Radiation Dose Reduction Strategies in Cardiac CT Angiography

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Introduction

With great advancements in technology especially in the field of medicine, computed tomography (CT) has developed to become a very important clinical tool (Smith-Bindman et al. 2009). The use of CT examinations has also substantially increased.

In the U.S., for example, the number increased from 3 million from the 80s to about 70 million by 2007. Computed tomography has been integrated into various healthcare procedures and has led to the dramatic improvement of patient health care. For this reason, CT has been considered among the most useful technologies in medicine.

The use of coronary CT angiography (CCTA) has been established to be a useful tool in the diagnosis of coronary artery disease (Goitein et al. 2011). This is basically because it allows for non-invasive evaluation of coronary arteries (Flohr, Raupach, and Bruder 2009).

It is currently used in the evaluation of acute chest pains. CCTA is also used in the elimination of coronary atherosclerosis. For this particular reason, it has been recognized to be valuable among those with intermediate cardiovascular risks (Torres et al. 2010).

Despite the importance of CT in medicine, it is known to be a great source of radiation that may be harmful to the human health. The radiation doses associated with this procedure is said to be much higher than those associated with the x-rays. A typical example is that of a chest CT scan.

It is said to deliver more than 100 times that of a conventional chest radiograph (Flohr, Raupach, and Bruder 2009). Therefore, the increased use of CT in medical examinations has led to the subsequent increase in the exposure of ionizing radiation to individuals.

This has led to great concerns since exposure to ionizing radiation has been associated with the development of cancer (Smith-Bindman et al. 2009). In a particular study, the amount of radiation that is exposed to the patient during a single CT scan was compared to that which the long-term Chernobyl survivors were exposed to (Smith-Bindman et al. 2009).

Radiations from cardiac CT angiography have increased the impact and prevalence of cardiac atherosclerosis among individuals. For this reason, regulation of the exposure of these elements has been necessary and has been done through adherence to the ALARA principle, which stands for ‘As Low As Reasonably Achievable’.

The ALARA principle came about as a measure to reduce the harmful effects of radiation during any procedure involving radiation (Stolzmann et al. 2008). This implies that reasonable methods should be employed in order to ensure that an individual encounters minimum radiation doses.

It is believed that each radiation dose increases the risk of genetic mutation or cancerous developments. However, since it is also necessary to maintain the image quality in terms of spatial and temporal resolution, it is important to consider both the radiation doses and image quality while performing CCTA.

Studies have shown how to comply with the ALARA principle while maintaining an image quality that would be useful in diagnosis (Torres et al. 2010).

Methodology

The method used for this paper included a thorough literature search to find relevant peer-reviewed articles. The various databases that were used included Medscape, ScienceDirect, PubMed and ProQuest. The keywords that were used included ‘Cardiac CT Angiography’, ‘CT radiation doses’ and ‘radiation reduction strategies in CT angiography’.

From the available articles, only those that were most relevant to the study were selected. Relevance of the articles was determined through the analysis of the articles’ titles, abstracts and conclusions.

The results were also checked and highlighted. In order to ensure up-to-date information, the search criterion was limited to articles published between 2008 and 2013 (not more than 5 years old). From the articles selected, various radiation reduction strategies were highlighted and may be summarised as shown below.

Tube current
Tube Potential
ECG Gating
Position of Patient
Z-axis Coverage
Automatic Exposure Control (ACE)
Collimation
Using of Dual Energy Technology
Exposing the Area of Interest Only (FOV)
Fast Gantry Rotation Time
Using Reconstruction Techniques
Pitch
Over-ranging

Objectives

To identify the various strategies to reduce radiation exposure in cardiac (coronary) CT angiography
To highlight some of the limitations associated with CCTA
To provide recommendations for the appropriate strategies to employ while performing coronary CT angiography

Tube current

Studies have indicated that the tube current within the CT suite should not be constant for all individuals (Leipsic et al. 2010). Several strategies have been studied to ensure optimal tube current during CCTA. One of the strategies employed is the weight-based approach.

