Physics/Radiopharm Flashcards
What are: a) isotope b) isotone c) isobar d) isomer
Nuclides with same: o isotopes – proton number o isobars – mass number o isomers – proton and neutron number, but different energy state o isotones – neutron number
What is KLL Auger electron? How do you calculate its energy
K-shell vacancy filled by L-shell electron, energy difference absorbed by another L-shell electron which is emitted from the atom
energy = KB – 2LBe
What are differences between Auger and Characteristic electrons? (They probably mean conversion electrons. No such thing as characteristic electrons.)
Property
Internal conversion
Auger
Source of energy
Excited/metastable nucleus
Orbital electron transition
Origin shell
Inner
Outer
Kinetic energy
Discrete (Eγ – BE)
Discrete (BEhole – BEtransition – BEAuger )
Name 2 methods of radionuclide production in a reactor.
Fission(1) & Neutron activation (2)
For the reaction (n,γ): what are the requirements for this reaction? What type of reaction occurs? What is the relationship between the target and product? Under what circumstances could high specific activity be obtained?
a. Need a neutron source (reactors)
b. Neutron activation (gains a neutron, and a gamma is emitted)
c. Same Z, so same chemical, but higher mass, which may be less stable.
d. Could be obtained if product underwent beta- decay to a different chemical (Z+1) or fission into multiple separable fission products.
What is the ‘effective positron range’?
In words: distance from nucleus to the line of annihilation photons (perpendicular), which is always shorter than the actual distance the positron travels
Start with 100MBq. T1/2 = 10 days. What is activity after 50 days?
Show calculation.
T1/2 = ln2 / λ
λ = ln 2 /10 = 0.0693147181
A(t) = A(o)e-λt = 100e-0.0693147181(50)
Describe in one or two sentences the Bateman equation. List two types of equilibrium as they relate to the Bateman equation. What is the ratio of 99Mo:99mTc at equilibrium? (exact wording, not sure activity ratio or molar ratio)
B2015-1: What is Bateman equation used for?
C2016-6. a)What is Bateman’s equation – give equation or describe
Differential equations describing nuclide quantities in a decay chain
· For 1-nuclide decay chain (parent-daughter), with branching ratio, BR, solution is:
< GET EQUATION>
Used for samples containing radionuclides having parent-daughter relationships. Equation for the activity of the daughter at time (t) which accounts for the fact that the daughter product is being formed (by decay of the parent) at the same time it is decaying.
1st part of the equation: activities of the parent Ap (t) and the daughter Ad (t) at the time t with their respective decay constants (λ) and branching ratio for the decay producing the daughter of interest when several decay modes are possible.
2nd part of the equation: residual activity of daughter that was present at time 0
Sorenson Chapter 4 page 39
List 2 equilibriums
Secular, Transient
Secular Equilibrium (Tp > 100x Td) At secular equilibrium, the Bateman equation dictates that the daughter will be produced at approximately the same rate that it decays, so a plateau of activity is reached.
Transient Equilibrium
Where BR = branching ratio which is 0.876 for Mo/Tc
At transient equilibrium, the Bateman equation dictates that the rate of decay of the daughter exceeds the rate of production, and so daughter activity will reach a peak after which it declines.
What is 99Mo:99mTc ratio?
Branching ratio = proportion of decay events resulting in a particular decay scheme: for 99Mo to 99mTc = 0.88
Calculate ratio of Mo99:Tc99m at equilibrium
< See notes>
Is the fraction of nuclei decaying by a specific decay mode
What is definition of transient and secular equil? What type of equil is Mo and Tc
a. Transient Equilibrium: when the daughter activity decays at the same rate as a parent whose half-life is 10-50x that of the daughter.
b. Secular Equilibrium: when the daughter activity decays at the same rate as a parent whose half-life is 100-1000x that of the daughter.
c. transient
How do you calculate decay constant.
THE DECAY CONSTANT = 0.693/T1/2
What are the physical half life and photon energy of I-123?
