Nuc Med Physics

ACVR Theoretical

What makes up an alpha particle?

2 protons and 2 neutrons

Describe alpha decay

• heavy radionuclides with too many protons and neutrons for stability (at the end of the graph)

• spontaneously emits an alpha particle

What is the outcome of alpha decay

• Atomic number decreases by 2, mass number decreases by 4

• A different element is formed

Describe beta-minus (negatron, betatron) decay

• Too many neutrons - above the line of stability

• A nuclear neutron becomes a proton and an electron

What is the outcome of beta-minus decay?

• Emits an electron (in the form of a beta particle) and an anti-neutrino

• atomic number increases by 1, mass number is unchanged

• a different element is formed = isobar because mass number is unchanged

Describe positron decay (Beta plus, positron emission)

• Too few neutrons - below the line of stability

• A proton becomes a neutron and a positron (beta plus) and a neutrino

What is the outcome of positron emission?

• atomic number decreases by 1, mass number unchanged

• creates a different element = isobar b/c mass number is unchanged

• positrons react violently w/ electron (annihilation) → energy and being emitted as oppositely directed (180deg) annihilation particles (PET)

Describe electron capture

• too few neutrons - below the line of stability

• inner shell electron is captured by the nucleus, proton becomes a neutron

• emits a neutrino and electron cascade to fill vacancy → characteristic x-rays

What is the outcome of electron capture?

• atomic number decreases by 1, mass number is unchanged

• produces a different element = isobaric b/c mass number is unchanged

What is the energy of positron energy?

511 keV

Describe isomeric transition

• internal transition from excited to stable state

• emits y-rays

What is the outcome of isomeric transition?

• atomic number is unchanged, mass number is unchanged

What is the energy of y-rays produced by Tc99

140 keV

How do you convert mCi to MBq?

1 mCi = 37 MBq

• multiply mCi by 37 to get MBq

• divide MBq by 37 to get mCi

At what time is the max production of a mo cow seen?

23 hours

• this is also when the max production and decay rates are equal = transient equililbrium

At equilibrium, what is the Tc99m activity?

97%

What is the max elution of a mo cow?

85%

Which type of collimator magnifies?

converging and pin-hole

Which collimator minimizes?

diverging and pin-hole

Which collimator inverts the the image?

pin-hole

In regards to the gamma camera scintillator crystals, what happens to efficiency as y-ray energy increases

Efficiency decreases as y-ray energy increases

Define half life

Time required for the number of radioactive atoms in a sample to decrease by ½

N=No/2^n

N= number of radioactive atoms remaining

No= initial number of radioactive atoms

n= number of ½ lives elapsed

Anything that increases collimator resolution will have what kind of effect on efficiency?

decrease efficiency

What type of frame mode is used for bone scans?

Static = a single image is acquired one at a time

• end acquisition can be based on

  • preset time interval

  • preset required count number

  • max number of counts/pixels

• main consideration = spatial resolution

• 256 × 256 = most common matrix used

How many numbers can 1 bit store?

2 numbers can be stored in 1 bit

2^n bits = range of numbers that can be stored

How many bits equal 1 byte?

8 bits = 1 byte

How many bytes and bits are in 1 word?

1 word = 2 bytes or 16 bits

How bits/pixel are stored in byte mode?

Byte mode - stores 8 bits/pixel

• 2^8 = 256 counts

How bits/pixel are stored in word mode?

Word mode - stores 16 bits/pixel

• 2^16 = 65,536 counts

What determines spatial resolution?

Matrix size - # rows and columns

What happens to display resolution when matrix size is increased?

increased matrix size → decreased size of pixel and increased display resolution

In order to avoid loss of resolution what should the pixel size be?

Pixel width should be < 1/3 the resolution limit of the gamma camera in order to avoid loss of resolution

Which artifact creates a “star” artifact?

Septal penetration - using a shield with too low energy for high energy photon

For F18 what is;

• decay process

• half life

• photon energy

• positron emission

• T ½ = 109 minutes

• Photon energy = 250 keV

For technetium what is;

• decay process

• half life

• photon energy

• isomeric transition from molybdenum 99

• T ½ = 6hrs

• photon energy = 140 keV

Which radiopharmaceutical is most commonly used for bone scans?

Tech - MDP

Which radiopharmaceutical is most commonly used for thyroid imaging?

Tech - pertechnetate

Which radiopharmaceutical is most commonly used for parathyroid imaging?

Tech - sestamibi

What is the common dose for MSK imaging?

Tc99m MDP

• 0.2 - 0.3 mCi/kg or 3.5-5.5 GBq/animal

What is the common timing of the 3 phases of bone scan imaging?

1) Vascular = 120 seconds or 2 minutes

2) Soft tissue = 3 - 15 minutes

3) Bone phase = 3 hours

What is a common dose for thyroid imaging?

Tc pertechnetate

• 20 - 30 mCi/horse

What is a common dose for parathyroid imaging?

Tc sestamibi

• 200 miC / horse

What are 4 time points for parathyroid imaging?

10 mins, 2.5 hrs, 5.5 hrs, and 24 hrs