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Centennial Dissertation |
1
Department of Radiology, New York Presbyterian Hospital, Cornell University
Medical College, Box 141, 525 E. 68th St., New York, NY 10021.
2
Department of Diagnostic Imaging, Yale University School of Medicine, 333
Cedar St., New Haven, CT 06510.
Received January 18, 2000;
accepted after revision February 16, 2000.
Honoring Henry K. Pancoast, MD and Sidney Lange, MD
Introduction
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When in the course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.
In the spirit of the preceding paragraph and in the name of all modern practitioners of genitourinary imaging, I propose to set forth below the events and reasons that now force a separation of the modern era of imaging urinary lithiasis from all that has gone before. A wonderful (and complete) review of the history of imaging of the urinary tract is given by Howard Pollack in the latest edition of the textbook he edited entitled "Clinical urography: an atlas and textbook of urological imaging [2]." In what follows, I have used this review as a guide and a source of historical references in the literature.
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It was also clear from the outset that radiography alone was inadequate to definitively diagnose a ureteral stone in a patient with acute flank pain and suspected renal colic. Even if an opacity was present along the anatomic course of the ureter, the ureter itself could not be directly seen on radiography. What was needed was some other means to force the ureter to reveal itself so that a specific opacity thought to be a stone could be located in (or outside of) the lumen of the ureter. In fact, this idea was tried within a year and the first "opacification" of the ureter was performed by inserting a metal wire into a ureteral catheter [5]. This technique was subsequently replaced by making the ureteral catheters themselves radiopaque.
Next, air was tried as a contrast agent to reveal the ureter on radiographs. Air was soon replaced by a liquid contrast agent containing a colloidal suspension of silver, and the first retrograde pyelogram was born [6]. The usefulness of this technique was quickly recognized but, unfortunately, so were the dangers associated with the silver-containing contrast agent. The search for safer materials began and sodium iodide solutions, first described by Cameron [7] in 1918, became the contrast agents of choice for retrograde pyelography.
The next step in this evolutionary process was to eliminate the need to directly introduce the contrast agent into the urinary system. An indirect means might be faster and safer. In fact, in 1923, Osborne et al. [8] described the use of a high dose of IV sodium iodide to reveal the ureters on radiography. This was a novel idea whereby the IV contrast agent was filtered by the kidneys and then excreted directly into the urine. Although images of the urinary system obtained in this manner were suboptimal, this led to the development of more robust agents that were IV administered and then excreted into the urine. The first such agents to yield higher quality urograms were iodinated pyridine compounds described by Swick [9] in 1929. In fact, iodinated pyridine compounds were routinely used to perform IV urography for the next 20 years. Then, in 1952, pyridine compounds were replaced by the even safer iodinated benzoic acid derivatives. These later agents are still sometimes used today, although the newer nonionic compounds are now the dominant force when IV contrast material is required for imaging studies of all types, including IV (or excretory) urography. However, despite the improved safety of the nonionic agents, even these carry the risk of allergic reactions, which vary from a few hives to respiratory distress and, in rare cases, death.
By obtaining radiographs before and after an IV contrast agent is administered, one can theoretically determine if an opacity seen on radiography is actually in the ureter. In addition, when a stone is obstructing the ureter there will be delayed filling of the collecting system (and ureter) as well as dilatation compared with the normal side. Would that things were so simple. Despite protest to the contrary [10,11,12,13], only about 60% of all ureteral stones will actually be visible on radiographs [14]. In addition, when significant obstruction is present, it may take an inordinate amount of time for contrast material to reach the level of obstruction. Even when contrast material does reach the level of obstruction, it still may not be possible to show that a specific opacity is the obstructing stone.
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Computerized axial tomography was first described by Hounsfield and Ambrose in 1973 [15, 16]. Nearly since its inception, it had been well-known that CT could reveal almost all renal (and hence ureteral) stones. In fact, it was even suggested that CT attenuation values could be used to determine the composition of renal stones [17]. At the same time, CT can reveal the ureter itself so that the relationship between a calcific density and the ureter can be precisely determined. This can even be done without the benefit of IV contrast material, so why not use CT to diagnose acute ureteral obstruction? This could have been done even on early CT scanners using relatively thick sections, but long scan times and respiratory variation between sections were apparently formidable obstacles to the advocacy of CT for this purpose. All of this changed, however, with the advent of helical CT scanners.