13.2 hours, 159keV
Aside from fission production of 223Ra, what is Health Canada’s approved production method
Actinium 227 generator
Name 3 alpha radionuclides with therapeutic potential. You are given a table to fill in with the headings of “Radionuclide,” “T1/2,” “Number of alpha particles,” “Total energy”
SEE NOTES
How do you image 223Ra? Or, how do you measure 223Ra in dose calibrator?
<2% of energy emitted is from photons – these can be imaged with gamma camera
Dose calibrator must be properly calibrated for RA223
Of F18-FDG, Rb82, and N13 ammonia, which has the best range, first pass extraction. Give their half-lives and positron ranges.
Shortest range: F18 FDG (t1/2 110 min, max range 2.3 mm)
Highest first pass extraction: N13 ammonia (t1/2 10 min, max range 5.1 mm)
First pass extraction: O-15 water > N-13 ammonia > Rb-82 > F-18 FDG
Range: Rb-82 (16 mm) > O-15 water (8 mm)> N-13 ammonia (5 mm) > F-18 FDG (2 mm)
Compare dose, time to imaging, urinary excretion, protein binding of NaF and MDP.
SEE NOTES
Calculate effective T1/2 for 99mTc. Biologic excretion (11% fecal, 35% renal, 4% etc.) Show formula and calculation. 11% cleared by feces, 4% by sweat, 35% by urine at 5 hours for 99mTc radiopharmaceutical. What is effective half life?
Effective 1/2 life: incorporates both the physical and biologic 1/2 lives. Effective 1/2 life is always shorter than the physical and biological 1/2 life.
Effective 1/2 Life = (physical 1/2 life X Biologic 1/2 Life) / (Physical + Biologic)
Biologic 1/2 Life: Refers to the time it takes an organism to eliminate half of an administered compound or chemical on a strictly biologic basis.
Physical 1/2 life of 99m Tc = 6 hrs
Biologic 1/2 life = 5 hours (given above) 648
Effective = 6 x 5 / 6 + 5 = 30/11 = approx 2.7 hours
90Y
a) Parent
b) T1/2
c) Mode of decay
Strontium-90
Half-life of 64.1 hours and a decay energy of 2.28 MeV
B- decay
Know generator daughter products (Table with the parent given, and asking for the corresponding daughter radionuc)
a. Mo99-Tc99m
b. Sr82-Rb82
c. Ge68-Ga68
d. Sr90-Y90
e. Rb81-Kr81m
f. Ac227-Ra223
Name 5 mechanisms of localization of radiopharmaceuticals in an organ.
PACRIM
P = passive diffusion
A = active transport or antbody/antigen complex
C = chemotaxis, compartmental localisation, cell sequestration, capillary blockage)
R = receptor binding
I = ionic exchange
M = metabolism
Describe how a generator works.
Consists of a parent-daughter radionuclide pair contained in an apparatus that permits separation and extraction of the daughter from the parent. The daughter product activity is replenished continuously by decay of the parent and may be extracted repeatedly.
Most important is the 99Mo-99mTc generator. Can be wet or dry:
- Wet Generator: A large reservoir of saline 500ml is connected to the generator. The generator is continuously bathed in saline. Techentium is eluted by attaching a sterile 30 ml evacuated vial to the elution port.
- Dry Generator: A 5 ml saline vial is attached before attaching the 30 ml evacuated vial. The 30 ml evacuated vial then draws 5 ml of saline through the generator to remove the Tc-99m activity followed by 25 ml of air to “dry” the column. Drying cuts down on radiolysis product formation. The air promotes oxidation to the pertechnetate state.
In both systems the evacuated vial contains sodium pertechnetate in normal saline.
Parts of cyclotron
Ion source, dees, gap, magnets, vacuum, stripping foil, target
Shielding method for MO QC from eluate
Mo99 breakthrough: Assessed with each elution by placing sample in a 10HVL lead shield (2mm for Tc99m; to attenuate 140keV gammas), and detecting 740 & 780keV photopeaks in dose calibrator. The upper limit is 0.15 kBq/MBq at the time of injection (may be exceeded in emergency situations). Because Mo99 decays slower, the ratio of Mo99 to Tc99m increases with time.
i. Can also perform with phenylhydrazine solution: Mo-Ph complex causes a color change, which can be quantified by colorimetric test.
