Approcci alla riduzione della dose in Computed Tomography
Dose and Radiation Risk in CT
Today, in industrialized countries, the radiation dose level from medical exposure is in the same range as the annual natural background radiation of 3.1 mSv. Radiation dose in medical imaging is an important topic of discussion. In the following section, in-depth information is provided about how radiation dose during a CT scan is estimated. Furthermore, the factors that affect it are discussed, including the difficulties in analyzing the real risk that can be attributed to CT scans and some interesting comparisons with environmental influences.
CT-Specific Dose Parameter: CTDI
During a CT scan, cross-sections (slices) of the body are irradiated. Nevertheless, the X-ray dose delivered to the body is not exactly confined to the user-defined slices, but extends outside this area due to scattering of the radiation (Figure 1).
The scattering of the X-rays must be included when calculating the absorbed dose D. The Computed Tomography Dose Index (CTDI) is the sum of the absorbed dose in the slices and the contributions outside (Figures 2 and 3), normalized to the nominal slice thickness.
Mathematically, the CTDI is calculated as the integral of the absorbed dose along the z-axis, divided by the nominal slice thickness.
CTDI is the measure of the dose deposited in a single axial slice of the patient. The unit used to measure it is the mGy (1 mGy = 1/1000 Gy).
In practice, the integration limits cannot be extended to infinity. E.g. CTDI as defined by the FDA (Food and Drug Administration) in the United States requires an integration length of 7 nominal slice thicknesses on either side of the irradiated slice1. The more common definition today, CTDI100, requires an integration range of 50 mm on either side of the irradiated slice. This is more practical, since most ionization chambers used to measure CTDI are 100 mm long. The ionization chambers are placed in the center and the periphery of Perspex® dummies of 16 cm diameter for the head and 32 cm diameter for the body (Figure 4).
There are different ways to calculate the CTDI. One of them is to consider the differences between the absorbed dose in the periphery and in the center of the patient’s body by a weighted sum of the central and peripheral CTDI values.
The resulting formula for the weighted CTDI (CTDIw) that takes into account this difference is:
1FDA October 20, 2006: Guidance for Industry, FDA Staff, and Third Parties – Provision for Alternate Measure of the Computed Tomography Dose Index (CTDI) to Assure Compliance with the Dose Information Requirements of the Federal Performance Standard for Computed Tomography
Important Parameters That Affect the Absorbed Dose in CT: CTDIvol and DLP
Volume CT scans include many sequential slices during a spiral scan. For this reason, the velocity with which the table moves must be considered: If the table moves slowly, the X-ray beam profiles will overlap (Figure 5). For a spiral scan, pitch is defined as the longitudinal distance in mm that the table travels during one revolution of the X-ray tube divided by the nominal irradiated width of the detector projected to the isocenter of the scanner.
For a spiral examination, the CTDIvol is:
CTDIvol = CTDIw 1/pitch
If the pitch is smaller than 1, the X-ray beam profiles overlap and the absorbed dose increases. If the pitch is larger than 1, the X-ray beam profiles do not overlap, there are gaps in the acquisition and the absorbed dose decreases. This is valid for both single-detector and multidetector row CT.
The expected CTDIvol is displayed on the user interface of the CT scanner prior to each scan. The operator can therefore easily observe on the screen the absorbed dose according to the parameters chosen for the scan (Figure 6).
In order to calculate the total absorbed dose for a complete CT examination, the range that is being examined must be taken into account (Figure 7).
The dose length product (DLP) is the product of CTDIvol and the examination range: DLP = CTDIvol · L It is measured in mGy cm. Both CTDIvol and DLP for each CT examination are stored with the patient protocol and are therefore readily available.
Another aspect to be considered is that the absorbed dose is also related to the size of the patient. If a patient is smaller than the 32-cm Perspex® phantom used to determine the body CTDI, the actual absorbed dose will be higher. If the patient is bigger, the actual absorbed dose will be lower.
If the patient’s shape/cross-section is similar to that of the CTDI phantom, CTDIvol can be used as an estimate for absorbed patient dose.
