Introduction to the Liver
The liver is an amazing structure in the abdominal cavity. It comprises just 2-5% of body weight yet receives ~25% of cardiac output via the hepatic artery with an additional ~60% from the portal vein. The right and left hepatic arteries deliver oxygenated blood from the aorta while the portal vein returns from the small intestine delivering deoxygenated blood but also nutrients (and possible toxic substances). It is a metabolic powerhouse: transforming, storing, detoxifying, and making to new substances needed by the body (e.g., clotting factors, albumin etc.). The liver is composed of hepatocytes (~60% of total cell number or 80% of liver volume) with the internal vascular and bile duct system comprising the rest. These hepatocytes and bile ducts are arranged into a functional unit called the portal triad. Kupffer cells are the liver resident macrophages which are present in the liver sinusoids where they can act on bacteria or their endotoxins arriving from the gut, release cytokines, proteinoids, nitric oxide and reactive oxygen species, and finally, remove old or senescent cells In the mouse, the arrangement of the portal tract is poorly defined histologically, comprising the hepatic artery, portal vein, and bile duct, extending into the hepatic parenchyma with flow toward the terminal hepatic vein (which returns to the heart via the vena cava). This zonal arrangement of the portal lobule, acinus, and hepatic lobule is discernable in human and non-human primates, and well-defined in the pig, which has lots of portal fibrous connective tissues. This zonation with its arterial and portal inputs establishes 3 functional zones: periportal (1), mid-zonal (2) and centrilobular (3), where the functional aspects are segmented. Activities for tasks such as urea formation, glutamine synthesis, glucose metabolism, and (drug) detoxification are linked with enzyme expression. OK, so why does this matter?
To understand non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH), you need to know what makes up the diagnostic criteria, and that means liver hematoxylin and eosin (H&E) histology from some sort of liver tissue biopsy (e.g., percutaneous or wedge resection) processed, stained, and scored by a trained (hepato)pathologist. Histology is broken into 3 separate sections, and the Likert-style scoring based on ranges is summed, with a total score of 4 or more indicating a diagnosis of NASH. The first score is related to steatosis (range 0-3), and the scoring is graded on the extent of the parenchymal involvement (the hepatocytes themselves), the fat distribution by zones, and the presence/absence of microvesicular patches. Inflammation is scored (range 0-3) above background levels of inflammatory cells, as assessed as lobular foci, microgranulomas, lipogranulomas in the portal areas, and outright portal inflammation. Lastly, we score for the presence of ballooning cells (range 0-2). Hepatocytes are predominantly bags of smooth endoplasmic reticulum, and all that surface area is critical to its metabolic function; but when there is cellular stress, degeneration, and death, these cells balloon and increase in size (not to mention leak enzymes into the blood stream which we can measure as increases in ALT/AST/GGT). Ballooning is scored relative to their size (often 1.5-2 times the normal diameter), frequency of few to many cells, prominent ballooning, acidophil bodies, pigmented macrophages, and megamitochondria. Ballooning can be thought of an important transition observation from NAFLD to NASH and often correlated with the increase in fibrosis. Special stains are often used to assess fibrosis, most often Masson’s trichrome and Sirius Red. Early NASH fibrosis starts with a centrilobular perivenular distribution and sinusoidal fibrosis in the perisinusoidal or periportal regions, often mild-moderate in Zone 3 and becoming portal/periportal. From here, the fibrosis worsens to be observed in the perisinusoidal and portal/periportal regions, with bridging fibrosis and eventually cirrhosis, representing diffuse fibrosis with hepatocytes confined to poorly functioning regenerative nodules. Advanced fibrosis is often considered irreversible and is often seen as a risk factor for progressing the hepatic cancer.
It is the current thinking of the FDA that the diagnosis and staging of NAFLD/NASH can only be accomplished by liver biopsy and histologic interpretation (Draft guidance Dec 2018 and June 2019). The FDA supports the use of noninvasive imaging biomarkers that can replace liver biopsies in liver studies, provided the sponsor proposes diagnostic criteria which have not been established for patient selection and efficacy assessment. For non-cirrhotic and non-alcoholic fatty liver and fibrosis, even when the invasiveness and logistics of using liver biopsies in a clinical trial are discussed, the FDA recommends that only histological improvements should be used as endpoints for their likelihood to predict clinical benefit. What can non-invasive imaging tell us about these cellular events?
MRI-PDFF as a fat fraction
Intracellular triglyceride can be stained in liver frozen sections using Oil red O, but is extracted during processing to paraffin block when stained with H&E. The hematoxylin stains cell nuclei blue, and eosin (which may contain some orange G and/or phloxine B) counterstains the extracellular matrix and cytoplasm pink, with other structures taking on different shades, hues, and combinations of these colors. Adipose scoring is then based on the absence of stain, and the extent of these adipose vacuoles is judged as a function of intracellular presence and distribution over the tissue slice. The common thickness of an H&E section is 6-7 microns, MRI slice thickness is in the range of 5-8 mm. When you consider that a routine liver MRI has a 38x38 cm FOV, acquired with a matrix of 192x192, if the slice thickness is 6 mm, this creates a voxel ~2x2x6 mm, or ~23.5 mm3. A hepatocyte is basically a cuboid cell, 25 microns on a side. If 80% of a voxel is hepatocytes, this would represent ~1.2 million hepatocytes. Validation of MRI-PDFF across platforms and magnet manufacturers is available in the literature, and confidence is high that hepatic fat fraction can be quantified with MRI technique. When we report the results of an MRI-PDFF analysis, our fat fraction is not accounting on the cellular level like a biopsy, but rather, on the total level of how much triglyceride is contributing to the fat signal from all cells. Using a multi-Couinaud region of interest analysis, we get a sampling/distribution of fat fraction across the entire liver, something we cannot do with biopsies for practical reasons. By using a 6 mm slice, it is possible that MRI provides a more general interpretation of the density and distribution fat present, not only in one hepatocyte but in a significant number of hepatocytes.
New 3D MRE Techniques to Assess Inflammation
MRI-PDFF methods capture the adipose domain of the NAS-CRN score. We currently use a 2D MR elastography method to estimate liver stiffness as a surrogate for liver fibrosis. Improvements in MRE techniques have identified a new multifrequency 3D elastography analysis method as a way to derive what to this point has been missing correlative data to biopsy histology. Current MRE methods utilize a single mechanical wave while this new technique uses 3, and it is the differences of the viscoelastic properties associated with edema, interstitial fluid pressures, and viscosity that can be correlated to states of inflammation and ballooning. Research into 3D MRE has identified the damping ratio as a possible biomarker, sensitive to inflammation and possibly ballooning. The damping ratio (loss modulus/storage modulus) correlated with lobular inflammation while the complex shear modulus, as a measure of shear stiffness, approximated hepatocellular ballooning. The use of 3D MRE requires further validation, but the prospect of developing a scanning protocol that derives the fat fraction from MRI-PDFF methods, the inflammation score from the 3D MRE damping ratio and the ballooning score from the shear stiffness, and traditional MRE for hepatic stiffness is quite appealing as an alternative to liver biopsies.
Liver structure and function - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4320379/
Liver blood supply - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5396263/
Liver histology - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5760001/
3D MRE - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5529282/