PET/ PET-CT Flashcards
(27 cards)
by combining PET and other imaging modalities, you can get:
- more precise anatomical localisation
- reduce total radiation exposure (than either modalities separately)
- reduce time
- reduce cost
compare the radioactive isotope emitted, isotope injected, collimator, detector shape, detector crystal used in SPECT/PET
SPECT:
- gamma emitter
- technetium 99m
- mechanical collimator
- 1/3 head collimator
- NaI crystal
PET:
- positron emitter
- carbon-11
- electronic collimator
- circular detector
- bismuth germinate crystal (BGO)
what is the difference in photon energy emission in SPECT/PET
spect = single photon at 60-600keV
pet = 2 photons at 511kev (with 180 degree)
functional imaging obtained with PET can be superimposed with anatomic imaging in CT
how long prior to scan should tracer be injected for PET-CT
1-2 hrs-
typical PET-CT scanning protocol
- tracer injected 1-2 hrs prior
- topogram/scout obtained for positioning
- patient move thru CT gantry/ retrieval of CT spiral scan scan (lower dose than diagnostic ct)
- patient then moved thru PET gantt, PET slices obtained in same located as CT slices
- CT attenuation correction factors (CTAC) generated
- CTAC applied to PET emission data for reconstruction
- PET/CT image fused
find image screenshot and label all parts of CT machine :
high voltage inverter
xray tube
detector
collimator and filter
gantry control computer
cathode/anode voltage tank
data acquisition system
slip ring
explain how PET-CT fusion occurs
- CT slices retrieved
- patient moves on to PET gantry
- whilst pet scanning happens, CT slices get reconstructed, downsampled, energy scaled and resolution mapped
- attenuation map is formed and this is forward projected onto the acquired PET sliced
- the now reconstructed (and attenuation corrected) PET slices fused with CT imaged
explain how positron emitters annihilate in body and what it forms
positron emitters injected
- annihilation of positron with electron = x2 511 Kev photons created
localisation of source of photons is referred to as what, how is this done
LOR, line of response
- the coincident detection of pair of photons (at the scintillator-photomultiplier detectors) can help localise the source of where these simultaneous photons came from
what is the composition of the detector blocks used in pet
matrix of 8x8 (64) scintillator crystals connected to 2x2 (4) photomultiplier tubes
- to determine x,y location of gamma photon, all signals from the 4 pmt are used (same logic as in anger camera)
note: if detection of temporal resolution is ultrafast, you can measure difference in time of arrival of the 2 photons, this improves the positioning of annihilation evens happened outside of line of response,
thus improving SNR and resolution
how does volume increase of scintillator crystals affect sensitivity
increases sensitivity
how does increase sensitivity, larger axial FOV improve PET scanning
increase sensitivity = shorter imaging per bed
larger FOV = fewer bed positions for same axial coverage
overall reduction in imaging time/dose for same image quality
what are the characteristics of the isotopes used in PET
- short half life
- low positron energy
because the nuclides are incorporated with other components e.g glucose or ammonia, they are referred to as radiotracers
whats the most commonly used radio tracer in PET and why
FDG flurodeoxy-glucose
(labelled with fluorine-18)
- very popular as it has a slightly longer half life than other tracers allowing more efficient transportation
where are these radio tracers made, why is the location of these places improtant
- made in a cyclotron
- due to the short half life (20mins) of the tracer, they need to be delivered from a cyclotron in close proximity
explain how a radionuclide is formed from a cyclotron
- cyclotron = particle accelerator
- x2 Dees (metal electrodes) face to face in chamber
- oscillating voltage (thousands of volts) applied to Dees
- ions released at centre, travel in circular path toward rims of dee due to magnetic field
- as it passes between the dees, it increases in acceleration each time
- increase in acceleration causes the ion to move in a larger radius (creating a spiral path)
- particle eventually reaches rim, passes out thru small gap and strikes a target forming radionuclide by NUCLEAR REACTION
difference b/w glucose, deoxyglucose and flurodeoxyglucose molecule?
H atom replaced OH atom in glucose = deoxyglucose (deoxygenated)
F-18 replaced H atom in deoxyglucose = flurodeoxyglucose
deoxyglucose and flurodeoxyglucose cant be further metabolised in cells as theres lack of OH atom
compare FBP and IR reconstruction in PET
refer to notes in note book
as CT scan is used instead of PET transmission scan for attenuation correction of PET data, many artefacts can appear
e.g metallic artefact, respiratory motion, contrast medium, truncation
what is truncation artefact/looks like?
Truncation artifact in CT is an apparently increased curvilinear band of attenuation along the edge of the image.
This artifact is encountered when parts of the imaged body part remain outside the field of view (e.g. due to patient body habitus), which results in inaccurate measurement of attenuation along the edge of the image.
(rounded edges in images as anatomy isnt fit within FOV)
What is contrast medium artefact/ looks like?
As contrast media is strongly attenuating in CT-imaging, it can thereby like metal cause severe artifacts (2-7). These artifacts commonly appear as hypo- and hyperdense areas surrounding the highly attenuating material.
