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15 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

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The following slides provide a brief overview
of Radiology, including physics, image
characteristics, films, processing,
radiobiology and digital radiography.

If you right click anywhere on the screen and
select “Full Screen” the slides will be easier
to view.

Radiology Review

oil

filter

timer

exposure


switch

collimator

PID

Step
-
up

Trans.

Step
-
down


Trans.

kVp

Auto

mA

The low
-
voltage circuit (green in diagram above) controls the
heating of the filament in the x
-
ray tube. The mA control regulates
the amount of voltage that passes through the step
-
down
transformer, which in turn reduces the voltage to about 5 volts;
this is enough to heat the filament and produce electrons.

X
-
ray Machine Components

The high
-
voltage circuit (red in diagram) controls the voltage across
the x
-
ray tube. It is regulated by the kVp selector (a rheostat) and the
step
-
up transformer, resulting in a very high voltage which pulls the
electrons from the filament to the target. The higher the kVp, the
greater the energy of the electrons

oil

filter

timer

exposure


switch

collimator

PID

Step
-
up

Trans.

Step
-
down


Trans.

kVp

Auto

mA

X
-
ray Machine Components

X
-
ray Tube

molybdenum focusing cup

tungsten filament

electron flow

tungsten target

copper sleeve

unleaded glass window

leaded glass

vacuum inside tube

electrical connections

x
-
ray

When the exposure switch is depressed, the filament is heated,
producing a cloud of electrons around the filament. The high
voltage between the cathode (filament, focusing cup) and the
anode (target, copper stem) pulls the electrons across the x
-
ray
tube to interact with the target to produce x
-
rays.

Bremsstrahlung X
-
ray Production

decelerated


electron

high
-
speed


electron

x
-
ray

+

The majority of x
-
rays produced are known as Bremsstrahlung.
These x
-
rays result from the attraction between the high
-
speed
electrons (negative charge) from the filament and the protons
(positive charge) in the nuclei of the target atoms. The attraction
causes the electron to slow down and change direction, resulting in
the release of energy in the form of an x
-
ray. The closer the electron
passes to the nucleus, the greater the energy of the resultant x
-
ray.
If the electron hits the nucleus, a maximum energy x
-
ray results.

With alternating current, x
-
rays are only produced during the
positive half of the cycle (red areas above). There is a large
fluctuation in the voltage between the filament and the target,
contributing to a wide range of x
-
ray energies. X
-
ray machines
with constant potential (“direct current”) are preferred over the
standard alternating current. This provides more efficient x
-
ray
production and less exposure time per radiograph. Most of the
newer x
-
ray machines utilize constant potential.

Constant Potential (800 cycles.sec.)

60
-
cycle Alternating Current

kVp determines the voltage across the x
-
ray tube. This
ultimately determines the energy (penetrating ability) of the x
-
ray beam. Higher kVp = higher average energy (dotted lines
above) and higher maximum energy. There is also an increase
in the number of x
-
rays produced when kVp is increased.
Increasing the kVp allows you to reduce exposure time (An
increase of 15 kVp allows you to cut the exposure time in half).
It is recommended that at least one x
-
ray machine in the office
have the capability of varying the kVp (to image children,
patients with tremors, etc., which requires minimum exposure
time). In general, a higher kVp is preferred, especially for
periapical and periodontal diagnosis.

An increase in the mA setting or the exposure time
results in an increase in the number of x
-
rays
produced. There is no change in the average energy
of the x
-
ray beam. A machine with variable mA
settings would normally be set at the highest mA,
allowing for a reduced exposure time.

Exposure time: 60 impulses = 1 sec.

Density

= degree of darkening. Affected by:


Exposure factors (Increase = increase in density)


Size of head (soft tissue, bone): Increase will result


in decrease in film density


Object density (bone, teeth, restorations): Increase


will result in decrease in film density


Film fog (scatter, storage): Results in increase in


overall film density


Contrast

= density differences. Increased by:


Lowering kVp


An increase in subject contrast


Technically, higher contrast (lower kVp) is preferred for
caries detection. Lower contrast (higher kVp) is
recommended for imaging periapical and periodontal
changes. For general use, a medium kVp (70
-
75) is usually
selected.