How does AL QC work
Al breakthrough: excess Al3+ induces flocculation (aggregation) of Tc99m-SC and agglutination of Tc99m-labeled RBCs, potentially resulting in liver and/or lung embolization, as well as image degradation. Al3+ is therefore limited to 10ug/mL by colorimetric spot test using aurin tricarboxylic acid or methyl orange, which is performed on each eluate.
How does AL in eluate affect radiopharm
Leads to flocculation of SC, agglutination of RBCs, and lung uptake in bone scan
4 drugs that can interfere with in vivo RBC labeling
Heparin, penicillin, iodinated contrast media, dextran, doxorubicin, hydralazine (can oxidize stannous ion)
With respect to Tc-SC – what is use of thiosulfate, gelatin, EDTA
a. Thiosulfate is the source of sulfur for SC formation in acid.
b. Gelatin is a protective colloid (a “stabilizer”) which coats the negatively-charged sulfur particles with a charged protein sheath, causing them to repel one another.
c. EDTA is added to the kit to chelate excess Al from Tc elution (Al flocculates SC).
Name 5 methods of radiolabelling.
B I E B E R : B= bifunctional chelating agents I= Introduction foreign label E= exchange of isotope B= Biosynthesis E= excitation labeling R= recoil labeling
What is most common method for producing I123. What is its half life and photon energy?
Indirect method:
Xe-124 (p,2n) Cs-123 -> Xe-123 -> I-123 This will be I-124 carrier free.
Direct method for producing I123
Proton bombardment of enriched Te-124 can yield I-123 but not ideal because can be contaminated by I-124 and I-125.
2 ways of making F-18? 1 adv and 1 disadv of each
a. O18(p,n) gives fluoride ion in aqueous solution, higher specific radioactivity and better for FDG production, but O18 water is expensive.
b. Ne20(d,α) gives elemental fluorine gas, cheap and better for F-dopa production, but lower specific radioactivity.
What is no carrier added. What are 2 advantages with MIBG?
No carrier added - A preparation of a radioactive isotope which is essentially free from stable isotopes of the element in question.
Advantages:
- Permits more efficient and high specific activity labeling with the therapeutically active 131-Iodine isotope which could result in maximum delivery of the therapeutic potential of I 131 MIBG
- Minimizes the amount of non-radioactive MIBG molecules administered to the patient, potentially reducing pharmacologic toxicities, especially cardiovascular events or toxicity, and possibly enabling better tolerated and effective treatment
Define Carrier. Define Specific activity. Calculate the carrier-free specific activity for I-131
a. Carrier: non-desirable contaminant radioactive or non-radioactive species in solution with the desired species.
b. Specific activity: proportion of a sample that contains only the radioactive form of the atom, expressed in Bq/g.
c. CFSA (Bq/g) = lambda*N = ln2/T1/2 * 6.022x1023/atomic weight (remember to convert T1/2 to sec)
Name 3 causes of radiochemical impurity.
Radiochemical purity is the fraction of total radioactivity in the desired chemical form in the radiopharmaceutical. Impurities include free and hydrolyzed Tc-99m TcO4-
Causes:
o Decomposition of the radiopharmaceutical due to the action of a solvent
o Changes in temperature or pH
o Light
o Presence of oxidizing or reducing agents
o Incomplete reaction
o Radiolysis
Name 5 ways to detect radiochemical impurities.
- solvent extraction
- ion exchange
- paper, gel, thin-layer or high performance liquid chromatography
- gel electrophoresis
- precipitation
- distillation
Define the following and give an example of each:
a) RN purity
b) RC purity
c) Chemical purity
d) Biological purity
A) fraction of total radioactivity in the desired radionuclide form. (Mo99 breakthrough)
B) fraction of total radioactivity in the desired chemical form. (HR and free tech.)
C) Presence or absence of contaminating chemicals (e.g. solvents in FDG synthesis).
D) Biologic purity: sterility and apyrogenecity.
Patient had 82Rb 4 days ago. Sets off radiation detector. What do you expect happened?
Sr–82 and Sr-85 breakthrough
What is the maximum quantity of Mo99 in Tc99m. How is it measured? When is it measured?
