Does fatty liver show up on PET scan? This is a common question for patients undergoing imaging investigations. Fatty liver disease affects approximately one in three UK adults, yet PET scans are not designed to detect or diagnose this condition. Positron emission tomography tracks metabolic activity using radioactive tracers, primarily for cancer detection, rather than identifying fat accumulation in liver cells. If you have concerns about fatty liver disease, your GP will arrange appropriate investigations such as blood tests, ultrasound scanning, or specialist referral. Understanding which imaging tests are suitable for hepatic steatosis helps ensure you receive the most effective and safe diagnostic pathway.
Summary: Fatty liver disease does not reliably show up on PET scans because these scans detect metabolic activity rather than fat accumulation in liver cells.
- PET scans use radioactive tracers to track glucose metabolism, not lipid deposits, making them unsuitable for diagnosing hepatic steatosis
- Ultrasound is the first-line imaging test recommended by NICE for detecting fatty liver disease in the UK
- MRI proton density fat fraction (MRI-PDFF) is the gold standard for accurately quantifying liver fat content
- NICE recommends the Enhanced Liver Fibrosis (ELF) blood test to assess advanced fibrosis risk in adults with NAFLD
- PET scans may be ordered for cancer staging or complex infections, and can incidentally reveal fatty liver on the CT component
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What Is Fatty Liver Disease and How Is It Diagnosed?
Fatty liver disease, medically termed hepatic steatosis, occurs when excess fat accumulates within liver cells. In the UK, this condition affects approximately one in three adults and exists in two main forms: non-alcoholic fatty liver disease (NAFLD), which develops in people who drink little or no alcohol, and alcohol-related fatty liver disease (ARLD), caused by excessive alcohol consumption. NAFLD is increasingly common due to rising rates of obesity, type 2 diabetes, and metabolic syndrome.
Most people with fatty liver disease experience no symptoms in the early stages, making diagnosis often incidental during investigations for other conditions. When symptoms do occur, they may include persistent fatigue, discomfort in the upper right abdomen, or unexplained weight loss. However, these are non-specific and typically appear only when the condition has progressed to more serious stages such as non-alcoholic steatohepatitis (NASH) or cirrhosis.
Diagnosis typically begins with:
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Blood tests – Liver function tests (LFTs) may show elevated alanine aminotransferase (ALT) and aspartate aminotransferase (AST), though these can be normal even with significant fat accumulation
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Clinical assessment – Your GP will evaluate risk factors including body mass index (BMI), waist circumference, blood pressure, and metabolic health markers
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Imaging studies – Ultrasound scanning is the first-line imaging modality recommended by NICE for detecting hepatic steatosis
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Fibrosis risk assessment – In primary care, your GP may use simple scoring tools such as the FIB-4 or NAFLD Fibrosis Score to estimate your risk of advanced liver scarring (fibrosis)
According to NICE guidelines (NG49), diagnosis involves a combination of clinical history, blood tests, and imaging to exclude other causes of liver disease. NICE recommends offering the Enhanced Liver Fibrosis (ELF) test to adults with NAFLD to assess for advanced fibrosis, with re-testing intervals determined by initial results and clinical context. If you have abnormal liver blood tests persisting for more than six months, high fibrosis risk scores, or features suggesting advanced liver disease, your GP may refer you to a hepatology specialist. Early detection is crucial because lifestyle modifications can reverse simple steatosis before it progresses to inflammation and scarring.
Why PET Scans Are Not Routinely Used for Fatty Liver Detection
Positron emission tomography (PET) scans are sophisticated nuclear medicine imaging studies that detect metabolic activity by tracking radioactive tracers, most commonly fluorodeoxyglucose (FDG), which accumulates in tissues with high glucose metabolism. Whilst PET scans excel at identifying malignancies and certain inflammatory processes, they are not designed or routinely used for diagnosing fatty liver disease for several important reasons.
Fatty liver disease is primarily a structural and biochemical condition characterised by lipid accumulation rather than altered glucose metabolism. The standard FDG-PET tracer does not specifically target or highlight fat deposits within hepatocytes. The normal liver typically shows moderate FDG uptake on PET imaging, and this uptake pattern does not reliably change with hepatic steatosis. The scan cannot differentiate between normal liver tissue and fatty infiltration with sufficient accuracy for diagnostic purposes.
