Iodinated compounds are frequently used for intravenous contrast. Iodine has a relatively high atomic number, causing it to give increased attenuation relative to the soft tissues of the body. This is particularly pronounced for kilovoltage imaging, which is reliant on the photoelectric effect (which is proportional to the atomic number cubed). Intravenous contrast allows easy differentiation between vessels, and can also highlight organs based on their capillary network.
ADVANTAGES OF IV CONTRAST FOR CT CHEST SIMULATION
The primary benefit of using intravenous contrast during CT simulation of the chest is that the great vessels of the mediastinum, their branches and their tributaries will become highly visible. This allows easy determination of the nature of structures in the mediastinum, and whether they are vascular or lymphatic/malignant in nature.
The benefit of this is that gross tumour volumes, particularly of lymph nodes, can be reduced by (on average) 25%, leading to smaller field sizes and less lung toxicity.
EFFECT OF IV CONTRAST ON DOSE CALCULATION
Unfortunately, the IV contrast may confuse the planning system as it increases the Hounsfield Units (HU) of the blood vessels. The planning system may calculate that the regions of increased HU attenuate the beam more than actually occurs, introducing a systematic error into dose calculation.
The two areas of concern are:
- Large increase of HU within blood vessels
- Smaller overall increase of HU within the lung due to contrast in capillaries
There are several strategies and theories regarding these concerns.
- A separate, non-contrast scan can be acquired before contrast is given. This provides correct density information for the lungs and great vessels. However, fusion of the two images is difficult unless gating is used for the image acquisition.
- Vessels containing contrast could be contoured as ‘water density’ on the treatment planning system, overriding the Hounsfield Units.
More recent reports suggest that although there may be 2% error in dose calculations when contrast is present, this is within clinically acceptable limits in most cases.
The reference for the last statement is:
Lees J, Holloway L, Muller M, Forstner D (2007) Effect of intravenous contrast on treatment planning system dose calculations in the lung. In Australasian Physical & Engineering Science in Medicine 28 (3) pp190-195
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