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Biophysics 2 (LAB) > 9. Isotope diagnostics > Flashcards

9. Isotope diagnostics Flashcards

1
Q

What are isotope diagnostics?

A

The host molecule labeled with the radioactive isotope (radiotracer) that is administered in the body accumulates in the target organ.

γ -radiation emitted by the isotope is detected outside the body and the spatial distribution of the isotope is reconstructed; alternatively, from the time sequence of the spatial distribution (the temporal change of activity), the isotope accumulation curve can be calculated.

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2
Q

In most cases pure ___ are used for “in vivo” isotope diagnostic applications to avoid the unnecessary exposure to 􏵸alpha- and 􏶀 beta-radiation.

A

gamma􏶲-radiating isotopes

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3
Q

Why are gamma􏶲-radiating isotopes mostly used in isotope diagnostic applications?

A

Because gamma rays escaping the body can be measured with high efficiency

→ thus it is possible to keep the number (concentration) of administered radioactive atoms low

→ consequently the radiation exposure of the body remains small.

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4
Q

The radiotracer technique needs typically 106 -108 times lower concentrations than any chemical microanalytical procedure.

→ Why can the isotopes of the most toxic heavy metal elements be utilized at this concentration?

A

even the isotopes of the most toxic heavy metal elements may be utilized because they do not cause toxicological symptoms.

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5
Q

One way of producing pure 􏶲-radiating isotopes is by using the ___

A

technetium-generator

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6
Q

4 aspects of isotope diagnostics

A
  1. only γ -emitting isotope, if possible
  2. physical half-life should match the biological half-life and the duration of the measurement→ should be as short as possible
  3. the photon energy of the emitted γ -radiation has to be high enough to pass through parts of the body,
  4. activity should be high enough to reach the required signal-to-noise ratio → sepia;d be the lowest possible.
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7
Q

Molybdenum decays into technetium, while beta􏶀- and 􏶲 gamma-radiation is emitted

In the first step the “parent” nucleus, 99Mo (further called Mo) decays via ___

negative 􏶀-decay, with a 66-hours half-life to technetium (“daughter” 99mTc, further called mTc).

A

negative beta􏶀-decay

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8
Q

Molybdenum decays into technetium, while beta􏶀- and 􏶲 gamma-radiation is emitted

The “daughter” mTc can be separated chemically from the “parent” Mo

→ thus for medical diagnostics we get an isotope of favorable properties, which emits (1)___, (2)____, (3)___

A
  1. soft
  2. monochromatic
  3. pure 􏶲gamma-radiation.
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9
Q

Molybdenum decays into technetium, while beta􏶀- and 􏶲 gamma-radiation is emitted

What is nuclear isomerism?

A

A nuclear isomer is a metastable state of an atomic nucleus, in which one or more nucleons occupy higher energy levels than in the ground state of the same nucleus.

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10
Q

Selection of the isotope atom and carrier compound

Is􏶲 gamma-radiating isotope usually administered in elementary form? Why?

A

gamma-radiating isotope is rarely administered into the body in its elementary form.

→ It is often attached to a carrier organic compound, called carrier pharmaceutical

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11
Q

Selection of the isotope atom and carrier compound

The role of the produced radiopharmaceutical

A

It is labeled with an isotope and accumulates in the examined organ

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12
Q

Selection of the isotope with proper half-life

2 factors that affect the effective activity of the isotope administered into an organ and measured on the body surface

A
  1. the physical decay
  2. the biological processes
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13
Q

Selection of the isotope with proper half-life

If we measure the time during which the activity decreases to its one half in the given organ, what do we obtain?

A
  1. The physical half-life (Tphys) in the absence of biological processes
  2. The biological half-life (Tbiol) if there is no physical decay
  3. The effective half-life (Teff) due to the simultaneous presence of biological processes and physical decay
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14
Q

Selection of the isotope with proper half-life

If we measure the time during which the activity decreases to its one half in the given organ, can we obtain the physical half-life (Tphys) in the presence of biological processes?

A

NO!

