Why use a direct drive motor on a CT gantry?

It removes the belt or gearing between motor and ring, so there is no compliance, backlash or belt wear. Kollmorgen describes direct drive motors moving massive imaging gantries, with servo loops closing in as little as 62 microseconds, to keep timing precise. Smooth speed matters because velocity ripple becomes timing error in the image data.

What is an MRI compatible motor?

Strictly, an item is labelled MR Safe, MR Conditional or MR Unsafe under ASTM F2503. Conventional permanent-magnet motors are unsuitable in or near the bore. MRI-guided robots instead use pneumatic, piezoelectric or ultrasonic actuators built from non-magnetic materials. One published prostate robot used non-magnetic ultrasonic motors and was described as MR Conditional after image-quality tests at 3 T.

What load does a CT patient table actuator need to carry?

Work from the table's rated patient load, the moving structure and a safety factor, at the worst cantilever position. Published tables reach 307 kg, as on the Siemens SOMATOM Force. The lift must hold the load on power loss, and the horizontal axis must move smoothly at scan speed with the patient on board, up to 737 mm/s on that scanner.

Which motions on a C-arm are motorised?

It varies by model and maker. Some systems motorise most axes; others motorise only a few, such as the column lift, and leave the rest manual with brakes. A published mobile C-arm offers +95° to −65° orbital, ±210° angulation, 450 mm vertical travel and ±12° wig-wag. Specify each axis with its range, load and holding needs.

What should I send for an imaging motion enquiry?

For a slip ring: bore, envelope, speed, every power circuit's voltage and current, data rate and direction, and service needs. For gantry, table and C-arm axes: load, range, speed, accuracy, duty and holding needs. Add the system's particular standard, such as IEC 60601-2-44 or 60601-2-54, EMC constraints, and whether anything sits near an MRI magnet.

CT slip rings, gantry motors and C-arm drives for imaging

Q: How does a CT slip ring transmit image data?

Usually without contact. Power crosses on metal-graphite brushes, while detector data crosses a capacitive or optical link between rotor and stator. SCHLEIFRING quotes up to 340 Gbit/s for its GigaCAP capacitive technology and bores up to 2,000 mm. Contactless links avoid wear and debris on the data path, which matters at the speeds and data rates of modern scanners.

Specify Motion technical library · Updated 30 October 2026 · How we prepare our content

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

CT gantry rotation time (published example)	Down to 0.25 s per turn, i.e. 240 rpm (Siemens SOMATOM Force)
CT gantry aperture and table load (same example)	78 cm bore; table load up to 307 kg; scan speed up to 737 mm/s
Large CT slip ring capability (published)	Bore up to 2,000 mm, 300+ rpm, up to 1,000 V DC and 300 A
Contactless capacitive data link (published)	Up to 340 Gbit/s (SCHLEIFRING GigaCAP)
Gantry servo loop closure (published)	As little as 62 µs (Kollmorgen)
Mobile C-arm motion (published example)	Orbital +95° to −65°, angulation ±210°, 450 mm vertical, ±12° wig-wag
MR labelling terms (ASTM F2503)	MR Safe, MR Conditional, MR Unsafe

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

Subsystem	Motion	Components	What decides the choice
CT gantry	Continuous rotation at scan speed	Direct drive ring motor or belt drive, bearing, slip ring, angle encoder	Inertia, velocity ripple, data rate
CT tilt (some models)	Limited tilt of the gantry	Gearmotor or actuator with brake	Holding torque, safe stop
Patient table	Vertical lift, horizontal travel	Electric cylinder, ballscrew module, absolute linear scale	Load, smoothness, power-loss behaviour
C-arm	Orbital, angular, vertical, wig-wag	Servo motors, low-backlash gears, brakes, absolute encoders	Offset load, stiffness, collision safety
Tube and detector stands	Vertical travel, tilt	Linear actuators, gearmotors, counterbalance	Positioning repeatability
MRI	Table travel, in-bore devices	Remote motors, non-magnetic actuators	MR labelling, image quality
Industrial CT	Part rotation and manipulation	Direct drive rotary stage, linear axes, slip ring	Accuracy, stiffness

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.

Inertia. J = 800 × 0.5² = 200 kg·m².
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.
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.
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.
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

Sizing the gantry for steady speed only. Spin-up, braking energy and centripetal load on rotor parts set the design.
Ignoring velocity ripple. It becomes timing error in the image.
Treating the table as one axis. Lift and travel have different loads, speeds and safety needs.
Calling a motor “MRI compatible” without an F2503 label and test conditions.
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.

Slip rings for CT scanners

A CT slip ring wraps the patient aperture and carries high-voltage generator power, control signals and detector data across a gantry that turns several times a second. Power runs on brushes; image data usually crosses a contactless capacitive or optical link, because sliding contacts cannot carry tens to hundreds of gigabits per second reliably. Medical CT rings are normally engineered to the scanner. Large-bore, hybrid fibre and pancake designs are starting points for industrial and security CT, intra-operative systems and retrofits. Specify bore, envelope, speed, power per ring, data rate and direction, debris limits and service intervals.

CT gantry: generator power to the X-ray tube
CT gantry: detector data off the rotor
Industrial and security CT: power and data on large rotating stages
Intra-operative CT: compact ring in a tight envelope