W7

Question 1

For diagnostic work it is usually necessary for the radioisotope to emit which rays and why?

Answer 1

gamma rays, b/c:

1. they can penetrate tissue and be detected outside the body => their distribution in the body can be determined

+ If the diagnostic test involves measuring samples taken from a patient, β particle emitters can be used.

Question 2

For therapeutic work it is usually more desirable for the radioisotope to emit which rays and why? (2)

Answer 2

β particles

1. they have a short range in tissue and
2. can deliver a high radiation dose to the location of the radioisotope

Question 3

Alpha (α) decay

particles released

Answer 3

α particles: high-E He nuclei consisting of 2 protons & 2 neutrons

Question 4

Beta (β) decay.

particles released

Answer 4

β particle - high-E electrons

• Electrons and Antineutrinos in β− decay
• Positrons (particles w/ the same mass as an electron but with 1 unit of positive charge) and Neutrinos in β+ decay

Question 5

Gamma (γ) decay and internal conversion (IC)

particles released

Answer 5

• γ-rays (very high E electromagnetic radiation) in γ decay
• Atomic orbital electrons in IC

Question 6

Spontaneous fission (SF)

particles released

Answer 6

• Neutrons (particles with the mass aproximately equal t that of a proton but with no charge),
• Heavier nuclei

Question 7

Neutron emission (NE) decay

particles released

Answer 7

Neutrons

Question 8

Proton emission (PE) decay

particles released

Answer 8

Protons

Question 9

5 physical quantities that must be conserved in each nuclear transformation:

Answer 9

• Total energy
• Momentum
• Charge
• Atomic number
• Atomic mass number (number of
  nucleons)

Question 10

N/Z in low Z atoms and high Z atoms

Answer 10

In low atomic number (Z) elements, nuclear stability is achieved when the number of neutrons (N) is approximately equal to the number of protons (Z).

As the atomic number Z increases, N/Z increases from 1 to about 1.5.

Question 11

β− decay

process

Answer 11

If a nucleus has N/Z too high for nuclear stability, it has an excess number of neutrons and is called neutron rich => decays through conversion of a neutron into a proton and emits an electron and antineutrino

Question 12

If the N/Z ratio is extremely high, …

Answer 12

a direct emission of a neutron is possible.

Question 13

β− decay

process

Answer 13

If a nucleus has N/Z too low for nuclear stability, it has an excess number of protons and is called proton rich => decays through conversion of a proton into a neutron and emits a positron and a neutrino

Question 14

Is direct emisson of a proton possible?

Answer 14

A direct emission of a proton is also possible, but is less likely, unless the nuclear imbalance is very high.

Question 15

Alpha decay

process of nuclear transformation

Answer 15

An unstable parent nucleus P attains a more stable nuclear configuration (daughter D) through ejection of an α-particle (4,2 He) => number of protons and neutrons is conserved by producing a He nucleus (α-particle) and lowering the parent’s A and Z by 4 and 2, respectively

Question 16

range of α-particles:

Answer 16

• in air about 1 cm to 10 cm
• in tissue about 10⁻³ cm and 10⁻² cm

Question 17

The most important example of radioactive decay in α decay

Answer 17

radium-226 (halflife of 1602 years) => radon-222 (half-life of 3.8 days)

Question 18

Does β decay lead directly to the ground state?

Answer 18

In many cases, β decay of a parent nucleus does not lead directly to the ground state of the daughter nucleus. It leads to an unstable or even metastable excited state of the daughter. The excited state de-excites through emission of γ-rays or through emission of internal
conversion electrons.

β decay can only take place when the binding energy of the daughter nucleus exceeds the binding energy of the parent nucleus.

