Letter to the Editor W14 | www.ajronline.org AJR:216, April 2021 Quantifcation of Liver Steatosis: Is CT Equivalent to PDFF? We read with great interest the article by Pickhardt et al. [1] and are concerned about their use of the term “PDFF”—that is, proton density fat fraction—to indicate the results obtained with CT. It is important to note that MRI PDFF and CT measure diferent pa- rameters; therefore, measurements are not transferable between the two diferent techniques. It should be made clear that PDFF was used as the reference standard and that CT results should not be reported as PDFF. Ideally, CT results should be reported as percentage of fat with confdence levels. The authors made their assumption on the basis of a proof-of- concept study in which they found a linear correlation between unenhanced CT liver attenuation values and MRI PDFF values [2]. However, it should be kept in mind that a correlation, even when it is high, does not mean an equivalence, as stated in the article; therefore, the conversion of attenuation values measured in Hounsfeld units into PDFF values is conceptually wrong and unacceptable. In this regard, we believe that the message to the readers is misleading, because an interchangeability between diferent techniques is implied in the article [1]. MRI PDFF is the best reference standard for an accurate quantifcation of liver steatosis. It must be highlighted that the accuracy of CT is much less promising, as shown in a recent article [3]. In fact, when us- ing MRI PDFF as the reference standard, investigators found that the sensitivity of CT for the detection of steatosis (defned as a PDFF > 5%) was only 75.9% [3]; that is, the diagnosis of steatosis was missed in almost one-quarter of subjects. Besides these concerns, there is an ethical issue in proposing CT as a screening technique for the assessment of liver steato- sis: The exposure to ionizing radiation cannot be overlooked. We agree that the availability of MRI PDFF is limited; however, it should be acknowledged that B-mode ultrasound, which does not cause any harm to the patients, is the frst-line examination for the evaluation of liver steatosis, with reported sensitivity and specifcity of 84.8% and 93.6%, respectively, for the detection of moderate to severe fatty liver [4]. Several new ultrasound tech- niques are available to measure liver fat with high accuracy [4]. We completely agree that if CT without or with contrast mate- rial is ordered for an appropriate indication, then determining liver fat content is reasonable given the extent of nonalcoholic fatty liver disease in the population; however, screening with CT should be avoided. Giovanna Ferraioli, MD Medical School University of Pavia Pavia, Italy giovanna.ferraioli@unipv.it Richard G. Barr, MD, PhD Northeastern Ohio Medical University Youngstown, OH The authors declare that they have no disclosures relevant to the subject matter of this letter. References 1. Pickhardt PJ, Blake G, Grafy PM, et al. Liver steatosis categorization on con- trast-enhanced CT using a fully-automated deep learning volumetric seg- mentation tool: evaluation in 1,204 heathy adults using unenhanced CT as reference standard. AJR 2020 Sep 16 [published online] 2. Pickhardt PJ, Grafy PM, Reeder SB, Hernando D, Li K. Quantifcation of liver fat content with unenhanced MDCT: phantom and clinical correlation with MRI proton density fat fraction. AJR 2018; 211:[web]W151–W157 3. Guo Z, Blake GM, Li K, et al. Liver fat content measurement with quantita- tive CT validated against MRI proton density fat fraction: a prospective study of 400 healthy volunteers. Radiology 2020; 294:89–97 4. Ferraioli G, Soares Monteiro LB. Ultrasound-based techniques for the diag- nosis of liver steatosis. World J Gastroenterol 2019; 25:6053–6062 doi.org/10.2214/AJR.20.25069 AJR 2021; 216:W14  ISSN-L 0361–803X/21/2164–W14  © American Roentgen Ray Society WEB—This is a web exclusive article. Letter to the Editor Downloaded from www.ajronline.org by 52.73.204.196 on 05/16/22 from IP address 52.73.204.196. Copyright ARRS. For personal use only; all rights reserved