Prostate cancer is the second most common cancer affecting American men, with approximately one in seven diagnosed in their lifetime. Most patients receiving conventional prostate cancer therapies such as surgery or radiation have good oncologic outcomes; however, these therapies are often associated with long-term erectile, urinary, and bowel complications that may significantly compromise quality of life. As a result, men are seeking treatment alternatives.
To address this issue, Profound Medical is commercializing a novel technology, TULSA-PRO™, which delivers a personalized and precise ablation of the prostate tissue in men with localized prostate cancer. TULSA-PRO combines real-time magnetic resonance imaging (MRI) with transurethral, robotically driven therapeutic ultrasound and closed-loop thermal feedback control. This enables targeted ablation of tissue while simultaneously protecting critical surrounding anatomy from potential side effects.
Achieving this ambitious goal, particularly in the MRI environment, involved more than a few challenges. The first was motive power. After all, conventional motors depend on the interaction of magnetic fields to operate, making them very poor fits for the MRI environment. To find a solution that would perform even under the most difficult conditions, Profound Medical turned to MICROMO and its technology partner PiezoMotor.
The MRI magnet room components of TULSA-PRO consist of an ultrasound applicator that delivers the therapeutic dose to the patient, the robotic positioning system of the applicator, and the interface box that includes power and drive electronics for the positioner. The ultrasound applicator features a probe with a linear array of ultrasound transducers near the tip. Controlled by a rotary and linear motor, the probe and array can be accurately positioned axially and rotationally to deliver the thermal energy exactly against the target boundary within the prostate as selected by the physician.
During treatment, the ultrasonic applicator is inserted in the urethra and rotated to deliver the ultrasonic energy generated by the transducers. This enables the precise ablation of the prostate tissue. Throughout the process, MR thermometry information is updated in real time to assess progress.
Operation in an MRI
The performance specifications require servo-motor-level performance. The problem is that the multi-tesla magnetic fields of an MRI machine would not only prevent proper operation of a dc servo motor, they would very likely demagnetize the permanent magnets. Early in the process, the Profound Medical team chose to go with piezoelectric positioners.
Piezoelectric ceramics expand incrementally under an applied voltage. When they are combined into laminated stacks, the effect is magnified enough to introduce nano- to micrometer-scale displacements. The effect can be leveraged in a variety of configurations to form piezoelectric motors capable of long linear travel and 360° rotations. When made with nonmagnetic housings, they are a good fit for the MRI environment.
The first piezoelectric motors the Profound Medical engineering team tested delivered adequate performance but were too large for the device. An MRI suite is already tightly constrained. This type of procedure requires multiple instruments and fixtures to be arrayed around the patient. The TULSA-PRO is situated directly adjacent to the patient. The goal of the Profound Medical team was to make the TULSA-PRO as compact as possible. That sent them searching for a better solution. They found it in Piezo LEGS® motors from MICROMO and PiezoMotor.
Piezo LEGS motors leverage friction and the piezoelectric effect to position to nanoscale spatial resolutions. Each device consists of a drive shaft placed on top of a bi-morph structure formed of piezoelectric stacks driven independently. When the voltage is applied to the trailing side of legs one and three, those stacks extend, causing the legs to bend forward slightly and move the load. When the voltage turns off, the stacks shrink, while voltage is now applied to the trailing side of legs two and four. Meanwhile, voltage is applied to the leading side of legs one and three, causing them to bend slightly backward before the whole cycle repeats again. When driven at the kilohertz level, this mechanism can move a load at a rate of centimeters per second.
The motors are available in both linear and rotary configurations. In the linear motors, the legs drive a straight driveshaft while in the rotary designs, the legs drive a flat disk connected to the motor axis. Both are built with housings of a nonmagnetic copper-nickel-zinc alloy.
The motion module that Profound Medical selected included both linear and rotary motors in a single housing. “The biggest advantage of the Piezo LEGS motors was first that all of the piezo elements were already packaged in the configurations that we had in mind and the overall footprint was drastically smaller than what we had before,” says Benny Yeung, mechanical engineering manager at Profound Medical.
Off-the-shelf, the rotary motors provided 50 mNm of torque but the application needed more. In the Piezo LEGS design, torque is a function of the static friction between the driveshaft and the tips of the individual legs. Static friction is in turn a function of the normal force, or internal preload applied to the driveshaft. By increasing the preload, the PiezoMotor team was able to boost the torque to 80 mNm. They also extended the shaft on the backside of the motor to make it easier to install. Finally, they modified the drive electronics to increase the speed.
MRI compatibility is a two-way issue. The MRI magnetic field should not affect motor performance but by the same token, the motor should not introduce image artifacts or compromise data transmission. MRI machines generate a significant amount of RF interference but the electrical signals from the motor driver can also be a problem for image integrity. This requires careful attention to shielding and filtering on both sides.