Question 19

radionuclides decaying by β− decay are used in medicine for: (2)

Answer 19

• external beam radiotherapy
• brachytherapy

Question 20

Mechanism of beta-decay in medicine

Answer 20

parent nuclide decays by β− decay into an excited daughter nuclide => it instantaneously or through a metastable decay process decays into its ground state => in the process of doing so it emits the excitation energy in the form of γ-ray photons => these are used for radiotherapy

Question 21

Beta-minus decay example:

Answer 21

cobalt-60 (half-life 5.26 years) => nickel-60 (excited, then instanteneously ground - emits 2 gamma rays)

Question 22

BETA PLUS DECAY in medicine

Answer 22

β+ decay = production of positrons => positron emitters => used in medicine for functional imaging with positron emission tomography (PET)

Nitrogen-13 labeled ammonia is injected intravenously and is mainly used in:
* cardiac imaging for diagnosis of coronary artery disease and myocardial infarction
* liver imaging
* brain imaging.

Fluorodeoxyglucose (FDG) labeled with radionuclide fluorine 18 is a sugar compound that can be injected intravenously into a patient for use in PET functional imaging => areas of increased glucose metabolism, FDG PET scan can:
* detect malignant disease.
* distinguish benign from malignant disease.
* be used for staging of malignant disease.
* be used for monitoring response to therapy of malignant disease

Question 23

β+ decay examples

Answer 23

nitrogen-13 => carbon-13
fluorine-18 => oxygen-18

Question 24

In alpha and beta decay, the daughter nucleus will reach its ground state through one of the following two processes:

Answer 24

1. Emit the excitation energy in the form of a γ photon in a decay process referred to as γ decay.
2. Transfer the excitation energy to one of its associated atomic orbital electrons in a process called internal conversion (IC).

Question 25

isomer -

Answer 25

nuclei that have the same atomic number Z and same atomic mass number A but differ in energy states

Question 27

γ decay process may be represented as follows:

Answer 27

X^* => X + γ + Q_γ | Excited nucleus => emission of gamma proton => relaxed nucleus

Question 28

internal conversion (IC) | definition/explanation and process representation

Answer 28

Nuclear de-excitation in which the de-excitation energy is transferred from the parent nucleus almost in full to an orbital electron of the same atom X^* => X⁺ + e^- + Q_IC => X

Question 29

spontaneous fission (SF) | definition/explanation

Answer 29

Nuclei with very large atomic mass numbers A may disintegrate by splitting into two nearly equal fission fragments, while also simultaneously emitting 2–4 neutrons SF can occur only in thorium, protactinium, uranium and transuranic elements (Z>92)

Question 30

What two different types of radiolabels are used when combining tomographic techniques with radiopharmaceuticals?

Answer 30

* gamma emitters for single photon emission computed tomography (SPECT) * positron emitters for positron emission tomography (PET)

Question 31

FDG: half life and typical application

Answer 31

Fluorodeoxyglucose, ¹⁸F - 2h Glu metabolism, particularly in. brain, b/c it crosses the BBB

Question 32

Sodium iodide: half life and typical application

Answer 32

^{123I - 13h thyroid}

Question 33

Pentetreotide: half life and typical application

Answer 33

^{111In - 2.8 days neuroendocrine tumors, b/c it's somatostatine analogue}

Question 34

Strontium chloride: half life and typical application

Answer 34

^{89Sr - 50 days Bone tumors}

Question 35

Apticide: half life and typical application

Answer 35

^{99mTc - 6h Acute trombosis (binds to platelets)}

Question 36

Penetetate: half life and typical application

Answer 36

^{99mTc - 6 h Kindney imaging (renal perfusion)}

Question 37

Krypton: half life and typical application

Answer 37

^{81mKr - 13 s Lung ventilation imaging}

Question 38

ideal radiopharmaceutical needs to: (9)

Answer 38

* have a short physical half-life * be eliminated from the body with an effective half-life approximately equal to the examination time * emit pure gamma rays with no subsequent change in the nucleus * emit mono-energetic gamma rays * have a high activity per unit mass (specific activity) * be able to localise largely and quickly at the target site * decay into a more stable daughter nucleus * easily and effectively be attached to the chemical compound at room temperature * be easily produced or found at the hospital site

Question 39

high-speed electron egected from a nucleus during radioactive decay -

Answer 39

beta particle

Question 40

the ejection of an alpha particle from a nucleus results in

Answer 40

decrease of the atomic number by TWO

Question 41

whicho of the following is more likely to be radioactive?

