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Nuclear medicine in diagnosis and therapy — An overview242
activated) crystal. This detector, with large field of view, was able to image the entire organ in
one view and also allowed dynamic imaging. Though the Anger camera was a major landmark in
the history of nuclear medicine, its optimal use was limited for the want of efficient electronics
and suitable radiotracer. The major breakthrough came in early 1970s with the advent of
Technetium labeled radiopharmaceuticals. Improvements in the collimator design, crystal, and
electronics resulted in scintillation detector far superior than its older version. Today the
scintillation camera serves as a workhorse for nuclear medicine all over the world. The other
important development in Nuclear Medicine includes the single photon emission computerized
tomography (SPECT) imaging. Kuhl and Edwards introduced the concept of SPECT in 1963 (6).
The detector was rotated around the patient’s organ of interest and planar images were acquired
at multiple angular projections. Acquisition and processing of image data was done with the help
of associated computer system. Ter-Pogossian et al (7) developed a multicrystal positron emission
transaxial tomographic (PET) system.
Diagnostic Nuclear Medicine
Diagnostic procedures in nuclear medicine can be broadly classified into a) in vitro tests, b) in
vivo - nonimaging procedures and c) in vivo imaging procedures.
In vitro tests
RIA
Schilling Test
Red Cell mass estimation
Blood volume estimation
Red Cell survival study
Radio respirometry (14C breath test for malabsorbtion syndrome-Urea Breath Test)
Radioimmunoassay (RIA) is a procedure that measures minute amount of a substance,
such as a hormone or drug, by quantitating the binding, or the inhibition of binding, of a
radiolabelled substance to an antibody. A typical radioimmunoassay is performed by the
simultaneous preparation of a series of standard and unknown mixtures in test tubes, each
containing identical concentrations of labelled antigen and specific antibody. After an appropriate
reaction time the antibody-bound (B) and free (F) fractions of the labelled antigen are separated
by one of a variety of techniques. The B/F ratios in the standards are plotted as a function of the
concentration of unlabelled antigen (standard curve), and the unknown concentration of antigen
is determined by comparing the observed B/F ratio with the standard curve.
Schilling test is used in patients with suspected vitamin B12 deficiency. The purpose of the
test is to determine the presence of pernicious anemia. The patient is given 57Co or 58Co labelled
vitamin B
orally followed by an intramuscular injection of unlabelled vitamin B12. The non-
12
Nuclear medicine in diagnosis and therapy — An overview 243
radiolabelled vitamin B12 saturates the tissue stores and prevents radioactive vitamin B12 binding
in body tissues. The patient’s urine is then collected over the next 24 hours. Normally, the
ingested radiolabelled vitamin B12 is absorbed in the body. Since the body already has an
adequate store of non-radiolabelled vitamin B12, much of the ingested vitamin B12 will be
excreted in the urine. A normal result shows at least 10% of the radiolabelled vitamin B12 in the
urine over the first 24 hours. In patients with pernicious anaemia or with deficiency due to
impaired absorption, less than 5% of the radiolabelled vitamin B12 is detected. If an abnormality
is found, the test is repeated with additional oral intrinsic factor to differentiate between pernicious
anaemia and other conditions like malabsorption resulting from coeliac disease, biliary disease,
Whipple’s disease or liver disease.
Red Cell Mass and Plasma Volume Estimation - Polycythemia describes an increased
proportion of red blood cells in the peripheral blood. In absolute polycythemia, there is increased
red cell mass with normal plasma volume, in contrast with apparent polycythemia, in which
there is increased or normal red cell mass and decreased plasma volume. In order to deliver the
appropriate treatment it is necessary to differentiate between the two conditions (8). Red cell
mass estimation can be done by the isotope dilution method. A small volume of the patient’s
whole blood or red cells is incubated with 51Cr. Once inside the red cell, the radioactive chromate
ion freely diffuses into the RBC where it is reduced to chromic ion that is retained in the cell and
re-injected into the patient. This is followed by withdrawing of the blood sample for counting
purposes. Simultaneously, plasma volume is measured using
125
I-labelled albumin; it is injected
and so diluted. Upon re-sampling, the volume of distribution can be determined for the known
initial red cell concentration. Erythrocyte mass can then be calculated. Obese patients can produce
odd results and so their results are often expressed in terms of ideal body weight or surface area.