Key limitations include:
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Lack of specificity – PET scans cannot quantify fat content or distinguish between simple steatosis and more serious steatohepatitis
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Radiation exposure – PET imaging involves ionising radiation from both the radiotracer and often an accompanying CT scan, making it inappropriate for a benign condition that can be assessed with safer methods
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Cost-effectiveness – PET scans are expensive and resource-intensive, requiring specialised equipment and radiopharmaceuticals
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Availability – PET scanners are limited to specialist centres and reserved primarily for oncology applications, with some use in neurology, cardiology, and complex infection or inflammation cases
NHS England PET-CT clinical indications and NICE guidance do not include fatty liver assessment. Current UK imaging pathways, as outlined in the Royal College of Radiologists' iRefer guidance, recommend ultrasound as the first-line investigation for suspected hepatic steatosis. If fatty liver is incidentally noted on a PET-CT scan performed for another indication (such as cancer staging), this would typically prompt further investigation with more appropriate imaging modalities and clinical assessment.
Best Imaging Tests for Diagnosing Fatty Liver in the UK
Several imaging modalities are available for detecting and quantifying hepatic steatosis, each with distinct advantages. Ultrasound scanning remains the first-line investigation recommended by NICE due to its accessibility, safety, and reasonable accuracy. During an ultrasound, fatty liver appears as increased echogenicity (brightness) compared to the kidney, with possible posterior attenuation of the ultrasound beam. This examination is non-invasive, involves no radiation, and is widely available across NHS and private facilities. However, ultrasound has limitations: it is operator-dependent, less sensitive in detecting mild steatosis (affecting less than 20% of hepatocytes), and sensitivity is reduced in people with higher body mass index. Ultrasound cannot reliably distinguish simple steatosis from NASH or assess the degree of fibrosis.
Magnetic resonance imaging (MRI) techniques, particularly MRI proton density fat fraction (MRI-PDFF), represent the gold standard for non-invasive fat quantification. MRI-PDFF can accurately measure hepatic fat content as a percentage and is highly reproducible. Magnetic resonance elastography (MRE) can additionally assess liver stiffness, helping to identify fibrosis. These advanced MRI techniques are increasingly used in specialist hepatology centres and research settings, though availability on the NHS may be limited outside tertiary centres.
Transient elastography (FibroScan®) combines ultrasound-based liver stiffness measurement with controlled attenuation parameter (CAP) scoring to assess both fibrosis and steatosis. This quick, non-invasive test is available in many NHS hepatology departments and specialist pathways. The CAP score quantifies fat content; thresholds indicating significant steatosis vary by device, protocol, and patient factors such as BMI, so interpretation should follow local protocols. NICE NG49 recommends offering the Enhanced Liver Fibrosis (ELF) blood test to assess advanced fibrosis in adults with NAFLD, rather than specifying FibroScan as the primary test, though transient elastography is commonly used in specialist hepatology practice.
CT scanning can detect moderate to severe fatty liver, which appears as decreased attenuation (darker) compared to the spleen. However, CT involves radiation exposure and is less sensitive than MRI for fat quantification. It is not routinely used solely for fatty liver assessment but may incidentally detect the condition during scans for other indications. Your GP or hepatologist will select the most appropriate imaging based on clinical context, local availability, and whether assessment of fibrosis is also required.
When Your Doctor Might Order a PET Scan Instead
Whilst PET scans are not used for diagnosing fatty liver disease itself, there are specific clinical scenarios where your doctor might order a PET scan if you happen to have known fatty liver disease. Understanding these situations helps clarify the distinct roles of different imaging modalities.
PET scans may be requested when:
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Staging of known cancers – If you are diagnosed with certain cancers elsewhere in the body (such as colorectal cancer, lung cancer, or lymphoma), PET-CT scanning is routinely used for staging according to NICE tumour-specific pathways and NHS England PET-CT clinical indications. Any pre-existing fatty liver would be visible on the CT component, though the PET component focuses on metabolically active tumour tissue.
