The present generation of digital radiography devices offers opportunity for improving both the quality and the productivity of x-ray examinations. A previous article considered digital radiography, CR and DR, detector technology, acquisition issues, display techniques, and data management (“Technical Advances in DR and CR: Part 1,” March 2004). In this article, we examine how these systems can improve work flow and reduce certain operating costs. These factors are important for justifying the higher capital expense of this technology. We also examine how the recently available digital radiography systems can improve the quality of radiographic studies. Finally, the manner in which these new detector technologies have been used to provide tissue discrimination and synthesized tomography is summarized.
Work-flow and Cost Benefits
CR acquisition devices have been used with film printers since they were introduced in 1981. When they are used to print films, the CR work flow is essentially the same as that for screen-film systems. The radiograph is recorded using a cassette. The cassette is then taken to a processing station to be read and the printed film placed in the patient’s folder. However, when CR devices are fully integrated with image and information management systems (HIS, RIS, and PACS), significant opportunities for improved work flow and increased productivity exist.
Modality worklist services that significantly improve the informatic steps of a procedure are now supported by all CR and DR suppliers. 1 Assuming that an order has been placed, a technologist can automatically retrieve all patient and examination information from a worklist server with a simple graphic selection. For departments that schedule procedures to individual rooms, the technologist can make this selection directly from a list of studies scheduled in the procedure room where they are currently working. When the examination is complete and the images are sent to the PACS system, the automatic entry of examination information including the assigned accession number ensures that the image data is properly associated with the correct patient, the examination order, and eventually the interpretive report.
Several work-flow enhancements are expected to be available on CR and DR systems as a part of the Integrating the Healthcare Enterprise efforts to improve the integration of radiology systems. 2-7 Notable are services to update the modality if patient information changes, ensure that the PACS system has received and stored images, and coordinate postprocessing such as computer-aided detection (CAD). For CR or DR systems that control the generator, services are now provided that automatically specify the radiographic views and technique factors based on the ordered examination.
Systems that are fully integrated with the x-ray generator and do not require the handling of a cassette offer additional improvements in work flow. Most DR systems are cassette-less with the detector integrated into a wall mount (chest), radiographic table, or C-arm assembly. Some CR systems integrate the phosphor screen into an automated changer and do not require the handling of a cassette. These systems permit the examination of the patient without the need for the technologist to leave the room. A technologist brings the patient into the room and selects the study from a scheduled list. For each view required, the exposure is made, the image quality is immediately validated, and the image is approved for transmission to the PACS system. When the examination is done, an examination-complete message is sent from the operator’s console to the information system to indicate that interpretation can begin. Improvements in productivity by a factor of 2 relative to traditional screen-film or CR-film processes have been reported by using fully integrated cassette-less imaging devices.
Image Quality Improvement
The image quality of traditional screen-film radiography and CR systems has been limited by light diffusion in the powdered phosphor screen. For thick screens that are efficient in detecting x-rays, excessive light diffusion causes image blur. High detail is obtained by using thin screens that have low radiographic speed. The first part of this article described how new DR technology can reduce light diffusion by using oriented structured phosphors or can eliminate the problem by using solid state detectors. Indirect detection DR devices are typically made with very thick cesium iodide (CsI) structured phosphors that improve detection efficiency by a factor of ~3 relative to powdered phosphor systems. However, the resolution of these CsI systems is only slightly better than that of powdered CR or screen-film systems. Direct detection DR devices typically use selenium, Se, for x-ray conversion. The lack of blur makes the resolution much better than with CsI DR systems. However, the detection efficiency of current Se DR systems is not as good as for the current CsI DR systems, although stillÂ a factor of ~2 better resolution than CR screen-film systems. The significance of these improvements for specific clinical studies is not fully understood.
For musculoskeletal radiography, the visualization of many bone features isÂ possible only when the radiograph has very high resolution. A classic example is the joint space narrowing and bone erosion observed in hand radiographs of patients with rheumatoid arthritis. Direct detection DR provides unmatched detail in these situations. Using studies from patients having metastatic bone surveys, Marnix Van Holsbeeck, MD, has recently compared the images from direct DR examination to those from prior CR examinations. These studies are done to evaluate disease progression in certain cancer patients. A total of 13 radiographic views of the skull, spine, pelvis, shoulder, and extremities are obtained. Improved visualization of lytic lesions was consistently demonstrated with the direct DR along with improved detail of irregularities at the inner cortical surface, trabecular detail, and certain ancillary findings such as the tooth root margins in lateral views of the mandible.
|Figure 1 A/B. A highly magnified view of a hand joint is compare for a DR recording (1A) and a CR recording (1B). The greatly improved visualization of fine detail in the DR recording is easily seen.|
In general, the images from indirect detection DR systems can be acquired more rapidly than those from direct detection DR systems. For Se DR devices, residual charge in the solid state conversion layer must be removed before the panel is ready for the next exposure, which makes it more difficult to rapidly acquire images. For CsI DR systems, the photodetectors can be sequentially read and the phosphor has minimal lag. This has led to the introduction of CsI DR detector panels operating at speeds up to 30 images per second. For cardiac and general angiography, systems are now available that replace the conventional image intensifier (II) with DR panels providing pulsed fluoroscopy or angiographic sequences. Compared with II devices, these panels have improved contrast due to the minimal veiling glare and no geometric distortion. High-speed CsI DR detectors are expected to enable new applications for such things as cone beam tomography and tomosynthesis.
Laminography is commonly used in general radiography to emphasize contrast in a particular layer of the subject. Specialized x-ray tube gantries and cassette holders are used to synchronously move the x-ray source and the screen-film cassette such that only structures in a specific plane are in focus and the recording of overlying tissue structures is blurred. Devices with a linear motion are commonly used for urological examinations. When the method is used with a digital detector, the receptor can remain fixed and computer processing used to simulate the motion of the detector.