Answer 41

nuclei with an odd number of protons and neutrons

Question 42

the most penetrating out of 3 common types of nuclear radiation is the

Answer 42

gamma ray

Question 43

a Geiger counter indirectly measures radiation by measuring

Answer 43

ions produced

Question 44

The emission of a gamma ray from a nucleus results in

Answer 44

no increase or decrease of the etomic number or atomic mass

Question 45

Which of the following isotopes is more likely to be radioactive? A) magnesium-24 B) calcium-40 C) astatine-210 D) ruthenium-101

Answer 45

C) astatine-210

Question 46

nucleons (2)

Answer 46

* positively charged protons and * neutral neutrons

Question 47

The components of the imaging system of Anger camera are:

Answer 47

1. collimator 2. camera head with 2.1) crystal 2.2) photomultiplyer tubes (PMTs) 3. electronics 4. computers f/ img acquisition & img processing

Question 48

Collimator's fn:

Answer 48

collimator restricts the rays from the source so that each point in the image corresponds to a unique point in the source: nuclides emit gamma ray photons in all directions, but the collimator allows **only those photons traveling directly along the long axis of each hole to reach the crystal**

Question 49

Collimators are composed of

Answer 49

thousands of precisely aligned holes, divided by septa, which absorb photons emitted in any other direction that the holes

Question 50

Why is collimator required?

Answer 50

Without a collimator in front of the crystal, the image would be unclear.

Question 51

Why diff types of collimators?

Answer 51

several types of collimators - designed to channel photons of different energies: high-sensitivity, low resolution: bigger holes; high-resolution, low sensititivity: smaller holes, **for the same bore length, the smaller the diameter, the higher the resolution** * By an appropriate choice of collimator, it is possible to magnify or reduce images. * It is also possible to select between imaging quality and imaging speed

Question 52

Anger camera head contains: (3)

Answer 52

* the crystal * PMTs * associated electronics enveloped and shileded in head housing, which includes a thin layer of lead; gantry supports the head

Question 53

Anger camera head crystal:

Answer 53

a large slab of thallium-“doped” NaI crystal thickness of a crystal affects its resolution as well as its sensitivity: **thicker crystals** => **higher sensitivity** but **lower resolution**, b/c gamma rays may be scattered and absorbed farther from the point at which they entered the crystal

Question 54

ANGER CAMERA: CAMERA HEAD PMTs

Answer 54

PMTs (60 or more): attached to the back surface of the crystal using light-conductive jelly; amnt of light received by a PMT is related to its proximity to the site of interaction of the gamma ray and crystal: PMT **closest** to the site of interaction receives **the greatest number of photons** and generates **the greatest output pulse**; the tube **farthest** from the nuclide source receives **the fewest light photons** and generates **the smallest pulse**

Question 55

number of resolvable points is limited to

Answer 55

the total number of PMTs (≈128 per camera), b/c image can be formed purely from the points corresponding to the PMT with the highest output at each photon interaction

Question 56

How to improve resolution with positioning algorithm?

