How to calculate CT effective radiation?

After the CT examination, the patient will get such a table of radiation dose.

In this form, we can get most of the scan-related information. There are two main parameters related to radiation dose, CTDI vol and DLP. So which is the effective radiation dose, if not, how is the effective radiation dose of the patient calculated?

How to calculate CT effective radiation?

Here, let me talk about some background knowledge:

Due to the two major biological effects of ionizing radiation: deterministic effects (which will only occur with a larger dose threshold, and its severity depends on the size of the exposure dose: such as cataracts caused by radiation) and random effects (there is no effect. Dose threshold, but the probability of occurrence is related to the size of the exposure dose: such as inducing tumors and genetic effects), the harm to the human body will increase correspondingly when the radiation dose increases. Generally speaking, CT scans have a larger dose than ordinary X-ray examinations, and the increase in radiation dose leads to an increase in the probability of random effects such as radiation-induced cancer.

In 2009, a patient at Cedars-Sinai Medical Center in Los Angeles, USA, experienced hair loss after undergoing a CT nerve perfusion scan. The hospital's investigation found that within 18 months from February 2008, a total of 206 patients were mistakenly given radiation doses up to 8 times the normal dose during the CT process. In order to regulate the behavior of CT examinations, the US Food and Drug Administration (FDA) recommends evaluating the radiation dose received by patients during CT examinations.

The Ministry of Health of China published a new version of "GBZ165-2012 Radiological Protection Requirements for X-ray Computed Tomography" in 2012, and for the first time announced the diagnostic reference level of CT examinations for different groups of people and different parts. The new version of the standard will be implemented on February 1, 2013, and the old version of the standard will be abolished at the same time. According to the "Protection Requirements", the diagnostic reference levels for the head, lumbar spine and abdomen of a typical adult patient are 50mGy, 35mGy, and 25mGy, respectively. The diagnostic reference level for the chest and head of a child from 0 to 1 year old is 23mGy and 25mGy, 10 The diagnostic reference levels for chest and head in children aged 2 years are 26mGy and 28mGy. The "Protection Requirements" proposes that CT staff should meet the needs of diagnosis while reducing the exposure dose to the examinee as much as possible. When conducting CT examinations, protect the non-examination parts and strictly control the scanning of parts outside the diagnostic requirements. It is forbidden to use adult radiation dose assessment standards to assess children's radiation dose.

CT Dose Index (CTDI)
CTDI refers to the radiation dose in the ray plane received by the subject in a CT examination. It is generally measured with a cylindrical water-filled phantom of 16cm (representing the head and limbs) and 32cm (representing the body) (unit: mGy), after it was first proposed by Shope in 1981, it was successively defined and adopted by many authoritative organizations such as FDA, IEC, CEC, IAEA, etc. It is currently a widely used CT dose index in the world, and my country's national standards also adopt this concept .

At present, there is no consensus on the characterization quantity and measurement method of CT dose (including the type of phantom) in the world. ICRP also pointed out that in order to avoid confusion, the difference between various CTDI definitions should be clarified.

There are currently three recognized CTDIs. The three indexes do not directly characterize the dose of the subject caused by various CT scans, but are closely related to the dose of the subject. It has the same dimension as the absorbed dose, with milligrays (mGy) as the unit.

1. CT dose index 100 (CTDI 100)

CTDI 100 is a basic characterization quantity that reflects the dose characteristics of CT scans widely used so far, and can be used to uniformly compare the performance of CT machines. It is defined as: CT rotates one circle, and the dose distribution D (z) parallel to the axis of rotation (z-axis, perpendicular to the plane of the fault) is integrated along the Z-axis from -50mm to +50mm, divided by the slice thickness T and the number of scan slices N The quotient of the product. which is:

How to calculate CT effective radiation?

CTDI 100 can use a thermoluminescence detector (TLD) to measure the dose distribution of each point in a dedicated TLD plug-in, and then obtain the dose distribution curve D (z), and then according to the half-width of the dose distribution curve (Full Width at Half Maximum, FWHM) CTDI is obtained by fitting calculation. CTDI 100 uses an integration interval from -50mm to +50mm, and can be easily measured in a general standard dose phantom with a pen-shaped ionization chamber with an effective length of 100mm, so as to facilitate the acceptance of the CT machine and regular quality control inspections.

