§ 175.08 Definitions.
RCNY § 175.08
The following definitions apply to Parts I and II (24 RCNY Health Code §§ 175.01 through 175.70) of this Article: "Absorbed dose (D)" means the energy imparted by ionizing radiation per unit mass of irradiated material. The SI unit of absorbed dose is joule per kilogram or gray (Gy). The previously used ("traditional") unit of absorbed dose is the rad (1 Gy = 100 rad). "Absorbed dose rate" means absorbed dose per unit time, for machines with timers, or dose monitor unit per unit time for linear accelerators. "Accelerator" means any machine capable of accelerating electrons, protons, or other charged particles in a vacuum and of discharging the resultant particulates or other radiation into a medium at energies usually in excess of 1 MeV. "Accessible surface" means the external or outside surface of the enclosure or housing of the radiation producing machine as provided by the manufacturer. This includes the high-voltage generator, doors, access panels, latches, control knobs, and other permanently mounted hardware and including the plane across the exterior edge of any opening. "Added filtration" means any filtration which is in addition to the inherent filtration. "Air kerma (K)" means the kinetic energy released in air by ionizing radiation per mass of air. The SI unit of air kerma is joule per kilogram or gray (Gy). "Air kerma rate (AKR)" means the air kerma per unit time. "As low as reasonably achievable (ALARA)" means making every reasonable effort to maintain exposures to radiation as far below the dose limits in this Article as practical, consistent with the purpose for which the licensed or registered activity is undertaken, taking into account the state of technology, the economics of improvements in relation to the state of technology, the economics of improvements in relation to benefits to the public health and safety, and other societal and socioeconomic considerations in the public interest. "Alert value" means a dose index (e.g., of CTDIvol (mGy) or DLP (mGy-cm)) that is set by the registrant to trigger an alert to the CT operator prior to scanning within an ongoing examination. The alert value represents a dose value well above the registrant's established range for the examination that warrants more stringent review and consideration before proceeding. "Aluminum equivalent" means the thickness of type 1100 aluminum alloy affording the same attenuation, under specified conditions, as the material in question. "Analytical x-ray equipment" means equipment that generates (by electronic means) and uses ionizing radiation for the purpose of examining the microstructure of materials, i.e. diffraction and spectroscopy (including fluorescence). "Annual" means at least once per year, at about the same time each year, plus or minus one calendar month. "Attenuation block" means a block or stack of type 1100 aluminum alloy, or aluminum alloy having equivalent attenuation, with dimensions 20 centimeters (cm) or larger by 20 cm or larger by 3.8 cm, that is large enough to intercept the entire x-ray beam. "Authorized user" means a licensed physician who is identified as a user of therapeutic equipment on a certified registration issued by the Department. "Automatic exposure control (AEC)" means a device which automatically controls one or more technique factors in order to obtain at a preselected location a required quantity of radiation. "Automatic exposure rate control (AERC)" means a device which automatically controls one or more technique factors in order to obtain, at a preselected location, a required quantity of radiation per unit time. "Background radiation" means radiation from cosmic sources; naturally occurring radioactive material, including radon (except as a decay product of source or special nuclear material); and global fallout as it exists in the environment from the testing of nuclear explosive devices or from past nuclear accidents such as Chernobyl that contribute to background radiation and are not under the control of the licensee or registrant. Background radiation does not include radiation from any regulated sources of radiation. "Barrier" has the same meaning as "protective barrier." "Beam axis" means a line from the source through the centers of the x-ray fields. "Beam-limiting device" means a device which provides a means to restrict the dimensions of the x-ray field. "Beam monitoring system" means a system designed and installed in the radiation head to detect and measure the radiation present in the useful beam. "Beam scattering foil" means a thin piece of material, usually metallic, placed in the beam to scatter a beam of electrons in order to provide a more uniform electron distribution in the useful beam. "Beam port" means an opening on the x-ray apparatus designed to emit a primary beam. This does not include openings on baggage units. "Biennial" means the test is done at least once every other year, at about the same time, plus or minus one calendar month. "Biweekly" means at least once per two consecutive weeks (see "Weekly"). "Bone densitometer" means a device intended for medical purposes to measure bone density and mineral content by x-ray or gamma ray transmission measurements through the bone and adjacent tissues. This generic type of device may include signal analysis and display equipment, patient and equipment supports, component parts, and accessories. "Bone densitometry" means a noninvasive measurement of