Computed tomography dose index

Computed Tomography Dose Index (CTDI) is a measure of the radiation dose received in Computed Tomography (CT) scanning. It is an important parameter for ensuring patient safety and optimizing the quality of CT images. The CTDI quantifies the dose distribution along the z-axis (the axis parallel to the patient's long axis) of a patient or a phantom that simulates the patient's absorption characteristics.

Overview

CTDI is designed to provide a standardized method for measuring radiation dose in CT scans, allowing for the comparison of radiation levels across different CT scanners and scanning protocols. It is expressed in units of milligrays (mGy). The CTDI takes into account the contributions from a single axial scan and the additional dose from adjacent CT slices due to beam overlap in helical CT scanning.

Calculation

The CTDI is calculated using the formula:

\[CTDI = \frac{1}{nT} \int_{-7T}^{7T} D(z) dz\]

where:

• \(D(z)\) is the dose profile along the z-axis measured in a cylindrical phantom (either 16 cm in diameter for head scans or 32 cm for body scans),
• \(T\) is the nominal slice thickness,
• \(n\) is the number of slices acquired in one rotation, and
• The integral limits represent the integration over a distance of 14T, which encompasses the primary beam and scatter radiation.

There are three main variants of CTDI:

• CTDI₁₀₀: Measures the dose over a 100 mm length phantom.
• CTDI_w: Weighted CTDI, which is a weighted average of the CTDI₁₀₀ measured at the center and periphery of the phantom. It provides a more realistic estimate of the dose a patient receives.
• CTDI_vol: Volume CTDI, which accounts for the pitch (the table feed per rotation divided by the total collimation width). CTDI_vol is considered the most relevant measure for patient dose in helical CT scanning.

Importance

Understanding and monitoring CTDI is crucial for minimizing the radiation dose to patients while maintaining the diagnostic quality of CT images. It helps in adhering to the As Low As Reasonably Achievable (ALARA) principle in medical imaging. Regulatory bodies and healthcare institutions use CTDI values to set dose limits, develop dose optimization protocols, and compare the performance of different CT scanners.

Limitations

While CTDI provides a standardized measure for radiation dose, it has limitations. It does not account for the entire dose distribution within the patient or phantom, nor does it reflect the varying sensitivity of different tissues to radiation. Additionally, the use of phantoms may not perfectly mimic the absorption and scattering properties of human tissues.

Conclusion

The Computed Tomography Dose Index is a fundamental tool in the field of radiology for ensuring patient safety and optimizing the quality of CT scans. By providing a standardized measure of radiation dose, CTDI enables the comparison, regulation, and reduction of patient doses across different CT scanning protocols and equipment.

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