This approach aims at adjusting the tube current based on the individual’s body weight or body mass index (Lee et al. 2012). This may be necessary to avoid the possibility of overexposing the individual to radiation. The tube current should be tailored to fit the individuals with different Body Mass Index (BMI) (Leipsic et al. 2010).

This is meant to correct for different body shapes in order to ensure minimal radiation exposure while maintaining diagnostic image quality (Tatsugami et al. 2008). Further research is underway to explore possibility of using diameter or thoracic shape (Shrimpton et al. 2009). This may provide a better alternative for future use as practitioners observe the ALARA principle.

Tube Potential

The tube potential also determines the amount of radiation exposure to patients (Lund et al. 2009). The relationship between the two is exponential. Increasing the tube voltage increases the radiation exposure by the square of that factor (Torres et al. 2010).

ECG Gating

For CCTA to be successful, “motionless” images of the coronary arteries should be acquired. For this to happen, all reconstructed images at a specific time must correspond to a specific point in the cardiac cycle. This may be made possible through ECG gating, which involves synchronizing with the ECG (Stolzmann et al. 2008).

One of the ways this is done is through retrospective ECG gating. This involves the activation of the x-ray beam throughout the entire cardiac cycle. Images are then reconstructed in any desired phase of the cycle.

This method allows for the selection of the appropriate images to reconstruct for each vessel. Despite the fact that this method allows for the assessment of any wall motion abnormality, valve function and ejection fraction, it exposes the patient to high radiation doses.

The alternative method, prospective ECG Gating, involves the same procedure but is only done in a specific period in the cycle (Wu, Budovec, and Foley 2009, 958). The interval between a whole cycle is referred to as the R-R interval. The x-ray tube is only activated at a particular point in the interval and is referred to as the phase window.

As the phase window is made narrower, the radiation dose is also decreased. Therefore, this method may be useful in minimizing exposure of radiation to the patient.

However, this also comes at a cost. Fewer phases of the cycle will be available for image reconstruction. For this reason, it may be difficult to obtain quality images with an increased heart rate (DeFrance et al. 2010).

Position of Patient

Another important factor to consider while reducing radiation dose is the position of the patient in the gantry. The patient should be placed in such a way as to ensure that the area of concern is placed at the isocenter.

This would ensure that the beam crossing that region in whatever direction would cross as much body tissue as possible. A well-positioned patient would facilitate constant image noise and avoid increased surface radiation dose (Bae et al. 2008).

Z-axis Coverage

The total radiation dose is greatly influenced by the craniocaudal length (Khan et al. 2011). This is basically due to the fact that it directly relates to the dose-length product. One way of reducing the radiation dose in CCTA is by limiting this length.

Automatic Exposure Control (ACE)

The ACE is a device that facilitates the termination of x-ray exposure. The x-ray terminal may be operated by man or automatically by the ACE. This device ensures that there is a consistent x-ray film. It may be useful in ensuring reduced radiation dose to the patient since it ensures a good consistent x-ray density in order to match the shapes and sizes of different individuals (Bae et al. 2008).

Collimation

The adaptive section collimation is a method that is promising to reduce radiation dose in CCTA (Deak et al. 2009). The results from a study by Deak and his colleagues showed that this method allowed great reduction of unnecessary exposure to radiation due to z-overscanning. They recommended that it could be used together with other strategies such as AEC and spectral optimization.

Using of Dual Energy Technology

The use of dual source CT has provided solutions to problems that were experienced during the implementation of CT angiography.

The main problem was the fact that imaging needed rapid volume coverage while resolving the disease in vessels. This technology employs the use of two x-ray sources and detectors and ensures double temporal resolution at twice the speed (Torres et al. 2010). Therefore, radiation exposure is minimized.

Exposing the Area of Interest Only (FOV)

Another way of decreasing radiation dose is by exposing the area of interest only for imaging. This ensures that there is an increased z-coverage requiring few images that maintain diagnostic image quality (Khan et al. 2011).

