Clinical and Health Affairs
Clinical-Imaging Evaluation Nephrolithiasis
An Update
By Terri J. Vrtiska, M.D., Amy E. Krambeck, M.D., Cynthia H. McCollough, Ph.D., Shuai Leng, Ph.D., Mingliang Qu, M.D., Lifeng Yu, Ph.D., and John C. Lieske, M.D.
Abstract
Advances in radiology have led to improvements in care for patients with urinary tract stones. One of the most promising imaging techniques is dual-energy CT, which enables more accurate characterization of stone disease than other imaging techniques and helps direct therapy at the time of the initial imaging evaluation. Improvements in percutaneous therapy have led to less-invasive and less-costly treatments for nephrolithiasis. This article describes some of these new approaches to diagnosing and caring for patients with renal stone disease.
Asymptomatic and symptomatic renal stone disease affects millions of people in the United States and costs billions of dollars annually. According to reports in 2000, care for renal stone disease resulted in total costs of more than $5.3 billion dollars.1
Diagnosis and treatment of patients with nephrolithiasis have traditionally involved imaging exams, metabolic evaluation, invasive treatment, and hospitalization. During the past several years, advances in imaging technology have led to improved diagnostic tools for identifying, characterizing, and differentiating stone disease types. One of the most promising imaging techniques is dual-energy (DE) CT, which enables more accurate characterization of stone disease than other imaging techniques and thereby helps direct therapy at the time of the initial evaluation. In addition, improvements in percutaneous therapy have led to less-invasive and less-costly treatments for nephrolithiasis. The following article describes these new approaches to the diagnosis and care of patients with renal stone disease.
Dual-Energy CT
The conventional methods for diagnosing renal stone disease are the abdominal plain film X-ray and intravenous urogram (IVU). Both can depict the size, location, number, and appearance of renal stones; they also allow for visualization of the urinary tract. More recently, the method of choice for imaging symptomatic renal stones has been nonenhanced CT using single-energy technology. With this technology, physicians have been able to rapidly identify the presence of stone material and urinary tract obstruction. Early studies showed that single-energy CT was 97% accurate in diagnosing ureteral stones. Subsequent studies of more advanced techniques showed sensitivities and specificities approaching 100%.2-5
Although early CT technology could accurately depict the absence or presence of stones, it was not helpful for determining the composition of the stones because nearly all urinary calculi (more than 99%), regardless of composition, appear as opaque calcifications on scans (Figure 1). Dual-energy CT is a recent advance that enables better material characterization and faster acquisition (5 to 10 seconds per acquisition with submillimeter resolution). Differences in the X-ray attenuation properties from the two X-ray sources allow for differentiation of certain renal stone compositions. In addition, with DECT, the various types of stones appear in different colors (Figure 2).
Initial research on DECT performed in vitro focused on distinguishing between stones composed of uric acid and calcium, as this distinction has an impact on treatment decisions.6-11 In particular, if it is known that a stone is composed of uric acid, a trial of urinary alkalinzation to dissolve it may be warranted. These in vitro studies showed a sensitivity of 88% to 100% depending on stone and anthropomorphic phantom sizes with an accuracy of 93% to 100%. Subsequent in vivo studies showed sensitivity of 74% to 100% and accuracy of 89% to 100%, including for DECT acquisitions done using a low-radiation-dose imaging protocol.12-14 Most recently, research has focused on using DECT to differentiate among several stone subtypes, eg, among those that contain uric acid, cystine, struvite, and calcium (calcium oxalate or calcium phosphate).15 Thus, this newer technology provides an opportunity for making an immediate determination about the stone subtype and the direction of the patient’s care.
An important consideration is the amount of radiation patients are exposed to with DECT. Advances in CT imaging in general have allowed images to be acquired using a radiation dose that is lower than that required by early scanners. The dose can be tailored to the patient’s size as well as the clinical indication. Importantly, the radiation dose in DECT exams is not double that in single-energy CT scanning, even though two X-ray sources rather than one are used. In fact, the radiation dose required for newer DECT imaging is lower than that used by earlier generations of CT scanners. Low-dose DECT techniques approximate those of single-energy CT scanning,14 yet the resolution and quality of the images produced with DECT are high.
Interventional Therapy
Over the past several decades, we have shifted from doing open operations to less invasive endourologic procedures for stone removal. Currently, stone disease can be treated with extracorporeal shockwave lithotripsy (ESWL), percutaneous nephrolithotomy (PCNL), and ureteroscopy (URS). Recent advances in whole-body imaging, specifically DECT scanning, are increasing the effectiveness and efficiency of each type of intervention.
Extracorporeal shockwave lithotripsy was developed in the early 1980s by Dornier, a German aerospace firm.16 Since its introduction in the United States in 1984, ESWL has quickly become the most widely utilized treatment for symptomatic stone disease.17 The procedure is done transcutaneously and can be performed with light sedation; however, its effectiveness is increased with use of general anesthesia.18 Shockwaves are produced by a generator that is coupled to the patient by a water barrier (originally the patient was placed in a water bath, today water balloons are used). Using ultrasound or fluoroscopic imaging, the shockwaves are focused on the stone. After a series of shockwaves are administered, the stone should fragment into pieces small enough to be passed spontaneously by the patient.
