Two recent reviews of radiation safety in the cardiac
catheterization laboratory outline in detail many of
the major issues (93,115,116).
The National Council on Radiation Protection and Measurement
(NCRPM) provides radiation exposure guidelines for medical
workers. There is no threshold below which harmful effects
may not occur. The use of ALARA—“as low as
reasonably achievable”—doses of radiation
should always be considered. A recent membership survey
performed by the Ad Hoc Committee on Women in Cardiology
sponsored by the ACC indicated that concerns surrounding
exposure to x-rays was a leading factor in the decision
of both men and women to avoid interventional cardiology
as a profession (93).
A. Terms for Understanding Radiation Exposure in
the Cardiac Catheterization Laboratory
In the cardiac catheterization laboratory, ionizing
radiation is produced by the interaction of x-rays and
matter. The basic units of measurement are summarized
below (93).
The measure of exposure is the roentgen (R). The roentgen
is a unit of radiation exposure defined by noting the
amount of ionization per mass of air due to x-rays or
gamma rays. It is described in terms of coulombs per
kg (C/kg).
The amount of energy absorbed per unit mass of material
is defined by the rad (radiation absorbed dose). It
is described in terms of gray units (Gy), in which 1
Gy = 100 rad.
The amount of energy absorbed by different materials
for the same exposure can vary depending on the type
of radiation and the atomic number of the material absorbing
the radiation. In radiation protection this is expressed
in terms of the rem. A rem is basically a rad multiplied
by some quality factor. In cardiology the quality factor
of x-rays and gamma rays is 1. Therefore, 1 rem = 1
rad. The units for rems are expressed in terms of sieverts
(Sv). There are 100 rems in 1 Sv. Because radiation
effects are often expressed in mSv; there are 10 mSv
per rem.
B. Biological Risks From Radiation Exposure
The biological risks from radiation exposure depend
on the amount of energy absorbed and whether there is
injury to the DNA or the cell itself. A stochastic effect
is an all-or-none effect that results in DNA injury.
This can lead to an increased risk of cancer or other
genetic effects. Stochastic effects occur with increasing
frequency as the cumulative radiation exposure increases,
but once the injury has occurred, a further increase
in the dose for that cell does not change the injury
afflicted. Nonstochastic effects, also referred to as
deterministic effects, are dose dependent and result
in cell death. Erythema, desquamation, cataracts, marrow
suppression, organ atrophy, gonadal injury, sterility,
and fibrosis are clinical expressions of this type of
injury. The greater the radiation exposure, the greater
the amount of injury that occurs with nonstochastic
or deterministic injury.
Table 14 summarizes current
recommendations and concerns. The average background
exposure that may be expected is about 0.1 rem per year.
During an average interventional cardiac catheterization
procedure, the physician operator receives about 0.004
to 0.016 rem of exposure (117-120).
In 1 review, the operating physicians in the cardiac
catheterization laboratory received from 0.2 to 6.0
rems per year, the nurses received from 0.8 to 1.6 rem
per year and the technologists about 0.2 rem per year,
as documented by collar and waist badges (120).
Ancillary personnel in the cardiac catheterization laboratory
thus receive about 10% to 30% of the dose received by
the primary operator. The maximum allowable occupational
exposure from all sources for medical workers is 5 rems
per year for the whole body. Over a total career, no
one should receive a cumulative exposure >1 rem X age
(or 50 rems) (93).
The cancer risk from radiation exposure is a stochastic
effect and appears to be related to the lifetime cumulative
dosage received. The risk of fatal cancer increases
by about 0.04% X rem of lifetime exposure (121).
The estimated risk of fatal cancer in the United States
is about 20% (122).
If a busy interventional cardiologist receives about
3 rems per year and practices for 25 years, the total
dose received would be 75 rems. This would translate
into an added risk of fatal cancer of 75 X 0.04% or
3%, resulting in a total projected overall lifetime
risk of 23% for development of a fatal cancer.
Radiation exposure of the fetus in the pregnant worker
is a special case that deserves comment. It is permissible
for a pregnant worker to make a decision as to whether
to continue working in the cardiac catheterization laboratory
during her pregnancy. There must be no repercussions
whether or not a pregnant worker chooses to work in
the laboratory itself (93).
Appropriate protection and monitoring must be provided.
The United Nations Scientific Committee on the Effects
of Atomic Radiation (UNSCEAR) estimates that the risk
of a congenital malformation or of developing a malignancy
after in utero exposure of 1 rem is about 0.2% (123).
Fetal exposure to high doses of radiation between weeks
8 and 15 may also result in mental retardation, as determined
from Japanese survivors of the atomic bomb. The risk
of mental retardation in this instance is about 0.4%
per rem of exposure (121).
