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New Research Shows MRI Can Differentiate Transurethral Bulking Agent from Serious Pathology

Key findings

  • The use of a bulking agent to treat stress urinary incontinence may not be documented in the patient's surgical history, and misinterpreting bulking agent as pathology can derail appropriate management
  • In a retrospective study of 50 patients, multiple lesions, upper to mid-urethral location, no internal fluid/low fluid level, no urethral connection, signal intensity and lesion shape helped distinguish bulking agents from urethral diverticula
  • The absence of urethral indentation or displacement was the most important finding that separated periurethral cysts from bulking agents and urethral diverticula
  • Bulking agent should be considered in the differential for every patient who requires MRI of the pelvis or lower abdomen

Transurethral or periurethral injection, a treatment for stress urinary incontinence, involves placing a bulking agent between the bladder neck and mid-urethra. This minimally invasive procedure is not always noted in a patient's surgical history, and patients may not think to report it to other health care professionals. If a patient later undergoes imaging of the pelvis or lower abdomen, the urethral bulking agent can be misinterpreted as pathology. Bulking agents have various imaging characteristics, based on the material and the amount of time elapsed since injection.

Elise J. B. De, MD, urologist with the Massachusetts General Hospital Department of Urology, and colleagues recently teamed up to determine how bulking agents can be distinguished from urethral pathology when patients undergo magnetic resonance imaging (MRI). They found that the absence of urethral indentation or displacement was the most important finding that separated periurethral cysts from bulking agents and urethral diverticula. Their findings are published in Abdominal Radiology.

Study and Diagnoses

Using a radiologic database at Mass General, the researchers reviewed 50 patients (with a total of 68 periurethral cystic lesions identified on MRI) who had good quality images and had confirmation of the findings by surgeons or pathologists between January 2001 and December 2017. Among those 68 lesions, they diagnosed 27 cases of bulking agents, 29 cases of urethral diverticula and 12 periurethral cysts.

Two board-certified abdominal radiologists, who were blinded to clinical, operative and histopathologic data, independently evaluated the MRI images.

Signal Characteristics

The bulking agent injection was collagen in 13 of the 16 patients, collagen and calcium hydroxylapatite in one, silicone particles in one and a chondrocyte/alginate mixture gel (being researched at Mass General) in one. Collagen bulking agents are no longer marketed in the United States, but they are still encountered on imaging.

Compared with muscles, collagen was isosignal on T1 weighted images (T1WI) and mildly hypersignal on T2 weighted images (T2WI), but had less signal than urine. Unexpectedly, in half of the patients, collagen was still present on MRI up to three years after injection.

Collagen, silicone and chondrocyte/alginate showed similar hyperintensity on MRI and could not be distinguished from each other. Calcium hydroxylapatite exhibited hypointensity.

Urethral Diverticula vs. Bulking Agents

Compared with urethral diverticula, bulking agents were significantly more likely to:

  • Appear as multiple lesions
  • Be located in the upper or upper-middle urethra
  • Lack internal fluid/fluid level
  • Lack urethral connection
  • Have T1 isointensity and T2 mild hyperintensity compared with muscles but lower T2 signal than urine
  • Have a wedge or circumferential shape

In most urethral diverticula, the signal intensity was similar to that of urine (T1 hypointensity and T2 marked hyperintensity). Round, oval and horseshoe-shaped lesions were more frequently urethral diverticula than bulking agents, whereas about equal numbers of the two types of lesions were crescent-shaped.

Periurethral Cysts vs. Other Diagnoses

The most important distinction between periurethral cysts and other diagnoses was that only 25% of cysts showed urethral indentation or displacement, versus 93% of bulking agents and 90% of urethral diverticula. Most cysts were oval (67%) and were located at the lower or mid-urethra (92%).

The average maximum diameter of cysts (15 mm) was about the same as for bulking agents (16 mm), but significantly less than for urethral diverticula (24 mm). The average angle (87°) was much smaller than that of urethral diverticula (150°) or bulking agents (154°).

Nine cysts (75%) showed hyperintensity on T1WI and marked hyperintensity on T2WI compared with the muscles, reflecting internal hemorrhage or high proteinaceous content. Three (25%) exhibited hypointensity on T1WI and marked hyperintensity on T2WI, suggestive of simple fluid. No cyst was isointense on T1WI.

The Bottom Line for Physicians

Radiologists—and urologists—should consider the bulking agent in the differential diagnosis for all patients who undergo MRI of the pelvis or lower abdomen, even when it is not recorded in the chart. Mischaracterization on the MRI report may lead to misdiagnosis, unnecessary referral and even unnecessary surgical intervention.

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