Jason Gonzalez, MS4
The Article: Gliga ML, Chirila CN, Podeanu DM, et al. Twinkle, twinkle little stone: an artifact improves the ultrasound performance! Medical Ultrasonography. 2017;19(3):272. doi:10.11152/mu-984.
The Idea: Ultrasound is frequently used in initial evaluation of patients with suspected renal calculi. However, there are many false negative and false positive results, specifically with stones that are less than 5mm in size. The twinkling artifact is the result of machine noise with color doppler ultrasound, resulting in a focus of alternating colors behind a reflective object (such as a calculi), giving the appearance of turbulent flow. This article evaluates the importance of using the twinkling artifact to detect renal stones that are less than 5mm in size.
The Study: This study was a prospective cohort study conducted at Mures County Hospital between March and May 2014. All examinations were performed by a single examiner with over 10 years of experience in renal US who was blinded to the patients’ history. One machine was used with specific parameters and standardized examination technique.
Inclusion Criteria: Uni/bilateral lumbar pain (colicative or not) or history of kidney stones.
Exclusion Criteria: Patients with stones larger than 5mm.
Primary Endpoint:
a) Sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) of the twinkle artifact. After the initial US investigation, the patients were evaluated with “gold standard” exams: either CT scan, intravenous urography (IVU) or the spontaneous elimination of the stones (urine sediment) to obtain the final statistical values.
Results: The presence of renal stones was confirmed in 123 out of 174 patients (37 patients using CT-scans, 20 patients using IVU, and 66 patients had history of lithiasis combined with the presence of urine crystals and spontaneous elimination of the calculi). The twinkling artifact was present in 113 of these patients. In 10 patients with confirmed stones, the twinkling artifact was not present and in 2 patients the twinkling artifact was present in the absence of stones on CT. Therefore, the sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) of the twinkle artifact in detecting renal stones of less than 5mm was 99.12%, 90.91%, 99.12% and 90.91% respectively.
Study Limitations:
The study only evaluated intra-renal stones and did not look for stones in the ureter or bladder. The researchers did not have much information on the chemical make-up of the stones. Researchers, however, did observe 4 patients with urate stones (all of which were larger than 1cm) and none of these stones produced twinkling artifact. The study used one US technician with 10+ years of experience in renal US, therefore it does not account for the large disparity in technician skill. The “gold standard” examinations were not standardized, for example the CT scans did not have a standardized slice thickness. The small sample size of 173 was another limitation.
The Takeaway: The paper proposes the routine use of color doppler in cases suspicious of renal stones to avoid expensive studies or exposure to unnecessary radiation. Being aware of the twinkling artifact can raise sensitivity when evaluating for renal calculi. This is a great starting point in the quick initial evaluation of renal calculi; however, we must keep in mind this study evaluated finding intra-renal calculi, with a trained renal US professional. While in the ED, most patients arrive symptomatic with stones in the ureter which are much harder to find on US. This will yield us a much lower “real-world” sensitivity, specificity NPV and PPV that this paper found.