Full English name: Computed Tomography Dose Index
Chinese explanation: CT Dose Index
? CTDI (CT Dose Index), unit: mGy, that is, the average radiation dose of CT in the whole scanning range. CTDIvol is used to evaluate the average dose of radiation per unit volume received by the subject during the scan.
CT Radiation Dose Knowledge Ramblings --- On the relationship between CTDIvol, DLP and mSv_Comprehensive News_News_News Information_Foshan Second People's Hospital
After the end of a CT scan, a one-page dose report will be automatically generated under the folder corresponding to the examined person, in which there are two indicated radiation dose indicators, CTDI and DLP.
1) CTDIvol: Volumetric CT dose index CTDIvol is used to reflect the average dose at the level of the entire spiral scan volume. CTDIvol? CTDIvol?= CTDIw / CT pitch (factor), CTDIW reflects the average dose in the serial scanning plane.
2) DLP:DLP is used to evaluate the total radiation dose received by a subject undergoing a single CT exposure ? The total radiation dose received after scanning. DLP = CTDIvol × L. CTDIvol is the volumetric CT dose index for a multi-row (layer) spiral CT scan, and L is the scan length along the Z axis.
When expressed in terms of a comparable magnitude of radiobiological damage, the unit mSv is required. mSv and DLP are related as
mSv?=DLP×k, and this k value is a factor (table below).
? As an example, a subject underwent coronary CT imaging (the subject received one of the largest x-ray radiation doses of any CT exam.) , although an exposure condition of 80kV and 220mAs was used, the actual mAs used was 144 due to the use of the automatic tube current technique, then since his CTDIvol value was only 0.67mGy, and considering that his scan length DLP value was only 13mGyNaN, and since the subject was an adult and the k-value was taken to be 0.014, the radiation dose from x-rays that he received was 0.014 × 13 = 0.18mSv, far lower than the average background radiation dose of 2.4mSv received by the earth's population in a year.
3) msv is the unit of calculation of radiation
Radiation dose is calculated as the amount of radiation energy absorbed by each unit of mass of the human body tissues and organs, and it is calculated in units of "sieverts", or "millisieverts". International safety standards state that the upper limit of radiation dose to the human body is 1 mSv per year in addition to the normal ambient background radiation.
International safety standards state that the upper limit of annual radiation dose to the human body beyond the normal ambient background radiation is 1 millisievert.
The unit of radiation dose is the sievert/sievert (Sv), which represents the total damage from radiation exposure to an individual. One joule absorbed per kilogram of human tissue is one sievert. The shee is a very large unit, and we more commonly use millisievert (mSv).
1Sv=1000mSv, 1mSv=1000μSv
How exactly is the radiation dose from a CT calculated? After first proposed by Shope in 1981, it has been defined and adopted by many authoritative organizations such as FDA, IEC, CEC, IAEA, etc. It is the most widely used CT dose index in the international arena, and our national standard also adopts this concept.
Currently, there are three recognized CTDIs, which do not directly characterize the dose to the subject due to various CT scans, but are closely related to the subject's dose. They have the same scale as the absorbed dose and are measured in milligray (mGy).
The volumetric CT dose index, CTDIvol, can be derived from the weighted CT dose index, CTDIw, which is the result of weighting the center of the dose modality with the CTDI100 measurements at four different locations on the periphery. Thus:
CTDI100 reflects the X-ray energy deposited at a point in the standard CT measurement module;
CTDIw describes the average dose profile in a given tomographic plane scanned by CT;
CTDIvol describes the average radiation dose of a multirow (layer) helical CT over the entire scanning volume.
CTDI100 is by far the most basic characterization quantity widely used to characterize the dose of a CT scan and can be used to compare CT machine performance in a uniform manner. It is defined as the quotient of the product of the layer thickness T and the number of scanned faults N, divided by the dose distribution D(z) integrated along the Z-axis from -50mm to +50mm parallel to the axis of rotation (z-axis, i.e., perpendicular to the tomographic plane) of the CT rotated by one week.
