LETTERS TO THE EDITOR 620 radiology.rsna.org n Radiology: Volume 269: Number 2—November 2013 Gadolinium-based versus US micro- bubble contrast media have very differ- ent mechanisms of action, thus reflect- ing different tumor properties. The former distributes in the extracellular space (intravascular and extracellular, extravascular spaces), whereas the lat- ter remains in the intravascular space and only reflects the vascular properties of HCC (1). The discordant appearance of HCC, especially at later phases of contrast-enhanced US compared to MR imaging, has been described (1,2). The importance of properly timed delayed phase imaging in MR imaging avoids misdiagnosis of intrahepatic cholangio- carcinoma as HCC (3), as it can have a very similar appearance to HCC at de- layed contrast-enhanced US (4). For these reasons, technical recom- mendations based on experience with contrast-enhanced US of HCC imaging cannot and should not be simply trans- ferred to MR imaging of HCC in our opinion. Disclosures of Conflicts of Interest: C.W. Fi- nancial activities related to the present article: none to disclose. Financial activities not related to the present article: receives payment from Philips Healthcare for board membership on the Radiology Medical Advice Network. Other relationships: none to disclose. M.W.R. No rel- evant conflicts of interest to disclose. J.K.H. No relevant conflicts of interest to disclose. H.K.H. Financial activities related to the present arti- cle: none to disclose. Financial activities not related to the present article: is a paid ad-hoc consultant for Bayer Healthcare. Other rela- tionships: none to disclose. E.A.P. No relevant conflicts of interest to disclose. J.B. Financial activities related to the present article: none to disclose. Financial activities not related to the present article: received a grant from Arqule and Bayer Shering Pharma; is a consultant on the advisory boards of Biocompatibles, Abbott, BMS, Chugai Pharma, Glaxo-Welcome, Im- clone, Daiichi Sankyo, Kowa, Lilly, Novartis, Roche, Sanofi, Terumo, and Wako. Other rela- tionships: none to disclose. References 1. Wilson SR, Kim TK, Jang HJ, Burns PN. Enhancement patterns of focal liver masses: discordance between contrast-enhanced so- nography and contrast-enhanced CT and MRI. AJR Am J Roentgenol 2007;189(1):W7– W12. 2. Burns PN, Wilson SR. Focal liver masses: en- hancement patterns on contrast-enhanced images—concordance of US scans with CT scans and MR images. Radiology 2007;242(1): 162–174. 3. Rimola J, Forner A, Reig M, et al. Cholangio- carcinoma in cirrhosis: absence of contrast washout in delayed phases by magnetic reso- nance imaging avoids misdiagnosis of hepato- cellular carcinoma. Hepatology 2009;50(3): 791–798. 4. Vilana R, Forner A, Bianchi L, et al. Intrahe- patic peripheral cholangiocarcinoma in cir- rhosis patients may display a vascular pattern similar to hepatocellular carcinoma on con- trast-enhanced ultrasound. Hepatology 2010; 51(6):2020–2029. Reproducibility of Dynamic Contrast- enhanced MR Imaging From Isabelle Thomassin-Naggara, MD, PhD,* Charles-André Cuenod, MD, PhD, † and Daniel Balvay, PhD † Department of Radiology, Hôpital Tenon, Assistance Publique–Hôpi- taux de Paris, Paris, France* e-mail: isabellethomassin@gmail.com Imaging Laboratory Research, Uni- versité Rene Descartes, UMR 970 INSERM, Paris, France † Editor: We read with much interest the arti- cle by Dr Heye and colleagues in the March 2013 issue of Radiology (1) re- garding the reproducibility of dynamic contrast material–enhanced magnetic resonance (MR) imaging quantifica- tion in the female pelvis. The authors show that although Kety-Tofts compart- mental models have been fairly stan- dardized, there remains considerable variability in the evaluation of pharma- cokinetic parameters depending on the software used. The principal cause for this variabil- ity appears to be the arbitrary selection of a theoretical arterial input function (AIF), well known to decrease repro- ducibility in extracellular extravascular volume fraction (v e ), volume transfer constant (K trans ) , and fractional plasma volume estimates and dependent in part on variations in body weights (allo- metric theory) (2–5). Indeed, the au- thors report on a population of 15 pa- tients with body weights ranging from 64.4 to 136.5 kg. One option to com- pensate for this limitation would be to correct the mean AIF for variations in body weight. The only software avail- able that provides weight correction (CADvue; iCAD, Nashua, NH) allows for decreased variability in v e estima- tion but is still associated with a large variability in K trans . This article shows another limitation of using a theoretical AIF: decreased re- producibility between software pack- ages. The similarities of the distribution between v e and the initial area under the gadolinium curve (iAUGC), both in a “U” pattern, suggest that the variability of v e is mainly due to the difficulties in trans- lating contrast agent concentration into MR contrast enhancement. Indeed, iAUGC is not affected by technical vari- ability in pharmacokinetic modelling so much as by signal calibration. Mean v e values follow directly the differences of signal calibration in mean iAUGC values. In our opinion, the study suffers from two main methodologic limitations. First, the relevance of the scaling factor (ie, signal calibration) should have been tested in a subpopulation for which large vessels are visible and where the v e should be equal to 100%. Bias and vari- ability of v e in this region would allow comparison of the software. Second, as mean parameter values are reported dif- ferently between software packages, the resulting concordance (Bland-Altman and intraclass correlation coefficient), by definition, is expected to be low. Per- haps if only correlation had been tested, the authors would have noted consis- tency between software beyond a cali- bration factor. This analysis of correla- tion is very important because if no correlation exists, the significance of any software analysis in routine clinical use can be questioned. Disclosures of Conflicts of Interest: I.T. Finan- cial activities related to the present article: none to disclose. Financial activities not related to the present article: received payment for develop- ment of educational presentations from ESRMB; institution received travel/accommodations/ meeting expenses unrelated to activities listed from General Electric. Other relationships: none to disclose. C.A.C. Financial activities related to the present article: none to disclose. Financial activities not related to the present article: re- Note: This copy is for your personal non-commercial use only. To order presentation-ready copies for distribution to your colleagues or clients, contact us at www.rsna.org/rsnarights.