Measures how well the details (boundaries) of
an object are reproduced on a radiograph


Increased by:


Source
-
object distance


Object
-
film distance


Film crystal size


Motion will decrease sharpness

Sharpness

Decreased by:


Source
-
object distance


Object
-
film distance

Magnification

Increasing the distance from the target of the x
-
ray tube (focal
spot, focus) to the object (teeth/film) (FFD = focus
-
film
distance) will result in an increase in sharpness and a
decrease in magnification. This results when a longer PID
(cone) is used.


Target

16” from film

Film


Target

8” from film

8” FFD image

16” FFD image

Moving the film closer to the teeth will also increase sharpness
and decrease magnification.

Most newer x
-
ray machines have a recessed target
(away from the PID). This helps to increase the focus
-
film distance (FFD), resulting in a sharper image and
less magnification without an increase in the length
of the PID (position indicating device). A longer PID is
effective, but it makes positioning the tubehead more
difficult.

8
"

FFD

12"

FFD

D
-
speed (Ultraspeed):

Probably the
most commonly used film in private practice.
Technically will give you sharper image,
because of the smaller crystal size.


F
-
speed (Insight):
Larger silver halide
crystals.
60% less radiation than D
-
speed
.


Intraoral Film

Clinasept Barriers (see above) seal the film inside
plastic, protecting the film from saliva. When the
films are separated out of the plastic, the films can
be handled for processing with minimal risk of
contamination. The cost for the film/barrier
combination is more expensive than film alone, but
the advantages in infection control are obvious.

Extraoral film for panoramic or cephalometric
radiographs comes in various types/sizes. T
-
Mat
film is available as G (high contrast), L (wide
latitude) and H (used when taking two films at
same time to provide extra film). In general, T
-
Mat
L film is used because exposure factors are not
as critical for slight variations in patient size
(wider latitude). These films are used with rare
earth screens (emit green light).


Ektavision screen/film combinations are also
available (G, H, L films). This system produces
images with more detail than the T
-
Mat system
(same speed). Ektavision screens also emit green
light.

Intensifying screens for panoramic or cephalometric
imaging should be a type of rare earth screen (green
-
light emitting). These screens require less radiation
exposure (than blue light emitting) with no loss of
image detail. Patient exposure is reduced.

Intensifying Screens


Calcium Tungstate

Barium Strontium Sulfate

Rare Earth


Lanex (used with T
-
Mat film)


Ektavision


Film Viewing Guidelines



Mask viewbox (block light around film mount)

No distractions

View films when alert, refreshed (not at the end


of the day)

Use magnifying lens

Vary illumination of viewbox if possible (rheostat)

Reduce room light

Radiobiology


Some patients are very concerned about the
amount of exposure they are getting from a
series of radiographs. It is important to
understand the effects of x
-
rays and what the
approximate doses are from individual films.
The following slides briefly describe the
effects and doses associated with routine
radiographic procedures.

When x
-
rays enter the body, they interact with the
atoms of the various tissues, causing ionization
(removal of an electron from the atom). This results
in the formation of ions and free radicals which are
very reactive and join with other atoms/molecules
to form undesirable combinations (mutations). If the
x
-
ray interacts with a critical molecule, especially
DNA, the molecule’s chemical bonds are broken
and this may alter the function of the DNA and
ultimately the cellular activity. This is the direct
effect of radiation. If the x
-
ray interacts with
another, non
-
critical molecule (usually water), the
ions and free radicals produced may in turn interact
with a critical molecule and cause damage. This is
the indirect effect.

According to the LNTH (Linear No
-
Threshold
Hypothesis; blue line above) any dose, no matter how
small, will result in some permanent damage within the
cells affected.


