Fat is graded qualitatively on ultrasound, estimated from unenhanced CT attenuation, and measured on MRI proton-density fat fraction; the report names the method and the number, and looks for focal fat and sparing that mimic lesions.
Orient first
- MRI-PDFF is the quantitative reference (fat fraction in %); ultrasound attenuation parameters (CAP, dB/m or dB/cm/MHz) are vendor-dependent.
- Each method has its own threshold — MRI fat fraction, CT attenuation against the spleen, ultrasound attenuation — so the report names the method with the value.
- Focal fat and focal sparing occur at typical sites: around the gallbladder fossa, the falciform ligament, segment IV anterior to the porta.
Acquire the study
- Curvilinear probe; compare liver and right kidney at the same depth; attenuation measurement per vendor protocol.
The manoeuvre
- Liver echogenicity compared with the renal cortex at the same depth.
- Beam attenuation: poor visualisation of the diaphragm and deep portal vein walls = higher grade.
- Report the attenuation value with its unit and vendor.
- Focal sparing: geographic hypoechoic areas at typical sites, no mass effect.
What confirms it
- A quantitative value (PDFF, attenuation or HU) above the method-specific threshold.
What licenses you to exclude it
- A normal PDFF excludes steatosis; a qualitative normal ultrasound does not exclude mild steatosis.
The classic misread
- Grading on a machine with gain set too high.