4.3 Non-Radiographic Imaging: Cardiac CT/MRI, Image Fusion & Ultrasound-Guided Access
Key Takeaways
- Task B15 requires acquiring and interpreting both radiographic and non-radiographic images, naming MRI, ultrasound, and the 3D mapping system explicitly.
- Cardiac CT is acquired at 60-80 percent of the R-R interval for a rate-controlled patient and defines pulmonary vein ostial anatomy, common ostia, and left atrial appendage morphology.
- Late gadolinium enhancement on cardiac MRI identifies ventricular scar and border zone that predicts the ventricular tachycardia substrate before the catheter enters the chamber.
- Ultrasound-guided venous access reduces access-site complications and confirms vein compressibility, patency, and the artery-vein relationship before the needle moves.
- An MRI-conditional CIED system requires the generator and every lead to be labelled conditional, with the device placed in an MRI mode before scanning and restored afterward.
4.3 Non-Radiographic Imaging: Cardiac CT/MRI, Image Fusion & Ultrasound-Guided Access
CCI task B15 — Acquire/interpret radiographic and non-radiographic (e.g., MRI, ultrasound, 3D mapping system) image is explicit that the RCES specialist works with more than fluoroscopy. Modern EP practice front-loads anatomy into cross-sectional imaging and then imports it into the mapping system, so the catheter time in the chamber is spent confirming rather than discovering.
1. Cardiac Computed Tomography
Acquisition principles. A contrast-enhanced, ECG-gated cardiac CT reconstructs the heart at a chosen point in the cardiac cycle.
| Parameter | Typical practice | Rationale |
|---|---|---|
| Gating phase | 60-80% of the R-R interval (mid-to-late diastole) | Least cardiac motion in a rate-controlled patient |
| Heart rate target | Often < 65 bpm, beta blockade if needed | Reduces motion blur and permits prospective gating |
| Contrast | Iodinated, timed by bolus tracking to the left atrium | Opacifies the chamber of interest |
| Reconstruction | 0.5-0.75 mm slices, then 3D segmentation | Enables surface-model export |
In atrial fibrillation the R-R interval varies, so a prospectively gated scan may misfire; retrospective gating with dose modulation or a wide-detector single-beat acquisition is used instead, at a higher dose cost.
What CT answers before an ablation:
- Pulmonary vein anatomy — number, ostial dimensions, and variants. A left common ostium occurs in roughly 20-30% of patients and changes both catheter selection and cryoballoon strategy; a right middle vein is a common accessory.
- Left atrial appendage morphology for closure planning — chicken-wing, windsock, cactus, cauliflower — plus ostial dimensions and landing-zone depth.
- Thrombus exclusion in the appendage (though transesophageal echocardiography remains the reference standard).
- Esophageal course relative to the posterior left atrial wall.
- Left atrial volume and wall thickness, which predict ablation outcome.
- Coronary and phrenic relationships to planned lesion sets.
2. Cardiac Magnetic Resonance
MRI's contribution is tissue characterization, which CT cannot provide.
| Sequence | What it shows | EP application |
|---|---|---|
| Late gadolinium enhancement (LGE) | Fibrosis and scar (gadolinium accumulates in expanded extracellular space and washes out slowly) | Ventricular tachycardia substrate: dense scar core, border zone, and conducting channels |
| Cine steady-state free precession | Wall motion, volumes, ejection fraction | Cardiomyopathy assessment, device candidacy |
| T1/T2 mapping and T2-weighted | Edema, inflammation, infiltration | Myocarditis, cardiac sarcoidosis, amyloid |
| MR angiography | Vascular anatomy | Pulmonary veins without iodinated contrast |
The scar distribution pattern is itself diagnostic and directs the mapping approach:
| Pattern | Typical cause | Mapping implication |
|---|---|---|
| Subendocardial or transmural in a coronary territory | Ischemic infarction | Endocardial substrate mapping usually sufficient |
| Mid-wall septal stripe | Non-ischemic dilated cardiomyopathy | Deep septal substrate; may need bipolar or septal approaches |
| Subepicardial lateral | Myocarditis, sarcoid, Chagas | Anticipate an epicardial approach |
| Patchy multifocal with edema | Active sarcoidosis | Treat inflammation; substrate may evolve |
A pre-procedure LGE map lets the team predict where late potentials and channels will be found, and the segmented scar can be imported into the electroanatomic map as a shell overlay.
