Contrast Flashcards
(16 cards)
What is Beta (β-) decay?
A neutron in the nucleus is transformed into a proton and electron. The electron and antineutrino are ejected. The resulting nucleus is in an excited and will also emit a gamma-ray.
Used in: SPECT
Z: increase
Compete: IC
What is Positron (β+) decay?
In an unstable parent nucleus, a proton decays to a neutron and emits a neutrino and a positron. The positron combines with an electron and annihilates emitting 2 511 keV photons.
Used in: PET
Z: decrease
Compete: EC
What is Internal Conversion (IC)?
In a metastable nucleus, energy is transferred to an orbital (inner shell) electron which is ejected. An outer shell electron will take its place and a characteristic x-ray will be emitted.
Used in: SPECT
Z: increase
Compete: β-
What is Electron Capture (EC)?
An orbital electron is “captured” by the nucleus and combines with a proton to form a neutron. An outer shell electron will take the inner shell electron’s place, emitting a characteristic x-ray. A neutron is also emitted.
Used in: PET
Z: decrease
Compete: β+
What does a low Z do?
more efficient decay
Photoelectric (PE) in PET, SPECT?
PE is most prominent in standard radiography energies, as the higher energy levels of both PET and SPECT do not easily support the low-energy photon dependency of the PE effect. This is still seen in SPECT imaging, but not as dominantly as in radiography.
Compton Scattering in SPECT, PET?
Compton scattering is the most prominent in SPECT imaging, as the soft tissues allow for the most scattering when compared to radiography and PET energies. This is still seen in PET imaging and radiography, causing degradation of the image.
What is photopeak energy resolution? Why do scintillation detectors have low res?
Photopeak energy resolution represents the scintillation detector’s ability to distinguish between different photon energies. A lower percentage means a sharper peak, therefore a better resolution.
Poor energy resolution can be a result of pulse pileup. The detector produces a single pulse, which is the sum of the individual signals from the two interactions, having higher amplitude than the signal from either individual interaction. Because of this effect, operating a pulse height spectrometer at a high-count rate causes loss of counts and misplacement of counts in the spectrum. Other factors such as the
efficiency of the crystal to absorb the full energy and the collection of the optical (scintillation) photons can also result in variation of the signal pulse heights collected by the PMT
Energy res equation
energy res = (FWHM / pulse height @ center) * 100
Why use F-18 in PET?
- F small enough to attach to glucose w/o affecting matter-tissue interaction
- longer half-life than other nuclides; increase SNR
Common SPECT radioisotopes
Tl-201 (Thallium): atomic number 81
I-123 (Iodine): 49
Tc-99m (Technetium): 43
Common PET radioisotopes
Cu-64 (Copper): atomic number 29
Ga-68 (Gallium): 31
C-11 (Carbon): 6
N-13 (Nitrogen): 7
O-15 (Oxygen): 8
F-18 (Fluorine): 9
Calculate how long will it would take to run the cyclotron to produce an activity of 18F at 75% of its saturation value?
A_F18(t) = A_0 (i - e^(-λ_F18 * T)
T_F18 = ~110 min
75%:
(A_F18) / A = 0.75 = 1 - e^(-λ_F18 * T)
T = [ln(1-0.75)]/λ_F18 = - [ln(0.25) / ln(2)] * T_F18
T = 2(110) = 220 min
If it takes 30 minutes to filter and conjugate deoxyglucose to produce 18FDG, what is its activity at the time of PET imaging? The proton beam current is 40 μA. The proton beam impinging onto the H218O target (100% pure; ρ~1.0 g/cm3; thickness of 1 mm) has a kinetic energy (Tp) of 10 MeV
T_p = 10 MeV
current = 40 μA
Saturation activity = 130 (mCi / μA)
A_0 = 130(mCi / μA) * 40μA = 5.2 Ci
30 mins:
A = 0.75 * A_0 * e^(-λ_F18 * 30) = 0.75 (5.2 Ci) * e^[-(ln(2) / 110 min) * 30 min] = 3.23 Ci
How is F-18 produced? (production reaction scheme)
CYCLOTRON | PET
18O (p,n) 18F where:
O-18: target / parent
p: impinging particle
n: product
F-18: daughter
How is Tc-99m produced? (production reaction scheme)
REACTOR | SPECT
235U (n,134Sn) 99Mo where:
U-235: target / parent
n: impinging
Sn-134: fissional product
Mo-99: fissional product