Answer 56

by combining signals from adjacent tubes: electrical pulse generated by each PMT is digitized by an ***analog-to-digital converter*** (ADC) => digital values are transmitted to the positioning algorithm (a part of the computer-processing equipment in the camera head) => computer (=positioning algorithm) knows the location of each PMT on the surface of the crystal and estimates the site of the gamma ray interaction in the crystal by weighing in the digital value of the amount of light each PMT receives

Question 57

**the sum** of the digital outputs from all the PMTs after each gamma photon-crystal interaction is proportional to

Answer 57

the energy of the gamma photon striking the crystal (summed output = Z-pulse)

Question 58

pulse-height analyzer fn:

Answer 58

it accepts only those Z-pulses that correspond to the gamma energy of interest; each accepted Z-pulse and its location are stored in the computer

Question 59

Nuclear medicine computers are used for the: (3)

Answer 59

* acquisition of data * storage of data (in digital form, can be visualized as a matrix) * processing of data.

Question 60

The greater the number of pixels:

Answer 60

* **the smaller is each pixel** for a given field of view * the **better** preserved is the **resolution** of the image

Question 61

SPECT:

Answer 61

Single-photon emission computed tomography: multiple ***planar views*** of the ***radioactivity in an organ*** => math processing => cross-sectional views of the organ

Question 62

gamma-emitting radionuclides used in SPECT: (3)

Answer 62

* ^99mTc (technetium) * ¹¹¹In (indium) * ¹²³I (iodine)

Question 63

2 additional features of SPECT camera:

Answer 63

1. the SPECT camera is constructed so that the head **can rotate about the patient to acquire multiple views**. 2. It is equipped with a **computer** that **integrates the multiple images to produce the crosssectional views of the organ**.

Question 64

SPECT is most used to help diagnose or monitor: (3)

Answer 64

1. **brain disorders** (dementia, seizures, head injuries, clogged blood vessels, epilepsy) 2. **heart problems** (clogged coronary arteries, reduced pumping efficiency) 3. **bone disorders** (areas of bone healing, cancer progression)

Question 65

PET

Answer 65

Positron emission tomography, diifers from SPECT in 1 way: **PET radionuclides are positron emitters**, so after the decay event, the positron travels a very short distance before colliding with an electron => annihilation event creates **two gamma photons** with an energy of **511 keV each** => gamma photons are traveling in **exactly opposite directions** => PET scanners use complete detector rings

Question 66

coincidence detection method:

Answer 66

in PET, almost-simultaneous detection of two γ -photons can be expected (*window of 5-15 ns*) => single events can be rejected as noise; line that connects the two PMTs that recorded the event = ***direction of projection*** => **PET has a dramatically better signal-to-noise ratio than SPECT**

Question 67

Noise in PET is recorded when: (2)

Answer 67

* two independent events occur outside the detection window * when a photon is scattered and changes its direction **PET's sensitivity is several orders of magnitude higher than that of a SPECT scanner**

Question 68

PET is used:

Answer 68

A) to inspect: A1. **blood flow** A2. **oxygen intake** A3. **metabolism** of organs and tissues B) to detect: B1. **cancer** B2. **brain disorders** B3. **central nervous system problems** B4. **heart problems** C) also, PET scanner shows problems at the **cellular level** and gives a better inside into diseases like: C1. **coronary artery disease** C2. **brain tumors** C3. **memory disorders** C4. **seizures**

Question 69

MULTI MODALITY IMAGING

Answer 69

Both SPECT and PET provide **functional information** but it is desirable to know the **location of radiopharmaceutical accumulation as exactly as possible** => structural image is often acquired with CT or MRI => reconstructed activity map can then be superimposed over the structural image 1st - CT/MRI 2nd - SPECT/PET

Question 70

What is the fn of scintillation crystal in Anger cameras?

Answer 70

to convert gamma ray into visible light

Question 71

What is the role of the PMTs used in PET imging?

Answer 71

to convert visible light into current

Question 72

Which component of a SPECT system is responsible f/ counting (or not counting) detected gamma pulses?

Answer 72

Pulse height analyser

Question 73

Out of CT, SPECT and PET, what technique has the highest resolution?

Answer 73

CT

Question 74

SPECT vs PET in s detection sensitivity and signal-to noise ratio:

Answer 74

PET is better