Medical literature shows that the most valuable echographic parameters for renal lithiasis, i.e. the posterior shadow cone and the hyperechogenic foci with marked echogenicity difference from the surrounding tissue have a low sensitivity compared to the total cases of renal lithiasis. This factor explains why CT is so often required in the diagnosis of suspected renal lithiasis.
The twinkling sign [14] is generated from the ‘noise’ stemming from rough interfaces composed of sparse reflectors, such as lithiasis or vascular calcifications, which split the sonography beam in a complex unit of waves. This produces a mix of red and blue pixels on color Doppler as turbulent flows [15, 16] (Figure 1).
As shown in literature, the twinkling sign can also be associated with vascular and/or parenchymal calcifications [17, 18]. It is possible to differentiate between these entities: when the twinkling sign is produced by vascular calcifications, it appears near a structure that at the same time produces a pulsing color signal due to the blood flow [19–22]. When the twinkling sign is produced by intraparenchymal calcifications, it differs from lithiasis due to its parenchymal location [23–25].
This study demonstrates that the twinkling sign is often associated with the presence of small renal lithiasis (_5 mm) and its identification can increase ultrasonography capabilities almost to CT levels. In our study, the twinkling sign was present in 177 (86%) out of 206 renal lithiasis identified with CT.
In the first group of lithiasis (CT positive and absolutely positive to grey-scale ultrasound, same side and location), the twinkling sign was always positive, proving to be as valuable and sensitive a sign as the other grey-scale parameters considered in medical literature. It is very interesting to compare the grey-scale images and the color-Doppler results of the second group of lithiasis (CT positive and doubtful positive on grey-scale images).
In the case of these lithiasis, which are very frequent due to the echostructural complexity of the renal medulla, the grey-scale parameters are not capable of providing a clear diagnosis. CT is required in order to do this. The twinkling sign is 100% positive in the lithiasis of this group, leading the way to a new diagnostic approach. It provides a new diagnostic tool for ultrasound to be used in those cases (71 out of 206 cases in our study, 34.5%) where classical ultrasound semeiotics only provide a doubtful diagnosis.
This means that the use of ionizing radiation is not necessary in the case of hyperechogenic foci with slight echogenicity differences and without posterior shadow cones on twinkling sign positive grey-scale images (Figure 2).
As regards the third group of lithiasis (CT positive but not visible on grey-scale ultrasound), the twinkling sign was positive in 8 out of 37 cases (22% sensitivity) (Figure 3). Although this method demonstrates low sensitivity in the detection of this group of lithiasis it is still useful as these lithiasis are not detectable by B-mode. The twinkling sign therefore provides a significant diagnostic advantage compared to grey-scale images, especially if we compare the sensitivity of the two techniques in the diagnosis of lithiasis in this specific group (21.6% vs 0%).
Moreover, the color-Doppler must be accurate and rigorous in order to identify the twinkling sign, aiming at finding a scanning plan in which the ultrasound beam is exactly perpendicular to the lithiasis. Only in this specific case does the lithiasis produce the twinkling sign. Identifying the twinkling sign is therefore much more difficult in color-Doppler because for these lithiasis, there are no grey-scale parameters that might raise some doubts and lead the physician to focus on a certain area of the kidney on color-Doppler. False negatives (29 out of 206) on the twinkling signs could be produced by the interference of obesity, intestinal meteorism and lack of cooperation by the patient with the ultrasound technique.
It may thus be inferred that, for a clear diagnosis, the twinkling sign is a much more sensitive parameter (86%) on the total number of lithiasis compared to the posterior shadow cone (36%) and to the marked echogenicity difference (31%), and also compared to both parameters (47.6%) (Tab.2).
As already mentioned, CT negative, grey-scale positive lithiasis (regardless of diagnostic certainty) were excluded from our study. Lithiasis might be CT negative because of the presence of false positives on grey-scale and, quite seldom, because of sampling mistakes due to CT parameters that were not compatible with the dimensions of the small lithiasis. In our opinion, including this group of lithiasis would imply a loss of objectivity in our study, leading our team to make philosophical speculations on the existence or non-existence of what might appear on the images. For this reason, we decided to focus on clearly diagnosed lithiasis, using CT as selection technique.
