Lecture 4 Flashcards
what is planar imaging
single projectiokn view at a selected angle. Entire 3D volume is projected onto a single 2D plane, and superposition of overlying and underlying activity. No image reconstruction is required
what is emission tomography
SPECT and PET: emission tomography. Multiple projection views acquired around the patient. Requires image reconstruction to generate cross-sectional images. Eliminates overlap of structures and increases contrast
what is SPECT
gamma camera rotates about axis of rotation, and circualr or body contour step and shoot (10-15s per projection or continuous. Matriz is 64x64 with 60 or 64 projections or 128x128 with 120 or 128 projections. Limited sensitivity camera acquires one projection at a time. Limited dynamic information
how is SPECT analyzed
the projections from a cardiac SPECT acquisition. Projections are input into reconstruction algorithm. Cross-sectional images are obtained for all axial locations within the FOV
what does SPECT represent when taking the image
the SPECt should represent the true 3D distribution of radiopharmaceuticals
what is the signal proportional to
it is proportional to the activity in the corresponding volume of tissue
what factors affect radio-pharmaceutical distribution
attenuation, scatter, spatial resolution, gamma camera, noise
what is attenuation
attenuation is the reduction of the number of detected photons due to photoelectric absorption and compton scatter in the patient.
what does attenuation depend on
depth of source within medium, energy of the photon, and composition of the medium
does attenutation cause an over estrimation or under estimation
attenuation causes an underestimation of activity concentration especially for deeper structures
why is attenuation correction sometimes bad for SPECT
two unknown functions are readiopharmaceutical distribution and attenuation coefficient distrubtion. no closed form analytic equation. It is difficult to reconstruct
how to do uniform attenuation correction
assume that the body is homogenous, draw a contour around object, assign a constant mu to pizels within contour, and most common algorithm is chang attenuation correction
what is required for accurate attenuation correction
it requires knowledge of the composition of the object. Measure the distribution of mu directly. Acquire transmission images using an external source of radiation. Use resulting transmission images in the attenuation correction of SPECt images
what is the most common way to get the attenuation correction
scanning line source and acquire emission and transmission scans simultaneously.
what contributes to scatter in SPECT
a compton scatter interaction within the medium which leads to a false positioning of the source of the detected photo, and adds background and results in loss of contrast
when is the scatter correction applied
once the emission data is corrected for attenuation
what are the two common approaches to scatter correction
use a broad beam instead of a true narrow beam value and this allows for detected scatter in projections- improving quantitative accuracy, and not really a scatter correction
-estimate the scatter in photopeak energy window by acquiring a scatter energy window and assumes scatter distribution in scatter window is the same as in the photopeak window
as the source moves away, what happens to spatial resolution
the spatial resolution degrades as the source is moved away from the collimator. Loss of resolution distorrts
are the corrections needed for quantification of uptake in SPECT
in routine clinical studies, corrections for attenuation, scatter, and resolution are not always applied. They are not needed for subjective visual interpretation of SPECT images. Therapy studies that incorporate dosimetry will require that SPECT images provide accurate quantification of uptake.
what are some acquisition parameters for SPECT image quality
angular, range, orbit, angular increment, matrix size, total counts acquired, reconstruction algorithm, reconstruction filter
what is angular range
only 180 degrees of projections are required for reconstruction. Bone and brain use 360 to reduce effect of variable reoslution. Cardiac SPECt uses 180 as myocardium is anterior and posterior have greater attenuation and little information
what is the orbit
elliptical and body contour orbits: allow gamma camera to pass closer to the patient and mimizes loss of resolution in anterior/posterior projections
what is uniformity
gamma camera nonuniformities create artifacts
what kinda artifacts can be created on gamma camera
ring artifacts
where does the center of rotation align
the mechanical center of rotation should align with the center of the acquisition matrix. Misalignment due to camera sag or wobble, mechnical slop in gantry
what is the rule of thumb for center of rotation
COR misalignment should be <0.5 pixel for a 64x64 matrix
what are dedicated organ SPECT systems for and what are the benefits
designed to perform one imaging procedure only, optimized for organ or interest. Benefits: increased sensitivity- collimators focus on the organ of interest. Increased spatial resolution- detector closer to the organ or interest, and smaller footprint does not need mutliple collimators
what is a D-SPECT design wise
uses 9 pixelated CZT detectors that swivel independently about the vertical axis. Higher sensitivity leads to reduced dose and acquisition time. Requires iterative reconstruction software to generate transaxial images.
what is an adaptive SPECT design
it is 12 independent swivel head detector arms comprised of CZT and flexible organ-specific acquisition- adaptive SPECT. It coveres 360 degree coverage. and it has 3x sensitivity and reduced dose, and acquisition time, and low energy imaging only
