Rationale and Objectives
Because it is imperative for understanding the performance of dual-energy computed tomography scanner to determine clinical diagnosis, we aimed to assess the accuracy of quantitative measurements using dual-energy computed tomography with fast kilovoltage switching.
Materials and Methods
Quantitative measurements were performed for 16 reference materials (physical density, 0.965–1.550 g/cm 3 ; diameter of rod, 2.0–28.5 mm; iodine concentration, 2–15 mg/mL; and calcium concentration, 50–300 mg/mL) with varying scanning settings, and the measured values were compared to their theoretical values.
Results
For high-density material, the maximum differences in Hounsfield unit values in the virtual monochromatic images at 50, 70, and 100 keV were −176.2, 61.0, and −35.2 HU, respectively, and the standard deviations over short- and long-term periods were 11.1, 6.1, and 3.5 HU at maximum. The accuracy of the Hounsfield unit measurement at 50 and 70 keV was significantly higher ( P < 0.05) with higher radiation output and smaller phantom size. The difference in the iodine and calcium measurements in the large phantom were up to −2.6 and −60.4 mg/mL for iodine (5 mg/mL with 2-mm diameter) and calcium (300 mg/mL) materials, and the difference was improved with a small phantom. Metal artifact reduction software improved subjective image quality; however, the quantitative values were significantly underestimated ( P < 0.05) (−49.5, −26.9, and −15.3 HU for 50, 70, and 100 keV, respectively; −1.0 and −17 mg/mL for iodine and calcium concentration, respectively) compared to that acquired without a metal material.
Conclusions
The accuracy of quantitative measurements can be affected by material density and the size of the object, radiation output, phantom size, and the presence of metal materials.
Introduction
Recent advances in scanner technology have led to increased clinical use of dual-energy computed tomography (DECT). DECT utilizes two different energy spectra to allow the analysis of energy-dependent changes in the attenuation of different materials. This use of high and low photon energies allows the differentiation of materials and makes possible the quantitative measurement of variables such as iodine and calcium concentrations, and thus, the DECT can tell us how much of these materials are present in the region of interest (ROI) . Although conventional computed tomography (CT) is routinely assessed in patterns of gray scale expressed as Hounsfield units, an iodine density map can provide physiological information, which can be helpful in diagnosis. Moreover, virtual monochromatic images (VMIs) at a specified photon energy level can be generated during the processing of material-density image data by calculating the linear attenuation coefficient. These images are less affected by the beam-hardening effect, and thus provide more accurate Hounsfield unit values than conventional CT scanners with a polychromatic energy beam . The beam-hardening effect has been recognized as one of the major concerns of inaccurate Hounsfield unit measurement because the low-energy photons within a polychromatic beam can reduce vascular enhancement in larger patients .
The advantages of DECT in clinical use are now an active area of research. Thieme et al. reported that DECT with pulmonary parenchymal iodine mapping identified pulmonary perfusion with a close correlation with perfusion single-photon emission CT and concluded that this technique might potentially enhance diagnostic accuracy . The advantages of the iodine mapping are its smaller spatial resolution compared to single-photon emission CT (in general, 10–20 mm) and a perfect spatial match of CT images with iodine mapping. Hu et al. performed unenhanced dual-energy head CT in the emergency department and analyzed the virtual noncalcium and calcium overlay images to distinguish calcification from hemorrhage. In the report, the accuracy of DECT for the detection of hemorrhage was 99%, whereas that of conventional CT was 87% . In a study by Cha et al., DECT with metal artifact reduction software (MARS) markedly reduced metallic dental artifacts and improved image quality in the buccal area and the tongue area . Patients with head and neck cancer are expected to benefit from the improvement of the image quality using MARS because accurate diagnostic staging considering the primary site, the size, and the existence of metastatic lymph nodes is imperative for proper treatment. The success of such investigations depends on how accurately DECT quantifies the concentration of materials such as iodine and calcium and Hounsfield unit values.
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Materials and Methods
Phantoms
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Table 1
Specification of Reference Materials of the Multienergy CT Phantom
Rod Number Reference Material Physical Density (g/cm 3 ) Theoretical CT Number (HU) 50 keV 70 keV 100 keV 1 Adipose 0.965 −107 −80 −66 2 Blood-40 1.060 53 40 35 3 Blood-70 1.097 82 72 69 4 Blood-100 1.130 109 104 102 5 Iodine 2 mg/mL 1.021 108 48 18 6 Iodine 2 mg/mL + Blood-40 1.066 161 92 57 7 Iodine 4 mg/mL + Blood-40 1.067 271 144 80 8 Iodine 5 mg/mL 1.029 274 127 52 9 Iodine 5 mg/mL with 10-mm core 1.029 274 127 52 10 Iodine 5 mg/mL with 5-mm core 1.029 274 127 52 11 Iodine 5 mg/mL with 2-mm core 1.029 274 127 52 12 Iodine 10 mg/mL 1.028 546 256 107 13 Iodine 15 mg/mL 1.032 821 387 164 14 Calcium 50 mg/mL 1.169 242 175 145 15 Calcium 100 mg/mL 1.245 492 320 241 16 Calcium 300 mg/mL 1.550 1499 901 625
CT, computed tomography.
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Image Acquisitions
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Data Analysis
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Table 2
Acquisition Parameters
Protocol Number Type of Scanning Field of View Scanning Field of View (mm) Rotation Time (s) Detector Coverage (mm) Average Tube Current (mA) CTDI vol (mGy) 1 Large body 500 1.0 40 600 32.5 2 Large body 500 0.7 40 260 9.1 3 Medium body 500 1.0 40 600 34.0 4 Medium body 500 0.6 40 275 9.7 5 Medium head 320 0.9 20 600 72.7 6 Medium head 320 0.8 20 260 26.2 7 Small head 320 0.5 20 630 39.7 8 Small head 320 0.8 20 260 23.1
CTDI vol , volume computed tomography dose index.
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Results
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Table 3
Effect of Scanning Parameter on Image Noise of Quantitative Measurements
Phantom size SFOV CTDI vol Mean Imaging Noise (SD) 50 keV (HU) 70 keV (HU) 100 keV (HU) Iodine (mg/mL) Calcium (mg/mL) Large Large body Max 25.0 14.0 10.0 0.5 5.8 Min 52.5 26.6 23.3 1.0 11.5 Medium body Max 25.4 13.7 9.7 0.5 6.3 Min 51.0 26.7 22.5 1.0 11.6 Medium head Max 34.4 17.0 16.0 0.8 11.1 Min 64.4 30.5 38.1 1.7 23.6 Small head Max 54.2 25.3 25.5 1.3 21.7 Min 62.1 32.3 39.5 1.5 25.5 Small Large body Max 10.8 5.5 4.5 0.2 2.1 Min 22.1 11.0 9.7 0.4 3.7 Medium body Max 10.9 5.5 4.6 0.2 1.9 Min 19.7 9.7 8.8 0.3 3.5 Medium head Max 12.1 7.0 7.2 0.2 2.9 Min 21.0 12.3 14.1 0.4 6.0 Small head Max 14.9 8.6 9.3 0.3 4.4 Min 21.1 12.6 14.6 0.4 6.3
SD, standard deviation; SFOV, scanning field of view.
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Discussion
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