certain physical characteristics of bone that reflect bone strength. Test results are typically reported as bone mineral content or density and are used for diagnosing osteoporosis, estimating fracture risk, and monitoring changes in bone mineral content. "C-arm fluoroscope" means a fluoroscopic x-ray system in which the image receptor and the x- ray tube housing assembly are connected or coordinated to maintain a spatial relationship. Such a system allows a change in the direction of the beam axis with respect to the patient without moving the patient. "Calibration" means the determination of the response or reading of an instrument relative to a series of known radiation values over the range of the instrument, or the strength of a source of radiation relative to a standard. "Cassette holder" means a device, other than a spot-film device, that supports or fixes the position of the cassette-based image receptor during a radiographic exposure. "Cathode ray tube" means any device used to accelerate electrons for demonstration or research purposes, except where such cathode ray tube is incorporated into a television or display monitor that is subject to, and has met applicable federal radiation safety performance standards in 21 C.F.R. §§ 1010 and 1020.10. "Certified Radiation Equipment Safety Officer" (CRESO) means an individual who holds an unexpired certificate as a radiation equipment safety officer issued by the New York State Department of Health. "C.F.R." or "10 C.F.R." means, except where a citation to a specific section is given, the regulations issued by the United States Nuclear Regulatory Commission contained in Chapter I of Title 10 of the Code of Federal Regulations. "Changeable filters" means any filter, exclusive of inherent filtration, which can be removed from the useful beam through any electronic, mechanical, or physical process. "Coefficient of variation (C)" means the ratio of the standard deviation to the mean value of a population of observations. It is estimated using the following equation: where: s = Estimated standard deviation of the population. = Mean value of observations in sample; x i = ith observation in sample; n = Number of observations sampled. "Collimator" means a device for restricting the useful radiation in one or more directions. "Computed radiography (CR)" means a digital x-ray imaging method in which a photo-stimulable phosphor is used to capture and store a latent image. The latent image is read out by stimulating the phosphor with a laser. Computed radiography systems may use cassettes to house the phosphor, or it may be integrated into a digital radiography system. See also, the definition of "digital radiography." "Computed tomography (CT)" means the production of a tomogram by the acquisition and computer processing of x-ray transmission data. "Computed tomography dose index (CTDI)" means the average absorbed dose, along the z-axis, from a series of contiguous irradiations. It is measured from one axial CT scan (one rotation of the x-ray tube), and is calculated by dividing the integrated absorbed dose by the nominal total beam collimation. The scattering media for CTDI consist of two (16 and 32 cm in diameter) polymethylmethacrylate (PMMA, e.g., acrylic or Lucite) cylinders of 14 cm length. The equation is: where: D(z) = the radiation dose profile along the z-axis, N = the number of tomographic sections imaged in a single axial scan. This is equal to the number of data channels used in a particular scan. The value of N may be less than or equal to the maximum number of data channels available on the system, and T = the width of the tomographic section along the z-axis imaged by one data channel. In multiple-detector-row (multi-slice) CT scanners, several detector elements may be grouped together to form one data channel. In single-detector-row (single-slice) CT, the z-axis collimation (T) is the nominal scan width. "Computed tomography (CT) scan" and "computerized axial tomography (CAT) scan" refer to an imaging procedure that uses x-rays to create cross-sectional images of the human body. "Cone Beam Computed Tomography (CBCT)" is a volumetric imaging modality. Volumetric data are acquired using two dimensional digital detector arrays and a cone-shaped x-ray beam (instead of fan-shaped) that rotates around the patient. Reconstruction algorithms can be used to generate images of any desired plane. "Contact therapy system" means a therapeutic radiation machine with a short target to skin distance (TSD), usually less than 5 centimeters. "Control panel" means that part of the x-ray control upon which are mounted the switches, knobs, pushbuttons, keypads, touchscreens, and other hardware necessary for manually setting the technique factors. "Controlled area" has the same meaning as "restricted area". "Conventional simulator" means any x-ray system designed to reproduce the geometric conditions of the radiation therapy equipment. "Cradle" means a removable device which supports and may restrain a patient above an x-ray table; or a device whose patient support structure is interposed between the patient and the image receptor during normal use; or which is equipped with means for patient restraint and which is capable of rotation about its long (longitudinal) axis. "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 this Article . "CT dosimetry phantom" means the phantom used for determination of the dose delivered by a CT x-ray system. The phantom