Limit is 0.15 uCi 99Mo/mCi 99mTc (0.15 kBq/MBq) at the time of administration. Saha, page 77
The 99Mo contamination is measured by detecting 740-keV and 780-keV photons of 99Mo in a dose calibrator or a NaI(Tl)detector coupled to a pulse height analyzer. The eluate vial is placed in a lead pot (about 6-mm thick) to stop all 140-keV photons from 99mTc and to count only 740-keV and 780-keV photons from 99Mo. The shielded vial is then assayed in the dose calibrator using the 99Mo setting. Molybdenum-99 along with 98Mo (from the molybdenum target) can also be detected by adding phenylhydrazine to the eluate and observing the color change due to the Mo-phenylhydrazine complex by the use of a colorimeter.
2 ways of 99mTc production
Using Fission MOLY in generator
99Mo → 99mTc + β− + νe
Cyclotron produced
100Mo(p,2n)99mTc
Tc eluate should be ___, ___, and be free of visible ___. It should have less than ___ kBq/MBq Mo-99 and ___ per mL of Al3+ ion. As the Tc-99m generator eluate does not contain bactericidal activity, how long can you keep it before it expires? A Mo/Tc generator expires ___ after manufacture.
The solution should be clear, colourless, and free of visible foreign material.
0.15 kBq/MBq, 0.10 ug/mL
12 hours
2 weeks
You elute a dry generator with 20 ml of saline, but only obtain 10 ml in the eluting vial. What would be the consequence if the remaining volume were left in the generator? What can you do to correct this?
It will cause oxidation and radiolysis. The Tc99m still in the generator will also decay, leaving more Tc99 and decreasing the specific activity of your next elution. Also, possibility of pseudo-channeling is created by leaving the column wet. You can try to elute remaining fluid with another vacuum-vial.
2 radiotracers produced by biosynthesis.
Co-57 cyanocobalamin (Vitamin B12)
C-14 Xylose
What are essential and tagged Tc99m tracers?
Tc-essential – Tc is integral part and for which the molecule would not be delivered to target in the absence of the Tc (prepared by integrated approach). Biodistribution depends of their physico-chemical properties (charge, size, lipophilicity) E.G. MAG-3, ECD, HMPAO, Sestamibi.
Tc tagged - non-substrate specific localization mechanisms. Technetium is labeled or “tagged” to a variety of molecular species that delivered technetium to organs of interest by simple diffusion, phagocytosis, entrapment, or cell sequestration mechanisms. Technetium is a passenger atom not essential for localization. include complexing agents (e.g. DTPA), particles (e.g. sulfur colloid), blood cellular elements (e.g. leukocytes), and proteins (e.g. human serum albumin).
Tc labeled radiopharmaceuticals can be “Tc-essential” or “Tc-tagged”. Define each, and give 4 examples for each.
Tc-essential is integral part of the molecule and without it, it will not have desired in vivo effect. Examples: HMPAO, ECD, MIBI, TETRAFOSMIN, HIDA, MAG3.
Tc-tagged does not need Tc atom to carry function. Tc is added to side-branch with no effect on receptor site or is attached to bifunctional chelate. MAA, SC, DTPA, RBC.
What is the oxidation state of MIBI?
1+
Tc oxidation states – Tc – Sc, DTPA, mibi and I think one more?
See notes
What are the possible oxidation states for Tc, and which is most stable? Describe the mechanism behind erythrocyte labeling with Tc?
+7 to -1 oxidation states are possible. +7 is the most stable (+4, +5, and +7 are the most common; +4 if I had to pick one, but not sure).
Add pyrophosphate-Sn to carry tin inside and reduce Hgb. Pertechnetate then added and goes inside cell and get reduced by Sn. The Tc then binds B-hemoglobin and heme.
What is the minimal number of MAA particles?
60 000 vs. 100000 (debated)
- What is a pyrogen? Which is best method to detect? Give 3 reasons?
