Computed tomography x-ray systems

1. Definitions. In addition to the definitions provided in sections 33.1-10-01-04 and 33.1-10-06-02, the following definitions are applicable to this section: a. "Computed tomography dose index" means the integral from -7T to +7T of the dose profile along a line perpendicular to the tomographic plane divided by the product of the nominal tomographic section thickness and the number of tomograms produced in a single scan, that is: where: z = Position along a line perpendicular to the tomographic plane. D(z) = Dose at position z. T = Nominal tomographic section thickness. n = Number of tomograms produced in a single scan. This definition assumes that the dose profile is centered around z = 0 and that, for a multiple tomogram system, the scan increment between adjacent scans is nT. b. "Contrast scale" means the change in the linear attenuation coefficient per computed tomography number relative to water, that is: where: μx = Linear attenuation coefficient of the material of interest. μw = Linear attenuation coefficient of water. (CTN)x = CTN of the material of interest. (CTN)w = CTN of water. c. "CS" (see "contrast scale"). d. "CT" means a radiologic imaging technique that produces images of "slices" through a patient's body. e. "CT conditions of operation" means all selectable parameters governing the operation of a CT x-ray system, including nominal tomographic section thickness, filtration, and the technique factors as defined in section 33.1-10-06-02. f. "CTDI" (see "computed tomography dose index"). g. "CT gantry" means the tube housing assemblies, beam-limiting devices, detectors, and the supporting structures and frames which hold these components. h. "CTN" (see "CT number"). i. "CT number" means the number used to represent the x-ray attenuation associated with each elemental area of the CT image. where: k = A constant (The constant has a normal value of one thousand when the Houndsfield scale of CTN is used.) μx = Linear attenuation coefficient of the material of interest. μw = Linear attenuation coefficient of water. j. "Dose profile" means the dose as a function of position along a line. k. "Elemental area" means the smallest area within a tomogram for which the x-ray attenuation properties of a body are depicted. (see also "picture element"). l. "Multiple tomogram system" means a computed tomography x-ray system which obtains x-ray transmission data simultaneously during a single scan to produce more than one tomogram. m. "Noise" means the standard deviation of the fluctuations in computed tomography number expressed as a percentage of the attenuation coefficient of water. Its estimate (Sn ) is calculated using the following expression: where: CS = Contrast scale. μw = Linear attenuation coefficient of water. s = Estimated standard deviation of the CTN of picture elements in a specified area of the CT image. n. "Nominal tomographic section thickness" means the full width at half maximum of the sensitivity profile taken at the center of the cross sectional volume over which x-ray transmission data are collected. o. "Picture element" means an elemental area of a tomogram. p. "Reference plane" means a plane which is displaced from and parallel to the tomographic plane. q. "Scan" means the complete process of collecting x-ray transmission data for the production of a tomogram. Data can be collected simultaneously during a single scan for the production of one or more tomograms. r. "Scan increment" means the amount of relative displacement of the patient with respect to the CT x-ray system between successive scans measured along the direction of such displacement. s. "Scan sequence" means a preselected set of two or more scans performed consecutively under preselected CT conditions of operation. t. "Scan time" means the period of time between the beginning and end of x-ray transmission data accumulation for a single scan. u. "Single tomogram system" means a CT x-ray system which obtains x-ray transmission data during a scan to produce a single tomogram. v. "Tomographic plane" means that geometric plane which is identified as corresponding to the output tomogram. w. "Tomographic section" means the volume of an object whose x-ray attenuation properties are imaged in a tomogram. 2. Requirements for equipment. a. Termination of exposure. (1) Means must be provided to terminate the x-ray exposure automatically by either de-energizing the x-ray source or shuttering the x-ray beam in the event of equipment failure affecting data collection. Such termination must occur within an interval that limits the total scan time to no more than one hundred ten percent of its preset value through the use of either a backup timer or devices which monitor equipment function. (2) A visible signal must indicate when the x-ray exposure has been terminated through the means required by paragraph 1. (3) The operator must be able to terminate the x-ray exposure at any time during a scan, or series of scans under computed tomography x-ray system control, of greater than one-half second duration. b. Tomographic plane indication and alignment. (1) For any single tomogram system, means must be provided to permit visual determination of the tomographic