In the rush to convert imaging departments to digital equipment, fluoroscopy has sometimes been ignored. Is there any reason to spend the money to acquire digital fluoroscopy, which can cost as much as three times what one would spend on an analog installation? What benefits does it offer? This article provides some tentative answers to these questions.

Digital Fluoroscopy: Advantages

The move to digital imaging systems has several drivers, one of them being the automatic storage of images and the ability to send them anywhere, including to personal computers. This feature is of less interest in many fluoroscopy applications, which generally involve real-time procedure guidance. However, there also is the appeal of superior contrast as a result of 10-bit depth (ie, 1,024 gray-scale levels) and displays with spatial resolution as high as 3,000 x 3,000 pixels.

 At least as important for some fluoroscopy applications are the numerous special processing and display features available on digital equipment that increase the information content of the data. Pan/zoom, background noise reduction, adjustable contrast and brightness, edge enhancement, quantitative analysis of vessel diameter and stenosis severity, subtraction capabilities, roadmapping, and bolus chase are common.  One version of the latter is “stepping digital subtraction angiography,” in which the patient table moves past the x-ray tube to track contrast medium all the way down the legs. Some systems display a previously acquired roadmap side by side with the live image. Another popular feature for vascular work is the ability to rotate the images and create three-dimensional reconstructions, permitting multiple views with a single contrast injection. Such features reduce the examination time and the contrast load.

In view of the continued growth in complex fluoroscopy-guided interventional procedures, the feature that may prove most important in driving digital fluoroscopy purchases is the numerous ways the equipment can reduce the radiation dose. Reports of fluoroscopy-associated radiation injuries began flowing into the Food and Drug Administration beginning in 1992, and the agency issued its first user guidelines on the subject in 1994. 1 (The guidelines are now being revised.) As the agency noted at that time, “even typical [radiation] dose rates can result in skin injury after less than one hour of fluoroscopy.” In 1996, Thomas B. Shope, PhD, director scientist at the FDA Center for Devices and Radiological Health, described several serious radiation injuries associated with intervention, including a burn necessitating a large skin graft in a man who, during coronary angioplasty, received a radiation dose that was estimated to exceed 20 Gy. 2 Other experts note that during “long procedures, differences in doses of 8 Gy or more are possible for some combinations of operational techniques,” depending on machine settings and hence operator choices. 3 Moreover, certain patients, such as those with diabetes mellitus and those who have already undergone high-dose interventional procedures, may have far less than normal radiation tolerance. 4

Digital fluoroscopy can reduce the radiation dose through improvements in image quality and machine features. Better image quality and the ability to manipulate contrast and brightness reduce the number of images that must be obtained to complete a procedure. They also shorten procedure times and enhance patient throughput. Also valuable are the dose-reduction features being incorporated by manufacturers of digital fluoroscopy systems. Some of these features are quite basic, such as filters that prevent delivery of much of the “soft” (and not useful) radiation. Other features are more elaborate. One example is anatomic programming, with which the machine parameters can be set automatically to obtain images of the minimal quality necessary for a given region of the body, reducing the likelihood of inappropriate setting choice by the operator. Also available is software that monitors the intensity of the radiation striking the detector and within milliseconds adjusts the x-ray tube output and exposure time to optimize the image quality and minimize the radiation dose. With digital (recursive) filtering, part of an image is constructed from previous images, such that less radiation is needed to acquire the live image. Frame grabbing avoids the need for many spot films. Another useful feature is the real-time display of cumulative dosage information, so the operator is constantly made aware of how much radiation the patient has received. Some of these systems are capable of outputting the total radiation dose with the images into the patient’s medical record.

The most important dose-reduction feature available with digital equipment is variable pulsed fluoroscopy. Rather than delivering a constant x-ray beam to maintain the image on the screen, the pulsed mode delivers x-rays intermittently, with the most recent image being displayed until the next one becomes available (last image hold). This capability was described by one expert group 3 as having “the greatest potential for maintaining radiation exposure at low levels.”

An example of the dose savings available with variable pulsed fluoroscopy was provided at last year’s European Congress of Radiology by a team from the Danube Hospital in Vienna. Kristina Lomoschitz, MD, and her associates performed double-contrast barium enema studies on 70 patients using pulsed digital fluoroscopy equipment with 10-bit gray-scale capability and compared the radiation doses with those in 35 studies on older equipment with the same beam filtration and kVp settings but without pulse capability. 5 At 7.5 pulses per second, there was a 75% reduction in the radiation dose with pulsed versus continuous fluoroscopy; at 3 pulses per second, there was a 90% reduction in the dose. The fluoroscopy time averaged 102 seconds for the pulsed study and 251 seconds for the traditional study. Overall, the median dose area product was 56% lower with pulsed fluoroscopy, the total radiation dose being 580 cGy/cm2 versus 1,310 cGy/cm2 with the continuous-beam examination. Moreover, these radiologists rated the images obtained with the pulsed system significantly superior, even with low-resolution display. The team concluded that such digital systems are “likely to be used in all fields of fluoroscopy examinations.”

