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I read an overview of urinary calculi imaging, but the text was not clear as to the benefits vs risks of using a contrast agent to support CT imaging. The text did discuss use of a contrast agent during non-CT radiology of the urinary tract. At one point the text seemed to be saying that 12% of CT radiography of the urinary tract used a contrast agent.

To what extent is an intravenous (iodine based) contrast agent useful or indicated for pelvic CT scans examing the urinary tract?

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Nephrolithiasis specifically refers to calculi (calcium stones) in the kidneys, but

renal calculi and ureteral calculi (ureterolithiasis) are often discussed in conjunction. Ureteral calculi almost always originate in the kidneys, although they may continue to grow once they lodge in the ureter.

When talking about intravenous contrast agents for radiological images of the urinary system, you are very likely talking about an Intravenous pyelogram (IVP) [Source: Radiologyinfo.org].

Intravenous pyelogram (IVP) is an x-ray exam that uses an injection of contrast material to evaluate your kidneys, ureters and bladder and help diagnose blood in the urine or pain in your side or lower back.

[...]

When contrast material is injected into a vein in the patient's arm, it travels through the blood stream and collects in the kidneys and urinary tract, turning these areas bright white on the x-ray images. An IVP allows the radiologist to view and assess the anatomy and function of the kidneys, ureters and the bladder.

[...]

The IVP exam can enable the radiologist to detect problems within the urinary tract resulting from:

  • kidney stones
  • enlarged prostate
  • tumors in the kidney, ureters or urinary bladder
  • scarring from urinary tract infection
  • surgery on the urinary tract
  • congenital anomalies of the urinary tract

With CT exams, the reason for the the contrast agent is the same as x-rays. Computed tomography (CT) is an imaging tool that combines x-rays with computer technology to produce a more detailed, cross-sectional image of your body. [Source: American Academy of Orthopaedic Surgeons]

CT images may add valuable information about the functioning tissue of the kidneys and surrounding structures nearby the kidneys, ureters and bladder. Small urinary tract tumors and stones are more easily identified on these examinations. [Source: Same Radiologyinfo.org link above]

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To help answer your question, if we are independently suspecting urinary calculi (whether it is within the kidney, ureter, or bladder), the imaging of choice is a CT scan without contrast. The reason for this is because the utilization of contrast can obscure the stone. What I mean is, the presence of contrast on a CT scan appears "white" (aka enhanced). So does a calculi. What if, at the moment the image was taken, the contrast is present behind, or in front of the stone (such as a vascular structure). Or perhaps oral contrast was also administered, and present within the intestine, and the stone is within the ureter, at the level of that portion of the intestine. You wont see the stone, because the contrast sitting in the intestine is now hiding it. Now of course, you have separate views and angles you can look at on the CT, but overall it makes it harder to find the stone. In a CT without contrast, the stone is much easier to find, because it will likely be the brightest shade of white (other than bone), in an area where it otherwise wouldn't belong. Also there are risks with contrast, such as contrast nephropathy. Therefore, why administer contrast, if what you are suspecting is a kidney stone? Contrast is also contraindicated if your kidney function is decreased (measured by your serum creatinine.)

Now let's say we are interested in the kidney's perfusion (amount of blood getting to it.) Or perhaps, if the kidney is infected (pyelonephritis.) In this scenario, IV contrast is beneficial. IV contrast will enhance the infected kidney, help discern if there is even potentially an abscess, and additionally you can see if there is any arterial narrowing within the supply to the kidney (such as renal artery stenosis). IV contrast is drawn to areas of high metabolic activity, and it comes from the intravascular space (because it is injected intravenously.) Pathologic diagnosis comes from either

  • Contrast is present where it doesn't belong
  • Contrast is much higher in a region that is within a structure
  • Contrast is 'smeared' near where it is expected to be within a vascular structure.
  • Contrast is higher in a focal region within a structure, with a central region of abrupt absence.

These examples are what providers usually look for when ordering an image with contrast. Contrast where it doesnt belong can mean many things, some examples are a vascular leak, or severe infection. Contrast much higher in a specific region within a structure usually indicates infectious. Contrast smeared near a vascular structure has a term called 'extrav' which means the vascular structure is perforated and leaking. Contrast with a higher focus of contrast within a structure, with a central region of abrupt absence may indicate an abscess (a walled off portion of the infection filled with pus, and doesn't have a vascular supply therefore no contrast gets to it.)

If you are worried about the function of motility within the urinary tract, this is when a pyelogram may be beneficial. For example, a ureteral stricture, or perhaps a ureteral leak.

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