How it works

Multislice computed tomography

An X-ray tube and a curved multi-row detector rotating around the patient on a slip-ring gantry, measuring attenuation from hundreds of angles and reconstructing cross-sectional images.

Course: CT Service — Level 1

Principle

A rotating-anode X-ray tube, fed by a high-voltage generator (typically 80-140 kV), produces a fan or cone beam that is shaped by filters, a bowtie filter and a collimator. After passing through the patient, the remaining radiation hits a curved array of detector elements, usually ceramic scintillators coupled to photodiodes, arranged in many rows. A data acquisition system (DAS) on the rotor digitises every channel for each angular position. Tube, generator, detector and DAS all sit on a rotor that turns continuously; power reaches it through slip rings and data leaves it through a contactless link. While the table moves through the gantry (helical scan), the reconstructor uses the projections and the exact table position to compute attenuation values, expressed in Hounsfield units, for every voxel. A practical rule for fault finding: problems 'before the patient' (tube, generator, filtration) change dose, noise and brightness of the whole image; problems 'after the patient' (detector, DAS, data link) produce localised, geometrically regular defects such as rings and streaks.

The chain, block by block

Block	What it does	How it fails
Mains supply, distribution and UPS	Three-phase supply feeds the gantry, table and computers through the system's power distribution unit; some sites add a UPS for the computers.	Voltage dips, phase imbalance or poor earthing cause intermittent, apparently random errors across the whole system; measure under load, not idle.
Slip rings and brushes	Transfer power and low-level control signals from the stationary frame to the continuously rotating rotor.	Brushes are a wear item: carbon dust inside the gantry, arcing, intermittent communication errors, streaks. Inspect brushes and clean rings at PM.
High-voltage generator	Produces tube voltage (kV) and current (mA) with fast regulation; on most modern scanners it is mounted on the rotor.	kV or mA regulation errors, high-voltage faults, breakdown in HV cables or connectors (tracking marks, smell of ozone).
X-ray tube and its cooling	Rotating-anode tube produces the beam; oil and heat exchanger remove the heat; counters record exposure time, scans and heat load.	Main consumable. End-of-life signs: rising arc (spit) errors, more image noise at the same protocol, longer cooling waits between patients, changed anode run-up sound. Filament may fail suddenly.
Beam shaping: filters, bowtie, collimator	Remove soft radiation, equalise dose across the patient and set slice coverage (z-collimation).	Wrong slice width or dose profile, collimator position errors, asymmetry in image noise.
Detector array	Rows of scintillator plus photodiode elements built from replaceable modules; converts X-rays to electrical signal per channel.	A bad or miscalibrated channel produces a ring at a fixed radius. Air calibration first: ring gone means calibration drift; ring stays means a module or its electronics. Temperature drift also causes rings.
DAS and rotor-to-stationary data link	DAS digitises each channel per view on the rotor; a contactless optical or capacitive link sends the raw data to the stationary side.	Missing or corrupted views: sharp streaks, torn projections, transfer errors in the log; dirt on the transmitter/receiver is a frequent cause.
Gantry mechanics: rotor, bearing, drive, tilt	Rotor carrying several hundred kilograms turns at sub-second speeds on a large bearing driven by belt or direct drive.	Bearing wear announces itself early: hum, vibration, sound change during run-up. Unbalance after tube replacement shortens bearing life; balancing is part of tube exchange.
Patient table	Moves the patient through the gantry with precise, encoder-measured position used in helical reconstruction.	Encoder or drive faults give wrong slice spacing, geometric distortion, mismatch with planning; check actual travel against commanded travel.
Reconstructor, console, network/PACS	Turns raw data into images, stores them and sends them to PACS.	Full disks, failed fans, network or DICOM configuration problems stop work while all X-ray hardware is healthy.

Block

What it does

How it fails

Mains supply, distribution and UPS

Three-phase supply feeds the gantry, table and computers through the system's power distribution unit; some sites add a UPS for the computers.

Voltage dips, phase imbalance or poor earthing cause intermittent, apparently random errors across the whole system; measure under load, not idle.

Slip rings and brushes

Transfer power and low-level control signals from the stationary frame to the continuously rotating rotor.

Brushes are a wear item: carbon dust inside the gantry, arcing, intermittent communication errors, streaks. Inspect brushes and clean rings at PM.

High-voltage generator

Produces tube voltage (kV) and current (mA) with fast regulation; on most modern scanners it is mounted on the rotor.

kV or mA regulation errors, high-voltage faults, breakdown in HV cables or connectors (tracking marks, smell of ozone).

X-ray tube and its cooling

Rotating-anode tube produces the beam; oil and heat exchanger remove the heat; counters record exposure time, scans and heat load.

Main consumable. End-of-life signs: rising arc (spit) errors, more image noise at the same protocol, longer cooling waits between patients, changed anode run-up sound. Filament may fail suddenly.

Beam shaping: filters, bowtie, collimator

Remove soft radiation, equalise dose across the patient and set slice coverage (z-collimation).

Wrong slice width or dose profile, collimator position errors, asymmetry in image noise.

Detector array

Rows of scintillator plus photodiode elements built from replaceable modules; converts X-rays to electrical signal per channel.

A bad or miscalibrated channel produces a ring at a fixed radius. Air calibration first: ring gone means calibration drift; ring stays means a module or its electronics. Temperature drift also causes rings.

DAS and rotor-to-stationary data link

DAS digitises each channel per view on the rotor; a contactless optical or capacitive link sends the raw data to the stationary side.

