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25 Pa. Code § 221.2. Definitions.

§ 221.2. Definitions.

 The following words and terms, when used in this chapter, have the following meanings, unless the context clearly indicates otherwise:

   AAPM—American Association of Physicists in Medicine.

   Air kerma—Kerma in air.

   Air kerma rate—Air kerma per unit time.

   Aluminum equivalent—The thickness of type 1100 aluminum alloy—the nominal chemical composition of type 1100 aluminum alloy is 99% minimum aluminum, 0.12% copper—affording the same attenuation, under specified conditions, as the material in question.

 Automatic exposure control—A device which automatically controls one or more technique factors to obtain at preselected locations a desired quantity of radiation.

 Beam axis—A line from the source through the centers of the X-ray fields.

 Beam-limiting device—A device providing a means to restrict the dimensions of the X-ray field.

 CBCT—Cone beam computed tomography—A digital volume tomography method used in some imaging applications using two-dimensional digital detector arrays and a cone-shaped X-ray beam, instead of fan-shaped, that rotates around to generate a high-resolution 3D image with high geometric accuracy. Reconstruction algorithms can be used to generate images of any desired plane.

 CINE—Cineradiography—A motion picture record of successive images appearing on a fluoroscopic screen.

 CR—Computed radiography—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. CR systems may use cassettes to house the phosphor or it may be integrated into a DR system.

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

 Cephalometric device—A device intended for the radiographic visualization and measurement of the dimensions of the human head.

   Certified components—Components of X-ray systems which are subject to regulations promulgated under the Radiation Control for Health and Safety Act of 1968 (42 U.S.C.A. § §  263b—263n).

   Certified system—An X-ray system which has one or more certified components.

   Changeable filter—A filter, exclusive of inherent filtration, which can be added to or removed from the useful beam through an electronic, mechanical or physical process.

   Coefficient of variation (C)—The ratio of the standard deviation to the mean value of a population of observations. It is estimated using the following equation:

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   Control panel—The part of the X-ray control upon which are mounted the switches, knobs, pushbuttons and other hardware necessary for manually setting the technique factors.

   DDR—Direct digital radiography—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.

   DR—Digital radiography

     (i)   An X-ray imaging method (or radiography) which produces a digital rather than film projection image.

     (ii)   The term includes CR and DDR.

   DRL—Diagnostic reference level—An investigational level, set as a standard by a recognized body (for example, the American College of Radiology, the American Association of Physicists in Medicine, the National Council on Radiation Protection and Measurements or similar), used to identify unusually high radiation doses for common diagnostic medical X-ray imaging procedures. DRLs are suggested action levels above which a facility should review its methods and determine if acceptable image quality can be achieved at lower doses. DRLs should not be applied to an individual patient.

   Dead-man switch—A switch so constructed that a circuit closing contact can be maintained only by continuous pressure on the switch by the operator.

   Dental panoramic system—A device intended to produce a radiographic image of both dental arches on one film.

   Diagnostic source assembly—The tube housing assembly with a beam-limiting device attached.

   Diagnostic X-ray system—An X-ray system designed for irradiation of a part of the human body for the purpose of diagnosis or visualization.

   Direct supervision—A licensed practitioner of the healing arts who exercises general supervision and is present in the facility and immediately available to furnish assistance and direction throughout the performance of the procedure. The licensed practitioner does not have to be present in the room when the procedure is being performed.

   Dose length product—The indicator of the integrated radiation dose from a complete CT examination. It addresses the total scan length by the following formula:

   DLP (mGy - cm) = CTDIvol (mGy) x scan length (cm)

   Electronic brachytherapy—A modality 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. X-ray devices specifically designed and solely used to treat skin cancer lesions are not considered electronic brachytherapy devices under this definition and must meet the applicable parts of this title pertaining to registration and use.

   Emerging technology—An innovative medical technology that uses an ionizing radiation source.

   Entrance exposure rate—The exposure in air per unit time at the point where the center of the useful beam enters the patient.

   FGI—Fluoroscopic-guided interventional procedures—An interventional diagnostic or therapeutic high-risk procedure performed by means of percutaneous or other access routes, usually with local anesthesia or intravenous sedation, which uses external ionizing radiation in the form of fluoroscopy to do all of the following:

     (i)   Localize or characterize a lesion, diagnostic site or treatment site.

     (ii)   Monitor the procedure.

     (iii)   Control and document the procedure.

   Field emission equipment—Equipment using an X-ray tube in which electrons are emitted from the cathode solely by the force between an electric field and the electrons.

