Technical Specifications for CT Scannersby Anastasia Zoldak
Computed tomography, or CT, uses multiple and layered x-ray images called slices to create a complete three-dimensional image used to diagnosis conditions or disease and monitor therapies. Prior to marketing new imaging devices, manufacturers must conform to the Radiation Control for Health and Safety Act , or RCHSA, of 1968 and the Medical Device Amendments, or MDA to the Federal Food, Drug, and Cosmetic Act of 1976 and 1990. To conform to these regulations, manufacturers have developed technical specification for CT scanners in compliance to federal code and radiation guidelines.
Patient Couch or Bed
Manufactured from carbon fiber to be metal free, patient beds move horizontally at speeds between .5 and 100 mm per second through the CT aperture. Horizontal positioning variances are slight, with a movement of no more than +\- 0.25 mm. Bed width and length is no more than 243 by 67.5 cm, with a weight capacity of 200 kg, or about 440 lbs..
The CT scanner aperture houses the x-ray tubes and generator needed to produce the multiple x-ray images as the patient and patient bed pass through the scanner gantry. Technical specifications for CT scanner by manufactures include the use of laser positioning lights to accurately position patients. Aperture openings range in size from 65.58 to 76.2 cm and have a title range average of +/- 300 cm.
Manufacture standards for the number of x-ray slices made by a single patient aperture pass range from six slices up to more advance CT scanners at 64. The more slices made by the CT scanner, the more detailed the three-dimensional image. New CT scanners also offer other imaging features including radiography, which creates two-dimensional x-ray viewing, fluoroscopy to create real-time two-dimensional moving images and cone beam imaging three-dimensional CT scans. The scan time technical specifications for CT scanners range from 0.5 seconds for partial scans to three minutes for full-body scans.
X-Ray Tubes and Radiation Levels
X-ray tubes generate radiation for imaging. Room temperatures require a humidity of 85% and all manufacturers recommend temperature control rooms for CT scanners to maintain the machines' integrity. Average x-ray tube shelf life is 130,000 scans, or 1 calendar year. Under the Radiation Control for Health and Safety Act of 1968 & 1990, CT devices must conform to the performance standards set for Ionizing Radiation Emitting Products. To comply with these regulations, the U.S. Food and Drug Administration under Federal Regulation 1020.31 have outlined a set of x-ray safety requirements for equipment such as CT scanners. X-ray tubes are required to be “source-skin distance” away from the patient. This means scanners operating above 50 kilovoltages of radiation must provide a distance of 18 cm. Scanners operating below kilovoltages of radiation require a distance of 10 cm for safety. The government also requires radiation limits on emissions from x-ray tube not exceeding an air kinetic energy released per unit mass of 0.26 micro gray units of radiation. To understand the small amounts of radiation a modern CT scanner generates, 5.0 or more gray units of radiation at one time results in radiation poisoning and death within 14 days. Most modern CT scanners fall below these recommendations, with radiation measurements as low as 0.1 to 0.5 microGy per scan.
Under federal regulation code Title 21, x-ray controls on CT scanners must provide a visual indication when the CT scanner is in use and an audible signal when terminating exposure. Technical specifications for CT scanner operator consoles must provide the operator with radiation dosage information such as dose length and strength along with proper monitoring tools to sense and notify radiation overages.
- Federal Drug Administration: Guidance for the Submission of 510(k)’s for Solid State X-ray Imaging Devices
- Imaging Performance Assessment of CT Scanners: Single Slice CT Scanner Comparison Report
- American Association of Medical Dosimertrists: Image-guided radiation therapy: Benefits and limitations
- US Food and Drug Administration: Code of Federal Regulations Title 21
- Jupiterimages/Polka Dot/Getty Images