Rationale and Objectives
To assess the performance of dual-energy computed tomography (DECT) equipped with the new tin filter technology to classify phantom renal lesions as cysts or enhancing masses.
Materials and Methods
Forty spherical lesion proxies ranging in diameter from 6 to 27 mm were filled with either distilled water ( n = 10) representing cysts or titrated iodinated contrast solutions with a concentration of 0.45 ( n = 10), 0.9 ( n = 10), and 1.8 mg/mL ( n = 10) representing enhancing masses. The lesion proxies were placed in a 12-cm diameter renal phantom containing minced beef and submerged in a 28-cm water bath. DECT was performed using the new dual-source CT system (Definition Flash, Siemens Healthcare, Forchheim, Germany) allowing for an improved energy separation by using a tin filter. DECT was performed at tube voltages of 140/80 kV without the tin filter (protocol A) and with tin filter (protocol B). The tube current time product was selected in each protocol to achieve a constant CTDI (computed tomography dose index) with both protocols of 19 mGy (full dose), 9.5 mGy (half dose), and 4.8 mGy (quarter dose). Two blinded readers classified each lesion as a cyst or enhancing mass by using iodine overlay (IO) images. One reader measured the CT numbers of each lesion at 120 kV, in the IO, linear blending (LB), and virtual noncontrast (VNC) images.
Results
The CT numbers of the lesions at 120 kV were 0.1 ± 0.7 HU (0 mg iodine/mL), 9.1 ± 0.7 HU (0.45 mg/mL), 18.1 ± 1.4 HU (0.9 mg/mL), and 37.6 ± 1.6 HU (1.8 mg/mL). Mean diameter of the lesion proxies filled with water or different iodine concentrations was similar ( P = 0.38). Image noise was not significantly different in protocols A and B at the corresponding dose levels. At full dose, protocol A had a sensitivity of 93% and a specificity of 60% for discriminating renal lesions. Sensitivity and specificity declined to 84% and 38% at quarter dose. In protocol B, sensitivity was 100% and specificity was 90% at full dose and 93% and 70% at quarter dose. All misclassifications occurred in cyst or low iodine concentration (0.45 mg/mL) lesion proxies. The differences between CT numbers at 120 kV and in the IO, VNC, and AW (average weighted) images were significantly lower in protocol B compared to protocol A (each P < .05).
Conclusions
DECT using the tin filter results in an improved sensitivity and specificity for discriminating renal cysts from enhancing masses in a kidney phantom model and demonstrates higher dose efficiency as compared to former dual energy technology without tin filters.
The computed tomography (CT) discrimination of renal lesions bases on the presence of contrast enhancement during the nephrographic phase of contrast enhancement. For this reason, the standard practice in genitourinary CT includes both a noncontrast CT acquisition to assess baseline attenuation of renal masses as well as a contrast-enhanced CT acquisition to measure contrast enhancement within the mass.
Up to two thirds of all renal cell carcinomas are discovered incidentally during CT studies obtained for nonurological indications , and often only a contrast-enhanced CT data set but no noncontrast CT data is available. In few cases a renal lesion can be categorized as a solid lesion based on the presence of heterogeneous hyperattenuation on the contrast-enhanced CT data alone . However, in most instances, a hyperattenuating cyst may be difficult distinguished from a slightly enhancing renal cell carcinoma . In cases of incidentally discovered renal lesions, the patient has to be recalled to a complete genitourinary CT workup (including a noncontrast CT data acquisition) to establish the diagnosis.
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Figure 1
Schematic drawing of the photon energy spectra of 80 kV tube voltage (- - -), 140 kV without tin filter (— — —), and 140 kV with tin filter (Sn140, ——). The tin filter increases energy separation by minimizing the overlap of high and low kVp spectra and reduces dose by blocking low energy photons from the high energy x-ray tube spectrum.
