Nuclear Medicine

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Nuclear Medicine

• Is the branch of medical specialities that uses
  special properties of isotopes in treatment and
  diagnosis

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Isotopes

• Isotopes of a given element have the same atomic
  number but differ in mass numbers.

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• There are 2450 isotopes of 100 elements
• Unstable isotopes attempt to reach the stability
  by emitting energy (radioactivity)
• Unstable isotopes ltgt radioactive isotopes ltgt
  radioisotopes .

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Radioactive Decay

• Fission Some heavy nuclei decay by splitting
  into 2 or 3 fragments.
• Alpha Decay Two protons and two neutrons leave
  the nucleus.
• Beta Decay - Positron Emission When the number
  of protons in a nucleus is in excess, the nucleus
  may reach stability by converting a proton into a
  neutron with the emission of a beta-plus particle
  which is known as a positron.

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Radioactive Decay

• Positrons annihilate with electrons to produce
  two back-to-back gamma-rays.
• Gamma Decay A nucleus in an excited state may
  reach its ground state by the emission of a
  gamma-ray.
• A gamma-ray is an electromagnetic photon of high
  energy.

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Radioactive Decay

• Radioactivity is the number of radioactive decays
  per unit time.
• Half Life The time taken for the number of
  radioactive nuclei in the sample to reduce by a
  factor of two.
• The Unit of radioactivity is the becquerel (Bq)
• 1 Bq one radioactive decay per second.

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Scintigraphy

• Is a form of diagnostic test where
  radioisotopes(Tracers) are given to the patient
  internally either intravenously or orally so ,the
  Pt. will be the source of radiation.
• Then, gamma cameras capture and forms two
  dimensional images from gamma rays emitted by the
  radioisotopes.

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Factors Which Affect the Choice of Tracer

• They will concentrate in the organ, or tissue,
  which is to be examined.
• They will lose their radioactivity (short )
• They emit gamma rays which will be detected
  outside the body.

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Factors Which Affect the Choice of Tracer

• Gamma rays are chosen since alpha and beta
  particles would be absorbed by tissues and not be
  detected outside the body.
• Technitium-99m is most widely used because it has
  a half-life of 6 hours.

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Why is a half-life of 6 hours important?
A half-life of 6 hours is important because

• A shorter half-life would not allow sufficient
  measurements or images to be obtained.
• A longer half-life would increase the amount of
  radiation the body organs or tissues receive.

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Tracers Used in Nuclear Medicine
Pharmaceutical Source Activity (MBq) Medical Use
Pertechnetate 99mTc 550 - 1200 Brain Imaging
Pyrophosphate 99mTc 400 - 600 Acute Cardiac Infarct Imaging
Diethylene Triamine Pentaacetic Acid (DTPA) 99mTc 20 - 40 Lung Ventilation Imaging
Benzoylmercaptoacetyltriglycerine (MAG3) 99mTc 50 - 400 Renogram Imaging
Methylene Diphosphonate (MDP) 99mTc 350 - 750 Bone Scans
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Nuclear Medicine Imaging Systems

• Conventional imaging with a gamma camera is
  referred to as Planar Imaging, i.e. a 2D image
  portraying a 3D object giving superimposed
  details and no depth information.

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The Gamma Camera
-A typical gamma camera is 40 cm in diameter
large enough to examine body tissues or specific
organs. -The gamma rays are given off in all
directions but only the ones which travel towards
the gamma camera will be detected.
-Gamma-cameras can have one, two or three
camera heads..

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DiagnosisStatic Imaging

• -There is a time delay between injecting the
  tracer and the build-up of radiation in the
  organ.
• -Static studies are performed on the brain, bone
  or lungs scans.

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DiagnosisDynamic Imaging

• -The amount of radioactive build-up is measured
  over time.
• -Dynamic studies are performed on the kidneys and
  heart.

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Single Photon Emission Computed Tomography(SPECT)

• SPECT uses a gamma camera which is rotating
  around the patient to acquire slice image.
• Modern gamma cameras which are designed for
  SPECT scanning consist of two camera heads
  mounted parallel to each other with the patient
  in between.

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Example Images

A SPECT slice of a patient's heart.
A series from a SPECT study of a patient's brain.

Images from a SPECT study of a patient's heart.
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Positron Emission Tomography (PET)

• produces images of slices through the body.
• Positrons are emitted from radioactive nuclei
  for stability, positrons do not last for very
  long in matter since they will quickly encounter
  an electron and a process called annihilation
  results.

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• In this process the positron and electron vanish
  and their energy is converted into two gamma-rays
  which are emitted at 180o degrees to each other.
  The emission is often referred to as two
  back-to-back gamma-rays .
• If these gamma-rays are detected, their origin
  will be detected by a ring of detectors which
  encircles the patient and tomographic images can
  be generated using a computer system.

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• The detectors are typically specialised
  scintillation devices which are optimised for
  detection of these gamma-rays.
• This ring of detectors, associated apparatus and
  computer system are called a PET Scanner

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• PET scanner

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PET/CT

• General Electric Medical Systems

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• The radioisotopes used for PET scanning are
  usually produced using an instrument called a
  cyclotron. These isotopes have relatively short
  half lives.
• PET scanning therefore needs a cyclotron and
  associated radiopharmaceutical production
  facilities located close by.

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