A red cell mass greater than 125% of that predicted is diagnosis of polycythaemia.
Red cell survival Study - Red cell survival and sequestration studies have been used to
confirm anaemia that results from either haemolysis or abnormal sequestration of RBC’s in the
spleen. The radiopharmaceutical used is Na
51
CrO4 and the RBC’s are labelled in the same
2
fashion as in blood volume studies except that a larger dose is used. For the estimation of RBC
half-life, blood samples are obtained at 24, 48 and 72 hours and on alternate days thereafter for
2-4 weeks. The samples are then counted together at the end of the study. For splenic sequestration,
regular monitoring of RBC uptake into the liver and the spleen and the comparison of this uptake
to the background vascular activity is done using a thyroid uptake probe. The counts are obtained
1,2 and 3 days after injection and then three times a week for another two weeks.
Urea breath test is a rapid diagnostic procedure used to identify infections by Helicobacter
pylori, a spiral bacterium implicated in gastritis, gastric ulcer, and peptic ulcer disease. Patients
swallow 14C- labelled urea. In the subsequent 10-30 minutes, the detection of isotope-labelled
carbon dioxide in exhaled breath indicates that the urea was split; thus indicating that urease (the
enzyme that H. pylori uses to metabolize urea) is present in the stomach, showing the presence
of H. pylori bacteria. The test should be performed 14 days after stopping proton pump inhibitors
Nuclear medicine in diagnosis and therapy — An overview244
or 28 days after stopping antibiotic treatment to avoid false negative results. It is also believed
that urease producing oral flora can result in a false positive result.
In vivo non-imaging procedures
Radioactive Iodine uptake.
T4 Suppression test
Perchlorate discharge test
Glomerular filtration rate (GFR) and Effective renal plasma flow (ERPF) estimation
Radioactive Iodine uptake is a commonly used test in the management of patients with
thyrotoxicosis and thyroid cancer. It is a diagnostic test used to determine how well the thyroid
gland takes up or absorbs iodine, and how iodine is distributed among various cells in the gland.
Radioactive iodine uptake can differentiate true hyperthyroidism from conditions such as excessive
intake of thyroid hormone and thyroiditis. It is also used in the detection of residual thyroid
tissue in operated patients of differentiated thyroid cancer.
T-4 Suppression Test Hot nodules of the thyroid are subdivided into TSH dependant thyroid
nodules, euthyroid autonomously functioning thyroid nodules (AFTN) with partial or total
suppression of the extranodular thyroid tissue and toxic AFTN. The thyroid suppression test is as
an adjunct in the diagnosis of autonomous functioning thyroid nodules.
Perchlorate Discharge Test is used to diagnose thyroid peroxidase deficiency (9).
Dyshormonogenesis is a cause of congenital hypothyroidism, the commonest abnormality being
absent or insufficient thyroid peroxidase enzyme. This can be measured by determining the
amount of radioiodine lost from the gland after the administration of potassium perchlorate
(Perchlorate discharge test); Potassium perchlorate is usually given 2 hours after the administration
of radioiodine. In unaffected individuals, the amount of radiolabelled iodine in the thyroid
remains relatively stable. This is due to the rapid oxidation of iodide to iodine and its subsequent
incorporation into thyroglobulin. In affected individuals the transport of iodine into the follicle
(and therefore its incorporation into thyroglobulin) is delayed, resulting in the leakage of iodide
into the bloodstream. The percentage released indicates whether a defect is partial or total. Total
iodide organification defects are characterized by discharge of more than 50% of the radioiodine
in the thyroid gland within two hours after administration of potassium perchlorate. Partial
defects are characterized by discharge of more than 10% of the accumulated radioiodine.
Glomerular filtration rate (GFR) is probably the most representative parameter of renal
function. It is routinely measured using tracers that are cleared exclusively by glomerular filtration,
the most common being 51Cr- EDTA and
99m
Tc- DTPA. A simple non-invasive measurement of
relative (split) or global (total) GFR can be made by the two available methods- blood sampling
and gamma camera methods. The blood sampling method measures global GFR accurately,
whereas the camera method measures relative renal function. The blood sample method is a
more accurate way of measuring the GFR than the gamma camera method. Single and double
Nuclear medicine in diagnosis and therapy — An overview 245
sample single compartmental methods are routinely practiced and both have been shown to
provide sufficient accuracy for most clinical purposes. The gamma camera method (Gates
method) has been used for its speed and convenience and its potential to provide the split renal
function of the right and left kidney. The Effective renal plasma flow (ERPF) can be measured
if the tracer is quantitatively and completely extracted by the kidney on the first passage through
the kidney and if no other organ clears the plasma of this tracer. The gold standard for the
measurement of ERPF is para amino hippuric acid (PAH). The other agents that can be used for
the measurement are OIH and MAG3.