Missing or corrupted views: sharp streaks, torn projections, transfer errors in the log; dirt on the transmitter/receiver is a frequent cause.

Gantry mechanics: rotor, bearing, drive, tilt

Rotor carrying several hundred kilograms turns at sub-second speeds on a large bearing driven by belt or direct drive.

Bearing wear announces itself early: hum, vibration, sound change during run-up. Unbalance after tube replacement shortens bearing life; balancing is part of tube exchange.

Patient table

Moves the patient through the gantry with precise, encoder-measured position used in helical reconstruction.

Encoder or drive faults give wrong slice spacing, geometric distortion, mismatch with planning; check actual travel against commanded travel.

Reconstructor, console, network/PACS

Turns raw data into images, stores them and sends them to PACS.

Full disks, failed fans, network or DICOM configuration problems stop work while all X-ray hardware is healthy.

Symptom → where to look first

Symptom	Suspect first
Concentric ring on images	Detector channel: run air calibration first; a persistent ring after calibration points to a detector module or its electronics. Ring radius indicates the channel.
Image noise rising at unchanged protocol	Tube: anode wear reduces output; check tube counters and arc log, then generator kV/mA accuracy.
Frequent 'cooling' waits between patients	Room climate and tube cooling first (air conditioning, filters, heat exchanger, oil circuit), then tube heat-storage behaviour near end of life.
Scan aborts with arc / spit errors	Tube vacuum degradation or HV cable/connector breakdown; trend error frequency, inspect HV connections.
Sharp streaks or missing projections	DAS and data link: dirt on contactless transmitter/receiver, link errors; slip-ring communication if errors coincide with rotation.
Intermittent errors from many unrelated subsystems	Power and slip rings: supply dips, earthing, worn brushes and carbon dust.
Wrong slice spacing or geometry mismatch	Table: encoder and drive accuracy, measured travel vs commanded.
Growing hum or vibration during rotation	Rotor bearing or unbalance (especially after tube change); trend the noise and plan replacement early.
Scanner works but images do not reach PACS / system freezes	Reconstructor and host: disk space, archive, network and DICOM settings.

Symptom

Suspect first

Concentric ring on images

Detector channel: run air calibration first; a persistent ring after calibration points to a detector module or its electronics. Ring radius indicates the channel.

Image noise rising at unchanged protocol

Tube: anode wear reduces output; check tube counters and arc log, then generator kV/mA accuracy.

Frequent 'cooling' waits between patients

Room climate and tube cooling first (air conditioning, filters, heat exchanger, oil circuit), then tube heat-storage behaviour near end of life.

Scan aborts with arc / spit errors

Tube vacuum degradation or HV cable/connector breakdown; trend error frequency, inspect HV connections.

Sharp streaks or missing projections

DAS and data link: dirt on contactless transmitter/receiver, link errors; slip-ring communication if errors coincide with rotation.

Intermittent errors from many unrelated subsystems

Power and slip rings: supply dips, earthing, worn brushes and carbon dust.

Wrong slice spacing or geometry mismatch

Table: encoder and drive accuracy, measured travel vs commanded.

Growing hum or vibration during rotation

Rotor bearing or unbalance (especially after tube change); trend the noise and plan replacement early.

Scanner works but images do not reach PACS / system freezes

Reconstructor and host: disk space, archive, network and DICOM settings.

Safety

Site and infrastructure

Electrical supply rated for peak generator demand, measured per phase under load; dedicated protective earth; emergency stop circuits functional.
Room air conditioning sized for the gantry and electronics heat load; temperature and humidity within manufacturer limits all year. Undersized cooling is a leading real-world cause of CT downtime.
Radiation shielding of the room built to the approved shielding design; doors, windows and warning lights / interlocks working; radiation protection survey on record.
Gantry and electronics cooling (air or water as designed): filters, heat exchanger, chiller where used.
Floor loading and level for gantry and table.
Network and storage: PACS connection, sufficient local archive, time synchronisation.

Preventive maintenance basics

Read and trend tube counters (scan seconds, exposures, heat units) and arc errors at every visit; forecast tube replacement before it fails.
Inspect slip-ring brushes and rings, clean carbon dust, check contactless data link for dirt.
Clean or replace gantry and electronics air filters; check heat exchanger, fans, oil leaks.
Run air calibration and the daily/periodic QC phantom checks: CT number of water, noise, uniformity, slice thickness, and compare with baseline.
Check gantry bearing sound and vibration, drive belt condition, tilt, lasers and emergency stops.
Verify table travel accuracy against commanded positions and check table drive, rails and weight limits.
Review error logs; check disk space, archive, PACS transfer.
Measure room temperature and humidity and supply voltage under load, and record them.

Safety

Ionising radiation: never be in the scan room during exposure; use the system's service modes only with the room door closed and warning light active; wear a personal dosimeter.
High voltage: the generator and tube operate at tens of kilovolts with stored energy; follow lockout, wait for capacitor discharge and verify zero voltage before touching HV parts.
Rotating mass: never work inside the gantry unless rotation is mechanically and electrically disabled; the rotor stores large kinetic energy.
Heavy parts: tubes, detectors and rotor components require proper lifting means and two people; rotor must be locked during component exchange and balanced afterwards.
Carbon dust from slip-ring brushes is conductive; clean with the manufacturer-approved method and avoid inhaling it.

Manufacturers

Siemens Healthineers · GE HealthCare · Philips · Canon Medical Systems · United Imaging · Neusoft Medical · Fujifilm (former Hitachi CT)