CTDI 100, the basic characteristic quantity, reflects the X-ray energy deposited in the air at a certain point measured in a standard methyl methacrylate phantom.

How to calculate CT effective radiation?

2. Weighted CT Dose Index (CTDI W)

Since the radiation doses at different positions in the same phantom are different, in order to better express the overall radiation dose level, the concept of weighted CT dose index (CTDI W) needs to be introduced, which can accurately reflect the average scanning plane dose.

How to calculate CT effective radiation?

CTDI 100 (center) is the measured value at the center of the phantom; CTDI 100 (outer periphery) represents the average of the measured values at four different positions around the phantom (at least 10mm below the surface of the phantom at 90° intervals) value.

How to calculate CT effective radiation?

Currently commonly used standard plexiglass dose phantom matched with a pen-shaped ionization chamber measuring instrument with an effective length of 100mm. There are two types: head phantom (diameter 160mm) and torso phantom (diameter 320mm), both of which are cylinders with a length of 140mm The center of the phantom and 10mm below the surface of the phantom are equipped with a special detection ionization chamber socket (the hole is inserted into a plexiglass rod of tissue equivalent when it is not measured).

The weighted CT dose index (CTDI w) has been selected as one of the indicators of the guiding (reference) level of CT diagnostic medical exposure. It can reflect the average dose of multi-slice continuous scanning (when pitch=1), but for discontinuous multi-slice scanning, CTDI w cannot accurately reflect its average dose.

3. Volumetric CT dose index (CTDI vol)

After the advent of spiral CT, CTDI w can no longer accurately characterize the level of radiation dose. The influence of the pitch on the scan dose needs to be considered:

CT pitch (factor) = Δd / N·T

Δ d is the moving distance of the examination table per revolution of the X-ray tube;

N is the number of slices generated by the rotation scan;

T is the scan layer thickness

How to calculate CT effective radiation?

The volumetric CT dose index CTDI vol reflects the average dose in the entire scan volume. This is also a parameter directly related to the dose in our dose table.

summary

The volumetric CT dose index CTDI vol can be obtained from the weighted CT dose index CTDI w, and CTDI w is the weighted result of the CTDI 100 measurements at the center of the dose phantom and four different locations around it. therefore:

CTDI 100 reflects the X-ray energy deposited at a certain point in the CT standard measurement phantom;

CTDI w describes the average dose status on a certain tomographic plane scanned by CT;

CTDI vol describes the average radiation dose of multi-slice (slice) spiral CT in the entire scanning volume.

How to calculate CT effective radiation?

Dose length product DLP

DLP is used to evaluate the total radiation dose from a complete CT scan of the subject. For sequence scan DLP (unit: mGy·cm), it can be expressed as:

DLP=i∑nCTDIw·nT·N·C

i is the number of X-CT scan sequence;

N is the number of rotations;

nT is the nominal limit beam collimation width per rotation (cm);

C is the product of tube current and exposure time per revolution of the X-ray tube (mAs);

nCTDI w represents the normalized weighted CT dose index (mGy·mA -1 ·s -1) corresponding to the used tube voltage and the total nominal beam collimation width.

For spiral scan DLP can be conveniently expressed as:

DLP = CTDI vol × L

CTDI vol is the volumetric CT dose index of multi-slice (slice) spiral CT scanning;

L is the scan length along the Z axis.

Effective dose D:

After the cumulative radiation dose is obtained, this parameter is not the radiation dose received by the final patient. The radiation dose of the subject will eventually be determined by the absorbed dose (D) of each tissue or organ. The absorbed dose of the tissue or organ is the X-ray product. The energy in the unit mass tissue or organ of the subject.