In vivo imaging procedures
Static imaging
Dynamic imaging
Gated imaging
SPECT imaging
PET imaging
In both in vivo non-imaging and in vivo imaging procedures, a radiopharmaceutical is
administered to the patient. Depending upon which type of scan is being performed, imaging is
done either immediately, a few hours later, or even several days after injection. The imaging time
varies, generally ranging from 20 to 45 minutes. The radiopharmaceutical is determined depending
upon the part of the body to be studied, since some compounds collect in specific organs better
than others. Depending upon the type of scan, it may take several seconds to several days for the
tracer to travel through the body and accumulate in the organ of interest.
Concepts of Gamma Camera Imaging
The Gamma Camera is the workhorse for all in vivo imaging procedures in nuclear medicine.
Sophisticated cameras with advanced computer systems are now available for this purpose both
with single and multiple detector heads. Cameras with multiple heads are now becoming popular
both for planar and SPECT imaging.
Static Imaging
Static acquisition is performed either as spot views or as whole body imaging depending upon
the need of the procedure. The acquisition parameters for static imaging include matrix size,
number of frames, method of terminating the study, pixel depth etc. The matrix size should be
large enough to obtain good resolution. For this purpose the pixel size should be at least half the
size of the smallest object that needs to be resolved in the image (sampling requirement). Any
number of frames can be acquired in a given study. The acquisition can be terminated by four
methods, which include preset total counts, preset acquisition time, preset count density, and
Nuclear medicine in diagnosis and therapy — An overview246
manually. The user has to decide whether to use byte or the word mode of acquisition so as to
avoid pixel saturation. The acquisition time should be reasonably practical without compromising
the image quality. The liver-spleen scan, renal cortical scan, thyroid scan, myocardial infarct
avid scan and ventilation-perfusion scans are some of the examples of static imaging.
Whole body distribution of the radiotracer is required in a number of studies. A whole body
bone scan is one such example. This is accomplished either by taking multiple spot views of the
entire body or by imaging the whole body in one view. The first method is time consuming,
needs adequate overlapping between images and difficult to interpret from many disjointed
images. The whole body imaging is achieved by the relative motion of the patient past the
detector with simultaneous data collection. A Gamma camera with two heads is advantageous in
such situations due to the simultaneous acquisition of anterior and posterior views thereby
decreasing the time of study considerably.
Dynamic imaging
Dynamic imaging is widely practiced in nuclear medicine. Sequential images are acquired over
a period of time after administration of the tracer into the body. The sequence of recording
temporal events can vary from a few milliseconds to hours. For example, first pass studies need
to be recorded in few milliseconds whereas renal studies are acquired for half an hour. Different
intervals with different frame rate can be used during the course of study. For example during a
renogram study the perfusion phase is acquired at the rate of one frame per 1-2 second for about
one minute to attain high temporal resolution, the subsequent frames are obtained at slower
frame rates. For high temporal resolution the spatial resolution is poor since the total counts in
each image are inadequate. Hence smaller matrix (6464) should be used in such studies. All the
acquired images are stored by the computer and processed. A region of interest (ROI) is marked
on the given image and the sequential images are passed through the ROI to generate timeactivity histogram/curve. All the required parameters are then derived from the time-activity
curve. The pattern and the area under the curve is frequently used to compare with the contralateral organ or compared over period of time in the same organ. Cross talk interference or
background scattering are the major problems with dynamic studies. Newer techniques are being
used to improve the methods of background correction. The second most important problem
with dynamic imaging is the count limitation (noisy images). This can be partially solved by
highly sensitive detection methods or increasing the administered activity albeit within the
permissible limit of the recommended dose (guidance level).