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Materials and methods
Renal Phantom
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Renal Cyst and Attenuating Mass Proxies
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Experimental Setup
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Table 1
Tube Voltage and Tube Current Settings in the Nine Dual-energy Computed Tomography Protocols
Tube A Tube B Protocol Tube voltage (kV) Tube current (mA) Tube voltage (kV) Tube current (mA) CTDI vol (mGy) A 140 70 80 385 18.8 A/2 140 35 80 193 9.6 A/4 140 17 80 94 4.7 B 80 430 Sn140 ∗ 233 19.0 B/2 80 215 Sn140 ∗ 116 9.5 B/4 80 108 Sn140 ∗ 58 4.8
CTDI vol , computed tomography dose index accounting the helical pitch or axial scan spacing
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Dual-energy Image Reconstruction
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CT Data Analysis
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Statistical Analysis
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Results
Renal Lesion Characteristics
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Image Noise
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Quantitative Measurements in Lesion Proxies
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Table 2
CT Numbers in the Lesion Proxies Using the Different Dual-energy CT Protocols
Iodine Concentration Protocol A Protocol A/2 Protocol A/4 Protocol B Protocol B/2 Protocol B/4 0 mg/mL Weighted average images (HU) -0.4 ± 1.7 (-2.5 to 2.3) -0.3 ± 1.6 (-2.0 to 2.1) -0.9 ± 2.0 (-3.1 to 2.0) -0.1 ± 0.7 (-1.1 to 1.2) 0.1 ± 1.5 (-1.9 to 2.4) 0.3 ± 1.7 (6.7 to 12.2) Virtual noncontrast (HU) -1.2 ± 2.0 (-3.6 to 2.6) -1.1 ± 2.5 (-3.6 to 3.4) -1.4 ± 2.6 (-4.1 to 3.3) -0.2 ± 0.7 (-1.1 to 0.7) -0.2 ± 1.1 (-2.1 to 1.5) -0.1 ± 1.7 (-2.6 to 2.6) Subtracted iodine (HU) 0.8 ± 2.2 (-2.9 to 4.0) 1.3 ± 2.4 (-3.1 to 4.2) 0.6 ± 3.0 (-3.3 to 4.2) -0.1 ± 1.4 (-1.2 to 2.0) 0 ± 1.5 (-1.6 to 2.0) -0.2 ± 1.9 (-2.5 to 3.0) 0.45 mg/mL Weighted average images (HU) 8.3 ± 1.4 (6.1 to 10.3) 8.3 ± 1.4 (6.2 to 10.9) 8.2 ± 1.5 (6.3 to 10.2) 9.2 ± 0.6 (8.8 to 10.4) 9.3 ± 1.4 (7.4 to 12.3) 9.1 ± 1.7 (6.7 to 12.2) Virtual non contrast (HU) -1.6 ± 2.1 (-5.4 to 1.0) -1.1 ± 2.3 (-4.0 to 2.1) -0.9 ± 3.1 (-5.0 to 3.0) 0.2 ± 1.4 (-1.5 to 1.9) 0.2 ± 1.4 (-1.3 to 2.8) 0.2 ± 1.1 (-1.0 to 2.2) Subtracted iodine (HU) 10.2 ± 2.7 (6.4 to 15.0) 10.8 ± 2.9 (5.5 to 14.0) 10.9 ± 1.9 (8.8 to 14.4) 8.9 ± 0.8 (8.0 to 10.1) 9.3 ± 1.5 (7.0 to 12.3) 9.6 ± 2.1 (6.9 to 12.5) 0.9 mg/mL Weighted average images (HU) 17.2 ± 1.3 (15.2 to 19.8) 17.1 ± 1.5 (15.5 to 19.7) 17.0 ± 2.1 (13.8 to 20.3) 17.9 ± 1.0 (16.5 to 19.1) 17.1 ± 1.3 (15.1 to 19.5) 17.2 ± 1.9 (14.1 to 20.0) Virtual noncontrast (HU) -1.7 ± 3.3 (-6.2 to 3.8) -1.5 ± 3.4 (-5.4 to .1) -1.0 ± 3.8 (-5.8 to 5.5) 0.3 ± 1.0 (-0.5 to 1.6) -0.1 ± 1.1 (-2.5 to 1.4) 0.5 ± 1.5 (-1.9 to 2.5) Subtracted iodine (HU) 19.7 ± 1.9 (17.0 to 22.9) 19.5 ± 2.1 (16.0 to 22.5) 20.4 ± 2.8 (15.5 to 24.7) 17.8 ± 1.0 (16.5 to 19.2) 18.8 ± 1.7 (16.2 to 21.5) 18.9 ± 2.6 (15.1 to 22.3) 1.8 mg/mL Weighted average images (HU) 36.4 ± 1.8 (33.2 to 39.1) 36.1 ± 2.5 (32.5 to 39.1) 36.5 ± 2.2 (32.8 to 39.9) 37.7 ± 1.2 (35.6 to 38.5) 37.3 ± 1.4 (34.2 to 39.2) 37.0 ± 1.8 (33.6 to 39.9) Virtual noncontrast (HU) -1.2 ± 2.6 (-5.8 to 3.4) -1.0 ± 3.1 (-5.9 to 4.6) -1.3 ± 3.4 (-4.9 to 4.7) 0.2 ± 1.2 (-1.3 to 1.3) 0.1 ± 1.1 (-1.3 to 1.4) -0.1 ± 2.1 (-3.6 to 2.8) Subtracted iodine (HU) 38.2 ± 2.0 (34.9 to 41.2) 38.4 ± 1.7 (35.1 to 42.1) 38.4 ± 2.4 (35.1 to 42.1) 37.4 ± 0.6 (37.2 to 38.8) 37.7 ± 1.8 (33.7 to 40.9) 38.1 ± 1.6 (35.8 to 41.1)
CT, computed tomography; HU, Hounsfield units.
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Qualitative Interpretation of the Lesion Proxies
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Table 3
Diagnostic Performance of the Different Dual-energy Computed Tomography Protocols for Discrimination of Cysts and Renal Masses
Protocol A Protocol A/2 Protocol A/4 Protocol B Protocol B/2 Protocol B/4 True positives 28 27 27 30 29 28 True negatives 6 6 5 9 9 7 False positives 4 4 5 1 1 3 False negatives 2 3 3 0 1 2 Sensitivity [%] 93.3 (77.9–99.2) 81.8 (64.5–93.0) 84.4 (64.2–94.7) 100 (88.4–100) 96.7 (82.8–99.9) 93.3 (77.9–99.2) Specificity [%] 60.0 (26.2–87.8) 42.9 (9.9–81.6) 37.5 (8.5–75.5) 90.0 (55.5–99.8) 90.0 (55.5–99.8) 70.0 (34.8–93.3)
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Discussion
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