15 15 IRIS 0.29 0.2 0.2 199 1/1 1/1 27 100 8 8 FBP 0.33 0.2 0.3 136 1/1 1/1 28 100 8 8 IRIS 0.33 0.2 0.3 134 1/1 1/1 29 80 8 8 FBP 0.15 0.1 0.1 285 1/1 1/1 30 80 8 8 IRIS 0.15 0.1 0.1 281 1/1 1/1
CTDI vol , volume computed tomography (CT) dose index; ED, effective dose; FBP, filtered back projection; ICRP, International Commission on Radiological Protection; IRIS, iterative reconstruction in image space.
The effective current–time product displayed after the CT scan on the operator’s console that is reduced compared to the nominal value due to automatic exposure control.
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Objective Image Quality and Radiation Dose
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Discussion
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