Image-Guided & Diagnostic Surgery

Definition

Navigation-Aided or Computer-Assisted Surgery (CAS) uses computer technology and 3D imaging to plan and guide surgical procedures in real time, showing instrument position relative to patient anatomy.

Principle

Combines preoperative imaging (CT/MRI) or intraoperative imaging with tracking systems to provide real-time 3D visualisation. The surgeon can see instrument position overlaid on the patient’s anatomical map at all times.

Components

1. Imaging system — CT, MRI, fluoroscopy (preoperative or intraoperative)
2. Tracking system — infrared/optical/electromagnetic sensors attached to instruments and patient reference frame
3. Computer workstation — registers imaging to patient anatomy; reconstructs 3D model
4. Display unit — shows real-time instrument position relative to anatomy on screen

Applications

Advantages

• Enhanced accuracy and precision in critical anatomical areas
• Minimally invasive approach possible with greater confidence
• Reduced complications and better outcomes
• Improved surgical planning and intraoperative decision-making

Limitations

• High cost and complex setup; requires dedicated equipment
• Registration errors — mismatch between preoperative image and intraoperative anatomy (brain shift, bowel movement)
• Requires technical expertise and training
• Limited tactile feedback

Definition

Stereotactic (stereotaxic) surgery is a minimally invasive, image-guided surgical technique that uses a 3D coordinate system to accurately localise and target specific areas inside the body for diagnosis or therapy.

Principle

• Combines medical imaging (CT, MRI, PET) with a computer-based coordinate system
• Provides three-dimensional spatial accuracy for instrument navigation
• Defines target using X, Y, Z axes relative to a fixed stereotactic frame or frameless (fiducial-based) system
• Allows accurate guidance of needles, probes, electrodes, or radiation beams to a precise intracranial or body target

Components

1. Stereotactic frame (Leksell, CRW) or frameless navigation system
2. Imaging — CT/MRI/PET for target localisation
3. Computer workstation for planning and coordinate calculation
4. Guidance instruments — needle, probe, electrode, radiation beam

Applications

Advantages

• High precision and reproducibility
• Minimally invasive — reduced morbidity compared to open craniotomy
• Shorter operative time and recovery
• Real-time 3D visualisation in critical anatomical areas

Limitations

• High cost and equipment dependence
• Requires expertise and dedicated imaging support
• Limited use in diffuse or mobile lesions (target must be a discrete, fixed point)

Definition

Image-Guided Surgery (IGS) refers to surgical procedures that use preoperative or intraoperative imaging to localise anatomy, pathology, or instruments in real time, improving accuracy, safety, and outcomes. In short: surgery performed with real-time imaging assistance.

Principle

Combines anatomical imaging (CT, MRI, USG, PET, NIR fluorescence) with computer-assisted navigation systems that track surgical instruments relative to the patient’s anatomy in real time.

System Components

• Imaging source: Preoperative — CT, MRI, PET; Intraoperative — USG, fluoroscopy, ICG-NIR, cone-beam CT
• Navigation system: Optical or electromagnetic trackers for instrument localisation
• Display/Software: 3D reconstruction and real-time overlay of anatomy and instrument position
• Surgeon interface: Monitor, head-mounted display (HMD), or robotic console

Applications by Specialty

Recent Advances in IGS

• Fluorescence-guided surgery (ICG-based) — perfusion and biliary mapping
• Augmented Reality (AR) and Virtual Reality (VR) overlays — projecting 3D anatomy onto the operative field
• Robot-assisted image guidance — da Vinci Firefly (ICG + robotic)
• AI-integrated navigation systems — automatic registration, predictive tissue identification
• Intraoperative MRI (iMRI) — real-time brain tumour margin assessment during craniotomy

Definition

Indocyanine Green (ICG) is a fluorescent dye that binds to plasma proteins and emits near-infrared (NIR) fluorescence, used for real-time visualisation of tissue perfusion, lymphatics, and anatomy during surgery.

