CT (Computed Tomography) and Radiation

CT (computed Tomography) and Radiation Dose

Computed Tomography (CT) and Radiation Dose

Computed Tomography (CT) can be described as an evaluation technique that is nondestructive used in the production of 2-dimensional or 3-dimensional cross-sectional images from an object image obtained through X-ray. Such a technique is powerful enough to display such details as, dimensions, internal defects, shape, and density of the object. Even though this technology has risks associated with radiation it is being used widely since the results obtained are better than those from other techniques. To address this concern the amount of radiation being used is being observed to ensure that the recommended radiation dose is not exceeded.

Since dose measurement is essential there are techniques that have been developed to help in achieving this. The most common technique used for estimating patient dose is the CT dose index (CTDI) which is a measure that uses average volume in cases where the incrimination of the table is done in conjunction with the rotation of the tube. When using this descriptor, the radiation dose that have been delivered within and beyond the scan volume are both integrated. Using the average across the field of view which also accounts for the dose absorbed from the center to the periphery of the object, a weighted CTDI is obtained as the descriptor and this is a representation of the average dose in the scan volume for CT scans that are adjacent (Kalra, et al. 2004). The measurement of CTDI is usually done using a pencil-shaped ionization chamber, although alternative detectors have also been used by other methods such as a solid-state real-time dosimeter. The patient dose is only approximated when the average dose to a homogenous cylindrical phantom is represented in this technique. The values for CTDI and point dose are very close for surface dose measurement from spiral CT, however, there is a dose overestimation by a factor of two or more with CTDI valuesfor surface dose measurement from perfusion CT when compared to values of dose values. In estimating patient dose and CT scanner output the use of both CTDI and point dose measurements are effective (Kalra, et al. 2004).

In order to achieve radiation dose savings it is necessary to alter a number of CT parameters such as tube current, and peak kilovoltage. When the tube current (mA) is lowered the dose decreases linearly and when the peak kilovoltage (kVp) is lowered the overall dose reduces by almost the power of 2, both of these dose reduction techniques have an effect on the quality of the image. Conversely, when bismuth shields are used it is possible to selectively reduce radiation dose, this takes place through the filtration of lower-energy photons from the polychromatic x-ray beam (Hurwitz, et al. 2009). These shields have proved very valuable in reducing radiation when the study involves organs such as the eye, breast, or thyroid which are known to be very complexly located in the body. When the thoracic organs are involved then a combination of bismuth shields, lower kVp, and automatic tube current modulation has been identified as effective. The use of tube current alteration (mA) to reduce radiation is more recommended compared to bismuth shields due to several factors. The use of mA creates a very little effect on the mean CT numbers, this is very valuable in studies that are quantitative in nature such as the assessment of lung density and coronary calcification. Secondly, mA reduced images have less noise than those on bismuth shields. The last advantage it has is that the streak artifacts that are usually experienced when shields are used do not affect images reduced by mA (Hurwitz, et al. 2009).

A good percentage of CT scans that are being performed everyday are performed on children and this frequency continues to increase. Even though this trend has benefits the radiation exposure has attracted attention and the several effects of the radiation can be viewed in different ways. The effects of ionizing radiation are numerous the most serious among them is the induction of fatal cancers. The estimation is that out of every 1000 children who have gone through CT scan 1 will develop a fatal cancer induced by radiation which is a higher percentage compared to the expected baseline lifetime risk of cancer (Shah, & Platt, 2008). Due to this a high-quality diagnostic imaging system having minimum possible amount of exposure has been developed to save the children. This risk of fatal cancer is increased when an individual undergoes multiple scan.

In order to safeguard the pediatric population unique considerations are being made and such considerations lead to their vulnerability to ionizing radiation effects. This is because the cells in a growing child divide rapidly making them more sensitive to the radiation effects, and also the lifetime for children is long thus radiation-related cancers get a lot of time to evolve. In order to reduce the amount of radiation that a child is exposed to the concept of 'as low as reasonably achievable' was recommended. This concept is to be applied having in mind that the efficiency and reliability of the study must be maintained thus a number of methods to achieve this have been developed. The first method is developing of weight-based protocols which help in reducing the total dose of radiation for every CT scan performed on a child. The significance of this is mostly felt for the very small, premature neonate; it is a shift from the conventional concept of 'more is better' which advocated for higher radiation dose in order to get a better image quality. Due to the risks of radiation that have already been identified pediatric radiologists are now working towards reducing dose without affecting reliability (Shah & Platt, 2008).

The second method used to reduce radiation exposure on children is considering alternative modalities that have no radiation such as magnetic resonance imaging (MRI) and ultrasound. These alternative modalities have also proved effective in disease evaluation and thus can be helpful in avoiding radiation exposure to children where they are applicable. However, before picking on such options, factors such as the experience and skill of the technician and radiologist, availability of the instruments, and the necessary study period must be considered and reassessed. The third method is improving shielding which can be enhanced by the technicians who have been educated and supported for this purpose. Currently there are shields that have been fabricated, such as thyroid and breast shields, these can be utilized for this purpose with no effect created on the scan quality. The fourth method is that of focusing and/or limiting the view of the scan study by identifying indications for limited-view studies. This method also requires that the technicians are educated on the necessary protocols. Lastly, repeat CT scans should be discouraged at best since repeat studies are often unnecessary. To avoid such repeat study cases, a radiologist may consider modifying a request to include just a single study instead of having a paired study (Shah, & Platt, 2008).

There have been several technological advances towards reducing radiation and these have led to the emergence of other various techniques. One of such techniques utilizes the X-ray beam which acts a pre-patient tracker and enhances the efficiency of the scanner thus reducing radiation exposure. The other method is X-ray filtration which decreases the 'soft x-rays' which are normally absorbed but do not contribute to the image since they do not reach the detector. There is also computer-simulated dose-reduction software which lowers radiations by increasing the noise in an image obtained from a specific tube current and simulating the images that are obtained from tube current that is lower (Shah, & Platt, 2008).