Applied Radiology

 Dr. Towbin is Radiologist-in-Chief, and Mr. Owen is PACS Administrator, Department of Radiology, Phoenix Children’s Hospital, Phoenix, AZ.

In the past three decades, the practice of radiology in general, and pediatric radiology in particular, has been transformed by imaging technology. Ultrasound, computed tomography (CT), and magnetic resonance imaging (MRI) have all contributed immensely to the care of children and led to a deeper understanding of both normal anatomy and disease processes. However, there has been no greater impact on pediatric radiology than the development of digital radiography (DR).

Plain radiography has evolved considerably over the past 20 years. Film screen radiography was the standard—the diagnostic centerpiece—of radiology departments for decades. By today’s standards, the technology was not too expensive and was able to create diagnostic images of good to excellent quality. But as technology advanced it became clear that there were several issues, including the need for film processing with the associated processing equipment, a dark room, chemicals and dedicated darkroom personnel. As a result, throughput was slow, repeat rates at times exceeded 10%, and the pressure was on the technologists to restrain, position, and make exposures that minimized motion artifacts in children who could be crying and/or unwilling to cooperate.

In 1985, computed radiography (CR) was introduced, providing an alternative to film-screen radiography. CR was able to use existing x-ray equipment to create and retain an image on a phosphor plate. Once exposed, the CR cassette was put into a reader, where a laser scanned the plate and converted the analog (A) image into a digital (D) format. This A-to-D conversion changed plain film radiography. The digital image could be fed into a computer and displayed on a PACS for review and interpretation. This simplified and decreased the expense of the entire process, since no photographic development was needed; film processors, dark rooms and associated personnel also were no longer necessary. This technology was widely accepted and utilized by radiology departments around the world. Once in a digital format, the images could be post-processed in a variety of ways to improve the diagnostic abilities of the radiologist and to promote rapid distribution of the imaging study to be immediately available to local and wide-area networks. In addition, once digitized, the images were immediately available on PACS and could be reviewed by the pediatric radiologist, who could assist the technologist with difficult cases and more rapidly provide a final reading to physicians caring for the child. The shortened turnaround time from image production to final reading improved patient care and radiology workflow, leading to customer satisfaction and potentially increased business.

Definitions of “DR”

The term ‘DR’ has two meanings in medical imaging. The first is “digital radiography,” which includes all methods of image acquisition, resulting in an image that can be displayed in a digital format. The hierarchy of digital radiography is divided into two major categories usually abbreviated as ‘CR’ and ‘DR’. This second use of the abbreviation ‘DR’ refers to ‘direct radiography,’ and it includes any system in which the image is created directly from a receptor. In direct radiography systems, the image is sent directly from the receptor for processing. Computed radiography is also referred to as indirect radiography because the image is read off the imaging plate through a discrete acquisition process. Generally speaking, techniques used in CR imaging can be compared to a 200 speed film/screen system while DR techniques may be compared to a 400 speed or higher film/screen system.^1,5 Essentially, a DR system requires approximately 50% or less technique than a CR system to produce a comparable image.

Direct radiography was introduced in the late 1990s. The substantial impact of DR on daily practice is multifaceted, and related in part to the high percentage of case volume represented by plain radiography. In our practice, and that of most departments, plain film radiography accounts for more than 50% of total imaging volume. As a result, this section of the department employs the most technologists. The high efficiency and rapid turnaround time [TAT] of digital radiography often lead to a reduction in the number of technologists by significantly increasing the number of studies performed per technologist. To better understand the effect of direct radiography in the pediatric radiology setting, we did a time-motion study that contrasted film screen radiography (FSR) and DR. We found that the average TAT for a 3-view skeletal examination was approximately 12 minutes for FSR and 3 minutes for DR. The effect on exam completion was more dramatic when all or part of an examination needed to be repeated. Other authors have documented similar experiences. An unanticipated outcome of the faster TAT was demonstrated in the relationship between radiology and clinical services. For example, with FSR or CR, the TAT was too slow to keep up with a busy orthopedic clinic, resulting in tension between the two groups. In contrast, with DR, the TAT is fast enough to keep up with the demands of “herd-type” scheduling and multiple orthopedists seeing patients simultaneously. This has dramatically improved relations between the two groups.

