Rationale and Objectives
To prospectively compare four contrast material injection protocols for dual-energy computed tomography (CT) pulmonary angiography (DE-CTPA) in patients with suspected pulmonary embolism (PE).
Materials and Methods
One hundred twenty consecutive patients were randomized to contrast material injection protocols defined by different iodine concentrations and iodine delivery rates (IDRs): (A) 80 mL iopromide 370/4 mL/sec = IDR 1.4 gI/sec; (B) 80 mL iopromide 370 at 3 mL/sec = IDR 1.1 gI/sec; (C) 98 mL iopromide 300 at 4.9 mL/sec = IDR 1.4 gI/sec; and (D) 98 mL iopromide 300 at 3.7 mL/sec = IDR 1.1 gI/sec. Attenuation values were measured in the inflow tract (subclavian vein–superior vena cava–right atrium), target tract (right ventricle–pulmonary trunk–pulmonary arteries), and outflow tract (left atrium–left ventricle–ascending aorta). Two readers assessed subjective image quality of CTPA images and iodine perfusion maps. The number of artifacts due to hyperdense contrast material on iodine perfusion maps was recorded.
Results
Target tract attenuation was highest for protocol A with 374 ± 98 Hounsfield units (HU) (highly concentrated contrast material/high IDR). This was significant compared to protocols B and D ( P = .0118, P = .0427) but not compared to protocol C ( P = .3395). No significant difference in target tract attenuation was found between protocols B (309 ± 80 HU), protocol C (352 ± 119 HU), and D (325 ± 74 HU). CTPA and iodine perfusion map image quality for protocol A was rated significantly higher compared to all other protocols (median score = 5/4; P < .0001 for both) with moderate interreader agreement (κ = 0.58/0.47). Protocols A and B displayed increased artifacts on iodine perfusion maps compared to protocols C and D (3 versus 2).
Conclusion
Despite increased artifacts on iodine perfusion maps, highly concentrated iodinated contrast material combined with high flow rates provides improved diagnostic image quality and has the highest target-tract attenuation for DE-CTPA protocols.
Diagnostic imaging plays a pivotal role in the diagnosis of pulmonary embolism (PE). In daily clinical routine, computed tomography (CT) pulmonary angiography (CTPA) has become the imaging modality of choice for the diagnosis of PE owing to its high spatial resolution, ready availability, and proven high diagnostic accuracy . CTPA allows for excellent diagnostic accuracy in detecting intravascular emboli in addition to providing valuable information concerning cardiopulmonary anatomy and pathology, including the presence of right ventricular dysfunction in patients with PE .
Dual-energy CT (DECT) examinations allow selective visualization of contrast material in tissues by taking advantage of the spectral properties of iodine . Thus, dual-energy CTPA (DE-CTPA) allows for a comprehensive pulmonary evaluation combining traditional CTPA for morphologic information and iodine-based perfusion maps for functional evaluation. The application of DE-CTPA in PE has shown promising results; however, all studies have reported a high incidence of contrast material–related beam-hardening artifacts . Beam-hardening artifacts may produce false-positive results or obscure true-positive findings. In turn, this limits both the sensitivity and the specificity of the technique . In an effort to enhance diagnostic quality of DE-CTPA scans, the contrast material delivery protocol has to be adjusted . DE-CTPA scans require a uniquely tailored protocol aimed at maximum pulmonary arterial and parenchymal enhancement while also minimizing contrast material–related beam hardening artifacts . In addition to a high attenuation in the large pulmonary arteries, a high attenuation of the pulmonary parenchyma is also required. Protocols strive to avoid high attenuation in the inflow vessels such as the subclavian vein and the superior vena cava to reduce beam-hardening artifacts.
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Materials and methods
Patients
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Contrast Material Injection Protocols
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Table 1
Contrast Material Injection Protocols
Group Contrast Material Iodine Concentration (mg/mL) Total Volume (mL) Injection Rate (mL/sec) Iodine Delivery Rate (gI/sec) Injection Time (s) Total Iodine Load (g) A Iopromide 370 370 80 4 1.4 20 29.6 B Iopromide 370 370 80 3 1.1 26.7 29.6 C Iopromide 300 300 98 4.9 1.4 20 29.4 D Iopromide 300 300 98 3.7 1.1 26.5 29.4
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Scanning Protocol
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Image Analysis
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Objective Image Quality
DE-CTPA images
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Iodine-based perfusion maps
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Subjective Image Quality
DE-CTPA image quality rating
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Iodine-based perfusion map rating
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Statistical analysis
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Results
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Table 2
Patient Demographics
Group N Male/Female Gender Age (yr) Height (cm) Weight (kg) BMI (kg/m 2 ) A 30 14/16 68 ± 11 167 ± 9 74.7 ± 16.2 27 ± 6 B 30 14/16 69 ± 16 169 ± 7 74.8 ± 18.1 26 ± 6 C 30 19/11 61 ± 12 170 ± 9 80.4 ± 16.9 27 ± 5 D 30 14/16 69 ± 15 170 ± 9 77.5 ± 18.5 27 ± 7
BMI, body mass index.
