Computed Tomography X-Ray Systems

Computed Tomography X-Ray Systems. 1. Definitions. In addition to the definitions provided in 1.1.2 and 1.6.2 of these regulations, the following definitions shall be applicable to 1.6.9: 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 CTN 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 conditions of operation" means all selectable parameters governing the operation of a CT x-ray system including, but not limited to, nominal tomographic section thickness, filtration, and the technique factors as defined in 1.6.2. e. "CTDI" (See "Computed tomography dose index"). f. "CT gantry" means the tube housing assemblies, beam-limiting devices, detectors, and the supporting structures and frames which hold these components. g. "CTN" (See "CT number"). h. "CT number" means the number used to represent the x-ray attenuation associated with each elemental area of the CT image. where: k = A constant5 μx = Linear attenuation coefficient of the material of interest. μw = Linear attenuation coefficient of water. i. "Dose profile" means the dose as a function of position along a line. j. "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"). 5 The constant has a normal value of 1,000 when the Houndsfield scale of CTN is used. k. "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. l. "Noise" means the standard deviation of the fluctuations in CTN expressed as a percentage of the attenuation coefficient of water. Its estimate (Sn) is calculated using the following expression: where: CS = Contract 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. m. "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. n. "Picture element" means an elemental area of a tomogram. o. "Reference plane" means a plane which is displaced from and parallel to the tomographic plane. p. "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. q. "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. r. "Scan sequence" means a preselected set of two or more scans performed consecutively under preselected CT conditions of operation. s. "Scan time" means the period of time between the beginning and end of x-ray transmission data accumulation for a single scan. t. "Single tomogram system" means a CT x-ray system which obtains x-ray transmission data during a scan to produce a single tomogram. u. "Tomographic plane" means that geometric plane which is identified as corresponding to the output tomogram. v. "Tomographic section" means the volume of an object whose x-ray attenuation properties are imaged in a tomogram. 2. Requirement for Equipment. a. Termination of Exposure. i. Means shall 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 shall occur within an interval that limits the total scan time to no more than 110 percent of its preset value through the use of either a backup timer or devices which monitor equipment function. ii. A visible signal shall indicate when the x-ray exposure has been terminated through the means required by 1.6.9(2)(a)(i). iii. The operator shall be able to terminate the x-ray exposure at any time during a scan, or series of scans under CT x-ray system control, of greater than one-half second duration. b. Tomographic Plane Indication and Alignment. i. For any single tomogram system, means shall be provided to permit visual determination of the tomographic plane or a reference plan offset from the tomographic plane. ii. 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. iii. If a device using a light source is used to satisfy 1.6.9(2)(b)(i) or (ii), the light source shall 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 500 lux. c. Beam-On and Shutter Status Indicators and Control Switches. i. The CT x-ray control and gantry shall provide visual indication whenever x-rays are produced and, if applicable, whether the shutter is open or closed. ii. Each emergency button or switch shall be clearly labeled as to its function. d. Indication of CT Conditions of Operation. The CT x-ray system shall be designed such that the CT conditions of operation to be used during a scan or a scan sequence shall 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 CT conditions of operation shall 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 shall not exceed that permitted by 1.6.4(3). f. Maximum Surface CTDI Identification. The angular position where the maximum surface CTDI occurs shall be identified to allow for reproducible positioning of a CT dosimetry phantom. g. Additional Requirements Applicable to CT X-Ray Systems Containing a Gantry Manufactured After September 3, 1985. i. The total error in the indicated location of the tomographic plane or reference plane shall not exceed 5 millimeters. ii. If the x-ray production period is less than one-half second, the indication of x-ray production shall be actuated for at least one-half second. Indicators at or near the gantry shall 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. iii. The deviation of indicated scan increment versus actual increment shall not exceed plus or minus 1 millimeter with any mass from 0 to 100 kilograms resting on the support device. The patient support device shall be incremented from a typical starting position to the maximum incremented distance or 30 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. iv. Premature termination of the x-ray exposure by the operator shall necessitate resetting of the CT conditions of operation prior to the initiation of another scan. 3. Facility Design Requirements. a. Aural Communication. Provision shall be made for two-way aural communication between the patient and the operator at the control panel. b. Viewing Systems. i. Windows, mirrors, closed-circuit television, or an equivalent shall be provided to permit continuous observation of the patient during irradiation and shall be so located that the operator can observe the patient from the control panel. ii. When the primary viewing system is by electronic means, an alternate viewing system (which may be electronic) shall be available for use in the event of failure of the primary viewing system. 