(b)Definitions. As used in this section, the following definitions apply: (1)Computed tomography dose index (CTDI) means the integral 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: View Image
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.
(2)Contrast scale means the change in linear attenuation coefficient per CT number relative to water; that is: View Image
where:
[MICRO]w = Linear attenuation coefficient of water.
[MICRO]x = Linear attenuation coefficient of material of interest.
(CT)w = CT number of water.
(CT)x = CT number of material of interest.
(3)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 § 1020.30(b)(36) .(4)CT number means the number used to represent the x-ray attenuation associated with each elemental area of the CT image.(6)CT dosimetry phantom means the phantom used for determination of the dose delivered by a CT x-ray system. The phantom shall be a right circular cylinder of polymethl-methacrylate of density 1.19±0.01 grams per cubic centimeter. The phantom shall be at least 14 centimeters in length and shall have diameters of 32.0 centimeters for testing any CT system designed to image any section of the body (whole body scanners) and 16.0 centimeters for any system designed to image the head (head scanners) or for any whole body scanner operated in the head scanning mode. The phantom shall provide means for the placement of a dosimeter(s) along its 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 a dosimeter(s) or alignment device at other locations may be provided for convenience. The means used for placement of a dosimeter(s) (i.e., hole size) and the type of dosimeter(s) used is at the discretion of the manufacturer. Any effect on the doses measured due 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.(7)Dose profile means the dose as a function of position along a line.(8)Modulation transfer function means the modulus of the Fourier transform of the impulse response of the system.(9)Multiple tomogram system means a CT x-ray system which obtains x-ray transmission data simultaneously during a single scan to produce more than one tomogram.(10)Noise means the standard deviation of the fluctuations in CT number expressed as a percent of the attenuation coefficient of water. Its estimate (Sn) is calculated using the following expression: View Image
where:
CS = Contrast scale.
[MICRO]w = Linear attenuation coefficient of water.
s = Estimated standard deviation of the CT numbers of picture elements in a specified area of the CT image.
(11)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.(12)Picture element means an elemental area of a tomogram.(13)Remanufacturing means modifying a CT system in such a way that the resulting dose and imaging performance become substantially equivalent to any CT x-ray system manufactured by the original manufacturer on or after November 29, 1984. Any reference in this section to "manufacture", "manufacturer", or "manufacturing" includes remanufacture, remanufacturer, or remanufacturing, respectively.(14)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.(15)Scan sequence means a preselected set of two or more scans performed consecutively under preselected CT conditions of operations.(16)Sensitivity profile means the relative response of the CT x-ray system as a function of position along a line perpendicular to the tomographic plane.(17)Single tomogram system means a CT x-ray system which obtains x-ray transmission data during a scan to produce a single tomogram.(18)Tomographic plane means that geometric plane which the manufacturer identifies as corresponding to the output tomogram.(19)Tomographic section means the volume of an object whose x-ray attenuation properties are imaged in a tomogram.