N.D. Admin. Code § 33.1-10-06-11
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:
$$CTDI = \frac{1}{nT} \int-7T+7T D(z)dz$$
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:
$$CS = \frac{\mu_x - \mu_w}{(CTN)_x - (CTN)_w}$$
where:
$\mu_x$ = Linear attenuation coefficient of the material of interest.
$\mu_w$ = 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.
$$CTN = \frac{k(\mu_x - \mu_w)}{\mu_w}$$
where:
k = A constant (The constant has a normal value of one thousand when the Houndsfield scale of CTN is used.)
$\mu_x$ = Linear attenuation coefficient of the material of interest.
$\mu_w$ = 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"). I. "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 (S$_n$) is calculated using the following expression:
$$Sn = \frac{100 \times CS \times s}{\muw}$$
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.
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:
[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.
History: Effective January 1, 2019.
General Authority: NDCC 23.1-03-04; S.L. 2017, ch. 199, § 1
Law Implemented: NDCC 23.1-03-03, 23.1-03-04; S.L. 2017, ch. 199, § 18