Chapter 68 Fluoroscopy and Radiation Safety Flashcards
Fluoroscopy is required in the advanced
procedures where precise needle placement is required
These procedures include interventions for
back pain such
as epidural steroid injection, facet joint injection, facet nerve
block and rhizotomy, sacroiliac joint injection, discography,
placement of spinal cord stimulator and the newer interventional
procedures such as biaculoplasty, nucleoplasty, and
vertebroplasty. Fluoroscopy is also used in lumbar paravertebral
sympathetic blocks as well as visceral sympathetic
blocks such as celiac plexus block, superior hypogastric
plexus block, and ganglion impar block. Blocks outside the
vicinity of the spine also benefit from fluoroscopic guidance
and include trigeminal nerve block and gasserian ganglion block.
For transforaminal epidural steroid injections
confirmation of correct needle placement
and spread of the dye in the anterior epidural space can only be demonstrated by fluoroscopy
in cervical area the lack of reliability of the loss-of-resistance technique may be partially due to
the lack of continuity of the ligamentum flavum in the
cervical area
plica mediana dorsalis
a thin septum not been demonstrated in the cervical region,
but in the lumbar and thoracic levels it has been shown to divide the posterior epidural
space into compartments hindering the free flow of
the injected solution
Why is the spread of the injectate in the anterior epidural space important
this is the location of the herniated intervertebral disc and the interface
between the herniated disc and the nerve root.
there are differences in the flow characteristics between the contrast media and the steroid
solution
the flow of the dye may not completely predict the flow of the steroid injectate. The steroid solution may be more limited in its distribution
because it tends to precipitate in its diluent which is typically either a local anesthetic or saline.
advantage of using fluoroscopy
confirmation of correct needle placement, the determination of the needle tip in an inadvertent location prior to injection. the documentation of the spread of contrast
whether it is unilateral, located in the ventral epidural space, or whether it reached the desired level of pathology.
Intravascular injection would be especially
hazardous via the
transforaminal route as arteries entering
the foramen supply the exiting nerve roots as well as the spinal cord depending on the level involved.
Digital subtraction angiography (DSA) can
further increase
sensitivity of live fluoroscopy for intravascular detection
There are several reasons for not utilizing fluoroscopy
in epidural steroid injections
avoidance of radiation; costs associated with the fluoroscopic equipment, its maintenance and technicians; inconvenient
scheduling; location of the x-ray facility; and allergy to contrast agents.
A current
measured in
milliamperes (mA), passes from an electrically heated negatively charged filament (the cathode) to an anode under a high voltage (kilovolt peak, kVp) within an x-ray tube.
As the electrons interact with the anode
energy is released
as both heat and photons called x-rays. These x-rays will then exit the tube and either become absorbed by or pass through the patient. The energy that passes through the
patient will enter an image intensifier where it is converted to a visible image that is displayed on a monitor screen and can be saved as a permanent record.
The important parts of the fluoroscopy machine include
the x-ray tube, image intensifier, C-arm, and the control panel
The x-ray tube fires the beam of electrons
through a
high-voltage vacuum tube, forming x-rays
that are emitted through a small opening. The image intensifier
collects the electromagnetic particles and translates them into a usable image that can be viewed on a television
monitor
The quality of image contrast depends on
the balance between the tube voltage (or kVp) and the tube current.
the tube voltage (kVp)
The kVp is the voltage through which the
electron beam passes in the x-ray vacuum tube. Increasing the kVp increases the penetrability of the x-ray beam through the patient and thereby decreases its absorption.
This will act to produce brighter, more exposed images but then also to decrease the contrast.
kVp values
of the spine of a normal sized adult starts with the kVp set at ~75; larger patients require a higher kVp. The typical settings are 80 to 100 kVp for the back, 50 kVp for the hands, and 70 kVp for the abdomen.
The tube current
reflects the number of electrons fired through the high voltage vacuum tube. Higher tube currents mean more
x-rays are produced and emitted. The tube current is set between 1 and 5 mA; lower settings are adequate for most
interventional fluoroscopy procedures.
The image contrast is obtained by
balancing the tube
voltage or kVp against the tube current. Higher kVp settings reduce the number of x-rays absorbed and decrease
exposure time. However, if the kVP settings are too high,
the image will lack the necessary contrast for a useful image.
Radiation
the process by which energy, in the forms
of waves or particles, is emitted from a source.
Radiation includes
x-rays, gamma rays, ultraviolet, infrared, radar,
microwaves, and radio waves
Radiation absorbed dose
rad
the unit of measure that expresses the amount of energy deposited in tissue from an ionizing radiation source.