The Phoenix Children’s experience

DR may be configured using single or dual detector systems. While both configurations work well and add efficiency at lower radiation doses, the technologists in our department prefer the dual-detector configuration because it is easier to position patients and requires fewer steps to complete a study with >2 views. However, this is not always a practical solution, since it is more costly—about $100,000. In 2011 Phoenix Children’s Hospital opened a new hospital building that included a new radiology department fitted with Philips imaging equipment. We made a commitment to use DR only and installed three DR units, one with a dual-detector system and two with single detectors. In addition, our satellites feature combination RF/DR rooms with single detectors.

As a children’s hospital, our facility is a strong advocate of the Image Gently^® movement with the goal of producing diagnostic studies at the lowest possible radiation dose. Our DR equipment supports these efforts by using lower mAs in most studies¹ and reducing the repeat rate. Other positive features of DR include faster TAT, more flexibility of the imaging device making it easier for the technologist to position the child resulting in shorter imaging times in our experience and that reported in the literature.^2,3 Compared to film/screen imaging, digital imaging systems are very forgiving of both under- and overexposure. Severely underexposed digital images can be grainy and unacceptable even after post-processing. In contrast, overexposed digital images can appear as if a correct technique had been used. This is a double-edged sword, since it eliminates a second exposure but may lead to exposure creep, one of the major problems of DR. Exposure creep is a tendency to increase technique to ensure that all images are diagnostic. Studies have shown DR images with exposure rates of 500% to 1000% can still produce a diagnostic quality image.⁴ Thus, a quality-assurance program that regularly monitors the technical output of DR to ensure the highest-quality imaging at the lowest possible dose is very important.

At Phoenix Children’s, the prevention of exposure creep has been addressed through two simple but effective measures: Technique charts and a film review program. Technique charts that build in substantive reductions in dose are employed in all our imaging systems. Coupled with the technique charts is a regular review of randomly selected studies to ensure compliance with the charts. A few examples of DR techniques include: a neonatal chest radiograph was typically obtained with CR using 58 Kvp and 2.0 mAs. With DR, the same examination is performed using 56 Kvp and 1.0-1.25 mAs. A 3-view ankle scan on a teenager (15-19 years old) on a CR system used 60 kVp at 4 mAs. The same study on DR uses 55 kVp at 1.5 mAs. An AP chest technique for a 6-month-old using CR required 70 kVp at 2-3 mAs. The same study on our DR system uses 60 kVp at 0.8 mAs. All examples show a reduction equal to or greater than 50% of patient dose.

In most CR systems, technique tracking can only be achieved through exposure indicators in the DICOM header. There is not an accurate way to track kVp, mA, or time. This is because a CR cassette has no connectivity to the x-ray generator. Consequently, there is no way to transfer study information from the x-ray generator to the CR cassette. CR system exposure indicators can be problematic. Every CR system manufacturer has a different methodology and scale to designate exposure indicator values. In addition, exposure indicators are a reference value representing the relative amount of radiation hitting the plate. Direct radiography systems do have the ability to track technique factors. With DR, the x-ray generator and receptor are part of a single, fully integrated system. Technique factors [mA, kVp, time] from the x-ray generator component of the DR system are included in the DICOM header. Patient and study information from the work list also becomes part of the DICOM header.

The pros and cons of DR and CR are summarized in Table 1.

In conclusion, DR has had a substantial positive impact on pediatric imaging by reducing radiation dose, imaging costs, and patient turnaround times. As a result of the image-acquisition advantages, post-processing toolbox, and cost savings, we anticipate that over time, DR will replace all other forms of plain film pediatric imaging.

References

1. Seibert JA. Medical Radiation Exposure Requirements for Digital Radiography. Presented: Digital Imaging Summit and Workshop for Veterinary Radiologists. San Luis Obispo, Calif. May 29-31, 2008.
2. Hermann T. Computed radiography and digital radiography: A comparison of technology, functionality, patient dose, and image quality. eRadimaging.com http://www.eradimaging.com/ site/article.cfm?ID=535. Accessed September 1, 2012.
3. Reiner Bruce I, et al. Multi-institutional analysis of computed and direct radiography: Part I. Technologist, Productivity, Radiology. 2005;236:413-419. Epub 2005 Jun 21.
4. Siebert J. The standardized exposure index for digital radiography: An opportunity for optimization of radiation dose to the pediatric population. Pediatr Radiol. 2011;41: 573–581. Published online 2011 April 14. doi: 10.1007/s00247-010-1954-6.
5. Willis, C. Computed radiography: A higher dose? SPR Seminar in Radiation Dose Reduction 2002. Ped Radiol. 2002;32:745-750.