Cool treatment

Fibroadenoma of the breast is a common benign lesion that can affect women during their reproductive years. Despite the fact that they are not life threatening, fibroadenomas can grow up to 4cm in diameter, cause physical deformity and may produce discomfort or emotional distress in those women who are afflicted with them.

Traditionally, such lesions have been surgically removed at the time of an open biopsy – but this is a lengthy and costly option, because patients need to undergo treatment in a hospital operating room under anaesthetic.

Recently, however, a faster, less expensive alternative to the procedure has been deployed that uses cryoablation guided by ultrasound to selectively freeze and destroy tissue in the target area. This treatment takes approximately 10 to 30 minutes, depending on lesion size, and can be performed in a doctor’s office under local anaesthesia.

While the idea seems simple, the systems engineers at Sanarus Medical, a small operation based in Pleasanton, California, were faced with a tough challenge – they had just four months to develop a prototype system that could perform such a procedure.

The engineers at the company were quickly able to map out the specifications of the new system. In operation, it would be required to deliver liquid nitrogen to a probe inserted directly into the lesion in the breast. Under the direction of an operator, the ultra cold temperature created at the tip of the probe would need to twice create an iceball of a specified size around the fibroadenoma to meet the US Food and Drug Administration (FDA) requirement that the lesion had been totally engulfed and destroyed.

Once the freezing procedure was complete, the probe tip would need to be heated very quickly to melt the ice adhered to it so that it could then be quickly removed from the tissue. Once the procedure was complete, the dead tumour would then be reabsorbed by the body over several months.

‘While we had a great deal of expertise in the field of cryoablation techniques, the small size of our design team meant that we had to look for an innovative way to control the system to get to market within a fourteen month period,’ admitted Jeff Stevens, Principal Systems Engineer.

Stevens knew that that this meant that he would need to look for alternatives to developing a custom control system from scratch to do the job. And when he was introduced to the CompactRIO data acquisition and control system from National Instruments and its LabVIEW graphical programming language, he realised that these off-the-shelf tools would enable him to develop the control system as well as the operator interface for the new product within that timeframe.

There was just one fly in the ointment. ‘CompactRIO is not cheap. In fact, it was well above the cost of goods requirement for the entire control subsystem,’ admitted Stevens. But certain that the system would meet his needs, Stevens put together a trade-off table to demonstrate the advantages of the off-the-shelf solution versus a custom design and presented the results to his management.

‘Although it was expensive, in every other respects, it had many advantages over a custom design,’ said Stevens, ‘Not only did it save us from the hassle of designing and manufacturing our own hardware and writing low-level software, it’s adaptable to late design changes, easy to package, and it has already been verified,’ he said.

Once Stevens had been given the go ahead, the rest was plain sailing. The software he developed around the LabVIEW programming language could not only be used to present the operator with a step-by-step guide to the medical procedure though a man-machine interface, but also control the operation of the entire machine to boot.

Stevens realised that the precise control of the temperature at the tip of the probe was key to the success of the system. That meant that he had to develop a closed-loop software program to control the flow of the cryogen into the cooling element using feedback from a temperature reference obtained from an in-built sensor at the probe’s tip. Not only that, but he also had to develop a similar program to control the heating of the element in the probe to the same degree of accuracy to ensure that the probe could be removed safely after the tumour had been killed.

To guarantee that the temperature at the tip of the probe could be synchronised to both the flow of the cryogen and the power supplied to the heating element inside it with both accuracy and speed, Stevens chose to implement the control software in a field programmable gate array (FPGA) built into the CompactRIO system precisely for such purposes.

With twelve systems in the field to date, the new system has already proven to be a success. However, as production of the new system ramps, Stevens has expressed a concern that the expense of the CompactRIO system might necessarily lead him back down the custom control system design route to lower the cost of the cryoablation unit further.

Fortunately, however, National Instruments has recently formed a relationship with design and fabrication house Flextronics, who can now produce less expensive custom single-board versions of the system, alleviating the need for Stevens to turn back to a custom route to achieve his cost-reduction goals once the system starts to sell in higher quantities.