The physics of Nuclear Imaging part 1
Outlines:
1-Radiopharmaceuticals
2-Radiopharmaceutical QC
3-Dose Calibrator QC
1-Radiopharmaceuticals:
·
Definition: Radiopharmaceuticals consist of a radioactive isotope, which
creates the image, and a pharmaceutical, which determines the physiological
behavior of the compound and, therefore, where the signal accumulates to form
the image.
·
Properties
of ideal radiopharmaceuticals:
1.
Easily
and firmly attached to the pharmaceutical at room temperature but has no effect
on its metabolism.
2.
Readily
available at the hospital site.
3.
A
high-specific activity
4.
High
target: non-target uptake ratio
5.
Easy
and cheap to produce
6.
Non-toxic
7.
Does
not alter physiology in order to give an accurate depiction of the patient’s
physiology
8.
The
effective half-life should be similar to the exam duration to reduce the
patient dose
9.
Pure
gamma emitter for imaging and pure particle radiation emitter for therapy
10.
Easily
to be eliminated from the body
·
Examples:
1.
99mTC-MDP (bone scan): MDP works as a pharmaceutical to be bonded
to a 99mTc radioisotope.
99mTC-MDP is accumulated at the highly active areas where
osteoblasts use the phosphate group to build the broken bone or repair the inflammatory sites because of
metastis occurance.
2- 99mTc-MIBI (cardiac scan): MIBI works as a pharmaceutical,
while 99mTc as a radionuclide, 99mTc-MIBI is going through the patient’s blood to
reach the mitochondria to form electrostatic binding (+ve 99mTC-MIBI and -ve
mitochondrial surface). that’s why you will find 99mTC -MIBI in the organs
containing mitochondria i.e heart, liver, and muscles ….
o The myocardial wall defects and cardiac functions
are diagnosed by 99mTc-MIBI
o the coronary arteries blockage prevents the
passing of 99mTc-MIBI to the myocardial wall.
o the stenosis or the occlusion of coronary arteries
reduces the blood flow to a certain segment of the left ventricle, so this
segment contains less amount of radioactive material which is shown as a cold
or hypoactive region in the image.
2-Quality control procedures in the
hot lab:
|
• Generator
(Alumina, Moly breakthrough, pH) • Radiopharmaceutical
(TLC labeling efficiency test) • Dose
Calibrator |
•
Generator (Alumina, Moly
breakthrough, pH) :
1-Alumina
breakthrough test
We have to assure
that alumina concentration doesn’t exceed 10PPM in the eluate.
Materials:
o
Alumina standard solution
o
Two Filter papers (A and B)
Procedure by
colorimetric method :
o
put a drop of alumina standard
solution on the filter paper (A) and monitor the color of the drop.
o
put a drop of the eluate (free
99mTc) on another filter paper (B)
o
compare the intensities of color
between A and B.
o
if the color intensity of B is
less than A, so it is an accepted result.
2-Mo-99 breakthrough
test
o
We have to assure that Mo-99
activity doesn’t exceed 0.15 uci per 1 mci of 99mTc at the time of
administration.
Material:
o
Lead shield or canister for Mo-99
o
Dose Activity meter
Procedure:
o
Put the eluate in the activity
meter and record the reading A by pressing the 99mTc button
o
Put the eluate in the lead
shield.
o
Put the shield containing the
eluate in the activity meter and record the reading B by pressing the Mo-99
button.
o
The moly activity is calculated
by the equation 1-((A-B)/A))
·
Radiopharmaceutical (TLC labeling
efficiency test) :
o
To assure the labeling efficiency
of 99mTc with pharmaceutics.
o
The free technetium 99mTc is able
to bind with the pharmaceutics under certain conditions.
o
The suitable condition of radiopharmaceutical
preparation varies with different types.
o
The preparation may require:
o
PH of the
solution, how much activity is required? , heating condition (Temp, heating
time ), how much volume is required?
o
Any change in the suitable
parameter cause reduction in the labeling efficiency of radiopharmaceutical
which disrupts the biodistribution.
o
There are three product results
after preparation: the free technetium 99mTc, the hydrolyzed reduced
technetium, and the labeled radiopharmaceutical.
o
Our target is to reach the
labeling efficiency of radiopharmaceuticals greater than > 90 %.
o
Thin Layer chromatography (TLC) is used to
discriminate between the three types of preparation products.
o
TLC permit to each product to
pass through it to reach a certain distance. we are using a solvent which able
to reach the maximum distance (solvent front) of TLC paper.
o
The ratio between the distance traveled
by the (product + solvent) to which traveled by the solvent only is called the
Rf value.
o
Each product bound to a solvent
has an Rf value. If Rf =1, the (product + solvent) reaches the same distance
traveled by the solvent alone.
o
The solvent type varies from one
radiopharmaceutical to another.
§ Procedure:
o
Prepare the radiopharmaceutical
solution
o
Put a drop of the solution at the
origin line of the TLC paper and let it dry.
o
Immerse the TLC paper vertically
in a specific solvent, but make the solvent surface below the origin line of
the TLC paper.
o
Wait for a few minutes, to let
the solvent pass through the TLC paper taking the product to a certain
distance.
o
Based on the Rf values, each
product will reach a certain distance.
o
Then, we cut the TLC paper to
different segments. each segment contains a certain product of the
radiopharmaceutical solution.
o
Each segment should be measured
by dose activity meter to record the dose for each segment.
o
We can calculate the labeling
efficiency, by measuring the dose of labeled product and dividing the result by
the total dose of all segments.
o
The result should be multiplied
by 100 to get the labeling efficiency in percent
o
Dose Calibrator:
o Constancy Test
o This test, performed at installation and on a daily basis, examines instrument precision and is intended to demonstrate that reproducible readings are produced day after day on all isotope settings that are likely to be utilized. In the dosage calibrator, a long-lived source, typically 30 yr Cs-137, is used. The activity is then measured on the Cs-137 setting (which actually represents a "mini" accuracy test) as well as all other daily settings. readings are entered in the proper logbook and compared to recent readings to assess if the instrument is consistently performing on a daily basis. Values measured must be within 10% of the standard value. It should be mentioned that, because we are reading the activity of a Cs137source on Tc99m and Tl201 settings,It should be emphasized that because we are reading the activity of a Cs137source on settings for Tc99m, Tl201, I123, Xe133, and other isotopes, we will acquire inaccurate values. We anticipate getting the same inaccurate reading day after day. The data acquired during a daily constancy test during a one-week period are shown in the table below. The dose calibrator was found to be accurate.
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