BERKELEY, CA (UroToday.com) - Though an artifact is usually a hindrance to proper visualization of structures, such as in CT and MRI, in several circumstances sonographers have learned to make “lemonade from lemons.” For example, “ring down” artifact helps to identify cholesterol crystals in gallbladder adenomyosis, “dirty shadowing” helps confirm presence of gas in a wall, and dense shadowing is used to look for calculi or metal clips. This “lemonade theory” also applies to the twinkling artifact for detecting calculi.
Since concluding our prospective study of twinkling artifact versus CT for identifying urinary tract calculi, use of this artifact has become a standard part of our routine abdominal ultrasound. After measuring kidney size and documenting presence of hydronephrosis, the technologist turns on the Doppler function on the machine and increases pulse repetition frequency to around 69 pulses/sec. The upper half of the kidney is evaluated followed by the lower half of the kidney, in two separate sweeps. If twinkling is identified, a 2-second cine clip is captured, demonstrating persistence of the artifact. This is done to prove that this is a real finding rather than a transient phenomenon. If twinkling persists, the area is identified as a probable calculus. Once the technologist has finished looking for twinkling artifact, the images are correlated to the gray-scale images. Even if an echogenic area does not demonstrate definite shadowing, as is traditionally expected of a calculus on gray-scale imaging, the area of twinkling artifact is still reported by the radiologist as a probable stone. The technologists complete their gray-scale evaluation of the kidney parenchyma to exclude renal masses and areas of cortical thinning. Identifying twinkling artifact in the kidneys and distal ureters has increased the technologists’ and radiologists’ confidence in diagnosing calculi. In outpatients, if this is an unexpected finding and management will change, ordering physicians at our institution may still request a CT scan. However, sonography is commonly requested in the emergency department in patients with history of previously documented renal calculi and acute presentation of typical flank pain. More studies are needed to assess possible weaknesses of twinkling artifact, such as effect of atherosclerosis on false positive rate. However in our patient study population, including those who were older and even those with BMI > 30, the artifact was still present in cases of calculi, allowing more confident diagnosis.
CT is currently still the gold standard for detecting urinary tract calcifications and calculi, but CT is not infallible. For instance, the renal pyramids can be dense on CT due to high concentration of urine, particularly in dehydrated patients, even on non-contrast enhanced images. This can decrease the contrast between small calculi and background renal parenchyma. Also, the use of ultra-low dose CT increases the background noise to the point where tiny calculi can be missed: this was demonstrated by Kim, et al. Though it probably cannot be said that use of twinkling artifact is equal to CT for detecting calculi at this time, with technologic improvements in the field of ultrasound, this may happen in the future. For now, we will enjoy our lemonade in moderation. Reference:
- Kim BS, Hwang IK, Choi YW, Namkung S, Kim HC, Hwang WC, Choi KM, Park JK, Han TI, Kang W. Low-dose and standard-dose unenhanced helical computed tomography for the assessment of acute renal colic: prospective comparative study. Acta Radiol. 2005 Nov;46 (7):756-63.
Ania Z. Kielar, MD, FRCPC as part of Beyond the Abstract on UroToday.com. This initiative offers a method of publishing for the professional urology community. Authors are given an opportunity to expand on the circumstances, limitations etc... of their research by referencing the published abstract.
Department of Radiology
Ottawa Hospital, C-1
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Ottawa, ON K1R 4E9, Canada