As depicted in the following video, over the years, cardiologists have relied upon invasive catheterization to diagnose cardiac-related blood flow problems and muscle wall damage. However, placing a catheter into blood vessels and guiding it into the heart is technique-sensitive, and is far more invasive and less diagnostic than the use of Positron Emission Tomography (PET).
This is true because instead of using a mechanical, wire-like catheter, cardiologists can inject a less intrusive “Rubidium-82 saline liquid” into patients’ blood vessels The short span radioactive liquid (having a very short 76 second half life) then combines with the blood that usually flows through the blood vessels. The Rubidium-82 liquid, also called a tracer, acts just like potassium. Rubidium-82 substitutes itself in place of potassium in the cells.
If the physician injects a potassium analogue tracer into a patient, the patient’s heart muscle cells will absorb the tracer. Note that blood vessels carry the tracer to the heart muscle, but while the tracer disappears in the blood, it collects in the heart muscle, providing a sharp contrast when viewed via a PET scanner that can photograph the emitting radiation.
Several minutes after the Rubidium-82 tracer injection, a PET image will show a concentrated, retained tracer in the heart, which contrasts with the low activity in the blood (because newly circulating blood is carrying away the excess tracer).
Since Rubidium-82 has a relatively short, 76 second half-life, after six half-lives (450 seconds or 7.5 minutes), investigators treat the tracer as completely decayed – because only a mere 1.6% of original radioactivity remains. This brevity of activity means that the tracer cannot be stored and transported between facilities. Instead, it must be produced on-site.
The usual way to produce tracers is by using an expensive cyclotron. Because the cyclotron is expensive and cumbersome, researchers investigated alternative ways to produce tracers with less expensive equipment that could be used on-site in the offices of cardiologists or other healthcare providers. The Cardiogen-82 Strontium-82 to Rubidium-82 generator, which General Electric manufactures and distributes for Bracco Diagnostics, Inc., is a relatively inexpensive piece of equipment that can produce Rubidium-82 from Strontium-82 (which has a longer half-life of 24 days). Strontium-82 is produced from Strontium-85 (having a half-life of about 65 days). Given the much longer half-lives, strontium isotopes would be too toxic for use as tracers in humans.
The CardioGen-82 generator employs a long tin oxide tube that contains the more toxic Strontium-82 (parent), and normal saline is pumped through the tin oxide tube, which results in the manufacture of the less toxic Rubidium-82 tracer. Of course, it is paramount that physicians prevent an occurrence called “breakthrough,” which happens when Strontium-82 is carried along with Rubidium-82 in the tracer liquid in larger than permissible amounts. Injecting Strontium along with Rubidium into a patient increases the risks of radiation poisoning.
The relatively short half-life of Rubidium-82 is both an advantage and a disadvantage. On the positive side, quick decay means shorter patient exposure to radiation. And, physicians can perform repeated PET scans every 10 minutes, because the radiation from the previous dose has disappeared by that time. However, fast decay shortens the maximum scan time and reduces the quality of the scans. That is the reason why on-site Rubidium-82 tracer manufacture and direct tracer injection into a patient, within a really brief period, is so necessary.
The information, above, makes it clear that both catheterization and Rubidium PET scanning have diagnostic pros and cons. While Rubidium PET is less mechanically invasive, and more diagnostic than catheterization, Rubidium PET is, nonetheless, invasive because of the use of radiation in patients and the complexity of technique required in order to use the product safely. If Strontium-82 were to break through during a procedure, it could be disastrous to a patient.