New molecular and cellular biology data demonstrate
that cellular control of massive natural DNA damage
rates contradicts the biological plausibility of the LNTH;
in other words, cellular repair takes care of any damage
below a certain dose. This is represented by the red line
above.

Permanent damage

Dose

It has been estimated that 8,000 to 10,000 DNA
-
damaging events occur spontaneously in each cell
every hour. This damage is successfully repaired
within minutes of its occurrence.


1 cGy (1 rad) of radiation produces 80 DNA damage
sites.


It is felt that if the cells can successfully repair the
damage from 8,000
-
10,000 spontaneous events, the
80 (1 %) more from x
-
ray exposure would also be
repaired.

Radiation Effects influenced by
:


Total Dose: Higher dose = greater effect

Dose Rate (all at once = greater effect or

spread over a period of time = less damage)

Area covered (volume of tissue): more

tissue = greater effect

Type of tissue (radiosensitive = greater

effect, radioresistant = reduced effect)

Age: greater effects in young people

Background Radiation

= 360 mrem/year

* Dental x
-
rays 0.1%

Radon 200 mrem (54%)

Cosmic (sun) 27 mrem (8%)

Rocks/soil 28 mrem (8%)

Internal 40 mrem (11%)

Medical x
-
rays* 39 mrem (11%)

Nuclear medicine 14 mrem (4%)

Consumer products 10 mrem (3%)

Other sources < 1 mrem (1%)

Everyone is exposed to certain amounts of background
radiation; the amount will vary depending on where you live,
the amount of outdoor activity, etc.. Radon is by far the largest
contributor to the background total and the radon levels in
your house should be checked. Dental x
-
rays contribute a very
small portion to the background total.

These numbers will vary depending on the source
of the information, but they are in the ballpark.
Keep in mind that these numbers don’t take into
account the total area covered; the next slide
gives a more accurate indication of the total
exposure a person receives.

Surface X
-
ray Exposure


Periapical/BW: 100 mR (F
-
speed)


250 mR (D
-
speed)

Panoramic: 500 mR

AFM: 2.0 R* (F
-
speed)

Lateral Skull: 200 mR

Chest: 20
-
40 mR


*1 R = 1000 mR

Effective dose is the approximate whole
-
body dose
received from the various x
-
ray procedures. A full
series of intraoral films, with round collimation and
F
-
speed film, results in less exposure than a chest
film, which uses intensifying screens and is
looking at soft tissue.


Effective Dose

AFM (round, F
-
speed) 6.7 mrem

AFM (rect., F
-
speed) 2.6 mrem

Panoramic 0.7 mrem

Skull 22.0 mrem

Chest 8.0 mrem

5 mSv (.5 rem) NCRP

50 mSv (5 rem) NCRP*

General Population

Occupationally exposed

M
aximum
P
ermissible
D
ose

MPD

The MPD represents the amount of radiation an
individual is allowed to receive from artificial
sources (such as x
-
ray machines). The values
listed below are per year.

*National Council on Radiation Protection

One
-
in one million chance of dying:





Obviously, we don’t want to mention dying in

connection with x
-
rays, but it puts things into

perspective regarding the effects/risks of taking

radiographs.

AFM

Smoking 1 cigarette

Riding a bicycle 10 miles

Driving a car 300 miles

Flying 1000 miles

The exposure a patient receives from a full
-
mouth
series of x
-
ray films is approximately the same as
the exposure received while traveling from New
York to LA on a plane. (Using F
-
speed film).


Patient Protection

Film ordering (Risk vs. Benefit)

Equipment reliability

Filtration

Collimation

Film/screen speed

Lead apron/thyroid collar

Technique

Processing


Ordering Films


Clinical exam


should be done before deciding
what films are needed.


Selection Criteria


developed by ADA and other
groups; these serve as guides in deciding what
films are necessary. (See next slide).


Professional Judgment


based on clinical
experience, didactic training, etc.