Unit: Gy, 1Gy=1 joule·kg -1 (J·kg -1) 100c Gy=100rad

The strict definition of the absorbed dose of a tissue or organ is a point quantity in the physical sense, that is, the absorbed dose refers to the quotient obtained by dividing the average energy of a substance in a certain volume element by ionizing radiation by the mass of the substance in the volume element. That is: D = dε / dm

The absorbed dose of tissues or organs is a complete characterization of the amount of X-ray exposure received by the examinee. However, in most cases it is impossible to directly measure it. It can be solved through phantom simulation research:

Using a simulated human body model, with the help of detectors such as TLD and other luminescence dosimeters, measure the absorbed dose and its distribution of the subject’s tissues or organs, and use Monte Carlo calculations to estimate the absorbed dose of tissues or organs.

The biological effect of the absorbed dose depends on the type of radiation and the exposure conditions. For the same absorbed dose, alpha rays are 20 times more harmful to living organisms than X-rays. In radiation protection, the absorbed dose actually received or likely to be received by an individual or collective is corrected according to the weight of the biological effect of the tissue, and the corrected absorbed dose is called the equivalent dose in radiation protection.

The unit of equivalent dose is the same as that of absorbed dose, that is, Joule·Kilogram -1 (J·kg -1 ), the proper name is Sv,

1Sv=1J·kg-1 (=1Gy)

When comparing the relative ionizing radiation risks of different types of radiological examinations, and taking into account the different radiation sensitivities of different tissues or organs, the effective dose E in sieverts (Sv) is used to characterize. The whole-body effective dose is a dose parameter that reflects the normalization of non-uniform exposure to the risk of whole-body exposure.

Effective Dose specifically refers to the weighted sum of the equivalent doses of all tissues or organs of the human body when the effect under consideration is a random effect (such as radiation-induced cancer, etc.), in the case of non-uniform irradiation of the whole body. which is:

E=∑WT·HT

HT is the equivalent dose received by the tissue or organ T; WT is the tissue weighting factor of T.

The effective dose is the weighted sum of the equivalent doses of organs and/or tissues according to the weighting factors of each tissue.

Effective dose of spiral CT

Use CTDI vol and its scan length L to calculate the dose length product DLP, and then multiply it by a specific conversion coefficient k to estimate the effective dose E:

E=k·DLP

The conversion factor k (mSv·mGy -1 ·cm -1) is related to the inspection site.

Different parts of the same individual have different sensitivity to the same radiation dose, which is specifically manifested in the different K values. The K value is the normalized effective dose weighting factor for different parts. In the same anatomical part, the older the age, the lower the K value; the K value of the head and neck of individuals of the same age is smaller than that of the abdomen and pelvis. In addition, different organs have different sensitivity to radiation. Sensitive organs include the lens, thyroid, breast, gonad and hematopoietic system. When exposed to unnecessary or excessive radiation, the human body is more likely to develop cataracts, thyroid cancer, and breast cancer.

How to calculate CT effective radiation?

Size-Specific Dose Estimates (SSDE)

The above evaluation methods are mainly based on the results of the phantom measurement. Because the actual patient's body shape is not a cylinder, and the density is different, the use of the above method will have errors in accurately reflecting the radiation dose received by the patient. In 2011, the American Association of Physicists in Medicine (AAPM) proposed a method for Size-Specific Dose Estimates (SSDE).

SSDE calculates the concept of effective diameter (Effectivediameter, ED), which refers to the diameter at a given position along the head and feet, assuming that the scanned patient has a circular cross-section. Although some parts of the body have approximately circular cross-sections, most of them are not. Therefore, the effective diameter can be considered as 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 after the patient's body size is corrected. It is based on the volumetric CT dose index CTDI vol displayed on the CT operation interface and is obtained through the body size-related conversion coefficient. Compared with the above evaluation method, SSDE is relatively accurate, but there is still a certain gap between the estimated value of SSDE and the true value of the patient's radiation.

Protective collars, eye masks, and breast surface shields can be used for protection during CT examinations. When setting scanning parameters, always pay attention to the ALARA principle. The ALARA principle is the guiding principle of the medical physical radiation safety plan worldwide, and continuously and scientifically examine the relationship between dose and image quality to continuously promote low dose and high quality The main driving force for the development of imaging technology. In short, the surface shielding method and the pre-filter method can be combined with other methods to reduce the dose, such as reducing the scan coverage, tube voltage adjustment, tube current adjustment, shortening the scan time, iterative algorithms and other technologies to further reduce sensitivity The radiation dose to the organ.