What is the safest approach to performing a PET/CT (positron emission tomography/computed tomography) in a 62‑year‑old man with liver metastases and impaired renal function?

PET/CT in a Patient with Liver Metastases and Renal Impairment

In a 62-year-old man with liver metastases and renal impairment, perform PET/CT without IV contrast (non-enhanced PET/CT), as this approach avoids nephrotoxic iodinated contrast while still providing valuable metabolic information for detecting extrahepatic disease, though it has significant limitations for liver lesion characterization. 1, 2

Primary Imaging Strategy

Non-Enhanced PET/CT as the Safest Option

• The ACR specifically addresses this clinical scenario, recommending either PET/CT or non-contrast MRI as options to evaluate for metastatic liver disease when patients cannot undergo contrast-enhanced CT or MRI due to renal dysfunction. 1
• Non-enhanced PET/CT avoids the nephrotoxic risk of iodinated CT contrast, which is particularly important given this patient's compromised renal function. 2
• The FDG component provides metabolic information about tumor viability and can detect unsuspected extrahepatic metastases, which occurs in 8-11% of patients and may change management. 1

Critical Limitations to Acknowledge

• Non-enhanced PET/CT has substantially reduced performance for liver lesion detection compared to contrast-enhanced imaging, with a detection rate of only 73.6% versus 90.9% for contrast-enhanced PET/CT and 95.4% for MRI. 3
• The sensitivity for characterizing detected liver lesions on non-enhanced PET/CT is only 67%, with specificity of 60% and accuracy of 66%, compared to 98% sensitivity and 100% specificity for MRI. 3
• The ACR explicitly warns that noncontrast CT for liver staging is usually not indicated and has severely limited sensitivity for metastases. 2

Alternative Approach: Non-Contrast MRI

When MRI is Feasible

• Non-contrast MRI may provide better soft-tissue detail than noncontrast CT when IV contrast is contraindicated, according to the ACR. 2
• MRI without IV contrast still has reasonable detection capability for liver metastases, rated as "may be appropriate" (appropriateness score 6) by the ACR. 1
• Diffusion-weighted imaging (DWI) sequences on non-contrast MRI can detect liver metastases without requiring gadolinium contrast. 1, 4

MRI Contraindications to Screen For

• Cardiac pacemakers or other implanted metallic devices
• Severe claustrophobia
• Inability to lie flat for extended periods
• Patient cooperation issues

Practical Implementation Algorithm

Step 1: Assess Renal Function Severity

• Determine if the renal impairment is severe enough to absolutely contraindicate iodinated contrast
• Consider discussion with nephrology regarding risk-benefit of contrast administration with appropriate hydration protocols

Step 2: Choose Imaging Modality

• If MRI is available and patient has no contraindications → Perform non-contrast MRI with DWI sequences
• If MRI is contraindicated or unavailable → Perform non-enhanced PET/CT
• Document in the radiology order that contrast is contraindicated due to renal impairment 1

Step 3: Optimize the Selected Study

For non-enhanced PET/CT:

• Use the spleen as the reference organ instead of the liver, since diffuse hepatic metastases may make the liver unsuitable as a reference standard. 5
• Acquire thin CT sections (3-5 mm) to reduce partial volume averaging, even without contrast. 2
• Focus interpretation on detecting extrahepatic disease and assessing metabolic activity of known liver lesions. 6

For non-contrast MRI:

• Include DWI sequences, which are essential for detecting metastases without contrast. 4
• Use T2-weighted and T1-weighted sequences to maximize lesion detection. 1

Critical Pitfalls to Avoid

Do Not Rely on Non-Enhanced CT Alone

• The ACR explicitly states that single-phase CT without proper timing misses significant disease burden and should never be relied upon for staging. 2
• The low-dose CT component of PET/CT is primarily for anatomic localization, not comprehensive liver evaluation. 3

Recognize False-Positive PET Findings

• Benign lesions including abscesses, cryptococcosis, hepatic adenomas, and inflammatory changes from cholecystitis can show increased FDG uptake. 7
• False-positive findings occur in up to 6-9% of patients and can lead to inappropriate management changes. 1

Understand Detection Limitations

• Non-enhanced PET/CT misses approximately 26% of liver lesions compared to contrast-enhanced imaging. 3
• Subcentimeter lesions are particularly likely to be missed on non-enhanced studies. 1

Documentation Requirements

• The ACR recommends documenting which reference region is used in the imaging report (spleen vs. liver), as this affects SUV calculations and serial comparison of disease burden. 5
• Note the limitation that renal impairment prevented optimal contrast-enhanced imaging. 1
• Recommend correlation with prior imaging if available to assess interval change. 5

When to Escalate Imaging

• If non-enhanced PET/CT or non-contrast MRI findings are equivocal or inadequate for surgical planning, discuss with the multidisciplinary team whether the benefits of contrast-enhanced imaging outweigh the renal risks with appropriate nephroprotective measures. 1
• Consider intraoperative ultrasound at the time of surgery for definitive assessment, which detects additional lesions in 11-27% of patients compared to preoperative imaging. 1