Optimization in Computed Tomography (CT)

FAQs for health professionals

» Does the patient exposure from a CT procedure depend on the number of slices?

Before answering this question, let’s compare two radiation quantities involved. First, dose, or absorbed dose, describes the amount of energy absorbed per unit mass of a specific tissue location. The other quantity is the total imparted energy to the body. For example, let’s assume that patient A received a scan of 10 slices, patient B received a scan of 20 slices, and all other factors were identical. The absorbed dose per unit mass to a tissue within the scan for Patient A is approximately the same as the absorbed dose per unit mass to a tissue within the scan for Patient B. This is because the energy from the additional 10 slices is absorbed in a different location. However, the total energy imparted to the body for Patient B is double the total energy imparted to the body for Patient A.

» Why are there special quantities to quantify radiation in CT, such as Computed Tomography Air Kerma Index?

Absorbed doses and organ doses cannot be measured directly within a patient, because it would require placing a dosimeter at the points of interest within the body. They can only be measured within a phantom. Also the air kerma at the axis of rotation can be measured. Tabulating the ratio between organ doses to various organs and air kerma in the axis of rotation of a scan for 1 mAs, provides a table of conversion coefficients. Once the Ca is known, it is straightforward to multiply it by the mAs value and the relevant conversion coefficients to obtain organ doses from a given examination. A more comprehensive and detailed discussion can be found in ICRU 74 and IAEA Code of Practice- TRS 457.

The quantity Ca, also called Computed Tomography Air Kerma Index can be measured free in air or in an air cavity within specific polymethylmethacrylate (PMMA) phantoms for head and body.
CTDI values for a variety of CT systems are available on the web at the ImPact site

(For a large number of years, Computed Tomography Dose Index (CTDI) has been used. Recent publications point out the experimental difficulty in determining the dose to air, especially in the vicinity of an interface, and that, in reality, the quantity measured by instruments is air kerma. For these reasons these publications recommend the use of air kerma rather than absorbed dose to air, and consequently, Computed Tomography Dose Index (CTDI) is to be replaced in future by the Computed Tomography Air Kerma Index (Ca). The reader should be aware, however, that most publications, existing so far still use quantities in terms of CTDI. The use of the new quantity does not change the method to determine organ doses from the conversion coefficients, nor their numerical values.)

» What is the major factor that limits how low we can reduce the radiation exposure of patients in CT?

In most scanning procedures, the exposure of patients can be varied over a wide range by changing the mAs. However, as the radiation absorbed in the tissue (the dose) is reduced, the visual noise in the image is increased. An optimized imaging protocol is one in which the mAs is adjusted to achieve an image noise level that is acceptable for clinical interpretation.

» If I scan with thinner slices, will that reduce the exposure of the patient?

No. Scanning with thinner slices will actually lead to an increased dose. It is actually an indirect effect that happens this way. When the slice thickness is reduced, that reduces the size (volume) of the individual tissue volume element (called voxels). The smaller voxels will absorb or capture less total radiation or number of X-ray photons. It is the number of photons absorbed in each voxel that affects the image noise. When the number of photons per voxel is reduced, the noise increases because of the statistical nature of the photon interactions.

Therefore, when slice thickness is reduced the dose has to be increased (usually with an increase in the mAs) to maintain the same level of noise as with the thicker slices.

» How can I determine the radiation exposure of a patient undergoing a CT examination?

Many modern CT systems calculate the Computed Tomography Air Kerma Index, Ca (*), and the Air Kerma Length Product, PKL, during the scanning procedure and display the values on the operator’s console. The Cvol is the Ca divided by the pitch factor; the PKL is obtained by multiplying the Ca by the length of body section covered by the scanning procedure.

If this feature is not available, an estimate can be calculated from the actual technique factors used (e.g. kV, mAs) and the data obtained from the beam calibration of the equipment (Ca and CVOL). This calibration and calculations are usually performed by a physicist.

» Does the use of 3D volume acquisition - compared to 2D slice acquisition - offer opportunities for dose reduction?

Not directly, but it does offer some possibilities in certain procedures. After a 3D volume acquisition scan is completed, the data can be used repeatedly for additional image reconstructions without any further exposure to the patient. For example, additional overlapping slice images can be reconstructed, or images for different thickness slices, or a variety of orientations.

» Can I reduce the radiation dose in spiral CT by increasing the pitch factor?

Yes but other factors must be considered. 

While increasing the pitch does reduce the dose if all other factors are the same, it also affects image quality. First, the pitch can place a limit on the maximum detail or spatial resolution that can be obtained in the axial slice thickness direction. Second, increasing pitch will increase image noise. However, most CT systems have a function that automatically increases the mAs and dose to maintain a specific noise level as other factors including slice thickness, matrix size, field of view, and pitch are changed. An advantage of increasing pitch is to reduce scanning time, not to reduce dose.

The appropriate action is to select pitch factor values that provide a balance among the image quality and scan time requirements and concerns for patient exposure. This comes from experience and the use of existing national or international guidelines and appropriate references.