Part 1: L6 Gamma Camera/SPECT Flashcards

1
Q

Overview of radiochemistry for SPECT:

A
  • e.g. Parent nucleus of U-238 emits alpha particles -> excited daughter particles which emits gamma ray
  • Alt: Beta and gamma decay modes - positron from unstable artificial isotopes
  • SPECT requires gamma particles
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2
Q

Radionuclides:

A
  • Primarily showing physiological function -> localising
  • Gamma-emitting radioisotopes which passes through tissue without being scattered, or causing much damage
  • Can enter cells
  • Generally administered in very low doses (ng)
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3
Q

SPECT technology:

A
  • Single photon emission computed tomography
  • Gamma camera
  • Radioactive event releases gamma particles -> collumate with lead fingers -> only photons directly upwards allowed -> Aluminium casing then scintillation crystal, photomultiply -> detect and trasnmit information to computer
  • Collimator is also one of the sources of blurring within the image, attenuates most (>99%) of incident photons
  • This limits the sensitivity
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4
Q

Nucleotide Limitations in SPECT:

A
  • Most gamma cameras are designed for specific energy windows (100-250 eV)
  • Lower than this: too scattered
  • Higher than this: difficult to collimate
  • Half-life must be long enough to synthesis and allow accumulation in target tissue, while allowing clearance through non-target organs -> short as possible to limit radiation dose
  • Desired half life is dependent on time required to localise to target tissue (heart/brain: shorter, tumour targeted radiolabelled monoclonal antibodies take longer)
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5
Q

Typical sourcing of radionuclides:

A
  • Generators considered ideal, preferably with low cost parent isotopes
  • Daughter can be easily separated from the parent via either ion exchange chromatography or solvent extraction
  • A few isotopes are produced in a nuclear reactor
  • Several can be generated by an accelerator or cyclotron - expensive - and can only produce one isotope at a time
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6
Q

Tc-99m qualities:

A
  • 6hr half-life
  • Generated in 99Mo/99mTc generator
  • Decays via isomeric transition
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7
Q

How are radionuclides administered?

A
  • Intravenous injection in liquid or aggregate form (majority)
  • Ingestion while combined with food
  • Inhalation as a gas or aerosol (Xenon-133m/Krypton-81m)
  • Some studies require the labelling of a patient’s own blood cells with a radionuclide leukocyte and red blood scintigraphy
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8
Q

Iodine-123 in SPECT

A
  • Half life of 13.13 hrs
  • Decays via gamma particle
  • Can be used for things like thyroid
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9
Q

Xenon-133m in SPECT:

A
  • Inhaled to assess pulmonary function and to image lungs
  • Also often used to image blood flow
  • Rapid clearance from brain
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10
Q

Technecium-99m in SPECT:

A
  • Most prevalent agent (85% of administered drugs)
  • Can be gaseous
  • Emits a 140 keV gamma-ray with 89% abundance and a 6hr half life
  • There is no stable isotope (middle of transition metals)
  • Versatile and simple chemistry (t.metal)
  • Made in 99Mo/99mTc generator
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11
Q

Thallium-201 in SPECT:

A
  • Historically used to image heart
  • Very toxic -> alopecia
  • Cyclotron produced
  • Half life 72.5hrs
  • TI decays emitting Hg X-rays and photons
  • Behaves similarly to potassium in body (Tl(I)) -> taken up well via sodium/potassium pump -> collects in myocardium then circulated out rapidly
  • Able to image heart in real time -> Thallium stress test
  • Has since been replaced by Sestimibi (Tc-99m based)
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12
Q

99m-Tc generation:

A
  • Molybdate (2- anion) decays to Pertechnetate in luminar column; single negative charge as Tc so able to wash off with 0.9% NaCl
  • Obtained in evac vial with lead shielding
  • Charge chromatography where Mo binds strongly and Tc elutes
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13
Q

Tc essential:

A
  • First class of 99mTc pharmaceuticals
  • Tc is an essential structural part of the introduced drug
  • Majority
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14
Q

Tct-tagged

A
  • Minority
  • The physiologically active targeting moiety such as an antibody, peptide, hormone etc has been labelled with 99mTc by a bifunctional chelate
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15
Q

Brain imaging with Technecium-99m

A
  • Kits are available for the radiographer with essential components for ‘one pot’ solution
  • Ceretec, neurolite etc -> Tc5+ formed by reduction with SnCl2, followed by complexation with the ligand in a saline solution
  • Capable of penetrating the intact BBB due to their neutral and lipophilic character
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16
Q

Tc SPECT agents in myocardial imaging;

A
  • Myoview (note lipophilic chains)
  • -> Kit of tetrafosmin (ligands), SnCl2, disodium, sulfosalicylate (balance acidity), sodium D-g, sodium hydrogen carbonate
  • Cardiolite (note pi-acceptor ligands) -> tumour uptake
17
Q

Research application of Tc agents:

A
  • Research into localisation of drugs
  • Assessing which are the active parts of drugs
  • Clinical trials