Units of gray (Gy)
preferred, instead of rad, in the International System (SI) of units. A gray is defined as the quantity of radiation that results in an energy deposition of 1 joule per kilogram (1 J/kg) within the irradiated material; 1 Gy is equivalent to 100 rad and to 1,000 mGy
To predict
occupational exposure from x-radiation
the term radiation absorbed dose (rad) is converted to radiation equivalent man (rem) in a 1:1 ratio. The unit of dose equivalent to rem in the SI system is the sievert (Sv); 1 rem is equivalent to 1 rad and 100 rem is equivalent to 1 Sv
The biologic effects of radiation are caused by
either the
direct disruption of macromolecules such as DNA, or by the ionization of water molecules within cells, producing
highly reactive free radicals that then damage macromolecules.
Acute effects (nonstochastic or deterministic) occur at
relatively high dose levels such as those given during radiotherapy treatments or in accidents. The term acute refers to not only the short time course but also to the high dosage involved.
Chronic effects
stochastic or nondeterministic. Doses lower than 1 Gy generally do not cause noticeable acute effects other than slight cellular
changes. However, there is increased probability of
induced cancer or leukemia in the exposed individual.
Effects of radiation doses
A radiation dose equivalent of 25 rem (0.25 Sv) may lead to
a measurable hematologic depression. A whole body total radiation dose exceeding 100 rem (1 Sv) may lead to nausea, fatigue, radiation dermatitis, alopecia, intestinal disturbances, and hematologic disorders.
The average annual radiation dose from medical x-rays
only approximately 40 mrem (0.4 mSv).
maximum permissible dose (MPD)
the upper limit
of allowed radiation dose that one may receive without the
risk of significant side effects.The annual whole-body dose
limit for physicians is 50 mSv.
For the fetus,
the annual maximum permissible dose is
0.5 rem or 5 mSv per year.
ALARA (as low as reasonably achievable) or ALARP
as low as reasonably practicable
principle involved in reducing the amount of radiation dose
precautions should be employed to minimize the exposure of the patient to radiation.
The beam-on time
should be reduced since radiation exposure increases linearly with time, and total exposure is equal to the exposure
rate multiplied times the time. It is recommended that the fluoroscopy machine be equipped with a laser pointer,
which is attached to the image intensifier. The
x-ray tube should be kept as far away from the patient as possible. Increasing the distance between the x-ray tube
and the patient reduces radiation to the patient. It has been recommended that the x-ray tube be at least 30 cm away from the patient.
The image intensifier
should be positioned
as close to the patient as possible while still maintaining the room required to perform the procedure.
Collimation
should be used to reduce the area being irradiated thereby reducing the amount of x-rays received by the patient. Collimation may also increase the
quality of the image by a reduction in radiation scatter.
magnification should be limited
since magnifying the image by a factor of 1 increases the amount of radiation 2.25 times while magnifying the
image by a factor of 2 increases the amount of radiation 4 times.
“10-day rule”
the “10-day rule” wherein it was thought that x-ray examination of the abdomen of a woman of childbearing
age should be carried out within 10 days of the
onset of menstruation because this time represents the least likelihood of conception taking place. If conception took place, the embryo would be most sensitive to the effect of radiation. The “10-day rule” is probably erroneous. The fetus is relatively insensitive to the effects of radiation
in the early stages of pregnancy. The period when the fetus
is most sensitive to radiation is between 8 to 15 weeks’ gestation, when the rate of proliferation of DNA within the brain is at a maximum.
Any significant deleterious effect of radiation during conception is
likely to lead to
spontaneous abortion.
The factors affecting radiation exposure to personnel include
the time or duration of x-ray exposure, distance from the
source of the x-rays, and protection from the radiation
the major source of radiation to the
personnel is the
patient or fluoroscopy table, which serves as
a conduit for scattered radiation.
The radiation dose to the
patient and subsequent scatter can be reduced by
using the lowest tube current (mA) compatible with a good x-ray image. The beam-on time should be kept to a minimum; there
is a 5-minute alarm in most fluoroscopy machines.
The intensity of ionizing radiation decreases exponentially as
the distance from the source is increased. The inverse-square
law states that the radiation is inversely proportional to the
square of the distance (the space between the individual and the x-ray source). Therefore, as the distance is doubled, the exposure rate is reduced by one fourth
The conventional undercouch fluoroscopy arrangement occurs when
the x-ray tube is located beneath the fluoroscopy table and the image intensifier is above the table. In this arrangement and with the table horizontal,
most of the scattered radiation is in the downward direction
and absorbed in the floor or the side panels of the table. Scattered radiation is 2 to 3 times
higher at the side of the x-ray tube.
