The platform: gantry, slip ring, X-ray chain, detector, table, console, power and network

Name every major assembly of a Revolution EVO or Revolution Maxima, say which side of the rotation boundary it sits on, how it is powered and connected, and which of its numbers you can take from the site documentation.

Which machines, and which documents

This course covers two GE 64-row systems that you will meet often in general hospitals: the Revolution EVO and the Revolution Maxima. The Maxima shares its site documentation with the Revolution Ace, the Optima CT680 Expert and the Maxima Select, so a lot of what you learn here also holds on those badges. Older GE lines such as BrightSpeed, LightSpeed and Optima follow the same overall logic, and Revolution ACT is a smaller sibling, but their boards, software and procedures differ. Do not carry part numbers or procedures across families on memory.

Before you touch a scanner, find out three things and write them on your job sheet: the exact system name on the label, the table type, and the software version shown on the console. On the EVO the table is a VT1700V, VT2000 or VT2000x; on the Maxima it is a GT1700V, GT2000 or the shorter Lite Table. The table type changes the room clearances, the patient weight limit and some service steps. The console is either the older NIO64 type or the Open Console built around an HP workstation (Z840, Z8G4 or, on later Maxima systems, Z8G5).

Two kinds of documents matter. The pre-installation manual tells you what the room, the power, the air conditioning and the network must provide; it is where most of the numbers in this lesson come from. The service manual and the procedures on the service desktop tell you how to test, calibrate and replace parts. In this course we name the procedures and explain why and in what order; the step-by-step method, the acceptance values and the part codes always come from the documentation for your system and software revision.

The gantry: what stands still and what turns

The gantry is the heaviest single unit: the EVO gantry is listed at about 1,820 kg and the Maxima gantry at about 1,560 kg, both standing on four pads. The EVO gantry tilts up to 30 degrees forwards and backwards (GE data sheet and site drawings); the Maxima site manual gives no tilt range and GE material describes a Digital Tilt done in reconstruction, so check in its documentation whether yours tilts mechanically. The gantry is the part that dictates most of the room: front and rear covers come off on dollies and need several metres of clear floor, and tube and detector changes use a boom assembly that needs ceiling height above the gantry.

On the stationary side you find the frame and bearing, the rotation drive, the tilt drive, the stationary half of the slip ring and data link, the gantry control electronics, the operator panels and displays on both sides of the bore, the emergency stop buttons and the positioning lasers. On the rotating side, as on GE CT systems in general, ride the X-ray tube with its cooling unit, the high-voltage tank, the collimator and filters, the detector with its data acquisition system (DAS), and the rotor electronics that run them. Whatever is on the rotor gets power and commands across the slip ring and sends its data back across it.

Air enters along the bottom of the gantry and leaves along the top. The intake passes a filter in the gantry heater assembly, and further filters protect the DAS, the tube cooling and the console. Dust from building work clogs these quickly, which is why the manufacturer insists that the scanner is the last thing installed in a finished room. Remember the airflow direction when you look at a room: a cabinet or a trolley parked against the lower edge of the gantry starves it of air.

Slip ring, power path and data path

The power distribution unit (PDU) in the scan room is the only power entry point of the system. From it, separate cables go to the gantry: a high-voltage DC supply (the cable data give plus and minus 350 V DC), a three-phase AC supply and a low-voltage AC supply. The console gets its own low-voltage feed from the PDU, and the table is fed from the gantry. Treat the DC supply as a stored-energy circuit: DC links like this usually hold charge in capacitors, so never assume its voltage has gone the moment a switch opens; verify it.

Brushes on the stationary side ride on rings on the rotor and carry power across the rotation boundary. On GE CT systems the detector data typically cross on a separate contactless link; how data and commands cross on your system is in its service documentation. Between gantry and console there is a fibre-optic cable for the data, a LAN cable and signal cables from the gantry control unit to the console and to the PDU. When the system has a communication fault, these are the physical paths you will trace, so learn where they run in your room: in the floor duct, under a raised floor or in a surface raceway.

On slip-ring CT systems the brushes and rings are wear parts, and their dust is conductive. Brush wear, contamination and ring run-out show up first as intermittent data or power faults during rotation, not as a clean failure. We return to them in lessons 3 and 4.