Gated Imaging
An electronic mechanism whereby scintillation events are collected during a specified time
interval is termed gating. The patient’s ECG is monitored for 5-10 seconds and an average R-R
interval is selected by the user or automatically by the computer interfaced to the camera. The
tallest wave in the ECG, the R Wave, is sensed by the computer and used as a signal to open the
gate for recording counts from then till the end of one cardiac cycle i.e. R to R interval. The pre-
Nuclear medicine in diagnosis and therapy — An overview 247
selected average R-R interval is divided into 16 or 32 temporal intervals. The data is acquired in
each of these 16/32 frames for one R-R interval and repeated in subsequent cardiac cycles. The
acquisition can be terminated on the basis of preselected time or counts. This kind of gated
acquisition is called “frame mode”. However, this mode of acquisition has limitations if the
patient has irregular heart beat. Modern day computers are equipped to deal with irregular
rhythms or arrhythmic beat rejection. One such method is to acquire the study in “list mode”. In
this technique, scintigraphic data from all cardiac cycles is collected and stored in the computer.
At the end of acquisition a histogram of R-R interval is constructed. The user then selects an
average R-R interval and only those cardiac cycles occurring within the preselected R-R interval
are used. From this acquired data, collected using either frame or list mode, a background
subtracted ventricular volume curve is generated. The gated blood pool study gives information
about the global and regional left ventricular ejection fraction and wall motion, absolute left
ventricular volume, right ventricular function and assessment of valvular regurgitation. In cardiac
first pass study a bolus of radiotracer is injected intravenously and images are acquired as the
bolus passes through the cardiac chambers. Since the images are collected when the bolus is in
each ventricle, it eliminates the problem of background activity and overlapping of the chambers
as encountered in gated blood pool study. First pass studies are acquired in byte mode at the rate
of typically 20 frame/sec. The data can be processed to obtain information about the right and
left ventricular ejection fraction and for shunt quantitation.
SPECT Imaging
A planar image portrays a two dimensional image with no depth information due to superimposed
information from overlying and underlying structures. SPECT imaging provides three dimensional
information about the distribution of radionuclide and significantly improves the contrast and
localization of radiotracer. This results in improved diagnostic utility of tomographic images.
SPECT is a set of static images acquired at equal angular intervals around the patient. From
these projections, transaxial, coronal and sagittal slices can be reconstructed. Software is now
available to reconstruct a 3-D section through the organ of interest. Although SPECT can be
acquired with a single head camera, cameras with two or three detector heads result in increased
sensitivity and decreased time of acquisition. These systems have the facility of acquiring data in
non-circular orbits (contouring) thereby positioning the detector head close to the patient resulting
in improved resolution. The SPECT system can acquire information in either step and shoot
mode or in continuous mode. In the step and shoot mode, the gantry rotates around the patient
and data is acquired in multiple angular projections whereas in the continuous rotation mode it
rotates and simultaneously acquires data. There are other parameters such as the matrix size,
number of angular increments, 180 or 360-degree arc rotation, type of collimator and the
acquisition time at each angular projection, which can be chosen by the operator during the
acquisition. During the reconstruction and processing of data, the most appropriate filter
(particularly in FBP technique) and method of attenuation correction are chosen. SPECT imaging
has now become a routine procedure for brain, bone, liver and kidney in most of the nuclear
Nuclear medicine in diagnosis and therapy — An overview248
medicine centers and especially in nuclear cardiology where SPECT imaging has now become a
standard procedure.
PET Imaging
Positron emission tomography is a technique for the non-invasive measurement of local tissue
concentration of injected radiotracer. Tracer kinetic model is applied to the acquired information
to quantify physiological function. In PET two gamma rays arise from the annihilation of a
positron and an electron, each with 511keV energy and travel in opposite direction. In PET
imaging the detectors on opposite sides of the subject register the coincidence events only when
the two pulses are recorded simultaneously. The PET scanner has ring(s) of detectors in circular
or polygonal array with associated electronic circuitry to process the signal. Unlike conventional
cameras, the detectors used here are of Bismuth germanate (BGO), lutetium oxyorthosilicate
(LSO), gadolnium oxyorthosilicate (GSO) etc. The significance of PET lies in its use of organic
molecules as tracers. Oxygen, carbon, nitrogen and hydrogen nuclides have positron-emitting
isotopes. It provides two major advantages. Firstly the organic molecules can be labelled with
natural radiotracers as compared to conventional imaging where the radiotracers are of different
atomic weight and chemical properties that can alter the physiological properties of the labelled
compound providing a poor analog of natural compound. Secondly the positron emitters have a
short half-life that delivers less radiation dose to the patient. Most of the compounds are labelled
with 18F, 13N, 15O, 11C, 68Ga, and 82Rb. A large array of radiopharmaceuticals are used for
measuring various physiological parameters in clinical settings. A summary of these
radiopharmaceuticals is given in table 1.
