Steatosis, marked by excessive fat in liver tissue, is an increasingly common clinical concern due to its association with metabolic dysfunction and the growing burden of related diseases. Traditionally diagnosed via invasive liver biopsy, steatosis now benefits from noninvasive imaging modalities such as MRI proton density fat fraction (PDFF). However, MRI is not universally available or accessible. A recent study published in Radiology evaluates the ability of photon-counting CT (PCCT) to quantify hepatic fat fraction on contrast-enhanced scans, validating its results against established modalities, including MRI PDFF, histopathology and the controlled attenuation parameter (CAP) from transient elastography.
Noninvasive Imaging and the Emergence of PCCT
While liver biopsy remains the reference standard for steatosis diagnosis, its invasiveness and susceptibility to sampling variability have led to the pursuit of noninvasive alternatives. MRI PDFF has emerged as the leading noninvasive standard, capable of quantifying a wide spectrum of fat deposition. CAP measurement via transient elastography offers an additional tool, though with reduced sensitivity at lower steatosis grades. Ultrasound remains common but is limited in detecting mild steatosis and prone to operator variability.
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Photon-counting CT introduces a novel approach by enabling spectral analysis of contrast-enhanced scans to virtually remove iodine and isolate hepatic fat content. Using a three-material decomposition algorithm, this technique quantifies fat in liver segments by excluding vascular and biliary structures. PCCT thus holds the promise of opportunistic screening for steatosis in routine clinical imaging, especially when MRI is not available.
Study Design and Comparative Performance
In a prospective study involving 178 participants with known or suspected liver disease, PCCT-derived liver fat fraction was compared with measurements from MRI PDFF, CAP and histopathology. Participants underwent contrast-enhanced abdominal PCCT with follow-up MRI, biopsy or CAP measurement within 120 days. Hepatic fat fraction was determined in four liver segments and cross-compared between modalities.
PCCT showed excellent agreement with MRI PDFF, yielding an intraclass correlation coefficient (ICC) of 0.91 overall, 0.84 in participants with fibrosis and 0.92 in those without. The correlation with histologic grading was moderate (ρ = 0.65), while the correlation with CAP was poor (ρ = 0.45). A PCCT threshold of 4.8% fat fraction best discriminated between participants with and without steatosis, achieving a sensitivity of 81% and specificity of 71%. These values suggest that PCCT may offer comparable accuracy to MRI PDFF in clinical practice, particularly for initial detection.
Clinical Implications and Limitations
The potential for PCCT to enable reliable liver fat quantification on contrast-enhanced scans marks a significant advance in steatosis assessment. Conventional CT struggles to quantify fat accurately once contrast is introduced, due to the difficulty of separating iodine from fat signals. Earlier dual-energy systems offered partial solutions, but photon-counting technology improves spectral resolution through better photon registration, reducing scatter and enabling clearer energy differentiation.
Despite these advances, the study acknowledges several limitations. PCCT systems remain limited in availability, and the participant pool included few individuals with higher steatosis grades, restricting the ability to validate grading thresholds beyond the binary distinction of presence or absence of steatosis. Moreover, the single-centre design and reliance on a single scanner limit generalisability. The high prevalence of fibrosis among biopsy participants also introduces potential confounding. Furthermore, the 120-day window between imaging modalities, though necessary to include sufficient participants, risks introducing changes in liver fat content due to lifestyle or treatment changes.
The study has validated the use of contrast-enhanced PCCT for noninvasive quantification of hepatic fat fraction, demonstrating strong agreement with MRI PDFF and moderate correlation with histopathology. PCCT could provide accurate, opportunistic detection of steatosis during routine clinical imaging without requiring additional scans. While limitations regarding availability and validation for higher steatosis grades remain, these findings support PCCT’s role in expanding access to reliable liver fat assessment. Future multicentre studies are needed to confirm these results and refine clinical thresholds across broader populations.
Source: Radiology
Image Credit: Freepik
