Demand for medical-imaging systems continues growing around the globe. As a result, imaging OEMs are turning to motion-control platforms that help control costs, streamline production on a global basis, and provide systems capable of delivering the most sophisticated 2D and 3D clinical images possible.

Imaging-system designers usually begin by defining the kinematics of the machine and even the specific motion algorithms. In this industry, motion-control systems fit into two categories:

Basic — single plane, horizontal X-Y movement such as a patient table moving in and out of an imaging cylinder, or raising and lowering a table for patient access

Complex — multi-axis coordinated motion, with extremely demanding kinematics

Complex systems typically consist of a patient table and an imaging arc or C-arc gantry — a large, C-shaped apparatus mounted on the floor or ceiling with the X-ray imaging beam source at one end of the arc, and the imaging detector on the opposing end.

Both elements have multiple axes of motion: the patient table moves horizontally, vertically, lifts the head or feet, and tilts side to side. The C-arc can make 180° arcs in three axes around the patient table to carry out diagnostic and clinical treatment tasks, such as real-time X-ray imaging of a cardiac catheterization procedure.

Some of the most complex kinematics are required when a test or procedure calls for isocentric motion: keeping the imaging beam on a point in the patient's body, while the imaging apparatus and patient table move independently through multiple passes to create a 360° image.

These require demanding kinematics loop computations that are unique to medical imaging: up to nine axes of motion, moving through X, Y, and Z planes, while retaining an extremely clear, fixed center-point image resolution that may only be a few millimeters in diameter — the size of a patient's blood vessel, for example.

In the past, motion-control subsystems were custom-made by the OEMs, which required diverting valuable engineering and programming resources to motion control, rather than the machine's core image processing functions. Specific control algorithms and kinematics are central to the OEM — as a must-have. Now, a new generation of motion controls offer “off-the-shelf” platforms with features such as:

• 32 KHz servo loop update rates

• Configuration tools that support rapid creation of complex control loops with interfaces to Matlab (a numerical computing environment and programming language) and optimized for features such as safety routines, accurate axis synchronization, gearing, and spline capability.

• Controller-level processing to ensure tighter integration with drives, I/O, and imaging functions.

• Open architecture C/C++ API

Most imaging systems use servo drives in the 500 W to 2 KW range to handle loads of the C-arc gantry and patient weights, which can range up to 400 lbs.

Most medical imaging systems also use a range of human-machine interfaces (HMIs) such as touchscreens, switch and dial panels, and joystick and handheld controls. Joystick control is similar to a teach pendant or handheld controller used in traditional robotics or industrial automation applications. In medical, the imaging system's main C program, which is responsible for executing the whole machine (imaging, motion, I/O), also translates the joystick motions/buttons into functions for the motion controller.

Open architecture

With wider use of off-the-shelf motion control platforms, there are advantages to selecting systems with open software architectures that support adaptive reuse of existing algorithms and intellectual property.

Unlike motion control in automotive and machine tools, which typically use the nearly universal IEC 1131 programming, medical-imaging systems need platforms with control loops optimized for complex applications such as isocentric motion. Plus, they need tight integration of other system functions such as I/O, safety systems, and operator controls. This advanced capability is often implemented in a C/C++ API by the system programmer.