Dual-Energy CT Spectral and Energy Weighted Data Sets

Rationale and Objectives

To evaluate dual-energy computed tomography spectral and energy weighted (W) image data sets concerning carotid stenosis and calcified plaque detection.

Materials and Methods

Ninety patients were evaluated using contrast media injection. Energy used for dual-energy computed tomography was tin filter with 140/80 kiloVoltage and effective milli Ampere second was 147.4/270.6. Image reconstruction was performed using D30f kernel and 0.0, 0.3, 0.6, 0.8, 1.0 weightings. Data sets were analyzed using both qualitative and quantitative methods.

Results

The signal-to-noise ratio, contrast-to-noise ratio, and figure-of-merit were significantly higher in 0.6-W compared to 140-kV or 80-kV data (all P < .05). Plaque thickness, span, and longitudinal diameters were different for 140-kV, 0.6-W, and 80-kV data (all P < .05). Stenotic intra-luminal diameter was significantly different among 140 kV, 0.6 W, and 80 kV data (all P < .05). A comparison between 0.6 W and digital subtraction angiography was nonsignificant ( P > .05) in normal lumen measurement.

Conclusions

The dimension of calcified plaque and carotid artery with contrast media decreased with increased energy. The percentage of carotid artery stenosis does not vary with different energy. Care must be taken for procedural planning like sizing of stents. Measured diameters of the 0.6 W were close to digital subtraction angiography; we suggest that planning should be based on the images acquired using 0.6 weighting.

Multiple imaging modalities such as ultrasound , magnetic resonance imaging , and computed tomography (CT) have proved their efficient role in the examination of vessels. CT has a major role in evaluating artery stenosis or calcified plaques and is almost equivalent to catheter angiography for assessment . Magnetic resonance angiography offers an equivalent accuracy in artery stenosis evaluation compared with digital subtraction angiography (DSA) . Recently, there has been a rapid increase in the use of dual-energy computed tomography (DECT). A recent article showed that DECT provides a good correlation of grading stenosis in comparison with the gold standard of catheter angiography. DECT was introduced in 2006, and this modality provides high temporal resolution of 83 ms in a monosegment reconstruction mode . A recent study showed that separation of objects such as water, adipose, iodine, and calcium concentrations are possible using multienergy CT images. The DECT system consists of two X-ray tubes, and it uses two different energies simultaneously for the imaging of patients. The two responsible radiation interaction mechanisms in the diagnostic energy are the Compton effect (CE) and the photoelectric effect (PEE).

The PEE is the mechanism that controls the image quality and radiation dose in diagnostic radiology. The PEE is directly related to the energy used for the imaging and the atomic number or density of the material under investigation. Contrast media (CM) consists of iodine that used for clinical examination of patients and calcified plaque consists of calcium; both are considered to be materials with a high atomic number compared to body tissues. Improvement of the cerebral artery blood flow and resolution of the neurological symptoms are highly dependent on the luminal dimension of the carotid artery. Treatment decisions often depend on carotid artery percent stenosis, and sizing of stents relies on an accurate measurement of luminal diameters. Hence, any possible variability in luminal diameter on the CT image potentially leads to a great clinical impact. In this study, we used a relatively new technology, the latest DECT, to study the luminal diameter of the extracranial carotid arteries using CM and calcified plaque detection. Here, the question arises of whether the DECT data sets produce similar results during diagnosis for the dimensions of calcification and the carotid artery measurements.

Materials and methods

Study Population

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Dual-Energy Protocol and Data Acquisition

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Table 1

Computed tomography (CT) imaging parameters used for the dual energy CT neck examination

Scan Parameters Protocol Scan mode Dual-source dual-energy Tube potential (kV) 80/Sn140 Effective tube current–time product (mAs) 270.6 ± 12 (262–280)/147.4 ± 14 (134–155) Automatic exposure control (care dose 4D) Yes Scan length (cm) 30 Rotation time (s) 0.28 Detector collimation (mm) 32 × 0.6 × 2 Pitch 1.2 Scan slice thickness (mm) 0.6 Reconstruction kernel D30f Reconstruction slice thickness (mm) 2 Matrix size 512 × 512 Field of view (mm) 200

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Image Reconstruction

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DSA Protocol

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Image Quality Assessments in DECT

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Objective method

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Subjective method

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Data Analysis

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Stenosis analysis

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Radiation Dose Estimation

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Statistical Analysis

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Results

Image Quality Analysis in DECT Data Sets

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Table 2

Assessment of the image quality parameters and quantitative analysis of the calcified plaque/carotid artery stenosis from the image data sets (category 1)

