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58 Cards in this Set

  • Front
  • Back
What is used to calculate the exposure
The ionizing chamber which measures the ionization in air produced by radiation.
What is measured with exposure
the amount of ionization that occurs in a unit of air (this is done by an ionizing chamber)
How many ionizations occur with exposure
Is the number of coluombs per kg of air small
yes
What is the enclosure for a air volume known as
an ionization chamber
What happens when a small volume of air interacts with ionizing radiation
The photons will interact with the atomic shell electrons and ions will form. This energy can be measured with an ionizing chamber
What contributes to the total amount of ionization that occurs in air
-the concentration of radiation photons

-the energy of the individual photons.
How many ions does 1 Roentgen produce
An exposure of 1 roentgen produces 2.08 x 109 ion pairs per cm3 of air at standard temperature and pressure
The official definition of the roentgen is the amount of exposure that will produce 2.58 x 10-4 C (of ionization) per kg of air
yes
For a specific given energy is the exposure proportional to the both the photon and energy concentration or fluence
yes
If the photon energy changes what happens to the relationship between exposure and photon/energy concentration
the relationship changes with photon energy because both the number of photons that will interact and the number of ionizations produced by each interacting photon is dependent on photon energy
Schematic showing the relationship between photon fluence and exposure at a given energy
Note that the value of exposure can be obtained from and ionizing chamber and if the keV photons is known then the energy and photon fluence can be calculated
At a given enery, say 60 keV what is 1 roentgen equivalent to in concentration of photons
If we assume a photon energy of 60 keV, a 1-R exposure is equivalent to a concentration of approximately 3 x 1010 photons per cm2
What is Air kerma used to define
Air kerma is another radiation quantity that is sometimes used to express the radiation concentration delivered to a point, such as the entrance surface of a patient's body
What is a kerma
It is a measure of the amount of radiation energy, in the unit of joules (J), actually deposited in or absorbed in a unit mass (kg) of air
What are the units of kerma
Therefore, the quantity, kerma, is expressed in the units of J/kg which is also the radiation unit, the gray (G)
What is another way of remembering air kerma
At this time we just need to recognize that air kerma is just the Absorbed Dose in air
What 2 quantities can an ionizing chamber measure
exposure (R or C/kg)
kerma (j/kg)
How does an ionizing chamber read exposure and kerma
Since the ionization produced in air by radiation is proportional to the energy released in the air by the radiation, ionization chambers actually measure air kerma as well as exposure
Are kerma and absorbed dose the same
no, but as far as i have to know they are. If we know air kerma measured (or calculated) at a point where soft tissue is located, the absorbed dose in the tissue will be just about equal to the air kerma
What is another name for surface intergral exposure
It is also referred to as the exposure-area product
What does SIE measure
the total amount of radiation delivered to the body
The total radiation delivered, or surface integral exposure (SIE), is determined by the exposure and the dimensions of the exposed area
yes
What r the units of SIE
roentgens-square centimeters (R-cm2)
If the radiation exposure is uniform over the entire area, how is the SIE calculated
If the radiation exposure is uniform over the entire area, the SIE is the product of the exposure in roentgens and the exposure area in square centimeters.
if the radiation exposure is not uniform how is the SIE calculated
If the exposure is not the same at all points in the exposed area, the SIE can be found by adding the exposure values for each square centimeter of exposed surface
What is the significance of the SIE
The significance of SIE is that it describes total radiation imparted to a patient, whereas exposure indicates only the concentration of radiation at a specified point
How does SIE not tell you the whole story
In both instances the SIE is 1,500 R- cm2. However, the exposure depends on how the x-ray beam was moved during the examination. In the first example the beam was not moved and the resulting exposure was 15 R. In the second example, the beam was moved to different locations so that the exposure was distributed over more surface area and the concentration became less.
how may this work in reverse
both recieved same exposure but one on the right covers 10 times the suface area and therefroe has 10 x SIE
Is DAP a similar concept to SIE
yes
What is the difference between the dose area product and the SIE
the units used (gy/cm2)
What is DAP
For a uniformly exposed area, the DAP is just the product of the air kerma, in Gy or mGy, and the exposed area in cm2.