We can only obtain the physical half-life (Tphys) in the absence of biological processes

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15
Q

Selection of the isotope with proper half-life

If we measure the time during which the activity decreases to its one half in the given organ, what is the condition to obtain the biological half-life (Tbiol)?

A

if there is no physical decay

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16
Q

Selection of the isotope with proper half-life

If we measure the time during which the activity decreases to its one half in the given organ, why we can obtain the effective half-life (Teff)?

A

due to the simultaneous presence of biological processes and physical decay

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17
Q

Selection of the isotope with proper photon energy

Which photons are absorbed more in the tissues of the body than those of high energy?

A

gamma-photons of low energy

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18
Q

Selection of the isotope with proper photon energy

What are the requirements for gamma-photons of low energy?

A

They needs to be high enough to…

  • be detectable
  • to avoid unwanted radiation exposure of tissues
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19
Q

Selection of the isotope with proper photon energy

What are the disadvantages of 􏶲 gamma-radiation of too high energy?

A

It is less effectively absorbed in the detector crystal, which reduces the efficiency of the detection.

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20
Q

Isotope accumulation curve

After the administration of the radiopharmaceutical, it ___ in the target organ.

A

accumulates

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21
Q

Isotope accumulation curve

What is isotope accumulation curve? (isotope uptake and elimination kinetics curve)

A

Time dependence of the activity in the organ of interest is represented in the isotope accumulation curve.

Characteristics of the function, or malfunction, of the examined organ are deduced from the shape and parameters of the curve.

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22
Q

Isotope accumulation curve

Time dependence of the activity (􏵿) in the investigated organ is represented by the ___(sometimes called time-activity curve, Fig. 8).

A

isotope accumulation curve.

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23
Q

Isotope accumulation curve

3 pieces of information that we can obtain from Isotope accumulation curve

A
  1. The biological half-life (Tbiol).
  2. the physical half life (Tphys).
  3. the effective half life (Teff).
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24
Q

What is biological half-life (Tbiol)?

A

The time during which the organ is able to eliminate the half of the initial amout of the administered radiopharmaceutical

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25
Q

What is the physical half life (Tphys)?

A

The time characteristic for the physical decay of the radioactice isotope

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26
Q

What is the effective half life (Teff)?

A

The half-life of the measured activity

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27
Q

What are the main parameters of the isotope accumulation curve?

A
  1. a sum of two exponentials, the rising (uptake to the target organ, or clearance from blood) and the decreasing (physical decay + biological elimination) exponential curve.
  2. Lag time before the appearance of activity (T0) in the target organ
  3. The slope
  4. At the time of the maximum (Tmax)
  5. The effective half-life (Teff)
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28
Q

Main parameters of the isotope accumulation curve

How do we measure Lag time before the appearance of activity (T0)?

A

Lag time before the appearance of activity (T0) in the target organ (minimal transit time) is measured from the introduction of the isotope

It is determined by the extrapolation of the ascending part of the curve.

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29
Q

Main parameters of the isotope accumulation curve

What does The slope of the ascending (first) part of the curve characterize?

A

the uptake rate of the organ (clearance).

30
Q

Main parameters of the isotope accumulation curve

At the time of the maximum (Tmax), are the uptake and excretion equal?

A

Yes

31
Q

Main parameters of the isotope accumulation curve

At the time of the maximum (Tmax), the uptake and excretion are equal.

Why is the value of the maximum activity (􏵿max) important?

A
  • it characterizes the uptake and elimination capacity of the organ.
  • it is especially important in comparing the functions of paired organs (e.g., kidney).
32
Q

Main parameters of the isotope accumulation curve

What does The effective half-life (Teff) characterize?

A
  • The most important parameter of the descending part of the curve
  • It characterizes both the physical decay of the radiopharmaceutical (Tphys) and the biological the transport rate of the labeled substance and its elimination from the target organ (Tbiol).
33
Q

Main parameters of the isotope accumulation curve

What does The effective half-life (Teff) characterize?

A

It can be calculated from the measured effective half-life and the known physical lifetime

  • Tbiol= (Teff ·Tphys) (Tphys-Teff)
34
Q

Main parameters of the isotope accumulation curve

Describe The area under the curve?