The digital version of laminography has a very important advantage in that the plane of focus can be determined after the data is acquired. Thus, a sequence of images can be produced that are similar to a stack of computed tomograms from a CT device. The method is thus commonly referred to as digital tomosynthesis. 8 Data is still collected with an x-ray source moved to many positions and a sequence of images is recorded using a digital receptor and the data reconstructed with a computer to form a tomographic stack of images that can be reviewed on a workstation. An additional advantage of digital tomosynthesis compared to traditional laminography is that computer processing methods can be used to minimize the contribution of the blurred, out of plane structures to the images deduced at each plane of interest. Presently, either iterative methods or methods that use matrix inversion are used. 9 The result is more like an image from a CT scanner than a traditional laminography image.
Presently, digital radiography receptors have improved to the point that a sequence of images can be rapidly obtained and digital tomosynthesis has become practical to implement. Using a digital chest detector, Dobbins has reported impressive chest tomosynthesis results in human subjects. 9 Using a digital mammography detector made by the same vendor, Rafferty has reported similarly impressive results for breast tomosynthesis performed in human subjects having breast cancer. 10 Using a simulation experiment, Duryea has recently suggested that tomosynthesis might be valuable in the assessment of hand joints for the evaluation of arthritis. 11 The emergence of commercial tomosynthesis devices in the next few years may lead to a variety of other useful applications where tomosynthesis may improve the performance of radiography without requiring a complete CT examination.
Dual energy radiography methods are used to generate separate images of bone and tissue structures from two exposures made using different radiographic techniques. The method is of particular value for chest radiography where the interference of ribs is removed from the lung tissue. The method is now relatively old in that a group at the University of Alabama reported excellent results 20 years ago using a scanned fan beam device. 13 More advanced methods to compute the bone and tissue images and new detector technologies have now made dual energy radiography a viable consideration for any center using a dedicated room for chest radiography.
Dual energy methods are most effective when both images are acquired simultaneously. One commercial CR system has used two CR plates separated by a metal filter to obtain the low and high energy data from the same x-ray exposures. The University of Chicago has used this single shot dual energy CT method extensively and recommends that it be used routinely with a conventional view available along with the tissue and bone views. 12 Improved detection of noncalcified pulmonary nodules has been reported by several groups. Additionally, it helps identify partially calcified nodules and pleural plaques, which can eliminate the need for CT in some cases. The method is now being implemented using flat panel DR detectors; however, these currently require two images to be obtained in rapid succession. While the flat panel dual energy systems produce images with less noise, they are prone to artifact from tissue motion occurring between the two exposures. 14
Michael J. Flynn, PhD, is senior research scientist, Department of Radiology, Henry Ford Health System, Detroit.
- Moore SM. Using the IHE Scheduled Work Flow Integration Profile to drive modality efficiency. Radiographics. 2003;23:523-529.
- Siegel EL, Channin DS. Integrating the Healthcare Enterprise: A primerPart 1: Introduction. Radiographics. 2001;21:1339-1341.
- Channin DS. Integrating the Healthcare Enterprise: A primerPart 2: Seven brides for seven brothers: The IHE Integration Profiles. Radiographics. 2001;21:1343-1350.
- Channin DS, Parisot C, Wanchoo V, et al. Integrating the Healthcare Enterprise: A primerPart 3: What does IHE do for ME? Radiographics. 2001;21:1351-1358
- Henderson M, Behlen FM, Parisot C, Siegel EL, Channin DS. Integrating the Healthcare Enterprise: A primerPart 4: The role of existing standards in IHE. Radiographics. 2001;21:1597-1603.
- Channin DS, Siegel EL, Carr C, Sensmeier J. Integrating the Healthcare Enterprise: A primer-Part 5: The future of IHE. Radiographics. 2001;21:1605-1608.
- Channin DS. Integrating the Healthcare Enterprise: A primerPart 6: The fellowship of IHE: Year 4 additions and extensions. Radiographics. 2002;22:15551560.
- Grant DG. Tomosynthesisa three dimensional radiographic imaging technique. IEEE Trans Biomed Eng. 1972; 19:20-28.
- Dobbins JT, Godfrey DJ, McAdams HP. Chest tomosynthesis. In: Samei E, Flynn MJ, eds. Advances in Digital Radiography: RSNA Categorical Course in Diagnostic Radiology Physics, 2003. Oak Brook, Ill: RSNA; 2003:211-217.
- Rafferty EA. Breast tomosynthesis. In: Samei E, Flynn MJ, eds. Advances in Digital Radiography: RSNA Categorical Course in Diagnostic Radiology Physics, 2003. Oak Brook, Ill: RSNA; 2003:219-226.
- Duryea J, Dobbins JT, Lynch JA. Digital tomosynthesis of hand joints for arthritis assessment. Med Phys. 2003; 30:325-333.
- MacMahon H. Dual-energy and temporal subtraction digital chest radiography. In: Samei E, Flynn MJ, eds. Advances in Digital Radiography: RSNA Categorical Course in Diagnostic Radiology Physics, 2003. Oak Brook, Ill: RSNA; 2003:181-188.
- Barnes GT, Sones RA, Tesic MM, Morgan DR, Sanders JN. Detector for dual-energy digital radiography. Radiology. 1985;156:537-540.
- Dobbins JT, Warp RJ. Dual-energy methods for tissue discrimination in chest radiography. In: Samei E, Flynn MJ, eds. Advances in Digital Radiography: RSNA Categorical Course in Diagnostic Radiology Physics, 2003. Oak Brook, Ill: RSNA; 2003:173-179.