Parameters X-ray energy used for imaging and its weighting factors Group 1 (G1) Group 2 (G2) Group 3 (G3) Group 4 (G4) Group 5 (G5)P 140 kV 0.3 W 0.6 W 0.8 W 80 kV Hounsfield unit (HU muscle ) 57 ± 7.2 (46–63) 64 ± 8.2 (62–69) 70 ± 11.3 (67–74) 75 ± 11 (72–79) 80 ± 13 (74–86) G1 vs G3 = .0011

G1 vs G5 = .0081

G3 vs G5 = .0092 Image noise (HU) 9 ± 2.8 (8–10) 7 ± 1.7 (6.1–7.9) 5 ± 0.8 (3.9–5.8) 8 ± 2 (7–9.3) 11 ± 2.4 (9.6–12.2) G1 vs G3 = .0053

G1 vs G5 = .0047

G3 vs G5 = .0434 Hounsfield Unit (HU artery ) 138 ± 25 (99–156) 197 ± 50 (167–234) 268 ± 64 (239–292) 312 ± 78 (286–337) 339 ± 80 (301–368) G1 vs G3 = .00099

G1 vs G5 = .00061

G3 vs G5 = .00071 Signal-to-noise ratio 7 ± 2.3 (6.2–7.8) 9 ± 3 (8.4–9.9) 12 ± 4 (11.1–12.9) 10 ± 2.4 (9.4–11) 8 ± 1.6 (7.2–8.9) G1 vs G3 = .0094

G1 vs G5 = .0205

G3 vs G5 = .0245 Contrast-to-noise ratio (HU) 11 ± 3.5 (4.4–14.5) 24 ± 9.9 (15.8–27.4) 38 ± 14 (29–46) 30 ± 15 (24.2–36.3) 23 ± 13 (19.4–28.7) G1 vs G3 = .0022

G1 vs G5 = .0104

G3 vs G5 = .0017 Figure of merit (FOM) 11 ± 7 (6–19) 52 ± 64 (34–67) 168 ± 140 (126–187) 121 ± 128 (102–139) 83 ± 110 (61–94) G1 vs G3 = .0044

G1 vs G5 = .0197

G3 vs G5 = .0002 Plaque thickness (mm) 3 ± 0.55 (2–3.9) 3.2 ± 0.68 (2.4–4.1) 3.6 ± 0.48 (2.8–4.5) 3.9 ± 0.54 (3.1–4.7) 4.2 ± 0.66 (3.3–5.1) G1 vs G3 = .0084

G1 vs G5 = .0062

G3 vs G5 = .0278 Plaque span (mm) 5.9 ± 0.69 (4.4–6.7) 6.3 ± 0.66 (4.8–7) 6.6 ± 0.76 (5.1–7.5) 6.9 ± 0.88 (5.9–7.9) 7.5 ± 0.81 (6.7–8.3) G1 vs G3 = .0432

G1 vs G5 = .0345

G3 vs G5 = .0443 Plaque longitudinal (mm) 7.3 ± 0.84 (5.6–8.5) 7.8 ± 0.89 (5.9–9.2) 8.4 ± 0.99 (6–9.9) 8.7 ± 0.88 (7.4–10) 9.2 ± 0.79 (7.9–11) G1 vs G3 = .0279

G1 vs G5 = .0185

G3 vs G5 = .0343 Narrowed lumen (mm; A) 4.1 ± 0.92 (2.9–5.8) 4.3 ± 0.85 (3.1–6) 4.5 ± 0.91 (3.3–6.4) 4.7 ± 0.93 (3.5–6.6) 5.2 ± 0.84 (3.8–6.9) G1 vs G3 = .0455

G1 vs G5 = .0389

G3 vs G5 = .0396 Normal lumen (mm; B) 7.1 ± 1.19 (5–9.3) 7.4 ± 1.42 (5.3–9.6) 7.8 ± 1.44 (5.6–10.1) 8.2 ± 1.21 (6.2–10.4) 9 ± 1.26 (6.5–10.7) G1 vs G3 = .0277

G1 vs G5 = .0194

G3 vs G5 = .0321 Percentage stenosis (%)

1 − (A/B) × 100 42.3 41.9 42.3 42.4 42.2 —

Table 3

Assessment of image quality parameters, calcified plaques, and carotid artery stenosis from the image data sets obtained using patients appeared for imaging of carotid artery due to atherosclerotic disease (category 2)

