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Letters to the Editor |
Department of Abdominal Radiology and Intervention, Massachusetts General Hospital, White 270, 55 Fruit Street, Boston, MA 02114
e-mail: dsahani@partners.org
Many thanks to Dr Kalra and colleagues and Dr Kremser and colleagues for their interest in our article, "Assessing Tumor Perfusion and Treatment Response in Rectal Cancer with Multisection CT: Initial Observations" (1). The imaging of tumor vascularity at dynamic CT is still evolving. Neither the precise parameters for data collection nor the kinetic model for data analysis has been standardized. Some of the assumptions necessary for perfusion modeling in the abdomen remain largely speculative. The calculations of CT perfusion are dependent on the scanning protocols and on various methods of mathematical analysis, such as the slope method, deconvolution, and Patlak analysis. Furthermore, the tissue permeability calculation is dependent not only on the duration of dynamic scanning but also on the rate and volume of contrast material administered, molecular size of the contrast material, sensitivity of the test (MR imaging vs CT), and physiologic state (eg, blood volume, cardiac output). Published literature on this topic is limited and also not well validated; by using a certain contrast material injection protocol, the duration of dynamic scanning selected by various investigators has ranged from 65 seconds to 10 minutes (2,3). A longer duration of dynamic scanning, although desirable, is associated with higher radiation burden and may also introduce motion artifacts. Patient motion in turn causes errors in the perfusion values. For these reasons, we performed dynamic CT scanning for 45 seconds, after a delay of 10 seconds following administration of 125 mL of contrast material at a rate of 7 mL/sec (1). In comparison, in the study by Goh et al (2), 100 mL of iodinated contrast material was injected at a rate of 5 mL/sec and, after 5 seconds delay, dynamic scanning was performed for 65 seconds (2).
According to the vendor's specifications, our protocol is considered acceptable for calculating tissue permeability with their software (Perfusion 2; GE Medical Systems). We have also addressed this issue in the discussion of our article: "We found that the duration of data collection for the arterial input concentration curve Ca(t) and the tissue curve Q(t) affected the accuracy and precision with which the permeability–surface area product was determined."
Although we agree that permeability may be better assessed with a longer duration of data acquisition, the minimum dynamic scanning time required for calculating permeability has not been established.
We thank Dr Kalra and colleagues for their suggestions regarding decreasing radiation dose by decreasing tube potential and tube current. However, such changes require validation studies to prove that they do not affect the results of perfusion parameters. Higher image noise generated by lowering the tube potential can negatively influence the data analysis.
We also thank Dr Kremser and colleagues for sharing their results on evaluation of rectal cancer by using MR perfusion. Although their initial study with a small cohort was inconclusive to explain the high perfusion index (arteriovenous shunts vs increased permeability–surface area), their present work indicates increased tumor permeability as a possible explanation for different response to therapy (4). However, more work is needed in this field to validate these hypotheses. We would encourage Dr Kremser's group to continue their excellent work on this topic.
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