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Biophysics I > Dosimetry > Flashcards

Dosimetry Flashcards

1
Q

The gas ionization chamber

A
  • Measures the radiation dose by measuring the charges produced between the two charged plates in the capacitor.
  • Charges move to oppositely charged electrodes, potential difference detected and represents a current pulse.
  • Size of the current is proportional to the energy of the incident radiation.
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2
Q

Thermoluminescent dosimetry

A
  • Ionizing radiation excites electron from valence band.
  • After electron relax, light is emitted.
  • Number of photons is proportional to the dose.
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3
Q

The Geiger-Müller counter

A
  • Based on a gas ionization chamber.
  • Voltage much higher.
  • Every radiation will form an avalanche of secondary electrons which form maximum current.
  • Sensitive to low energy radiation, due to high voltage.
  • Counts the number of particles that interact with the device.
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4
Q

Semiconductor detectors in dosimetry

A
  • Uses a diode connected in reverse bias, no current flowing in circuit.
  • Ionizing particle causes electron-hole pair in depletion region.
  • Current flows. Current is evidence of presence of radiation.
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5
Q

Physical, chemical and biological phases of radiation effects

A

Physical: Ionization
Chemical: Free radicals
Biological: DNA damage

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6
Q

The absorbed dose

A
  • Amount of energy absorbed per mass. (J/Kg) (Gy)
  • Difficult to measure since even a lethal dose of 6Gy corresponds to an unnoticeable temperature change.
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7
Q

Converting exposure in air to absorbed dose in tissue

A

1 C/Kg (Exposure) = 34 Gy in air

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8
Q

The stochastic radiation effect

A
  • Radiation damage that can occur due to absorbed dose.
  • Valid for damage that can occur at low levels of absorption.
  • Random, proportional to increasing dose.
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9
Q

The exposure

A
  • Measures positive charge produced in air with certain mass by ionization. (C/Kg)
  • Can be measured by ionization chamber.
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10
Q

Weighting factors in dosimetry

A

1) Type of radiation - Different radiation cause biological damage in different severity.
2) Which tissue exposed - Different tissue have different sensitivity to radiation.

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11
Q

The deterministic radiation effect

A
  • Probability of radiation damage increases abruptly over a threshold dose.
  • Severity of damage above threshold is proportional to the dose.
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12
Q

The equivalent dose

A
  • Different types of radiation influences tissues in different severity associated with a particular dose. (J/Kg) (Sievert Sv)
  • Wr = Radiation weighing factor, how many times greater effect with certain type of radiation compared to gamma radiation.
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13
Q

ALARA-principle

A

ALARA - As Low As Reasonably Achievable. Reducing exposure to the minimal by:
- Minimal time near source.
- Distance from source should be maximal.
- Person dealing with radioactive material should wear protective shield.
- Source must not cause any deterministic effect.

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14
Q

The direct and indirect effects of ionizing radiations

A
  • Direct: Biologically important macromolecules are damaged (DNA).
  • Indirect: Radiation ionizes water first, causing free radicals (OH.) which combine to form H2O2, chemically attacking DNA.
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15
Q

The effective dose

A
  • Measures the absorbance dose, taking into account sum of radiation types and weighing factors, also type of organ and its weighing factor (probability of stochastic damage)
  • Sum of all tissue weighing factors is 1.
  • If radiation effects more than one organ, effective doses should be summed. (Sv)
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16
Q

Typical dose values and dose limits

A
  • Lethal dose is 4 - 6 Gy
  • Max. Limit for body is 20mSv/year, for skin and limbs is (500mSv/year)
  • Background radiation: 2.4mSv/year
  • X-ray image: 0.2 - 1 mSv
  • CT scan: 2 - 8 mSv
17
Q

The dose rate

A

Ratio of dose and time of irradiation (Gy/h) (Sv/h)

18
Q

Information obtained by isotope diagnostics

A
  • Organ function: Metabolism, how fast it absorbs and expels the tracker.
  • Circulation in the organ if tracker placed in blood.
  • Air circulation in organ if tracker is inhaled.
19
Q

Principles of selecting the isotope for diagnostics according to half- life

A
  • Its half-life should match the biological half-life.
  • Needs to be as short as possible.
20
Q

Principles of selecting the isotope for diagnostics according to radiation type and energy.

