NUCLEAR MEDICINE (ALL NOTES)

Atom

a collection of protons, neutrons, and electrons

nucleus

the protons and neutrons are found in the _______

discrete energy levels

shells of electrons orbit the nucleus with _________

Z

____ = atomic number (number of protons)

A

_____ = atomic mass number (sum of N and Z)

_____ = number of neutrons

Transmutation

a change in element when decay involves change in the number of protons

Radionuclides

isotopes attempting to reach stability by emitting radiation

Nuclear Instability

may result from either neutron or proton excess

nucleus

it seeks greater stability by decay or disintegration of the ______ to a more stable form

Alpha - particle emission

consists of two protons and two neutrons, is released from the nucleus, with a resulting decrease in the atomic mass number (A) by four and reduction of both Z and N by two

Alpha-particle emission

essentially useless for imaging purposes

Beta (β−) particle emission

emitted from the nucleus accompanied by an antineutrino; as a result, one of the neutrons may be thought of as being transformed into a proton, which remains in the nucleus

Beta (β−) particle emission

decreases the number of neutrons (N) by one and increases the number of protons (Z) by one, so that A remains unchanged.

Positron (β+) emission

too many protons in the nucleus (a neutron-deficient nuclide), decay may proceed in such a manner that a proton may be thought of as being converted into a neutron, which is always accompanied by a neutrino

Positron (β+) emission

increases N by one and decreases Z by one, again leaving A unchanged

Positron (β+) emission

cannot occur unless at least 1.02 MeV of energy is available to the nucleus

Electron Capture

occurs in a neutron-deficient nuclide when one of the inner orbital electrons is captured by a proton in the nucleus, forming a neutron and a neutrino

Electron Capture

can occur when not enough energy is available for positron emission, and electron capture is therefore an alternative to positron decay

electron capture

_________ may be accompanied by gamma emission and is always accompanied by characteristic radiation, either of which may be used in imaging

Isomeric Transition

If, in any of these attempts at stabilization, the nucleus still has excess energy, it may be emitted as nonparticulate radiation, with Z and N remaining the same.

Isomeric Transition

any process in which energy is given off as gamma rays and in which the numbers of protons and neutrons are not changed

Internal Conversion

the excess energy of the nucleus is transmitted to one of the orbital electrons; this electron may be ejected from the atom, which is followed by characteristic radiation when the electron is replaced.

Internal Conversion

usually competes with gamma-ray emission and can occur only if the amount of energy given to the orbital electron exceeds the binding energy of that electron in its orbit

Metastable

a gamma-ray photon is emitted almost instantaneously after particulate decay

it is the intermediate excited state of an isotope if there is a measurable delay in the emission of the gamma-ray photon and the resulting decay process is an isomeric transition

99mTc (the m refers to metastable)

the most well-known metastable isotope is _______.

Activity

the amount of radioactivity present (the number of disintegrations per second)

curie (Ci)

conventional unit of radioactivity has been the ______

becquerel (Bq)

The SI unit for Activity is ________

Biologic Half-life

refers to the time it takes an organism to eliminate half of an administered compound or chemical on a strictly biologic basis

Effective Half-life

accounts for both physical and biologic half- lives

effective half-life

the ________ is always shorter than either the physical or biologic half-life

Radionuclide Production

most radioactive material that does not occur naturally can be produced by particulate bombardment or nuclear fission

unstable isotope

both methods alter the neutron-to-proton ratio in the nucleus to produce an _________

cyclotron

bombardment essentially consists of the irradiation of the nuclei of selected target elements with neutrons in a nuclear reactor or with charged particles (alpha particles, protons, or deuterons) from a _________

Radionuclide Generator

a number of radionuclides of interest in nuclear medicine are short-lived isotopes that emit only gamma rays and decay by isomeric transition

ion exchange column

a radionuclide “parent” with a relatively long half-life is firmly affixed to an ________

daughter

because the _______ is only loosely bound on the column, it may be removed, or washed off, with an elution liquid such as normal (0.9%) saline

normal (0.9%) saline

because the daughter is only loosely bound on the column, it may be removed, or washed off, with an elution liquid such as ________.

Wet System

most common in commercial radiopharmacies

Wet System

has a saline reservoir and a vacuum vial that draws saline across the column

Dry system

most common in imaging clinics

Dry System

a specific amount of saline in a vial is placed on the generator entry port and drawn across by a vacuum vial

residual parent isotope

after the daughter is separated from the column, the buildup process is begun again by the _________

Breakthrough

removal of some of the parent isotope or alumina from the column during elution and appears in the eluate containing the daughter isotope

elution times

to make efficient use of a generator, _______ should be spaced appropriately to allow for reaccumulation of the daughter isotope on the column.

Radionuclide Purity

is the percentage of activity present that is due to the radionuclide of interest

molybdenum generator

Because 99mTc normally is obtained by eluting or “milking” a __________, there must be assurance that only 99mTc is eluted

99Mo (740- and 780-keV) photons

the amount of 99Mo contamination, or breakthrough, during elution is normally determined by placing the eluate from the generator in a lead shield and measuring the penetration of any ___________.