Pyrogenicity: ability of substance to induce fever
- Rabbit test and limulus amebocyte lysate test
BEST IS Limulus amebocyte lysate test
- o No live animal testing
- o Rapid result (1 hour)
- o Less radiopharmaceutical needed
Current standard for testing pyrogenicity? 3 reasons this is better than rabbit testing
LAL/ BET test derived from horseshoe crab blood cells (amebocytes): Gram-negative endotoxin activates proenzyme to coagulase, which in turn activates coagulogen to coagulin. The gel-clot method and the chromogenic method are comparable.
b. More sensitive than rabbit testing; cheaper; faster; easier; more ethical (?)
Name 2 conditions that require reducing the number of MAA particles
Pulmonary Hypertension
Pediatric
Known R to L Shunt
2014-1. RBC labelling , which is most efficient method and what is the labeling efficiency..what reduces Tc in RBC
a. In vitro is most at >97%, modified in vivo >95%, in vivo ~85%.
b. Sn2+
3 types of QC you would do on eluate? (specifically said eluate..radionuc purity, biologic, etc - not sure if radiochemic would count, as that’s more after prep)
a. Mo99 breakthrough (radionuclide purity)
b. Al breakthrough and pH (chemical purity)
c. HR tech (radiochemical purity)
Cyclotron production of radionuclides: What is the typical charge of the bombarding particle? What is the change in the nuclear charge? Where are products in relation to line of stability? What types of decay occur in the product? What is the isotopic purity? Radionuclidic yield? Cost of produced radionuclides?
- Bombarding particle typically has +1 (proton) or +2 (deuteron) charge (though most cyclotrons now are negative-ion cyclotrons)
- In a (p,n) reaction, presumably you’re gaining a charge.
- They are shifted to the proton-heavy side
- They decay by positron or EC.
- Isotopic purity is high.
- Radionuclidic yield is low.
- Cost is high.
. 4 advantages of a negative ion cyclotron
a. stripping foil is near 100% efficient, so get less radioactivity in the housing.
b. Better beam optics.
c. The foil can also split a beam, so you can create 2 different products at once.
d. Smaller than positive ion cyclotrons
e. Disadvantage: need a lower-pressure vacuum.
Define chelation.
a. Chelation: The formation of attractive interactions between two or more separate binding sites within the same ligand and a single central atom (usually a metal).
b. BFCA: reagents containing a strong metal-chelating group and a chemically reactive functional group
110 particles in 1x1 mm square on hemocytometer, how many particles / ml of MAA?
1x1x0.1mm = 0.1mm3 = 0.0001mL, so 110/0.0001mL = 1,100,000/1mL
Pittsburgh protein B (PiB) has been investigated for binding to what substance? What radionuclide has been used to label PiB?
B-amyloid plaques, labeled with 11C.
What is ionization? What is delta ray?
a. In an ionization event, an electron (delta ray) is ejected from its atomic orbit by an incident charged particle, and has enough energy to cause secondary ionizations on its own.
2 types of interactions a photon greater than 511 Kev would have
a. Compton scatter
b. Photoelectric absorption
- What is iodine escape peak
Iodine escape peak: photoelectric absorption of an incident photon by iodine in the crystal emits a 30keV characteristic x-ray. When this x-ray is not absorbed, the resultant energy deposited is 30keV less than the photopeak. This happens more commonly for low-energy photons (<100keV).
What is the photoelectric effect. Describe its relationship to Z and E
What is photoelectric effect. Relationship to Z and photon energy for photoelectric effect
It is one of the possible interactions between photons and matter, where the atom absorbs all energy of incident photon and an orbital electron is ejected.
It is the dominating effect in heavy elements at low photon energies.
Probability of photoelectric effect α Z3/E3.
The photoelectric component increases abruptly at energies corresponding to orbital electron binding energies of the absorber elements (K absorption edges)
Compton scatter:
a) How does this affect image quality
b) What does it do?
c) How does Compton scatter affect camera performance?
