Cardiopulmonar y Imaging • Original Research Bogot et al. Low-Energy Pulmonary CT Angiography and Image Quality

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Cardiopulmonary Imaging Original Research

Image Quality of Low-Energy Pulmonary CT Angiography: Comparison With Standard CT Naama R. Bogot 1,2,3 Alexander Fingerle 4 Dorith Shaham1 Izhak Nissenbaum1 Jacob Sosna1,5 Bogot NR, Fingerle A, Shaham D, Nissenbaum I, Sosna J

Keywords: CT angiography, dual-energy CT, pulmonary embolism DOI:10.2214/AJR.10.5318 Received July 13, 2010; accepted after revision December 30, 2010.

OBJECTIVE. The purpose of this article is to prospectively compare visualization of central and peripheral pulmonary arteries on simultaneously acquired low-energy and standard pulmonary CT angiography. SUBJECTS AND METHODS. Thirty-three consecutive patients (20 women and 13 men; mean age, 55.6 years; range, 21–92 years) with suspected pulmonary embolism (PE) were scanned (140 kVp; 250–300 mA) on a single-source dual-layer dual-energy MDCT scanner. Attenuation and image noise were measured at the main and segmental pulmonary arteries. Signal-to-noise ratios were calculated. Two blinded experienced radiologists assessed segmental and subsegmental artery visibility in consensus, using slab maximumintensity-projection (MIP) reconstructions. Nonparametric sign test and kappa statistic were used for statistical analysis. RESULTS. PE was detected in three patients (9.1%); two segmental vessel and subsegmental emboli were seen in the low-energy images only. Higher attenuation was noted in low-energy versus standard images for all arteries evaluated, with a mean (± SD) increase of 66.6 ± 4.4 HU (p < 0.0001). Low-energy images improved visualization of segmental and subsegmental arteries from 97.0% to 99.2% and from 88.0% to 93.9%, respectively. A larger number of subsegmental vessels was seen on low-energy MIP reconstructions in 69.7% (95% CI, 36.5–71.89%) of studies compared with 9.1% on the standard images. Visualization of subsegmental vessels was superior in 55.5% of cases using low-energy imaging. The mean image noise increased by 9.7 ± 0.6 HU (p < 0.0001). The mean signal-to-noise ratio showed no significant difference in the low-energy (8.2) versus standard (8.1) CT images (p = 0.7759). CONCLUSION. Improved visualization of central and peripheral arteries can be obtained with low-energy pulmonary CT angiography, without a substantial decrease in image quality.

1

Department of Radiology, Hadassah-Hebrew University Medical Center, PO Box 12000, Jerusalem 91120, Israel. Address correspondence to J. Sosna ([email protected]).

2

Department of Radiology, Shaare Zedek Medical Center, Jerusalem, Israel. 3 Department of Radiology, University of Michigan Hospitals, Ann Arbor, MI. 4 Department of Radiology, Klinikum rechts der Isar, Technische Universität München, Munich, Germany. 5 Department of Radiology, Beth Israel Deaconess Medical Center, Boston, MA.

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DCT angiography is the procedure of choice for diagnosing pulmonary embolism (PE). Sensitivity of 83–100% and specificity of 89–98% have been reported [1–3]. However, several studies have shown that indeterminate examinations occur in 0.03–10% of pulmonary CT angiography (CTA) studies [4, 5], limiting accuracy and sensitivity and hampering correct patient management [6]. The most common causes of reduced image quality are poor contrast opacification and excessive movement [7]. Additional factors contributing to inadequate enhancement of pulmonary arteries include technical factors, such as poor bolus timing and choice of contrast material, and patient factors, such as contrast material dilution in large patients, reduced cardiac output, and interruption of contrast material [8– 10]. Standard pulmonary CTA examinations

deliver a relatively high effective radiation dose to the patient [11]. Low-kilovoltage studies have been shown to provide improved pulmonary artery attenuation in CTA [12–14]. With this technique, the effective energy of the x-ray beam is in the range of maximum absorption close to the k-edge of iodine, leading to a relative increase in attenuation of contrast material. Recently, spectral imaging capability has been added to MDCT. With dual-energy CT, low-energy and conventional CT images can be obtained. Previous low-kilovoltage pulmonary CTA studies have compared low- and high-kilovoltage protocols in different patient populations [12–14]. Our purpose was to prospectively quantify changes in contrast attenuation and image noise in the pulmonary arteries and to compare visualization of the peripheral pulmonary arteries

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Bogot et al. in low- and regular-energy spectral CT images acquired in a single clinically indicated study for each participating patient.

