Motion control for CT, MRI and X-ray imaging systems

In short:Imaging motion is about timing and smoothness under heavy, fast-rotating or patient-carrying loads. A CT gantry spins a heavy ring several times a second and sends its image data through a slip ring; tables and C-arms move patients and detectors precisely and must stop safely. Near an MRI magnet, conventional motors are replaced by non-magnetic actuators.

Typical requirements

Machinery we specify motion for

• CT scanner gantries
• Intra-operative and mobile CT
• CT and radiology patient tables
• X-ray C-arms and mobile fluoroscopy
• X-ray tube and detector positioning
• MRI patient handling and in-bore devices
• Radiography stands and ceiling suspensions
• Industrial and security CT

What imaging systems demand from motion

This page builds on our medical and laboratory overview, which covers IEC 60601-1 component evidence and the basics of CT slip ring specification. Here we go through the whole imaging system: gantry, slip ring, patient table, C-arm and the special case of MRI.

Imaging motion has one overriding requirement: the system must know exactly where everything is when each image sample is taken. Kollmorgen’s imaging case study puts it as microns and microseconds making the difference between a clear image and artefacts. In a 320-slice cardiac CT, images are captured only while the heart is at rest between beats, so gantry motion, X-ray timing and ECG data must line up. Kollmorgen says its servo loops close in as little as 62 µs for this kind of work.

The loads are large. A modern CT gantry carries the X-ray tube, generator parts and detector around a 78 cm bore, and tables carry patients of up to 307 kg on published scanners such as the Siemens SOMATOM Force, which also quotes a rotation time down to 0.25 s and a scan speed of up to 737 mm/s.

Where motion sits in an imaging system

CT gantries: direct drive and the forces involved

A gantry at 0.25 s per turn is spinning at 240 rpm, or about 25 rad/s. Two consequences follow for motion design.

Worked example: gantry drive and centripetal load

Assume a rotating assembly of 800 kg with a radius of gyration of 0.5 m, bearing and brush friction of 50 Nm, and a spin-up to 240 rpm in 20 s. These are illustrative figures, not a scanner’s data.

1. Inertia. J = 800 × 0.5² = 200 kg·m².
2. Acceleration torque. α = 25.1 / 20 ≈ 1.26 rad/s², so T = 200 × 1.26 ≈ 251 Nm. Add 50 Nm friction: about 300 Nm during spin-up.
3. Power. At the end of spin-up, 300 Nm × 25.1 rad/s ≈ 7.5 kW. At steady speed only friction remains: 50 × 25.1 ≈ 1.3 kW.
4. Braking. Stopping in the same 20 s puts about ½ × 200 × 25.1² ≈ 63 kJ back into the drive, so plan a braking resistor or regenerative supply.
5. Centripetal load. A component at 0.6 m radius sees ω²r ≈ 25.1² × 0.6 ≈ 380 m/s², about 39 g. Brush blocks, connectors and any encoder read head on the rotor must be rated for that.

A direct drive suits this duty because the control loop sees the ring itself with no belt in between, as Kollmorgen describes for imaging gantries. Arc-segment ironless motors such as the Akribis ACR are built for large radii, with no cogging and no attraction force. Check the stray field near detectors, and ask for velocity ripple at scan speed.

CT slip rings beyond the basics

The overview page covers bore, surface speed, power and data. Two developments are worth knowing. First, contactless data rates keep rising: SCHLEIFRING quotes up to 340 Gbit/s for its GigaCAP capacitive technology, alongside power up to 1,000 V DC and 300 A on brushes, bores up to 2,000 mm and speeds of 300 rpm and above. Second, CT is moving into theatres. SCHLEIFRING describes an intra-operative CT that must send real-time image data from two sources in a very tight installation space, which pushes ring designers towards thinner, lighter rotors.

Medical CT rings are engineered to the scanner. For industrial or security CT, retrofits and prototypes, catalogue units such as the Large Size Slip Ring (bores quoted up to 1,000 mm) or a fibre optic integrated slip ring are a starting point. See our slip rings for medical imaging page and the large-diameter type.

Patient tables and C-arms

Tables have two very different axes. The lift carries the full patient load slowly and must hold it on power loss. The horizontal axis moves the patient through the beam smoothly at scan speed. Specify them separately, with an absolute linear scale such as the Bogen LMSA where the table must know its position at power-up.

C-arms move heavy, offset loads around the patient. A published mobile unit from MXR Imaging offers +95° to −65° orbital rotation, ±210° angulation, 450 mm vertical travel and ±12° wig-wag. Harmonic Drive lists precise, safe positioning of X-ray generators, detectors and C-arms among its medical uses. Size each axis on the unbalanced moment at its worst angle, use low-backlash gearing and holding brakes, and define what each axis does on a collision or emergency stop. IEC 60601-2-54 covers radiography and radioscopy equipment such as these.

Industrial and security CT

The same gantry and slip ring technology appears outside medicine: SCHLEIFRING lists security inspection and industrial CT alongside medical scanners. Industrial CT often rotates the part on a precision stage instead of spinning the source, so a housed direct drive with an integrated encoder, plus stiff linear axes for magnification, does the job. The duty is slower, but accuracy and stiffness are tighter.

MRI: when motors cannot go near the magnet

ASTM F2503 defines three labels. MR Safe means no known hazards in any MR environment; MR Conditional means demonstrated safety within defined conditions; MR Unsafe means unacceptable risks. Conventional permanent-magnet motors, steel gearboxes and many encoders are unsuitable near the bore.

Two approaches work. Keep motors well away from the magnet and transfer motion mechanically, or use non-magnetic actuators. In an MRI-guided prostate robot, Eslami et al. built all components from non-ferrous, non-magnetic materials and drove each axis with a non-magnetic ultrasonic motor of up to 1 Nm. Signal-to-noise varied by no more than 15.35% across the first six test states at 3 T, and the authors describe the robot as classifiable as MR Conditional. Ask any supplier for a non-magnetic build and test evidence, never assume it.

Typical pitfalls

1. Sizing the gantry for steady speed only. Spin-up, braking energy and centripetal load on rotor parts set the design.
2. Ignoring velocity ripple. It becomes timing error in the image.
3. Treating the table as one axis. Lift and travel have different loads, speeds and safety needs.
4. Calling a motor “MRI compatible” without an F2503 label and test conditions.
5. Forgetting EMC. Generators and detectors are both noisy and sensitive; screen and filter drive cables.

What to send us

Send the axis list with loads, ranges, speeds and duty, the slip ring circuit and data list, and the envelope. Add the system’s particular standard, EMC constraints and any MR environment. Our specialist engineering and supply partners will come back with a proposal for your design review.

Standards to have in view

• IEC 60601-1 (medical electrical equipment, basic safety and essential performance)
• IEC 60601-1-2 (EMC for medical electrical equipment)
• IEC 60601-2-44 (X-ray equipment for computed tomography)
• IEC 60601-2-54 (X-ray equipment for radiography and radioscopy)
• IEC 60601-2-33 (magnetic resonance equipment for medical diagnosis)
• ASTM F2503 (marking items for safety in the MR environment)
• ISO 14971 (medical device risk management)

Which standards apply depends on the end product and its market. Your engineer will confirm what the component itself needs to meet.