Table 1: PET Radiopharmaceuticals
Blood Flow
Blood Volume
Myocardial free fatty acid metabolism
Myocardial metabolism
Oxygen metabolism
Protein synthesis
11
C-acetylene,
11
C-butanol, 13N-ammonia, 13N-nitrous oxide,
15
O-water, 15O-carbon dioxide, 18F-ethanol
11
C-carbonmonoxide, 15O-carbon monoxide
11
C-palmitate
11
C-pyruvate, 11C-lactate,
15
O-oxygen, 15O-carbondioxide
11
C-l-l-leucine, 11C-l-S-methionine, 11C-l-l-tyrosine,
11
Cl-l-tryptophan
11
C-methane,
18
F-FDG
Receptor imaging Benzodiazepine-11Cflunitrazepam
Opoid Dopamine -
11
C carfentanil
11
C-l-DOPA
11
C-meththylspiperone
18
F-flourodopamine
Serotonin - 11C-methylspiperone
11
C-nicotine,
Nuclear medicine in diagnosis and therapy — An overview 249
Therapeutic Nuclear Medicine
Beirwaltes (10) made a strong statement that “the therapeutic approach of internally administered
radiopharmaceuticals offers the potential to outmode the present approaches of conventional
radiation therapy and chemotherapy”. This confident assertion was based on the 40 years of
worldwide experience in the use of radioactive iodine for the treatment of thyrotoxicosis and
thyroid cancer. Since then many new applications have developed and the use of radionuclides
for therapeutic purpose has increased significantly.
Radionuclide therapy is based on the principle of tracer localization. For example, iodine is
used as a precursor for thyroid hormone synthesis; radioiodine administered to the patient is
taken up by the thyroid cells, which causes its destruction. Similarly 32P taken up by proliferating
malignant cells for DNA synthesis cause their destruction. The amount of radiation dose delivered
to organ of interest or metastatic deposit is selective and enormous as compared to the conventional
radiotherapy, thus the surrounding normal tissue suffers with little or no side effect by this mode
of therapy. However, radionuclide therapy has some of its own limitations such as:
The patient becomes a source of radiation and may require strict isolation from radiation
safety point of view. Specially designed isolation rooms are required for high dose
radioiodine therapy. The radioactive waste generated in the process becomes an additional
problem.
Different therapeutic applications require different radionuclides based on their
characteristics whereas in external beam therapy a single source can treat most of the
tissues.
Some of the radionuclides like 89Sr are expensive and not readily available.
Characteristics of therapeutic radionuclides
Radionuclides emitting particulate radiation (alpha, beta) of moderate to high energy
Low energy gamma emission with low abundance along with beta rays is suitable for
dosimetry and scintigraphy
Long biological and physical half life (long residence time)
Least toxic effects on normal cells
Selective uptake by the target tissue
Rapid uptake by the target tissue and fast clearance from the blood/marrow to limit the
blood/marrow dose to as less as possible (<2Gy)
Easily available and inexpensive
Nuclear medicine in diagnosis and therapy — An overview250
Radioiodine therapy
131
I is a radionuclide used for the management of thyrotoxicosis and differentiated thyroid
carcinoma (11). It is well tolerated, simple to administer and has been proved in multiple studies
to be safe and effective. In cases of thyrotoxicosis, before the initiation of therapy, it is essential
that the diagnosis of thyrotoxicosis has been confirmed clinically and biochemically. The nature
of thyrotoxicosis should also be confirmed so as to differentiate Grave’s disease and Plummer’s
diseases, which are successfully treated with radioiodine, from thyroiditis and iodine induced
thyrotoxicosis, which should not be treated with radioiodine. The dose administered for this
purpose ranges from 74 MBq (2 mCi) to a maximum of 550 MBq (15mCi). Out of the three
modalities available (surgery, antithyroid drugs and radioiodine therapy)
preferred choice of treatment. Sufficient data has been accumulated over the last 4-5 decades,
which provides convincing evidence of its safety.