Can reduce intrinsic spatial resolution. Intrinsic spatial resolution is the limit of spatial resolution achievable by the detector and electronics, ignoring additional blurring due to the collimator. It is limited primarily by 2 factors:
Detection of compton scattered events reduces image quality
Compton scatter: photon interaction with outer shell electron resulting in deflection of photon and ejection of electron
Can reduce intrinsic spatial resolution. Intrinsic spatial resolution is the limit of spatial resolution achievable by the detector and electronics, ignoring additional blurring due to the collimator. It is limited primarily by 2 factors:
Multiple scattering of photons within the detector: if a photon undergoes Compton scattering within the detector crystal and the residual scattered photon is also detected, but at some distance away, the two events are recorded as a single event occurring at a location along the line joining the two interaction sites. For photon energies less than 300 keV this is not significant.
What is HVL? What type of photon beam geometry?
HVL is the half value layer or the amount of a material necessary to attenuate half of the incident radiation
*Dependent on type/density of material and energy/type of incident radiation
*Assumes narrow beam geometry
Examples in lead:
99mTc: 0.03 cm
67Ga: 0.07 cm
60Co: 1.6 cm
What is HVT? What factors affect/influence its value
a. Half-value thickness (thickness of a material that absorb 50% of an incident beam)
b. Z and density of material, and energy and type of incident beam
When is constancy of dose calibrator performed
at installation, daily, after servicing. Use Cs-137 (long-lived) +/- 10%
List 4 things to perform for dose calibrator quality control. Which of these does the shielding method test for?
Accuracy, linearity, constancy, geometry. Shielding is used for linearity.
QC for survey meters
a. Battery check – display on screen
b. Background – performed in an area remote from radioactive sources (to ensure meter is not contaminated)
c. Constancy – using long lived source, should be within +/- 10%
List 3 daily quality control tests for ionization survey meter?
Battery check
Background
Constancy?
Physical inspection (EANM)
How does a Geiger Mueller counter work?
· High sensitivity counting-type ionizing radiation detector
· Ionizations in gas chamber (argon + quenching gas) result in avalanches due to high voltage operation (accelerated electrons -> excited gas molecules -> UV photons -> more ionizations)
GM counter is a gas-filled (argon + quenching gas) detector designed with high voltage across the chamber.
· when ionization occurs, the accelerating electrons in addition to ionizing gas molecules, also cause excitation of gas molecules which in turn release UV photons. The UV photons undergo photoelectric absorption and more electrons are emitted, causing an avalanche ionization. The Geiger discharge continues until a band of slow-moving positive charges around the anode reduce the effective electric field, dropping it below the level needed for gas amplification.
· GM counter CANNOT distinguish different energies
What are characteristics of a quenching gas?
- Give up electrons easily
- When they are neutralized by electrons entering higher energy orbits, they deenergize themselves by dissociating into molecular fragments rather than by emitting UV photons
- Strong absorbers of UV radiation
Well counter - Name 2 reasons why the volume and geometry must be kept constant (2 marks)
- To be able to compare counting rates between two samples, because sample volume/geometry has a significant effect on counting rates.
- To maintain constant geometric efficiency.
The fraction of gamma rays escaping the well (and the geometric efficiency) depends on the position of the source in well. Differences in volume and geometry affect the position of the source in the well and will therefore affect counting rates.
Sorenson p. 197
Name QC tests for dose calibrators and well counters and give their frequency.
Well Counter:
DAILY Q/C: background adjustment, constancy (with long-lived reference source)
QUARTERLY Q/C: Energy resolution (FWHM)
ANNUAL Q/C: Efficiency (cpm/Bq) ref source +/- 5%
Dose Calibrator: says +/-10 % in Saha??
DAILY Q/C: Constancy (reference source +/- 5%)
QUARTERLY Q/C: Linearity (shielding or decay method +/- 5%), Accuracy (2 radionuclides +/- 5%)
After repair/recalibration: Geometry (+/- 5%)
What is the range of activity measured in a well counter. What are the primary factors that determine maximum and minimum detectable activity?
Bq-kBq up to 37 kBq. Crystal thickness and dead time are the primary factors determining minimum and maximum detectable activity
The detection efficiency D (see Chapter 11, Section A) of the NaI(Tl) well counter for most γ-ray emitters is quite high, primarily because of their near 100% geometric efficiency g. The combination of high detection efficiency and low background counting levels makes the well counter highly suitable for counting samples containing very small quantities (Bq–kBq) of γ-ray-emitting activity. The geometric efficiency for small (environ 1-mL) samples in the standard well counter is approximately 93% (see Fig. 11-3).