Implementing Digital

The Technology

The most popular type of digital fluoroscopy at present is an analog to digital converter (ADC) inserted into the imaging chain. The ADC samples the analog signal at regular intervals and converts it to binary numbers to store the image. For those who wish to purchase a truly digital fluoroscopy system, there are two choices: indirect and direct capture.

The indirect capture systems, including some flat panel displays that can be integrated into analog systems, are similar conceptually to the traditional film-screen technology. In one version, a cesium iodide scintillator captures the x-rays as they exit the patient and converts them to light. This light is turned into electronic signals by a matrix of amorphous silicon sensors. Each sensor corresponds to a single pixel of the image and is connected to a readout line. The signals from each cell in the matrix are read out in sequence row by row to obtain the image. High information transfer rates permit the display of moving images.

In direct capture or direct to digital systems, x-ray energy is not converted to light. Instead, it is captured by a thin film transistor matrix of a material such as amorphous selenium that changes it into electronic signals. No intensifying screen is required, and none of the energy is lost through scatter, as happens when x-ray energy is converted to light on its way to display of an image.
-J. Bronson

There are several options for the department that wishes to change to digital fluoroscopy. The simplest is to retrofit one’s analog system with some type of digital image-capture ability (see box), leaving most of the equipment in place. Other options are universal radiographic/fluoroscopy systems, some of which also are equipped with ultrasound capabilities. Some manufacturers have digital radiographic/fluoroscopic systems configured for particular specialties such as urology, and one sells a unit for cardiovascular studies having two C-arms, both capable of digital fluoroscopy. Not unexpectedly, the cost of the latter option is more than twice the cost of a retrofit.

Because of its lower radiation dose, digital imaging has been particularly appealing when fluoroscopy is required in children. At Massachusetts General Hospital, among others, all pediatric gastrointestinal and genitourinary fluoroscopy studies are now performed with digital equipment. M. Ikeda, MD, and associates of Nihon University School of Medicine in Tokyo found digital fluoroscopy valuable for examining infants and toddlers who were believed to have swallowed radiolucent foreign bodies. Although the specificity for the object itself was low, digital fluoroscopy was very sensitive to the distortions of the anatomy caused by the objects. 6 R. Hanas, MD, and colleagues from Uddevalla Hospital in Sweden used digital fluoroscopy to solve problems with indwelling subcutaneous insulin catheters in seven children aged 5 to 11 years. 7 In a presentation at the Radiological Society of North America meeting in 1999, Håkan Geijer, MD, and coworkers of Örebro Medical Centre Hospital, also in Sweden, reported that pulsed digital fluoroscopy provided images adequate for Cobb angle measurements in scoliosis patients with an effective radiation dose of 0.017 mSv versus 0.087 mSv for film-screen radiography. 8 R. Waugh, MD, and associates of South Cleveland Hospital in Cleveland, UK, used digital grid-controlled pulsed fluoroscopy for abdominal imaging in children and found a reduction in the total radiation dose of as much as 83% compared with standard radiography. 9 Those investigators also found that obtaining the necessary hard-copy images by frame grabbing rather than digital spot imaging provided additional dose savings with no loss of image quality.

Because of its speed and image quality, Brian D. Kavanagh, MD, and colleagues of the Medical College of Virginia sought to exploit digital fluoroscopy to guide the placement of low-dose-rate gynecologic brachytherapy devices. 10 Although there were some problems obtaining adequate images of very obese patients, in most cases, digital fluoroscopy permitted almost real-time assessment of device position, along with data suitable for dosimetric calculations. The set of four images needed for dosimetry could be stored on a single 3.5-inch floppy disk. These investigators commented on the versatility of the digital images, noting, for example, that images of suitable size could be printed on plastic transparencies and overlaid on simulations to design external-beam boost fields. They did caution that each fluoroscopy unit would require assessment of any intrinsic image distortion, as well as calibration according to standard radiographs, before use to guide brachytherapy.

Judith Gunn Bronson, MS, is a contributing writer for Decisions in Axis Imaging News.


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