must be a right circular cylinder of polymethyl-methacrylate of density 1.19±0.01 grams per cubic centimeter. Except for pediatric dose measurements, the phantom must be at least 14 centimeters in length and must 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 must provide means for the placement of a dosimeter 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. "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, a normal value of 1,000 when the Houndsfield scale of CT number is used; µ x = Linear attenuation coefficient of the material of interest; µ w = Linear attenuation coefficient of water. "CTDI100" means the accumulated multiple scan dose at the center of a 100-mm scan and underestimates the accumulated dose for longer scan lengths. It is thus smaller than the equilibrium dose. The CTDI100 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 and one of the two standard CTDI acrylic phantoms (16 and 32 cm diameter) and a stationary patient table. The equation is: "CTDIvol" see "Volume Computed Tomography Dose Index (CTDIvol)" "CTDIw" see "Weighted Computed Tomography Dose Index (CTDIw)" "CT x-ray system" is technology that is used to perform CT scans and includes, but is not limited to: a control panel; image display device; gantry; x-ray tube; collimating device with filters; high voltage transformer; and, a data acquisition system. This includes CBCT systems that are used for other than dental x-ray scans. "CT scanner" refers to technology used to perform and interpret CT scans and includes, but is not limited to: a control panel; gantry; high voltage generator; x-ray tube; table; and, display devices that are used for image interpretation. "Cumulative air kerma" means the total air kerma accrued from the beginning of an examination or procedure and includes all contributions from fluoroscopic and radiographic irradiation. "Dead-man switch" means a switch so constructed that a circuit closing contact can be maintained only by continuous pressure on the switch by the operator. "Detector" has the same meaning as "radiation detector." "Declared pregnant worker" means a worker who has voluntarily informed their employer, in writing, of their pregnancy and the estimated date of conception. The declaration remains in effect until the declared pregnant worker withdraws the declaration in writing or is no longer pregnant. "Department" means the New York City Department of Health and Mental Hygiene. "Diagnostic source assembly" means the tube housing assembly with a beam-limiting device attached. "Diagnostic x-ray system" means an x-ray system designed for irradiation of any part of the human or animal body for the purpose of diagnosis or visualization. "Diaphragm" means a device or mechanism by which the radiation beam is restricted in size. "Digital radiography (DR)" means an x-ray imaging method or radiography which produces a digital rather than analog image. DR includes both computed radiography and direct digital radiography. "Direct digital radiography (DDR)" means an x-ray imaging method in which a digital sensor, usually incorporating a thin-film transistor, is used to capture an x-ray image. Some DDR systems use a scintillator to convert x-rays to light and a photodiode array to convert light to charge, while others use a photoconductor to convert x-rays directly to charge, which is stored on the thin-film transistor. See also the definitions of "computed radiography" and "digital radiography." "Direct scattered radiation" or "direct scatter radiation" means that scattered radiation which has been deviated in direction only by materials irradiated by the useful beam. See also the definition of "scattered radiation." "Direct personal supervision" means that the qualified practitioner must be present in the room when the procedure is being performed and is immediately available to provide assistance and direction throughout the performance of the procedure. "Direct supervision" means a qualified practitioner must exercise general supervision and be present in the facility and immediately available to furnish assistance and direction throughout the performance of the procedure. It does not mean that the qualified practitioner must be present in the room when the procedure is being performed. "Dose" means the absorbed dose. "Dose equivalent" means the product of the absorbed dose in tissue, quality factor, and all other necessary modifying factors, at the location of interest. Appropriate quality factors may be found in 10 C.F.R. § 20.2004. "Dose area product (DAP) or "kerma-area product (KAP)" means the product of the air kerma and the area of the irradiated field and is typically expressed in Gy-cm 2 , so it does not change with distance from the x-ray tube. "Dose length product (DLP)" means the indicator of the integrated radiation dose from a complete CT examination. It addresses the total scan length by the formula: DLP (mGy-cm) = CTDIvol (mGy) x scan length (cm) "Dose monitor unit (DMU)" means a unit response from the beam monitoring system from which the absorbed dose can be calculated. "Dose profile" means the dose as a function of position along a line. "Effective dose equivalent" is the sum of the products of the dose equivalent to the organ or tissue (HT) and the weighting factors (WT) applicable to each of the body organs or tissues that are irradiated: HE = Σ (WT HT). "Electronic brachytherapy" means a method of radiation therapy where an electrically generated