Indocyanine Green (ICG) Fluorescence Imaging: Route, Timing, Half-Life and Interpretation

For example:

Proposed bowel transection line

↓

IV ICG

↓

5–15 seconds

↓

Observe perfusion

↓

Change transection line if necessary

↓

Repeat ICG assessment

↓

Construct anastomosis

↓

Optional postoperative/anastomotic reassessment

The key advantage is that ICG can provide real-time functional information before an irreversible surgical decision.

However, fluorescence should complement—not replace—clinical judgement, anatomy, haemodynamic assessment and established surgical principles.

8. ICG for biliary fluorescence cholangiography

This is where timing becomes particularly important.

After IV administration, ICG reaches the liver, is taken up by hepatocytes and is excreted into bile.

The bile ducts therefore become fluorescent later than the blood vessels.

Current prescribing information recommends:

2.5 mg IV ≥45 minutes before surgery

for extrahepatic biliary duct visualization, with fluorescence expected in the biliary tree within approximately 45 minutes. (DailyMed)

Why not inject immediately before looking for the bile duct?

Because immediately after IV injection there is substantial fluorescence within:

• Blood
• Liver
• Vascular structures

This can produce a high background signal.

The objective of delayed biliary imaging is to allow:

Blood-pool fluorescence ↓

while:

Biliary fluorescence ↑

thereby improving the bile duct-to-liver contrast.

9. Timing of ICG for cholecystectomy

There is no universally accepted single timing protocol for fluorescence cholangiography.

Published studies have used administration intervals ranging from approximately 15 minutes to 24 hours before surgery, with substantial variation in dose and timing. A 2026 systematic analysis of 54 studies involving 3,070 patients found IV administration to be the predominant route, while emphasizing the continuing heterogeneity of protocols. (PubMed)

Expert consensus has favoured giving ICG at least 45 minutes before the procedure in order to reduce intense hepatic background fluorescence. (PubMed)

Some clinical studies have investigated administration on the day before surgery to further improve bile duct-to-liver contrast. (PubMed)

Practical interpretation

For routine biliary fluorescence:

Short interval

→ more blood/liver background

≥45 min

→ useful biliary fluorescence with reduced blood-pool component

Several hours / overnight

→ potentially improved duct-to-liver contrast in some protocols

But:

Longer is not automatically better.

The optimal timing depends on dose, liver function, inflammation, body habitus, equipment, indication and the fluorescence characteristics of the imaging system.

10. Difficult cholecystectomy: what ICG can and cannot do

ICG fluorescence can help identify biliary structures before and during dissection.

Potential advantages include:

• Real-time visualization
• No ionising radiation
• No need for duct cannulation
• Repeated imaging
• Improved anatomical orientation
• Assistance in obese patients
• Assistance in inflammatory cases

A systematic review found that ICG fluorescence cholangiography improves visualization of biliary anatomy, although there remains heterogeneity regarding dose, timing and administration protocols. (PubMed)

Importantly:

ICG fluorescence is an adjunct—not a substitute for sound surgical anatomy and critical-view principles.

Severe inflammation, impacted stones, previous biliary intervention and other factors can reduce visualization quality. (PubMed)

11. Lymphatic mapping: a different route, a different clock

Lymphatic imaging is fundamentally different from IV perfusion imaging.

ICG is injected intradermally, subcutaneously or interstitially, depending on the procedure.

The dye then travels through lymphatic channels toward regional lymph nodes.

For example, the current prescribing information describes:

Cervical/uterine lymphatic mapping

→ 5 mg interstitially

→ four injections

→ superficial and deep cervical injection sites

→ lymphatic vessels and nodes should begin to become visible within approximately 1 minute. (DailyMed)

The exact technique, concentration and dose are indication-specific.

Therefore:

Never transfer an IV vascular dose directly to a lymphatic protocol.

12. A practical “ICG clock” for surgeons

0 seconds — IV injection

ICG enters bloodstream.

5–15 seconds

Vascular fluorescence

Best for:

• Arterial inflow
• Vessel visualization
• Tissue perfusion
• Anastomotic assessment

(DailyMed)

1–5 minutes

The circulating signal begins to decline because of the short plasma half-life.

Useful interpretation:

Dynamic perfusion rather than static staining.

30–45+ minutes

ICG has undergone hepatic uptake and biliary excretion.

Biliary fluorescence becomes clinically useful.

The current labelled protocol specifies ≥45 minutes for extrahepatic biliary imaging. (DailyMed)

Several hours

Biliary fluorescence can remain useful, depending on the clinical situation and imaging system.

However, signal quality is influenced by:

• Hepatic function
• Dose
• Timing
• Inflammation
• Tissue thickness
• Obesity
• Liver background
• Imaging distance
• Camera angle
• Device sensitivity

Hours later

The clinical interpretation should focus on where the ICG has moved, rather than assuming that the original intravascular signal remains unchanged.