Mechanism

Physiological Basis of Applications

Uses in Surgery

Dose

• Perfusion assessment (fluorescence angiography): 0.2–0.5 mg/kg IV
• Biliary mapping (fluorescence cholangiography): 2.5–5 mg IV given 45–60 min before surgery
• Sentinel node mapping: 0.5–1 ml of 0.5 mg/ml injected peritumoural

Advantages

• Real-time, non-invasive perfusion assessment
• Reduces ischaemic complications — detects microvascular compromise not visible to naked eye
• No ionising radiation
• Short half-life allows repeat assessment
• Enhances anatomical visualisation of biliary tree without bile duct puncture

Limitations

• Requires specialised NIR camera system (not universally available)
• Limited tissue penetration (~5–10 mm) — deep structures not visualised
• Contraindicated in iodine allergy (iodine-containing dye)
• Transient visualisation window; background liver fluorescence if biliary mapping done too early
• False negatives in severe vasospasm

1. Fluorescence Angiography (Perfusion Assessment)

Principle: After IV ICG injection (0.2–0.5 mg/kg), NIR camera detects tissue perfusion in real time. Well-perfused areas appear bright; ischaemic regions appear dark.

Applications:

• Colorectal surgery: Assess bowel perfusion before anastomosis → reduces anastomotic leak rates
• Plastic/Reconstructive surgery: Evaluate flap viability and skin perfusion before inset
• Vascular surgery: Assess bypass graft patency or distal flow after revascularisation
• Gastro-oesophageal surgery: Assess gastric conduit perfusion in oesophagectomy

2. ICG Biliary Mapping (Fluorescence Cholangiography)

Principle: ICG is exclusively hepatically excreted in bile, highlighting the biliary tree under NIR imaging without contrast injection into the duct.

Technique: IV ICG 2.5–5 mg given 45–60 min pre-op; NIR camera visualises biliary anatomy in real time during dissection.

Applications:

• Laparoscopic cholecystectomy: Identify Calot’s triangle structures (cystic duct, CBD, CHD); detect anomalous or accessory hepatic ducts; especially valuable in difficult cases (acute cholecystitis, Mirizzi syndrome)
• Liver surgery: Map segmental bile ducts; assess bile leaks post-resection

Advantage over intraoperative cholangiogram (IOC): Real-time continuous visualisation; no duct puncture; no radiation; can be repeated.

3. Sentinel Lymph Node Mapping

Peritumoral injection of ICG → lymphatic uptake → NIR camera identifies first-echelon (sentinel) nodes glowing green → targeted biopsy without formal lymphadenectomy.

Used in: breast cancer, gastric cancer, colorectal cancer, melanoma.

4. Hepatic Tumour Identification

Hepatocytes take up ICG and excrete it; tumour cells retain ICG longer → tumour nodules appear brighter than surrounding liver tissue. Used for detecting small HCC nodules and intrahepatic metastases during liver resection.

Principle

CT uses X-ray attenuation to generate cross-sectional images. The X-ray tube rotates around the patient; different tissues absorb X-rays differently (bone = white/high attenuation; soft tissue = grey; air/fat = black/low attenuation). Computer algorithms reconstruct 3D cross-sectional images from the acquired data.

Types of CT Scanners

Surgical Uses

Advantages

• High spatial resolution — excellent anatomical detail
• Fast and widely available
• 3D reconstruction possible for surgical planning
• Detects bone, soft tissue, and vascular lesions simultaneously

Disadvantages

• Ionising radiation — especially important in children and repeated scans
• Iodine contrast risk — nephrotoxicity, allergy
• Less sensitive than MRI for soft tissue characterisation
• Not portable; patient must go to scanner

Principle

MRI uses a strong magnetic field and radiofrequency (RF) pulses to align and then perturb hydrogen proton nuclei in tissue. As protons relax back to alignment, they emit RF signals that are detected and used to construct images. Different tissues have different T1 and T2 relaxation times, providing tissue contrast without ionising radiation.

