Reducing Body Radiation Dose At CT

 CT Scanner, maintenance & installation, Radiology

The explosive spread of CT can be attributed to its speed and diagnostic benefits. Recently it was estimated that more than 70 million CT examinations were performed each year in the United States. Although it accounts for only about one tenth of total radiologic examinations, CT is responsible for more than two thirds of the total radiation dose associated with medical imaging. Public concern with radiation exposure escalated when widely publicized articles claimed that the cancer risk in United States, attributed to CT radiation, has grown owing to the substantial increase in use of CT.

Based on studies of atom bomb survivors, the thought process is that the risk of getting cancer is elevated from an exposure of radiation between 50-100 MSV. There are no definitive answers in the comparisons between the bomb survivors and the recipients of radiation exposure from a medical test- cognizance of these tests have been noted. .

In the last few years a systematic plan of action has been in place for the decreasing of exposure due to a CT, these include cardiac gating, regulating the dosage as per the patients physical attributes, automatic exposure control as well as clinical indications. Whilst the above actions have resulted in increased lowering of dosage, more so with regards to pediatric patients, however the problem that arises is that with the reduction in dosage of CT, so the quality of the images will decline. ASIR – Adaptive Statistical Iterative Reconstruction, has recently been implemented in order to solve this decline in image quality.

FBP – Filtered Back Projection is the accepted methodology for the construction of CT images. As speed is a prerequisite in clinics the process of FBP is advantageous as its requirements are lower than the repetitive methods. This procedure is problematic with lower dosage situations. There is an alternative that is available, namely MBIR – Model Based Iterative Reconstruction this has the ability of incorporating the physical CT into the process of reconstruction. Yes, this will enhance the image quality more so with regards to the lower dosage CT-however with the computer technology that is currently available it takes many hours to put an image together.

The procedure of ASIR is reliant on the exact modeling of the sound distribution of the data that is received, instead of that of the systems optics resulting in an algorithm that is speedy as well as being competent in the reduction of the noise and thereby allowing the reducing of dosages that would previously not be possible.

The user has the ability to choose a setting for the amount of ASIR between 10-100% and the resultant image formed is a combination of ASIR and FBP, the finer details are totally dependant on these methods. The procedure of ASIR will increase the ultimate quality of images by decreasing the level of noise, the amount to be reduced is supported by the settings in the image construction.

It is not uncommon that radiologists experience varied sums of image noise and it is also possible that this could stifle their ability in the detection of any abnormalities. It is the advised settings of 20-60% in most cases, the increased amounts especially for that of younger patients who are an elevated risk to radiation exposure, as well as anyone that requires repetitive CT testing and finally when a low dosage is require , for example the detection of kidney stones. A higher setting is optional in a non-contrast examination prior to an angiogram. In making the selection of elevated ASIR settings, the said dosage may be lowered by 45-80% this is dependant on the current indicators.

Heavily overweight patients are likewise assisted by the process of ASIR in that the quality of the images is increased dramatically. In overweight patients the images are, compared to that or regular individuals, normally of a poor quality being attributed to the increased level of noise. With the low level of worry regarding the dosage in overweight patients, there is an automatic modulator that is utilized to maintain the standard of the images i.e. the dosage will increase for overweight and decrease according to size and therefore the utilization of ASIR the images are pristine and this is achieved without the overloading of the radiation.

Multiple studies have shown noise reduction does improve image quality. However, low dose images without noise reduction show the same CT findings. Iterative reconstruction has an advantage in accurately modeling the system geometry, incorporating physical effects like beam spectrum, noise, beam hardening effect, scatter and incomplete data sampling. It may improve spatial resolution and reduce image artifacts such as beam hardening, windmill, and metal artifacts. However, the cost is high computation load!