eduKate Learning Manual
Science | Veterinary World
Define the Perfusion Question → Establish White-Light Anatomy → Administer Intravascular ICG → Illuminate With Near-Infrared Light → Observe Dye Arrival, Distribution and Washout → Quantify or Grade Fluorescence → Check Dose, Timing, Camera Geometry and Haemodynamics → Integrate With Tissue Viability and Other Perfusion Evidence

Veterinary Indocyanine Green Fluorescence Angiography

Why Normal Tissue Colour Does Not Prove Adequate Perfusion

Wait, What? A Skin Flap Can Look Pink to the Eye While Its Vascular Supply Is Uneven

Surgeons routinely judge tissue using colour, capillary refill, bleeding and visible vessels. These observations matter, but they are indirect.

Indocyanine green—ICG—fluorescence angiography adds a dynamic intravascular tracer. After intravenous injection, ICG binds strongly to plasma proteins and remains largely within the vascular compartment during its first pass. Near-infrared illumination excites the dye, and a specialised camera records fluorescence as blood carries it through tissue.

normal-looking tissue ≠ proven adequate vascular delivery.

The Scientific Job

This page owns one Veterinary World job:

How should veterinarians interpret ICG near-infrared fluorescence angiography as evidence of vascular delivery and tissue perfusion while separating dye arrival, fluorescence intensity, vessel anatomy and tissue viability?

Veterinary Contrast-Enhanced Ultrasound retains ultrasound microvascular perfusion. Veterinary Laser Speckle Contrast Imaging retains superficial speckle-based perfusion mapping. This page owns the narrower job of intravascular near-infrared fluorescence angiography with ICG.

Quick Answer

ICG fluorescence angiography visualises blood-borne dye in real time using near-infrared imaging. Veterinary studies have used the technique to map canine axial-pattern-flap vessels, modify flap planning, visualise superficial brachial flap perfusion and guide tumour or biliary procedures. The strongest interpretation is dynamic: when does fluorescence arrive, where does it spread, how strong is it relative to surrounding tissue, and how does it wash out? Fluorescence does not equal viability automatically. Camera distance, dye dose, injection timing, hepatic clearance, systemic haemodynamics and tissue depth all influence what is seen.

A 2024 Veterinary Surgery study in 15 client-owned dogs showed that near-infrared ICG angiography could identify vascularisation of several axial pattern flaps and changed surgical margin planning compared with surface landmarks alone. A 2022 report in two dogs demonstrated real-time assessment of superficial brachial flap vascularisation during reconstruction.

Explore 2024 Canine Axial-Pattern-Flap Angiography Study →

Explore Canine Flap Case Report Using ICG Angiography →

Primary Entry — ICG Is an Intravascular Optical Tracer

ICG absorbs and emits light in the near-infrared range. Near-infrared wavelengths penetrate tissue more deeply than visible light and experience less interference from haemoglobin absorption than much of the visible spectrum.

After intravenous administration, protein-bound ICG travels with plasma. A fluorescence camera therefore sees where dye-bearing blood arrives and distributes.

Part 1 — Arrival Time Carries Haemodynamic Information

Fluorescence does not appear everywhere simultaneously. Arterial inflow, tissue perfusion and venous clearance create a time sequence.

Delayed arrival can reflect altered inflow, but timing also depends on injection technique, cardiac output and camera acquisition. A slow appearance is therefore not automatically a local arterial obstruction.

Part 2 — Intensity Is Not a Universal Perfusion Unit

Brighter fluorescence may correspond to more dye within the imaged field, but raw intensity also depends on camera sensitivity, distance, angle, ambient light, tissue depth and dose.

Quantitative work therefore often uses relative intensity, time-to-peak, inflow slope or contrast-to-background rather than treating one brightness value as absolute blood flow.

fluorescence brightness ≠ absolute perfusion unless the acquisition and calibration model support that claim.

Part 3 — Vessel Anatomy and Tissue Perfusion Are Related but Different

ICG can make a direct cutaneous artery visible and can also show the downstream fluorescence pattern through a flap.

Seeing the vessel confirms vascular anatomy. Seeing homogeneous tissue fluorescence adds evidence that perfused blood reaches the tissue field. The two observations should not be collapsed.