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Objective Image Quality
DE-CTPA attenuation values
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Table 3
Attenuation Values (in Hounsfield Units) at Various Anatomic Levels as Measured on 120-kV “Virtual” Computed Tomography Angiography of the Lungs
Vessel Segment Group A Group B Group C Group D Subclavian vein 1538 ± 843 1816 ± 613 1465 ± 667 1923 ± 443 Superior vena cava 815 ± 355 824 ± 361 775 ± 408 780 ± 207 Right atrium 437 ± 126 363 ± 115 402 ± 156 387 ± 106 Right ventricle 352 ± 110 295 ± 96 352 ± 120 305 ± 72 Pulmonary trunk 388 ± 101 319 ± 77 358 ± 127 334 ± 77 Right pulmonary artery 379 ± 101 311 ± 79 347 ± 117 329 ± 77 Left pulmonary artery 377 ± 96 311 ± 76 351 ± 125 332 ± 75 Left atrium 287 ± 74 242 ± 65 314 ± 75 261 ± 44 Left ventricle 241 ± 84 217 ± 63 275 ± 73 236 ± 39 Ascending aorta 303 ± 83 250 ± 67 316 ± 82 273 ± 46 Inflow tract 930 ± 344 1001 ± 318 880 ± 300 1030 ± 199 Target tract 374 ± 98 309 ± 80 352 ± 119 325 ± 74 Outflow tract 277 ± 75 236 ± 64 302 ± 74 257 ± 41 Pulmonary parenchyma 27 ± 7 23 ± 5 24 ± 5 23 ± 5
To provide comprehensive results, vessel segments were consolidated as follows—inflow tract: subclavian vein, superior vena cava, and right atrium; target tract: right ventricle, pulmonary trunk, and pulmonary arteries; outflow tract: left atrium, left ventricle, and ascending aorta. Note that a high target tract attenuation in combination with a low inflow tract attenuation should indicate superior image quality.
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Table 4
Individual Protocol Comparison
A (370-4 mL/sec) vs B (370-3 mL/sec) A (370-4 mL/sec) vs C (300-4.9 mL/sec) A (370-4 mL/sec) vs D (300-3.7 mL/sec) B (370-3 mL/sec) vs C (300-4.9 mL/sec) B (370-3 mL/sec) vs D (300-3.7 mL/sec) C (300-4.9 mL/sec) vs D (300-3.7 mL/sec) Inflow tract 930 ± 344 vs 1001 ± 318 930 ± 344 vs 880 ± 300 930 ± 344 vs 1030 ± 199 1001 ± 318 vs 880 ± 300 1001 ± 318 vs 1030 ± 199 880 ± 300 vs 1030 ± 199P value .42 .51 .32 .09 .49.01 Target tract 374 ± 98 vs 309 ± 80 374 ± 98 vs 352 ± 119 374 ± 98 vs 325 ± 74 309 ± 80 vs 352 ± 119 309 ± 80 vs 325 ± 74 352 ± 119 vs 325 ± 74P value.01 .33.04 .34 .43 .64 Outflow tract 277 ± 75 vs 236 ± 64 277 ± 75 vs 302 ± 74 277 ± 75 vs 257 ± 41 236 ± 64 vs 302 ± 74 236 ± 64 vs 257 ± 41 302 ± 74 vs 257 ± 41P value.01 .20 .42.0009 .07.008
Inflow tract includes subclavian vein, superior vena cava, and right atrium; target tract includes right ventricle, pulmonary trunk and pulmonary arteries; outflow tract includes left atrium, left ventricle, and ascending aorta.
Note that bold print indicates significant results.
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Iodine-based perfusion map attenuation
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Table 5
Iodine-Based Perfusion Map and Dual-Energy Computed Tomography Pulmonary Angiography (DE-CTPA) Subjective and Objective Image Quality Criteria
Iodine-Based Perfusion Maps Group A Group B Group C Group D Parenchyma overlay (HU ± SD) 26.7 ± 6.8 23.4 ± 5.5 24.0 ± 5 22.8 ± 5.5 Average number of beam hardening artifacts (minimum–maximum) 3 (1–4) 3 (3–3) 2 (1–4) 2 (2–2) Image quality Reader A (minimum–maximum) 4 (3–5) 3 (3–4) 3 (2–4) 3 (2–3) Image quality Reader B (minimum–maximum) 5 (3–5) 3 (3–5) 3 (3–5) 3 (2–3) DE-CTPA image quality median score (minimum–maximum) Reader A 5 (3–5) 3 (2–5) 4 (3–4) 3 (3–4) Reader B 5 (3–5) 3 (3–5) 3 (3–5) 3 (2–4)
HU, Hounsfield units; SD, standard deviation.
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Subjective Image Quality
DE-CTPA image quality rating
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Iodine-based perfusion map image quality and number of beam-hardening artifacts
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Discussion
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