|Absorbed dose: The energy
imparted per unit mass by ionizing radiation to matter at
a specified point. The SI unit of absorbed dose is the
joule per kilogram. The special name for this unit is the
Air kerma: The energy released per unit mass of a small
volume of air when it is irradiated by an x-ray beam. For
diagnostic x-rays, air kerma is the same as the absorbed
dose delivered to the volume of air in the absence of
scatter. Air kerma is measured in Gy.
Biologic variation: With respect to radiation, the
differences among individuals in the threshold dose
required to produce a deterministic effect, or the
differences in degree of effect produced by a given dose.
Biologic variation may be idiopathic or due to underlying
disease. Different areas and types of skin also differ in
C-arm fluoroscopic system: A fluoroscopic system
consisting of a mechanically coupled x-ray tube and image
receptor. Such systems typically have two rotational
degrees of freedom (left-right and cranial-caudal). Most
such systems have an identifiable center of rotation
called the isocenter. An object placed at the isocenter
remains centered in the beam as the C-arm is rotated.
Cumulative dose (CD): The air kerma accumulated at a
specific point in space relative to the fluoroscopic
gantry (the interventional reference point) during a
procedure. CD does not include tissue backscatter and is
measured in Gy. CD is sometimes referred to as cumulative
Deterministic effect: A radiation effect characterized by
a threshold dose. The effect is not observed unless the
threshold dose is exceeded. (The threshold dose is
subject to biologic variation.) Once the threshold dose
is exceeded in an individual, the severity of injury
increases with increasing dose. Examples of deterministic
effects include skin injury, hair loss, and cataracts.
Dose: As used in this document, "dose" is the same as the
absorbed dose unless specified as "equivalent dose" or
Dose–area–product (DAP): The integral of air kerma
(absorbed dose to air) across the entire x-ray beam emitted
from the x-ray tube. DAP is a surrogate measurement for the
entire amount of energy delivered to the patient by the beam.
DAP is measured in Gy·cm2.
Effective dose: The sum, over specified tissues, of the
products of the equivalent dose in a tissue and the
tissue weighting factor for that tissue. Effective dose
is measured in Sieverts (Sv). Stochastic risk factors are
usually stated relative to effective dose.
Equivalent dose: A quantity used for radiation protection
purposes that takes into account the different
probability of effects that occur with the same absorbed
dose delivered by radiations with different radiation
weighting factors. Effective dose is measured in Sv.
Fluorographic image: A single recorded image obtained
using an image intensifier or flat digital panel as the
image receptor. A digital angiographic "run" consists of
a series of fluorographic images.
Fluoroscopy time: The total time that fluoroscopy is used
during an imaging or interventional procedure.
Interventional reference point (IRP): For C-arm–type
fluoroscopic systems with an isocenter, the IRP is
located along the central ray of the x-ray beam at a
distance of 15 cm from the isocenter in the direction of
the focal spot. The IRP is defined by International
Electrotechnical Commission (IEC) standard 60601-2-43
Isocenter: For C-arm–type fluoroscopic systems, the point
in space between the focal spot and the image receptor through
which the central ray of the x-ray beam passes, regardless of
Kerma: Kinetic Energy Released in
Matter; the amount of energy transferred from the
x-ray beam to charged particles per unit mass in the
medium of interest. For diagnostic x-rays, this is
equivalent to absorbed dose in the specified medium (eg,
air, soft tissue, bone). Kerma is measured in Gy.
Peak skin dose (PSD): The highest dose at any portion of a
patient’s skin during a procedure.
Stochastic effect: A radiation effect whose probability of
occurrence increases with increasing dose, but whose
severity is independent of total dose. Radiation-induced
cancer is an example.
Threshold dose: The minimum radiation dose at which a
specified deterministic effect can occur. Threshold doses
differ among individuals as a result of biologic
variation. The threshold dose for skin injury also
differs in different anatomic sites on the same
Although practicing physicians should strive to achieve perfect
compliance, in practice, all physicians will fall short of
this ideal to a variable extent. Indicator thresholds may
be used to assess the efficacy of ongoing
quality-improvement programs. For the purposes of these
guidelines, a threshold is a specific level of an
indicator that should prompt a review. When compliance
rates fall below a minimum threshold, a review should be performed
to determine causes and implement changes if necessary. If
recording patient radiation dose data is one measure of
the quality of radiation dose management, compliance
rates lower than the defined threshold should trigger a
review of policies and procedures within the department
to determine the causes and implement changes to improve
quality. Thresholds may vary from those listed here; for
example, patient referral patterns and selection factors
may dictate a different threshold value for a particular indicator
at a particular institution. Because institutions and
interventional fluoroscopic units vary widely in their
ability to measure various metrics of patient dose,
radiation dose data may be recorded with use of one or
more of four different dose metrics: fluoroscopy
time/number of fluorographic images, DAP, CD, and PSD. Therefore,
setting universal thresholds is very difficult and each
department is urged to alter the thresholds as needed to
higher or lower values to meet its own
quality-improvement program needs.