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Subjects and Methods This study was approved by our institutional review committee before patient recruitment. Informed consent was obtained from all patients.

Patient Population The study group consisted of 33 consecutive patients (20 women and 13 men; mean age, 55.6 years; range, 21–92 years) with clinically suspected acute PE who underwent pulmonary CTA examinations. Twenty-three patients were inpatients, and 10 were emergency department patients. None were outpatients.

Single-Source Dual-Energy CT Scanner The study was performed on a single-source dual-energy MDCT scanner (Orion N, Philips Healthcare) based on simultaneous acquisition with twolayer detectors. The upper detector layer primarily absorbs the low-energy x-ray spectrum, whereas the lower layer absorbs the high-energy spectrum. Each layer has 32 detector rows and a 50-cm FOV. Data from each layer are independently reconstructed, creating separate CT images corresponding to the low- and high-energy x-ray spectrum. A combination of the two signals provides a conventional CT image. Because the MDCT scanner uses a single-source dual-detector layer, only a single peak kilovoltage is given; separation of low- and high-energy images occurs at the detector level.

CT Protocol and Image Reconstruction The scan protocol was as follows: tube voltage, 140 kV; tube current, 250–300 mA; rotation time, 0.5 second; collimation, 32 × 0.625 mm; and pitch, 0.66. Scans were performed in a cra­ niocaudal direction, from the lung apices through the lung bases, in suspended respiration. Nonionic IV contrast material (100 mL iohexol; Omni­ paque 330, GE Healthcare) was injected at a flow rate of 4 mL/s with a dual-head power injector via a 20-gauge needle inserted into an antecubical vein, followed by a flush of 20 mL of normal saline. Scan initiating time was determined using 12 mL of test bolus followed by 20 mL of saline chase, with the region of interest (ROI) placed on the main pulmonary artery. Images were reconstructed with 1-mm slice thickness, 0.5-mm slice increment, and 512 × 512 image matrix. The FOV covered the entire thorax.

Image Analysis All studies were reviewed on a workstation (Extended Brilliance 3.5, Philips Healthcare). Low-

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energy and combined standard CT images were compared using quantitative and qualitative parameters. The low-energy and standard CT images were unmarked and presented in a randomized order on a different workstation (Extended Brilliance 3.1, Philips Healthcare) using the same windowing settings (center, 60 HU; width, 360 HU). Randomization was performed by a research assistant. Contrast attenuation—Measurements were performed by a research fellow with 3 years of experience in CT analysis. Using ROI measurements, average contrast attenuation in the lowenergy and standard CT images was measured and compared. Measurements were performed in five predetermined areas: at the main pulmonary artery, and at the segmental arteries of the left and right and upper and lower lobes. ROIs were drawn to encompass the maximal possible area in each artery, while avoiding vessel movement or streak artifacts. Care was taken to use ROIs of comparable size and placement in low-energy and standard images for each vessel in each patient. The mean (± SD) area of ROIs was 249 ± 121 mm 2 in the main pulmonary artery and 23 ± 13 mm2 in the segmental arteries. In each vessel, the average attenuation (measured in Hounsfield units) and the average noise (calculated as the SD of attenuation) were measured; signal-to-noise ratios (SNRs; i.e., average attenuation divided by average noise) were also calculated. Differences in attenuation and noise were calculated for the low-energy versus standard CT images. Quantitative analysis of image quality—Two attending radiologists specializing in thoracic and abdominal imaging (with 14 and 15 years of experience, respectively), with expertise in the interpretation of angiographic studies, evaluated the studies in consensus. The images were not marked. The segmental and subsegmental pulmonary arteries were determined as analyzable or nonanalyzable. Analysis was performed in eight predetermined locations: anterior segmental and subsegmental arteries in the right and left upper lobes, anterior segmental and subsegmental arteries in the right lower lobe, and anterior-medial segmental and subsegmental arteries in the left lower lobe. If a vessel was determined to be nonanalyzable, the reason was recorded. Image quality of the low-energy and standard CT images was compared in the lungs, mediastinum, and pleura using a 4-point ordinate scale, where 1 indicates poor, 2 indicates moderate, 3 indicates good, and 4 indicates excellent quality. Subsegmental vessel visibility in the low-energy and standard CT images was compared in 30mm slab coronal maximim-intnesity-projection (MIP) reconstructions at the level of pulmonary artery bifurcation. Visibility assessment was per-

formed with each set of unmarked images positioned side by side in a randomized order, with the readers blinded to the type of images. For each pair of images, they were asked to evaluate the number and visibility of subsegmental vessels. This assessment was performed with blinding as to the energy level of the images. The images were then unblinded by a research fellow, who accordingly scored them on 3-point ordinate scales by the radiologists. For number of vessels, 1 indicates more vessels seen on the low-energy image, 2 indicates similar number of vessels seen on both images, and 3 indicates more vessels seen on the standard CT image; for vessel visibility, 1 indicates better visualization on low-energy image, 2 indicates equal visualization, and 3 indicates better visualization on the standard CT image. Pulmonary emboli were determined to be visible or nonvisible on low-energy and standard CT images.

Statistical Analysis The nonparametric sign test was used to compare pulmonary vessel attenuation, average image noise, and SNRs. Image quality comparisons between the low- and dual-energy CT reconstructions were made using the kappa statistic. Visualization of vessels on MIP coronal views was compared using 95% CIs. A p value of less than 0.05 was considered significant.

Results Three studies (9.1%) were positive for PE. Emboli were found in the main pulmonary artery in one patient and in both segmental and subsegmental arteries in two patients; overall, there were one central and four peripheral emboli. All emboli were seen in the low-energy images; however, in two patients, two subsegmental emboli and one segmental embolus were not visible on standard CT images (Fig. 1). Image Analysis Attenuation and noise measurements—All pulmonary arteries showed a higher mean attenuation on low-energy images compared with standard CT images (Fig. 2). The overall mean increase was 66.6 ± 4.4 HU (p < 0.0001) (Table 1). The mean noise of 9.7 HU (range, 8.8– 10.5 HU) was higher for the low-energy images than for the standard CT images in all pulmonary arteries (p < 0.0001) (Table 2). The mean SNR was higher for the lowenergy images than for the standard CT images, in the right upper, left upper, and lower segmental arteries, and was lower in the main pulmonary artery and right lower segmental artery (Table 3). Overall, there was a

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Low-Energy Pulmonary CT Angiography and Image Quality

A

B

Fig. 1—61-year-old man with pulmonary embolism. A and B, Left lower lobe segmental and subsegmental emboli (arrow) were detected on low-energy image (A) but were overlooked on standard image (B).

nonsignificant increase in the mean SNR of 0.1 ± 0.5 in the low-energy images compared with standard CT images (p = 0.7759). Segmental and subsegmental vessel visibility—In axial views, four of 132 segmental arteries (3.0%) and 16 of 132 subsegmental arteries (12.2%) were not analyzable on standard CT images. Three of these segmental arteries and eight subsegmental arteries were analyzable on the low-energy images, improving visualization of segmental and subsegmental arteries from 97.0% to 99.2% and from 88.0% to 93.9%, respectively. All segmental and subsegmental arteries that were analyzable on the standard CT images were also well visualized on low-energy images. On low-energy images, the inability to analyze arteries was a result of movement or vascular attenuation secondary to emphysema. Assessment of image quality—There was good correlation for image quality scores be-

tween the low-energy and standard CT images in all regions (Table 4). Coronal MIP Reconstruction Evaluation On coronal MIP reconstructions, more subsegmental arteries were seen on low-energy images in 69.7% of patients (95% CI, 51.29– 84.41%), whereas more subsegmental arteries were seen on the standard CT images in 9.1% of patients (Fig. 3). A total of 55.5% (95% CI, 36.50–71.89%) of subsegmental arteries in the coronal slab MIP reconstructions were better seen on low-energy images, compared with only 3.0% that were better seen on standard CT images (Fig. 3). Discussion In our study, which compared low-energy and standard pulmonary CTA in the same population, increased attenuation was noted

A

in low-energy images compared with standard CT images for all arteries evaluated, with an average increase of 66.6 HU. Compared with standard CT images, low-energy images improved visualization of segmental and subsegmental arteries; in two patients, two subsegmental and one segmental emboli were seen only in low-energy images. Mean image noise increased at low-energy, but there was no change in mean SNR. A low-kilovoltage technique has been shown to improve visualization of central and peripheral pulmonary arteries in CTA, which may increase diagnostic accuracy for the detection of PE. Schueller-Weidekamm et al. [14] showed an increase in the percentage of evaluable central and peripheral pulmonary arteries with CTA for a 100-kVp protocol compared with a standard 140-kVp protocol. Matsuoka et al. [13] found that vascu-

B

Fig. 2—33-year-old woman with suspected pulmonary embolism. A and B, There is increased attenuation on low-energy image (A) relative to standard image (B). Measured attenuation in left lower lobe segmental artery is 481.1 HU on low-energy image, compared with 371.4 on standard image. On lower-energy image, noise increased from 27.5 to 37.2 HU.

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Bogot et al. TABLE 1: Average Attenuation in Pulmonary Vessels: Comparison of Low-Energy and Standard CT Images

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Artery

Standard CT Images

Differencea

Low-Energy Images

Main pulmonary artery



256.4 ± 70.8

329.6 ± 91.0

73.1 ± 24.9

Right upper lobe segmental artery



199.6 ± 68.9

263.2 ± 85.0

63.7 ± 22.9

Right lower lobe segmental artery



241.4 ± 64.9

312.0 ± 80.3

70.6 ± 22.0

Left upper lobe segmental artery



198.38 ± 71.6

261.1 ± 88.6

62.7 ± 24.4

Left lower lobe segmental artery



241.2 ± 68.7

311.9 ± 88.9

62.7 ± 24.5

Note—Data are mean ± SD Hounsfield units. ap < 0.0001 for all comparisons (sign test).

TABLE 2: Average Noise in Pulmonary Vessels: Comparison of Low-Energy and Standard CT Images Artery

Differencea

Standard CT Images

Low-Energy Images

26.4 ± 8.5

36.5 ± 12.6



10.0 ± 5.0

Main pulmonary artery



Right upper lobe segmental artery



50.1 ± 52.3

60.6 ± 57.2



10.5 ± 8.9

Right lower lobe segmental artery



34.6 ± 16.4

44.5 ± 18.4



9.9 ± 5.8

Left upper lobe segmental artery



42.6 ± 40.4

51.4 ± 40.7



8.8 ± 6.3

Left lower lobe segmental artery



33.7 ± 16.4

43.1 ± 22.0



9.4 ± 7.8

Note—Data are mean ± SD Hounsfield units. ap < 0.0001 for all comparisons (sign test).

lar attenuation is significantly increased in segmental and subsegmental arteries for the low-kilovoltage images in a comparison of 100–110 kVp and standard 120–130 kVp protocols in 400 patients. Sangwaiya et al. [15] compared images obtained with 80 and 140 kVp using a dual-source scanner in 65 patients. They found that vascular attenuation and diagnostic confidence increased on the low-energy images. However, in that study, the reference tube current greatly differed between the low-energy images (400 mA) and the standard images (140 mA). Objective and subjective image quality did not substantially decrease with lowkilovoltage imaging in either study. However, in both studies [13, 14], comparisons involved different sets of patients for the low- and standard-kilovoltage techniques. In our study, we circumvented this limita-

tion, with simultaneous acquisition of lowand standard-energy (peak kilovoltage) CT images that included the entire chest, in the same group of patients. At lower energy levels, iodine-based CT contrast agents are expected to have increased attenuation because of the maximum of absorption at the k-edge of iodine [12–14]. Consistent with this theory, in our study, the mean attenuation in the main pulmonary artery on low-energy images was 330 HU, which was 73 HU higher (p < 0.0001) than that in the standard-energy images. Matsuoka et al. [13] found significantly different mean attenuation values of 376 HU and 309 HU in the main pulmonary artery for the low- and standard-kilovoltage images, respectively. The higher mean attenuation values in their study may be the result of their use of a greater contrast agent vol-

TABLE 3: Average Signal-to-Noise Ratio in Pulmonary Vessels: Comparison of Low-Energy and Standard CT Images Artery

Standard CT Images

Low-Energy Images

Difference

p (Sign Test)

Main pulmonary artery

10.7 ± 0.7

10.0 ± 0.6

−0.7 ± 0.9

0.4692

Right upper lobe segmental artery

6.4 ± 0.7

6.8 ± 0.8



0.4 ± 1.1

0.6956

Right lower lobe segmental artery

8.4 ± 0.8

8.2 ± 0.7

−0.2 ± 1.0

0.8557

Left upper lobe segmental artery

6.6 ± 0.7

7.0 ± 0.8



0.3 ± 1.0

0.7386

Left lower lobe segmental artery

8.2 ± 0.6

8.9 ± 0.9



0.8 ± 1.1

0.4727

Note—Except for p values, data are mean ± SD signal-to-noise ratio.

