Quantification of liver fat content in liver and primary liver lesions using triple-echo-gradient-echo MRI
Nougaret S, Monsonis B, Molinari N, Riviere B, Piron L, Kassam Z, Cassinotto C, Guiu B
Eur Radiol. 2020 Sep;30(9):4752-4761. doi: 10.1007/s00330-020-06757-1. Epub 2020 Apr 22. PMID: 32318848
Incidence of focal liver lesions (FLL) detection parallels growth in imaging utilization. Hemangiomas, focal nodular hyperplasias (FNH), and adenomas (HCA) are the most commonly solid benign lesions that arise in noncirrhotic liver. Metastases are the most common malignant lesions in noncirrhotic livers. Hepatocellular carcinoma (HCC) and intrahepatic cholangiocarcinoma (ICC) occur in the setting of chronic liver disease. Despite FLL usually having typical imaging characteristics, differential diagnosis by visual assessment only can be difficult sometimes or it requires contrast media administration mandatorily.
However, quantitative MRI imaging is an emerging research field that is increasingly assuming an important role in the characterization of FLL with unenhanced MRI protocol.
Authors, with their study, aim to quantify and compare whole liver parenchymal and intralesional fat fraction in primary liver lesions, such as HCC, HCA and FNH, using a triple-echo-gradient-echo-sequence.
Chemical shift MRI techniques are a non-invasive method for fat quantification within a specified volume [1, 2]. Routinely dual-echo in-phase (IP)/out of phase (OP) imaging is used for subjective evaluation of intralesional fat in liver lesions. However, the dual-echo technique is hampered by T1 and T2* relaxation times which may underestimate fat fraction when T2* is short (for example in the setting of iron-containing lesions) [3]. A triple-echo technique has been developed, which consists of a breath-hold, triple-echo spoiled gradient-echo sequence with consecutive first IP, OP, and second IP echo times. This method enables the quantification of fat fraction, corrected for T2* decay and minimizing T1-related effects (by decreasing the flip angle at 20°) [4-6].
It is a retrospective study that included 128 consecutive patients that underwent a liver MRI for lesion characterization. For each patient, a triple-echo sequence was performed before contrast medium injection on 1.5T scanner for fat evaluation. Then, the obtained images were post-processed and analyzed quantitatively and qualitatively by a radiologist. Quantitative analysis was performed by a Radiologist, with 3 years of experience in gastrointestinal imaging, by drawing regions of interest (ROIs) around the liver lesion on each slice of the fat fraction map, referring to the other sequences to delineate the whole tumor volume. Care was taken to avoid contamination of these ROIs by the normal liver parenchyma. Further, a large ROI representing the “normal” liver parenchyma was drawn for each patient. The same radiologist performed a qualitative analysis of fat quantification by visual evaluating the decrease in signal intensity on OP/IP images.
Among the 128 included patients, there were 47 HCCs, 25 HCA, and 56 cases of FNH suspected on MRI. Forty-nine patients had histologic confirmation of the diagnosis following ultrasound-guided biopsy. Where biopsy was not performed (27 patients in the HCC group (57%), 47 patients in the FNH group (84%), and 5 in the HCA group (20%)), lesion characterization was made on MRI in conjunction with clinical history, follow-up MRI, and laboratory results.
Results confirmed that this technique is accurate to quantify fat fraction in a tumor volume. According with pathologic findings, results demonstrated the relatively higher intralesional fat content of HCC and HCA compared to FNH. Furthermore, the previously published 5.56% threshold to assess liver steatosis might not be applicable in analysis of a solitary liver lesion when used to distinguish between fat-containing and non-fat-containing lesions because was solely used to determine liver steatosis. Authors, in their cohort, found that a lower cutoff of 2.7% was able to accurately diagnose FNH over HCA.
This study had some limitations: due to the retrospective nature not all data (e.g. BMI) were available. Pathology correlation could not be obtained for all patients. The heterogeneity of fat distribution within the lesion was not evaluated. Inter/intraobserver reproducibility was not evaluated.
In conclusion this study quantified for the first-time intralesional fat fraction using triple-echo chemical shift gradient-echo approach. An intralesional fat fraction of less than 2.7% may indicate high likelihood of FNH over HCA.
This study has important implication in abdominal radiology as an aid in the study and characterization of liver lesions also because triple-echo-gradient-echo-sequence sequences have short acquisition time so could be routinely inserted and performed in clinical liver study protocols.
.
References:
- Reeder, S.B., et al., Quantitative Assessment of Liver Fat with Magnetic Resonance Imaging and Spectroscopy. J Magn Reson Imaging, 2011. 34(4): p. 729-749.
- Dixon, W.T., Simple proton spectroscopic imaging. Radiology, 1984. 153(1): p. 189-94.
- Westphalen, A.C., et al., Liver fat: effect of hepatic iron deposition on evaluation with opposed-phase MR imaging. Radiology, 2007. 242(2): p. 450-5.
- Guiu, B., et al., Multiecho MR imaging and proton MR spectroscopy for liver fat quantification. Radiology, 2008. 249(3): p. 1081.
- Guiu, B., et al., Mapping of liver fat with triple-echo gradient echo imaging: validation against 3.0-T proton MR spectroscopy. Eur Radiol, 2009. 19(7): p. 1786-93.
- Guiu, B., et al., Liver methylene fraction by dual- and triple-echo gradient-echo imaging at 3.0T: Correlation with proton MR spectroscopy and estimation of robustness after SPIO administration. J Magn Reson Imaging, 2011. 33(1): p. 119-27.
Francesco Pucciarelli is a third-year radiology resident at Sant’Andrea Hospital, Sapienza University of Rome and graduated at Università Cattolica del Sacro Cuore, Policlinico Gemelli of Rome with merit in 2017. Dr. Pucciarelli is an ESGAR member and he was awarded as “ESGAR Top 20 presenter” on the occasion of the ESGAR 2020 virtual event. He has a wide range of interests in abdominal diagnostic imaging and is currently involved in scientific research centered on quantitative MRI of focal and diffuse liver diseases.
Comments may be sent to