Fluoroscopic imaging is valued as a useful tool for practitioner guidance during surgery or other interventional or diagnostic procedure. Using fluoroscopic apparatus, the practitioner can obtain real-time video feedback on aspects of a surgical procedure, on positioning of tubing or other hardware, flow of a contrast agent, or activity of a particular organ. In conventional fluoroscopic imaging systems, the radiation source and detector are mechanically coupled, so that their position, relative to each other, has a rigid, fixed geometry. Alignment of the x-ray source to detector is not adjustable to the operator, but is fixed by the imaging system mechanics.
The advent of portable digital radiography (DR) detectors advances radiographic imaging and makes it possible to improve patient access to imaging services, such as in situations where it can be risky or awkward to move the patient for a procedure. An area where this can be of particular utility is in the intensive care unit (ICU), where multiple support systems may need to be used for a particular patient.
One difficulty with the use of portable DR detectors relates to data on relative source-to-detector positioning, which is no longer inherently provided by the imaging system. Aspects of positioning that are of particular importance for fluoroscopy include positioning the source so that it is perpendicular with the detector and field limitation, controlling the radiation field size and direction so that the primary radiation field lies fully on the detector.
Various source-detector alignment approaches have been proposed for systems having the DR detector mechanically decoupled from the source. These approaches have included the use of various instruments to detect skew, orientation, source-image distance, and other aspects of positioning for these components. Other approaches have included the use of an initial exposure that directs a pattern of alignment beams toward the detector for sensing and calculation of skew error. While there is some merit in such approaches, however, they are largely directed to portable radiography itself, rather than to specific requirements of fluoroscopy. Characterized by lower levels of radiation and real-time display of image content at lower resolution, fluoroscopy is a guidance tool that helps the practitioner to visualize the progress of a procedure rather than to diagnose the condition of internal bone or tissue.
There are a number of considerations and requirements of conventional radiographic practice that are poorly suited for the fluoroscopy environment. This can be appreciated, for example, in considering differences between serial radiography, in which a series of exposures is acquired in a timed sequence using standard radiographic strictures, and fluoroscopy, in which a series of exposures is rapidly acquired under very different working conditions and generally at lower dosage and, consequently, lower resolution. In serial radiography, the radiographer and any attending staff move away from the imaged subject until the series of exposures completes and the images are generated and displayed. In fluoroscopy, on the other hand, the sequence of exposures is acquired and displayed with the practitioner and staff positioned closely about the patient and, consequently, very near to the radiation field.
Distinctions between radiography and fluoroscopy environments and practices have been recognized by regulatory agencies, along with an awareness of the advantages and risks of portable detector use. As one result, there are different requirements for beam accuracy in light of these differences.
FIG. 1 summarizes some of the separate regulatory requirements that have been adopted for radiographic and fluoroscopic devices. Under the radiography regulations certain exemptions are given for devices using portable detectors. The exemptions relate to risk/benefits concerns for the patient, obtained when source and detector are uncoupled, which enables imaging at a beside but limits the ability to meet certain regulations. Specifically, exemptions deal with field limitation and radiation path perpendicularity. As such, there are challenges, when using fluoroscopy with a portable detector, in meeting equivalent requirements related to conventional fluoroscopic regulations.
It may not always be possible to fix the relative positions of source and detector relative to the patient for the full fluoroscopy sequence. For example, in ICU and other environments in which fluoroscopy is used, there can be unwanted movement by the patient and the need for readjustment of source position during the imaging session. Repositioning may alternately be needed in fluoroscopy for various reasons, such as when the source or its supporting structure is accidentally jostled and shifted in position during a procedure.
Thus, it can be recognized that there is a need for solutions that meet regulatory guidelines for beam coverage and perpendicularity with portable DR detectors used in fluoroscopy applications. Of particular concern is the need to provide fluoroscopy using portable DR detectors while limiting exposure to the treatment region and away from the attending medical team and from other regions of patient anatomy.