A

The area under the curve

→ the integral of the curve between the first and last moment of the chosen time interval

→ gives the mean isotope content of the organ during that period.

35
Q

What are the 4 methods of medical imaging?

A
  1. Planar scintigraph
  2. Planar scintigraph with gamma camera
  3. SPECT (Single Photon Emission Computed Tomography)
  4. PET (Positron Emission Tomography)
36
Q

Imaging techniques in nuclear medicine

Do radiopharmaceuticals bound to the cells of the target organ emit 􏶲gamma-radiation randomly any time?

A

YES

37
Q

Imaging techniques in nuclear medicine

Radiation passes through the tissues of the body and becomes detected from the outside with a ___

A

detector

38
Q

Imaging techniques in nuclear medicine

The indices of refraction of the different materials for 􏶲gamma -radiation are ___ (n = 1)

A

identical

39
Q

Imaging techniques in nuclear medicine

What are collimators? Their role?

A

thick plates made usually of lead with one or many holes.

→ allows the passage only of those gamma􏶲-photons that travel along the axes of the holes

→ determine the isotope distribution of the high- activity centers in the target organs by moving the detector in x-y directions.

40
Q

Imaging techniques in nuclear medicine

What determine the spatial resolution? What is the consequence?

A

The width and density of the collimator hole

→ The smaller the hole, the better the resolution, but at the same time more radioactive material needs to be administered to reach the same efficiency of detection, as the number of detected photons is smaller.

41
Q

Imaging techniques in nuclear medicine

The smaller the hole of collimator, the better the resolution, but at the same time more radioactive material needs to be administered

WHY?

A

The width and density of the collimator hole

→ The smaller the hole, the better the resolution, but at the same time more radioactive material needs to be administered to reach the same efficiency of detection, as the number of detected photons is smaller.

42
Q

Imaging techniques in nuclear medicine

The size of the collimator hole is finally the result of a compromise between these two antagonistic requirements.

What are they?

A

spatial resolution and sensitivity

43
Q

Describe Planar scintigraphy

A

A single detector equipped with a single collimator scans the entire examined area.

The activity values measured at different locations are recorded and plotted in the form of spots of different size, brightness or color.

44
Q

Describe the image produced by Planar scintigraphy

A

The isotope distribution of the examined body part is imaged in two dimensions (2D) on a grey scale.

45
Q

Describe PLANAR SCINTIGRAPH WITH GAMMA CAMERA

A

A gamma camera detector head uses a collimator plate (lead plate with numerous holes), a large scintillation crystal and tens of photomultiplier tubes.

The image of the isotope distribution (2D) is calculated by computer.

46
Q

Describe image produced by PLANAR SCINTIGRAPH WITH GAMMA CAMERA

A

A gamma camera detector head uses a collimator plate (lead plate with numerous holes), a large scintillation crystal and tens of photomultiplier tubes.

The image of the isotope distribution (2D) is calculated by computer.

47
Q

Describe SPECT

(Single Photon Emission Computed Tomography)

A

Improved version of the planar scintigraph with a gamma camera.

Computer reconstructs the three-dimensional image of the isotope distribution form projections recorded by one or more gamma camera heads at different directions.

48
Q

Describe PET

(Positron Emission Tomography)

A

A positron from a short-half-life isotope simultaneously generates, via annihilation, two gamma photons of opposite directions, which are captured by scintillation detectors without collimators, placed in a ring around the patient.

Computer analyzes the coincidence of the detected photons and calculates the location of the isotope.

The two- or three-dimensional images of the distribution of the isotopes are reconstructed and displayed.

49
Q

PET (Positron Emission Tomography)

Principle of operation: a short-half-life (several minutes), (1) ____ isotope is administered into the patient.

The emitted positron is promptly (2)___ adjacent to the place of decay (< 1 mm), while two 􏶲gamma -photons with (3)___ energies, 511keV but (4)____ directions are created

A
  1. positive 􏶀beta-radiation- emitting
  2. annihilated
  3. Identical
  4. Opposite
50
Q

PET (Positron Emission Tomography)

More annihilation at the same place is represented by straight lines, which cross at the location of the ___

A

radiation source

51
Q

PET (Positron Emission Tomography)

Is the sensitivity of PET is high?