Effective Dose in CT
The effective dose in CT takes into account the direct and scattered radiation for all organs in the scan volume. It cannot be calculated exactly for each patient, but a close estimate can be obtained by means of Monte Carlo simulations, assuming an idealized “average” patient. It is based on a mathematical adult hermaphrodite phantom of the kind used for Monte Carlo simulations of effective doses by the UK National Radiological Protection Board (NRBP) in 1989.
The formula for the effective dose E is: E = Σ Dorg · worg
The effective dose in CT is therefore a measure of the mean radiation burden based on a patient group, not a measure of the radiation burden of an individual patient, who normally deviates from the idealized “average” patient).
The effective dose is the sum of the doses for all organs, multiplied by the respective tissue weighting factors. For different scan ranges, the effective dose E can be calculated approximately from the dose length product (DLP)1:
E = DLP · f
The mean weighting factor f (average between male and female models) is used for different regions of the human body:
1. Head: f = 0.0021 mSv/(mGy · cm)
2. Neck: f = 0.0059 mSv/(mGy · cm)
3. Thorax: f = 0.014 mSv/(mGy · cm)
4. Abdomen and Pelvis: f = 0.015 mSv/(mGy · cm)
Table 1 shows typical examples of the effective dose for different CT routines.
1Jessen KA et al. EUR 16262: European Guidelines on Quality Criteria for Computed Tomography. Office for Official Publications of the European Communities, Luxembourg, 2000.
Radiation Risk in CT
The effective doses typically used during CT routines (e. g. head 1.9 mSv, thorax 3.4 mSv, abdomen 4.9 mSv) are far below the thresholds that are commonly associated with deterministic damage (Table 2).
However, the risk of stochastic damage after one CT scan remains uncertain. There are only a few assumptions and models to quantify this risk.
The most important study that addresses this issue was conducted on 105,000 radiation victims in Hiroshima and Nagasaki, of which 35,000 received radiation doses between 5 and 200 mSv.1 Unfortunately, this study revealed a high statistical uncertainty in the low dose range that applies to CT scanning. Furthermore, uncertainties remain about the shape of the dose-response, both for cancer and for non-cancer diseases, below about 100 mSv.2 The assumption today is a linear relationship between the radiation dose and the additional cancer risk with no dose threshold (linear no-threshold model, or LNT) and that risk depends strongly on the age at the time of irradiation (the younger the child, the higher the potential risk).
A recent publication by Brenner et al estimated the lifetime risk of death from cancer attributable to a CT scan as shown in Figure 9A and 9B. 3
The International Commission on Radiological Protection (ICRP) of 1990 assumed an excess lifetime cancer mortality risk of about 5% per Sv. Based on this assumption, a CT examination with 10 mSv may increase cancer mortality risk by about 0.05%. This value is in reasonable agreement with Brenner’s assumptions (Figures 9A and 9B). However, this risk has to be framed appropriately. According to Smith, the average cancer mortality risk in Western society is about 25%. After a CT examination with 10 mSv, it is increased by only 0.05% (25.05%). This is the same increase of mortality risk as living in Central London for 450 days (death caused by air pollution) or living in the same apartment with a smoker for 540 days.4
The estimated lifetime risk of death from various sources is shown in Table 3.
Therefore, if clinically indicated, the benefit of a CT examination far outweighs the additional radiation risk for the patient. Nevertheless, Siemens’ ultimate goal is to adhere to the ALARA (As Low As Reasonably Achievable) principle, i.e. to use the reasonably achievable dose to obtain the required diagnostic quality images.
1 Preston DL et al. Solid cancer incidence in atomic bomb survivors: 1958–1998. Radiat Res. 2007 Jul;168(1):1-64.
2 Muirhead CR. Studies on the Hiroshima and Nagasaki survivors, and their use in estimating radiation risks. Radiat Prot Dosimetry. 2003;104(4):331-5.
3 Brenner DJ et al. Computed tomography – an increasing source of radiation exposure. N Engl J Med. 2007 Nov 29;357(22):2277-84.
4 Smith JT. Are passive smoking, air pollution and obesity a greater mortality risk than major radiation incidents? BMC Public Health. 2007 Apr 3;7:49.