CTDI100 can be obtained by using a thermoluminescent detector (TLD), measuring the dose distribution at each point in the special TLD plug-in, and then deriving the dose distribution curve D(z), and then calculating the CTDI by fitting the dose distribution curve according to the FullWidthatHalfMaximum (FWHM). The CTDI100 uses an integration interval from -50mm to +50mm, which can be conveniently measured by a pen-shaped ionization chamber with an effective length of exactly 100mm in a universal standard dose model, thus facilitating the acceptance of CT machines and regular quality control testing.
The most basic characterization, CTDI100, reflects the X-ray energy deposited in air at a point measured in a standard methyl methacrylate die.
Since the radiation dose varies from location to location within the same mold, in order to better express the overall radiation dose level, it is necessary to introduce the concept of a weighted CT dose index (CTDIW), which accurately reflects the average dose in the scanning plane.
At present, the commonly used standard Plexiglas dosimetry module with an effective length of 100mm pen-shaped ionization chamber detection instrument, divided into the head module (diameter 160mm) and torso module (diameter 320mm) two kinds of, were 140mm cylindrical length, the center of the mold body and its surroundings under the surface of the 10mm there are dedicated detection of the ionization chamber jacks (the hole does not measure that is). (the hole is not measured that is inserted into the organization equivalent of a plexiglass rod).
The weighted CT dose index (CTDIw) has been chosen as one of the quantities to characterize the guideline (reference) level of CT diagnostic medical exposure. It can reflect the average dose of multilayer continuous scans (at pitch=1), but for discontinuous multilayer scans, CTDIw does not accurately reflect their average dose.
After the introduction of spiral CT, CTDIw can no longer accurately characterize the level of radiation dose, and the effect of pitch on the scanning dose needs to be considered:
CT Pitch (Factor) = Δd/N-T
Δd is the distance traveled by the examining bed for each week of rotation of the X-ray tube;
N is the number of tomograms produced by a rotational scan;
T is the scan layer thickness
The volume CT dose index, CTDIvol, reflects the average dose over the entire scan volume. It is also the first parameter in our dose reporting form that is directly related to dose.
The DLP is used to evaluate the total radiation dose to the subject for a complete CT scan . For a sequence scan DLP (in mGy-cm) can be expressed as:
DLP=i∑nCTDIw-nT-N-C
i is the number of X-CT scan sequences;
N is the number of rotations;
nT is the nominal beam-limiting collimation width in cm per rotation;
C is the tube current and exposure time of the X-ray tube per C is the product of tube current and exposure time (mAs) for each rotation of the X-ray tube;
nCTDIw is the normalized weighted CT dose index (mGy-mA-1-s-1) corresponding to the tube voltage used and the total nominal beam-limiting collimation width.
For spiral scans DLP can be conveniently expressed as:
DLP=CTDIvol×L
CTDIvol is the volumetric CT dose index for a multirow (layer) spiral CT scan;
L is the scan length along the Z-axis.
After the cumulative radiation dose has been obtained, this parameter is not the final radiation dose received by the patient; the radiation dose to the subject is ultimately realized as the absorbed dose (D) to the tissues or organs in question, which is the amount of energy that the X-rays have accumulated per unit mass of tissue or organ in the subject.
Unit: Gy, 1Gy=1 Joule kg-1 (J-kg-1) 100cGy=100rad
The absorbed dose to a tissue or organ is strictly defined as a point quantity in the physical sense, i.e., the absorbed dose is the quotient of the average energy imparted by ionizing radiation to the material in a given volume of elemental matter, divided by the mass of the material in that volume of elemental matter. That is, D=dε/dm
The absorbed dose to a tissue or organ is the most complete characterization of the amount of X-ray exposure received by a subject, however, in most cases it is not possible to measure it directly, which can be solved by a body model simulation study:
The absorbed dose and its distribution to the subject's tissues and organs are measured by using simulated human body models with the help of detectors such as TLDs and other luminescent dosimeters. The absorbed dose and its distribution are measured in tissues or organs of the subject with the help of detectors such as TLDs and other luminescent dosimeters, and the absorbed dose of the tissues or organs is estimated by MonteCarlo calculations.