13. Why “half-life” should not be confused with “imaging duration”

This is one of the most important teaching points.

Plasma half-life ≠ fluorescence duration

A drug with a 3–5 minute plasma half-life does not necessarily produce only 3–5 minutes of useful surgical fluorescence.

Why?

Because ICG is moving between physiological compartments.

Blood

→

Liver

→

Bile

→

Intestine

The surgeon is observing different compartments at different times.

Therefore:

The timing of the image determines the physiological question being answered.

14. Route × timing × question

A useful mental model is:

The exact protocol should always follow the specific ICG product label, imaging system and surgical indication.

15. How to interpret a “weak” ICG image

A weak image does not automatically mean poor perfusion.

Possible explanations include:

Patient factors

• Obesity
• Thick tissue
• Poor cardiac output
• Low arterial inflow
• Hypotension
• Liver dysfunction for biliary applications

Surgical factors

• Severe inflammation
• Oedema
• Haematoma
• Tissue depth
• Excessive distance from camera
• Incorrect camera angle

Technical factors

• Insufficient dose
• Incorrect timing
• Excessive ambient/background signal
• Device settings
• Poor exposure geometry

A systematic review of fluorescence cholangiography demonstrated that fluorescence intensity is affected by several variables, including obesity and inflammation. (PubMed)

Therefore:

Interpret the fluorescence image in context—not as a binary “green = good / no green = bad” test.

16. ICG with modern NIR systems

Modern fluorescence platforms can present ICG information in different visualization modes.

For example, systems may provide:

• White-light anatomy with fluorescence overlay
• Grayscale fluorescence visualization
• High-contrast fluorescence display

The purpose is not merely aesthetic.

Different visualization modes can change the surgeon’s ability to distinguish a weak fluorescence signal from background anatomy or liver fluorescence.

The most important principle remains:

The imaging system detects the fluorescence; the surgeon must understand the physiology behind the fluorescence.

17. Practical checklist before injecting ICG

Before administration, ask five questions:

1. What am I trying to see?

Perfusion? Bile duct? Lymphatics?

2. What is the correct route?

IV or local/interstitial?

3. When should I inject?

Seconds before vascular assessment?

≥45 minutes before biliary imaging?

Procedure-specific timing for lymphatic mapping?

4. What should I expect to see?

Know the expected fluorescence window before injecting.

5. What does a weak signal mean?

Consider:

Timing + dose + physiology + tissue depth + inflammation + equipment

before concluding that the anatomy or perfusion is abnormal.

18. The surgeon’s takeaway

ICG fluorescence is best understood as a time-dependent physiological imaging technique.

Its short plasma half-life is not a limitation—it is one of its advantages.

For vascular imaging:

Inject → seconds → assess perfusion

For biliary imaging:

Inject → hepatic uptake → biliary excretion → wait → identify ducts

For lymphatic imaging:

Local injection → lymphatic transport → visualize channels/nodes

The same molecule therefore answers very different surgical questions depending on route, dose, timing and imaging modality.

The simplest rule to remember

ICG is not simply a dye. It is a dynamic tracer.

Seconds = blood flow.

Tens of minutes = biliary mapping.

Local injection + minutes = lymphatic mapping.

And the most important interpretive principle:

Always interpret the fluorescence according to where the ICG should physiologically be at that moment.

Key references

1. SPY AGENT GREEN / Indocyanine Green prescribing information. Current labeling specifies 5–15-second vascular fluorescence and ≥45-minute biliary imaging after IV administration. (DailyMed)
2. Systematic review of ICG near-infrared fluorescence in laparoscopic cholecystectomy. Demonstrates benefits for biliary visualization while highlighting heterogeneity in dose and timing. (PubMed)
3. International Delphi consensus on ICG fluorescence during laparoscopic cholecystectomy. Supports NIR fluorescence for biliary visualization and discusses dose/timing considerations. (PubMed)
4. Systematic review and ex-vivo evaluation of ICG fluorescence cholangiography. Examines factors affecting fluorescence intensity and image quality. (PubMed)
5. 2026 systematic analysis of ICG fluorescence during cholecystectomy. Reviews route, dose, timing and detection outcomes across 54 studies and 3,070 patients. (PubMed)

Bottom line

ICG timing should be planned backward from the surgical question.

Perfusion → inject immediately before assessment.

Biliary anatomy → inject sufficiently early, with ≥45 minutes being the labelled approach for extrahepatic biliary imaging.

Lymphatic mapping → use the procedure-specific local/interstitial protocol.

Never interpret ICG fluorescence without considering the time elapsed since injection.

This can also be converted into a 6–8 slide “ICG: The Surgeon’s Clock” presentation, with a single timeline graphic from 0 seconds → 15 seconds → 45 minutes → several hours, which would work particularly well for a surgical video presentation.