   Filter—Material placed in the useful beam to modify the spectral energy distribution and flux of the transmitted radiation and preferentially absorb selected radiation.

   Filtration—The amount of material placed in the useful beam to modify the radiation’s characteristics, typically expressed in terms of millimeters of aluminum or copper equivalent.

   Fluoroscopic imaging assembly—A subsystem in which X-ray photons produce a fluoroscopic image. The term includes the image receptors such as the image intensifier and spot-film device, electrical interlocks, if any, and structural material providing linkage between the image receptor and diagnostic source assembly.

   Fluoroscopic system—See fluoroscopic imaging assembly.

   Focal spot—The area projected on the anode of the X-ray tube by the electrons accelerated from the cathode and from which the useful beam originates.

   General supervision—The overall direction and control of a licensed practitioner of the healing arts. The licensed practitioner is not required to be present during the performance of the procedure.

   HVL—Half-value layer

     (i)   The thickness of specified material which attenuates the exposure rate by 1/2 when introduced into the path of a given beam of radiation. In 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.

     (ii)   The term is used to describe the penetrating ability of the radiation.

   Healing arts screening—The testing of human beings using X-ray machines for the detection or evaluation of health indications when the tests are not specifically and individually ordered for the purpose of diagnosis or treatment by a licensed practitioner of the healing arts legally authorized to prescribe the X-ray tests.

   Health physics—An application of physics concerned with protection of people and the environment from the biological effects of radiation.

   High-risk procedure—Any radiologic procedure that uses energies of less than 1 million electron volts that could exceed skin doses of 200 rad (2.0 Gy).

   IORT—Intraoperative radiation therapy—A modality of therapy in which therapeutic levels of ionizing radiation are applied to a target area, such as a cancer tumor, while the area is exposed during surgery.

   Image intensifier—An image receptor with electronic amplification, installed in its housing, which instantaneously converts an X-ray pattern into a corresponding light image of higher energy density.

   Image receptor—A device, such as a fluorescent screen or radiographic film, 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.

   Intensifying screen—A fluorescent screen which transforms incident X-ray photons into a visible image.

   Intraoral dental radiography—A modality of dental radiography in which the image receptor is placed inside a patient’s oral cavity.


   kVp—Peak tube potential (see kilovolts peak).

   Kerma—A measure of energy transferred from radiation to matter and means kinetic energy released per unit mass. It is related to, but not the same as, absorbed dose. Unit of measure is gray.

   Kilovolts peak (kVp)—The maximum value of the potential difference across the X-ray tube during an exposure.

   Lead equivalent—The thickness of lead affording the same attenuation, under specified conditions, as the material in question.

   Leakage radiation—Radiation emanating from the diagnostic or therapeutic source assembly except for the following:

     (i)   The useful beam.

     (ii)   Radiation produced when the exposure switch or timer is not activated.

   Leakage technique factors—The technique factors associated with the tube housing assembly which are used in measuring leakage radiation defined as follows:

     (i)   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 charge per exposure being 10 millicoulombs—10 milliampere seconds—or the minimum charge obtainable from the unit, whichever is larger.

     (ii)   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.

     (iii)   For other equipment, the maximum-rated peak tube potential and the maximum-rated continuous tube current for the maximum-rated peak tube potential.

   Licensed practitioner of the healing arts—An individual licensed by the Commonwealth to practice the healing arts, which for the purposes of this article shall be limited to medicine, surgery, dentistry, osteopathy, podiatry and chiropractic.

   Light field—The area defined by the intersection of the light beam with a plane parallel with the plane of the image receptor. The edge of the field is defined by the points at which the light intensity is 25% of the maximum light intensity in the plane.

   Line-voltage regulation—The difference between the no-load and the load line potentials expressed as a percent of the load line potential calculated using the following equation:

    Percent line-voltage regulation = 100 (Vn - V1)/V1


    Vn = No-load line potential and

    V1 = Load line potential.

   Low-risk procedure—Any radiologic procedure that is not a high-risk procedure.


   mAs—Milliampere second.


   Maximum line current—The root-mean-square current in the supply line of an X-ray machine operating at its maximum rating.

   Medical physics—An application of physics that addresses the needs of medicine or health care. Subfields of medical physics include the following:

     (i)   Therapeutic medical physics.

     (ii)   Diagnostic medical physics or imaging.

     (iii)   Nuclear medical diagnostic or molecular imaging and therapy.

     (iv)   Medical health physics or radiation protection.

   Mobile X-ray system—See X-ray equipment.

   Patient—An individual subjected to healing arts examination, diagnosis or treatment.