Selection criteria are used to identify teeth
which are potentially at risk (or are to be used
as abutments) and require periapical films to
identify periapical changes.

Selection Criteria

Symptomatic teeth

Fracture/chipped tooth

Large caries

Large restorations

Abutment teeth

Gingival condition


Equipment Reliability


Leakage Radiation

Timer Accuracy

X
-
ray production (kVp, mA)


State Inspection by Ohio Department of Health


X
-
ray machines are required to be inspected
every five years in Ohio. If you take over an
existing practice, I recommend that you contact
the ODH to schedule an inspection if one has
not been done recently.

The amount of filtration is regulated by the government
(NCRP). Any machine capable of operating at a kVp of
70 or above must have 2.5 mm aluminum equivalent of
total filtration. If the maximum kVp of the machine is 65,
only 1.5 mm aluminum equivalent is needed. The
manufacturer automatically provides this on x
-
ray
machines.

Filtration

Glass window

of x
-
ray tube

Inherent

Oil/metal barrier

1.5 mm

2.5 mm

70 kVp

Total

Aluminum filter(s)

Added

Collimation controls the size of the x
-
ray beam.
Allowed beam size is a maximum of 2.75 inches
(7 cm) in diameter at skin surface.


6 cm round


# 2 film

(4.5 cm long)

entrance

entrance

exit

exit

6 cm

7 cm

If you switch from a 7
cm round PID to a 6
cm round PID, the
patient receives 25%
less radiation.

Switching from 7cm
round PID to
rectangular PID
reduces dose by
55%.

I recommend a lead apron with a separate, but
attached, thyroid collar for intraoral films. For
panoramic or ceph films, a double
-
sided apron with
no collar is used. There are multiple sources for
lead aprons.


Lead Apron/

Thyroid Collar

Some people are suggesting

that lead aprons are not

needed. However, most feel

that any reduction in patient

exposure is beneficial and, since the cost and time
of placement of the apron are minimal, the use of
the apron is encouraged
.

….the fetal exposure is only about 1 microGray for a
full
-
mouth series. Accordingly, the guidelines for
ordering films can be used with pregnant patients
just as with other patients.


The unborn child is very sensitive to ionizing
radiation. Limit radiographic examination during
pregnancy to cases with a specific diagnostic
indication. Postpone elective procedures until the
termination of the pregnancy.


The apparently contradictory views above came from the
same textbook. Bottom line: Do what you think is best
for your pregnant patient.

Pregnancy


X
-
ray Protection for the Operator



Do not hold films for patient


• Utilize barriers if possible


Door with leaded glass


Wall of room (drywall adequate protection);


need mirror mounted opposite doorway


so that you can see patient in operatory


• Adhere to position
-
and
-
distance rule


if no barriers available (see next slide)

If no barriers are available, you should stand at
least six feet away from the patient at an angle of
90
-
135 degrees to the direction of the x
-
ray beam,
on either side of the patient (footprints in diagram
above).

Position and Distance Rule

135
°

135
°

90
°

90
°

Film badges are only required if you expect to exceed
10% of the MPD (0.05 Sv or 5 rem) during the year. Since
you should not exceed this amount if you follow routine
radiation protection procedures, film badges are not
required in the State of Ohio. However, I feel it is a good
idea to provide badges for your assistants/hygienist for
their peace of mind, at least for a 3
-
month period. If no
exposure is recorded during that time, it is unlikely that
any future exposure will occur and the badges can be
discontinued.

Ohio Department of Health

Radiologic Health Program

Registration


For purchase, transfer, or disposal


of x
-
ray equipment


Biennial registration fee


Inspection


Every five years (private practice)


Inspection fee per machine

Contact the Ohio Department of Health, Radiologic
Technology section for information and guidelines to
satisfy above requirements. (614) 752
-
4319.


State Requirements


Radiation Safety Officer (Dentist, hygienist,


or assistant)

Notice to Employees (Must be prominently


displayed); available from state.