plane or a reference plane offset from the tomographic plane. (2) For any multiple tomogram system, means shall be provided to permit visual determination of the location of a reference plane. This reference plane can be offset from the location of the tomographic planes. (3) If a device using a light source is used to satisfy paragraph 1 or 2, the light source must provide illumination levels sufficient to permit visual determination of the location of the tomographic plane or reference plane under ambient light conditions of up to five hundred lux. c. Beam-on and shutter status indicators and control switches. (1) The computed tomography x-ray control and gantry must provide visual indication whenever x-rays are produced and, if applicable, whether the shutter is open or closed. (2) Each emergency button or switch must be clearly labeled as to its function. d. Indication of computed tomography conditions of operation. The computed tomography x-ray system must be designed such that the computed tomography conditions of operation to be used during a scan or a scan sequence must be indicated prior to the initiation of a scan or a scan sequence. On equipment having all or some of these conditions of operation at fixed values, this requirement may be met by permanent markings. Indication of computed tomography conditions of operation must be visible from any position from which scan initiation is possible. e. Extraneous radiation. When data are not being collected for image production, the radiation adjacent to the tube port may not exceed that permitted by subsection 3 of section 33.1-10-06-04. f. Maximum surface computed tomography dose index identification. The angular position where the maximum surface computed tomography dose index occurs must be identified to allow for reproducible positioning of a computed tomography dosimetry phantom. g. Additional requirements applicable to computed tomography x-ray systems containing a gantry manufactured after September 3, 1985. (1) The total error in the indicated location of the tomographic plane or reference plane may not exceed five millimeters. (2) If the x-ray production period is less than one-half second, the indication of x-ray production must be actuated for at least one-half second. Indicators at or near the gantry must be discernible from any point external to the patient opening where insertion of any part of the human body into the primary beam is possible. (3) The deviation of indicated scan increment versus actual increment may not exceed plus or minus one millimeter with any mass from zero to one hundred kilograms resting on the support device. The patient support device must be incremented from a typical starting position to the maximum incremented distance or thirty centimeters, whichever is less, and then returned to the starting position. Measurement of actual versus indicated scan increment may be taken anywhere along this travel. (4) Premature termination of the x-ray exposure by the operator shall necessitate resetting of the computed tomography conditions of operation prior to the initiation of another scan. h. Facility design requirements. (1) All CT capable systems shall be required to have the x-ray control permanently mounted in a protected area during the entire exposure (see appendix B). (2) Aural communication. Provision must be made for two-way aural communication between the patient and the operator at the control panel. i. Surveys, calibrations, spot checks, and operating procedures. (1) Surveys. (a) All computed tomography x-ray systems installed after March 1, 1992, and those systems not previously surveyed shall have a survey made by, or under the direction of, a qualified expert. In addition, such surveys must be done after any change in the facility or equipment which might cause a significant increase in radiation hazard. (b) The registrant shall obtain a written report of the survey from the qualified expert, and a copy of the report must be made available to the department upon request. (2) Radiation calibrations. (a) The calibration of the radiation output of the computed tomography x-ray system must be performed by, or under the direction of, a qualified expert who is physically present at the facility during such calibration. (b) The calibration of a computed tomography x-ray system must be performed at intervals specified by a qualified expert and after any change or replacement of components which, in the opinion of the qualified expert, could cause a change in the radiation output. (c) The calibration of the radiation output of a computed tomography x-ray system must be performed with a calibrated dosimetry system. The calibration of such system must be traceable to a national standard. The dosimetry system must have been calibrated within the preceding two years. (d) Computed tomography dosimetry phantoms must be used in determining the radiation output of a computed tomography x-ray system. Such phantoms must meet the following specifications and conditions of use: [1] Computed tomography dosimetry phantoms must be right circular cylinders of polymethyl methacrylate of density one point nineteen plus or minus point zero one grams per cubic centimeter. The phantoms