   Peak tube potential—The maximum value of the potential difference across the X-ray tube during an exposure.

   Performance phantom—A device specifically approved by the QMP or QE for evaluation of operational conformance with tolerances established by the QMP, QE or manufacturer.

   Personal supervision—A licensed practitioner of the healing arts who exercises general supervision and is present in the room or adjacent control area during the performance of the procedure.

   Phototimer—A method for controlling the radiation exposures to an image receptor by measuring the radiation which reaches a radiation monitoring device. The radiation monitoring device is part of an electronic circuit which controls the duration of time the tube is activated.

   Portable radiation system—See X-ray equipment.

   Portable X-ray system—See X-ray equipment.

   Position indicating device (PID)—A device on dental X-ray equipment used to indicate the beam position and to establish a definite source-surface (skin) distance.

   Positive beam limitation—The automatic or semiautomatic adjustment of an X-ray beam to the size of the selected image receptor, whereby an X-ray exposure cannot be made without an adjustment.

   Protective apron—An apron incorporating radiation absorbing materials.

   Protective barrier—A barrier of radiation absorbing material used to reduce radiation exposure. The term includes the following types:

     (i)   Primary protective barrier—Material used to reduce radiation exposure from the useful beam.

     (ii)   Secondary protective barrier—Material used to reduce exposure from stray, leakage or scattered radiation.

   QE—Qualified expert—The term as defined in §  215.2 (relating to definitions).

   QMP—Qualified medical physicist—An individual who is competent to independently provide clinical professional services and practices only in health or radiological physics, or in the subfields of medical physics.

     (i)   A QMP meets all of the following credentials:

       (A)   Certified in the field of medical physics, radiological physics, medical health physics or health physics by an appropriate national certifying body recognized by the Department.

       (B)   Complies with the certifying body’s requirements for continuing education and recertification.

       (C)   Provides clinical professional services and practices only in health/radiological physics or in one or more of the subfields of medical physics, consistent with the individual’s training and experience, and in accordance with the individual’s respective certifying body’s code of ethics.

     (ii)   An individual who does not meet the requirements of subparagraph (i) shall meet each of the following credentials to qualify as a QMP:

       (A)   Has earned a master’s or doctoral degree, or both, in physics, medical physics, biophysics, radiological physics, health physics or equivalent disciplines from an accredited college or university.

       (B)   Has 3 years of documented relevant clinical training and experience in each of the subfields in the definition of ‘‘medical physics,’’ under the supervision of a QMP who is qualified to practice in the same subfield, for each of the areas in which the individual intends to practice.

       (C)   Completes the continuing education requirements of an applicable certifying body of health/radiological physics or in one or more of the subfields of medical physics in which the individual practices.

     (iii)   An individual who has been practicing as a QMP in health/radiological physics or in one or more of subfields of medical physics for at least 5 years prior to January 24, 2019, is exempt from the requirements of subparagraphs (i) and (ii). Documentation of at least 5 years of practicing as a QMP in health/radiological physics or in one or more of the subfields of medical physics must be maintained for each of the fields or subfields, or both, in which the individual practices. As of January 24, 2019, an individual who qualifies as a QMP under this subparagraph shall meet the continuing education requirements in subparagraph (ii)(C).

   Radiation therapy simulation system—A radiographic or fluoroscopic X-ray system intended for localizing the volume to be exposed during radiation therapy and confirming the position and size of the therapeutic irradiation field.

   Radiograph—An image receptor on which an image is created directly or indirectly by an X-ray pattern and results in a permanent record.

   Radiographic imaging system—A system whereby an image is produced on an image receptor by the action of ionizing radiation.

   Radiological physics—See health physics.

   Rating—The operating limits specified by the component manufacturer.

   Registrant—A person who is legally obligated to register with the Department under this article and the act.

   Research—One of the following:

     (i)   Theoretical analysis, exploration or experimentation.

     (ii)   The extension of investigative findings and theories of a scientific or technical nature into practical application for experimental and demonstration purposes, including the experimental testing of models, devices, equipment, materials and processes. The term includes the external administration of X-ray radiation to human beings for diagnostic or therapeutic purposes or in an equivalent manner as a diagnostic or therapeutic procedure.

   SID—Source-image receptor distance—The distance from the source to the center of the input surface of the image receptor.

   SRDL—Substantial radiation dose level—An appropriately selected dose used to trigger additional dose-management actions during a procedure and medical follow-up for a radiation level that might produce a clinically relevant injury in an average patient.

   SSD—The distance between the source and the skin of the patient.