USNRC and USP

the ______ and ______ regulations allow no more than 0.15 μCi (0.005 MBq) of 99Mo per 1 mCi (37 MBq) of 99mTc at the time of administration

alumina

if, during elution, sufficient _______ breaks through, the eluate may become cloudy

indicator paper (pH paper)

small amounts of aluminum ion may be detected with an _________. If aluminum ion is present, a red color develops

red

small amounts of aluminum ion may be detected with an indicator paper (pH paper). If aluminum ion is present, a ____ color develops

10 μg/mL of 99mTc

the maximum permissible amount of aluminum ion is __________ eluate with a fission generator

Radiochemical Purity

the percent of the total radioactivity present in the desired chemical form in a radiopharmaceutical preparation

successful tagging

assessment of chemical purity of 99mTc radiopharmaceuticals is performed by determining the degree of _______ of the agent contained in the kit and the amount of residual (unbound 99mTc) in the preparation

thin-layer chromatography

one reason for performing __________ before patient injection is that simple errors can cause the radiolabeling to be completely ineffective

Geiger-Mueller counters

are handheld, very sensitive, inexpensive survey instruments

Geiger-Mueller counters

used primarily to survey for the presence of radiation and radioactivity, and to detect small amounts of radioactive contamination

Geiger-Mueller counters

usually limited to exposure rates of up to about 100 mR (2.5 × 10-5 C/kg)/hr

Ionization Chambers

handheld survey instruments used to measure low or high exposure rates

Ionization Chambers

they have an air- or gas-filled chamber but a low efficiency for detection of gamma rays

Dose Calibrator

a special form of an ionization chamber

Dose Calibrator

essentially a well-type, cylindrical ionization chamber filled with a defined volume of pressurized gas (usually argon) capable of measuring quantities in the millicurie (37 MBq) range.

Sodium Iodide Well Counter

gasless devices common in nuclear medicine laboratories for performing in vitro studies as well as quality control and assurance procedures

Sodium Iodide Well Counter

can typically count activity only up to about 1 μCi (37 kBq)

Single Probe Counter

primarily used for measuring thyroid uptake of radioactive iodine

Gamma Scintillation Camera

the most widely used imaging devices in nuclear medicine

Gamma Scintillation Camera

converts photons emitted by the radionuclide in the patient into a light pulse and subsequently into a voltage signal

Voltage Signal

this signal is used to form an image of the distribution of the radionuclide

Collimator

made of perforated or folded lead approximately 1 to 2 inches (2.5–5 cm) thick and is interposed between the patient and the scintillation crystal

Collimator

allows the gamma camera to localize accurately the radionuclide in the patient’s body

Collimator

absorbs and stops most radiation except that arriving almost perpendicular to the detector face

Pinhole Collimator

radiation must pass through its aperture to be imaged, and the image is always inverted on the scintillation crystal

Pinhole Collimator

because little radiation coming from the object of interest is allowed to pass through over a given period, it has very poor sensitivity

Pinhole Collimator

collimator sensitivity refers to the percentage of incident photons that pass through the collimator

Pinhole Collimator

the poor sensitivity makes placement near the organ of interest critical, and bringing the object of interest close magnifies the image

magnification

because _______ is a function of distance, the image may be distorted if the object of interest is not relatively flat or thin

Pinhole Collimator

routinely used for very high resolution images of small organs, such as the thyroid, and for certain skeletal regions, such as hips or wrists, especially in pediatric patients

Multihole Collimator

parallel hole collimator is the most widely used multihole collimator in nuclear medicine laboratories

Multihole Collimator

consists of parallel holes with a long axis perpendicular to the plane of the scintillation crystal

Multihole Collimator

the lead walls between the holes are referred to as septa

septa

the ______ absorb most gamma rays that do not emanate from the direction of interest; therefore a collimator for high-energy gamma rays has much thicker septa than does a collimator for low-energy rays

parallel hole collimators

________ should be chosen to correspond to the energy of the isotope being imaged

150 keV

low-energy collimators generally refer to a maximum energy of ______

400 keV

medium-energy collimators have a maximum suggested energy of about ______

Scintillation Crystal

radiation emerging from the patient and passing through the collimator typically interacts with a thallium- activated sodium iodide crystal

orbital electron (photoelectric absorption)

interaction of the gamma ray with the crystal may result in ejection of an _________, producing a pulse of fluorescent light (scintillation event) proportional in intensity to the energy of the gamma ray.

aluminum housing

because sodium iodide crystals are fragile and hygroscopic, they must be encased in a sealed ________ to protect them from moisture, extraneous light, and minor physical damage.

Photomultiplier Tube

converts a light pulse into an electrical signal of measurable magnitude

Photomultiplier Tubes

situated behind the sodium iodide crystal and may be placed directly on the crystal, connected to the crystal by light pipes, or optically coupled to the crystal with a silicone -like material.

Computer

converts the output signals into an image display

Pulse Height Analyzer

discards signals from background and scattered radiation and/or radiation from interfering isotopes, so that only primary photons known to come from the photopeak of the isotope being imaged are recorded

the photopeak is the result of total absorption of the major gamma ray from the radionuclid

the _______ is the result of total absorption of the major gamma ray from the radionuclide

voltage discriminator

Signal intensity information is matched in the PHA against an appropriate window, which is really a _______

• Brain Perfusion Imaging

• Cerebrospinal Fluid Imaging

Clinical Applications:

Central Nervous System

Tc-99m DTPA

Tc-99m HMPAO (first-pass uptake)

Tc-99m ECD

PET: F-18 FDG

Brain Perfusion Imaging

Radiopharmaceutical:

In-111 DTPA

Cerebrospinal Fluid Imaging

Radiopharmaceutical:

Thyroid Gland

gives a useful clinical index of thyroid function