Radiation Dosimetry in CT Scan

Thursday, December 8, 2016

Radiation Dosimeter

It is the responsibility of each radiologic technologist or imaging professional to understand the benefit and risk of any procedure and attempt to maximize the positive and minimize the negative effect of radiation. Medical physicist is usually used a special dosimeter called a pencil ionization chamber to measure the CTDI. This 100 mm long thin cylindrical device is long enough to span the width of 14 contiguous 7 mm CT slices. This provides a better estimate of MSAD for thin slices than of the single-slice method. When this method is used it is referred to as the CTDI.

Radiation Dose Concepts

The rational use of CT scan relative to patient care involves two components: appropriate patient selection and minimization of the radiation dose without compromising the diagnostic image quality. In an effort to address the need for appropriate patient selection, the American College of Radiology ‘ACR’ has created guidelines that are widely available. Although appropriate patient selection is essential, it falls primarily to the radiologist to enforce ACR guidelines.

Ionization Radiation Measurement Terminology

The ionizing radiation used in CT is an xray with maximum energy from 120 to 140 keV and an average energy near 70 keV. The unit of xray exposure in air is the roentgen ‘R’. When the xray from a CT scanner strike a patient and interact with tissue, most of the energy is absorbed and some of it passes through to the detector.

Unit of Radiation Dose

The unit of absorbed dose is called the radiation absorbed dose, or rad. This unit describes the amount of energy absorbed per unit mass. The International System of Units is a newer system that is used internationally, both in every day commerce and in science. The SI unit of absorbed dose is the gray ‘Gy’. There are 100 rad in 1 Gy. A centigray ‘cGy’ equals to 1 rad.

Physicist measure the energies of fast moving particles like those in xrays, cosmic rays and particle accelerators in the units called electron volts ‘eV’. An eV is the amount of energy that one electron gains when it is accelerated by an electrical potential of 1 volt. – A flashlight battery has about 1.5 volts. 1keV = 1000eV.

Quality Factor

Quality factor is another recognized health effects to x-ray and applied to the absorbed dose. This factor is responsible for calculations of different biologic effect produced from different types of ionizing radiation. The quality factor is 1 for the diagnostic xrays that are used in CT scan.

Dose Equivalent 

When the quality factor has been applied to the radiation absorbed dose the new quantity is called the dose equivalent. The dose equivalent is the rem, or roentgen equivalent man. The SI unit is equivalent to Sievert ‘Sv’. There are 100 rem in 1 Sv.

Equivalent Dose 

A newer quantity that is quite similar to the dose equivalent is called the equivalent dose ‘H’. It is the product of the absorbed dose and a radiation weighted factor. The radiation weighted factor is analogous to the Q. The unit for the equivalent dose is also the rem or the Sv.

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