Material Decomposition Images Generated from Spectral CT

Rationale and Objectives

To evaluate the detectability of urinary calculi on material decomposition (MD) images generated from spectral computed tomography (CT) and identify the influencing factors.

Materials and Methods

Forty-six patients were examined with true nonenhanced (TNE) CT and spectral CT urography in the excretory phase. The contrast medium was removed from excretory phase images using water-based (WB) and calcium-based (CaB) MD analysis. The sensitivity for detection on WB and CaB images was evaluated using TNE results as the reference standard. The signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) on MD images were evaluated. Using logistic regression, the influences of image noise, attenuation, stone size, and patient’s body mass index (BMI) were assessed. Threshold values with maximal sensitivity and specificity were calculated by means of receiver operating characteristic analyses.

Results

One hundred thirty-six calculi were detected on TNE images; 98 calculi were identified on WB images (sensitivity, 72.06%) and 101 calculi on CaB images (sensitivity, 74.26%). Sensitivities were 76.92% for the 3–5-mm stones and 84.51% for the 5-mm or larger stones on both WB and CaB images but reduced to 46.15% on WB images and 53.85% on CaB images for small calculi (<3 mm). Compared to WB images, CaB images showed lower image noise, higher SNR but similar CNR. Larger stone sizes (both >2.71 mm on WB and CaB) and greater CT attenuation (>280 Hounsfield units [HU] on WB, >215 HU on CaB) of the urinary stones were significantly associated with higher stone visibility rates on WB and CaB images ( P ≤ .003). Image noise and BMI showed no impact on the stone detection.

Conclusions

MD images generated from spectral CT showed good reliability for the detection of large (>2.71 mm) and hyperattenuating (>280 HU on WB, >215 HU on CaB) urinary calculi.

Material characterization with the use of dual-energy computed tomography (DECT) has been described in the late 1970s . However, DECT was not adopted widely for clinical use until recently because of the limited technology. Currently, the two common approaches to realize DECT scanning are the use of dual-source, dual-detector assembly for the simultaneous generation of low- and high-energy CT images and the use of single-source, single-detector system for the simultaneous acquisition of the low- and high-energy projection sets in a single examination. DECT provides material decomposition (MD) images for material characterization and has been shown to be useful for determining the composition of urinary stones .

Traditionally, nonenhanced CT is used for the detection of urinary stones because urinary stones are often obscured by high-attenuating iodinated contrast material in the renal parenchyma or collecting system in the contrast-enhanced CT images . However, with the use of virtual nonenhanced (VNE) images generated from DECT, iodine can be subtracted from the contrast-enhanced CT images and be used to depict urinary stones submerged in iodine solutions , and detect urinary stones in the pyelographic phase images . Therefore, with the creation of VNE CT scans, nonenhanced CT during CT urography could be achieved without obtaining a true nonenhanced (TNE) scan for the detection of urinary stones to reduce radiation dose and scanning time. However, the accuracy of stone detection on VNE images could not be exactly equivalent to TNE images. Recently, it has been shown that VNE images in the dual-source DECT (dsDECT) generated from the excretory phase enables the depiction of urinary stones larger than 5 mm with high sensitivities; however, there are limitations regarding smaller stone sizes (sensitivity of 16%–29% for <3-mm stones) . Urinary calculi with a diameter of <3 mm can cause symptoms such as pain and microscopic hematuria. Unfortunately, this cannot be assured in the current state of VNE dsDECT imaging.

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Materials and methods

Patient Population

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CT Protocol

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Image Interpretation

Quantitative analysis

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Qualitative analysis

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Statistical Analyses

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Results

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Table 1

Calculus Diameter Measurements on WB, CaB, and TNE Images

Parameter WB Images ( n = 98) TNE images ∗ ( n = 98)P † CaB Images ( n = 101) TNE Images ‡ ( n = 101)P § Long-axis diameter (mm) 8.09 ± 8.08 8.40 ± 8.26 .041 7.91 ± 8.11 8.21 ± 8.20 .083 Short-axis diameter (mm) 4.68 ± 3.91 5.05 ± 3.81 <.001 4.63 ± 3.91 4.96 ± 3.79 .015

CaB, calcium-based; TNE, true nonenhanced; WB, water-based.

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Table 2

ROC Analyses of the Parameters Influencing the Stone Detection Rate on WB and CaB Images

Parameter WB Images CaB Images Threshold Sensitivity (%) Specificity (%) AUC Threshold Sensitivity (%) Specificity (%) AUC Maximum attenuation of calculi (HU) 280 82.65 81.58 0.845 215 88.12 74.29 0.839 Long-axis diameter(mm) 2.71 69.39 76.32 0.769 2.71 67.33 74.29 0.745

AUC, area under the curve; CaB, calcium-based; HU, Hounsfield units; ROC, receiver operating characteristics; WB, water-based.

Table 3

Sensitivity of WB and CaB Images for Detecting Urinary Calculi by Calculi Size and Patient BMI

WB Images CaB Images <3 3–5 ≥5 Any <3 3–5 ≥5 Any Any 46.15 (18/39) 76.92 (20/26) 84.51 (60/71) 72.06 (98/136) 53.85 (21/39) 76.92 (20/26) 84.51 (60/71) 74.26 (101/136) <24 50 (6/12) 70 (7/10) 86.67 (26/30) 75 (39/52) 60 (6/10) 70 (7/10) 86.67 (26/30) 78 (39/50) 24–28.9 45.83 (11/24) 78.57 (11/14) 82.05 (32/39) 70.13 (54/77) 52.78 (13/26) 78.57 (11/14) 82.05 (32/39) 70.89 (56/79) ≥29 33.33 (1/3) 100 (2/2) 100 (2/2) 71.43 (5/7) 66.67 (2/3) 100 (2/2) 100 (2/2) 85.71 (6/7)

BMI, body mass index; CaB, calcium-based; D, diameter; WB, water based.

Numbers in parentheses are numbers of calculi.

*Diameter, in millimeter.

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Discussion

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Conclusion

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Acknowledgment

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References

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Material Decomposition Images Generated from Spectral CT

Rationale and Objectives

Materials and Methods

Results

Conclusions

Materials and methods

Patient Population

CT Protocol

Image Interpretation

Quantitative analysis

Qualitative analysis

Statistical Analyses

Results

Discussion

Conclusion

Acknowledgment

References

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