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Nuclear medicine
The nucleus of an atom contains positively charged protons and neutrally charged neurons
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In a stable nuclei there is a balance between
nuclear force (strong force) and electrical force (repulsion of protons)
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Elements can have
different numbers of neutrons, referred to as isotopes
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Isotope stability is related to the proton-neutron ratio
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Radioactivity is a natural and spontaneous process
in which unstable atoms emit or radiate excess energy as particles or waves
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The more unstable the isotope the greater the rate of decay
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Half life is the time taken for the number of
radioactive atoms to decrease by half
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Alpha radiation occurs when an atom undergoes radioactive decay, giving off an alpha
particle consisting of two protons and two neutrons (structurally equivalent to the nucleus of
a helium atom)
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Alpha particles are relatively slow and heavy
compared with other forms of nuclear radiation
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Thus alpha particles are unsuitable for use as
radiotracers in nuclear imaging, as they are unable to penetrate through biological tissue and
emit a signal that could be detected by a detector, additionally, as alpha particles are highly
ionising they can cause a great deal of biological damage
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Beta particles have a
small mass and can be released with high energy, as such they can achieve relativistic
speeds
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Beta particles are much less ionising than alpha particles and generally do less damage for
a given amount of energy deposition
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A gamma ray, or gamma radiation, is a penetrating electromagnetic radiation
arising from the radioactive decay of atomic nuclei
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Gamma radiation typically
results from alpha or beta decay, or nuclear fission or fusion
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Gamma radiation is most typically used in nuclear medicine
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A radioactive-labelled pharmaceutical (radiopharmaceutical;) is administered intravenously
to a patient
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Most radiopharmaceuticals used in nuclear
medicine and SPECT are labelled with radionuclides that emit gamma-ray photons
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The scintillation

camera consists of a lead collimator that allows photons travelling in a given direction to
pass through a large area scintillator that converts the energy of the gamma ray photons to
lower energy photons which are in turin converted to electric signals by photomultiplier tubes
(PMTs)
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The
scintillation camera provides a two dimensional projection image of the three dimensional
radioactivity distribution or radiopharmaceutical uptake within the patient
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The amount of radiopharmaceutical that can be
administered is limited by the allowable dose of radiation to the patient
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Also, the acceptance for
the projection data
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Technetium 99m (99mTc) is the most commonly used radionuclide used in
nuclear medicine
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The emission of
a beta electron leaves behind an excited nucleu, which returns to its ground state emitting a
gamma photon
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99m Tc emits a relatively low dose of radiation, thus is safe to administer to a
patient
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99m Tc is also relatively inexpensive
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SPECT in combination with CT enables a direct
correlation of anatomic information and functional imaging, resulting in better ionization and
definition of scintigraphic findings
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The most often used
radiopharmaceutical 99m Tc- methylene phosphate binds to bone by chemisorption to the
hydroxyapatite crystal
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Bone is a frequent site of secondary metastasis, especially in breast and
prostate cancer, thus the diagnosis of bone metastasis, via SPECT, can inform prognosis
and signal the need for intensified or additional treatment
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SPECT/CT can define suspicious bone lesions and guide
subsequent biopsy
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Myocardial perfusion scintigraphy (MPS) using SPECT with radiopharmaceuticals
is widely used for noninvasive diagnosis of obstructive coronary artery disease (CAD)
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MPS
evaluates regional myocardial perfusion as well as giving information about functional
parameters such as transient ischemic dilation and extent of perfusion defect
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99m Tc-sestambi and 99m Tc-tetrofosmin are
radiopharmaceuticals used for MPI by SPECT
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99m Tc-sestamibi, is there long myocardial retention because of the positive molecular
charge and fast liver clearance kinetics
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99m Tc-3SPboroxime is an important radiotracer
considering is initial heart uptake, myocardial retention time, liver clearance kinetics,
heart/liver ratios, and SPECT imaging quality
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99m
Tc-3SPboroxime can be used to; detect perfusion defects, accurately quantify and determine
regional blood flow rate
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SPECT advantages
Advantages of SPECT include; yields both structural and functional information, dynamic
studies are possible, lower radiation dose in comparison to PET, moderate throughput due
to fast imaging time (minutes) especially in comparison to MRI (minutes to hours), yields
quantifiable results, greater sensitivity (10-100 pmol) especially in comparison to PET (1010
pmol), high patient comfort, inexpensive compared to MRI and PET, and use of only tracer
quantities of drugs
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Positron emission tomography principles
Positron emission tomography (PET) is a technique that measures physiological function by
looking at blood flow, metabolism, neurotransmitters, and radiolabeled drugs
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The technique is based on the
detection of radioactivity emitted after a small amount of a radioactive tracer is injected into a
peripheral vein
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The advantages of the
radiopharmaceuticals utilised for PET include; utilisation of physiologically relevant atoms,
the ability to label biological compounds without altering properties, and the fact that they
can be given in extremely small quantities
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In a PET scan, the nucleus of the radioisotope emits a positron
(positive electron), this collides with an electro nin the tissue and in the process converts
mass to energy in the form of two gamma ray photons, this is an annihilation reaction
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The PET camera uses scintillation crystals
placed around the patient to detect these photons
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PET and CT can be co-registered to
provide both functional and anatomical information
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Accumulation of the radiolabeled glucose analogue 18-fluorodeoxyglucose (FDG)
allows measurement of the rate of consumption of glucose
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One clinical use of
this is to distinguish between benign and malignant tumours, as malignant tumours
metabolise glucose at a faster rate than benign tumours
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Other applications of PET include looking at blood flow and oxygen
consumption in different parts of the brain, for example, in assessing strokes and dementia
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PET has further applications in cardiology, in
pretransplantation assessment of viable myocardium, and in distinguishing recurrent tumors
from radiation necrosis and surgical scarring, and in a variety of cancers
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Compared to conventional loan administration of FDG in a tatic PET
scan, functional PET provides a significantly higher temporal resolution to study the
dynamics of glucose uptake
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PET can be co-registered with MRI
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However PET/CT remains more widely available,
inexpensive, and offers a higher throughput
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PET disadvantages
Disadvantages of PET include; the use of ionising radiation as the dose administered is
higher than that administered in any other imaging modality, very expensive it carries the
highest cost of all imaging modalities, limited availability, motion artefacts, and a lower
resolution (4-5mm) in comparison to CT (0
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1mm), and a lower sensitivity
(1-10 pmol) in comparison to SPECT (10-100 pmol) and MRI (10-100 pmol)

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