source of ionizing radiation is placed in or near the tumor or target tissue to deliver therapeutic radiation dosage. "Electronic brachytherapy device" means the system used to produce and deliver therapeutic radiation including the x-ray tube, the control mechanism, the cooling system, and the power source. "Electronic brachytherapy source" means the x-ray tube component used in an electronic brachytherapy device. "Emergency procedure" means the written pre-planned steps to be taken in the event of actual or suspected exposure of an individual in excess of administrative or regulatory limits including the names and telephone numbers of individuals to be contacted. "Equipment" means x-ray equipment, unless the specific context clearly indicates otherwise. "Exposure rate" means exposure per unit of time, such as roentgen per minute. For purposes of this definition, "exposure" means the quotient dQ/dm, where dQ is the absolute value of the total charge of the ions of one sign produced in air and dm is the mass of air. "External beam radiation therapy" means therapeutic irradiation in which the source of radiation is at a distance from the body. "Extremity" means hand, elbow, arm below the elbow, foot, knee and leg below the knee. "Facility" means the location, building, vehicle, or complex under one administrative control, at which one or more radiation machines are installed, located or used. "Field of view (FOV)" means, in fluoroscopy, the diameter for circular image receptors or the diagonal for rectangular image receptors. "Field emission equipment" means equipment which uses an x-ray tube in which electron emission from the cathode is due solely to the action of an electric field. "Field-flattening filter" means a filter used to homogenize the absorbed dose rate over the radiation field. "Filter" means material placed in the useful beam to preferentially absorb selected radiations. "Fluoroscopic imaging assembly" means a subsystem in which x-ray photons produce a set of fluoroscopic images or radiographic images recorded from the fluoroscopic image receptor. It includes image receptors, electrical interlocks, if any, and structural material providing linkage between the image receptor and diagnostic source assembly. "Fluoroscopic irradiation time" means the cumulative duration during an examination or procedure of operator-applied continuous pressure to the device, enabling x-ray tube activation in any fluoroscopic mode of operation. "Fluoroscopically-Guided Interventional (FGI) procedures" means an interventional diagnostic or therapeutic procedure performed via percutaneous or other access routes, usually with local anesthesia or intravenous sedation, which uses external ionizing radiation in the form of fluoroscopy to localize or characterize a lesion, diagnostic site, or treatment site, to monitor the procedure, and to control and document therapy. "Fluoroscopy" means a technique for generating x-ray images and presenting them simultaneously and continuously as visible images. This term has the same meaning as the term "radioscopy" in the standards of the International Electrotechnical Commission. "Focal spot" means the area projected on the anode of the x-ray tube bombarded by the electrons accelerated from the cathode and from which the useful beam originates. "Gantry" means that part of a radiation therapy system supporting and allowing movements of the radiation head about a center of rotation. For computed tomography, "gantry" means tube housing assemblies, beam-limiting devices, detectors, and the supporting structures, frames, and covers which hold or enclose these components within a computed tomography system. "General purpose radiographic x-ray system" means any radiographic x-ray system which, by design, is not limited to radiographic examination of specific anatomical regions. "General supervision" means the procedure is performed under the overall direction and control of the qualified practitioner but who is not necessarily required to be physically present during the performance of the procedure. "Gonad shield" or "gonadal shield" means a protective barrier for the ovaries or testes. "Gray (Gy)" means the SI unit of absorbed dose, kerma, and specific energy imparted equal to 1 joule per kilogram. Also see "Absorbed Dose". "Half-value layer (HVL)" means the thickness of specified material which attenuates the beam of radiation to an extent such that the AKR is reduced by one-half of its original value. For the purposes of this definition, the contribution of all scattered radiation, other than any which might be present initially in the beam concerned, is deemed to be excluded. "Hand-held x-ray equipment" or "hand-held x-ray system" means x-ray equipment that is designed to be hand-held during operation. "Healing arts" means the use of radiation for purposes of medical diagnosis or treatment of humans or animals. "Healing arts screening" means the testing of human beings using x-ray machines for the detection or evaluation of health indications when such tests are not specifically and individually ordered by a licensed practitioner of the healing arts legally authorized to prescribe such x-ray tests for the purpose of diagnosis or treatment. "Image intensifier" means a device, installed in its housing, which instantaneously converts an x- ray pattern, in an analog fashion, into a corresponding light image of higher intensity. "Image receptor" means any device, such as