Is it a tumor? Magnetic Resonance Imaging (MRI) can tell. Your head is full of tiny radio transmitters (the nuclear spins of the hydrogen nuclei of your water molecules). In an MRI unit, these little radios can be made to broadcast their positions, giving a detailed picture of the inside of your head.
Section summary
Radiopharmaceuticals are compounds that are used for medical imaging and therapeutics.
The process of attaching a radioactive substance is called tagging.
[link] lists certain diagnostic uses of radiopharmaceuticals including the isotope and activity typically used in diagnostics.
One common imaging device is the Anger camera, which consists of a lead collimator, radiation detectors, and an analysis computer.
Tomography performed with
-emitting radiopharmaceuticals is called SPECT and has the advantages of x-ray CT scans coupled with organ- and function-specific drugs.
PET is a similar technique that uses
emitters and detects the two annihilation
rays, which aid to localize the source.
Conceptual questions
In terms of radiation dose, what is the major difference between medical diagnostic uses of radiation and medical therapeutic uses?
One of the methods used to limit radiation dose to the patient in medical imaging is to employ isotopes with short half-lives. How would this limit the dose?
A neutron generator uses an
source, such as radium, to bombard beryllium, inducing the reaction
. Such neutron sources are called RaBe sources, or PuBe sources if they use plutonium to get the
s. Calculate the energy output of the reaction in MeV.
Neutrons from a source (perhaps the one discussed in the preceding problem) bombard natural molybdenum, which is 24 percent
. What is the energy output of the reaction
? The mass of
is given in
Appendix A: Atomic Masses , and that of
is 98.907711 u.
The purpose of producing
(usually by neutron activation of natural molybdenum, as in the preceding problem) is to produce
Using the rules, verify that the
decay of
produces
. (Most
nuclei produced in this decay are left in a metastable excited state denoted
.)
(a) Two annihilation
rays in a PET scan originate at the same point and travel to detectors on either side of the patient. If the point of origin is 9.00 cm closer to one of the detectors, what is the difference in arrival times of the photons? (This could be used to give position information, but the time difference is small enough to make it difficult.)
(b) How accurately would you need to be able to measure arrival time differences to get a position resolution of 1.00 mm?
The activities of
and
used in thyroid scans are given in
[link] to be 50 and
, respectively. Find and compare the masses of
and
in such scans, given their respective half-lives are 8.04 d and 13.2 h. The masses are so small that the radioiodine is usually mixed with stable iodine as a carrier to ensure normal chemistry and distribution in the body.
(a) Neutron activation of sodium, which is 100%
, produces
, which is used in some heart scans, as seen in
[link] . The equation for the reaction is
. Find its energy output, given the mass of
is 23.990962 u.
(b) What mass of
produces the needed 5.0-mCi activity, given its half-life is 15.0 h?
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