BERKELEY, CA (UroToday.com) - Kidney stones are formed through complex pathophysiologic processes, with the size, shape, composition, and internal and surface characteristics providing important clues to the specific cause of a given patient’s stone formation. While non-contrast CT examinations provide excellent sensitivity and specificity for the detection of stones, and can provide important information regarding the size, location, and total stone burden, direct assessment of stone composition requires waiting for a stone to pass or a stone removal procedure. Urine and blood tests, as well as patient history, provide many indirect clues to the composition of an in vivo stone, but the interpretation of these data is subjective, relying to a great extent on the practitioner’s experience, as well as the willingness of the patient to comply with 24-hour urine collection protocols. The long-term goal of our team’s research is to change this paradigm by providing objective, quantitative measures of stone characteristics to providers before the stone is passed or removed. The information can be obtained during the same CT examination used to confirm the diagnosis of a stone event, and to determine number, size, and location of stones, without any additional radiation to the patient. We are pursuing this goal with a new type of CT technology, known as dual energy CT.
Non-contrast enhanced CT exams of the abdomen and pelvis are the most sensitive and specific energy exam for stone disease. Dual energy CT builds upon this strength by using different energy systems on the CT system to detect specific characteristic behaviors of different stone types. For example, uric acid stones are made of primarily low atomic number elements. Their dual energy CT signature is readily distinguished from calcium-containing stones because the relatively higher atomic number of calcium responds very differently than uric acid stones. However, there are a number of calcium-containing stone types having very similar average atomic numbers. Despite similar elemental composition, these calcium-containing stones do not all result from the same pathophysiologic processes, nor do they all respond similarly to specific treatment. Thus, we have been motivated to find additional ways to differentiate calcium-containing stones, for example, calcium oxalate monohydrate (COM) and calcium oxalate dihydrate (COD) stones. In our review of the literature, we noted that previous investigators have tried to use morphological features to differentiate COM and COD stones. This motivated us to develop a technique capable of differentiating COM and COD stones based on their distinct morphological properties.
In previous studies regarding stone morphology, stones were inspected by human observers visually (in an ex vivo environment) or using CT images (in an in vivo environment). This method was time consuming and subjective. Thus, previous results were likely limited by inter- and intra-observer variations. Additionally, previous studies attempting to evaluate stone morphologies using CT image data were likely limited by the relatively poor spatial resolution of previous generations of CT systems. CT technology has advanced markedly over the last decade, and modern CT systems can provide sub millimeter spatial resolution in three dimensions, allowing us to explore morphological details of the stone surface and anterior using CT datasets.
Our technique uses a novel method to quantify stone surface morphology using a metric that describes surface curvature. This and other metrics evaluated originated in the field of computer vision. Importantly, our method automatically calculates this quantitative morphology information without user involvement, providing a fast, automated, objective, and quantitative method for describing stone morphology. We originally investigated several morphological metrics using very high spatial resolution micro CT images and a set of sample COM and COD stones. The promising results were translated to clinical CT images, where we found that the technique was sensitive to the differences in surface morphology between COM and COD stones, even though the CT image data had much poorer spatial resolution compared to micro CT data. We plan to continue to explore the relationship between stone type and morphology, extending our work into characterization of the stone interior. Based on previous literature, we are optimistic that the quantitative measures of interior stone structure will provide valuable information that can be used to predict stone fragility. As with our previous work on discriminating uric acid and non-uric acid stones using dual energy CT, and more accurately measuring stone volume, we intend to rapidly translate our morphologic characterization techniques into clinical practice in order to provide important quantitative information about stone characteristics to those physicians caring for patients suffering from urinary stone disease.
Written by:
Xinhui Duan, PhD and Cynthia H. McCollough, PhD 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.
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