Fatty liver disease affects a wide range of Americans in both of its forms. Alcoholic fatty liver affects 90-100% of heavy drinkers.1 For those affected by alcohol-related liver disease, the American Liver Foundation recommends quitting drinking. Alcohol is the fourth leading preventable cause of death in the United States. However, there are still liver diseases that affect those people who do not drink. Nonalcoholic fatty liver disease affects approximately 30% of Americans in all age ranges, including kids.
Nonalcoholic fatty liver disease (NAFLD) occurs when more than 5%-10% of the liver's weight is fat in patients who drink little to no alcohol.1,2 The more serious form of NAFLD is nonalcoholic steatohepatitis (NASH) and is characterized by swelling of the liver. Both NAFLD and NASH can lead to cirrhosis, or late stage scarring in the liver. NAFLD does not typically cause symptoms, so it is diagnosed if an abnormal tests, either blood work or ultrasound, are observed. In the case of blood tests, ultrasound is the most common confirmation method. However, magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) have emerged as excellent diagnostic tools for physicians.
Ultrasound provides a qualitative image that allows the radiologist to see whether there is a large amount of fat in the liver.3 Because the ultrasound is dependent on the machine and the radiologist, it is difficult to replicate and reproduce. Additionally, obese patients have the highest risk of developing NAFLD but produce issues with beam penetration and liver visualization.
Accuracy of MR
Water and fat resonate at different peak magnetic frequencies.3,4 Fat has a much slower peak frequency than water. This allows magnetic resonance spectroscopy (MRS) to measure fat proton signals and differentiate between water and fat composition. Other compounds can be measured using MRS, such as sodium and phosphorus, but proton spectroscopy is easiest and handiest. The signal-to-noise ratio is highest for protons as well. MRS helps to detect and characterize fat in the liver mostly using the same sequences as MRI. Instead of creating a spatial arrangement image of tissues, MRS identifies chemical compounds inside of the liver, because it doesn't use a gradient like an MRI.5 Intrahepatic lipid (lipids in the liver) accumulation can be detected even in small amounts using MRS, unlike ultrasound or CT imaging.
Like MRS, MRI measures the difference between water and fat resonance. MRI uses in-phase and out-of-phase (IOP) imaging to determine fat content.3,4,5,6 This is done by taking images in peak (in-phase) and out-of-peak (out-of-phase). Because two echo images are taken, this is sometimes referred to as dual echo imaging. The computer and radiologist then compare the two images to see where and how much fat there is. This is one of the most accurate non-invasive imaging methods for quantification of liver fat content.
Another MRI technique that can be used to measure the amount of fat in a liver is done with fat-suppressed imaging. Fat-suppressing pulses are emitted during these MRIs that allow the radiologists to see the difference in signal between when the fat signal is being suppressed and when it is not. If fatty liver is present, the image of the area with fat is much darker in a fat-suppression image, because the fat signal is being blocked by the pulse.
Finally, multi-parametric MRI has recently emerged as another technique to quantify liver fat content.7,8 This procedure takes multiple images using different techniques to combine into an overall image of the liver which could be used to determine whether liver biopsy is needed. Liver biopsy is currently the method to determine NAFLD versus NASH. Multi-parametric MRI is the only imaging option that provides a high level of accuracy for staging steatosis, thus preventing the need of biopsy in patients with NAFLD whose conditions weren't as severe as NASH.
Overall, MRS and MRI, including but not limited to IOP, fat-suppression, and multi-parametric, have a substantially higher accuracy rate than ultrasound in most patients and has the ability to quantify and qualify liver fat, leading to significantly less liver biopsies. MR is not reliant on a specific machine and technologist and, therefore, can be reproduced, unlike ultrasound. MR should be considered for use after abnormal tests to accurately diagnose NAFLD patients, especially as a third of the population has this disease.
1. "Liver Disease Statistics." American Liver Foundation. 26 October 2017. Web. 29 October 2018. <https://liverfoundation.org/liver-disease-statistics/#alcohol-related-liver-disease-and-cirrhosis>.
2. Mayo Clinic Staff. "Nonalcoholic fatty liver disease." Mayo Clinic. 7 March 2018. Web. 29 October 2018. <https://www.mayoclinic.org/diseases-conditions/nonalcoholic-fatty-liver-disease/symptoms-causes/syc-20354567>.
3. Scott B. Reeder & Claude Sirlin. "Quantification of Liver Fat with Magnetic Resonance Imaging." Magn Reson Imaging Clin N Am. 1 August 2010; 18(3): 337-357. Web. 29 October 2018. <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3002753/>.
4. Kiran Gangadhar, et al. "MRI evaluation of fatty liver in day to day practice: Quantitative and qualitative methods." EJRNM. September 2014; 45(3): 619-626. Web. 29 October 2018. <https://www.sciencedirect.com/science/article/pii/S0378603X14001090>.
5. Michael Dieckmeyer, et al. "MR spectroscopy." Radiopaedia.org. Web. 30 October 2018. <https://radiopaedia.org/articles/mr-spectroscopy-1>.
6. Okka W. Hamer, et al. "Fatty Liver: Imaging Patterns and Pitfalls." RadioGraphics. 1 November 2006; 26(6): 1637-1653. Web. 29 October 2018. <https://pubs.rsna.org/doi/10.1148/rg.266065004?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub%3dpubmed>.
7. Staff News Brief. "Assessing non-alcoholic fatty liver disease with multiparametric MRI." Appl Radiol. 5 March 2018. Web. 29 October 2018. <https://appliedradiology.com/articles/assessing-non-alcoholic-fatty-liver-disease-with-multiparametric-mri>.
8. Stefan Neubauer. "Multiparametric Magnetic Resonance Imaging (MRI as a Safe, Non-invasive Method for Assessing Liver Health." LiverLine.com. 22 June 2017. Web. 30 October 2018. <https://liverline.com/multiparametric-magnetic-resonance-imaging-mri-as-a-safe-non-invasive-method-for-assessing-liver-44b834cb964d>.