3. Image Integration and Fusion
Segmentation converts a CT or MR volume into a 3D surface model of the chamber of interest. Registration aligns that model to the patient's real-time position within the mapping system's coordinate space.
| Registration method | How it works | Weakness |
|---|---|---|
| Landmark (fiducial) registration | Match discrete anatomic points — vein ostia, appendage, CS os — acquired with the catheter to the same points on the model | Depends on the accuracy of each point |
| Surface registration | Least-squares fit of a cloud of acquired points to the model surface | Requires broad, even sampling |
| ICE-based | Contours traced from intracardiac echo build or align the shell in real time | Operator-dependent tracing |
Sources of registration error are heavily tested because they are the practical failure mode: the CT was acquired days earlier in a different rhythm (sinus versus AF), at a different volume status, at a different respiratory phase, and in a different body position. A shell registered in sinus rhythm will misalign once the patient is in atrial fibrillation with a dilated atrium. Registration is verified — not assumed — by touching a known landmark and confirming the catheter icon lands on it, and it is re-verified after any patient movement, cardioversion, or large fluid administration.
The reference patch or reference catheter must not move; if it does, the entire acquired geometry shifts, producing duplicate or displaced anatomy.
4. Ultrasound-Guided Vascular Access
Ultrasound guidance has become the default for femoral access in the EP lab because it converts a landmark-and-palpation procedure into a visualized one.
Distinguishing artery from vein in the short axis:
| Feature | Vein | Artery |
|---|---|---|
| Compressibility | Collapses with light probe pressure | Remains round, resists compression |
| Pulsatility | Non-pulsatile (may transmit) | Pulsatile |
| Wall | Thin | Thick, echogenic |
| Doppler | Low-velocity, phasic with respiration | High-velocity, pulsatile |
| Response to Valsalva | Distends | No change |
| Position at the groin | Medial to the artery | Lateral |
Technique. Sterile probe cover and sterile gel; identify the common femoral vein above the bifurcation and below the inguinal ligament; use a short-axis (transverse) view for orientation and a long-axis view to track needle depth; advance under continuous visualization until the tip tents and then enters the anterior wall; confirm the wire is intraluminal in long axis before dilating.
Value: reduced inadvertent arterial puncture, fewer attempts, fewer hematomas, and reliable success where anatomy is difficult — obesity, prior scarring, chronic occlusion, or a small vein. Ultrasound also detects an anatomic variant such as an anteriorly overlying artery, chronic femoral vein thrombosis, or an unexpectedly high bifurcation before the needle is used, which no landmark technique can do.
5. MRI in Patients With Cardiac Implantable Electronic Devices
Historically an absolute contraindication, MRI is now routinely performed under controlled protocols. The distinction the exam tests is between MRI-conditional and legacy systems.
MRI-conditional requires that every component — generator and every lead — is labelled MRI-conditional by the manufacturer, and that the specific scanner field strength, gradient, and SAR conditions in the labelling are met. A conditional generator with one legacy lead is not a conditional system. An abandoned or capped lead makes a system non-conditional because it cannot be programmed and can heat at its free end.
| Hazard | Mechanism | Mitigation |
|---|---|---|
| Lead-tip heating | RF energy couples into the lead, concentrating at the electrode | Conditional lead design; SAR limits; scan-region restrictions |
| Oversensing / inhibition | Gradient and RF noise interpreted as intrinsic activity | Program asynchronous pacing in a dependent patient |
| Inappropriate ICD therapy | Noise detected as ventricular arrhythmia | Suspend tachytherapy for the scan |
| Force and torque | Static field acting on ferromagnetic components | Modern components are largely non-ferromagnetic |
| Reed switch behavior | Static field may close a magnet-sensitive switch | Conditional devices use Hall sensors and a defined MRI mode |
Workflow: interrogate and document baseline parameters; confirm the patient is not pacing-dependent, or program to an asynchronous mode if they are; enable MRI mode; monitor continuously with ECG and pulse oximetry during the scan with trained personnel and a resuscitation plan; then exit MRI mode and re-interrogate, restoring the original programming and verifying thresholds, sensing, and impedances. Leaving a device in MRI mode after the scan is a documented and preventable adverse event.
A pre-ablation cardiac CT for atrial fibrillation was acquired three weeks earlier while the patient was in sinus rhythm. In the lab the patient is in atrial fibrillation, and after landmark registration the mapping catheter icon sits 8 mm outside the segmented left atrial shell at the left superior pulmonary vein. What is the most likely explanation?
During ultrasound-guided femoral access the specialist sees two circular structures in the short-axis view. Light probe pressure collapses the medial structure completely while the lateral structure stays round. What do these findings establish?
A patient with an MRI-conditional pacemaker generator and one MRI-conditional right ventricular lead also has an abandoned, capped right atrial lead from a prior system. The patient needs a lumbar spine MRI. How should the system be classified?