Radiation and CBCT

Radiation and CBCT

what is a normal radioactive
dose??
We live in a radioactive world!
We can not see or feel it but it is all around us.
The standard measurement is the mrem.
An average annual dose of 620 mrem is common.
International standards allow up to 5000 mrems.
Medical and Dental xrays increase the normal dose.

environmental radiation
american Nuclear society July 2010
Cosmic radation at sea level 62 (mrems).
Elevation above sea level add 2, 5, 9, 15, 21, 29,
40, 53, 70 (mrems) for each 1000 additional feet.
Atlantic Coasts add 16 (mrems).
Colorado Plateau Area (Denver) 63 (mrems).
Stone, Brick or Concrete Building 7 (mrems).
From food , water and air 268 (mrem).

Entry Dose
• Entry Dose is a measured amount of radiation at the
tissue surface as the radiation enters the body. It is
easily quantifiable using thermoradiolucent sensors
on the surface of the skin.

medical/dental radiation
Absorbed dose: fundamental quantity for describing
effects of radiation in a tissue. It is the energy
deposited in the volume of tissue the beam passes
through divided by the mass of the tissue.
(joules/kilogram) joules = xray energy, kilogram =
mass or weight of tissue. 1 joule/kilogram = 1 gray
(Gy)
In older terms 1 Gy = 100 rad..
1 Gy = 1,000,000 microsieverts

Medical/dental
radiation
Equivalent dose: Biological effects in tissue vary by
the type of radiation given whether it is xrays, gamma
rays, beta rays, neutrons of other particles. Each type
of radiation interacts with tissues differently. This
difference in reaction with tissues is described by a
“radiation weighting factor” for each type of
radiation. The “absorbed dose” (in Gy) averaged over
the entire tissue is multiplied by the “radiation
weighting factor” = the “equivalent dose. The
equivalent dose is measured by the sievert (Sv).

Sv Gy and Radiation
weighting factors
For x ray energy encountered in CT the radiation
weighting factor is 1.0.
equivalent dose (Sv) = absorbed dose (Gy) x radation
weighting factor 1 (for CT)
so for CT absorbed dose in a tissue in Gy is equal to
equivalent dose in Sv.

effective dose
or risk of cancer
The risk of cancer from an equivalent dose depends
on the organ receiving the dose. Each tissue has a
specific tissue weighting factor based on its weight,
density and other characteristics that account for the
variations in cancer risk.
The product of equivalent dose x tissue weighting
factor = effective dose. It is a calculated
measurement not a measured quantity.
Effective dose is also measured by the sievert (Sv).

eview
Absorbed dose: 1 joule/kilogram = 1Gy
Equivalent dose: radiation weighting factor x
absorbed dose (Gy) = equivalent dose (Sv).
Effective dose: tissue weighting factor x equivalent
dose. (Sv).

Conversions
Radiation Units
The rem (rem) is replaced by the sievert (Sv)
1 kilorem (krem) = 10 sievert (Sv)
1 rem (rem) = 10 millisievert (mSv)
1 millirem (mrem) = 10 microsievert (µSv)
1 microrem (µrem) = 10 nanosievert (nSv)
The sievert (Sv) replaces the rem (rem)
1 sievert (Sv) = 100 rem (rem)
1 millisievert (mSv) = 100 millirem (mrem)
1 microsievert (µSv) = 100 microrem (µrem)
1 nanosievert (nSv) = 100 nanorem (nrem
Multiples
kilo (k) 1,000 = 10^3 = 1 thousand
hecto (h) 100 = 10^2 = 1 hundred
deka (da)10 = 10 = ten
1
deci (d) 0.1 =10^-1 = 1 tenth
centi (c) 0.01 = 10^-2 = 1 hundredth
milli (m) 0.001 = 10^-3 = 1 thousandth
micro (µ) 0.000 001 = 10^-6 = 1 millionth

Online converter
online unit converter pro @
http://online.unitconverterpro.com

New vs old numbers
A somewhat new article (2008) by John Ludwig and
others in Oral Surg Oral Med Pathol Radiol Endod
2008 has brought to light new radiation dose
numbers that differ from current numbers noted by
some CBCT manufacturers.
What do they mean to the doctor and patient?

Things to know
The ICRP - International Commission of Radiological
Protection.
ICRP has recently revised the tissue weighting
factors and have included the salivary glands and the
brain as a weighted tissue (remember: tissue
weighting x equivalent dose = effective dose.
Old effective doses were from 1990 new doses are
from the 2007 recommendations.

x ray is cumulative
Because x-ray risks are cumulative doctors need
strategies to reduce dose.
2007 recommendations have not been adequately
reported.
Tissue weighted factors: 2 new independent tissues,
brain and salivary glands and remainder tissues from
11 total to 14 total.

