Electrophysiologic Applications of Cardiac CT

Cardiac CT Imaging in Patients with Atrial Fibrillation

Cardiac CT (CCT) scanning remains problematic in atrial fibrillation (AF) in several ways because of inadequate temporal resolution:

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  The heart rate tends to be higher in patients with AF ( Table 22-1 ).

  TABLE 22-1

  CTA Characteristics of Patients in Sinus Rhythm and in Atrial Fibrillation

  Data from Wolak A, Gutstein A, Cheng VY, et al. Dual-source coronary computed tomography angiography in patients with atrial fibrillation: initial experience. J Cardiovasc Comput Tomogr . 2008;2(3):172-180.

  	ATRIAL FIBRILLATION ( n = 24)	SINUS RHYTHM ( n = 119)	P
  Pre-scan heart rate	54 ±14 bpm	62 ±112 bpm	<.01
  Postinjection heart rate	103 ± 30 bpm	71±17 bpm	<.01
  Nondiagnostic CTA studies	8%	10%	ns
  Good or excellent CTA image quality	54%	79%	.01
  Radiation exposure	26 ± 8 mSv	8 ± 4 mSv	<.01

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  The cardiac cycle length is always irregular.

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  The increment in heart rate is greater during contrast injection and during image acquisition.

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  Opacification of the left atrial appendage (LAA), which usually is dilated and nearly effectively stagnant, is delayed well beyond that of the left heart cavities and coronary arteries, potentially resulting in pseudothrombus in the left atrium, the result of incomplete opacification of the LAA due to enlargement of the LAA and altered flow characteristics in the left atrium due to AF ( Table 22-2 ).

  TABLE 22-2

  Left Atrial Appendage Volume and the Incidence of Pseudothrombus

  Data from Saremi F, Channual S, Gurudevan SV, Narula J, Abolhoda A. Prevalence of left atrial appendage pseudothrombus filling defects in patients with atrial fibrillation undergoing coronary computed tomography angiography. J Cardiovasc Comput Tomogr . 2008;2(3):164-171.

  	ATRIAL FIBRILLATION	SINUS RHYTHM
  Left atrial appendage volume	15 ± 7 mL	7 ± 3 mL	0.02
  (Proven) pseudothrombus in the left atrial appendage during CTA without delayed imaging	71%	0

With different cardiac cycle lengths, multicycle/segmented acquisition image quality is seriously compromised. Using 64-slice CCT, overall only half (54%) of coronary CT angiography (CTA) image studies of AF are of adequate or good quality. Use of retrospective imaging at full tube output increases radiation exposure. Although feasible, cardiac/coronary imaging during AF with CT scanning requires further development to obtain increased yield and to lessen radiation exposure.

Structural noncoronary studies such as pulmonary venous and coronary venous anatomy studies are more likely than coronary CTA studies to be adequate.

However, using 320-slice CCT, the large majority of cases can be adequately imaged despite the presence of AF ( Table 22-3 ).

The distal left circumflex artery appears to pose the most problems in evaluating for the presence and severity of coronary artery disease (CAD).

Identification of Left Atrial or Left Atrial Appendage Thrombus

The presence of underlying AF challenges the accurate detection of atrial appendage thrombi. In contrast to patients in sinus rhythm, the variable cardiac cycle length and greater and more variable heart rate increase in response to the stimuli of scanner acceleration and contrast injection of patients in AF excessively tax the ability of 64-slice CT scanners to accurately assess the LAA. The left atrium, whose imaging is less affected by AF than is that of the appendage, is more readily assessed for the presence or absence of thrombus. The stagnation of flow within the LAA necessitates a delayed scan at 2 or more minutes post-injection, with additional radiation exposure.

Stagnation of flow within the LAA is a major confounder of assessment of the LAA for thrombosis. The delayed clearance of non–contrast-enhanced blood within the LAA has established the high incidence of false positive studies without delayed images. For example, Martinez et al. found that in 402 consecutive patients undergoing non–ECG-gated CCT and transesophageal echocardiographic (TEE) pre-pulmonary venous isolation, CCT identified 40 LAA filling defects, of which only 9 were confirmed by TEE. False negatives also are common; for example in Tang et al.’s study, only 4 of 10 thrombi seen on TEE were seen on CCT. Some false-positive defects persist even after delay.

There are few data to this point, but small early studies have shown correlation between TEE and contrast cardiovascular CT (CVCT) measurements of LAA size and correlation of findings of thrombi larger than 7 mm. Higher heart rates and irregular rhythm (the norm in AF) challenge feasibility at this point. ECG gating is important. CVCT is unable to show left atrial or appendage spontaneous contrast.

Small accessory LAAs are recognized by 64-slice CCT, but these, due to their novel recognition, are still of unknown significance. Accessory LAAs have been reported in 10% to 15% of the adult population undergoing cardiac CT ( Figs. 22-1 through 22-4 ).

Pre-Procedure Cardiac Mapping: Assessment of Pulmonary Vein Anatomy for Planning of Pulmonary Vein Ablation for Atrial Fibrillation

Atrial fibrillation usually is initiated by ectopic activity within the proximal pulmonary veins. Catheter-based techniques, in which access to the left atrium occurs via a transseptal puncture of the interatrial septum, are able to deliver radiofrequency current (or cryotherapy) to the sites that have been identified by mapping techniques as responsible for AF, electrically isolating them from the remainder of the atria. The usual success rate in achieving sinus rhythm at 1 year with two or three attempts is 75% to 85%—a marked improvement compared to medical therapy, which affords about 20% success.

