SPECT / SPECT-CT Flashcards
(33 cards)
what does SPECT and PET stand for
single photon emission
position emission
what is spect
generates 3D rep of radiopharmaceutical distribution in body
(spect is to planar imaging, what ct is to plain film xray)
(initially improved planar imaging of skeleton by visually separating overlying structures e.g scapula and rib)
how is signal in SPECT acquired
- rotating gamma cameras
what are the 2 main SPECT detector configurations and what is each used for
H mode (360 degrees) - for oncology
L mode (180 degrees) - for cardiac imaging
SPECT acquisition parameters: projections? matrix?
128 projections (approx) at different angles (3 degree angle differences)
128x128 or 64x64 pixel matrix
how long is the imaging time for each projection
20-30s
- thus each scan doesnt show much detail but when you combine all 128 projections together, there is detail
what are 2 methods of camera movement in SPECT
step and shoot (image, move 3 degrees and shoot etc)
continuous (camera moves continuously and acquire continuously)
what is the issue with continuous shoot than step and shoort
may suffer from motion blurring (unless processing can correct)
important to display raw data to ensure no patient movement will effect reconstruction, how can this be assessed, how does it work
using CINE motion images of projections
- cine data shows distance ‘bounce’ during image where motion is, guidelines show the up and down movement
another method of evaluating raw projections to negate motion affect in SPECT is via sinogram, what is this
displays each image angle and each activity position of all projections in one
(accumulation of all projectional data at each angle forms sin wave (sinogram))
2 methods of spect image reconstruction are?
FBP and IR
what is star artefact that can be found in post processing of images
multiple angles form star artefact of unfiltered BP, caused by the high energy gamma rays being able to pass through the walls of the collimator. It looks like a star because of the way that the collimators are constructed
blurring caused by back projection is improved with what?
ramp filter
- this is a high pass filter that does not permit low frequencies that cause blurring to appear in the image.
- applied to each raw image prior to back projection
what is a downside to using ramp filters
as it increase amp of high freq / decrease low freq changes, it increase image noise
what is used to decrease the increased image noise from using ramp filters to remove blurring of SPECT image
low pass filter (soft kernel)
with FBP, final product is 3D pixels known as voxels
to make sagittal and coronal planes, reorientate the voxels
SPECT more commonly uses IR instead of FBP now, what is the name of the commonly used IR method
OSEM
ordered subset expectation maximisation
(refined mathematical technique thats reduced number of iteration needed to reach convergence)
how has OSEM improved from normal IR method
processes groups of image projection rather than whole set at a time
how does attenuation correct treat attenuation artefact in SPECT
- corrects absorption/scatter of gamma rays inside body (lower activity for deep tissue vs more superficial ones)
- CT gives information of patient size and internal tissue distribution
- probability for photon transmission thru patient calculated using that info
- calculated length of expected line considering tissue density along line helps form correction factor thats applied for each projection view
when would attenuation correction (AC) be applied for FBP/IR post procession
FBP= applied before OR after reconstruction
IR= applied INTO reconstruction
how does scatter artefact happen and how does scatter correction work
- gamma rays scatter and not absorbed by body, this can be part of image registered in wrong location
- during scattering process, they lose energy but can still be included in image so
- measure scatter photons in separate lower activity window and subtract this contribution
what term is used to describe the 90 degree/right angle of photon detection at the detector
orthogonal
e.g direct primary photon from body to detector = orthogonal primary photon,
primary photon from body received at detector (not at right angle) = non-orthogonal primary photon
what is the use of CT in SPECT-CT hybrid imaging
- ct can provide attenuation map (customised for each subject)
- dose is low from CT and initial scout tomogram done to optimise exposure (for each patient)
huge pro from SPECT-CT is u can get physiology/function AND anatomical detail with patient lying in same position
but NM only gets physiology no anatomy
so in spect ct e.g you have have CT heart image then overlay function/physiology image from SPECT
