Scintigraphy (Video)
Nuclear Medicine groups together all of the medical applications that use radioelements, that is radioactive sources.
Scintigraphy is an imaging technique that involves tracking the distribution, in the patient’s body, of radioactive elements that have been introduced in very small quantities.
To understand how this works, let’s review some Physics:
The majority of the chemical elements found in Nature are stable. But certain elements … called “radioactive”, are unstable. That is, with the passage of time and in a random manner, they disintegrate in order to transform themselves into another atom with more stable properties.
Of all the different kinds of known disintegrations, they all have the release of energy in common. In scintigraphy, the one that emits gamma radiation is the most interesting because gamma photons can be detected with a gamma camera.
More than the slow progress of the radioactive element in the human body, it is its fixation inside certain tissues that we seek to detect. Thus the activities of certain organs or pathological processes can be explored in a very targeted manner. It is for this reason that we speak of functional imagery, as opposed to an atomical imagery which shows the contours and densities of organs.
The radioactive element is chosen according to its affinity for the organ that is to be explored
We see here, for example, the fixation of a tracer in the lungs, which enables us to evaluate pulmonary irrigation.
The examination begins well before the patient is brought in, with the early morning delivery, or the fabrication on site, of different radiotracers in a radiopharmacy.
Let’s consider bone scintigraphy as an example. The radiopharmacist will create a bone tracer by tagging a medication (diphosphonate) with a radioactive element (Technetium 99m).
The handling of medications, and even more so, radioactive ones, requires a very strict protocol and is carried out in a protected environment. The radioelements are always stored and transported in lead containers. When the final product is ready to be administered to the patient, it is put in a syringe, itself inside a tungsten syringe protector. Because the radioactivity being handled is very weak, all of these measures serve primarily to protect the personnel, who are subject to the most exposure. (Personnel or Staff ??)
The radiotracer is then administered to the patient by injection. It takes time for it to diffuse and be captured by the targeted organ, in this case, the skeleton.
For the next step, the patient is placed on an examination table under a gamma camera, which will measure the radiation emitted by the patient. By progressively moving the table, one can achieve a full body scan in order to obtain an image of the skeleton.
The heads of the gamma cameras are mobile and, for certain examinations, they can be made to turn around the patient to provide images in sections; this is known as tomoscintigraphy.
This is the case for this heart examination, carried out at rest or after a stress test.
The appearance of the images obtained depends on the organ being studied.
With the thyroid, for example, which is an endocrine gland located at the base of the neck, one will inject the patient with a radioactive isotope of iodine (iodine 123), which is used in hormone production.
The static image, which is taken 2 hours after the injection, gives us, on one hand, information about the gland’s morphology, but above all about its functioning.
In this photograph, thyroid activity is normal. On this one, by contrast, we see that the gland is larger and displays a diffuse hyperfixation related to overactivity on the part of the gland, a hormonal disorder namedhyperthyroidism.
In the opposite case, the very weak fixation of iodine in this photograph, taken with the same exposure time, indicates a slowed down thyroid function. This is hypothyroidism.
On these two other photographs one can see abnormal, rounded structures in the interior of the gland. These are nodules. On the left we have hyperfixing “hot nodules” and on the right the hypofixing “cold nodules”.
In the case of bone scintigraphy, the tracer used is one with an affinity for bone. Here is what we see in a healthy subject. Cancer cells have higher level of metabolic activity than healthy ones, so the pockets of hyperfixation reveal the presence of metastases.
Here is a final example, myocardial scintigraphy. After computer analysis, images in section are obtained with colored zones that indicate good irrigation of the myocardium.
The same observation made after a stress test reveals an area that has not taken up the tracer (in this case thallium 201). Thus an ischemia is diagnosed, that is to say, poor oxygenation resulting from a coronary stenosis.
PET Scan
The letters “P” “E” “T” are the acronym for Positron Emission Tomography. The principle is the same in that we inject a radioactive tracer into the patient’s body, but here it is a beta+ disintegration that will be detected indirectly.
If we take this normal image, primarily the brain and the myocardium can be seen, which are heavy consumers of glucose, as well as the urinary passages, indicating the elimination of the tracer. In these other images, pockets of cancer can be detected because cancer cells require a lot of energy in relation to their activity of proliferation.
The images obtained with this technique can be superimposed over the tomodensitometric image of the same region, provided by a CT-scan linked to the apparatus. An anatomic image : Scan ... and a functional image: PET are thus combined.
Risks and benefits
Scintigraphy helps us to visualize the functioning of organs in a simple and only slightly invasive manner. For a patient stricken with a neurodegenerative disease, for example, MRI or a CT-scan only show normal structure whereas scintigraphy reveals those areas of the brain that are no longer functioning.
The use of very weak doses of radioactive elements has not shown any undesirable effects.
As a precautionary measure this examination is always contraindicated for pregnant or nursing women.
The personnel are equipped with dosimeters (and rings) in order to ensure that their exposure to radiation remains within the norms.