a fluorescent screen, radiographic film, x-ray image intensifier tube, solid-state detector, or gaseous detector which transforms incident x-ray photons either into a visible image or into another form which can be made into a visible image by further transformations. In those cases where means are provided to preselect a portion of the image receptor, the term "image receptor" shall mean the preselected portion of the device. "Individual monitoring devices" ""means devices designed to be worn by a single individual for the assessment of effective dose equivalent, such as film badges, thermo-luminescent dosimeters (TLDs), optically-stimulated luminescent dosimeters (OSLDs), pocket ionization chambers etc. "Inherent filtration" means the filtration of the useful beam provided by the permanently installed components of the tube housing assembly. "Inspection" means an official examination or facility observation including, but not limited to, reviews of tests, reports, assessments, specifications, surveys and monitoring to determine compliance with applicable rules, regulations, orders, requirements and conditions enforceable by the Department. "Intensity Modulated Radiation Therapy (IMRT)" means radiation therapy that uses non-uniform radiation beam intensities which have been determined by various computer-based optimization techniques. "Interlock" means a device or engineered system that precludes access to an area of radiation hazard either by preventing entry or by automatically removing the hazard. "Interruption of irradiation" means the stopping of irradiation with the possibility of continuing irradiation without resetting of operating conditions at the control panel. "Irradiation" means the exposure of matter to ionizing radiation. "Isocenter" means the center of the smallest sphere through which the beam axis passes when the equipment moves through a full range of rotations about its common center. "Kerma" means the quantity defined by the International Commission on Radiation Units and Measurements. The kerma, K, is the quotient of dEtr by dm, where dEtr is the sum of the initial kinetic energies of all the charged particles liberated by uncharged particles in a mass dm of material; thus K=dEtr/dm, in units of J/kg, where the special name for the unit of kerma is gray (Gy). When the material is air, the quantity is referred to as "air kerma." "Kerma-area product (KAP)" has the same meaning as "dose area product." "Kilovolt (kV)" or "(kilo electron volt (keV))" means the energy equal to that acquired by a particle with one electron charge in passing through a potential difference of 1,000 volts in a vacuum. (Note: current convention is to use kV for photons and keV for electrons.) "Kilovolt peak (kVp)" means the maximum value in kilovolts of the potential difference of a pulsating generator. When only one-half of the wave is used, the value refers to the useful half of the wave. "kWs" means kilowatt second. "Last-image hold (LIH) radiograph" means an image obtained either by retaining one or more fluoroscopic images, which may be temporarily integrated, at the end of a fluoroscopic exposure or by initiating a separate and distinct radiographic exposure automatically and immediately in conjunction with termination of the fluoroscopic exposure. "Lead equivalent" means the thickness of lead affording the same attenuation, under specified conditions, as the material in question. "Leakage radiation" means all radiation coming from within the source housing, except the useful beam or radiation produced when the exposure switch or timer is not activated. "Leakage technique factors" means the technique factors associated with the diagnostic source assembly which are used in measuring leakage radiation and defined as follows: (a) For diagnostic source assemblies intended for capacitor energy storage equipment, the maximum-rated peak tube potential and the maximum-rated number of exposures in an hour for operation at the maximum-rated peak tube potential with the quantity of charge per exposure being 10 millicoulombs (or 10 mAs) or the minimum obtainable from the unit, whichever is larger; (b) For diagnostic source assemblies intended for field emission equipment rated for pulsed operation, the maximum-rated peak tube potential and the maximum-rated number of x- ray pulses in an hour for operation at the maximum-rated peak tube potential; and (c) For all other diagnostic source assemblies, the maximum-rated peak tube potential and the maximum-rated continuous tube current for the maximum-rated peak tube potential. "Licensed radiologic technologist (LRT)" means an individual who is licensed and operates in compliance with Article 35 of the New York State Public Health Law and Part 89 of Title 10 of the New York Articles, Rules and Regulations, or any successor law or regulations. "Licensee" means any person who is licensed by the Department in accordance with this Article or any person who possesses radioactive material which is subject to the licensure requirements of this Article. "Light field" means that area of the intersection of the light beam from the beam-limiting device and one of the set of planes parallel to and including the plane of the image receptor, whose perimeter is the locus of points at which the illumination visually appears to be approximately one-fourth of the maximum in the intersection. "Limited-use system" means a personnel screening system that is capable of delivering