Organs included in the study
Individual organs used for calculating E or effective dose
(tissue weighting factor x equivalent dose).
Bone Marrow, esophagus, thyroid, bone surface, skin
dose, brain, salivary glands, oral mucosa, lymphatic
nodes, muscle, and extrathoracic regions.
Remember effective dose is a calculated amount not an
actual measurement. If you examine the charts in the
article you will see in Table V. very high numbers in the
columns. Using the equation above gives us the
effective doses we see in Table VI.

comparing the 1990 and the
2007
FOV
Increase in effective
dose
Large NewTom 3G 42 68 62%
Medium
Manufacturer
Effective dose
1990 micro sv
Effective dose
2007 micr sv
Classic i-CAT
extended scan 125 235 87%
Next Generation
Portrait mode 37 74 100%
Classic i-CAT
standard scan 47 102 112%
Next Generation
Landscape mode 36 87 139%
Galileos default
exposure 28 70 148%
Galileos maximum
exposure 52 128 148%
Somatron 64
MDCT 453 860 90%
Somatron 64
MDCT care dose 285 534 87%

FOV Manufacturer 1990 effective dose 2007 effective dose % change
Small
Promax 3D
small adult 151 488 224%
Promax 3D
large adult 203 488 222%
PreXion 3D
standard 512 66 189 187%
PreXion 3D
high res 1024 154 388 151%
PreXion 3D
V-11 38 (85) New Mode
Large FOV had an 83% increase
Medium FOV had a 115% increase
Small FOV had a 196% increase

Effective doses of the i-CAT
using several modes
British J of radiology 2009 Jan:82(973):35-40
This study was done in the UK and shows similar results as
the study we just reviewed. It is more complete as it gives
values for many of the different scans that can be done on
the i-CAT.
1990 ICRP 2007 ICRP
Full Head 92.8 206.2
13 cm jaws 39.5 133.9
6cm high res mand 47.2 188.5
6cm high res max 18.5 93.3
6cm standard mand 23.9 96.2
6cm standard max
9.7 58.9

Evaluating the new data
Greater increases in effective doses are seen in the smaller FOV This is due to the
concentration of the beam or focus in areas where weighted tissues are found. These effects
are diluted in the larger FOV scans.
The comparison in terms of multiples of panoramics has surprising results. Even with the
increase in effective doses of CBCT the multiples of panorexes decreases. It can be
explained similarly to small FOV CBCT scans. Panoramic scans have rotational centers in
the center of the floor of the mouth for scanning the anterior jaws. This rotation center
coincides with the location of the parotid, submandibular and sublingual glands continuously
exposing the same areas to radiation. The effective dose from this panoramic imaging will
be higher than the more even distribution of the CBCT.
Average Small FOV Scanner
Scan to Panorex 1990 10
Scan to Panorex 2007 5

Risks of CBCT
Still 1.5 to 12X less than MDCT.
Considered a dose sparing technique.
ALARA as-low-as-reasonably-achievable should
always be considered.
Dose vs Diagnostic task should be evaluated.
1990 vs 2007 where do we go?????

Has Radiation Really
Increased??
• No. All machines still produce the same amount of
radiation. We are using new parameters to measure
effective dose. All numbers listed should be
considered and averaged. Effective dose will vary
based on the area of the scan and the mode
incorporated in the scan acquisition.
• Remember that resolution and radiation go hand in
hand. To achieve the best image quality of any
machine the higher dose of radiation is required.

Other Factors in Clear Images
• Radiation in not monochromatic. Some photons are weaker and
do not reach the sensor.
• Absorption and reflection of photons in the target tissues.
• Noise to Signal ratio: the actual energy reaching the sensor is
the signal. With low energies used in conebeam the noise ratio
can increase rendering a “fuzzy” image.
• Focal Point improves image quality: smaller = better.
• Pixel size of the sensor.
• Software processing.

Radiation in Perspective
• As we see from the tests, radiation can vary from 60-
500 msev with the typical scan.
• Radiation treatment for a head and neck tumor
approachs 250,000 msev.
• We owe our patients the lowest possible dose with
the corresponding acceptable diagnostic image.

Conclusion
• This is not meant to be an all inclusive report on
radiation. It should stimulate you the doctor to learn
more about the radiation and the diagnostic
information you getting from your current radiology
practices. It is our obligation to use not the lowest
radiation, but the best radiology mode to acquire the
most complete and diagnostic information for our
patients. Learn more and prosper.
• Dr Dan McEowen
e-mail: drdan1310@gmail.com