Hepatic fat and iron deposition

Chapter 12
Hepatic fat and iron deposition

Ersan Altun¹, Mohamed El-Azzazi^1,2,3,4, and Richard C. Semelka¹

¹The University of North Carolina at Chapel Hill, Department of Radiology, Chapel Hill, NC, USA

²University of Dammam, Department of Radiology, Dammam, Saudi Arabia

³King Fahd Hospital of the University, Department of Radiology, Khobar, Saudi Arabia

⁴University of Al Azhar, Department of Radiology, Cairo, Egypt

Fatty liver disease

Fatty liver disease is the most common chronic liver disease in North America and its prevalence is about 15% in the general population.
It results from the accumulation of triglycerides within the cytoplasm of hepatocytes.
The two most common conditions associated with fatty liver are alcoholic liver disease and nonalcoholic fatty liver disease.
Alcoholic liver disease is caused by excess alcohol consumption.
Nonalcoholic fatty liver disease is associated with insulin resistance and metabolic syndrome. Therefore, diabetes mellitus and obesity are risk factors for nonalcoholic fatty liver disease. Additionally, viral hepatitis, some toxins, drugs including steroids, rapid weight loss, malnutrition, and some congenital disorders including glycogen storage disorders also cause fatty liver.
Clinical presentation:
- Usually asymptomatic
- May be associated with hepatomegaly, right upper quadrant abdominal pain, or elevated liver function tests
Forms:
1. Diffuse fatty deposition
2. Focal deposition with focal sparing
3. Multifocal deposition
4. Subcapsular deposition
On CT:
- Fatty deposition in the liver causes lower attenuation in the liver, which is lower than that of the spleen. Quantitatively, absolute liver density measuring less than 40 HU or more than 10 HU relative density decrease of the liver compared to the spleen suggests fat deposition in the liver on precontrast images. Additionally, more than 25 HU density decrease of the liver compared to the spleen suggests fat deposition in the liver on postcontrast images on the hepatic venous phase.

On MRI:

Three different techniques are used for fat detection and quantification.

Chemical shift imaging
Fat suppression
MR spectroscopy

Chemical shift imaging including in-phase and out-of-phase gradient echo imaging is the most commonly used fat detection method in the liver. Chemical shift imaging is based on the detection of signal loss on out-of-phase T1-gradient echo sequence compared to in-phase T1-gradient echo sequence due to the presence of microscopic fat. This arises from the different precessional frequencies of fat and water protons; the opposite phase of fat and water protons located in the same voxel leads to signal loss of fat-and-water containing structures, compared to in-phase T1-gradient echo sequence, in which fat and water protons are in-phase with one another. The relative signal loss on out-of-phase image, which is approximately 16%, suggests the presence of fat with high accuracy. The presence of macroscopic/abundant fat does not cause signal loss on out-of-phase images, but creates a signal void (also termed as India ink) artifact at water-fat interfaces. To minimize T2* signal decay effects, chemical shift imaging should be performed with the shortest possible TE times and at 1.5 T. The out-of-phase gradient echo sequence should be acquired with a shorter TE than the in-phase sequence.

Fat suppression techniques include either fat saturation or water excitation. With the use of these techniques, tissues containing macroscopic fat show maximal fat signal loss when fat represents near 100% of the tissue on T1- and T2-weighted sequences. This is in contrast to chemical shift imaging where signal loss is maximal when fat and water are approximately 50% each in the volume of tissue.

These three methods are also used for fat quantification. Chemical shift imaging is a successful method for fat quantification. The accuracy of chemical shift imaging for fat quantification is increased with the use of techniques minimizing T1-weighting and correcting T2* signal decay. MR spectroscopy is the most specific technique for fat quantification, although it is not practical due to its small volume sampling (single voxel technique) and long acquisition times (multivoxel technique). The accuracy of fat quantification with MR spectroscopy can be improved by obtaining spectra at multiple echo times and correcting for the T2* decay.

Fat quantification is performed with the calculation of fat signal fraction with the following formula: Fat Signal Fraction = Fat Signal/Water Signal + Fat Signal.

Chemical shift imaging may quantify fat content in the liver with the calculation of fat signal fraction based on the following formulae:

Signal (In-phase) = Water signal + Fat signal
Signal (Out-of-phase) = Water signal − Fat signal
Fat Signal Fraction = Signal (In-phase) − Signal (Out-of-phase)/2 Signal (In-phase)

Diffuse Fat Deposition

Diffuse fat deposition may be uniform or heterogeneous.