131
I is also effective in the management of
well-differentiated thyroid cancer. Following the surgical removal of the tumor with total
thyroidectomy and excision of any nodal disease, radioiodine has a major role in achieving
complete cure. It is administered for the ablation of any residual thyroid tissue and in larger
doses for the treatment of metastatic disease. Therapy doses range from 1110-7400 MBq (30200 mCi).
MIBG (Metaiodobenzylguanidine) therapy
131
I treatment is the
MIBG is used in the diagnosis and treatment of tumors derived from the neural crest. It’s high
sensitivity and specificity especially in pheochromocytoma and neuroblastoma led to the
therapeutic use of the agent in these conditions (12). It can be safely applied, even in children,
provided the bone marrow is free of tumor cells. It is an effective radiopharmaceutical for
diagnosis and therapy of neuroblastoma and phaechromocytoma. Both
for diagnosis and
131
I is used for the therapy. MIBG may detect extensive bone and bone marrow
123
I and
131
I can be used
involvement in neuroblastoma, in the absence of findings on bone marrow aspiration and biopsy,
plain radiograph and bone scintigraphy.
131
I-MIBG is used in the palliation of advanced
neuroblastoma and phaechromocytoma. Early therapeutic intervention in pediatric neuroblastoma
appears promising.
Bone pain palliation
Bone metastases are most common in patients with primary carcinoma of the breast, prostrate
and lung. Pain in patients with cancer metastatic to bone is a significant cause of morbidity. The
modalities available for bone pain palliation include radiotherapy, hormone therapy, chemotherapy
and analgesics. Bone seeking radiopharmaceuticals have provided a new option for effective
pallination of bone pain. The radiopharmaceuticals used should show selective uptake at metastatic
sites relative to normal bone, rapid clearance from soft tissue and normal bone, should have
maximum beta energy between 0.8-2.0 MeV, besides being stable, easily available, easy to
transport and cost effective. Its distribution should be predicted from
Radiopharmaceuticals used for palliative therapy are listed in the table 2.
99m
Tc-MDP bone scan.
Nuclear medicine in diagnosis and therapy — An overview 251
Table 2: Radionuclides in Radiation Synovectomy
Radionuclide Most abundant Half Life Maximum Therapeutic Range
Beta minus (days) Range in in soft tissue
energy (MeV) soft tissue (mm) (mm)
Y-90 0.935 (100%) 2.8 10.08 2.8
Re-186 0.309 (22%) 3.8 4.5 1.0
0.362 (71%)
Er-169 0.098 (45%) 9.4 1.0 —
0.101 (55%)
Re-188 0.729 (25%) 0.7 10.1 2.1
0.795 (71%)
Ho-166 0.651 (48%) 1.1 8.7 2.1
0.693 (51%)
Sm-153 0.199 (34%) 1.9 3.1 0.7
0.224 (44%)
0.263 (21%)
Radiation synovectomy
Recently radiosynovectomy has generated a lot of interest. Radiosynovectomy, originally termed
radiosynoviorthesis by Delbarre et al (13), means the restoration of the synovium by local
application of radionuclides. This can serve as an alternative to surgical synovectomy. The ideal
radiopharmaceutical for radiosynovectomy should have beta particle energy sufficient to ablate
the synovia without damaging the articular cartilage or overlying skin. The particle size should
be small enough to be phagocytosed but should not leak out of the joint (approx. 2-5m). This
mode of therapy is useful in rheumatoid disease (Rheumatoid arthritis, psoriatic arthritis),
Villonodular synovitis, Haemarthrosis in the hemophiliac, knee and hip arthroses. The
radiopharmaceuticals commonly used in the radiosynovectomy are listed in table 3.
Table 3: Radionuclides in Bone Pain Palliation
Radionuclide Maximum Beta Mean Beta Half life Maximum range Additional gamma
Energy (MeV) Energy (MeV) (days) in tissue (mm) photon (keV)
Sr-89 1.46 0.583 50.5 6.7 -
P-32 1.71 0.695 14.3 8.0 Re-186 1.07 0.349 3.8 4.7 137 (9%)
Sm-153 0.8 0.224 1.95 3.4 103 (28%)
Sn-117m Conversion 0.129, 0.153 13.6 0.3 159 (86%)
Electron
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