[Usefulness of subtraction computed tomography angiography employing orbital synchronized helical scanning for diagnosis of lower extremity arteries with vessel wall calcification]

[Accuracy Verification for Slice Sensitivity Profile Measurement Method by Averaging the Multiple Tilted Wires in Computed Tomography Image]

PURPOSE

Several studies present the unsuitability of the tilted-wire method for slice sensitivity profile (SSP) in helical scan. We compared the accuracy for SSP by the tilted-wire averaging method using multiple wire profiles and by the conventional micro-coin method.

METHODS

A micro-coin phantom positioned at the center or the off-center was scanned using a 64-detector row CT scanner in different positions where an X-ray tube starts scanning. In the same way, tilted-wire averaging phantoms, approximately 70 mm in diameter, in the shape of a donut, 8 wires tilted from the circumference toward the center, were scanned. Images were reconstructed with a slice thickness of 0.5 mm.

RESULTS

The relative errors of full width at half maximum (FWHM) by the tilted-wire averaging method were -0.015 mm to -0.004 mm (-1.98% to -0.56%) at the center compared to those by the micro-coin method, and it is almost the same value regardless of the number of wires. Relative errors were 0.001 mm to 0.029 mm (0.11% to 3.74%) at the upper 8 cm from the center, and 0.014 mm to 0.078 mm (1.86% to 10.25%) at the upper 16 cm, and the value of relative errors increased as it got farther from the center and as the number of wires went fewer.

CONCLUSION

This study indicated that accurate measurement of SSP may be achieved by using 4 (arranged every 90 degrees) or more averaging wires.

[Method for Measuring Noise Power Spectrum Depending on X-ray Tube Angle in Spiral Orbit of Helical CT Scan]

PURPOSE

The noise power spectrum (NPS) in computed tomography (CT) images potentially varies with the X-ray tube angle in a spiral orbit of the helical scan. The purpose of this study was to propose a method for measuring the NPS for each angle of the X-ray tube.

METHODS

Images of the water phantom were acquired using a helical scan. As a conventional method, we measured the two-dimensional (2D) NPS from each image and averaged them; the obtained 2D-NPS was referred to as NPSconventional. In the proposed method, we made the X-ray tube angle θ (0°≤θ<360°) to correspond to the image according to each slice position of the images that located within the travel distance of the CT scan table per 360° rotation of the X-ray tube. We obtained the 2D-NPS from each image and assigned the θ (0°, 30°, 60°, 90°, 120°, 150°, 180°); the obtained 2D-NPS was referred to as NPSsθ. The NPSsθ was compared to the NPSconventional. Also, we investigated the dependency of the NPSsθ on the θ.

RESULTS

The NPSconventional was found to be isotropic, and in contrast, the NPSsθ was anisotropic. The NPSsθ showed a continuously rotational change while increasing the θ. There was an excellent correlation (R2>0.999) between the rotation angle of NPSθ and the θ.

CONCLUSION

The proposed method was demonstrated to be effective for evaluating anisotropic noise characteristics depending on the X-ray tube angle.

Diagnostic performance of computed tomography digital subtraction angiography of the lower extremities during haemodialysis in patients with suspected peripheral artery disease

INTRODUCTION

With intra-arterial digital subtraction angiography (DSA) considered as the gold standard, we compared the diagnostic value of computed tomography angiography (CTA) and computed tomography-digital subtraction angiography (CT-DSA in hemodialysis (HD) patients suspected of having lower limb peripheral artery disease (PAD).

METHODS

In this retrospective study, we enrolled 220 HD patients with suspected PAD. CT-DSA images were obtained by subtracting unenhanced images from enhanced images. The research team calculated the area under the curve (AUC), sensitivity, specificity, positive and negative predictive value (PPV, NPV), and recorded the diagnostic accuracy between the CTA and CT-DSA images using the DSA as gold standard. Visual evaluation of calcifications in the peripheral arteries were also compared between CTA and CT-DSA images.

RESULTS

At the above-knee level, the CTA AUC [95% confidence interval (CI)] was 0.68 (CI 0.64-0.72), sensitivity and specificity were 60 and 81%, PPV and NPV were 85 and 53%, and accuracy was 67%. Below the knee, these values were 0.66 (CI 0.62-0.70), 71 and 79%, 79 and 47%, and 66%. For CT-DSA, above-knee, the AUC [95% CI] was 0.88 (CI 0.85-0.91), sensitivity and specificity were 84 and 92%, PPV and NPV were 89 and 97%, and accuracy was 93%. Below the knee, these values were 0.95 (CI 0.93-0.97), 95 and 93%, 96 and 83%, and 93%. The scores for the visualization of calcification in the peripheral arteries was significantly higher for CT-DSA than CTA (p < 0.05).

CONCLUSIONS

CT-DSA helps to assess stenotic PAD with high calcification in the lower extremities of HD patients.

IMPLICATIONS FOR PRACTICE

On CT-DSA images, the severity of vascular calcification can be assessed for HD patients suspected of PAD of the lower extremities.