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ume, 125 mL, versus 100 mL in our study. The ability to use a decreased volume of contrast agent with low-kilovoltage imaging has been assessed previously. Sigal-Cinqualbre et al. [16] found that the contrast agent dosage can be lowered by up to 54% without significant loss of image quality, depending on body weight. Godoy et al. [17] compared dual-source dual-energy images, obtained at 80 and 140 kVp, in three patient groups: standard pulmonary CTA group (higher volume of contrast material at higher injection rate with bolus tracking), nonpulmonary CTA group (lower contrast volume at lower injection rate without bolus tracking), and lowervolume pulmonary CTA group (low volume of contrast material at pulmonary CTA technique). They found that in the 80-kVp images in the nonpulmonary CTA group, vascular enhancement was equivalent to that in the standard pulmonary CTA images. For the detection of PE in pulmonary CTA, it is important to visualize the pulmonary arteries at least to the level of their subsegmental branches. Although peripheral emboli may not pose an immediate threat to the patient, treatment may be required to avoid long-term problems such as chronic PE and pulmonary artery hypertension, particularly in patients with an underlying hypercoagulable state or decreased cardiorespiratory status [18]. The disadvantage of the low-energy technique is an increase in image noise, especially in areas with more attenuating tissue, such as the abdomen, or in large patients [18]. Lowenergy protocols may also deliver increased radiation to the skin [18]. Image noise increases with low tube current settings and low peak voltage; however, this is only a minor problem in the lungs. In a recent study, SzucsFarkas et al. [19] showed that image quality is not significantly reduced in low-kilovoltage pulmonary CTA with body size up to 100 kg. Our data show a mean 10 HU increase in image noise for the low-energy images in both the main and the subsegmental pulmonary arteries (p < 0.0001). However, such an increase in image noise may be negligible because the measured attenuation in the pulmonary arteries ranged from 261.1 to 329.6 HU. To objectively assess the effect of image noise on image quality, we calculated the SNR for low- and standard-energy images. The mean SNR was not significantly different in low-energy compared with standardenergy images, and the increase in image noise did not reduce subjective image quality. Image quality in the lung, mediastinum,

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Low-Energy Pulmonary CT Angiography and Image Quality TABLE 4: Image Quality Scores in Low-Energy Images Versus Standard CT images

stantial changes in image quality, in a comparison of low- and standard-energy CT images acquired simultaneously with a single-source dual-energy MDCT. This finding further supports the potential use of low-kilovoltage imaging for improved visualization of the central and peripheral pulmonary arteries.

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Low energy Standard κ, mean (95% CI)

Lung

Mediastinum

Pleura

3.2 ± 0.6

3.3 ± 0.5

3.3 ± 0.6

3.2 ± 0.7

3.4 ± 0.6

3.4 ± 0.6

0.8821 (76.50–100.00)

0.8821 (72.40–100.00)

0.89 (73.97–100.00)

Acknowledgment We thank Shifra Fraifeld, a Research Associate in the Department of Radiology at the Hadassah Hebrew University Medical Center, for her contribution to database development and management and her editorial assistance in the preparation of this manuscript.

Note—Data are mean ± SD image quality score.

References

A

B

Fig. 3—66-year-old man with suspected pulmonary embolism who underwent pulmonary CT angiography. A and B, Shown are thick-slab coronal maximum-intensity-projection reconstructions on low-energy image (A) and standard image (B). Increased number of subsegmental vessels is visualized on low-energy image.

and pleura showed good correlation between the two sets of images (κ = 0.89). Delineation of segmental and subsegmental vessels was superior in most cases, both in the axial and coronal low-energy images. One subsegmental and two segmental emboli were seen only on the low-energy images. This finding supports our impression that low-energy images can improve diagnostic sensitivity; however, this impression could not be statistically confirmed because of the small number of positive cases. These findings are in agreement with previous data. Gorgos et al. [20] found higher SNRs and contrast-to-noise ratios on peripheral vessels on PE studies performed at lower peak kilovoltage. One important advantage of the use of low-kilovoltage imaging is the reduced radiation dose. In 2006, an estimated 62 million CT procedures accounted for 15% of the total number imaging procedures (excluding dental procedures), but over half of the collective dose [21]. It is important to establish CT protocols that provide uncompromised diagnostic capability with reduced radiation. Our findings suggest that lower kilovoltage may be used in patients undergoing CTA for the detection of PE. A second important ad-

vantage may be the potential to reduce the volume of contrast agents used for pulmonary CTA at lower energy levels [17]. Our study had several limitations. The study population was relatively small. However, we were able to evaluate all 33 patients at both energy levels because of the capability of the single-source dual-energy CT to acquire lowand standard-energy images in a single acquisition of the entire chest. Because of the small study population, the number of positive cases of PE was relatively low, with emboli found in only three of 33 patients (9%); however, this rate is within the range reported in other series. We also did not assess the influence of body weight on image quality. Several studies have shown that body weight has to be taken into account with low-kilovoltage imaging [22]. Because of the scanner configuration, we could also not assess the radiation dose of the low-energy CT separately from that of the standard CT [16]. Brighter attenuation on the low-energy images could potentially bias the readers on the side-by-side blinded comparison with the standard CT images. In conclusion, improved visualization of central and peripheral arteries was obtained with low-energy pulmonary CTA, without sub-

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