A

Yes

52
Q

Describe image produced by PET (Positron Emission Tomography)

A

it is capable of 3D reconstruction of the isotope distribution if several rings of detectors are used

53
Q

What is pair production?

A

It occurs only if the energy of incident gamma photon is greater than 1.022 MeV

→ The photon disappears and an electron and position pair is created from its energy

→ The position then collides with an electron

→ Their mass annihilates and 2 gamma photons of 511 keV are created

54
Q

3 steps of diagnos-tic procedure

A

(1) Radioactive material introduced into the patient
(2) Distribution and alteration of activity is detected
(3) Monitoring of physiological pathways and/or identification and localization of pathological changes

55
Q

How appropriate radioisotope for nuclear imaging

A
  • Maximize the information - Minimize the risk
  • For that find the optimal
    • type of radiation
    • photon energy
    • half-life
    • radiopharmacon
56
Q

how to to minimize absorption effects in body tissue

A

decay via photon emission

57
Q

Which type of radiation has sufficient penetration depth for body tissue?

A

gamma radiation

58
Q

equirements for photon energy

A

Photon must have sufficient energy to penetrate body tissue with minimal attenuation

BUT!

Photon must have sufficiently low energy to be regis- tered efficiently in detector and to allow the efficient use of lead collimator systems (must be absorbed in lead)

59
Q

What is a suitable physical half-life for selection of radioisotopes?

A

Activity is directly proportional to number of undecayed nuclei and decay constant which is proportional to the reciprocal of half life T
=> Selection must be
- Activity must be smaller
- Shorter half life (it has to be long enough for monitoring the physiological organ functions to be studied)
- Smaller number of undecayed nuclei (dose consid- erations for patients)

60
Q

What is radiopharmaceutical?

A

substance that contain one or more radioactive atoms and are used for diagnosis or treatment of disease.

→ made of two components, the radionuclide and the chemical compound to which it is bound

61
Q

3 requirements of pharmacons

A
  • Selectively accumulates in certain tissues (depending on their biochemical characteristics)
  • high target / non-target ratio
  • have no pharmacological or toxicological effects which may interfere with the organ function under study.
62
Q

3 Factors responsible for the ultimate distribution of the radioisotope

A

- blood flow (percent cardiac input/output of a specific organ)

- availability of compound to tissue, or the proportion of the tracer that is bound to proteins in the blood

- basic shape, size, and solubility of molecule which controls its diffusion capabilities through body mem- branes

63
Q

3 types of image?

A
  • *Static image**
  • *Dynamic image -**
  • *Static and dynamic image**
64
Q

Characteristics of static image

A

spatial distribution of isotope / activity at a certain time (over)

65
Q

Characteristics of dynamic image

A

variation of the amount of isotope / activity in time

66
Q

Amplitude of electric pulses varies in a wide range when it comes to gamma camera. Why?

A
  • *- absorption of one gamma-photon induces electric signals in more then one tubes**
  • *- attenuation mechanism can be photoeffect and Compton-scattering.**
67
Q

Describe Pulse amplitude spectrum of gamma camera

A

Amplitude of an electric pulse generated by a gam- ma-photon absorption in photoeffect is proportion to the photon energy.

→ They can be distinguished by Differential discriminator

68
Q

How to achieve better image quality?

A

several multipliers are fitted to the crystal scintillation

69
Q

Describe true coincidence

A
  • *(1) One annihilation**
  • *(2) Straight path photons in opposite direction**
70
Q

Describe scatter coincidence

A

(1) One annihilation
(2) Photons scatter
(3) Measured line of response places annihilation reaction along artefactual projection

71
Q

Describe random coincidence

A
  • *(1) More than 1 annihilation**
  • *(2) Photons from different annihilations are detected stimultaneously**
  • *(3) Artefactual line of response calculated**
72
Q

The role of gamma camera (focus on image)

A

Detects 2D projection of radiation emitted by decay of diagnostic radiopharmacons introduced into body

Biophysics 2 (LAB) (13 decks)

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