Shockwave lithotripsy is ideal for stones that are not impacted and are less than 2 cm in size. Certain types of stones including cystine, calcium oxalate monohydrate, and brushite are resistant to ESWL and will not easily fragment. Overall, the success rate of ESWL ranges from 33% to 90%, depending on stone size and location.18
Although ESWL is minimally invasive and does not require an incision, it is not without potential consequences. In general, most complications are minor and include pain, fever, and hematuria; however, major acute complications involving every abdominal organ have been reported.19 Long-term complications have not been clearly defined and are somewhat controversial. To date, no association between ESWL and the development of renal insufficiency has been identified. Some authors have identified an association between ESWL and the long-term development of hypertension and even diabetes mellitus;20 yet, others have not found such associations.21
Given these considerations, preoperative detection of stone composition can limit the unnecessary use of ESWL or increase its effectiveness. If DECT scanning identifies calcium oxalate monohydrate or brushite stone composition in vivo, ESWL can be avoided and a more appropriate treatment option such as URS or PCNL utilized. Furthermore, if struvite stones, which are associated with urinary tract infections, are detected preoperatively, the surgeon can choose a treatment option such as URS or PCNL that will immediately render the patient free of the infected stones. By limiting the inappropriate use of ESWL, rare, but potentially severe, complications can be avoided.
Compared with ESWL, URS is a more invasive surgical treatment. It is ideal for stones 1 cm or less in size located in the kidney or ureter. With the patient under general anesthesia, a small endoscope is advanced through the urethra up the ureter to the stone; no incision is necessary. The endoscope may be rigid, in the case of distal ureteral stones, or flexible, for proximal ureteral and renal stones. Once the stone is encountered, it is either removed intact using a basket or broken into smaller pieces using a holmium laser. These smaller pieces are often extracted using the basket or left to be passed spontaneously. The stone-free rate for URS is very high (78% to 97%).22 The procedure is generally performed as an outpatient surgery. In most cases, it requires that a ureteral stent be left in place postoperatively for a time to avoid ureteral obstruction secondary to swelling. Unfortunately, these stents are associated with patient discomfort and, in general, are poorly tolerated. Possible complications associated with URS include pain, urinary tract infection, ureteral injury, and ureteral stricture.
Although URS is highly effective, it should be used appropriately. Many uric acid stones less than 1 cm in size can be effectively dissolved using urine alkalinization therapy alone. DECT is capable of identifying uric acid stones in vivo, which may save a patient the unnecessary discomfort of a ureteroscopic procedure. Furthermore, some patients with ureteral stones first receive a ureteral stent to decompress the urinary system prior to URS. In some cases, the stone will pass with the stent in place; but since the stone appears to be the same radiopacity as the stent, its passage is not always recognized before URS is performed. Dual-energy CT scanning can easily differentiate the ureteral stent (red in appearance) from a ureteral stone (blue in appearance) and can accurately determine if the stone has passed prior to URS.
Percutaneous nephrolithotomy is the most invasive endoscopic treatment option for urinary stone disease. It is used to treat multiple or complicated stones or those greater than 2 cm in size. This surgical intervention involves creating a tract from the skin to the kidney through the flank using ultrasound and/or fluoroscopic guidance. Once the tract is in place, it is dilated to a size large enough to admit an endoscope into the kidney. The stone is then removed using a combination of ballistic, ultrasound, and suction techniques. The patient is left with a nephrostomy tube overnight, then discharged home.
The most effective way to ensure that the patient is rendered stone-free is PCNL. Most patients return to normal activity within two weeks of surgery, and scarring is minimal. Potential complications associated with PCNL including pain, bleeding (with a transfusion rate of 1% to 10%), injury to surrounding organs (0.5%), pleural injury (1% to 10% of upper pole tracts), and infection.18
The ability of DECT scanning to identify stone types can influence the technique and guide appropriate utilization of PCNL. In general, all types of stones can be effectively removed using PCNL; however, removal of certain types of stones such as those containing cystine, calcium oxalate monohydrate, and brushite can be time-consuming and may require a combination of techniques and/or multiple procedures. Other types of stones, such as those composed of struvite, are easily removed, and in many instances large branching stones can be removed through one percutaneous tract using a combination of ultrasound and laser technology. Uric acid stones may grow very large and appear to require removal with PCNL; however, by using a combination of URS with laser fragmentation to increase stone surface area and medical alkalinization therapy, these stones can be dissolved and the patient can avoid invasive therapy.
Conclusion
Ongoing improvements in CT technology including dual-energy acquisitions are providing better information about the size, location, and composition of renal calculi. This information can guide surgeons and internists toward the most appropriate treatment for a patient. MM
Terri Vrtiska is in the department of radiology, Amy Krambeck is in the department of urology, and John Lieske is in the division of nephrology and hypertension and department of laboratory medicine and pathology at Mayo Clinic. Cynthia McCollough, Shuai Leng, Mingliang Qu, and Lifeng Yu are in Mayo’s department of radiology/medical physics.
Acknowledgement
The authors acknowledge the contributions of the Mayo Clinic CT Innovation Center. Drs. Vrtiska, Lieske Krambeck, McCollough, and Leng are supported by the National Institutes of Health O’Brien Urology Research Center at Mayo Clinic (P50 DK83007). Dr. McCollough receives research support from Siemens Healthcare.
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