Due to the two biological effects of ionizing radiation: deterministic effects (with large dose thresholds occurring, and their severity depends on the size of the irradiated dose: cataracts such as radiation) and random effects (there is no effect) The dose threshold, but the probability of occurrence is related to the size of the irradiated dose: such as the induction of tumors and genetic effects, the increase in radiation dose will increase the harm to the human body. In general, CT scans are larger than normal X-rays, and an increase in the dose of radiation results in an increased incidence of random effects such as radiation-induced cancer.

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

In 2012, the Ministry of Health of China announced the new edition of GBZ165-2012 X-ray computed tomography radiological protection requirements, and published the diagnostic reference level for CT examination of different populations and different parts for the first time. The new version of the standard will be implemented on February 1, 2013, and the old version will be abolished at the same time. According to the “Protection Requirements”, the diagnostic reference levels of the head, lumbar vertebrae and abdomen of typical adult patients are 50mGy, 35mGy and 25mGy, respectively. The reference level of chest and head diagnosis for children aged 0-1 is 23mGy and 25mGy,10 The reference level for chest and head diagnosis in aged children is 26mGy and 28mGy. The Protection Requirements state that CT staff should minimize the exposure dose of the subject while meeting the diagnostic needs. When performing CT examinations, do a good job of protecting non-inspected parts and strictly control the scanning of parts outside the diagnostic requirements. It is prohibited to use adult radiation dose assessment criteria to assess a child's radiation dose.

CT Dose Index (CTDI)

CTDI refers to the radiation dose in the ray plane received by the subject during CT examination. It is generally measured by a cylindrical water-filled phantom of 16cm (representing the head and limbs) and 32cm (representing the body). mGy), first proposed by Tope in 1981, has been defined and adopted by many authoritative organizations such as FDA, IEC, CEC, IAEA, etc. It is currently the most widely used CT dose indicator in the world, and is adopted by China's national standards. concept.

At present, there is no international consensus on the amount of CT dose and the measurement methods (including the type of motif). ICRP also pointed out that in order to avoid confusion, the differences between various CTDI definitions should be clarified.

There are currently three recognized CTDIs. The three indices do not directly characterize the dose of the subjects caused by various CT scans, but are closely related to the dose of the subjects. It has the same dimensions as the absorbed dose, in milliGore (mGy).

The CTDI 100 is the most basic characterization of the CT scan dose characteristics that has been widely used to date and can be used to uniformly compare CT machine performance. It is defined as: CT rotates one week, and the dose distribution D (z) parallel to the axis of rotation (z axis, ie perpendicular to the plane of the fault) is integrated from -50 mm to +50 mm along the Z axis, divided by the layer thickness T and the number of scanning faults N The product of the product. which is: