Cardiac CT: Looking Beyond the Coronaries

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Transversal (small image left) and oblique (large image) images from a CT scan demonstrating cardiac perforation of the right ventricular lead (black and white arrows). Source: Clin Res Cardiol 2009; 98(8):517–520.

Advances in CT scanner hardware and software make it possible to image patients while delivering less than 5 mSv of radiation dose, which opens broader pathways for CT to be used for structural heart and electrophysiology (EP) indications.

CRT & LVADs

Up to 30 percent of patients who receive a cardiac resynchronization therapy (CRT) device do not respond appropriately. Researchers have been trying to determine anatomical and/or physiological characteristics that are predictive of device failure and CT imaging could play a prominent role.

Uebleis et el used PET/CT to determine that lead placement in relation to scar tissue was the best predictor of device success. The next step, they said at the 2010 SNM meeting, is to evaluate patients prior to therapy to determine the optimal location for lead placement.

Quynh A. Truong, MD, a cardiologist specializing in CT imaging at Massachusetts General Hospital (MGH) in Boston, and colleagues found that CT-derived changes in left ventricular (LV) wall thickness correlated best with the extent of dyssynchrony seen in heart failure patients with wide and narrow QRS, followed by LV wall motion and then LV volume (J Am Coll Cardiol Img 2008;1:772-781).

They have since begun a trial to determine which metric correlates best with clinical outcomes for device therapy. “The ideal CT exam would be done before implantation and would assess the pulmonary veins, evaluate the ventricles for dyssynchrony to target the area that is most delayed and identify scar tissue to help facilitate lead placement,” says Truong.

CT also can help assess the position and orientation of left ventricular assist devices (LVADs) after implantation, says Wm. Guy Weigold, MD, director of cardiac CT at the Washington Hospital Center (WHC) in Washington, D.C. “Patients can experience problems as to how the device is situated in the heart and CT can help us determine if it is perhaps rubbing against the LV wall or if there is a flow obstruction.”

In patients with longer-term LVADs who are going onto a heart transplant, CT can reveal whether there are adherent structures at risk for trauma when opening the chest. “CT allows us to look at the devices moving in real time during the cardiac cycle and detect evidence of adhesions,” he says.

Atrial fibrillation

Patients scheduled for atrial fibrillation (AF) ablation at WHC undergo either CT or MRI prior to the procedure to ascertain variances in pulmonary veins and related structural anatomy, Weigold says. Having the option to use either modality allows referrers and imagers to choose the appropriate technique based on patient characteristics. Patients with devices that are not MRI compatible can undergo CT, while those with compromised kidney function who are unable to tolerate iodinated contrast media can undergo MRI. Recently, however, the risk of nephrogenic systemic fibrosis from gadolinium contrast media puts imagers on alert for utilizing MRI in certain patients with severely compromised kidney function.

The use of CT to image the pulmonary veins is quite mature, Weigold offers. In fact, most coronary artery postprocessing programs have an EP package for pulmonary vein analysis. Pre-procedural CT imaging can reveal variances in the pulmonary veins, as well as in the atrial septum, which would inform left atrium access. EPs can then integrate these images with electrical signal mapping and fluoroscopy images, giving them a 3D view of the heart during ablation.

Using CT prior to AF ablation typically includes viewing 3D volumetric images, in contrast to coronary CT, which often utilizes axial and curved reformatted images, says Jeffrey M. Schussler, MD, an interventional cardiologist at Baylor University Medical Center in Dallas. “The relatively larger pulmonary veins are beautifully displayed in 3D volumetric images, giving electrophysiologists an excellent overview of their relationship to other cardiac structures.”

AF ablation patients can sometimes present postoperatively with respiratory symptoms that frequently mimic more common diseases. “The most frequent cause, however, is pulmonary vein stenosis and CT is an excellent test to detect or exclude stenosis,” Schussler says. “While 3D volumetric images provide an overview of the pulmonary