In the overcouch fluoroscopy arrangement, the position of the x-ray tube and image intensifier
reversed or the oblique
and lateral views are employed.
Shielding
refers to radiation protection afforded by equipments that absorb x-rays. The categories of shielding include fixed, mobile, and personal shielding.
Fixed shielding
includes the
thickness of walls, which should have a
lead equivalence of 1 to 3 mm, the doors, and protective cubicles.
Mobile shielding
appropriate during fluoroscopy procedures in which a member of staff needs to remain near the patient
Personal shielding
includes lead aprons, gloves, thyroid shields, and glass spectacles.
lead aprons generally
have shielding equivalence equal to
0.25- to 0.5-mm
lead barrier and will only attenuate the radiation. Lead
aprons absorb 90% to 95% of scattered radiation that reaches them.
Lead rubber gloves
usually have a minimum lead equivalence of
0.25 mm since
thicker leaded gloves make manipulations that require dexterity difficult.
The use of leaded glasses with side shields may reduce the
risk of
cataract formation. A single dose of 200 rem (2 Sv) or a total exposure of 800 rem (8 Sv) has been related to cataract formation and
the latent period between the radiation exposure and the appearance of cataracts is approximately 8 years.
Federal and state regulations in the United States require that anyone who works in a station where he or she may receive over
25% of the allowable quarterly limit (1.25 rem or 1250 mrem) must be supplied with
monitoring equipment or
a radiation badge or film badge.
A radiation badge
is a pack of photographic film that measures radiation exposure for personnel monitoring. It measures the quantity and the quality of radiation (beta or gamma radiation). It is read with a densitometer and the amount of darkening of the film is proportional to the amount of radiation absorbed by the film. The film inside the badge is easily damaged by pen or moisture and the badge cannot be used for periods exceeding
8 weeks because the image fades.
Iodine
the only element that has proved satisfactory
for general use as an intravascular radiological contrast
medium. Its radio-opacity is conferred by its high molecular
weight.
The maximum recommended concentration
of iodine is
300 mg iodine per ml and the maximum recommended dose is 3 g of iodine. Its mean half-life is 12 hr and 80% to 90% is
excreted via the kidneys within 24 hr.
two kinds of contrast media with respect to their osmolality:
the high-osmolality contrast media (HOCM) and the
low-osmolality contrast media (LOCM)
High-osmolality contrast media (HOCM)
ionic monomers and include various concentrations of sodium, meglumine, or sodium-meglumine
salts of diatrizoic and iothalamic salts. These media provide 3 iodine atoms for 2 ions, giving an iodine:particle ratio of 3:2; their osmolalities
range between 433 mOsm/kg and 2400 mOsm/kg
Low-osmolality contrast media (LOCM)
nonionic monomers, that is, a molecule
that does not dissociate in solution. The LOCM provide an iodine:particle ratio of 3:1 and their osmolalities range between 411 mOsm/kg and 796 mOsm/kg.
LOCM vs. HOCM
The LOCM
cause less nausea and vomiting, produce less pain on peripheral
arterial injection, and are associated with a lower incidence of mild, moderate, and severe adverse reactions
compared to the HOCM.
Patients considered at greater risk of
an adverse reaction to the contrast media
Patients with history of a previous adverse reaction to radiologic
contrast media (excluding mild flushing, nausea)
Asthmatic patients
Allergic and atopic patients
Cardiac patients with decompensation, unstable arrhythmia, recent
MI
Renal failure, diabetic nephropathy
Feeble infants and the elderly
Patients with severe general debility or dehydration
Patients with metabolic hematologic disorders
Patients who have a history of allergic reaction to the radiologic contrast media should
be premedicated. recommended that the patient be given three doses of oral prednisone 50 mg at 13, 7, and 1 hr before the procedure. It has also been recommended that oral diphenhydramine (Benadryl) 50 mg be given 1 hr before injection of the contrast
Ten Measures for Minimizing Risks
from Fluoroscopic x-rays
- Dose rates are greater and dose accumulates faster in larger
patients. - Keep the tube current as low as possible.
- Keep the kVp as high as possible (and mA as low as possible) to
achieve the appropriate compromise between image quality and
low patient dose. - Keep the patient at maximum distance from the x-ray tube.
- Keep the image intensifier as close to the patient as possible.
- Do not overuse geometric or electronic magnification.
- If the image quality is not compromised, remove the grid
during procedures on small patients or when the image
intensifier cannot be placed close to the patients. - Always collimate down to the area of interest.
- Personnel must wear protective aprons, use shielding, monitor
their doses, and know how to position themselves and the
machines for minimum dose. - Keep beam-on time to an absolute minimum.