The biological effect of absorbed dose depends on the type of radiation and exposure conditions. For example, for the same absorbed dose, alpha rays are 20 times more harmful to organisms than X-rays. In radiation protection, the individual or collective actually received or may receive the absorbed dose according to the organization of the biological effect of weighted correction, after correction of the absorbed dose in radiation protection is called the equivalent dose.
The unit of equivalent dose is the same as absorbed dose, i.e., joule-kilogram-1 (J-kg-1), and the proper name is Sv,
1Sv=1J-kg-1 (=1Gy)
When comparing the relative risk of ionizing radiation from different types of radiological examinations and taking into account the varying radiosensitivities of different tissues or organs, it is used in the form of the equivalent dose in terms of Sivert (Sv,Sv,Sv,Sv,Sv,Sv,Sv,Sv,Sv,Sv,Sv). Sivert,Sv) units to characterize the effective dose E. The whole-body effective dose is a dose parameter that reflects the risk of non-uniform exposure normalized to whole-body exposure.
EffectiveDose refers specifically to the weighted sum of the equivalent doses to all tissues or organs of the human body under non-uniform whole-body irradiation when the effect under consideration is a stochastic effect (e.g., radiation-induced cancer, etc.). That is:
E=∑WT-HT
HT is the equivalent dose to tissue or organ T; WT is the tissue weighting factor for T.
The effective dose is the sum of the equivalent doses to organs and/or tissues weighted by each tissue weighting factor.
Effective dose for spiral CT:
The effective dose E is estimated by calculating the dose-length product DLP using the CTDIvol and its scan length L, multiplied by a specific conversion factor k:
E=k-DLP
The conversion factor k (mSv-mGy-1-cm-1) is related to the examination site.
Different parts of the same body have different sensitivities to the same radiation dose, as evidenced by differences in k. The k value is a normalized effective dose weighting factor for different sites. For the same anatomical part, the older the age, the smaller the K-value; the K-value of the head and neck of individuals of the same age group is smaller than that of the abdomen and pelvis. In addition different organs have different sensitivities to radiation; sensitive organs include the eye crystals, thyroid, breast, reproductive glands and hematopoietic system. When exposed to unnecessary or excessive radiation, the probability of the human body to develop cataract, thyroid cancer and breast cancer will increase.
Size-Specific Dose Estimates (SSDE)
The above methods are based on the results of mold measurements, but because the actual patient's body is not cylindrical and density varies, there are inaccuracies in how accurately they reflect the radiation dose received by the patient. In 2011, the American Association of Physicists in Medicine (AAPM) proposed the method of Size-Specific Dose Estimates (SSDE).
SSDE calculates the concept of effective diameter (ED), which refers to the diameter at a given location along the cephalad direction of the scanned patient, assuming that the patient has a circular cross-section. Although some parts of the body have an approximate circular cross-section, most locations do not. Therefore the effective diameter can be considered to be the diameter of a circle equal to the cross-sectional area of the patient's body.SSDE refers to the estimated CT dose received by the patient corrected for patient body size, and is based on the volumetric CT dose index, CTDIvol, as displayed on the CT operator interface, and obtained by a body-size-dependent conversion factor. SSDE is relatively accurate compared to the above assessment methods, although there is still a gap between the SSDE estimate and the true value of radiation received by the patient.
Protective bibs, eye shields, and breast surface shields can be used for protection during CT examinations. When setting scanning parameters, always be mindful of the ALARA principle, which is the guiding principle of radiation safety programs in medical physics around the world, and the primary motivation for continually and scientifically examining the link between dose and imaging quality in order to continually promote the development of low-dose, high-quality imaging techniques. In conclusion, both the surface shielding method and the pre-filter method can be applied in conjunction with other dose reduction methods, such as reduced scan coverage, tube voltage modulation, tube current modulation, shorter scan times, iterative algorithms, and other techniques to further reduce the radiation dose to sensitive organs.