Physics: Radiation Quantities And Units 3

Front 1

What is CDTI

Back 1

The Computed Tomography Dose Index, CTDI, is the special dose quantity that is used extensively to express absorbed dose in CT.

Front 2

How is exposure converted to absorbed dose

Back 2

you most know the ratio at a given photon energy. the ratio is called f

Front 3

What does CDTI take into account

Back 3

the direct radiation and the radiation from adjacent slices that result from scatter

Front 4

What is the mean glandular dose

Back 4

The Mean Glandular Dose (MGD) is the special dose quantity used in mammography.

Front 5

What is the definition of mean glandular dose

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It is defined as the mean, or average, dose to the glandular tissue within the breast.

Front 6

Why is glandular tissue considered important

Back 6

The assumption is that the glandular tissue, and not the fat, is the tissue at risk from radiation exposure

Front 7

Is it possible to determine the radiation dose to glandular breast on a case by case basis

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it is just about impossible to determine the actual dose to the glandular tissue during a specific mammographic procedure because of variations in breast size and distribution of glandular tissue within the breast.

Front 8

What is MGD based on

Back 8

The MGD is based on some standard breast parameter

Front 9

What is the first step when measuring MGD

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The first step is to determine the entrance surface exposure, or air kerma, to the breast. This can be measured directly with small dosimeters placed on the breast or calculated from the known calibration factors for the mammography equipment

Front 10

What is the second step when measuring MGD

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Then, the MGD is determined by multiplying the surface exposure value by published dose factors. The dose factor values are tabulated according to breast size and composition and the penetrating characteristics of the x-ray beam as determined by the anode material, filtration, and KV.

Front 11

What standardized breast is used for for comparison of imaging techniques, evaluation of equipment performance, general dose management, and regulatory and accreditation purposes

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A "standard" breast is used. The standard is a 4.2cm thick compressed breast consisting of 50% glandular tissue and 50% fat.

Front 12

What does the standard breast correspond to

Back 12

This corresponds to the standard phantom that is used for image quality evaluation and comparative dose determinations

Front 13

What is the integral dose

Back 13

Integral dose is the total amount of energy absorbed in the body. It is determined not only by the absorbed dose values but also by the total mass of tissue exposed

Front 14

What are the conventional unites of integral dose

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The conventional unit for integral dose is the gram-rad, which is equivalent to 100 ergs of absorbed energy

Front 15

Theoretically how is the intergral dose calculated

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The concept behind the use of this unit is that if we add the absorbed doses (rads) for each gram of tissue in the body, we will have an indication of total absorbed energy

Front 16

What is the SI unit of integral dose

Back 16

Since integral dose is a quantity of energy, the SI unit used is the joule

Front 17

What is the conversion of integral dose from SI to conventional units

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1 J = 1,000 gram-rad

Front 18

Why is integral dose the the radiation quantity that most closely correlates with potential radiation damage during a diagnostic procedure.

Back 18

This is because it reflects not only the concentration of the radiation absorbed in the tissue but also the amount of tissue affected by the radiation.

Front 19

How is the integral dose calculated

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There is no practical method for measuring integral dose in the human body. However, since most of the radiation energy delivered to a body is absorbed, the integral dose can be estimated to within a few percent from the total energy delivered to the body

Front 20

Back 20

Computed tomography can be used to demonstrate integral dose, as illustrated below. We begin with a one-slice examination and assume that the average dose to the tissue in the slice is 5 rad. If there are 400 g of tissue in the slice, the integral dose will be 2,000 gram-rad. If we now perform an examination of 10 slices, but all other factors remain the same, the dose (energy concentration) in each slice will remain the same. However, the integral dose (total energy) increases in proportion to the number of slices and is now 20,000 gram-rad. In this example we made the simplifying assumption that no radiation is scattered from one slice to another. In reality, some radiation is exchanged between contiguous slices but that does not affect the concept presented.

Front 21

What are dose equivalents used for

Back 21

Dose equivalent (H) is the quantity commonly used to express the biological impact of radiation on persons receiving occupational or environmental exposures. Personnel exposure in a clinical facility is often determined and recorded as a dose equivalent.

Front 22

What is the dose equivalent proportional to

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Dose equivalent is proportional to the absorbed dose (D), the quality factor (Q), and other modifying factors (N) of the specific type of radiation

Front 23

What is the quality factor for most stuff in radiology

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Most radiations encountered in diagnostic procedures (x-ray, gamma, and beta) have quality and modifying factor values of 1

Front 24

What is a major exception to this rule

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For example, alpha particles have a quality factor value of approximately 20

Front 25

continue with this if necessary

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http://www.sprawls.org/ppmi2/RADQU/#ABSORBED DOSE

Front 26

What is equivalent dose

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equivalent dose, is a computed average measure of the radiation absorbed by a fixed mass of biological tissue, that attempts to account for the different biological damage potential of different types of ionizing radiation.

Front 27

What are the units of equivalent dose

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Equivalent dose is dimensionally a quantity of energy per unit of mass, and is usually measured in sieverts or rems.

Front 28

What is the reason for effective dose

Back 28

The concept of effective dose is used in radiation protection as an estimate of the stochastic effect that a non-uniform radiation dose has on a human.

Front 29

What quantity is used to express relative risk to patients

Back 29

Effective dose is becoming a very useful radiation quantity for expressing relative risk to humans, both patients and other personnel

Front 30

What does effective dose take in to consideration

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It takes into account the specific organs and areas of the body that are exposed

Front 31

Why is important to consider organs individually

Back 31

The point is that all parts of the body and organs are not equally sensitive to the possible adverse effects of radiation, such as cancer induction and mutations

Front 32

How are different organs weighted differently

Back 32

For the purpose of determining effective dose, the different areas and organs have been assigned tissue weighting factor (wT) value

Front 33

What is the formula for effective dose

Back 33

Effective Dose (Gy) = Absorbed Dose (Gy) x wT

Front 34

What happens if more than one organ is irradiated during a procedure

Back 34

f more than one area has been exposed, then the total body effective dose is just the sum of the effective doses for each exposed area.

Front 35

Is effective dose useful for comparing different types of radiologic exams

Back 35

There is often a need to compare the amount of radiation received by patients for different types of x-ray procedures, for example, a chest radiograph and a CT scan. The effective dose is the most appropriate quantity for doing this. Also, by using effective dose it is possible to put the radiation received from diagnostic procedures into perspective with other exposures, especially natural background radiation

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effective dose

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Front 37

sample weighting factors

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