§ 221.201. Definitions.

 In addition to the definitions in § §  215.2 and 221.2 (relating to definitions), the following words and terms, when used in this section and § §  221.202—221.205, have the following meanings, unless the context clearly indicates otherwise:

   Alert value—A dose index value (for example, CTDIvol (mGy) or of DLP (mGy-cm)) that is set by the registrant or licensee, or both, to trigger an alert to the operator prior to scanning within an ongoing examination. The alert value represents a value well above the registrant’s or licensee’s established range for the examination that warrants more stringent review and consideration before proceeding.

   CS—Contrast scale—The change in the linear attenuation coefficient per CT number relative to water; that is:

   CS = (Ux - Uw)/((CT)x - (CT)w)


   Ux = Linear attenuation coefficient of the material of interest

   Uw = Linear attenuation coefficient of water

   (CT)x = CT number of the material of interest

   (CT)w = CT number of water

   CT—Computed tomography—The production of a tomogram by the acquisition and computer processing of X-ray transmission data.

   CT conditions of operation—The 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 this chapter.

   CT dosimetry phantom—The phantom used for determination of the dose delivered by a CT X-ray system.

   CT number—The number used to represent the X-ray attenuation associated with each elemental area of the CT image:



 k = A constant, a normal value of 1,000 when the Hounsfield scale of CTN is used.

 µx = Linear attenuation coefficient of the material of interest.

 µw = Linear attenuation coefficient of water.

   CTDI—Computed tomography dose index

     (i)   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.



 z = Position along a line perpendicular to the tomographic plane.

 D(z) = Dose at position z.

 T = Nominal tomographic section thickness (cm).

 N = Number of tomograms produced in a single scan.

     (ii)   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.

   CTDI100—An accumulated multiple scan dose at the center of a 100-mm scan that requires integration of the radiation dose profile from a single axial scan over specific integration limits. In the case of CTDI100, the integration limits are +50 mm, which corresponds to the 100-mm length of the commercially available ‘‘pencil’’ ionization chamber. CTDI100 is acquired using a 100-mm long, 3-cc active volume CT ‘‘pencil’’ ionization chamber, one of the two standard CTDI acrylic phantoms (16 and 32 cm diameter) and a stationary patient table.

   CTDIvol—Volume Computed Tomography Dose Index—A radiation dose parameter derived from the CTDIw (weighted or average CTDI given across the field of view), that is:

 CTDIvol = (N)(T)(CTDIw)/I,


 N = number of simultaneous axial scans per X-ray source rotation,

 T = thickness of one axial scan (mm), and

 I = table increment per axial scan (mm).


 CTDIvol = (1 / pitch) x CTDIw

   CTDIw—Weighted Computed Tomography Dose Index—The estimated average CTDI100 across the field of view. The equation is:

 CTDIw = 1/3 + 2/3 CTDI100.edge

 Where 1/3 and 2/3 approximate the relative areas represented by the center and edge values derived using the 16 cm or 32 cm acrylic phantom. CTDIw uses CTDI100 and an f-factor for air (0.87 rad/R for exposure or 1.0 mGy for air kerma measurements).

   Detector—A device that provides a signal or other indication suitable for measuring one or more quantities of incident radiation.

   Dose profile—The dose as a function of position along a line.

   Elemental area—The smallest area within a tomogram for which the X-ray attenuation properties of a body are depicted.

   Gantry—The tube housing assemblies, beam-limiting devices, detectors, transformers, if applicable, and the supporting structures and frames which hold these components.

   Lux—A unit illumination equivalent to 1 lumen per square centimeter or 0.0929 foot-candles.

   Modulation transfer function—The modulus of the Fourier transform of the impulse response of the system.

   Multiple tomogram system—A CT X-ray system which obtains X-ray transmission data simultaneously during a single scan to produce more than one tomogram.

   Noise—The standard deviation of the fluctuations in the CT number expressed as a percentage of the attenuation coefficient of water. Its estimate (Sn) is calculated using the following expression:

 Sn = 100 x CS x S/Uw


 CS = Contrast scale

 Uw = Linear attenuation coefficient of water.

 S = Estimated standard deviation of the CT number of picture elements in a specified area of the CT image.

   Nominal tomographic section thickness—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.

   Notification value—A dose index value (for example, CTDIvol (mGy) or DLP (mGy-cm)) that is set by the registrant to trigger a notification to the operator prior to scanning when the dose index exceeds the established range for the examination.

   Performance phantom—A phantom which has a capability of providing an indication of CS, noise, nominal tomographic section thickness, the resolution capability of the CT system for low and high contrast objects, and measuring the mean CT number for water or other reference materials.

   Picture element—See elemental area.

   Pixel—See elemental area.

   Reference plane—A plane which is at a known fixed distance—which could be zero—to the tomographic plane and parallel to it.

   Scan—The complete process of collecting X-ray transmission data for the production of a tomogram. Data may be collected simultaneously during a single scan for the production of one or more tomograms.

   Scan increment—The amount of relative displacement of the patient with respect to the CT X-ray system between successive scans measured along the direction of the displacement.

   Scan sequence—A preselected set of two or more scans performed consecutively under preselected CT conditions of operation.

   Scan time—The period of time between the beginning and end of X-ray transmission data accumulation for a single scan.

   Sensitivity profile—The relative response of the CT X-ray system as a function of position along a line perpendicular to the tomographic plane.

   Single tomogram system—A CT X-ray system which obtains X-ray transmission data during a scan to produce a single tomogram.

   Technique factors—The conditions of operation, specified as follows:

     (i)   For CT equipment designed for pulsed operations, peak tube potential, scan time in seconds, X-ray pulse width in seconds and the number of X-ray pulses per second or per mAs.

     (ii)   For CT equipment not designed for pulsed operation, peak tube potential, and either tube current and scan time in seconds or the product of tube current and exposure time in mAs.

   Tomogram—The depiction of the X-ray attenuation properties of a section through a body.

   Tomographic plane—The geometric plane which is identified as corresponding to the output tomogram.

   Tomographic section—The volume of an object whose X-ray attenuation properties are imaged in a tomogram.


   The provisions of this §  221.201 issued and amended under sections 301 and 302 of the Radiation Protection Act (35 P.S. § §  7110.301 and 7110.302); and section 1920-A of The Administrative Code of 1929 (71 P.S. §  510-20); and amended under the Radon Certification Act (63 P.S. § §  2001—2014).


   The provisions of this §  221.201 adopted October 2, 1998, effective October 3, 1998, 28 Pa.B. 4894; amended May 16, 2008, effective May 17, 2008, 38 Pa.B. 2243; amended October 26, 2018, effective January 24, 2019, 48 Pa.B. 6791. Immediately preceding text appears at serial pages (333947) to (333950).

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