an effective dose equivalent greater than 0.25 µSv (25 µrem) per screening but cannot exceed an effective dose equivalent of 10 µSv (1 mrem) per screening. Limited-use systems require additional controls and documentation to ensure that annual individual dose limits required by H.12e. appropriate regulations or national standards are not exceeded. "Line-voltage regulation" means the difference between the no-load and the load line potentials expressed as a percent of the load line potential: Percent line-voltage regulation = 100 (V n - V l ) / V l ) where: V n = No-load line potential; and V l = Load line potential. "mA" means milliampere. "mAs" means milliampere second. "Medical event" means a situation (except for an event that results from patient intervention) in which the administration of radiation from any radiation source results in: (a) a dose that differs from the prescribed dose by more than 0.05 Sv (5 rem) effective dose equivalent, 0.5 Sv (50 rem) to an organ or tissue, or 0.5 Sv (50 rem) shallow dose equivalent to the skin; and (1) the total dose delivered differs from the prescribed dose by 20 percent or more; or (2) the fractionated dose delivered differs from the prescribed dose, for a single fraction, by 50 percent or more.
(b)a dose that exceeds 0.05 Sv (5 rem) effective dose equivalent, 0.5 Sv (50 rem) to an organ or tissue, or 0.5 Sv (50 rem) shallow dose equivalent to the skin from any of the following: (1) an administration of a dose to the wrong individual or human research subject; (2) an administration of a dose delivered by the wrong mode of treatment.
(c)a dose to the skin or an organ or tissue other than the treatment site that exceeds by 0.5 Sv (50 rem) to an organ or tissue and 50 percent or more of the dose expected from the administration defined in the written directive; (d) the administration of a CT or CBCT scan in which any of the following occur: (1) a CT or CBCT scan is performed on the wrong person; (2) a CT or CBCT scan is performed on the wrong body part, such that the patient dose from the scan to the wrong body part results in an effective dose equivalent exceeding 2.5 mSv (250 mrem), or 50 mSv (5 rem) to an organ or tissue; (3) a CT or CBCT scan that results in damage to an organ, organ system or results in hair loss or erythema as determined by a physician; or (e) Any event resulting from intervention of a patient or human research subject in which the radiation from any radiation source results or will result in unintended permanent functional damage to an organ or a physiological system, as determined by a physician. "Medical institution" means a "hospital" as defined in New York State Public Health Law § 2801(1). "Megavolt (MV)" or "mega electron volt (MeV)" means the energy equal to that acquired by a particle with one electron charge in passing through a potential difference of 1,000,000 volts in a vacuum. (Note: current convention is to use MV for photons and MeV for electrons.) "Minor" means an individual less than 18 years of age. "Mobile electronic brachytherapy service" means transportation of an electronic brachytherapy device to provide electronic brachytherapy at an address that is not the address of record. "Mobile x-ray equipment" has the meaning ascribed to it in the definition of "x-ray equipment." "Mode of operation" means, for fluoroscopic systems, a distinct method of fluoroscopy or radiography provided by the manufacturer and selected with a set of several technique factors or other control settings uniquely associated with the mode. The set of distinct technique factors and control settings for the mode may be selected by the operation of a single control. Examples of distinct modes of operation include normal fluoroscopy (analog or digital), high-level control fluoroscopy, cineradiography (analog and digital), digital subtraction angiography, electronic radiography using the fluoroscopic image receptor, and spot film recording. In a specific mode of operation, certain system variables affecting kerma, AKR, or image quality, such as image magnification, x-ray field size, pulse rate, pulse duration, number of pulses, source-image receptor distance (SID), or optical aperture, may be adjustable or may vary; their variation per se does not comprise a mode of operation different from the one that has been selected. "Monitor unit (MU)" has the same meaning as "dose monitor unit." "Monthly" means at least once per calendar month. "Moving beam radiation therapy" means radiation therapy with any planned displacement of radiation field or patient relative to each other, or with any planned change of absorbed dose distribution. It includes arc, skip, conformal, intensity modulation and rotational therapy. "Noise" in CT means the standard deviation of the fluctuations in CT number expressed as a percentage of the attenuation coefficient of water. Its estimate (Sn) is calculated using the following expression: where: CS = Linear attenuation coefficient of the material of interest. µ W = Linear attenuation coefficient of water. s = Estimated [S]standard deviation of the CT numbers of picture elements in a specified area of the CT image. "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. "Nominal treatment distance" means: (a) for electron irradiation, the distance from the scattering foil, virtual source, or exit window of the electron beam to the entrance surface of the irradiated object along the central axis of the useful beam.