Part 4 — Flap Planning Shows Why Surface Landmarks Are Incomplete

Axial pattern flaps are traditionally designed using known anatomical landmarks. Individual vascular anatomy can still vary.

In the 2024 canine study, fluorescence imaging added patient-specific information and influenced planned flap dimensions. That is a direct example of a dynamic imaging signal adding information beyond visual inspection and palpation.

Part 5 — Tissue Colour Can Be Misleading

Colour depends on oxygenation, venous congestion, surface lighting, tissue thickness and the observer. A pink surface can still contain marginally perfused regions; a dark surface can be congested rather than completely non-perfused.

ICG does not make clinical judgement unnecessary. It gives an independent tracer-based observation.

Secondary Deepening — Dose and Timing Matter

Too little dye may produce weak fluorescence. Repeated injections can create residual background fluorescence. The timing of imaging relative to injection determines whether the camera captures arterial inflow, tissue phase or venous washout.

Serial comparisons therefore require careful attention to injection and imaging timing.

Part 6 — Systemic Cardiac Output Changes Local Arrival

ICG reaches tissue through the circulation. Low cardiac output, hypotension or vasoconstriction can delay or reduce fluorescence even when local arterial anatomy is intact.

A local fluorescence map therefore belongs inside the patient’s broader haemodynamic state.

Part 7 — ICG Is Cleared by the Liver

ICG is extracted predominantly by the liver and excreted into bile. Hepatic function and repeated dosing can affect circulating background levels and washout.

This is one reason ICG is also used for biliary fluorescence applications, which are a different scientific job from tissue perfusion.

Explore Canine Near-Infrared Fluorescence Cholangiography Pilot Study →

Part 8 — Perfusion and Viability Are Not Identical

Tissue can fluoresce because blood reaches it, yet still be injured, infected or metabolically compromised. Conversely, low fluorescence under poor acquisition conditions does not prove irreversible necrosis.

Viability remains an integrated clinical outcome involving perfusion, oxygen delivery, metabolism and tissue integrity.

JC Deepening — A Fluorescence Curve Has Kinetic Information

Rather than looking only at a still image, the fluorescence signal can be plotted through time. Useful descriptors include onset, inflow slope, peak intensity, time to peak and washout.

These variables reflect different components of vascular delivery and clearance. Two tissues can reach similar peak brightness through very different kinetic paths.

Part 9 — Camera Geometry Changes Quantitation

Near-infrared fluorescence intensity falls with distance and is influenced by viewing angle and tissue curvature. The same tissue imaged from a different distance can produce a different raw intensity.

Quantitative protocols should therefore standardise camera geometry where possible.

Part 10 — Depth Limits What the Camera Sees

Near-infrared light penetrates farther than visible light, but not infinitely. Deep vessels and deeply buried perfusion abnormalities may be attenuated by overlying tissue.

A negative surface fluorescence pattern does not automatically exclude deep perfusion.

Part 11 — Tumour Fluorescence Is a Different Job From Perfusion Angiography

Some canine oncology studies have used delayed ICG fluorescence to help identify superficial tumour tissue. That application depends on tumour vascular permeability, retention and timing and should not be confused with first-pass perfusion angiography.

Explore Fluorescence-Guided Surgery in Dogs With Superficial Solid Tumours →

Part 12 — Quantitative Thresholds Are Still Developing in Veterinary Surgery

Veterinary literature demonstrates feasibility and decision support, but universal canine fluorescence thresholds that guarantee tissue survival are not established across tissues and devices.

That means the strongest current use is as an additional real-time perfusion signal interpreted with anatomy and clinical judgement.

How Do We Know?

Veterinary evidence includes canine flap studies, clinical reconstructive cases, tumour-fluorescence studies and biliary applications. These show that ICG near-infrared imaging can reveal vascular anatomy and dynamic dye delivery that are not apparent from white-light inspection alone. They also show why fluorescence must remain method-, timing- and tissue-specific.

Observation vs Inference

• Observation: one distal flap region fills later and less intensely than the proximal region.
• Inference: vascular delivery is relatively reduced there; irreversible necrosis is not yet proven.
• Observation: a named cutaneous vessel becomes visible after ICG injection.
• Inference: patient-specific vascular anatomy has been identified.
• Observation: fluorescence is globally delayed in a hypotensive patient.
• Inference: systemic haemodynamics may explain part of the pattern.