4. Surveys, Measurements, Spot Checks and Operating Procedures. a. Surveys. i. All CT x-ray systems installed after the effective date of these regulations and those systems not previously surveyed shall have a survey made by, or under the direction of, a qualified expert. In addition, such surveys shall be done after any change in the facility or equipment which might cause a significant increase in radiation hazard. ii. The registrant shall obtain a written report of the survey from the qualified expert, and a copy of the report shall be made available to the Agency upon request. b. Radiation Measurements. i. The measurements of the radiation output of the CT x-ray system shall be performed by, or under the direction of, a qualified expert. ii. The measurement of the radiation output of the CT x-ray system shall be performed annually and after any change or replacement of components which could cause a change in the radiation output. iii. The measurement of the radiation output of the CT x-ray system shall be performed with a calibrated dosimetry system. The calibration of such system shall be traceable to a national standard. The dosimetry system shall have been calibrated within the preceding 2 years. iv. CT dosimetry phantom(s) shall be used in determining the radiation output of a CT x-ray system. Such phantom(s) shall meet the following specifications and conditions of use: i. CT dosimetry phantom(s) shall be right circular cylinders of polymethyl-methacrylate of density 1.19 plus or minus 0.01 grams per cubic centimeter. The phantom(s) shall be at least 14 centimeters in length and shall have diameters of 32.0 centimeters for testing CT x-ray systems designed to image any section of the body and 16.0 centimeters for systems designed to image the head or for whole body scanners operated in the head scanning mode. ii. CT dosimetry phantom(s) shall provide means for the placement of a dosimeter(s) along the axis of rotation and along a line parallel to the axis of rotation 1.0 centimeter from the outer surface and within the phantom. Means for the placement of dosimeters or alignment devices at other locations may be provided. iii. Any effects on the doses measured to the removal of phantom material to accommodate dosimeters shall 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. iv. All dose measurements shall be performed with the CT dosimetry phantom placed on the patient couch or support device without additional attenuation materials present. v. The measurement of the radiation output shall be required for each type of head, body, or whole-body scan performed at the facility. vi. Measurement of the radiation output shall meet the following requirements: i. The dose profile along the center axis of the CT 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 shall be performed for each available nominal tomographic section thickness. ii. The CTDI6 along the two axes specified in 1.6.9(4)(b)(iv)(ii) shall be measured. The CT dosimetry phantom shall be oriented so that the measurement point 1.0 centimeter from the outer surface and within the phantom is in the same angular position within the gantry as the point of 6For the purpose of determining the CTDI, the manufacturer's statements as to the nominal tomographic section thickness for that particular system may be utilized. maximum surface CTDI identified. The CT conditions of operation shall correspond to typical values used by the registrant. iii. The spot checks specified in 1.6.9(4)(c) shall be made. vii. Procedures for the measurement of radiation output shall be in writing. Records of measurements performed shall be maintained for inspection by the Agency. c. Spot Checks. i. The spot-check procedures shall be in writing and shall have been developed by a qualified expert. ii. The spot-check procedures shall incorporate the use of a CT 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 CTN for water or other reference material. iii. All spot checks shall be included in the measurement required by 1.6.9(4)(b) and at time intervals and under system conditions specified by a qualified expert. iv. Spot checks shall include acquisition of images obtained with the CT dosimetry phantom(s) using the same processing mode and CT conditions of operation as are used to perform measurements required by 1.6.9(4)(b). The images shall be retained, until a new measurement is performed, in two forms as follows: i. photographic copies of the images obtained from the image display device; and ii. images stored in digital form on a storage medium compatible with the CT x-ray system. v. Written records of the spot checks performed shall be maintained for inspection by the Agency. d. Operating Procedures. i. The CT x-ray system shall not be operated except by an individual who has been specifically trained in its operation. ii. Information shall be available at the control panel regarding the operation of the system and measurements of radiation output. Such information shall include the following: i. dates of the latest measurements and spot checks and the location within the facility where the results of those tests may be obtained; ii. instructions on the use of the CT dosimetry phantom(s) 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; iii. the distance in millimeters between the tomographic plane and the reference plane if a reference plane is utilized; and iv. a current technique chart available at the control panel which specifies for each routine examination the CT conditions of operation and the number of scans per examination. iii. If the measurement or spot check of the CT x-ray system identifies that a system operating parameter has exceeded a tolerance established by the qualified expert, use of the CT x-ray system on patients shall be limited to those permitted by established written instructions of the qualified expert.