Safe Operating Procedures (List of x
-
ray


machines, settings, usage, etc.)

Instruction of Individuals (Signed form


indicating employee familiar with x
-
ray


procedures)


Ohio State Dental Board


(614) 466
-
2580


Assistants need Radiographer’s
license to take radiographs. Need 7
hours of CE followed by in
-
office
training. Two hours of radiology CE
then required every two years.

Digital Radiography Advantages


Reduced patient exposure (Intraoral)

Ability to manipulate image

Patient consultation/education

“Instant” image (CCD, CMOS)

No chemical processing

Environmentally friendly (no processing


chemicals, silver, lead)

Remote consultation

Lower long
-
term cost?


Digital Radiography Components


X
-
ray machine: Standard intraoral or


pan/ceph machine

Sensor (CCD, CMOS, PSP; see next slide)

Laser scanner (PSP only)

Computer with monitor and modem or


high
-
speed cable connection

Printer

Direct Digital



(Sensor connected directly to computer)



CCD: Charged Coupled Device


CMOS: Complimentary Metal Oxide


Semiconductor

Indirect Digital


(Requires laser scan of sensor)



PSP: Photo
-
Stimulable Phosphor



sensor

sensor

plastic

sleeve*

to computer

* Protects film from saliva

covered with plastic sleeve and
finger cot before placing in mouth

PSP

CCD, CMOS


CCD, CMOS



These digital sensors are composed of a pure
silicon chip divided into an array of pixels (picture
elements). When x
-
rays strike the surface of the
sensor, energy is stored in the pixel; the amount
of energy stored is determined by the strength of
the x
-
ray hitting a particular pixel. These charges
are then removed electronically, in sequence,
creating an output signal with a voltage
proportional to the energy stored in each pixel.
These signals produce the digital image seen on
the monitor.


PSP

(Photostimulable phosphor)



These phosphors absorb x
-
ray energy in a manner
similar to the phosphors used in intensifying
screens. X
-
rays striking the phosphor excites
electrons in the atoms, some of which produce
light but the majority of which are trapped within
the phosphor. When the sensor is scanned with a
ruby laser, the trapped electrons are released,
causing emission of shorter
-
wavelength light in
the blue region of the spectrum. The more x
-
rays
absorbed by the phosphor, the brighter the light.
The emitted light is detected by a photomultiplier
tube and the information is digitized to form the
image.

CCD and CMOS systems produce an
“instant” image and can be useful for
endo or emergencies. The sensor is
very thick and rigid and can be more
difficult to place in the mouth. The
sensors cost several thousand dollars
to purchase or replace. Pan/ceph
systems are much more expensive.


PSP systems require laser scanning of the
sensors which takes several minutes (time to
load films in scanner and scan). Sensors are
actually thinner than x
-
ray film and are more
comfortable for the patient. Intraoral sensors
cost about $35; you would need to have enough
sensors for at least two full series in order to
function effectively. Pan/ceph sensors cost
around $800
-
900.

The initial cost of the PSP system is higher than
CCD or CMOS systems because of the cost of
the laser scanner. (This assumes you only have
one CCD/CMOS sensor).

Schick has developed a wireless sensor that will
produce an “instant” image as do the corded
sensors. However, the wireless sensor is much
thicker and more expensive.


Several systems use laptop style computers for
imaging. This allows easy portability between
operatories.


The old
-
style CRT monitors will give you better x
-
ray images, but many feel the overall space
-
saving and great color for intraoral pictures
make flat
-
panel monitors the best choice in the
operatory.

All systems differ slightly in their
software but all allow you to change
brightness and contrast, reverse
black and white, colorize, measure,
etc.. In choosing a system, evaluate
size of sensor, # of different sensor
sizes available (#0, 1, 2), overall cost
of system, extended warranty costs,
tech support, etc..

Below are two good sources for
information on digital radiography.


http://www.odont.au.dk/rad/Digitalx.htm


http://www.learndigital.net