Key MRI Sequences in Surgery

Surgical Uses

CT vs MRI

Contraindications to MRI

Absolute: Pacemaker/defibrillator; ferromagnetic aneurysm clips; cochlear implants; metallic foreign body in eye.

Relative: Claustrophobia; first trimester pregnancy; renal failure (gadolinium → nephrogenic systemic fibrosis); inability to lie still.

Definition

PET-CT is a hybrid imaging technique combining metabolic information from PET (Positron Emission Tomography) with anatomical detail from CT in a single scan, providing both functional and structural data.

Principle

PET component: Uses radioactive tracers (radiopharmaceuticals) that emit positrons (β¹ particles). Commonest tracer: FDG (Fluorodeoxyglucose) — a glucose analogue. Metabolically active cells (tumours, inflammation) take up more glucose → higher FDG uptake → bright on PET (SUV = Standardised Uptake Value).

CT component: Provides anatomical localisation of the metabolic hotspots detected by PET. Fusion of both images gives precise tumour location and extent.

Surgical Applications

Limitations

• Not reliable for HCC — variable FDG uptake (well-differentiated HCC = low uptake)
• False positives: inflammatory conditions, recent surgery, granulomas (sarcoid, TB)
• False negatives: mucinous tumours, well-differentiated carcinoids (low metabolic activity)
• High cost; limited availability; radiation exposure
• Requires fasting ≥6 hrs; blood glucose <8 mmol/L before injection

Principle

Ultrasound uses high-frequency sound waves (2–15 MHz for diagnostic imaging) emitted by a piezoelectric transducer. Sound waves reflect differently at tissue interfaces; returning echoes are detected and processed into real-time images. No ionising radiation.

Special Surgical Applications

Advantages

• No ionising radiation — safe in pregnancy and children
• Real-time, portable, bedside available
• Inexpensive and widely available
• Guides procedures (biopsy, drainage, central line insertion)

Limitations

• Operator dependent — significant learning curve
• Bowel gas and obesity hamper visualisation
• Poor for deep retroperitoneal or intracranial structures
• Limited soft tissue characterisation compared to MRI

Definition

Endoscopic Ultrasound (EUS) is a minimally invasive technique combining endoscopy and high-frequency ultrasonography (5–20 MHz) to produce high-resolution images of the GI wall and adjacent organs (pancreas, bile duct, lymph nodes, vessels). It can also be used for fine-needle aspiration (EUS-FNA) and therapeutic procedures.

Principle

An ultrasound transducer is built into the tip of a flexible endoscope. The endoscope is inserted into the GI lumen (oesophagus, stomach, duodenum, or rectum) → placing the probe in close proximity to the target organ. This minimises interference from air and bone, allowing high-resolution imaging of deep structures. Linear probes allow needle passage for real-time guided biopsy or drainage.

Types of EUS Scopes

Surgical Applications

EUS-Guided Interventions

• EUS-FNA / FNB: Tissue sampling of pancreatic masses, lymph nodes, submucosal lesions
• EUS-guided drainage: Pseudocyst cystogastrostomy; hepaticogastrostomy in biliary obstruction when ERCP fails
• Coeliac plexus neurolysis (CPN): Alcohol injection for pancreatic cancer pain via EUS guidance
• EUS-guided fiducial placement: Gold markers for stereotactic radiotherapy planning
• EUS-guided rendezvous: Combined with ERCP for difficult biliary cannulation

Advantages over Other Modalities

• Highest resolution for peripancreatic structures and GI wall layers
• Allows simultaneous tissue sampling (FNA/FNB)
• No radiation; real-time guidance
• Superior to CT/MRI for T-staging of GI tumours and small pancreatic lesions

Limitations

• Invasive (endoscopic procedure); requires sedation
• Expertise-dependent; steep learning curve
• Limited field of view compared to CT/MRI
• Cannot assess distant metastases