Flat Panel Technology
St. Luke’s Hospitals’ clinical experience during the first 1,000 procedures using the Innova 4100.
To view the figures related to this article, please refer to the print version of our November/December issue, page 34.
From 1960 to the present, image-intensifier technology has been a perfectly acceptable imaging modality for interventional radiology and has been used with great success. With this technology, physicians diagnose and treat a number of conditions such as blockages in arteries, aneurysms, and congenital malformations of the arteries and veins. Although image-intensifier technology was considered the “gold standard” for many years, it has become clear that advances have been made with digital flat panel detector technology to improve image capture, field of view, dynamic range, and dose management. Physicians are marrying their skills with these advances, and patients are benefiting in terms of faster minimally invasive techniques.
IMPROVING ON THE IMAGING STANDARD OF THE PAST
Although image-intensifier technology has many advantages, it also has multiple analog stages prior to digital conversion. Each component in the imaging chain degrades the x-ray image to some extent through the introduction of distortion, noise, artifacts, and nonuniformity (Figure 1). It is important to note that these stages are linked in series and information is lost or distorted at every stage, which adds up to significant loss and reduced image quality. An analogy for cumulative losses would be the transfer of water through a series of funnels (Figure 2), each accounting for some loss. Because of loss at every funnel stage, the total yield at the output is 87% of the original volume, even though each individual stage yields more than 93%. This problem becomes more apparent when imaging large patients or positioning the image intensifier at steep angles.
The resolution of the conventional system’s round detector is not uniform throughout the full field of view, which results in a “sweet spot” in the center of the image. This can result in difficulty visualizing small catheters and guidewires. The area of interest may require radiation bolus filtering and multiple acquisitions to gain diagnostic patient information. In addition, the circular field of view in the conventional image-intensifier system provides less coverage than the square field of view of a flat panel system.
Image-intensifier systems also have limited dynamic range. The system is unable to display the full range of densities, represented by many gray levels from thin to thick anatomical areas, which can lead to problems in imaging vessels over varied anatomical backgrounds. More radiation may be required to achieve optimal visualization, which has the potential to increase dose levels to the patient and staff. Many conventional systems have unintelligent dose management controls that use the same protocol for a specific anatomical part in patients of all sizes. Such protocols could provide higher-than-required doses to thinner patients.
NEW GOLD STANDARD
The Innova 4100 (GE Medical Systems, Waukesha, WI) is a large, digital, flat panel detector system (41 cm X 41 cm) dedicated to angiography and interventional procedures. The Innova 4100 received FDA 510(k) marketing approval in November 2002. St. Luke’s Hospital & Health Network in Bethlehem, Pennsylvania, which acted as the beta site for the Innova 4100, has performed more than 1,000 procedures with the system.
By eliminating most of the individual analog stages, the flat panel system reduces the loss of information from stage to stage (Figure 3), thus increasing the reliability of the image information transfer. The ability to transfer information from the point that the x-ray beam hits the detector is known as “detective quantum efficiency.” Use of this new flat panel digital technology results in lower dose and better image quality.
Larger Field of View
The resolution is constant over the entire panel, thus eliminating the need for panning or multiple runs. The square surface configuration of the flat panel provides approximately 40% more coverage. The “sweet spot” is in the entire field of view and provides a crisp, detailed edge even at the corners of the field (Figure 4).
Increased Dynamic Range
The dynamic range of the flat panel system is approximately five to 10 times greater than that of a conventional image-intensifier system, which allows interventionists to customize and improve visualization of the vasculature and view the placement and measurement of stents as well as other devices, such as vena cava filters, with greater precision (Figures 5 and 6).
To translate the benefit of image quality and contrast resolution of the system into patient benefits, protocols have been modified to offer less contrast agent loads, such as dilutions of 3:1, 2:1, and even a 1:1 ratio of contrast agent to heparinized saline (Figures 7A-C). These protocols have allowed for an approximate 30% reduction in contrast use, which is of significant importance for imaging high-risk patients, such as those with existing renal failure.
One of the most significant dose-saving features of the Innova 4100 is the technique of Auto-Exposure (AutoEx). With AutoEx, effective patient thickness is calculated in each exposure. The technique and spectral filtration are automatically chosen to provide the patient with the lowest dose while providing the physician with the image quality needed. Thus, for a given setting, the interventionist is able to obtain the most optimal dose for the patient.
The Innova 4100 has integrated spectral filters in the collimator, allowing the filtration of soft radiation out of the beam (known as hardening of the beam). This filtration helps to minimize skin dose to the patient and scatter radiation to staff while maintaining image quality and sharpness.
This system was developed to ensure reliability. According to Donald Norder, RN, CRN, manager, Interventional Radiology at St. Luke’s, “The room has not been down since the equipment was installed in August 2002. We have, of course, had planned downtime for initial installation and upgrades. As clinical evaluation continues, upgrades are expected and planned for accordingly.”
The GE Innova 4100 has allowed our department to reduce radiation dose and allowed for an approximate 30% reduction in contrast load. Also, more consistent imaging is possible due to the larger field of view, eliminating the need for multiple runs. The increased dynamic range allows physicians to place small catheters and stents with more precision and visualize detail not seen on conventional image-intensifier–based systems.
Hal Folander, MD, is Chairman of the Department of Radiology, St. Luke’s Hospital and Health Network, Bethlehem, Pennsylvania. He holds no financial interest in any product or manufacturer mentioned herein. Dr. Folander may be reached at (610) 954-4808; firstname.lastname@example.org.
Renee Martino, RN, CRN, is Director of Clinical Management, Department of Radiology, St. Luke’s Hospital and Health Network, Bethlehem, Pennsylvania. She holds no financial interest in any product or manufacturer mentioned herein. Ms. Martino may be reached at (610) 954-6080; email@example.com.