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DOI: 10.1148/radiol.2242011819
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(Radiology 2002;224:613-614.)
© RSNA, 2002

Letters to the Editor

Underestimation of Renal Cortical Perfusion Calculated from Dynamic CT Data

Yoshito Tsushima, MD,*, Jun Aoki, MD{dagger} and Keigo Endo, MD{dagger}

Department of Radiology, Motojima General Hospital, 3-8 Nishi-Honcho, Ohta, Gunma 373-0033, Japan* e-mail: yoshito@xa2.so-net.ne.jp
Department of Diagnostic Radiology and Nuclear Medicine, Gunma University Hospital, Gunma, Japan{dagger}

Editor:

We commend Dr Paul and colleagues for their article in the October 2001 issue of Radiology (1), in which they illustrated their innovative application of contrast material–enhanced functional computed tomography (CT) in the diagnosis of renovascular hypertension. Their data indicated that analysis of time-attenuation curves may help distinguish between renal artery stenosis with and that without preserved perfusion. Although we agree with their result as a new diagnostic tool for renovascular hypertension, we believe that renal cortical perfusion in their study may be underestimated.

In their study, cortical perfusion was calculated by using the maximum-slope method proposed by Miles (2). Tissue perfusion can be calculated by dividing the peak upward slope of the tissue time-attenuation curve by the peak aortic CT number increase. This technique is attractive, since only early arterial-phase data from dynamic CT are necessary. However, this algorithm depends on several assumptions, one of which is that none of the bolus of contrast medium has left the region of interest (renal cortex) at the time of the calculation of data points. In other words, there should be no venous washout at the time of the peak upward slope. If contrast medium has started to leave the region of interest, use of the maximum-slope method will result in underestimation of perfusion. We suppose that this assumption does not hold when renal cortical perfusion is measured (3,4).

We present typical dynamic CT data of normal kidneys (Figs 1, 2). A bolus of 40 mL of contrast material (ioversol 320 mg/mL, Optiray; Yamanouchi, Tokyo, Japan) was given at 8 mL/sec (faster than the bolus injection used in their study) via an 18-gauge intravenous catheter in the antecubital fossa. Images were obtained every 2 seconds. In this case, renal venous washout started before the time of the peak upward slope, which suggests that, at least in some patients, renal cortical perfusion may be underestimated. However, the mean transit time in the renal cortex may depend on the patient’s age and cardiac output.



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  		Figure 1. Dynamic CT image obtained at 8 seconds after initiation of image acquisition in a 23-year-old woman with a pancreatic head tumor shows left renal vein enhancement (long arrow); however, no enhancement of the portal vein (short arrow) is shown.

 


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  		Figure 2. Time-attenuation curves demonstrate that renal vein enhancement was observed at the time of the peak upward slope (arrow) of the left renal cortex. Note that portal venous enhancement started later than the enhancement in the renal vein.

 
Mean transit time through the liver parenchymal bed was estimated to be approximately four times as long as that in the renal cortex (5). The other abdominal solid organs are also suspected to have longer mean transit times than do the kidneys. In fact, no venous washout was observed in portal (Figs 1, 2), hepatic (not shown), or splenic (not shown) veins. Therefore, it is not a problem when perfusion of the liver, spleen, and pancreas is calculated with this technique in a majority of cases.

Perfusion calculation may not have been the authors’ primary intention in this study, but our comments are a suggestion for any further studies of perfusion CT.

REFERENCES

  1. Paul JF, Ugolini P, Sapoval M, Mousseaux E, Gaux JC. Unilateral renal artery stenosis: perfusion patterns with electron-beam dynamic CT—preliminary experience. Radiology 2001; 221:261-265.[Abstract/Free Full Text]
  2. Miles KA. Measurement of tissue perfusion by dynamic computed tomography. Br J Radiol 1991; 64:409-412.[Abstract]
  3. Miles KA. Ideal contrast medium bolus for perfusion measurement in dynamic lung CT (letter). Radiology 1998; 209(P):583.[Free Full Text]
  4. Blomley M, Tsushima Y, Dawson P, Peters M. Ideal contrast medium bolus for perfusion measurement in dynamic lung CT. Radiology 1998; 209:583-584.
  5. Blomley MJK, Coulden R, Dawson P, et al. Liver perfusion studied with ultrafast CT. J Comput Assist Tomogr 1995; 19:424-433.[Medline]

Dr Paul responds:

Jean-Franois Paul, MD

Department of Radiology, Hôpital Marie Lannelongue , 133 avenue de la Résistance, Le Plessis-Robinson 92350, France e-mail: pauljf@ccml.com

We appreciate the interest of Dr Tsushima and colleagues in our study (1). We agree with their comment about a possible underestimation of cortical renal perfusion with use of the gradient technique with dynamic CT data. Although, in our experience, venous washout seems negligible at the time of maximum upslope.

The observation that substantial washout at the time of maximum upslope may be due to differences between individuals, as well as to technical factors, such as interscan delay, curve fitting, and the rate of injection. All of these factors are different in this well-illustrated case. In our study, values of cortical renal perfusion in controls were in the normal range (mean, 4.1 mL/min/mm3); therefore, we believe that venous washout was not a cause of substantial underestimation.

Interestingly, if there was any systematic underestimation of the estimates of cortical perfusion values calculated from dynamic CT data, then it would not affect the differences of cortical perfusion between nonstenotic and stenotic kidneys, as reported in table 1 (1). Thus, this value would be credible in the assessment of hypoperfusion associated with renal artery stenosis.

Finally, as noted by Dr Tsushima and colleagues, even if determination of the values of cortical perfusion in stenotic kidneys was not our primary intention, we found their comments useful for further evaluation of separate values of cortical perfusion with use of dynamic CT data.

REFERENCES

  1. Paul JF, Ugolini P, Sapoval M, Mousseaux E, Gaux JC. Unilateral renal artery stenosis: perfusion patterns with electron-beam dynamic CT—preliminary experience. Radiology 2001; 221:261-265.



This article has been cited by other articles:


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Y. Tsushima, K. Endo, C. Weidekamm, and T. Bader
Portal Perfusion Measurement on Dynamic CT in Patients with Liver Cirrhosis
Am. J. Roentgenol., September 1, 2005; 185(3): 813 - 813.
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