Because of their high intrinsic and geometric efficiencies (resulting from the use of a thick crystal and a well-type counting geometry, respectively), well counters are extremely sensitive and, in fact, can reliably be used only for counting activities up to approximately 37 kBq; at higher activities, dead-time counting losses become prohibitive and the measured counts inaccurate.
Why measure background before and after counting in a well counter.
- Measure background before and after to have more reliable (mean) background?
- Identify spills( “ Further, even trace contamination of the counting well will produce inaccurately high counting-rate values. Accordingly, a blank (i.e., an empty counting tube or vial) should always be included to determine the current background count.”)
What is detector efficiency and give a formula?
D = ratio of detector counting rate to emission rate of a source = g x ε x f x F
o g = geometric efficiency
o For point source far away ~ Α/(4π r2)
§ A = area of detector
§ r = distance from point source
o For point source close ~ ½ (1 – cos θ)
o ε = intrinsic efficiency
o f = fraction of output signals within PHA window
o F = factor for absorption/scatter occurring within source or between source and detector
Detector Efficiency: D = g x e x f x F Explain each term.
g: geometric efficiency of the detector – efficiency with which detector intercepts radiation emitted from the source, determined mostly by detector size and distance from source.
e: intrinsic efficiency with which detector absorbs incident photons and converts them to potentially usable detector output signal
f: fraction of output signals produced by the detector recorded by the counting system, an important factor in energy-selective counting, when a pulse-height analyzer is used to select signals in a desired amplitude (energy) range
F: Absorption and scatter within the source itself, or by material between source and detector
Define intrinsic efficiency. What factors affect it?
Intrinsic efficiency (ε) – efficiency with which detector absorbs incident radiation events and converts them into potentially usable detector output signal; determined by detector thickness and composition/density and by type and E of radiation – with increased E need increased crystal thickness
o Decreases with increasing E
o Increases with increasing thickness of detector
o Increases as detector density and Z increase (stopping power)
How does geometric efficiency change in relation to volume and position?
Geometric efficiency decreases at a rate equivalent to the square of the distance
o g = geometric efficiency
o For point source far away ~ Α/(4π r2)
§ A = area of detector
§ r = distance from point sourceUncertain about volume component…if they meant area then it increases in proportion to the area of the detector
“The coaxial Ge(Li) detector was developed in order to increase overall detector volume, and thus detection efficiency”
What is the purpose of Tl in the NaI crystal?
Tl doping (0.1-0.4 mol %) adds activation centres that makes NaI crystal an efficient scintillator at room temperature (vs. liquid nitrogen temperatures)
Name 4 components of a liquid scintillation camera.
organic solvent – dissolves scintillator material (the primary solute) and radioactive sample; absorbs most radiation from sample and transfer energy to scintillator molecules
primary solute (aka scintillator or fluor) – absorbs energy from solvent and emits light
secondary solute (waveshifter) – absorb emissions from primary solute and re-emit photons of different wavelength which are better matched to PMT response
additives – improve some aspect of LS performance, eg energy transfer efficiency from solvent to primary solute
What are benefit of semiconductor with multiple pinholes as you ask for admin to pay for this equipment
a. Faster scan time (more scans performed in a day)
b. Lower dose administered to the patient
c. Better spatial resolution (improved images for more accurate diagnosis)
d. Patient comfort (some scanners pt in sitting position and scan faster to acquire)
e. Smaller size of the scanner – needs less room in hospital
What is Z pulse
The summed signal from all PMTs from a single scintillation event, which is then fed to the pulse-height analyzer for analysis.
List three advantages of semiconductor detectors over NaI detectors. List 3 disadvantages of semiconductor detectors. What is the energy resolution in FWHM of a semiconductor detector vs. a NaI detector?
Semiconductor High energy resolution Direct conversion to current Higher efficiency than gas-filled detectors Expensive Requires supercooling Poor stopping power
CZT
Room temperature operation
High Z, good stopping power
NaI Good stopping power 50-250 keV High light output Transparent to own scintillations Inexpensive Scintillation wavelength well matched to PMT