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Suspected malignancy requiring whole-body assessment – In selected cases where there is clinical suspicion of cancer and whole-body metabolic imaging is indicated, PET-CT may be appropriate. However, if you develop a focal liver lesion on ultrasound or CT, the standard UK pathway involves further characterisation with multiphasic contrast-enhanced CT or MRI, not PET-CT. These modalities are the mainstay for diagnosing hepatocellular carcinoma (HCC), which can develop in cirrhotic livers, including those with advanced NAFLD. FDG-PET has limited sensitivity for well-differentiated HCC and is not routinely used for liver lesion characterisation.
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Complex infection or inflammation – In rare cases where there is diagnostic uncertainty about systemic inflammatory or infectious conditions affecting multiple organs, PET scanning may be considered, though this would not be related to the fatty liver itself.
It is important to note that if a PET-CT scan is performed for any of these indications, the CT component may incidentally reveal fatty liver changes (decreased liver attenuation), but this is a secondary finding rather than the scan's purpose. If you have concerns about liver health or have been told you need a PET scan, discuss with your GP or specialist exactly why this particular test has been recommended. For straightforward fatty liver assessment and monitoring, your doctor will arrange appropriate blood tests, ultrasound, ELF testing, or specialist referral instead.
Seek urgent medical attention by calling 999 or attending A&E immediately if you develop red-flag symptoms such as vomiting blood, passing black or tarry stools, severe confusion or drowsiness, yellowing of the skin or eyes (jaundice) with abdominal swelling, or severe abdominal pain, as these may indicate serious liver complications requiring emergency assessment. Contact your GP promptly for new persistent symptoms such as unexplained weight loss, ongoing upper abdominal discomfort, or worsening fatigue.
Frequently Asked Questions
Can a PET scan detect fatty liver disease?
No, PET scans cannot reliably detect fatty liver disease because they track metabolic activity using glucose-based tracers rather than identifying fat deposits. The standard FDG-PET tracer does not specifically highlight hepatic steatosis, and normal liver uptake patterns do not change predictably with fat accumulation, making PET unsuitable for diagnosing this condition.
What scan is best for diagnosing fatty liver in the UK?
Ultrasound scanning is the first-line imaging test recommended by NICE for detecting fatty liver disease due to its safety, accessibility, and reasonable accuracy. For more precise fat quantification, MRI proton density fat fraction (MRI-PDFF) is the gold standard, whilst transient elastography (FibroScan) can assess both fat content and liver stiffness in specialist settings.
Will fatty liver show up if I have a PET-CT scan for cancer?
Fatty liver may appear incidentally on the CT component of a PET-CT scan as decreased liver attenuation (darker appearance), but this is a secondary finding rather than the scan's purpose. The PET component tracks metabolic activity for cancer staging and does not specifically identify hepatic steatosis, so any fatty liver changes would be noted on the CT images alone.
How do I get tested for fatty liver disease on the NHS?
Start by booking an appointment with your GP, who will arrange blood tests (liver function tests) and assess your risk factors such as BMI, diabetes, and metabolic health. If fatty liver is suspected, your GP will typically request an ultrasound scan and may offer the Enhanced Liver Fibrosis (ELF) blood test to assess for advanced scarring, with specialist referral arranged if needed.
What is the difference between ultrasound and MRI for fatty liver?
Ultrasound is widely available, radiation-free, and can detect moderate to severe fatty liver, but it is operator-dependent and less sensitive for mild steatosis. MRI techniques, particularly MRI-PDFF, provide highly accurate fat quantification as a percentage and are reproducible, making them the gold standard, though they are typically reserved for specialist centres due to cost and limited NHS availability.
When would a doctor order a PET scan if I have fatty liver?
A PET scan would be ordered for separate clinical indications such as staging known cancers (colorectal, lung, lymphoma) or investigating complex infections, not for assessing the fatty liver itself. If you develop a suspicious liver lesion, UK pathways recommend multiphasic contrast-enhanced CT or MRI for characterisation rather than PET-CT, as FDG-PET has limited sensitivity for liver tumours like hepatocellular carcinoma.
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