Fast Gantry Rotation Time

In order to facilitate underexposure of radiation to the patient, some scanners have the capability of sub-second gantry rotation times. When this is coupled with tube current modulation, it causes tube current saturation. This means that the tube current works at maximum capacity (Israel et al, 2008).

Using Reconstruction Techniques

The reconstruction method used is also an important consideration. There are two classes that include the surface-based and thresholding-based reconstruction techniques. The thresholding-based reconstruction is preferred (Leipsic et al. 2010). This is due to its speed and its use of relatively small amount of computational power.

Pitch

The high-pitch CT angiography is a technology that has helped achieve reduced radiation exposure to patients. This is mainly because they have significantly reduced the scan time and allowed greater volumes to be covered at a time (Apfaltrer et al. 2012).

Over-ranging

Over-ranging is another strategy for reducing radiation dose exposure. It facilitates the covering of large areas during scanning in order to ensure that the examination is done at a shorter period. The wide-range detectors may be used to facilitate this.

Limitations of CCTA

Despite the many advantages associated with cardiac CT angiography, this technology has its limitations. CT scanners produce ionizing radiation that come into contact with the patient while under examination. Increasingly high exposure to radiations from these procedures increases risks of cancer.

This is even more worrying since the number of people exposed to these radiations has increased over time since CCTA procedures are done virtually everywhere. Some studies have shown the possibility of CT angiography false-detecting coronary obstruction where it actually does not exist (Nissen 2008). Such a high false-positive rate may be dangerous since it may lead to unnecessary procedures.

For CCTA to be successful, the heart rate must be about 65 beats per minutes (McCollough 2008). When this heart rate is exceeded, the quality of images may be affected. Foods and drinks containing caffeine may increase heart rate. Therefore, patients should be encouraged not to take them before examination.

Patients also need to be trained on proper breath-holding techniques in order to get diagnostic images (Hausleiter et al. 2009).

Another limitation is the fact that contraindicated contrast material may lead to misinterpretation of results. Lack of clinical and radiological experts is another limitation since not many are trained in the field. Since the machines used for CCTA are expensive, lack of funding may lead to the use of poor equipment.

Discussion

The technology behind CCTA has proved very beneficial in the field of medicine. Several improvements have been made to make examination more effective and efficient.

This includes reduction of radiation exposure and improved image quality. Despite the several studies that have been done in order to reduce radiation dose, several limitations still exist. This calls for further research in the field.

Recommendations

Several strategies have been studied but each has its pros and cons. While using ECG gating, for example, it would be recommended to use the prospective ECG gating instead of using retrospective ECG gating (Miller, Rochitte, and Dewey 2008, 2330).

Among the various protocols used in CCTA, the 64-row MDCTCA has been recommended (Johnson, Pannu & Fishman, 2009). It is also recommended that patient preparation should be done before the patient undergoes examination.

During examination, exposure parameters such as tube potential and tube current should be adapted to the individuals BMI. Proper positioning of the patient in the scanner is also recommended in order to ensure optimization of CCTA.

Conclusion

Great advancements in technology led to the introduction of coronary CT angiography. This technology allowed for the non-invasive imaging of coronary arteries. The multi-detector computed tomography (CT) scanners were used to perform this examination. Within few years, this technology was in use in virtually every medical situation.

This procedure promised to provide safe and painless diagnosis of coronary disease. However, it was not long before some of the limitations of this procedure were discovered.

The most worrying of the issues associated with CCTA was exposure to ionizing radiations that were linked to cancerous developments. For this reason, the ALARA principle was introduced in order to ensure that the patient is exposed to minimal radiation dose.

Several strategies have been suggested and used to reduce the radiation dose in cardiac CT angiography (Raff et al. 2009). However, some problems are still encountered with the strategies that are in place. Therefore, more research is required to find more effective and safe methods of diagnosing coronary artery disease.

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