Radiation Dose Reduction Strategies in CT-guided Interventions

Introduction

Computed tomography is also known as computed axial tomography, CT scan, or CAT (computerized axial tomography) scan. It is a diagnostic tool that has been used worldwide by doctors to identify various types of health problems. This requires special X-ray equipment to create high-quality images of the structures inside the body. The computed scan also exposes the body to some radiation, which does carry some amount of risk and is important to consider before going for a scan.

What Is Radiation?

Computed tomography uses ionizing radiation. This type of radiation is powerful enough to pass through the body and create images on the computer screen. This type of radiation can be harmful and may increase the chances of having cancer at some point in the future. However, this is a rare chance and ionizing radiation is present everywhere in the environment, such as cosmic rays from outer space.

The dose of radiation in CT is very low and does not cause any harm usually, however, at higher doses, there may be a potential risk of cancer.

How Is Radiation Measured?

The radiation dosage is measured as the effective dose, which means how much the body absorbs the dose. Different tissues of the body have different levels of absorption, such as the amount of absorption of radiation dosage for the abdomen will be different from the scan of the head. The radiation dosage is measured in units called millisieverts (mSv). The mSv dosage is compared by the doctors to how much time it would take to absorb the same amount of radiation from the environment.

How Does a CT System Work?

A CT scan can be done on any part of the body and does require special equipment and set-up to perform the scan.

• For this, a motorized table is used that moves the patient through a circular opening in the computed tomography system.

• The patient has to lie on the table which is then moved inside the opening. Then an X-ray source and detector assembly present in the system revolves around the patient.

• A single rotation takes around a second or less to complete.

• During this, the X-ray source creates a narrow fan-shaped beam of X-rays that passes through the body of the patient.

• These beams are received by the detectors present in the opposite row and the images of the patient’s body are created. The images are taken at different angles and are collected during one complete rotation.

• In this way, multiple images are taken and sent to a computer system to reconstruct the image of the body part by collecting all the cross-sectional images of the organs and tissues.

• The CT images provide detailed and high-resolution images as compared to other imaging tests such as X-rays.

What Are the Effective Dosage for Different Scans?

The effective dosage values for some of the common imaging examinations include -

The effective dosage values for computed tomography for different imaging procedures comparing to the natural background radiation exposure is as follows -

1. Computed Tomography (CT) - Abdomen and Pelvis

• Approximate effective radiation dose - 7.7 mSv.
• Comparable to natural background radiation for 2.6 years.

2. Computed Tomography (CT) - Abdomen and Pelvis (Repeated With and Without Contrast Material)

• Approximate effective radiation dose - 15.4 mSv.
• Comparable to natural background radiation for five years.

3. Computed Tomography (CT) - Colonography

• Approximate effective radiation dose - 6 mSv.
• Comparable to natural background radiation for two years.

4. Computed Tomography (CT) - Brain

• Approximate effective radiation dose - 1.6 mSv.
• Comparable to natural background radiation for seven months.

5. Computed Tomography (CT) - Brain (Repeated With and Without Contrast Material)

• Approximate effective radiation dose - 3.2 mSv.
• Comparable to natural background radiation for 13 months.

6. Computed Tomography (CT) - Head and Neck

• Approximate effective radiation dose - 1.2 mSv.
• Comparable to natural background radiation for five months.

7. Computed Tomography (CT) - Spine

• Approximate effective radiation dose - 8.8 mSv.
• Comparable to natural background radiation for 2.6 years.

8. Computed Tomography (CT) - Chest

• Approximate effective radiation dose - 6.1 mSv.
• Comparable to natural background radiation for two years.

9. Computed Tomography (CT) - Lung Cancer Screening

• Approximate effective radiation dose - 1.5 mSv.
• Comparable to natural background radiation for six months.

What Are the General Dose Reduction Strategies?

Lots of effort has been put into improving the dose efficiency for computed tomography scans. This is related to multiple components of the CT system, such as the detector, collimator, and beam-shaping filter.

1. X-ray Detector - This is one of the most important factors in determining the dose performance of the CT system. The X-ray detector depends on two factors: quantum detection efficiency and geometrical efficiency, which help describe the effectiveness of the detector in converting the X-ray energy into signals. One obstacle faced by X-ray detectors in reducing the radiation dosage is image noise.

The image noise is composed of two systems: quantum noise and electronic noise. Quantum noise means the number of photo particles collected by the detector, and electronic noise means fluctuations in the electronic components of the data-collecting system. When the number of photon particles gets reduced (reducing radiation exposure), the image quality is degraded, and to improve the image quality and reduce the radiation dosage refinement of the electronic component system is needed.

2. Collimators - There are two types of collimators: pre-patient collimators and post-patient collimators.

Pre-patient collimators are those which are placed between the X-ray source and the patient to define the X-ray beam coverage and avoid unnecessary radiation doses to patients.

Post-patient collimators are present between the patient and the detector, usually just in front of the detector, to reject scattered radiation, which improves the image quality but sacrifices dose efficiency. Therefore, careful assessment has to be done between the dose and image quality to optimize the design of scatter-rejection collimators.

3. X-ray Beam-shaping Filter - This is an important factor for the dose performance of a CT system. This is a physical object that reduces and hardens the X-ray beam to penetrate the human body to provide a detailed image. Some of the X-ray filters are designed in special shapes to reduce the radiation dose to the patient, especially the skin dosage of radiation. In modern CT systems, various shapes of X-ray beam filters are used in the case of different scans such as head, body, pediatric and cardiac to reduce the peripheral radiation dosage.

4. Scan Range - This is directly related to the total radiation dosage delivered to the patient. Therefore it is important to maintain the scan range as low as possible to avoid direct radiation exposure to any region of the body.

What Can a Person Do to Reduce Radiation Exposure?

The risk of unwanted radiation exposure can be reduced by following some of the below-mentioned steps -

• Asking the doctor about the need for the test and to go for an alternative imaging test such as ultrasound to avoid radiation exposure.

• In case the scan reports are required at another facility then it is better to ask for the digital copies of the scan instead of going to another scan.

• If a person has taken multiple scans then they should keep a record of them so that the doctor knows how many times a person is exposed to radiation.

• Patients should not go for scans just to be sure of any problem, as CT is a powerful tool that should only be used when it is necessary.

Conclusion

Computed tomography is one of the best tools for detecting multiple conditions in the body. Additionally, many efforts have been made to reduce the radiation dose from the CT scan. To avoid unwanted exposure, patients should not go for unnecessary scans, and the radiation doses should be kept as low as possible. Therefore, when the radiation dosage per each examination is reduced by introducing novel techniques, which may further improve the benefits of the technique.