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Powering portable medical diagnostic equipment

06 May 2015

Digital radiography greatly benefits the healthcare industry, but it also places high demands on medical equipment manufacturers. Neil Oliver explores how smart batteries can help meet these challenges.

Batteries designed by Accutronics specifically for portable X-ray machines

Thanks to digital radiography (DR), medical imaging is being transformed by better detectors, more powerful computers, sharper displays, faster processing and more efficient archiving. The benefits are great and wide ranging; an image can be acquired in seconds and viewed on any computer monitor; it can be accepted or deleted at the click of a button; it can be easily shared with other medical professionals and archived in an online database. It also allows for greater precision, and the balance between image quality and radiation dose can be controlled more accurately - making it a safer process for the patient.

The initial cost of a DR system, compared with conventional film-based systems, is high. However, hospitals save money by eliminating film costs and reducing the storage space required to archive the images. Because it is a faster and more efficient process, perhaps the biggest saving is time. Fewer operators are required to man the service and patients can be seen more quickly.

With the advent of digital radiography an electronic detector is used instead of photographic film, the image being subsequently processed by a computer, rather than being chemically developed. The basic principle is very similar to that of a digital camera. Both the conventional camera and its digital counterpart work by using a series of lenses that focus light to create an image. In the digital camera, instead of focusing the light onto a piece of film, it focuses it onto a semiconductor device that records light electronically. A computer then breaks this electronic information down into digital data.

In digital radiology, amorphous silicon detectors are combined with a caesium iodine or gadolinium oxysulphide scintillator to convert X-rays into light. The light is channelled through the amorphous silicon photodiode layer where it is converted to a digital output signal. The signal is then read by thin film transistors or fibre coupled CCDs before being sent to a computer for processing and display. 

The portable detector
In digital radiography, the X-ray source will often be built into a medical trolley. The battery allows the detector to be completely portable, so that if the patient is in a hospital bed, the trolley can be wheeled to them and the detector can be slipped underneath the patient to take the X-ray. Portable X-ray detectors need to be lightweight and compact but extremely robust.

The comparison with the digital camera ends when it comes to the size of the detector required for each device. For example, the detector on your smart phone camera is about 3mm across, whereas a DR detector can be the size of a briefcase. 

Furthermore, the batteries within smart phones usually consist of a single lithium-ion cell whereas the large portable detector panels require a far more complicated multi-cell battery pack.

Although the batteries for detectors need to be larger, they must still maintain a slender profile if they are to fit into the rear of the detector – the need for thinness in such a large footprint creates a number of challenges for the battery designer.

The lithium-ion cells used within such batteries are more commonly of the ‘pouch’ type which cannot tolerate bending or twisting. The average thickness of a rechargeable battery for the DR market is now less than seven millimetres but has a footprint similar to a sheet of A5 paper and must be flat within 0.2mm.

The battery case must provide enough torsional rigidity to prevent damage to the cells which may cause an internal short circuit, leading to overheating or even fire. The use of modern plastics technology can aid in the design of such cases but clever mechanical design can also ensure that rigidity is built into the structure while still maintaining the thinness that the market demands.

Removable and rechargeable
Some digital radiography detectors contain an embedded battery similar to that used in handheld tablet devices, but the consequence is that, once the battery reaches its end of life, the hospital has to send the detector back to the manufacturer for the power source to be replaced. This is a costly exercise which puts a vital piece of medical equipment out of service. The solution is a removable, rechargeable battery, enabling the hospital to achieve continuous use. 

However, batteries used in critical environments in this way must provide genuinely accurate ‘fuel gauging’ to ensure that the remaining battery life indicators are reliable. This provides the user with predictive run time of the device, giving them the ability to know how many images they can take.

For clinical environments, Accutronics has developed multi-bay smart charger technology which means that multiple batteries can be charged at a time, resulting in a quick changeover and a device that can remain in continuous use. 

These power products contain the latest smart battery technology, including active and passive protection circuits that prevent over-temperature, over- and under-voltage, overload and short-circuit. They are capable of accurate fuel gauging and are built to international regulatory standards, all features especially useful in the increasingly dynamic nature of modern hospital care.

Digital radiography's future
Because of the initial cost of a DR system, it is mostly being adopted in countries with a tier-one health system. However, digital technology is evolving at a rapid rate and becoming more flexible and affordable. More than two billion people in the world now have access to the Internet and five billion of them have mobile phones. Like the transition from film-based to digital cameras, it's conceivable that digital radiography will also, one day soon, have a global appeal.

Neil Oliver is technical marketing manager, Accutronics

Batteries for use in medical devices need to meet stringent regulations and comply with international safety standards. Accutronics tests its batteries, chargers and power supplies to meet minimum standards such as IEC 62133, UL2054 and IEC60950-1. The testing process goes through three stages: internal design verification to check the product is fit for purpose, external testing such as ingress protection (IP) testing for use in medical environments and customer specific testing; for example, the required testing can be dependent on which country the product is destined. 

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