Babeh Edi

CT SCAN Equipment

• Gantry
• Patient Couch
• Contrast Injector
• CT console
• Slip Rings

Gantry

The gantry is the 'donut' shaped part of the CT scanner that houses the components necessary to produce and detect x-rays to create a CT image. The x-ray tube and detectors are positioned opposite each other and rotate around the gantry aperture. Continuous rotation in one direction without cable wrap around is possible due to the use of slip rings. The images below are of a Toshiba Aquilion 16 CT scanner with the external and internal components of the gantry, and the control panel labelled. This gantry weighs a total of 1750kg (3858lb)!

CT Gantry External View

1. gantry aperture (720mm diameter)
2. microphone
3. sagittal laser alignment light
4. patient guide lights
5. x-ray exposure indicator light
6. emergency stop buttons
7. gantry control panels
8. external laser alignment lights
9. patient couch
10. ECG gating monitor

CT Gantry Internal View

1. x-ray tube
2. filters, collimator, and reference detector
3. internal projector
4. x-ray tube heat exchanger (oil cooler)
5. high voltage generator (0-75kV)
6. direct drive gantry motor
7. rotation control unit
8. data acquisition system (DAS)
9. detectors
10. slip rings
11. detector temperature controller
12. high voltage generator (75-150kV)
13. power unit (AC to DC)
14. line noise filter

CT Gantry Control Panel

1. gantry tilt (+/-30 degrees)
2. laser alignment lights on/off
3. couch in/out
4. free (manual) couch movement
5. zero couch position
6. couch up/down
7. home button (couch out & down)

Patient Couch

1. CT gantry
2. couch top
3. couch pedestal
4. couch control pedals (up/down & in/out)
5. velcro patient immobilisation strap

The patient lies on the couch (also known as a table) and is moved through the CT gantry aperture during the CT examination. Depending on the body part to be scanned and the protocol selected the patient may be positioned supine or prone and either head or feet first. A weight limit for the couch of approximately 205kg (450lb) is specified by the manufacturer beyond which the movement of the table is not guaranteed to be accurate and may even result in damage.
The couch top is usually made of carbon fibre due to its strength and low x-ray attenuation properties. A thin radiolucent mattress and a pillow are placed on the couch top to increase patient comfort. Detachable velcoro straps can be used to help immobilise and secure an active patient. The couch top must be capable of moving at least 1800mm to allow the patient to be scanned from 'head-to-toe' without having to be repositioned.
The couch pedestal (or base) houses the electronic and mechanical components that allow the couch to move in both the horizontal (longitudinal) and vertical direction . The pedestal allows the height of the table to be altered in the vertical direction to make it easier for patients with varying levels of mobility to access it. The foot pedals offer an alternate to using the controls on the CT gantry to move the couch up/down and in/out. The couch can also be moved remotely by the CT operator from the control console.
During conventional slice-by-slice scanning the couch is indexed (moved) between each scan depending on the slice thickness and slice incrementation (degree of overlap or separation) that has been selected for that examination. For spiral/helical CT, including multislice CT, the couch is translated through the gantry at a constant speed depending on the length of the area to be scanned, the total scan time, and the pitch that has been selected.

Slip Rings

A slip ring is electromechanical technology that enables the transmission of power and electrical signals from a stationary to a rotating structure. This transmission of power/data is made possible through electrical connections made by stationary brushes pressing against rotating circular conductors. This technology is also called "rotary electrical joint" and "electric swivel" technology. A slip ring can be used in any electromechanical system that requires unrestrained, intermittent or continuous rotation while transmitting power and / or data. In Radiology, Slip Ring technology is found in CT, MRI, high-resolution ultrasound and digital mammography systems. In additional to medical systems, Slip Rings are also found in Electro-Optic Systems, Helicopters, Propellers, Wind Turbines and some Vehicles. CT Slip ring technology was introduced to enable helical (continuous rotating) scanning. Prior to the introduction of Slip Rings, only Axial scanning was possible (which had the need to stop / reverse direction of rotation, after no more than 700 degrees rotation due to the finite length of the attached cables.) Slip ring technology eliminated the need for cables and enabled the continuous rotation of the gantry components.

CT Control Console

Slip rings used to bring power to x-ray tube on rotating gantry of a helical CT machine and, for some designs, to acquire information from the detector array. (a) The shiny metal strips carry electric signals that are swept off by special brushes. (b) The brushes are not in the form of bristles but rather of metal blocks (in this case a silver alloy). The five pairs of larger brushes provide the voltage required by the x-ray tube, and the three pairs of smaller ones transfer signals from the gantry controller.