What is DAP an estimation of
DAP provides a good estimation of the total radiation energy delivered to a patient during a procedure
Where is DAP used
Both radiographic and fluoroscopic machines can be equipped with devices (DAP meters) or computer programs that measure or calculate the DAP for each procedure. It is the most practical quantity for monitoring the radiation delivered to patients.
Does exposure express the total radiation delivered to the body
The important point to remember is that exposure (roentgens) alone does not express the total radiation delivered to a body. The total exposed area must also be considered
What are some of the units of radiation used in quanties for radiation dosage
joules, ergs, kiloelectron volts
What is energy fluence
Energy fluence (concentration) is the amount of radiation energy delivered to a unit area.
What are the units of energy fluence
The units for expressing radiation energy concentration are either the millijoule (mJ) per square centimeter or erg per square centimeter
For a specific photon energy, energy fluence is proportional to exposure
yes
Why does the relationship of energy fluence and exposure change at a given photon energy level
The relationship changes with photon energy because of the change in photon interaction rates
If we assume a photon energy of 60 keV, the energy fluence for a 1-R exposure is approximately 0.3 mJ/ cm2.
yes
The energy delivered by an x-ray beam can be put into perspective by comparing it to the energy delivered by sunlight, as shown below. For the x-ray exposure we will use the fluoroscopic factors of 5 minutes at the rate of 3 R/min. This 15-R exposure delivers x-ray energy to the patient with a concentration (fluence) of 4.5 mJ/cm2 if we assume an effective photon energy of 60 keV.
The energy delivered by the sun depends on many factors including geographic location, season, time of day, and atmospheric conditions; a typical midday summer exposure on a clear day in Atlanta produces approximately 100 mJ/sec/cm2. In 5 minutes a person would be exposed to an energy fluence of 30,000 MJ/cm2. We see from this example that the energy content of an x-ray beam is relatively small in comparison to sunlight. However, x-ray and gamma radiation will generally produce a greater biological effect per unit of energy than sunlight because of two significant differences: x- and gamma radiation penetrate and deposit energy within the internal tissue, and the high energy content of the individual photons produces a greater concentration of energy at the points where they are absorbed within individual atoms.
What does the portion of the x ray beam that is absorbed depend on
The portion of an x-ray beam that is absorbed depends on the penetrating ability of the radiation and the size and density of the body section exposed
What percent of the x-ray beam is usually absorbed
>90%
What is absorbed dosage
Absorbed dose is the quantity that expresses the concentration of radiation energy absorbed at a specific point within the body tissue
Is all the dosage abosorbed by radiation the same as radiation passes through
no, Since an x-ray beam is attenuated by absorption as it passes through the body, all tissues within the beam will not absorb the same dose
Where is the absorbed dose the greatest
The absorbed dose will be much greater for the tissues near the entrance surface than for those deeper within the body.
What is the definition of absorbed dose
Absorbed dose is defined as the quantity of radiation energy absorbed per unit mass of tissu
What are the SI units of absorbed dose
The conventional unit for absorbed dose is the rad, which is equivalent to 100 ergs of absorbed energy per g of tissue
100ergs/g
is the same as a 1 rad
fix prior that is conventional
ok
What are the SI units of absorbed dose
The SI unit is the gray (Gy), which is equivalent to the absorption of 1 J of radiation energy per kg of tissue.
1 rad = 100 erg/g = 0.01 J/kg = 0.01 Gy
1 rad = 100 erg/g = 0.01 J/kg = 0.01 Gy
1 Gy = 100 rad
1 Gy = 100 rad
Is the absorbed dose proportional to the exposure given to the tissue
yes
Does the exposure to absorbed dose converstion change of a given photon energy range
yes
What is the conversion of exposure to absorbed dose for bone and soft tissue over a given photon energy range
The ratio, f, between dose (rads) and exposure (roentgens) is shown below for soft tissue and bone over the photon energy range normally encountered in diagnostic procedures
What is the absorbed dose is soft tissue
The absorbed dose in soft tissue is slightly less than 1 rad/R of exposure throughout the photon energy range