Evidence Boundaries

• normal tissue colour ≠ adequate perfusion proven.
• bright fluorescence ≠ absolute blood flow.
• low fluorescence ≠ irreversible necrosis proven.
• visible vessel ≠ uniform downstream tissue perfusion.
• tissue perfusion ≠ tissue viability automatically.
• one device’s intensity ≠ another device’s intensity.
• delayed tumour fluorescence ≠ first-pass perfusion angiography.
• ICG finding ≠ treatment instruction.

Common Misconceptions

Misconception	Better model
The flap looks pink, so it is fully perfused.	Visual colour is indirect; fluorescence adds tracer-based vascular information.
Brighter fluorescence means more mL/min of blood flow.	Raw intensity is strongly dependent on dose, timing and camera geometry.
No fluorescence means dead tissue.	Technical and haemodynamic explanations must be excluded before inferring viability.
ICG angiography replaces clinical examination.	It adds an independent dynamic perfusion layer to clinical judgement.

Unfamiliar Transfer

Dog A has a visually normal flap but delayed distal fluorescence. Dog B has homogeneous fluorescence but later develops a non-perfusion-related complication. Dog C has globally delayed dye arrival during low cardiac output. Dog D shows different raw brightness when the camera is moved closer.

A strong learner asks whether the difference came from blood delivery, tissue depth, systemic circulation, imaging geometry or the clinical tissue state.

Checkpoint Questions

1. Why is ICG useful for near-infrared angiography?
2. What does fluorescence arrival time represent?
3. Why is raw brightness not absolute flow?
4. How is vessel anatomy different from tissue perfusion?
5. Why can tissue colour mislead?
6. How can cardiac output affect local fluorescence?
7. Why does repeated dosing matter?
8. Why is perfusion not identical to viability?
9. How does camera distance affect quantitation?
10. Why are veterinary thresholds still tissue- and device-specific?

Answer key

1. It travels intravascularly and fluoresces under near-infrared excitation.
2. The timing of dye-bearing blood reaching the imaged region under the current haemodynamic conditions.
3. Intensity depends on acquisition geometry, dose, timing and device processing.
4. A named vessel can be present while downstream distribution remains uneven.
5. Colour is influenced by oxygenation, congestion and lighting rather than flow alone.
6. ICG reaches tissue through the systemic circulation.
7. Residual circulating dye can raise background fluorescence.
8. Viability also depends on oxygenation, metabolism and tissue injury.
9. Optical intensity changes with geometry and distance.
10. Veterinary outcome-linked cut-offs are not universally validated across tissues and systems.

Edge Science — Can Quantified ICG Kinetics Predict Canine Flap Survival?

Future systems could analyse arrival time, inflow slope and washout automatically across a flap and compare those trajectories with later healing.

The key requirement is outcome validation. A quantitative map becomes clinically powerful only when specific kinetic patterns reliably predict tissue fate across dogs and devices.

Veterinary World Direction Graph

Veterinary ICG fluorescence angiography → perfusion question → ICG injection → near-infrared excitation → arrival/distribution/washout → camera/dose/systemic-state audit → clinical tissue assessment → relative perfusion interpretation → outcome follow-up.

Research Sources and Further Reading

• Near-Infrared ICG Angiography of Canine Axial Pattern Flaps
• ICG Angiography for Real-Time Flap Vascularisation in Two Dogs
• Fluorescence-Guided Surgery Using ICG in Dogs With Superficial Solid Tumours
• Near-Infrared Fluorescence Cholangiography in Dogs

Educational boundary: Suspected tissue ischaemia, compromised surgical flaps or intraoperative perfusion concerns require direct veterinary surgical assessment. This manual explains imaging interpretation only and does not provide dye dosing, flap revision, resection-margin or treatment instructions.

Teaching Guide for Parents, Tutors and Teachers

For the people who teach because somebody depends on them.

Use a road-map analogy. A neighbourhood can look normal from above, but a fluorescent delivery van reveals which streets actually receive traffic and how quickly it arrives. Appearance and delivery are different measurements.

look at the tissue → follow the tracer → measure timing and distribution → check the whole circulation → separate perfusion from viability.

The mastery target is a learner who understands that dynamic tracer movement can reveal function hidden behind ordinary appearance.