On the basis of these data, it is recommended that fetal
exposure to radiation, as monitored by a waist dosimeter
worn under the outer lead apron, should be no more than
0.5 rem for the entire pregnancy or <0.05 rem per month
(93).
This can be accomplished through careful attention to
radiation dose exposure.
The greatest concern about the nonstochastic effect
of long-term occupational exposure is the formation
of cataracts. The risk is small, but cataracts have
been associated with single doses in the range of 200
rads (124).
Cumulative doses of up to 750 rads have also been reported,
with no evidence for cataracts (124).
Cataracts usually form after a latent period of several
years. The recommended maximum exposure to the eye lens
is 15 rems per year. Eye protection is therefore warranted.
Leaded eyeglasses help reduce the exposure risk.
C. Measuring Radiation Exposure
Radiation exposure is commonly measured by 1 of two
methods, either a film badge or a transluminescent dosimeter
(TLD) badge. The film badge basically contains a piece
of x-ray film. The magnitude of exposure to x-rays is
derived based on the density of the exposed photographic
film within the badge compared with densities from film
that has had known amounts of exposure. The TLD badge
contains a disk with lithium fluoride crystals. The
lithium fluoride crystal absorbs the x-rays, raising
the electrons to a higher state. When heated, the electrons
in the crystal return to their baseline state, releasing
light in proportion to the x-ray exposure.
Proper measurement of radiation exposure requires the
dosimeter badge to be worn with the front of the badge
in the direct line of the scattered x-rays. If a single
badge is worn, it is usually placed on the thyroid collar.
It is recommended, however, that 2 badges be worn during
all cardiac catheterizations, with the second badge
placed under the protective lead at the waist (93).
Cardiac angiographers receive the highest exposure on
the hands, but these are not usually monitored because
of sterility issues with wearing a ring badge. When
a ring badge is worn, it should be placed with the label
(the TLD chip) palm side down (93).
D. Minimizing Occupational Exposure
The 3 tenets for reducing occupational exposure to radiation
are time, distance, and barriers. X-ray scatter is also
reduced by minimizing the number of magnified views,
using digital-only cine acquisition, keeping the image
intensifier as close to the patient as possible, and
selecting the highest kVp that will provide acceptable
image contrast. Obviously, reducing the time of exposure
results in a reduction in overall radiation exposure.
Scatter exposure is reduced by using lower framing rates
and pulsed fluoroscopy and by minimizing both fluoroscopic
and cine time. Although fluoroscopic radiation may result
in 1/10 or so of the exposure per sec compared to cine,
the far greater duration of fluoroscopic use during
procedures in the cardiac catheterization laboratory
can result in a 6-fold greater radiation exposure from
fluoroscopy than from cine for many interventional procedures
(125).
The radiation beam attenuates based on the inverse square
law (1/d2), and distance becomes an important
means of reducing operator radiation exposure from scatter
radiation. Most x-ray scatter occurs at the entry surface
of the patient. The nearer the operator is to the x-ray
tube (not image intensifier), the greater the x-ray
exposure. For instance, in the cranial LAO view, where
the operator is closest to the x-ray tube and the bottom
of the table, the operator exposure may be 2.6 to 6.1
times that observed in the caudal RAO view, where the
x-ray tube is on the other side of the table (125).
Finally, barriers are important. Proper collimation
of the x-ray beam and copper filters helps reduce exposure
from the source. Shielding such as side table drapes,
properly positioned door- or ceiling-mounted acrylic
shields, lead aprons, thyroid collars, and protective
eyeglasses are all important in limiting occupational
radiation exposure.
E. Minimizing Radiation Exposure to the Patient
Radiation to the patient can be minimized by some simple
rules. Some of the same concepts apply here as noted
above for reducing exposure to the medical worker. Because
the patient is directly in the x-ray beam, the patient
benefits primarily from measures that reduce x-ray dose.
These include collimation of the x-ray beam, use of
pulsed fluoroscopy, copper filters, digital-only cine
acquisition, limiting magnified views, reduction of
fluoroscopy and cine times and framing rates, keeping
the source-to-image distances as narrow as possible,
using the highest kVP acceptable to maintain the lowest
mA possible, and direct shielding of sensitive areas,
such as the gonadal regions.
F. Quality Management
Quality management in the catheterization laboratory
must include (1) an effective and ongoing educational
program in the diagnostic use of x-rays, (2) accurate
monitoring and timely reporting of personnel exposure,
and (3) modification of procedural conduct in those
cases where exposure levels are of concern. These issues
are addressed earlier in this document (see Quality
Assurance Issues in the Cardiac Catheterization Laboratory).
