Isotopes for Diagnosis

Normal thyroid scan. Equal uptake in both lobes 4h post-dose
Normal thyroid scan. Equal uptake in both lobes 4h post-dose

This subject is vast and growing, so we’ll just take a very quick look.

When introducing ionising radiation into the body there are a few rules of thumb. These are as follows:

Metabolically indistinguishable thus metabolized as if stable

Short-lived, both physically and biologically

Non-toxic especially if a non-metabolite.

For imaging, beta or gamma emitters are preferred of medium energy (hence range) so as they leave the body they can be detected

There are very many techniques routinely used and new developments happen frequently. Here’s a few examples. The ones in red are medical applications.

Two are popularly used – the following is expanded from the above link:

Technetium – 99m: The most widely used radioactive isotope for diagnostic studies in nuclear medicine. Different chemical forms are used for brain, bone, liver, spleen and kidney imaging and also for blood flow studies. M = metastable. Eluted (milked) as sodium pertechnetate from a molybdenum “cow”, it has a convenient half life of 6h and energy a useful 140keV. An injected dose of the radiopharmaceutical is then able to either be followed around the body (dynamic renal studies), uptaken into an organ which can be externally imaged with a gamma camera or blood protein metabolism when tagged to serum albumen studied by repeated sampling later and measurement of radiation content in a scintillation counter.

Screen Shot 2014-04-19 at 8.45.13 PM

Screen Shot 2014-04-19 at 8.46.13 PM

Screen Shot 2014-04-19 at 8.50.04 PMThe reduced uptake – cold spots – in this Tc 99m bone scan are clearly visible.

The later  image is on the left – notice the fuller bladder.

Iodine – 131: half life just over 8 days – a bit long for today’s clinical studies – and an energy of 360keV is used to diagnose and treat thyroid disorders. (Former President George Bush and Mrs. Bush were both successfully treated for Graves’ disease, a characteristic enlargement of the thyroid, with radioactive iodine.)

 

A small quantity of the isotope is given orally as dilute sodium iodide. The thyroid uptakes the iodine exclusively to metabolize two hormones which regulate iodine metabolism in the body. A scintillation detector is used to measure the count rate over the gland after specific time intervals, from which important diagnostic information is obtainable. A deficiency or overactivity in production causes characteristic symptoms which require either surgical or biochemical intervention.

In much larger doses, iodine 131 is an effective agent for treatment – its ionizing properties are restricted to the surrounding thyroid tissue and renders cancer cells ineffective.

Work through  Q6, p710

In conclusion – questions needn’t necessarily be restricted to these two. For example, Xe 133 is completely inert and has a very short half life. Patient takes a deep breath infused with a small quantity of the isotope, stands in front of the gamma camera for a few seconds and a lung perfusion image is created. Here’s an image of two healthy lungs with no cold spots.Screen Shot 2014-04-19 at 10.16.29 PM

 Effective Half Life, T

Assuming both decay and elimination from the body are exponential decay processes, we can write:

In calculations, make sure that both half lives are being measured in the same units
In calculations, make sure that both half lives are being measured in the same units

So, if iodine 131 has a radiological half life of 8 days and a biological half life of 24 days, the effective half life is 6d.

 

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