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Viewpoint |
1 From the Department of Radiology, University of Virginia Health System, 1215 Lee St, Charlottesville, VA 22908 (D.J.S., G.D.H.); and the Department of Radiology, Dotter Interventional Institute, Oregon Health Sciences University, Portland. Received and accepted September 5, 2001. Address correspondence to D.J.S. (e-mail: djs4m@virginia.edu). 2 9*: Vascular system, location unspecified.
Index terms: Angiography, contrast media, 9*.1211 • Contrast media, complications • Contrast media, effects • Contrast media, toxicity • Gadolinium • Iodine and iodine compounds • Uremia
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INTRODUCTION |
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 TOP INTRODUCTION REFERENCES |
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The assumption that contrast media must be "compared at equal-attenuating concentrations or doses" (1) is reasonable from a stoichiometric point of view, but that is not the way contrast media are used in vivo. Rather, contrast media are used the way they are supplied. The concept of diluting iodinated contrast medium is not new to the interventionalist. However, once diluted much beyond one-half strength, image contrast is typically unsatisfactory, particularly when penetration of considerable soft tissue is required or when attempting to image medium- or large-sized high-flow vessels.
Nyman et al state that 7–15 mL of a commonly used solution with 300 mg of iodine per milliliter should not result in any decline in renal function (1). Anecdotal reports of a decrease in renal function after performing studies with low volumes (20–30 mL) of iodinated contrast medium are not uncommon (2). Indeed, we have witnessed the occurrence of worsening renal function with the use of as little as 10 mL of iodinated contrast medium. It is uncertain whether higher doses are safe in patients with renal insufficiency, and it is difficult to determine the exact volume of contrast medium that will result in nephrotoxicity, owing to the multifactorial nature of this complication. No one really knows how much contrast medium one can safely administer to a specific patient with renal insufficiency.
Not many interventional studies can be performed with 7–15 mL of full-strength contrast medium. Dilution of this volume to the typical 40–50 mL required for most low-volume contrast-enhanced studies would result in iodinated contrast medium containing 75–100 mg of iodine per milliliter (one-fourth to one-third strength). The angiographic image quality achieved with iodinated contrast medium at this strength in a standard patient is typically worse than that achieved with full-strength gadolinium-based contrast media, which provides images more like those obtained with 100–150 mg of iodine per milliliter (3).
An important consideration is the concern among clinicians that iodinated contrast medium may cause a worsening of renal failure. Even if we assume that the rate of worsening renal function after iodine-enhanced studies is in the 10%–20% range (4,5), contrast medium–induced nephrotoxicity in some of these patients may lead to delays in surgery or other hospital procedures, occasional dialysis, a failure to return to baseline renal function, and the potential for increased in-hospital mortality (5,6). Many clinicians and patients are not reassured by statistics when an individual’s well-being is at stake. These concerns cause many clinicians to avoid any study that involves iodinated contrast media in patients at risk for contrast medium–induced nephrotoxicity, even if most patients who experience worsening renal function eventually achieve a return to their baseline function. The use of gadolinium chelates or other alternative contrast media in these patients prevents the withholding of needed diagnostic studies or interventions.
Nyman et al state that a 0.5 mol/L solution of gadolinium-based contrast medium will attenuate x rays to the same extent as a solution with 63 mg of iodine per milliliter, if it is assumed "that iodine and gadolinium atoms attenuate x rays to the same extent" (1). However, iodine and gadolinium do not attenuate x rays to the same extent. Many variables affect attenuation. These variables include beam energy, k edge of the attenuating atom, and tissue thickness that the x-ray beam must penetrate.
For an 80-kVp digital subtraction angiography spectrum without attenuating soft tissue, the mean energy is 47 keV and the effective energy is 34 keV. This finding is consistent with the authors’ "rule of thumb" that the majority of the x-ray spectrum is in a range that iodine attenuates more than gadolinium does. However, when one calculates the x-ray spectrum of a typical angiography x-ray tube filtered with 3.5 mm aluminum and 0.2 mm copper, with 20 cm of water to simulate soft-tissue attenuation, the mean energy is 66 keV and the effective energy is 62 keV. The majority of the x-ray spectrum under these circumstances is in the range that gadolinium attenuates more than iodine does. The mean and effective energies are further increased when higher kilovolt peak energies are used. Typically, we use 96 kVp for gadolinium-enhanced imaging. Indeed, not only is this the theoretical consideration demonstrated by the phantom model in figure 1b in our Viewpoint article (7) but it is also the case in clinical practice with full-strength gadolinium-based contrast medium, as shown in figure 2b and 2c in our article (7). We have found that a 0.5 mol/L solution of gadolinium chelate is equivalent to approximately 75 mg of iodine per milliliter with 0 cm of water as an attenuator and to 100–150 mg of iodine per milliliter with 20 cm of water as an attenuator. Therefore, in vivo, a 0.5 mol/L gadolinium chelate provides image contrast that more closely approximates the contrast on images obtained with half-strength iodinated contrast medium (150 mg of iodine per milliliter) in a typical abdomen or pelvis.
Finally, in the study by Spinosa et al (8) referred to by Nyman et al (1), true randomization was not used because the investigators wanted the option to use gadolinium-based contrast media instead of iodinated contrast media in those patients thought to be at the highest risk for contrast medium–induced nephrotoxicity. This introduced a bias in favor of iodinated contrast media rather than gadolinium-based contrast media.
In summary, we believe that Dr Nyman and colleagues have made an important contribution to the study of alternative radiographic contrast media. As clinicians faced daily with patients at risk for contrast medium–induced renal failure, we differ in our final conclusions. We believe that gadolinium-based media are a useful tool for small-volume contrast-enhanced angiographic studies in patients with renal insufficiency. Their safety profile as a contrast medium when used at these volumes in patients with renal insufficiency has been shown extensively in magnetic resonance imaging reports and in several angiographic reports. Clinical studies evaluating the safety of small volumes of iodinated contrast medium are needed to determine the incidence of contrast medium–induced nephrotoxicity in patients with renal insufficiency. We have undertaken an institutional review board–approved randomized study to determine if small volumes (20–30 mL) of iodinated contrast medium diluted to half strength are as safe as gadolinium chelates for diagnostic and interventional renal angiographic studies in patients with renal insufficiency. Unfortunately, recruitment is slow because of the fear by both patients and referring physicians regarding contrast medium–related nephrotoxicity. The results of these studies will provide to patients and their clinicians a clearer picture of the true risk of contrast medium–induced nephrotoxicity in angiography with iodinated contrast media.
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FOOTNOTES |
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