Stone disease of the urinary tract is one of the most frequent referrals to CT in the emergency department. Typically it affects the somewhat “younger” population from a medical point of view, in which reduction to potentially harmful X-ray exposure is more relevant than in elderly or oncological patients.
Low-dose CT without intravenous contrast medium application has widely replaced conventional radiography or intravenous excretory pyelography for this clinical question because it has a substantially higher detectability rate for even tiniest calculi and overall superior diagnostic accuracy. A stone – regardless of its composition – typically presents as high-attenuating structure with very high contrast towards surrounding soft tissue and fat in the abdomen. For this, only little radiation dose is required to answer the clinical question (stone yes/no, location, size, obstruction) since perfect image quality for parenchymal organ evaluation is not required. So called low-dose CT for urinary stone detection has been established many years ago and can be regarded as standard of care.
However, CT technology has made tremendous advances within the last 2-5 years. A bundle of new achievements from improved detector sensitivity and dose effectiveness to sharper and less noisy images achieved by iterative reconstruction algorithms has been established across many platforms. Scanners have also become smarter in terms of the automated selection of the optimal tube voltage and tube current combination customized to the patient’s body habitus. A rather novel technique was introduced to the market two years ago and affects the X-ray spectrum the patient is exposed to. The X-rays that are produced in an X-ray beam are polychromatic, meaning there is a wide variety of X-ray beam energies. Some are very “soft”, meaning they penetrate the human body rather bad and “get stuck” in tissue, contributing nothing to the actual image but only to patient exposure. An additional filtration of that polychromatic X-ray beam by means of a tin filter is able to get rid of these low-energy X-rays and allows only photons of a certain, higher energy to pass (so called “hardening” or “spectral shaping”). This reduces patient exposure substantially and does not negatively affect image quality.
Some researchers found promising results in CT of the lung with exposure equivalent to that of standard two plane chest X-ray. Our results indicate that we are well competing with abdominal X-ray for stone detection with a mean exposure of our group that was examined with that tin filter technique of only 1.5 mSv. These results represent a major strengthening and support of un-enhanced low-dose CT of the abdomen as a first-line imaging test in patients with suspected urinary stones disease: it is fast, accurate and with ever ongoing technical developments in radiology less and less likely to cause harmful side-effects of ionizing radiation.
Prof. Dr. Ralf W. Bauer, MD, EBCR
By Patricia Dewes, Claudia Frellesen, Jan-Erik Scholtz, Sebastian Fischer, Thomas J. Vogl, Ralf W. Bauer and Boris Schulz
From the University Hospital Frankfurt, Department of Diagnostic and Interventional Radiology, Frankfurt, Germany