   Scattered radiation—Radiation that, during passage through matter, has been deviated in direction.

   Screening—See the definition of ‘‘healing arts screening.’’

   Serial radiography—Radiographic images produced in regular sequence.

   Shutter—A device attached to the tube housing assembly which can totally intercept the useful beam and which has a lead equivalency not less than that of the tube housing assembly.

   Source—The focal spot of the X-ray tube.

   Specific prescription—A written or oral directive authorizing a radiographic or fluoroscopic examination of a specified individual.

   Spot check—A procedure to assure that a previous calibration continues to be valid.

   Spot film—A radiograph which is made during a fluoroscopic examination to permanently record conditions which exist during that fluoroscopic procedure.

   Spot-film device—A device intended to transport or position a radiographic image receptor between the X-ray source and fluoroscopic image receptor. The term includes a device intended to hold a cassette in front of the input end of an image intensifier for the purpose of making a radiograph.

   Stray radiation—The sum of leakage and scattered radiation.

   Technique factors—The following conditions of operation:

     (i)   For capacitor energy storage equipment, peak tube potential in kV and quantity of charge in mAs.

     (ii)   For field emission equipment rated for pulsed operation, peak tube potential in kV, number of X-ray pulses and either tube current or product of tube current and time.

     (iii)   For other equipment, peak tube potential in kV and either tube current in mA and exposure time in seconds or the product of tube current and exposure time in mAs.

   Therapeutic X-ray system—A system design for irradiation of a part of the human body for the purpose of treatment or alleviation of symptoms of disease.

   Timer—An electronic device which is capable of measuring an X-ray exposure.

   Tube—An X-ray tube, unless otherwise specified.

   Tube housing assembly—The tube housing with the X-ray tube installed. The term includes high-voltage or filament transformers, or both, and other appropriate elements when contained within the tube housing.

   Unintended dose—A radiation dose in diagnostic or interventional X-ray resulting from an error in procedure or equipment malfunction.

   Useful beam—The radiation which passes through the tube housing port and the aperture of the beam-limiting device when the exposure switch or timer is activated.

   Visible area—The portion of the input surface of the image receptor over which incident X-ray photons are producing a visible image.

   Wedge filter—An added filter effecting continuous progressive attenuation on all or part of the useful beam.

   X-ray control—A device which controls input power to the X-ray high-voltage generator or the X-ray tube, or both. The term includes equipment such as timers, phototimers, automatic brightness stabilizers and similar devices, which control the technique factors of an X-ray exposure.

   X-ray equipment—An X-ray system, subsystem or component thereof. Types of X-ray equipment are as follows:

     (i)   Mobile X-ray equipment—X-ray equipment mounted on a permanent base with wheels or casters for moving while completely assembled.

     (ii)   Portable X-ray equipment—X-ray equipment designed to be hand-carried.

     (iii)   Stationary X-ray equipment—X-ray equipment which is installed in a fixed location or vehicle.

   X-ray field—The area defined by the intersection of the useful beam with a plane parallel with the plane of the image receptor. The edge of the field is defined by the points at which the exposure rate is 25% of the maximum exposure rate in the plane.

   X-ray high-voltage generator—A device which transforms electrical energy from the potential supplied by the X-ray control to the tube operating potential.

   X-ray subsystem—A combination of two or more components of an X-ray system.

   X-ray system—An assembly of components for the controlled production of X-rays. The term includes minimally an X-ray high-voltage generator, an X-ray control, a tube housing assembly, a beam-limiting device and the necessary supporting structures. Additional components which function with the system are considered integral parts of the system.

   X-ray tube—An electron tube which is designed to be used primarily for the production of X-rays.


   The provisions of this §  221.2 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 the Radon Certification Act (63 P.S. § §  2001—2014).


   The provisions of this §  221.2 adopted February 1, 1972, effective February 2, 1972, 2 Pa.B. 212; amended December 18, 1987, effective December 19, 1987, 17 Pa.B. 5235; amended October 2, 1998, effective October 3, 1998, 28 Pa.B. 4894; amended July 16, 2004, effective July 17, 2004, 34 Pa.B. 3823; 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 (333927) to (333934).

Cross References

   This section cited in 25 Pa. Code §  216.1 (relating to purpose and scope); 25 Pa. Code §  219.229 (relating to diagnostic or interventional procedure medical reports); 25 Pa. Code §  221.35a (relating to fluoroscopic X-ray systems); 25 Pa. Code §  221.201 (relating to definitions); and 25 Pa. Code §  227a.2 (relating to definitions).

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