Frequently asked questions about computed tomography scans

Are the X-rays used in CT dangerous?

The amount and strength of the X-rays used in CT have a very small risk of causing damage to the cells of our body. We could say that the dose of radiation received in a CT scan would be the same that we would receive naturally, from our environment, in approximately three years.

However, despite the low risk, it is a procedure that is not recommended for pregnant women.

What is the preparation before the test?

The preparation for the CT scan will vary depending on the area of the body to be seen, or the pathology to be studied.

There are studies that do not require any preparation, such as osteoarticular studies, others will require a six-hour fast (three in the case of diabetics), and in some abdominal studies, you will also be asked to drink water or an oral contrast solution before entering the room.

Normally you will be asked to remove your clothes and will be provided with a gown, to avoid possible metallic elements that may interfere with the image.

In general, jewelry or metallic elements will only interfere with the image if they are in the area to be explored, so for example, in studies of the head we will ask you to remove earrings or mouth prostheses, but it will not be necessary to remove your watch or rings.

What are contrasts and why do I have to sign an informed consent form?

The CT shows us images of the different organs or structures of our body, differentiating very well the tissues of different densities, but being not very subtle when the tissues have a very similar density. In an abdominal CT image we will differentiate very well a vertebra from a kidney, since they have very different densities, but within the image of the kidney sometimes we cannot differentiate structures with a similar density, such as a cyst or a nodule.

In order to be able to differentiate structures with similar densities and thus perform a correct CT study, it is sometimes necessary to use radiological contrast agents.

By definition, a radiological contrast is a chemical substance that has the capacity to absorb X-rays to a greater or lesser degree than soft tissues. Depending on the route by which they are administered, we can classify them into digestive contrast (oral or rectal route) and endovenous contrast (through venous puncture).

The oral digestive contrast can be hydric (water), barium sulfate or iodine, and normally you have to start drinking between 45 minutes and 1 hour before starting the study. When the digestive contrast has to be introduced through the rectal route, in addition to the three types of contrast mentioned above, air can also be used (mainly in CT virtual colonoscopy), and it is administered inside the examination room.

Endovenous contrast is composed of iodine and is usually administered through a vein in the arm.

It is common for us to use both types of contrast (oral and intravenous) in abdominal studies.

The use of contrast, especially intravenous contrast, is not without risk. When we inject a contrast through a vein, there is a low probability of suffering an adverse reaction, that is why a radiologist will supervise the patient during the entire test and will intervene if necessary by administering the appropriate treatment for the reaction suffered.

Before entering the CT scanning room, you will be provided with an informed consent form, which is an explanatory document explaining the benefits and possible risks of the contrast injection.

The law dictates that this document must be signed before the test is performed, whether the contrast injection is accepted or refused, to ensure that the patient has received the necessary information.

What happens if I am diabetic or suffer from renal insufficiency?

Until a few years ago, diabetic patients treated with Metformin were recommended to suspend the treatment a few days before and after the test. Nowadays, the ESUR guide, which is the reference guide for the injection of radiological contrasts, has shown that there is no interaction between Metformin and the intravenous contrast, so it is no longer necessary to suspend treatment. The only difference in the preparation of diabetic patients is that in the studies that require fasting, instead of 6 hours, this fasting may be 3 hours.

In cases of chronic renal insufficiency requiring dialysis, intravenous contrast may be administered, since there is no longer a risk of renal lesion.

When the renal insufficiency is mild or moderate, and is not treated with dialysis, the patient should bring a blood test in which the Creatinine and Glomerular Filtration are reflected, since here there is a risk of damaging the renal system. It will be the radiologist who will evaluate the analysis and decide whether it is safe or not to inject the contrast.