CT Protocol for Adrenal Gland Evaluation with Contrast
For adrenal gland evaluation, perform an unenhanced CT first (measuring Hounsfield units), followed by contrast-enhanced CT with delayed imaging at 15 minutes if the initial unenhanced scan shows attenuation >10 HU, as this protocol achieves >95% sensitivity and >97% specificity for distinguishing benign from malignant lesions. 1
Initial Imaging Protocol
Step 1: Unenhanced CT Abdomen and Pelvis
- Always begin with unenhanced (non-contrast) CT to measure baseline attenuation 1
- Measure Hounsfield units (HU) using region of interest over the adrenal lesion 1
- If attenuation ≤10 HU: the lesion is highly specific for adenoma (96-100% specificity), and no further imaging is needed 1
- If attenuation >10 HU: proceed to contrast-enhanced protocol with delayed washout imaging 1
Step 2: Contrast-Enhanced CT with Delayed Washout (if HU >10)
- Administer IV contrast and obtain immediate post-contrast images 1
- Obtain delayed images at 15 minutes post-contrast (some protocols use 30 minutes, but 15 minutes achieves sensitivity >95%, specificity >97%) 1
- Calculate contrast washout: adenomas wash out >60% at 15 minutes, while malignant lesions retain contrast 1
- This delayed enhancement technique is superior to unenhanced CT alone and may be superior to MRI 1
Renal Function Considerations
Pre-Contrast Assessment
- Check current GFR before administering contrast 2
- GFR >60 mL/min: contrast can be administered with minimal risk 2
- GFR 30-60 mL/min: implement pre-procedural hydration with isotonic saline, use low-osmolar or iso-osmolar contrast agents, minimize contrast volume 2
- GFR <30 mL/min: consider MRI with chemical shift imaging or non-contrast alternatives 2
Critical Pitfall to Avoid
- Never skip the unenhanced phase when evaluating adrenal masses - in one study, unenhanced CT detected parenchymal abnormalities in only 1.4% of cases when performed after contrast, missing critical diagnostic information 1
- Performing only contrast-enhanced CT without baseline unenhanced images reduces sensitivity for adenoma detection from 56% to approximately 10-36% (even with histogram analysis) 1
Alternative Protocol: Chemical Shift MRI
If the adrenal mass is discovered incidentally on contrast-enhanced CT (no baseline unenhanced images available), chemical shift MRI is the preferred next step rather than repeating CT 1
- Chemical shift MRI detects lipid content in adenomas with 96-100% accuracy 1
- Particularly useful when CT density is indeterminate (10-30 HU), where MRI correctly characterizes 89% of lesions 1
- Avoids additional radiation and contrast exposure 1
Size-Specific Protocols
Lesions <3 cm
- Unenhanced CT (rating 8/9) or delayed enhancement CT if HU >10 (rating 8/9) 1
- Chemical shift MRI (rating 8/9) if discovered on contrast-enhanced study 1
- Follow-up imaging at 6-12 months if indeterminate 1
Lesions 3-5 cm
- Same protocol as above, but follow-up at 3-6 months if indeterminate 1
- Biopsy consideration increases (rating 6/9) if non-invasive imaging inconclusive 1
Lesions >4 cm
- Suspect adrenal carcinoma if >4 cm with irregular margins or internal heterogeneity 1
- Perform adrenal protocol CT: unenhanced for HU measurement, then contrast-enhanced with 15-minute washout 1
- If HU >10 and washout <60% at 15 minutes, malignancy is likely 1
Common Pitfalls
Failing to obtain unenhanced images first - this is the single most important error, as it eliminates the ability to accurately measure baseline attenuation 1
Not waiting long enough for delayed images - imaging before 15 minutes reduces sensitivity for distinguishing adenomas from metastases 1
Ignoring renal function before contrast administration - pre-existing renal impairment is the principal risk factor for contrast-induced nephropathy 2, 3
Using histogram analysis incorrectly - if ≥5% of pixels are <0 HU on post-contrast images, sensitivity improves from 10% to 36%, but this still underperforms compared to proper unenhanced imaging 1