Evaluation of the CT Parameters to Suppress Renal Cysts Pseudoenhancement Effect: Influence of the Virtual Monochromatic Spectral Images, the Model-based Iterative Reconstruction Algorithm and the Aperture Size in Phantom Model

Pseudoenhancement in Cystic Renal Lesions - Impact of Virtual Monoenergetic Images of Photon-Counting Detector CT on Lesion Classification

RATIONALE AND OBJECTIVES

To investigate the impact of virtual monoenergetic images (VMI) from photon-counting detector CT (PCD-CT) on the enhancement and classification of renal cysts.

MATERIALS AND METHODS

Adults with renal cysts (≥7 mm) who received a triphasic examination on a clinical PCD-CT (120 kVp; IQ level 68) between July 2021 and March 2022 were retrospectively identified. Only non-enhancing cysts (enhancement<10 HU between unenhanced and venous phase at 70 keV) were included. VMI from 40 to 190 keV with increments of 10 keV were reconstructed from the venous phase. Enhancement was measured to classify each lesion as non-enhancing (<10 HU), equivocally enhancing (10-19 HU), and definitely enhancing (≥20 HU). Classification changes as a function of VMI were assessed. Pearson correlation coefficient, the Kruskal-Wallis and the Chi-square test were used.

RESULTS

A total of 86 patients (mean age, 74 ± 9 years; 74 male) with 160 non-enhancing renal cysts (17.6 ± 10 mm) were included. CT attenuation of the cysts increased from higher to lower VMI levels with a mean attenuation of 4 ± 11 HU at 190 keV to 36 ± 17 HU at 40 keV. Mean attenuation of the renal parenchyma was 43 ± 4 HU at 190 keV and 414 ± 71 HU at 40 keV. No cyst exhibited enhancement from 70 keV to 190 keV. At 40, 50, and 60 keV, 35% (56/160), 29% (47/160) and 9% (15/160) of cysts showed equivocal and 46% (74/160), 10% (16/160), and 0% (0/160) definite enhancement, respectively. There was no significant influence of size (P=.13), cyst location (P=.9) and BMI (P=.19) on enhancement classification.

CONCLUSION

VMI has a relevant impact on enhancement and classification of renal cysts with misclassification in a large number of cases at energy levels below 70 keV.

Effect of Ultra-High Resolution CT on Pseudoenhancement in Renal Cysts: A Phantom Experiment and Clinical Study

Background: Ultra-high resolution CT (U-HRCT) allows acquisition using a small detector element size, in turn allowing very high spatial resolutions. The high resolution may reduce partial-volume averaging and thereby renal cyst pseudoenhancement. Objective: To assess the impact of U-HRCT on renal cyst pseudoenhancement. Methods: A phantom was constructed that contained 7-, 15-, and 25-mm simulated cysts within compartments simulating unenhanced and nephrographic-phase renal parenchyma. The phantom underwent two U-HRCT acquisitions using 0.25- and 0.5-mm detector elements, with reconstruction at varying matrices and slice thicknesses. A retrospective study was performed of 36 patients (24 men, 12 women; mean age, 75.7±9.4 years) with 118 renal cysts who underwent renal-mass protocol CT using U-HRCT and the 0.25-mm detector element, with reconstruction at varying matrices and slice thicknesses; detector element size could not be retrospectively adjusted. ROIs were placed to measure cysts' attenuation increase from unenhanced to nephrographic phases (to reflect pseudoenhancement), and SD of unenhanced-phase attenuation (to reflect image noise). Results: In the phantom, attenuation increase was lower for the 0.25 mm than 0.5 mm detector element for the 15-mm cyst (4.6±2.7 HU vs 6.8±2.9 HU, p=.03) and 25-mm cyst (2.3±1.4 HU vs 3.8±1.2 HU, p=.02), but not the 7-mm cyst (p=.72). Attenuation increase was not different between 512×512 and 1024×1024 matrices for any cyst size in the phantom or patients (p>.05). Attenuation increase was not associated with slice thickness for any cyst size in the phantom or for ≥5-to-<10 mm and ≥10 mm cysts in patients (p>.05). For cysts <5 mm in patients, attenuation increase showed decreases with thinner slices (3 mm: 23.7±22.5, 2 mm: 20.2±22.7 HU, 0.5 mm: 11.6±17.5 HU, 0.25 mm: 12.6±19.7 HU; p<.001). Smaller detector element size, increased matrix size, and thinner slices all increased image noise for cysts of all sizes in the phantom and patients (p<.05). Conclusion: U-HRCT may reduce renal cyst pseudoenhancement through a smaller detector element size and, for <5 mm cysts, very thin slices; however, these adjustments result in increased noise. Clinical Impact: Although requiring further clinical evaluation, U-HRCT may facilitate characterization of small cystic renal lesions, thereby reducing equivocal interpretations and follow-up recommendations.