Tube, generator and the detector

The X-ray chain is sized by the generator option. The site documentation for both models gives a 100 kVA configuration that reaches 140 kV at 515 mA and a 75 kVA configuration; the EVO manual quotes 120 kV at 400 mA for the 75 kVA option, the Maxima manual 120 kV at 460 mA, and the Maxima also lists a 50 kVA option at 120 kV and 280 mA. The option fitted decides the maximum technique, so check it before you blame a protocol that 'should' run. The GE data sheet for the EVO names the Performix 40 Plus tube with a liquid bearing and 0.35 s as the fastest (optional) rotation; confirm the tube type on the label of the one you have.

The detector is GE's Clarity detector. The EVO data sheet describes 64 channels of 0.625 mm covering 40 mm, with overlapped reconstruction giving 128 slices per rotation in axial modes, and an integrated detector and DAS with the conversion electronics next to the photodiodes. GE's page for the Maxima Select lists 20 mm and 40 mm imaging and 64 slices; check what yours has. As on CT detectors in general, the detector is held at a controlled temperature by heaters and fans. A temperature difference across the detector is a real fault: it changes the gain of the modules and produces rings and bands long before any error appears.

From the scatter data in the manual you can also learn how the beam is shaped: the scatter surveys are made at 140 kV, 100 mA, 1 second and 40 mm collimation with a 32 cm body phantom and a 16 cm head phantom, and the manual gives correction factors for other kV and aperture settings. That is the information the radiation protection expert needs; for you, it is a reminder of how much scatter the room holds during a service exposure.

Table, console and reconstruction

The table is a positioning machine with vertical and longitudinal drives, position sensing and brakes. Its weight rating depends on type: the VT1700V and VT2000 are listed for patients up to 227 kg, the VT2000x up to 306 kg, the Maxima Lite Table up to 205 kg. The table and gantry exchange signals through a gantry-to-table cable, and the table takes its supply from the gantry, so a gantry power fault often looks like a table fault too.

The operator console holds the host computer that runs the user interface, the scan control, the database and the image storage, together with the reconstruction hardware. On the Open Console this is an HP workstation in a wheeled cabinet that needs working room behind it. The console talks to the gantry over the fibre and LAN, to the hospital network for PACS, RIS and printing, and, where the hospital allows it, to GE's remote service platform.

From a fault-finding point of view, divide the console into the scan path (prescription, scan control, raw data), the reconstruction path (raw data to images) and the image path (database, review, network). A scanner can acquire a topogram and raw data perfectly and still show no images if the reconstruction path has failed.

Power distribution, cooling and network

The building side is part of the system. A dedicated feeder should come from the main distribution panel to a lockable main disconnect, called A1/MDP in the drawings, which the manuals require within 10 m of the PDU and visible to someone working there. The same panel carries the overcurrent protection and at least one emergency off switch inside the scan suite near the console. The X-ray warning light and the door interlock are wired to a terminal block in the PDU. Many sites also have a partial UPS, typically a two-phase 10 kVA unit, that keeps the computers and some gantry functions alive through a short outage so they can shut down properly.

The manuals set limits you can measure: total load regulation at the PDU input no more than 6 percent, line-to-line imbalance no more than 2 percent of the lowest line voltage, transients limited to 1,500 V peak, and earthing such that the PDU ground is within 0.5 ohm of the facility earth and within 2 ohms of earth overall. Idle demand is about 5 kVA without rotation and X-ray, rising to the full rating for a fraction of a second during a high-power exposure. Measuring a supply with no load tells you nothing about how it behaves at peak.

The room must stay between 18 and 26 degrees C, 22 recommended, and between 30 and 60 percent relative humidity without condensation, day and night. The heat load is large: at worst about 5.5 kW from the gantry alone and about 7.7 kW for the whole system, with the peak reached during the long calibration after a tube change. The network connection is copper Ethernet at 100 Mbit/s or 1 Gbit/s, with the run from the hospital switch to the wall outlet no longer than 88 m. The remote service agent on the Maxima only needs an outbound HTTPS connection on port 443.

On-site checklist

• System name, table type, console type and software version recorded
• Generator option (100, 75 or 50 kVA) confirmed
• A1/MDP, emergency off, PDU and any UPS located; which phases the UPS carries known
• Cable routes for power, fibre, LAN and signal cables known
• Room temperature and humidity measured against 18 to 26 degrees C and 30 to 60 percent
• Free space for cover dollies, tube change box and boom checked
• Network outlet, IT contact and remote service status noted

Check yourself