must be at least fourteen centimeters in length and must have diameters of thirty-two centimeters for testing computed tomography x-ray systems designed to image any section of the body and sixteen centimeters for systems designed to image the head or for whole-body scanners operated in the head scanning mode. [2] Computed tomography dosimetry phantoms must provide means for the placement of a dosimeter along the axis of rotation and along a line parallel to the axis of rotation one centimeter from the outer surface and within the phantom. Means for the placement of dosimeters or alignment devices at other locations may be provided. [3] Any effects on the doses measured due to the removal of phantom material to accommodate dosimeters must be accounted for through appropriate corrections to the reported data or included in the statement of maximum deviation for the values obtained using the phantom. [4] All dose measurements must be performed with the computed tomography dosimetry phantom placed on the patient couch or support device without additional attenuation materials present. (e) The calibration shall be required for each type of head, body, or whole-body scan performed at the facility. (f) Calibration must meet the following requirements: [1] The dose profile along the center axis of the computed tomography dosimetry phantom for the minimum, maximum, and midrange values of the nominal tomographic section thickness used by the registrant shall be measurable. Where less than three nominal tomographic thicknesses can be selected, the dose profile determination must be performed for each available nominal tomographic section thickness. [2] The computed tomography dose index (For the purpose of determining the computed tomography dose index, the manufacturer's statement as to the nominal tomographic section thickness for that particular system may be utilized.) along the two axes specified in item 2 of subparagraph d must be measured. The computed tomography dosimetry phantom must be oriented so that the measurement point one centimeter from the outer surface and within the phantom is in the same angular position within the gantry as the point of maximum surface computed tomography dose index identified. The computed tomography conditions of operation must correspond to typical values used by the registrant. [3] The spot checks specified in paragraph 3 of subdivision i must be made. (g) Calibration procedures must be in writing. Records of calibrations performed must be maintained for inspection by the department. (3) Spot checks. (a) The spot check procedures must be in writing and must have been developed by a qualified expert. (b) The spot check procedures must incorporate the use of a computed tomography dosimetry phantom which has a capability of providing an indication of contrast scale, noise, nominal tomographic section thickness, the resolution capability of the system for low and high contrast objects, and measuring the mean computed tomography number for water or other reference material. (c) All spot checks must be included in the calibration required by paragraph 2 and at time intervals and under system conditions specified by a qualified expert. (d) Spot checks must include acquisition of images obtained with the computed tomography dosimetry phantoms using the same processing mode and computed tomography conditions of operation as are used to perform calibrations required by paragraph 2 of subdivision i. The images must be retained, until a new calibration is performed, in two forms as follows: [1] Photographic copies of the images obtained from the image display device; and [2] Images stored in digital form on a storage medium compatible with the computed tomography x-ray system. (e) Written records of the spot checks performed shall be maintained for inspection by the department. (4) Operating procedures. (a) The computed tomography x-ray system may not be operated except by an individual who has been specifically trained in its operation. (b) Information must be available at the control panel regarding the operation and calibration of the system. Such information must include the following: [1] Dates of the latest calibration and spot checks and the location within the facility where the results of those tests may be obtained; [2] Instructions on the use of the computed tomography dosimetry phantoms, including a schedule of spot checks appropriate for the system, allowable variations for the indicated parameters, and the results of at least the most recent spot checks conducted on the system; [3] The distance in millimeters between the tomographic plane and the reference plane if a reference plane is utilized; and [4] A current technique chart available at the control panel which specifies for each routine examination the computed tomography conditions of operation and the number of scans per examination. (c) If the calibration or spot check of the computed tomography x-ray system identifies that a system operating parameter has exceeded a tolerance established by the qualified expert, use of the computed tomography x-ray system on patients must be limited to those uses permitted by established written instructions of the qualified expert.