The 3D revolution hasn’t just taken over Hollywood. Access to volumetric 3D imaging of the heart is changing the way interventional procedures are performed, particularly in patients with challenging and complicated anatomy. Meanwhile, the rapid adoption of 3D transesophageal echocardiography is a result of its usefulness to surgeons, but what will be its ultimate role?
The next wave in 3D
Cardiac catheterization is the standard for assessing anatomy and physiology in congenital heart disease (CHD), and fixed projection angiography is typically used to guide congenital cardiovascular interventions. There are drawbacks, however, to the effectiveness of fixed projection angiography, including limitations in soft tissue visualization and imprecise characterization of segmental branch pulmonary arteries, coronary arteries and other complex structures.
“Understanding...three-dimensional anatomy becomes absolutely key in performing these procedures appropriately.”
|Jamil Aboulhosn, MD, director of the Ahmanson/UCLA Congenital Heart Disease Center|
Enter rotational angiography, which has advanced cardiac catheterization by enhancing imaging. In contrast to fixed projection 2D angiography, rotational angiography swings the camera 240 degrees around the patient over four seconds at 30 frames/second during expiratory breath-hold, according to Thomas E. Fagan, MD, of Children’s Hospital Colorado in Aurora.
“With one angiogram, by viewing these structures in such different projections, we start to gain a lot more information depending on the type of structure,” says Fagan.
Studies have shown the technique provides diagnostic quality imaging without increasing contrast or radiation exposure (JACC Cardiovasc Imaging 2010;3:1149-57). It can aid clinicians by revealing the relationship between adjacent structures and helps when carrying out procedures with a risk of serious complications.
Fagan breaks rotational angiography down into three modalities, the first being a single, standard 2D angiogram. The 2D x-ray projection images from the rotational angiogram can then be reconstructed into a 3D image volume set, similar to a CT scanner. The third modality is multiplanar reformatting, which is the simultaneous display of 2D image slices from several orientations simultaneously, allowing the user to scroll through the reconstructed volume one slice at a time.
Recently, Fagan and colleagues evaluated the overall diagnostic utility of all three rotational angiography modalities. A pair of reviewers assigned to each modality subjectively rated the utility compared with standard angiograms, and in more than 60 percent of cases, more information was gained using rotational angiography. In 80 percent of cases, it was at least as useful as the standard.
Next steps involve looking at those 20 percent of cases where there was no added benefit and determining when the technology should be applied. There are also some limitations with temporal distortions caused by cardiac motion. Logistically, having the camera in motion requires forethought when setting up a procedure. “You’ve got a very large piece of equipment moving very quickly around a patient,” says Fagan, which means extra care must be taken to make sure people along with cables, tubes and other equipment don’t interfere with the camera’s movement.
The 3D roadmap
3D rotational angiography, CT or MR images can be integrated with fluoroscopy to provide a roadmap to help interventionalists. “Once you bring [the 3D image in] and you step on the fluoro, the 3D image will appear right over the structures that we intend to work with,” says Fagan.
Jamil Aboulhosn, MD, director of the Ahmanson/UCLA Congenital Heart Disease Center, and colleagues have been pushing the envelope on the utilization of 3D overlays, and he explains the leap in technology is akin to the advancement in video games from pixelated Pac-Man in the 1980s to the realistic 3D games of today.
These 3D techniques are invaluable in cases of patients with unusual anatomy such as an upside-down heart or in people born with single ventricle defects. “For all of these things, traditional imaging and assumptions are challenged,” says Aboulhosn. “Understanding roadmaps and three dimensional anatomy becomes absolutely key in performing these procedures appropriately.”
Without the overlay, performing cath lab procedures can be like guiding a wire through shadows and shades of grey. Bringing in a 3D model is similar to dropping the roadmap and icons over the satellite images in Google Maps, though lining up the image takes a highly trained eye.
“If you get it right, it’s fantastic…but you have to take the time to try and get it right and make sure the model is the right size, to make sure the structures are in the right place [in each model],” says Aboulhosn.
The 3D models do not move along with the heart, though techniques are in development that could involve dynamic CTs to help match the movement on the model to the x-ray motion screen.
Tiny ultrasound, big potential
Another 3D whiz kid on the horizon that promises to aid in interventional procedures is the development of a catheter that can generate live, streaming 3D ultrasound images from inside the heart. Called a live volumetric imaging (LVI) intracardiac catheter, the technology could improve transcatheter valve therapies and cardiac ablation for arrhythmias.
David E. Dausch, PhD, senior research engineer at RTI International, Research Triangle Park, N.C., where the technology was developed, says the main innovation is a new type of matrix transducer array called a piezoelectric micromachined ultrasound transducer.
“This allows us to actually take the ultrasound transducer array and miniaturize it to the point where we can fit it inside an intracardiac catheter,” says Dausch.
LVI offers wider field of view than existing intracardiac echo catheters and because the 3D view is generated from within the heart, this means better resolution and less shadowing due to calcification compared with transesophageal echo. Dausch adds that 3D transesophageal echo requires general anesthesia because of the large probe running down the esophagus and an echocardiographer in the operating room to drive the device and run the imaging. LVI, because it’s a catheter device, would put imaging back in the hands of the interventionalist. This all results in reduced costs for a procedure, says Dausch.
Dausch and colleagues presented a feasibility study of the device in a porcine model at the 2013 American College of Cardiology annual meeting in San Francisco. The device also received a Cardiovascular Innovation Award at the 2013 Cardiovascular Research Technologies Annual Symposium in Washington, D.C.
Between the adoption of 3D echo and the continued development of rotational angiography, 3D overlays and 3D ultrasound, the multidimensional picture of the heart is becoming clearer by the day.