Since its first description in 1994 (Essenmacher KR, Smith RC, Rosenfield AT. Annual Meeting of the Society of Uroradiology, January 1994) and first publication 1 year later [18], unenhanced helical CT has quickly become the imaging study of choice to examine patients with acute flank pain and suspected urinary lithiasis. Unenhanced helical CT takes only minutes to perform and is highly accurate and completely safe. CT reveals the size and location of virtually all ureteral stones. These are the two most important factors used for patient treatment. In addition, CT can diagnose nearly all other serious conditions that mimic renal colic.
To paraphrase (and give a modern interpretation of) the last paragraph of the Declaration of Independence:
We, therefore, the Representatives of the new generation of Genitourinary Radiologists, in Public, Assembled, appealing to the Supreme Judge of the Radiology World (i.e., the Editor of the AJR) for the retitude of our intentions, do in the Name, and by Authority of the good people of radiology, solemnly publish and declare, That the practitioners of Genitourinary Radiology are, and of right ought to be, free and Independent; that they be absolved from all Allegiance to the Excretory Urogram (formerly known as the "IVP") and that all connection between past and present practices are and ought to be totally dissolved; and that as Free and Independent Thinkers, they have full Power to Levy war on the excretory urogram, conclude that it is unnecessary for the diagnosis of ureteral obstruction, contract new Allegiances, establish new Procedures and to do all other Acts and Things which Independent Thinkers may of right do. And for the support of this Declaration, with a firm reliance on the protection of divine Providence, we mutually pledge to each other our Lives, our Fortunes our new Allegiance to Unenhanced CT and our sacred Honor.
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On CT, secondary signs of obstruction can aid in the diagnosis of acute renal colic when a stone is not readily apparent. The frequency of CT secondary signs of obstruction on CT has recently been shown to correlate with the duration of pain [20]. This should be kept in mind when interpreting CT studies, particularly in patients whose duration of pain is less than 2 hr. In addition, secondary signs on CT can be used to make the diagnosis of a recently passed stone [21]. In the latter case, some stones will actually be revealed while still present in the bladder. All patients should be scanned in the prone position so that a stone that has already passed into the bladder is not confused with a stone still lodged in the ureter at the ureterovesical junction [22].
Once the diagnosis of obstruction by a ureteral stone has been made on CT, prognosis and patient treatment can be guided on the basis of the site of obstruction and the size of the stone. In fact, the CT scout view can itself often be used as a baseline study in patients requiring followup imaging and in patients who will undergo lithotripsy [23]. We performed an exhaustive review of the literature in an attempt to find a single study that has shown that the degree of obstruction (as determined on excretory urography) can be used to guide patient treatment or determine prognosis. Our review failed to reveal any such article. In addition, several recent studies strongly suggest that the secondary signs of obstruction on CT can in fact be used to determine the degree of obstruction as well as help predict the likelihood of stone passage [24, 25]. It would indeed be the ultimate irony if CT findings are shown to more accurately reflect the degree of obstruction and predict the likelihood of stone passage than excretory urography.
With negative findings for acute ureterolithiasis on unenhanced helical CT, radiographers can confidently exclude the diagnosis of clinically significant stone disease and many other causes of acute flank pain. In addition, despite the lack of IV or oral contrast material, unenhanced CT can reveal many other causes of acute flank pain that are unrelated to the urinary system, such as pelvic masses, appendicitis, and diverticulitis. Unenhanced CT may also reveal abnormalities of the urinary system unrelated to stone disease, such as pyelonephritis [26]. In some patients, it may occasionally be necessary to repeat the CT scan after the administration of IV or oral contrast material to make a diagnosis. Examples include patients with renal vein or renal artery thrombosis and patients with renal infarcts. We would never hesitate to administer contrast material when necessary and appropriate.
The one remaining pitfall in the interpretation of unenhanced helical CT is the confusion of a phlebolith with a ureteral stone, especially in the pelvis. Two prior studies have addressed this issue [27, 28], but the pitfall remains. However, with experience and by using the secondary signs of obstruction, this difficulty can usually be overcome.
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