DR News: Staying Flexible
By Dan Cathie
Radiology Today
Vol. 25 No. 6 P. 6

Are flexible X-ray detectors the future of diagnostic radiology?

Despite most of the human body being made up of different shapes and sizes of bones, ligaments, muscles, organs, and tissue, most digital X-ray equipment requires radiographers to carefully position patients against a flat and rigid detector surface.

However, recent advances in X-ray technology have made it possible to create curved detectors that wrap around industrial pipes for internal inspection. A variant of this new type of detector could, therefore, be made to fit around the curvature of the body—similar to radiographic film before the advent of digital detectors. This kind of flexible X-ray detector offers a range of benefits for patients and health services.

Rigid Radiology
Since the early 2000s, when digital flat panel X-ray detectors largely replaced radiographic film, there has been a dramatic improvement in workflow for radiographers. Although the digitization of radiography has brought significant benefits, it has a fundamental limitation in terms of its rigidity. The form factor of these digital flat panels is a direct result of the underlying electronics that are used to capture and digitize the images. These electronics reuse the same basic manufacturing technology as displays (such as TVs and computer monitors), which are fabricated on rigid glass backplanes. Even with newer complementary metal-oxide-semiconductor (CMOS) flat-panel detectors, which are used for “live” high-resolution imaging, they are manufactured on silicon wafers, which are also rigid.

While this limitation has not hindered the advancement of digital radiology, the advent of plastic electronics has recently opened the door to the potential for flexible X-ray detectors. This is again built on the same electronic technology used in curved display monitors and foldable mobile phone screens. With the potential in medical imaging to create detectors that can be shaped to fit around the human body, in which medical fields might this new possibility be deployed?

Reframing Treatment
In radiotherapy, preserving healthy tissue is of the utmost importance while treating a tumor. This is essential when it comes to sparing organs that are particularly sensitive to radiation or where loss of function may occur—for example, in the brain.

To ensure patients are positioned in line with a known reference to the radiotherapy treatment beam, a head frame is usually screwed into the patient’s skull to restrain them. With flexible X-ray detectors, a personalized skull cap with an embedded printed radiation detector on it could accurately monitor—and potentially even correct—the position of the treatment beam in real time. The benefits are clear: improved patient comfort and accurate treatment delivery.

Gafchromic film is currently used for calibrating the accuracy of Gamma Knife systems for brain stereotactic radiosurgery. This analog film is inserted into a head phantom, irradiated, and then scanned in for digital 3D analysis. This laborious workflow could be replaced by using a head phantom with integrated digital X-ray sensors inside. These sensors would provide a direct “live” readout of the beam’s trajectory and provide medical physicists with an instant 3D image. The benefits are significant in terms of productivity improvements, leading to more equipment availability for patient treatment.

Compressless Mammography
Millions of women miss out on vital breast cancer screening, with discomfort often given as one of the reasons for avoiding it. Companies, such as AB-CT, are seeking ways to overcome this fundamental challenge, for example, by having the patient lie face down on a bed with a mini-CT scanner circling the breast to image it. With the opportunity presented with flexible X-ray detectors, another approach would be to create a detector that is shaped to conform around the breast. The discomfort would then be eliminated as the breast would no longer have to be compressed to conform with a rigid flat-panel X-ray detector while maintaining close contact with the breast to maximize dose efficiency (minimal radiation) and provide the best image quality (maximum resolution).

Open Wide
Dental radiographs are valuable diagnostic aids for oral health care, and intraoral examinations are commonly used by dental practices worldwide for routine checks or dental health, particularly for the detection of cavities and for examination prior to an extraction procedure.

There are several opportunities for curved or flexible X-ray detectors in this segment, the first being in panoramic dental imaging, where radiographic film was replaced by charge-coupled device (CCD) line scanners, which circle the patient’s jaw over the course of a few seconds. As CCD line scanners are being made obsolete, equipment manufacturers are pushed towards using more expensive options (such as CMOS line scanners). By providing a curved digital X-ray detector, the overall cost would be reduced, and there would be no more need for automated moving parts without loss of image quality.

Another opportunity within dental radiography would be for intraoral sensors. It is well known that many patients experience discomfort during intraoral radiographs, with a significant number unable to tolerate the intraoral film/sensor placement in their mouth due to the gag reflex. This number is reported to have increased with the advent of digital radiography because of the rigidity and thickness of the radiographic sensor. For such cases, new flexible sensors with the consistency of a marshmallow could be developed. Patient discomfort would be eliminated, leading to improved oral hygiene and earlier detection of dental issues.

Managing Costs
Although the advantages of CT imaging are evident for diagnosis, the cost of the systems and the dose rate required to capture the detailed 3D-reconstructed X-ray images means it can only be prescribed to a small percentage of patients.

One of the primary cost drivers for such systems is the high-speed robotic scanning system, which circles the patient in a helical movement around and down the area to be scanned, taking hundreds of images per rotation. The engineering challenges to this set-up are quite significant, particularly as consultant radiologists seek to view more detail.

Considering the potential of flexible detectors, if a curved detector could be developed to circle the patient, the rotational movement would no longer be required. This is partly what cone beam CT equipment manufacturers have attempted, albeit by abutting large flatpanel detectors together side by side. The imaging performance deteriorates due to scattered radiation, but the cost comes down by an order of magnitude. With a single curved linear detector to replace multiple flat-panel detectors, the cost would likely become comparable to existing DR systems for standard X-rays without compromising image quality. In fact, if multiple detectors could be used with filters between them, considering their low cost, energy information would be retained, and color X-ray images could be produced.

A Flexible Future
In summary, although the need for flexible X-ray detectors in medical radiography may not be immediately obvious, there are multiple instances in which their adoption could be useful. The advantages fall largely around an improvement in patient comfort and in the savings from medical equipment or productivity improvements. Only time will tell whether such innovation will remain only of benefit to industrial, security, and portable applications or whether dental and medical radiography will also embrace and adopt it in the coming years.