Nuclear Med extra flashcards
Definition Becquerel
number of nuclei decaying per second
Particles and mass: describe mass of positrons and photons
-Positrons have very little mass (esp compared to protons), they are equal to electrons in mass.
-Photon: can be considered to have no mass.
Tc99m: radiation emitted, A and Z number, origin particle
-Mass number 99
-Atomic number: 43
-Is created from Mo-99 when it undergoes Beta minus decay
-Tc 99 has a half life of 200k (so it isn’t technically stable)
Effective doses for: VQ scans, bone scans
-Lung ventilation: 0.1-0.6mSv
-Lung perfusion: 1mSv
Total imaging for V/Q: under 1.6mSv
-Bone: effective dose 6mSv
Bone scan: what radionuclide is used? How much activity administered? How long have to wait post injection to look for mets
-Effective dose is 6mSv
-99mTc-MDP is used
-Activity: 555-750 Mbq
-For mets: time to scan following injection time is 3-4 h.
What do we call the process of new element formation (post alpha decay)
Transmutation
Shielding: what can you use for gamma, x-rays, apha and beta
-Gamma: concrete, barium plaster, lead
-X-ray: lead aprons
-Alpha: sheet of paper
-Beta: plastic
18-F: what is it normally attached to? Half life and where is it made?
-18-F: attached to FDG (fluorodeoxyglucose) to produce 18-FDG and is metabolised by tissues with a high metabolic activity
and glucose update —> used in diagnosis, staging and monitoring of cancer.
-Fluorine-18: Most common used radionuclide
-Half life: 110 minutes
-Radionuclides have short half lives, produced in cyclotrons
What is the half life of Nitrogen 13?
10 mins
What is the half life of Carbon 11?
20 mins
What is the half life of Oxygen 15?
2 mins
What is the half life of Iodine 131?
8 days
What is the half life of Iodine 123
13h
What is the half life of Xenon 133
5 days
What is the half life of thallium 201
3 days
Name nuclides produced in a cyclotron
fluoride18, 67 Ga, 111ln, 123 I, 201 TI
Name a nuclide produced in a generator
99mTc
Name a nuclide produced in a nuclear reactor
125 I
Name nuclides that are fission products
131 I, 133 Xe
By how much can lead glass sleeves used for syringes can reduce the radiation?
75%
Do gamma cameras only detect gamma rays?
-Can detect more than gamma rays - anything’s that has radiation will stimulate the crystal but the PHA will limit their inclusion in the image if they are not of the expected energy.
Gamma camera: what is the crystal made of? How does thickness affect resolution and sensitivity
-Large NaI crystal is typically 9-12mm thick.
-Spatial resolution is increased using a thinner crystal
-Typical SR is 5mm
-Thicker crystal: improves sensitivity
Gamma camera: what is the typical sensitivity of collimators?
1%
What is the HVL of lead for Tc-99?
0.3mm thick
PMTs: a flash of 1 gamma photon usually releases X light photons
5000 light photons
How does increasing PHA affect sensitivity/spatial resolution?
-increasing the PHA increases sensitivity but reduces spatial resolution due to increased scatter of image.
T/f only ever used 1 PHA window
What is the window width normally set at?
-False
-Can use multiple PHA windows (eg when more than one gamma ray is emitted in In-III)
-Window is usually set at +/- 10%
What is the greatest limiting factor of gamma camera images?
Noise
SPECT: what is the SR and how does this compare to PET?
-15-20mm
(SR of PET is 4.5mm in centre and 5mm in periphery)
What reconstruction algorithm is used in SPECT?
-FBP
How does SPECT work?
-Measures gamma rays released from decay of radio-isotopes (same ones detected in regular gamma imaging).
-Multiple axial sections are created simultaneously from one gamma camera
-Camera seaming stationary for up to 30 secs and images are collected in 6 degree increments.
-Can have up to 3 cameras detecting simultaneously —> obtain scan faster which reduces the effect of patient movement or increases count rate for same scan time. Both lead to improved resolution of the image.
SPECT: what is the max accepted variation in count rate?
-Quality control: max accepted variation in count rate is 2%
Describe how PET works
-F-18 decays by releasing a positron - this is due to neutron deficit in the atom (Beta plus decay).
-Distribution of the radiotracer is determined by the detection of coincident gamma rays
-PET scanners consist of a stationary ring of detectors that surround the patient
What kind of detectors are used in PET?
-Scintillation detectors: most common detector used in PET to detect coincident gamma rays
-Bismuth germinate: short decay time (300 nanoseconds): allows for rapid measurements and very little dead time.
Why is CT done alongside PET?
-Structures with little/no FDG update will not be seen on PET scan normal tissues are not clearly represented, this is why PET
is often fused with CT.
-CT allows for anatomical correlation and attenuation corrections to be made on PET images.
-Scan acquisition time: usually 30 mins.
What is the resolution of PET?
-8-10mm
-Negative PET scan cannot reliably exclude serious pathology in lesions smaller than this
-Lesions below this size with a high metabolic activity can still be detected.
Proton rich or proton poor nuclides result in PET emission? How are the photons made?
Proton rich radionuclides
-Achieve stability by converting a proton into a neutron, with the positive charge being carried away from the nucleus by a
positron
-After a few mm, the positron combines (annihilates) with an electron outside the radioactive nucleus and releases 2 photons
at 180 degree trajectories, each with an energy of approx 511 keV. The positrons do not have that energy.
What are the most radiosensitive and resistant phases of the cell cycle?
-Most radioresistant phase of the cell cycle is: S phase
-Most radiosensitive phase: Late G2 and M phases
Describe a Geiger Muller counter (outside, sensor, walls, anode)
-Gas filled radiation detector
-Sensor: inert gas-filled tube (helium, neon or argon) that briefly conducts an electrical current when a particle or photon of radiation temporarily ionises the gas.
-Walls: metal or coated to form cathode.
-Anode: thin central wire
Can a Geiger Muller Counter be used for monitoring dose?
-Used for quality control, NOT for monitoring dose.