Anatomic features are not adequate to identify the site responsible for AF.

Conventionally, real-time retrograde injection of contrast dye into the left atrium has been used during the procedure to visually guide catheter placement. However, opacification of the right inferior pulmonary vein is difficult to achieve by reflux venography. Other modalities such as intracardiac echocardiography and TEE are less robust than reflux venography, which is less robust than CCT in detecting normal and variant anatomy (e.g., supernumerary and common ostia and branch variation) ( Table 22-4 ).

Variations in pulmonary venous anatomy are fairly common. The most common variant is a separate right middle pulmonary vein. Pulmonary venous variants include:

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  Three to five right-sided ostia (28%)

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  One or two separate middle lobe pulmonary venous ostia (26%)

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  A single right ostium on the right side (2%)

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  A single left-sided ostium (14%)

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  A right superior supranumerary pulmonary vein (2.2%), which passes behind the bronchus intermedius and drains mainly the posterior segment of the right upper lobe, but also receives a few subsegmental branches from the superior segment of the right lower lobe (mean diameter: 5.1 mm)

Factors Affecting Pulmonary Venous Size

The size (diameter) of pulmonary veins depends on:

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  Volume status

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  Respiratory phase (inspiration decreases pulmonary venous return and size). Expiratory phase images provide a better match to MRI depiction of pulmonary veins.

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  The phase of the cardiac cycle (by 25%–45%):

  • Maximal size of pulmonary veins: 35%

  • Minimal size of pulmonary veins: 85%

The use of image guidance during pulmonary venous isolation/ablation procedures reduces fluoroscopy times and increases success.

CCT has become the principal noninvasive diagnostic test to depict venous anatomy of the heart for the following reasons:

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  Especially when compared with TEE, intracardiac echocardiography, and reflex venography, it is less operator dependent.

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  Unlike cardiac MR (CMR), it can be performed on patients with pacemakers and implantable cardioverter defibrillators.

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  Its results are reproducible.

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  It can depict the thickness of the wall of the pulmonary veins.

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  It can characterize the lung parenchyma for complications such as pulmonary edema and infarction.

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  It can image the mediastinum both before and after ablation. This can assist in visualizing the relation of the esophagus to the left atrium and in detecting the presence of large intrathoracic hiatus hernias.

CCT can offer several forms of depiction of pulmonary venous and left atrial anatomy that may be useful to interventional electrophysiologists before they undertake catheter-based ablation procedures and to interventional cardiologists:

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  Three-dimensional (3D) reconstruction of surface views depicting the coronary venous structures on the myocardium

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  “Endoscopic” views to depict pulmonary veins from a perspective within the left atrium

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  3D views of pulmonary veins and the left atrium, with or without the remainder of the heart

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  Diagnostic imaging of pulmonary vein stenosis, achieved with multiplanar reconstructions, maximal intensity projections, and volume-rendered images

Examples of normal pulmonary veins are shown in Figures 22-5 and 22-6 .

The size of pulmonary veins is variable and is influenced by several factors ( Table 22-5 ).

TABLE 22-5

Factors Influencing Size of Pulmonary Veins

LARGER	SMALLER
Chronic long-term atrial fibrillation	Chronic short-term atrial fibrillation
Chronic atrial fibrillation	Paroxysmal atrial fibrillation
Larger LA	Smaller LA
Mitral disease	No mitral disease
LV disease	No LV disease
Volume loaded	Volume depleted
Male	Female

LA, left atrium; LV, left ventricle.

There is a wide range of variant pulmonary venous anatomy, and image examples of variant pulmonary venous anatomy are seen in Figures 22-7 through 22-12 .

Impact of respiration on pulmonary venous morphology:

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  Inspiration:

  • Increases the intervein distance (2–4 mm)

  • Increases the angle between (“splays”) the veins

  • Decreases the diameter of the veins

Impact of the cardiac cycle on pulmonary venous ostia:

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  Enlarges at end-systole versus end-diastole (35–45%)

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  Maximum diameter, on average, at 35% RR interval

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  Minimum diameter, on average, at 85% of RR interval (atrial contraction post–early diastolic emptying into the LV)

In pre-ablation cases, CCT affords more imaging information than does CMR, albeit at the expense of increased radiation exposure ( Table 22-6 ).

TABLE 22-6

Comparison of CMR and CT Pulmonary Venograpphy

	CMR	CTA
Radiation dose (mSv)	0	0.7–6
Spatial resolution (mm)	1.5–3	0.5–0.7
Gating/motion compensation	None	May be ECG-gated
Evaluation of lungs	Limited	Excellent, within the obtained FOV
Evaluation of mediastinum	Limited	Excellent, within the obtained FOV
Evaluation of coronary artery anatomy	Poor	If gated, especially if 320-slice CCT, excellent
Evaluation of sinoatrial coronary branch	None	If gated, and heart rate is low, can be imaged

CCT, cardiac computed tomography; CMR, cardiac magnetic resonance; CTA, computed tomographic angiography; ECG, electrocardiographically, FOV, field of view.