A
  • Gamma radiation isotopes are used frequently due to long effective range, low absorbance in body tissue, and high energy to be easily detectable.
  • Energy of gamma should be high enough to penetrate body and be detected, but not too high as it will be less effectively absorbed by detector. (More damaging to patient)
  • Should be absorbed to some extent for contrast image.
  • Activity needs to be high enough to detect sufficient photons.
21
Q

Parts and function of Tc-generator

A

Machine which produces gamma radiation isotope from parent isotope with a relatively long half-life. Parts:
- Lead container
- Saline container
- Generator columb
- Tc container

22
Q

Cost-benefit principle in isotope diagnostics

A

If the risk of not having examination is higher than that caused by the radiation exposure, the procedure can be done.

23
Q

Definition of radiopharmaceutical

A

Chemical agent/drug having radioactivity. (Labeled with radioisotope for diagnostic and therapeutic purposes)

24
Q

Determination of the biological half-life of an organ

A

Biological transport rate and elimination from target organ. 1/Teff = 1/Tphys + 1/Tbiol

25
Q

Relative depth dose

A

Ratio of absorbed dose in certain depth within body to absorbed dose at reference point of the body central ray.
- The higher the better
- More dose to a cancerous tissue in the depth vs lower dose to a healthy tissue.

26
Q

Parts and function of the gamma-camera

A
  • Collimator: Only allows gamma photons of particular direction to enter.
  • Scintillation crystal
  • Photomultiplier tube
  • Amplifier
  • Computer and electronics (Discriminator and counter unit)
27
Q

Scintigraphy

A

Examination where low quantity of radiopharmaceutical inserted to body. Aggregation (clustering) in target organ gives us information about it.
Gamma energy is detected with Gamma camera, precise signal mapping can be formed.

28
Q

Single Photon Emission Computer Tomography (SPECT)

A
  • Gamma radiopharmaceutical injected into body.
  • Detector gamma camera is rotated around body axis.
  • Gamma photons detected.
  • Multiple 2D images from different angles are acquired.
  • Images computed using tomography algorithm.
  • 3D image yielded.
29
Q

Teletherapy, geometric viewpoints

A

Teletherapy: High gamma radiation induced (relative depth dose) from many different directions in order to create a high dose on the cancer, but split the dose between healthy tissues to reduce damage.

30
Q

Interpretation of a typical isotope accumulation curve.

A
  • Lag time: Time from introduction of isotope to uptake. (Before curve starts)
  • Ascending curve: Uptake of isotope.
  • T-max: Max activity.
  • Descending curve: Physical + Biological decay (Effective half life)
  • Area under curve: Mean isotope content of organ.
31
Q

Parts and working principle of Positron Emission Tomograph (PET).

A

PET is a functional imaging method, provides information about distribution of Positron radioactive isotope. Emitted positrons interact with electrons producing 2 gamma rays with opposite direction, detected by a ring detector.
Parts:
- Positron radiating isotope
- Ring detector (Scintillator crystal, Photomultiplier)

32
Q

Role of collimators in radiation therapy, gamma-knife

A

Collimator: A perforated lead plate allowing passage of gamma photons traveling along axis of hole. The smaller the hole the better resolution, however less photons are detected.
- Scintigraphy: Collimator with only one hole.
- Gamma camera and SPECT: Collimator with thousands of holes.
Gamma knife: Radiation treatment with high intensity focused gamma beam precisely. (Can attack brain tumor)

33
Q

Multimodal imaging: PET/CT and SPECT/MRI

A
  • Method where Superimposed functional images formed (PET or SPECT)
  • Structural imaging method (CT or MRI)
    Products is a high resolution image with identifiable anatomical structure and information about functional status.
34
Q

Principles of brachytherapy

A
  • Insertion of sealed source of radiation into patient’s body near/in the site of tumor.
  • Radiation harms the tumor cells with low damage to healthy surrounding tissue.
  • Treats: Cervical, Prostate, Breast and Skin cancers.

Biophysics I (7 decks)

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