Strategies for Reducing Radiation Dose in CT Scan

General Strategies

Because a combination of factors is responsible for the total radiation dose delivered to the patient during a CT scan examination, a variety of methods for reducing dose are available. The following options can be used in any combination according to the specific clinical situation. Ideally, appropriate strategies are chosen and used in conjunction to reduce the dose as much as possible without sacrificing the image quality necessary to answer the clinical questions posed.

Adjusting mAs

At this point in the situation of radiation dose, the need to adjust mAs to suit individual patient size should be apparent. Small bodies require a lesser dose, and large bodies require higher dose. Numerous authors have documented the ability to adjust mAs, and therefore dose, without compromising image quality. Although some researchers have used the patient’s weight to adjust mAs, others prefer using the diameter of the patient to determine optimal mAs setting. Both approaches have proved successful results.

Automatic Tube Current Modulation

More recently manufacturer have provided user with another method to reduce patient dose. Some systems have an option that will make changes in tube current the mA, based on the estimated attenuation of the patient at a specific location. The estimation are derived from scout view done in both the anteroposterior AP and lateral projections or from these views, the mA will be programmed to vary by location along the length of the patient. The exact details of the option vary by manufacturer.

Avoid Increasing kVp

Increasing the xray tube potential increases both the radiation dose and penetration of the xrays through the body. In general, increases beyond 120kVp should be avoided, except when imaging obese patients. However, an increase in kVp could be accompanied by a reduction in tube current mA to offset the increased dose.

Increased Pitch

Another method for reducing radiation dose with helical scanning is to increase the pitch of the examination. Vade and co-workers showed that increasing the pitch from 1.0 to 1.5 decreased the dose by 33% without apparent loss of diagnostic information.

Limit the Use of Thin Slices

Using a large number of thin adjacent CT slices results in 30% to 50% more radiation dose to the patient than using fewer thicker slices to scan the same anatomy. Although it is not always possible to avoid using thin slices, radiologic technologist and radiologist should be aware of the consequences.

Limit Repeat Scans

Because the effect of repeat scans of the same area are cumulative, redundant or multiphase studies should be performed only when clinically indicated. Numerous authors have shown that detection of liver lesions can be improved by multiple scans taken during different phases of contrast injection. Although multiphasic studies are clearly indicated to evaluate for liver abnormalities, they should not be done in all circumstances. Additionally, it has been recommended that triple phase studies for the evaluation of kidney lesions be reserved for patients in whom a question arises on a routine study or other examination rather than as a standard protocol.

Newer Reconstruction Methods

A newer method of image reconstruction known as iterative reconstruction has been recently introduced for the use in CT scan image reconstruction and, compared with standard filtered back-projection methods, can reduce the dose by as much as 50%.