Parameters X-ray energy used for imaging and its weighting factors Group 1 (G1) Group 2 (G2) Group 3 (G3) Group 4 (G4) Group 5 (G5)P 140 kV 0.3 W 0.6 W 0.8 W 80 kV Hounsfield unit (HU muscle ) 56 ± 7 (41–64) 62 ± 8 (55–73) 68 ± 11 (61–81) 75 ± 11 (69–87) 79 ± 12 (71–95) G1 vs G3 = .0122

G1 vs G5 = .0041

G3 vs G5 = .0092 Image noise (HU) 11 ± 3.1 (9–14) 7.5 ± 2 (6–9) 6 ± 1 (4–7.5) 8.6 ± 2 (7–9.5) 12 ± 3 (10–14) G1 vs G3 = .0033

G1 vs G5 = .0497

G3 vs G5 = .0034 Hounsfield unit (HU artery ) 144 ± 34 (123–169) 209 ± 53 (184–239) 277 ± 69 (243–296) 331 ± 82 (289–351) 358 ± 87 (324–387) G1 vs G3 = .0009

G1 vs G5 = .0001

G3 vs G5 = .0002 Signal-to-noise ratio 5.1 ± 2.2 (4.4–5.6) 8.3 ± 3 (7.2–8.9) 11.3 ± 3.7 (9–14) 8.7 ± 2.5 (6–10) 6.6 ± 1.5 (5–8) G1 vs G3 = .0004

G1 vs G5 = .0405

G3 vs G5 = .0045 Contrast-to-noise ratio (HU) 8 ± 4 (6.5–9.7) 19.6 ± 10 (16–24) 34.8 ± 12.5 (29–38) 29.8 ± 14 (25–34) 23.3 ± 13 (18–27) G1 vs G3 = .0002

G1 vs G5 = .0004

G3 vs G5 = .0191 Figure of merit (FOM) 6.6 ± 6.5 (4.3–8) 39.76 ± 52 (32–46) 125.4 ± 111 (113–141) 92 ± 63 (81–101) 56.2 ± 32 (38–67) G1 vs G3 = .0004

G1 vs G5 = .0029

G3 vs G5 = .0002 Plaque longitudinal (mm) 18.9 ± 1.2 (16–24) 19.9 ± 1.4 (17–25) 20.5 ± 1.5 (18–26) 21.2 ± 1.4 (18–26) 22.1 ± 1.3 (19–28) G1 vs G3 = .0375

G1 vs G5 = .0145

G3 vs G5 = .0346 Plaque thickness (mm) 5.4 ± 0.6 (4.4–6) 5.8 ± 0.55 (4.9–6.5) 6.2 ± 0.65 (5.5–6.9) 6.6 ± 0.62 (6–7.3) 7.3 ± 0.6 (6.6–7.9) G1 vs G3 = .0373

G1 vs G5 = .0232

G3 vs G5 = .0388 Narrowed lumen (mm; A) 1.9 ± 0.9 (1.5–2.5) 2.2 ± 1 (1.9–2.6) 2.3 ± 0.93 (2–2.7) 2.5 ± 0.97 (2.2–2.9) 2.7 ± 0.9 (2.4–3.2) G1 vs G3 = .0455

G1 vs G5 = .0129

G3 vs G5 = .0366 Normal lumen (mm; B) 7.2 ± 1.3 (6.8–7.5) 8.2 ± 1.2 (7.9–8.4) 8.5 ± 1.4 (8.3–8.7) 9.2 ± 1.5 (8.8–9.4) 10 ± 1.4 (9.7–10.3) G1 vs G3 = .0411

G1 vs G5 = .0264

G3 vs G5 = .0425 Percentage stenosis (%)

1 − (A/B) × 100 73.6 73.2 73.2 72.8 73 —

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Stenosis and Plaque Evaluation Using DECT

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Comparison Between DECT (0.6 W) and DSA

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Dose Estimation

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Table 4

Technical as Well as dose parameters used for digital subtraction angiography examination

Parameter: DSA Kilovoltage 71.5 ± 5 (66–78) Current (mA) 461.6 ± 51 (413–488) Time (sec) 8.06 ± 2.72 (5–12) Number of frames 14.2 ± 5 (9.1–21) Frame rate (sec) 2 Filter thickness (mm) 0.1 Skin entry dose (mGy) 30 ± 20 (24.3–52.6) Dose–area product (Gy/cm²) 13.44 ± 5.5 (7.1–20.4)

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Discussion

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Conclusion

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